a&p module 4
TRANSCRIPT
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Text: Marieb and Hoehn, 8th edn – 2010
I J i H i i Mi h ll
Biology 141 – Human Anatomy and Physiology
Instructor: Jamie Heisig-Mitchell
Lecture Outline: Chapters 10 - 14
Skeletal Muscles: Functional Groups
1. Prime movers Provide the major force for producing a specific
movement
2. Antagonistsg Oppose or reverse a particular movement
Skeletal Muscles: Functional Groups
3. Synergists Add force to a movement
Reduce undesirable or unnecessary movement
4 Fixators4. Fixators Synergists that immobilize a bone or muscle’s origin
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Muscle Mechanics: Lever Systems
Components of a lever system Lever—rigid bar (bone) that moves on a fixed point or
fulcrum (joint)
Effort—force (supplied by muscle contraction) applied to a lever to move a resistance (load)
Load—resistance (bone + tissues + any added weight) moved by the effort
Effort x length of effort arm = load x length of load arm(force x distance) = (resistance x distance)
Effort
Effort
10kg
25 cm
0.25 cm
10 x 25 = 1000 x 0.25250 = 250
(a) Mechanical advantage with a power lever
Load
Fulcrum
Load
1000 kg
Fulcrum
Figure 10.2a
Load
Effort
Effort
100 kg
25 cm
50 cm
Figure 10.2b
100 x 25 = 50 x 502500 = 2500
(b) Mechanical disadvantage with a speed lever
Fulcrum
Load
50 kg
Fulcrum
50 cm
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Classes of Lever Systems
First class Fulcrum between load and effort
(a) First-class leverArrangement of the elements is
load-fulcrum-effort
Fulcrum
Load Effort
Figure 10.3a (1 of 2)
Example: scissors
Load
FulcrumEffort
(a) First-class leverArrangement of the elements is
load-fulcrum-effort
Figure 10.3a (2 of 2)
In the body: A first-class lever systemraises your head off your chest. Theposterior neck muscles provide the effort,the atlanto-occipital joint is the fulcrum,and the weight to be lifted is the facialskeleton.
Load
Fulcrum
Effort
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Classes of Lever Systems
Second class Load between fulcrum and effort
(b) Second-class leverArrangement of the elements is
fulcrum-load-effort
Load
Fulcrum Effort
Figure 10.3b (1 of 2)
Example: wheelbarrow
Load
Effort
Fulcrum
(b) Second-class leverArrangement of the elements is
fulcrum-load-effort
Effort
Figure 10.3b (2 of 2)
In the body: Second-class leverage isexerted when you stand on tip-toe. Theeffort is exerted by the calf musclespulling upward on the heel; the joints ofthe ball of the foot are the fulcrum; andthe weight of the body is the load.
Load
Fulcrum
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Classes of Lever Systems
Third class Effort applied between fulcrum and load
(c) Third-class leverArrangement of the elements is
load-effort-fulcrum
Load Effort
Fulcrum
Figure 10.3c (1 of 2)
Example: tweezers or forceps
Fulcrum
Load
Effort
Fulcrum
(c) Third-class leverArrangement of the elements is
load-effort-fulcrum
Effort
Figure 10.3c (2 of 2)
In the body: Flexing the forearm by thebiceps brachii muscle exemplifiesthird-class leverage. The effort is exertedon the proximal radius of the forearm, thefulcrum is the elbow joint, and the load isthe hand and distal end of the forearm.
Load
Fulcrum
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Functions of the Nervous System
1. Sensory input Information gathered by sensory receptors about
internal and external changes
2. Integrationg Interpretation of sensory input
3. Motor output Activation of effector organs (muscles and glands)
produces a response
Sensory input
Figure 11.1
Motor output
Integration
Divisions of the Nervous System
Central nervous system (CNS) Brain and spinal cord
Integration and command center
Peripheral nervous system (PNS) Peripheral nervous system (PNS) Paired spinal and cranial nerves carry messages to and
from the CNS
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Peripheral Nervous System (PNS)
Two functional divisions1. Sensory (afferent) division
Somatic afferent fibers—convey impulses from skin, skeletal muscles, and joints
Visceral afferent fibers—convey impulses from visceral Visceral afferent fibers convey impulses from visceral organs
2. Motor (efferent) division Transmits impulses from the CNS to effector organs
Motor Division of PNS
1. Somatic (voluntary) nervous system Conscious control of skeletal muscles
Motor Division of PNS
2. Autonomic (involuntary) nervous system (ANS) Visceral motor nerve fibers
Regulates smooth muscle, cardiac muscle, and glands
Two functional subdivisionsv Sympathetic – “Fight or Flight”
Parasympathetic - digestion, urination, defecation, etc
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Central nervous system (CNS)Brain and spinal cordIntegrative and control centers
Peripheral nervous system (PNS)Cranial nerves and spinal nervesCommunication lines between theCNS and the rest of the body
Motor (efferent) divisionMotor nerve fibersConducts impulses from the CNSto effectors (muscles and glands)
Sensory (afferent) divisionSomatic and visceral sensorynerve fibersConducts impulses fromreceptors to the CNS
Somatic nervoussystem
Somatic motor(voluntary)Conducts impulsesfrom the CNS toskeletal muscles
Autonomic nervoussystem (ANS)
Visceral motor(involuntary)Conducts impulsesfrom the CNS tocardiac muscles,
Somatic sensoryfiber Skin
Figure 11.2
Parasympatheticdivision
Conserves energyPromotes house-keeping functionsduring rest
skeletal muscles
Sympathetic divisionMobilizes bodysystems during activity
cardiac muscles,smooth muscles,and glands
StructureFunctionSensory (afferent)division of PNS Motor (efferent) division of PNS
Visceral sensory fiber
Motor fiber of somatic nervous system
StomachSkeletalmuscle
Heart
BladderParasympathetic motor fiber of ANS
Sympathetic motor fiber of ANS
Histology of Nervous Tissue
Two principal cell types1. Neurons—excitable cells that transmit electrical
signals
Histology of Nervous Tissue
2. Neuroglia (glial cells)—supporting cells: Astrocytes (CNS) Microglia (CNS) Ependymal cells (CNS) Oligodendrocytes (CNS) Oligodendrocytes (CNS) Satellite cells (PNS) Schwann cells (PNS)
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Astrocytes
Most abundant, versatile, and highly branched glial cells
Cling to neurons, synaptic endings, and capillaries
Support and brace neurons Support and brace neurons
Astrocytes
Help determine capillary permeability
Guide migration of young neurons
Control the chemical environment
P ti i t i i f ti i i th b i Participate in information processing in the brain
Capillary
Neuron
Figure 11.3a
(a) Astrocytes are the most abundantCNS neuroglia.
Astrocyte
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Microglia
Small, ovoid cells with thorny processes
Migrate toward injured neurons
Phagocytize microorganisms and neuronal debris
NeuronMicroglial
Figure 11.3b
(b) Microglial cells are defensive cells inthe CNS.
Microglialcell
Ependymal Cells
Range in shape from squamous to columnar
May be ciliated Line the central cavities of the brain and spinal column
Separate the CNS interstitial fluid from the Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities
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Brain or
Ependymalcells
Fluid-filled cavity
Figure 11.3c
Brain orspinal cordtissue
(c) Ependymal cells line cerebrospinalfluid-filled cavities.
Oligodendrocytes
Branched cells
Processes wrap CNS nerve fibers, forming insulating myelin sheaths
N
Myelin sheath
Process ofoligodendrocyte
Figure 11.3d
(d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers.
Nervefibers
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Satellite Cells and Schwann Cells
Satellite cells Surround neuron cell bodies in the PNS
Schwann cells (neurolemmocytes) Surround peripheral nerve fibers and form myelin Surround peripheral nerve fibers and form myelin
sheaths
Vital to regeneration of damaged peripheral nerve fibers
Schwann cells(forming myelin sheath)
Cell body of neuronSatellitecells
N fib
Figure 11.3e
(e) Satellite cells and Schwann cells (whichform myelin) surround neurons in the PNS.
Nerve fiber
Neurons (Nerve Cells)
Special characteristics: Long-lived ( 100 years or more) Amitotic—with few exceptions High metabolic rate—depends on continuous supply of
d loxygen and glucose Plasma membrane functions in: Electrical signaling Cell-to-cell interactions during development
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Cell Body (Perikaryon or Soma)
Biosynthetic center of a neuron
Spherical nucleus with nucleolus
Well-developed Golgi apparatus
R h ER ll d Ni l b di ( h t hili Rough ER called Nissl bodies (chromatophilic substance)
Cell Body (Perikaryon or Soma)
Network of neurofibrils (neurofilaments)
Axon hillock—cone-shaped area from which axon arises
Clusters of cell bodies are called nuclei in the CNS Clusters of cell bodies are called nuclei in the CNS, ganglia in the PNS
Dendrites(receptive regions)
Cell body(biosynthetic centerand receptive region)
Nucleolus
Figure 11.4b
Nucleus
Nissl bodies
Axon(impulse generatingand conducting region)
Axon hillock
NeurilemmaTerminalbranches
Node of Ranvier
Impulsedirection
Schwann cell(one inter-node)
Axonterminals(secretoryregion)
(b)
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Processes
Dendrites and axons
Bundles of processes are called Tracts in the CNS
Nerves in the PNS Nerves in the PNS
Dendrites
Short, tapering, and diffusely branched
Receptive (input) region of a neuron
Convey electrical signals toward the cell body as graded potentials graded potentials
The Axon
One axon per cell arising from the axon hillock
Long axons (nerve fibers)
Occasional branches (axon collaterals)
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The Axon
Numerous terminal branches (telodendria)
Knoblike axon terminals (synaptic knobs or boutons) Secretory region of neuron
Release neurotransmitters to excite or inhibit other cells Release neurotransmitters to excite or inhibit other cells
Axons: Function
Conducting region of a neuron
Generates and transmits nerve impulses (action potentials) away from the cell body
Axons: Function
Molecules and organelles are moved along axons by motor molecules in two directions: Anterograde—toward axonal terminal Examples: mitochondria, membrane components, enzymes
Retrograde—toward the cell body Examples: organelles to be degraded, signal molecules,
viruses, and bacterial toxins
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Myelin Sheath
Segmented protein-lipoid sheath around most long or large-diameter axons
It functions to: Protect and electrically insulate the axon Protect and electrically insulate the axon
Increase speed of nerve impulse transmission
Myelin Sheaths in the PNS
Schwann cells wraps many times around the axon Myelin sheath—concentric layers of Schwann cell
membrane
Neurilemma—peripheral bulge of Schwann cell p p gcytoplasm
Myelin Sheaths in the PNS
Nodes of Ranvier Myelin sheath gaps between adjacent Schwann cells
Sites where axon collaterals can emerge
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Schwann cellcytoplasm
Axon Schwann cellnucleus
Schwann cellplasma membrane
1
2
A Schwann cellenvelopes an axon.
The Schwann cell thenrotates around the axon, wrapping its plasma
Figure 11.5a
(a) Myelination of a nervefiber (axon)
Neurilemma
Myelin sheath
3
wrapping its plasma membrane loosely around it in successive layers.
The Schwann cellcytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath.
Unmyelinated Axons
Thin nerve fibers are unmyelinated
One Schwann cell may incompletely enclose 15 or more unmyelinated axons
Myelin Sheaths in the CNS
Formed by processes of oligodendrocytes, not the whole cells
Nodes of Ranvier are present
No neurilemma No neurilemma
Thinnest fibers are unmyelinated
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N
Myelin sheath
Process ofoligodendrocyte
Figure 11.3d
(d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers.
Nervefibers
White Matter and Gray Matter
White matter Dense collections of myelinated fibers
Gray matter Mostly neuron cell bodies and unmyelinated fibers Mostly neuron cell bodies and unmyelinated fibers
Structural Classification of Neurons
Three types:1. Multipolar—1 axon and several dendrites
Most abundant
Motor neurons and interneurons
2. Bipolar—1 axon and 1 dendrite Rare, e.g., retinal neurons
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Structural Classification of Neurons
3. Unipolar (pseudounipolar)—single, short process that has two branches: Peripheral process—more distal branch, often associated
with a sensory receptor
Central process branch entering the CNS Central process—branch entering the CNS
Table 11.1 (1 of 3)
Table 11.1 (2 of 3)
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Functional Classification of Neurons
Three types: 1. Sensory (afferent)
Transmit impulses from sensory receptors toward the CNS
2. Motor (efferent) Carry impulses from the CNS to effectors
Functional Classification of Neurons
3. Interneurons (association neurons) Shuttle signals through CNS pathways; most are entirely
within the CNS
Table 11.1 (3 of 3)
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Neuron Function
Neurons are highly irritable
Respond to adequate stimulus by generating an action potential (nerve impulse)
Impulse is always the same regardless of stimulus Impulse is always the same regardless of stimulus
Principles of Electricity
Opposite charges attract each other
Energy is required to separate opposite charges across a membrane
Energy is liberated when the charges move toward Energy is liberated when the charges move toward one another
If opposite charges are separated, the system has potential energy
Definitions
Voltage (V): measure of potential energy generated by separated charge
Potential difference: voltage measured between two pointsp
Current (I): the flow of electrical charge (ions) between two points
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Definitions
Resistance (R): hindrance to charge flow (provided by the plasma membrane)
Insulator: substance with high electrical resistance
Conductor: substance with low electrical resistance Conductor: substance with low electrical resistance
Role of Membrane Ion Channels
Proteins serve as membrane ion channels
Two main types of ion channels1. Leakage (nongated) channels—always open
Role of Membrane Ion Channels
2. Gated channels (three types): Chemically gated (ligand-gated) channels—open with binding
of a specific neurotransmitter Voltage-gated channels—open and close in response to
changes in membrane potential Mechanically gated channels—open and close in response to Mechanically gated channels open and close in response to
physical deformation of receptors
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Na+Na+
Receptor
Na+
Na+
Neurotransmitter chemicalattached to receptor
Chemicalbinds
Membranevoltageh
Figure 11.6
(b) Voltage-gated ion channels open and close in responseto changes in membrane voltage.
Closed Open
(a) Chemically (ligand) gated ion channels open when theappropriate neurotransmitter binds to the receptor,allowing (in this case) simultaneous movement of Na+ and K+.
K+
K+
Closed Open
changes
Gated Channels
When gated channels are open: Ions diffuse quickly across the membrane along their
electrochemical gradients Along chemical concentration gradients from higher
concentration to lower concentrationconcentration to lower concentration Along electrical gradients toward opposite electrical charge
Ion flow creates an electrical current and voltage changes across the membrane
Resting Membrane Potential (Vr)
Potential difference across the membrane of a resting cell Approximately –70 mV in neurons (cytoplasmic side of
membrane is negatively charged relative to outside)
G d b Generated by: Differences in ionic makeup of ICF and ECF Differential permeability of the plasma membrane
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Resting Membrane Potential
Differences in ionic makeup ICF has lower concentration of Na+ and Cl– than ECF
ICF has higher concentration of K+ and negatively charged proteins (A–) than ECF
Resting Membrane Potential
Differential permeability of membrane Impermeable to A–
Slightly permeable to Na+ (through leakage channels)
75 times more permeable to K+ (more leakage 75 p ( gchannels)
Freely permeable to Cl–
Resting Membrane Potential
Negative interior of the cell is due to much greater diffusion of K+ out of the cell than Na+ diffusion into the cell
Sodium-potassium pump stabilizes the resting p p p gmembrane potential by maintaining the concentration gradients for Na+ and K+
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Suppose a cell has only K+ channels...K+ loss through abundant leakagechannels establishes a negativemembrane potential.
The permeabilities of Na+ and K+ across the membrane are different.
The concentrations of Na+ and K+ on each side of the membrane are different.
Na+
(140 mM )K+
(5 mM )
K+ leakage channelsK+K+
Outside cell
Inside cellNa+-K+ ATPases (pumps) maintain the concentration gradients of Na+ and K+
across the membrane.
The Na+ concentration is higher outside the cell.
The K+ concentration is higher inside the cell.
K+
(140 mM )Na+
(15 mM )
Figure 11.8
Finally, let’s add a pump to compensate for leaking ions.Na+-K+ ATPases (pumps) maintain the concentration gradients, resulting in the resting membrane potential.
Now, let’s add some Na+ channels to our cell...Na+ entry through leakage channels reducesthe negative membrane potential slightly.
Cell interior–90 mV
Cell interior–70 mV
Cell interior–70 mV
Na+
Na+-K+ pump
K+K+
K+
Na+
K+
K+K
Na+
K+K+ Na+
K+K+
Membrane Potentials That Act as Signals Membrane potential changes when:
Concentrations of ions across the membrane change
Permeability of membrane to ions changes
Changes in membrane potential are signals used to Changes in membrane potential are signals used to receive, integrate and send information
Membrane Potentials That Act as Signals Two types of signals
Graded potentials Incoming short-distance signals
Action potentials Long-distance signals of axons
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Changes in Membrane Potential
Depolarization A reduction in membrane potential (toward zero)
Inside of the membrane becomes less negative than the resting potential
Increases the probability of producing a nerve impulse
Depolarizing stimulus
Insidepositive
Insidenegative
Depolarization
Figure 11.9a
Time (ms)
Restingpotential
Depolarization
(a) Depolarization: The membrane potentialmoves toward 0 mV, the inside becoming less negative (more positive).
Changes in Membrane Potential
Hyperpolarization An increase in membrane potential (away from zero)
Inside of the membrane becomes more negative than the resting potential
Reduces the probability of producing a nerve impulse
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Hyperpolarizing stimulus
Figure 11.9b
Time (ms)
Restingpotential
Hyper-polarization
(b) Hyperpolarization: The membranepotential increases, the inside becomingmore negative.
Graded Potentials
Short-lived, localized changes in membrane potential
Depolarizations or hyperpolarizations
Graded potential spreads as local currents change Graded potential spreads as local currents change the membrane potential of adjacent regions
Depolarized region
Stimulus
Figure 11.10a
Plasmamembrane
(a) Depolarization: A small patch of the membrane (red area) has become depolarized.
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Figure 11.10b
(b) Spread of depolarization: The local currents (black arrows) that are created depolarize adjacent membrane areas and allow the wave of depolarization to spread.
Graded Potentials
Occur when a stimulus causes gated ion channels to open E.g., receptor potentials, generator potentials,
postsynaptic potentials
M i d i di l ( d d) i h i l Magnitude varies directly (graded) with stimulus strength
Decrease in magnitude with distance as ions flow and diffuse through leakage channels
Short-distance signals
Active area(site of initialdepolarization)
bran
e po
tent
ial (
mV
)
Figure 11.10c
Distance (a few mm)
–70Resting potential
(c) Decay of membrane potential with distance: Because current is lost through the “leaky” plasma membrane, the voltage declines with distance from the stimulus (the voltage is decremental). Consequently, graded potentials are short-distance signals.
Mem
b
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Action Potential (AP)
Brief reversal of membrane potential with a total amplitude of ~100 mV
Occurs in muscle cells and axons of neurons
Does not decrease in magnitude over distance Does not decrease in magnitude over distance
Principal means of long-distance neural communication
1 2 3
4
Resting state Depolarization Repolarization
Hyperpolarization
The big picture
3
enti
al (m
V)
Actionpotential
Hyperpolarization
1 1
2
4
Time (ms)
ThresholdMem
bran
e po
te
Figure 11.11 (1 of 5)
Generation of an Action Potential
Resting state Only leakage channels for Na+ and K+ are open
All gated Na+ and K+ channels are closed
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Properties of Gated Channels
Properties of gated channels Each Na+ channel has two voltage-sensitive gates Activation gates
Closed at rest; open with depolarization
Inactivation gates Open at rest; block channel once it is open
Properties of Gated Channels
Each K+ channel has one voltage-sensitive gate
Closed at rest
Opens slowly with depolarization
Depolarizing Phase
Depolarizing local currents open voltage-gated Na+ channels
Na+ influx causes more depolarization
At threshold (–55 to –50 mV) positive feedback At threshold ( 55 to 50 mV) positive feedback leads to opening of all Na+ channels, and a reversal of membrane polarity to +30mV (spike of action potential)
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Repolarizing Phase
Repolarizing phase Na+ channel slow inactivation gates close
Membrane permeability to Na+ declines to resting levels
Slow voltage-sensitive K+ gates open
K+ exits the cell and internal negativity is restored
Hyperpolarization
Hyperpolarization Some K+ channels remain open, allowing excessive K+
efflux
This causes after-hyperpolarization of the membrane (undershoot)
Actionpotential
3
The AP is caused by permeability changes inthe plasma membrane
oten
tial
(m
V)
ne p
erm
eabi
lity
Time (ms)
1 1
2
4
Na+ permeability
K+ permeability
Mem
bran
e po
Rel
ativ
e m
embr
an
Figure 11.11 (2 of 5)
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Role of the Sodium-Potassium Pump
Repolarization Restores the resting electrical conditions of the neuron
Does not restore the resting ionic conditions
Ionic redistribution back to resting conditions is Ionic redistribution back to resting conditions is restored by the thousands of sodium-potassium pumps
Propagation of an Action Potential
Na+ influx causes a patch of the axonal membrane to depolarize
Local currents occur
Na+ channels toward the point of origin are Na channels toward the point of origin are inactivated and not affected by the local currents
Propagation of an Action Potential
Local currents affect adjacent areas in the forward direction
Depolarization opens voltage-gated channels and triggers an APgg
Repolarization wave follows the depolarization wave
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Voltageat 0 ms
Recordingelectrode
Figure 11.12a
(a) Time = 0 ms. Action potential has not yet reached the recording electrode.
Resting potential
Peak of action potential
Hyperpolarization
Voltageat 2 ms
Figure 11.12b
(b) Time = 2 ms. Action potential peak is at the recording electrode.
Voltageat 4 ms
Figure 11.12c
(c) Time = 4 ms. Action potential peak is past the recording electrode. Membrane at therecording electrode is still hyperpolarized.
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Threshold
At threshold: Membrane is depolarized by 15 to 20 mV
Na+ permeability increases
Na influx exceeds K+ effluxN
The positive feedback cycle begins
Threshold
Subthreshold stimulus - weak local depolarization that does not reach threshold
Threshold stimulus - strong enough to push the membrane potential toward and beyond threshold p y
AP is an all-or-none phenomenon - action potentials either happen completely, or not at all
Coding for Stimulus Intensity
All action potentials are alike and are independent of stimulus intensity How does the CNS tell the difference between a weak
stimulus and a strong one?
Strong stimuli can generate action potentials more often than weaker stimuli
The CNS determines stimulus intensity by the frequency of impulses
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Actionpotentials
Figure 11.13
ThresholdStimulus
Time (ms)
Absolute Refractory Period
Time from the opening of the Na+ channels until the resetting of the channels
Ensures that each AP is an all-or-none event
Enforces one-way transmission of nerve impulses Enforces one-way transmission of nerve impulses
Absolute refractoryperiod
Relative refractoryperiod
Depolarization(Na+ enters)
Figure 11.14
Stimulus
Time (ms)
Repolarization(K+ leaves)
After-hyperpolarization
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Relative Refractory Period
Follows the absolute refractory period Most Na+ channels have returned to their resting state Some K+ channels are still open Repolarization is occurring
Threshold for AP generation is elevated Exceptionally strong stimulus may generate an AP
Conduction Velocity
Conduction velocities of neurons vary widely Effect of axon diameter
Larger diameter fibers have less resistance to local current flow and have faster impulse conduction
Effect of myelination Continuous conduction in unmyelinated axons is slower
than saltatory conduction in myelinated axons
Conduction Velocity
Effects of myelination Myelin sheaths insulate and prevent leakage of charge Saltatory conduction in myelinated axons is about
30 times fasterV lt t d N + h l l t d t th d Voltage-gated Na+ channels are located at the nodes
APs appear to jump rapidly from node to node
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Size of voltage
Voltage-gatedion channel
Stimulus
Stimulus
(a) In a bare plasma membrane (without voltage-gatedchannels), as on a dendrite, voltage decays becausecurrent leaks across the membrane.
Figure 11.15
Stimulus
Myelinsheath
Node of Ranvier
Myelin sheath
(b) In an unmyelinated axon, voltage-gated Na+ and K+
channels regenerate the action potential at each pointalong the axon, so voltage does not decay. Conduction is slow because movements of ions and of the gatesof channel proteins take time and must occur beforevoltage regeneration occurs.
(c) In a myelinated axon, myelin keeps current in axons(voltage doesn’t decay much). APs are generated onlyin the nodes of Ranvier and appear to jump rapidlyfrom node to node.
1 mm
Multiple Sclerosis (MS)
An autoimmune disease that mainly affects young adults Symptoms: visual disturbances, weakness, loss of muscular
control, speech disturbances, and urinary incontinence Myelin sheaths in the CNS become nonfunctional scleroses
Sh i d h i i i f i l Shunting and short-circuiting of nerve impulses occurs Impulse conduction slows and eventually ceases
Multiple Sclerosis: Treatment
Some immune system–modifying drugs, including interferons and Copazone: Hold symptoms at bay
Reduce complicationsp
Reduce disability
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Nerve Fiber Classification
Nerve fibers are classified according to: Diameter
Degree of myelination
Speed of conductionSp
Nerve Fiber Classification
Group A fibers Large diameter, myelinated somatic sensory and motor
fibers
Group B fibersp Intermediate diameter, lightly myelinated ANS fibers
Group C fibers Smallest diameter, unmyelinated ANS fibers
The Synapse
A junction that mediates information transfer from one neuron: To another neuron, or
To an effector cell
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The Synapse
Presynaptic neuron—conducts impulses toward the synapse
Postsynaptic neuron—transmits impulses away from the synapsey p
Types of Synapses
Axodendritic—between the axon of one neuron and the dendrite of another
Axosomatic—between the axon of one neuron and the soma of another
Less common types: Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrite to soma)
Dendrites
Cell body
Axon
Axodendriticsynapses
Axoaxonic synapses
Axosomaticsynapses
(a)
Figure 11.16
Axosomaticsynapses
Cell body (soma) ofpostsynaptic neuron
Axon
(b)
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Electrical Synapses
Less common than chemical synapses Neurons are electrically coupled (joined by gap
junctions)
Communication is very rapid, and may be unidirectional or bidirectional
Are important in: Embryonic nervous tissue
Some brain regions
Chemical Synapses
Specialized for the release and reception of neurotransmitters
Typically composed of two parts Axon terminal of the presynaptic neuron, which contains Axon terminal of the presynaptic neuron, which contains
synaptic vesicles
Receptor region on the postsynaptic neuron
Synaptic Cleft
Fluid-filled space separating the presynaptic and postsynaptic neurons
Prevents nerve impulses from directly passing from one neuron to the next
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Synaptic Cleft
Transmission across the synaptic cleft: Is a chemical event (as opposed to an electrical one)
Involves release, diffusion, and binding of neurotransmitters
Ensures unidirectional communication between neurons
Information Transfer
AP arrives at axon terminal of the presynaptic neuron and opens voltage-gated Ca2+ channels
Synaptotagmin protein binds Ca2+ and promotes fusion of synaptic vesicles with axon membraney p
Exocytosis of neurotransmitter occurs
Information Transfer
Neurotransmitter diffuses and binds to receptors (often chemically gated ion channels) on the postsynaptic neuron
Ion channels are opened, causing an excitatory or p , g yinhibitory event (graded potential)
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Action potentialarrives at axon terminal.
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Ca2+ entry causesneurotransmitter-
Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters.
Ca2+
Axon
Mitochondrion
Postsynapticneuron
Presynapticneuron
Presynapticneuron
Synapticcleft
Ca2+
Ca2+
Ca2+
1
2
3
Figure 11.17
containing synapticvesicles to release theircontents by exocytosis.
Synapticvesicles
Axonterminal
Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane.
Binding of neurotransmitteropens ion channels, resulting ingraded potentials.
Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse.
Ion movement
Graded potentialReuptake
Enzymaticdegradation
Diffusion awayfrom synapse
Postsynapticneuron
4
5
6
Termination of Neurotransmitter Effects Within a few milliseconds, the neurotransmitter
effect is terminated Degradation by enzymes
Reuptake by astrocytes or axon terminal p y y
Diffusion away from the synaptic cleft
Synaptic Delay
Neurotransmitter must be released, diffuse across the synapse, and bind to receptors
Synaptic delay—time needed to do this (0.3–5.0 ms) )
Synaptic delay is the rate-limiting step of neural transmission
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Postsynaptic Potentials
Graded potentials
Strength determined by: Amount of neurotransmitter released
Time the neurotransmitter is in the area Time the neurotransmitter is in the area
Types of postsynaptic potentials 1. EPSP—excitatory postsynaptic potentials
2. IPSP—inhibitory postsynaptic potentials
Table 11.2 (1 of 4)
Table 11.2 (2 of 4)
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Table 11.2 (3 of 4)
Table 11.2 (4 of 4)
Excitatory Synapses and EPSPs
Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na+
and K+ in opposite directions
Na+ influx is greater that K+ efflux, causing a net g , gdepolarization
EPSP helps trigger AP at axon hillock if EPSP is of threshold strength and opens the voltage-gated channels
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An EPSP is a localdepolarization of the postsynaptic membranethat brings the neuroncloser to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowingThresholdan
e po
tent
ial (
mV
)
Figure 11.18a
ion channels, allowing the simultaneous pas-sage of Na+ and K+.
Time (ms)(a) Excitatory postsynaptic potential (EPSP)
Threshold
Stimulus
Mem
bra
Inhibitory Synapses and IPSPs
Neurotransmitter binds to and opens channels for K+
or Cl–
Causes a hyperpolarization (the inner surface of membrane becomes more negative)g )
Reduces the postsynaptic neuron’s ability to produce an action potential
An IPSP is a localhyperpolarization of the postsynaptic membraneand drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.
Thresholdane
pote
ntia
l (m
V)
Figure 11.18b
Time (ms)(b) Inhibitory postsynaptic potential (IPSP)
Threshold
Stimulus
Mem
bra
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Integration: Summation
A single EPSP cannot induce an action potential
EPSPs can summate to reach threshold
IPSPs can also summate with EPSPs, canceling each other outother out
Integration: Summation
Temporal summation One or more presynaptic neurons transmit impulses in
rapid-fire order
Spatial summationp Postsynaptic neuron is stimulated by a large number of
terminals at the same time
Threshold of axon ofpostsynaptic neuron
Resting potential
E1 E1
Figure 11.19a, b
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
Resting potential
E1 E1 E1 E1
(a) No summation:2 stimuli separated in time cause EPSPs that do notadd together.
(b) Temporal summation:2 excitatory stimuli closein time cause EPSPsthat add together.
Time Time
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E1
E2 I1
E1
E1 + E2 I1 E1 + I1
(d) Spatial summation ofEPSPs and IPSPs:Changes in membane potential can cancel each other out.
(c) Spatial summation:2 simultaneous stimuli atdifferent locations causeEPSPs that add together.
Time Time
Figure 11.19c, d
Integration: Synaptic Potentiation
Repeated use increases the efficiency of neurotransmission
Ca2+ concentration increases in presynaptic terminal and ostsynaptic neuron
Brief high-frequency stimulation partially depolarizes the postsynaptic neuron Chemically gated channels (NMDA receptors) allow Ca2+
entry
Ca2+ activates kinase enzymes that promote more effective responses to subsequent stimuli
Integration: Presynaptic Inhibition
Release of excitatory neurotransmitter by one neuron may be inhibited by the activity of another neuron via an axoaxonic synapse
Less neurotransmitter is released and smaller EPSPs are formed
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Neurotransmitters
Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies
50 or more neurotransmitters have been identified
Classified by chemical structure and by function
Chemical Classes of Neurotransmitters Acetylcholine (Ach)
Released at neuromuscular junctions and some ANS neurons
Synthesized by enzyme choline acetyltransferase
Degraded by the enzyme acetylcholinesterase (AChE)
Chemical Classes of Neurotransmitters Biogenic amines include:
Catecholamines Dopamine, norepinephrine (NE), and epinephrine
Indolamines Serotonin and histamine
Broadly distributed in the brain Play roles in emotional behaviors and the biological
clock
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Chemical Classes of Neurotransmitters Amino acids include:
GABA—Gamma ()-aminobutyric acid
Glycine
Aspartate
Glutamate
Chemical Classes of Neurotransmitters Peptides (neuropeptides) include:
Substance P Mediator of pain signals
Endorphins Act as natural opiates; reduce pain perception Act as natural opiates; reduce pain perception
Gut-brain peptides Somatostatin and cholecystokinin
Chemical Classes of Neurotransmitters Purines such as ATP:
Act in both the CNS and PNS
Produce fast or slow responses
Induce Ca2+ influx in astrocytes
Provoke pain sensation
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Chemical Classes of Neurotransmitters Gases and lipids
Nitric oxide (NO) Synthesized on demand
Activates the intracellular receptor guanylyl cyclase to cyclic GMP
Involved in learning and memory
Carbon monoxide (CO) is a regulator of cGMP in the brain
Chemical Classes of Neurotransmitters Gases and lipids
Endocannabinoids Lipid soluble; synthesized on demand from membrane lipids
Bind with G protein–coupled receptors in the brain
Involved in learning and memory
Functional Classification of Neurotransmitters Neurotransmitter effects may be excitatory (depolarizing)
and/or inhibitory (hyperpolarizing) Determined by the receptor type of the postsynaptic neuron GABA and glycine are usually inhibitory Glutamate is usually excitatory Glutamate is usually excitatory Acetylcholine Excitatory at neuromuscular junctions in skeletal muscle Inhibitory in cardiac muscle
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Neurotransmitter Actions
Direct action Neurotransmitter binds to channel-linked receptor and
opens ion channels
Promotes rapid responses
Examples: ACh and amino acids
Neurotransmitter Actions
Indirect action Neurotransmitter binds to a G protein-linked receptor
and acts through an intracellular second messenger
Promotes long-lasting effects
Examples: biogenic amines, neuropeptides, and dissolved gases
Neurotransmitter Receptors
Types1. Channel-linked receptors
2. G protein-linked receptors
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Channel-Linked (Ionotropic) Receptors Ligand-gated ion channels
Action is immediate and brief
Excitatory receptors are channels for small cations
N + i fl t ib t t t d l i ti Na+ influx contributes most to depolarization
Inhibitory receptors allow Cl– influx or K+ efflux that causes hyperpolarization
Ion flow blocked Ions flowLigand
(a) Channel-linked receptors open in response to binding of ligand (ACh in this case).
Closed ion channel Open ion channel
Figure 11.20a
G Protein-Linked (Metabotropic) Receptors Transmembrane protein complexes
Responses are indirect, slow, complex, and often prolonged and widespread
Examples: muscarinic ACh receptors and those that Examples: muscarinic ACh receptors and those that bind biogenic amines and neuropeptides
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G Protein-Linked Receptors: Mechanism Neurotransmitter binds to G protein–linked receptor
G protein is activated
Activated G protein controls production of second messengers, e.g., cyclic AMP, cyclic GMP, g , g , y , y ,diacylglycerol or Ca2+
G Protein-Linked Receptors: Mechanism Second messengers
Open or close ion channels
Activate kinase enzymes
Phosphorylate channel proteins p y p
Activate genes and induce protein synthesis
1 Neurotransmitter (1st messenger) binds and activates receptor.
ReceptorG protein
Closed ionchannelAdenylate cyclase
Open ion channel
cAMP changes membrane permeability by opening or closing ion
5a
2 Receptoractivates G protein.
3 G proteinactivates adenylate cyclase.
4 Adenylate cyclase converts ATP to cAMP (2nd messenger).
y p g gchannels.
5b cAMP activates enzymes.
5c cAMP activates specific genes.
Active enzyme
GDP
(b) G-protein linked receptors cause formation of an intracellular second messenger (cyclic AMP in this case) that brings about the cell’s response.
Nucleus
Figure 11.17b
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Developmental Aspects of Neurons
The nervous system originates from the neural tube and neural crest formed from ectoderm
The neural tube becomes the CNS Neuroepithelial cells of the neural tube undergo
differentiation to form cells needed for development
Cells (neuroblasts) become amitotic and migrate
Neuroblasts sprout axons to connect with targets and become neurons
Axonal Growth
Growth cone at tip of axon interacts with its environment via: Cell surface adhesion proteins (laminin, integrin, and nerve
cell adhesion molecules or N-CAMs)
N h l h h Neurotropins that attract or repel the growth cone
Nerve growth factor (NGF), which keeps the neuroblast alive
Astrocytes provide physical support and cholesterol essential for construction of synapses
Cell Death
About 2/3 of neurons die before birth Death results in cells that fail to make functional
synaptic contacts
Many cells also die due to apoptosis (programmed cell death) during development
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Central Nervous System (CNS)
CNS consists of the brain and spinal cord
Cephalization Evolutionary development of the rostral (anterior)
portion of the CNSp
Increased number of neurons in the head
Highest level is reached in the human brain
Embryonic Development
Neural plate forms from ectoderm
Neural plate invaginates to form a neural groove and neural folds
Embryonic Development
Neural groove fuses dorsally to form the neural tube
Neural tube gives rise to the brain and spinal cord
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Embryonic Development
Anterior end of the neural tube gives rise to three primary brain vesicles Prosencephalon—forebrain
Mesencephalon—midbrainp
Rhombencephalon—hindbrain
Embryonic Development
Primary vesicles give rise to five secondary brain vesicles Telencephalon and diencephalon arise from the
forebrain
Mesencephalon remains undivided
Metencephalon and myelencephalon arise from the hindbrain
Embryonic Development
Telencephalon cerebrum (two hemispheres with cortex, white matter, and basal nuclei)
Diencephalon thalamus, hypothalamus, epithalamus, and retinap ,
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Embryonic Development
Mesencephalon brain stem (midbrain)
Metencephalon brain stem (pons) and cerebellum
Myelencephalon brain stem (medulla oblongata) Myelencephalon brain stem (medulla oblongata)
Central canal of the neural tube enlarges to form fluid-filled ventricles
(d) Adult brainstructures
(c) Secondary brainvesicles
Diencephalon(thalamus, hypothalamus,epithalamus), retina
Cerebrum: cerebralhemispheres (cortex,white matter, basal nuclei)
Diencephalon
Telencephalon
Third ventricle
Lateralventricles
(e) Adultneural canalregions
Spinal cord
Cerebellum
Brain stem: medullaoblongata
Brain stem: pons
Brain stem: midbrain
epithalamus), retina
Myelencephalon
Metencephalon
Mesencephalon
Central canal
Fourthventricle
Cerebralaqueduct
Figure 12.2c-e
Effect of Space Restriction on Brain Development Midbrain flexure and cervical flexure cause
forebrain to move toward the brain stem
Cerebral hemispheres grow posteriorly and laterallyy
Cerebral hemisphere surfaces crease and fold into convolutions
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Regions and Organization of the CNS Adult brain regions
1. Cerebral hemispheres
2. Diencephalon
3. Brain stem (midbrain, pons, and medulla)( , p , )
4. Cerebellum
Regions and Organization of the CNS Spinal cord
Central cavity surrounded by a gray matter core
External white matter composed of myelinated fiber tracts
Regions and Organization of the CNS Brain
Similar pattern with additional areas of gray matter
Nuclei in cerebellum and cerebrum
Cortex of cerebellum and cerebrum C
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CerebrumCerebellum
Migratorypattern ofneurons
Cortex ofgray matterInner graymatter
Gray matter
Outer whitematter
Central cavity
Central cavity
Region of cerebellum
Figure 12.4
Inner gray matter
Gray matter
Outer white matter
Central cavity
Inner gray matter
Outer white matter
Brain stem
Spinal cord
Ventricles of the Brain
Connected to one another and to the central canal of the spinal cord
Lined by ependymal cells
Ventricles of the Brain
Contain cerebrospinal fluid Two C-shaped lateral ventricles in the cerebral
hemispheres
Third ventricle in the diencephalon
Fourth ventricle in the hindbrain, dorsal to the pons, develops from the lumen of the neural tube
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Cerebral Hemispheres
Surface markings Ridges (gyri), shallow grooves (sulci), and deep grooves
(fissures)
Five lobes
Frontal
Parietal
Temporal
Occipital
Insula
Cerebral Hemispheres
Surface markings Central sulcus Separates the precentral gyrus of the frontal lobe and
the postcentral gyrus of the parietal lobe Longitudinal fissure Longitudinal fissure Separates the two hemispheres
Transverse cerebral fissure Separates the cerebrum and the cerebellum
Cerebral Cortex
Thin (2–4 mm) superficial layer of gray matter
40% of the mass of the brain
Site of conscious mind: awareness, sensory perception, voluntary motor initiation, communication, memory storage, understanding
Each hemisphere connects to contralateral side of the body
There is lateralization of cortical function in the hemispheres
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Functional Areas of the Cerebral Cortex The three types of functional areas are:
Motor areas—control voluntary movement
Sensory areas—conscious awareness of sensation
Association areas—integrate diverse informationg v
Conscious behavior involves the entire cortex
Motor Areas
Primary (somatic) motor cortex
Premotor cortex
Broca’s area
F t l fi ld Frontal eye field
Primary Motor Cortex
Large pyramidal cells of the precentral gyri
Long axons pyramidal (corticospinal) tracts
Allows conscious control of precise, skilled, voluntary movementsmovements
Motor homunculi: upside-down caricatures representing the motor innervation of body regions
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MotorMotor map inprecentral gyrus
Posterior
Anterior
Figure 12.9
Toes
Swallowing
Tongue
Jaw
Primary motorcortex(precentral gyrus)
Premotor Cortex
Anterior to the precentral gyrus
Controls learned, repetitious, or patterned motor skills
Coordinates simultaneous or sequential actions Coordinates simultaneous or sequential actions
Involved in the planning of movements that depend on sensory feedback
Broca’s Area
Anterior to the inferior region of the premotor area
Present in one hemisphere (usually the left)
A motor speech area that directs muscles of the tonguetongue
Is active as one prepares to speak
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Frontal Eye Field
Anterior to the premotor cortex and superior to Broca’s area
Controls voluntary eye movements
Sensory Areas
Primary somatosensory cortex
Somatosensory association cortex
Olfactory cortex
Gustatory cortex
Visceral sensory area
Vestibular cortex Visual areas
Auditory areas
Primary Somatosensory Cortex
In the postcentral gyri
Receives sensory information from the skin, skeletal muscles, and joints
Capable of spatial discrimination: identification of Capable of spatial discrimination: identification of body region being stimulated
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SensorySensory map inpostcentral gyrus
Posterior
Anterior
Figure 12.9
Genitals
Intra-abdominal
Primary somato-sensory cortex(postcentral gyrus)
Somatosensory Association Cortex
Posterior to the primary somatosensory cortex
Integrates sensory input from primary somatosensory cortex
Determines size texture and relationship of parts Determines size, texture, and relationship of parts of objects being felt
Visual Areas
Primary visual (striate) cortex Extreme posterior tip of the occipital lobe
Most of it is buried in the calcarine sulcus
Receives visual information from the retinasv v
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Visual Areas
Visual association area Surrounds the primary visual cortex
Uses past visual experiences to interpret visual stimuli (e.g., color, form, and movement)
Complex processing involves entire posterior half of the hemispheres
Auditory Areas
Primary auditory cortex Superior margin of the temporal lobes Interprets information from inner ear as pitch, loudness,
and location
A d Auditory association area Located posterior to the primary auditory cortex Stores memories of sounds and permits perception of
sounds
OIfactory Cortex
Medial aspect of temporal lobes (in piriform lobes)
Part of the primitive rhinencephalon, along with the olfactory bulbs and tracts (Remainder of the rhinencephalon in humans is part of (Remainder of the rhinencephalon in humans is part of
the limbic system)
Region of conscious awareness of odors
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Gustatory Cortex
In the insula
Involved in the perception of taste
Visceral Sensory Area
Posterior to gustatory cortex
Conscious perception of visceral sensations, e.g., upset stomach or full bladder
Vestibular Cortex
Posterior part of the insula and adjacent parietal cortex
Responsible for conscious awareness of balance (position of the head in space)(p p )
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Multimodal Association Areas
Receive inputs from multiple sensory areas
Send outputs to multiple areas, including the premotor cortex
Allow us to give meaning to information received Allow us to give meaning to information received, store it as memory, compare it to previous experience, and decide on action to take
Multimodal Association Areas
Three parts Anterior association area (prefrontal cortex)
Posterior association area
Limbic association area
Anterior Association Area (Prefrontal Cortex) Most complicated cortical region
Involved with intellect, cognition, recall, and personality
Contains working memory needed for judgment Contains working memory needed for judgment, reasoning, persistence, and conscience
Development depends on feedback from social environment
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Posterior Association Area
Large region in temporal, parietal, and occipital lobes
Plays a role in recognizing patterns and faces and localizing us in spaceg p
Involved in understanding written and spoken language (Wernicke’s area)
Limbic Association Area
Part of the limbic system
Provides emotional impact that helps establish memories
Lateralization of Cortical Function
Lateralization Division of labor between hemispheres
Cerebral dominance Designates the hemisphere dominant for language (left Designates the hemisphere dominant for language (left
hemisphere in 90% of people)
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Lateralization of Cortical Function
Left hemisphere Controls language, math, and logic
Right hemisphere Insight visual-spatial skills intuition and artistic skills Insight, visual-spatial skills, intuition, and artistic skills
Left and right hemispheres communicate via fiber tracts in the cerebral white matter
Cerebral White Matter
Myelinated fibers and their tracts
Responsible for communication Commissures (in corpus callosum)—connect gray matter
of the two hemispheres p
Association fibers—connect different parts of the same hemisphere
Projection fibers—(corona radiata) connect the hemispheres with lower brain or spinal cord
Basal Nuclei (Ganglia)
Subcortical nuclei
Consists of the corpus striatum Caudate nucleus
Lentiform nucleus (putamen + globus pallidus) Lentiform nucleus (putamen + globus pallidus)
Functionally associated with the subthalamic nuclei (diencephalon) and the substantia nigra (midbrain)
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Functions of Basal Nuclei
Though somewhat elusive, the following are thought to be functions of basal nuclei Influence muscular control
Help regulate attention and cognitionp g g
Regulate intensity of slow or stereotyped movements
Inhibit antagonistic and unnecessary movements
Diencephalon
Three paired structures Thalamus
Hypothalamus
Epithalamusp
Encloses the third ventricle
Thalamus
80% of diencephalon
Superolateral walls of the third ventricle
Connected by the interthalamic adhesion (intermediate mass)(intermediate mass)
Contains several nuclei, named for their location
Nuclei project and receive fibers from the cerebral cortex
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Thalamic Function
Gateway to the cerebral cortex Sorts, edits, and relays information
Afferent impulses from all senses and all parts of the body Impulses from the hypothalamus for regulation of emotion
and visceral functionand visceral function Impulses from the cerebellum and basal nuclei to help direct
the motor cortices
Mediates sensation, motor activities, cortical arousal, learning, and memory
Hypothalamus
Forms the inferolateral walls of the third ventricle Contains many nuclei
Example: mammillary bodies Paired anterior nuclei Olfactory relay stations
Infundibulum—stalk that connects to the pituitary gland
Hypothalamic Function
Autonomic control center for many visceral functions (e.g., blood pressure, rate and force of heartbeat, digestive tract motility)
Center for emotional response: Involved in pperception of pleasure, fear, and rage and in biological rhythms and drives
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Hypothalamic Function
Regulates body temperature, food intake, water balance, and thirst
Regulates sleep and the sleep cycle
Controls release of hormones by the anterior Controls release of hormones by the anterior pituitary
Produces posterior pituitary hormones
Epithalamus
Most dorsal portion of the diencephalon; forms roof of the third ventricle
Pineal gland—extends from the posterior border and secretes melatonin Melatonin—helps regulate sleep-wake cycles
Brain Stem
Three regions Midbrain
Pons
Medulla oblongataM g
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Brain Stem
Similar structure to spinal cord but contains embedded nuclei
Controls automatic behaviors necessary for survival
Contains fiber tracts connecting higher and lower Contains fiber tracts connecting higher and lower neural centers
Associated with 10 of the 12 pairs of cranial nerves
Midbrain
Located between the diencephalon and the pons
Cerebral peduncles Contain pyramidal motor tracts
Cerebral aqueduct Cerebral aqueduct Channel between third and fourth ventricles
Midbrain Nuclei
Nuclei that control cranial nerves III (oculomotor) and IV (trochlear)
Corpora quadrigemina—domelike dorsal protrusions Superior colliculi—visual reflex centers
Inferior colliculi—auditory relay centers
Substantia nigra—functionally linked to basal nuclei
Red nucleus—relay nuclei for some descending motor pathways and part of reticular formation
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Dorsal
Cerebral aqueduct
Superiorcolliculus
Oculomotor
Periaqueductal graymatter
Tectum
Figure 12.16a
Reticular formation
Crus cerebri ofcerebral peduncle
Ventral
Fibers ofpyramidal tract
Substantianigra
(a) Midbrain
Rednucleus
Mediallemniscus
nucleus (III)
Pons
Forms part of the anterior wall of the fourth ventricle
Fibers of the pons Connect higher brain centers and the spinal cord
Relay impulses between the motor cortex and the cerebellum
Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial)
Some nuclei of the reticular formation
Nuclei that help maintain normal rhythm of breathing
Reticularformation
Trigeminal mainsensory nucleus
Superior cerebellarpeduncle
Fourthventricle
Figure 12.16b
Trigeminalnerve (V)
Pontinenuclei
Fibers ofpyramidaltract
Middlecerebellarpeduncle
sensory nucleus Trigeminalmotor nucleus
Medial lemniscus(b) Pons
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Medulla Oblongata
Joins spinal cord at foramen magnum Forms part of the ventral wall of the fourth ventricle Contains a choroid plexus of the fourth ventricle Pyramids—two ventral longitudinal ridges formed y g g
by pyramidal tracts Decussation of the pyramids—crossover of the
corticospinal tracts
Medulla Oblongata
Inferior olivary nuclei—relay sensory information from muscles and joints to cerebellum
Cranial nerves VIII, X, and XII are associated with the medulla
Vestibular nuclear complex—mediates responses that maintain equilibrium
Several nuclei (e.g., nucleus cuneatus and nucleus gracilis) relay sensory information
Medulla Oblongata
Autonomic reflex centers
Cardiovascular center Cardiac center adjusts force and rate of heart
contraction
Vasomotor center adjusts blood vessel diameter for blood pressure regulation
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Medulla Oblongata
Respiratory centers Generate respiratory rhythm
Control rate and depth of breathing, with pontine centers
Medulla Oblongata
Additional centers regulate Vomiting
Hiccuping
Swallowing S g
Coughing
Sneezing
Choroidplexus
Fourth ventricle
Inferior cerebellarpeduncle
Cochlear
Vestibular nuclearcomplex (VIII)
Solitarynucleus
Dorsal motor nucleusof vagus (X)
Hypoglossal nucleus (XII)
Figure 12.16c
PyramidMedial lemniscus
Inferior olivarynucleus
Nucleusambiguus
peduncle nuclei (VIII)
(c) Medulla oblongata
LateralnucleargroupMedialnucleargroupRaphenucleusR
etic
ula
r fo
rmat
ion
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The Cerebellum
11% of brain mass
Dorsal to the pons and medulla
Subconsciously provides precise timing and appropriate patterns of skeletal muscle contractionappropriate patterns of skeletal muscle contraction
Anatomy of the Cerebellum
Two hemispheres connected by vermis
Each hemisphere has three lobes Anterior, posterior, and flocculonodular
Folia—transversely oriented gyri Folia—transversely oriented gyri
Arbor vitae—distinctive treelike pattern of the cerebellar white matter
Arborvitae
Cerebellar cortex
Anterior lobe
Figure 12.17b
(b)
Medullaoblongata
Flocculonodularlobe
Choroidplexus offourth ventricle
Posteriorlobe
Cerebellarpeduncles• Superior• Middle• Inferior
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Anteriorlobe
Posterior
Figure 12.17d
(d)
Posteriorlobe
Vermis(d)
Cerebellar Peduncles
All fibers in the cerebellum are ipsilateral Three paired fiber tracts connect the cerebellum to
the brain stem Superior peduncles connect the cerebellum to the
dbmidbrain Middle peduncles connect the pons to the cerebellum Inferior peduncles connect the medulla to the
cerebellum
Cerebellar Processing for Motor Activity Cerebellum receives impulses from the cerebral cortex of
the intent to initiate voluntary muscle contraction
Signals from proprioceptors and visual and equilibrium pathways continuously “inform” the cerebellum of the b d ’ d body’s position and momentum
Cerebellar cortex calculates the best way to smoothly coordinate a muscle contraction
A “blueprint” of coordinated movement is sent to the cerebral motor cortex and to brain stem nuclei
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Cognitive Function of the Cerebellum Recognizes and predicts sequences of events during
complex movements
Plays a role in nonmotor functions such as word association and puzzle solvingp g
Functional Brain Systems
Networks of neurons that work together and span wide areas of the brain Limbic system
Reticular formation
Limbic System
Structures on the medial aspects of cerebral hemispheres and diencephalon
Includes parts of the diencephalon and some cerebral structures that encircle the brain stem
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Corpus callosum
Septum pellucidum
Diencephalic structuresof the limbic system•Anterior thalamicnuclei (flanking3rd ventricle)•Hypothalamus
Fiber tractsconnecting limbic system structures
•Fornix•Anterior commissure
Cerebral struc-tures of the limbic system
Figure 12.18
Olfactory bulb
•Mammillarybody
y
•Cingulate gyrus•Septal nuclei•Amygdala•Hippocampus•Dentate gyrus•Parahippocampalgyrus
Limbic System
Emotional or affective brain Amygdala—recognizes angry or fearful facial
expressions, assesses danger, and elicits the fear response
Cingulate gyrus plays a role in expressing emotions Cingulate gyrus—plays a role in expressing emotions via gestures, and resolves mental conflict
Puts emotional responses to odors Example: skunks smell bad
Limbic System: Emotion and Cognition The limbic system interacts with the prefrontal lobes,
therefore: We can react emotionally to things we consciously
understand to be happening
We are consciously aware of emotional richness in our lives
Hippocampus and amygdala—play a role in memory
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Reticular Formation
Three broad columns along the length of the brain stem Raphe nuclei
Medial (large cell) group of nuclei( g ) g p
Lateral (small cell) group of nuclei
Has far-flung axonal connections with hypothalamus, thalamus, cerebral cortex, cerebellum, and spinal cord
Reticular Formation: RAS and Motor Function
RAS (reticular activating system) Sends impulses to the cerebral cortex to keep it
conscious and alert
Filters out repetitive and weak stimuli (~99% of all stimuli!)
Severe injury results in permanent unconsciousness (coma)
Reticular Formation: RAS and Motor Function
Motor function Helps control coarse limb movements
Reticular autonomic centers regulate visceral motor functions Vasomotor
Cardiac
Respiratory centers
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Radiationsto cerebralcortex
Figure 12.19
Visualimpulses
Reticular formation
Ascending generalsensory tracts(touch, pain, temperature)
Descendingmotor projectionsto spinal cord
Auditoryimpulses
Electroencephalogram (EEG)
Records electrical activity that accompanies brain function
Measures electrical potential differences between various cortical areas
Brain Waves
Patterns of neuronal electrical activity
Generated by synaptic activity in the cortex
Each person’s brain waves are unique
C b d i t f l b d Can be grouped into four classes based on frequency measured as Hertz (Hz)
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Types of Brain Waves
Alpha waves (8–13 Hz)—regular and rhythmic, low-amplitude, synchronous waves indicating an “idling” brain
Beta waves (14–30 Hz)—rhythmic, less regular waves occurring when mentally alert
Theta waves (4–7 Hz)—more irregular; common in Theta waves (4–7 Hz)—more irregular; common in children and uncommon in adults
Delta waves (4 Hz or less)—high-amplitude waves seen in deep sleep and when reticular activating system is damped, or during anesthesia; may indicate brain damage
Alpha waves—awake but relaxed
Beta waves—awake, alert
1-second interval
Figure 12.20b
Theta waves—common in children
Delta waves—deep sleep
(b) Brain waves shown in EEGs fall intofour general classes.
Brain Waves: State of the Brain
Change with age, sensory stimuli, brain disease, and the chemical state of the body
EEGs used to diagnose and localize brain lesions, tumors, infarcts, infections, abscesses, and epileptic , , , , p plesions
A flat EEG (no electrical activity) is clinical evidence of death
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Epilepsy
A victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking
Epilepsy is not associated with intellectual impairmentsp
Epilepsy occurs in 1% of the population
Epileptic Seizures
Absence seizures, or petit mal Mild seizures seen in young children where the
expression goes blank
Tonic-clonic (grand mal) seizures(g ) Victim loses consciousness, bones are often broken due
to intense contractions, may experience loss of bowel and bladder control, and severe biting of the tongue
Control of Epilepsy
Anticonvulsive drugs
Vagus nerve stimulators implanted under the skin of the chest can keep electrical activity of the brain from becoming chaoticg
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Consciousness
Conscious perception of sensation
Voluntary initiation and control of movement
Capabilities associated with higher mental processing (memory logic judgment etc )processing (memory, logic, judgment, etc.)
Loss of consciousness (e.g., fainting or syncopy) is a signal that brain function is impaired
Consciousness
Clinically defined on a continuum that grades behavior in response to stimuli Alertness
Drowsiness (lethargy)( gy)
Stupor
Coma
Sleep
State of partial unconsciousness from which a person can be aroused by stimulation
Two major types of sleep (defined by EEG patterns) Nonrapid eye movement (NREM) Nonrapid eye movement (NREM)
Rapid eye movement (REM)
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Sleep
First two stages of NREM occur during the first 30–45 minutes of sleep
Fourth stage is achieved in about 90 minutes, and then REM sleep begins abruptlyp g p y
Awake
REM: Skeletal muscles (except ocular muscles and diaphragm) are actively inhibited; most dreaming occurs.NREM stage 1:Relaxation begins; EEG shows alpha waves, arousal is easy.
NREM stage 2: Irregular
Figure 12.21a(a) Typical EEG patterns
NREM stage 2: IrregularEEG with sleep spindles (short high- amplitude bursts); arousal is more difficult.
NREM stage 3: Sleep deepens; theta and delta waves appear; vital signs decline.
NREM stage 4: EEG is dominated by delta waves; arousal is difficult; bed-wetting, night terrors, and sleepwalking may occur.
Sleep Patterns
Alternating cycles of sleep and wakefulness reflect a natural circadian (24-hour) rhythm
RAS activity is inhibited during, but RAS also mediates, dreaming sleep, g p
The suprachiasmatic and preoptic nuclei of the hypothalamus time the sleep cycle
A typical sleep pattern alternates between REM and NREM sleep
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Awake
REM
Stage 1
Stage 2NonREM Stage 3
Figure 12.21b
(b) Typical progression of an adult through onenight’s sleep stages
REM Stage 3
Stage 4
Time (hrs)
Importance of Sleep
Slow-wave sleep (NREM stages 3 and 4) is presumed to be the restorative stage
People deprived of REM sleep become moody and depressed
REM sleep may be a reverse learning process where superfluous information is purged from the brain
Daily sleep requirements decline with age
Stage 4 sleep declines steadily and may disappear after age 60
Sleep Disorders
Narcolepsy Lapsing abruptly into sleep from the awake state
Insomnia Chronic inability to obtain the amount or quality of Chronic inability to obtain the amount or quality of
sleep needed
Sleep apnea Temporary cessation of breathing during sleep
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Language
Language implementation system Basal nuclei Broca’s area and Wernicke’s area (in the association
cortex on the left side)A l i i d d Analyzes incoming word sounds
Produces outgoing word sounds and grammatical structures
Corresponding areas on the right side are involved with nonverbal language components
Memory
Storage and retrieval of information
Two stages of storage Short-term memory (STM, or working memory)—
temporary holding of information; limited to seven or p y g ;eight pieces of information
Long-term memory (LTM) has limitless capacity
Outside stimuli
General and special sensory receptors
Data permanentlylost
Afferent inputs
F t
Data selectedfor transfer
Automatic
Temporary storage(buffer) in cerebral cortex
Figure 12.22
Data transferinfluenced by:
ExcitementRehearsalAssociation ofold and new data
Long-termmemory(LTM)
Retrieval
Forget
Forgetfor transfermemory
Data unretrievable
Short-termmemory (STM)
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Transfer from STM to LTM
Factors that affect transfer from STM to LTM Emotional state—best if alert, motivated, surprised,
and aroused
Rehearsal—repetition and practice
Association—tying new information with old memories
Automatic memory—subconscious information stored in LTM
Categories of Memory
1. Declarative memory (factual knowledge) Explicit information
Related to our conscious thoughts and our language ability
Stored in LTM with context in which it was learned
Categories of Memory
2. Nondeclarative memory Less conscious or unconscious
Acquired through experience and repetition
Best remembered by doing; hard to unlearny g;
Includes procedural (skills) memory, motor memory, and emotional memory
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Brain Structures Involved in Declarative Memory Hippocampus and surrounding temporal lobes
function in consolidation and access to memory
ACh from basal forebrain is necessary for memory formation and retrieval
Smell
Basal forebrain
Prefrontal cortex
Taste
Thalamus
Touch
Hearing
Vision
Figure 12.23a
Hippocampus
Thalamus
Prefrontalcortex
Basalforebrain
Associationcortex
Sensoryinput
ACh ACh
Medial temporal lobe(hippocampus, etc.)
(a) Declarativememory circuits
Brain Structures Involved in Nondeclarative Memory Procedural memory
Basal nuclei relay sensory and motor inputs to the thalamus and premotor cortex
Dopamine from substantia nigra is necessary
Motor memory—cerebellum
Emotional memory—amygdala
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Dopamine
Thalamus Premotorcortex
Substantianigra
Associationcortex
Basalnuclei
Sensory andmotor inputs
Premotorcortex
Figure 12.23b
ThalamusSubstantia nigra
Basal nuclei
(b) Procedural (skills) memory circuits
Molecular Basis of Memory
During learning: Altered mRNA is synthesized and moved to axons and
dendrites
Dendritic spines change shape
Extracellular proteins are deposited at synapses involved in LTM
Number and size of presynaptic terminals may increase
More neurotransmitter is released by presynaptic neurons
Molecular Basis of Memory
Increase in synaptic strength (long-term potentiation, or LTP) is crucial
Neurotransmitter (glutamate) binds to NMDA receptors, opening calcium channels in postsynaptic p , p g p y pterminal
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Molecular Basis of Memory
Calcium influx triggers enzymes that modify proteins of the postsynaptic terminal and presynaptic terminal (via release of retrograde messengers)
Enzymes trigger postsynaptic gene activation for synthesis of synaptic proteins, in presence of CREB (cAMP response-element binding protein) and BDNF (brain-derived neurotrophic factor)
Protection of the Brain
Bone (skull)
Membranes (meninges)
Watery cushion (cerebrospinal fluid)
Bl d b i b i Blood-brain barrier
Meninges
Cover and protect the CNS
Protect blood vessels and enclose venous sinuses
Contain cerebrospinal fluid (CSF)
F titi i th k ll Form partitions in the skull
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Meninges
Three layers Dura mater
Arachnoid mater
Pia mater
Skin of scalpPeriosteum
Arachnoid mater
Duramater Meningeal
Periosteal
Bone of skull
Superioritt l i
Figure 12.24
Falx cerebri(in longitudinalfissure only)
Blood vesselArachnoid villusPia matersagittal sinus
Subduralspace
Subarachnoidspace
Dura Mater
Strongest meninx
Two layers of fibrous connective tissue (around the brain) separate to form dural sinuses
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Dura Mater
Dural septa limit excessive movement of the brain Falx cerebri—in the longitudinal fissure; attached to
crista galli
Falx cerebelli—along the vermis of the cerebellum
Tentorium cerebelli—horizontal dural fold over cerebellum and in the transverse fissure
Falx cerebri
Superiorsagittal sinus
Straightsinus
Crista galliof theethmoid
Tentoriumcerebelli
Figure 12.25a
bone
Pituitarygland
Falxcerebelli
(a) Dural septa
Arachnoid Mater
Middle layer with weblike extensions
Separated from the dura mater by the subdural space
Subarachnoid space contains CSF and blood vessels Subarachnoid space contains CSF and blood vessels
Arachnoid villi protrude into the superior sagittal sinus and permit CSF reabsorption
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Pia Mater
Layer of delicate vascularized connective tissue that clings tightly to the brain
Cerebrospinal Fluid (CSF)
Composition Watery solution
Less protein and different ion concentrations than plasma
Constant volume
Cerebrospinal Fluid (CSF)
Functions Gives buoyancy to the CNS organs
Protects the CNS from blows and other trauma
Nourishes the brain and carries chemical signals N g
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Choroid Plexuses
Produce CSF at a constant rate
Hang from the roof of each ventricle
Clusters of capillaries enclosed by pia mater and a layer of ependymal cellslayer of ependymal cells
Ependymal cells use ion pumps to control the composition of the CSF and help cleanse CSF by removing wastes
Ependymalcells
Capillary
Connectivetissue of
Sectionof choroidplexus
Figure 12.26b
pia mater
Wastes andunnecessarysolutes absorbed
(b) CSF formation by choroid plexuses
Cavity ofventricle
CSF forms as a filtratecontaining glucose, oxygen, vitamins, and ions(Na+, Cl–, Mg2+, etc.)
Superiorsagittal sinus
Arachnoid villus
Subarachnoid spaceArachnoid materMeningeal dura mater
Periosteal dura mater
Right lateral ventricle(deep to cut)Choroid plexus
Choroidplexus
Interventricularforamen
1
4
Figure 12.26a
Choroid plexusof fourth ventricle
Central canalof spinal cord
Third ventricle
Cerebral aqueductLateral apertureFourth ventricleMedian aperture
(a) CSF circulation
CSF is produced by thechoroid plexus of eachventricle.
1
CSF flows through theventricles and into the subarachnoid space via the median and lateral apertures. Some CSF flows through the central canal of the spinal cord.
2
CSF flows through thesubarachnoid space. 3
CSF is absorbed into the dural venoussinuses via the arachnoid villi. 4
2
3
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Blood-Brain Barrier
Helps maintain a stable environment for the brain
Separates neurons from some bloodborne substances
Blood-Brain Barrier
Composition Continuous endothelium of capillary walls
Basal lamina
Feet of astrocytesy Provide signal to endothelium for the formation of tight
junctions
Capillary
Neuron
Figure 11.3a
(a) Astrocytes are the most abundantCNS neuroglia.
Astrocyte
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Blood-Brain Barrier: Functions
Selective barrier Allows nutrients to move by facilitated diffusion
Allows any fat-soluble substances to pass, including alcohol, nicotine, and anesthetics
Absent in some areas, e.g., vomiting center and the hypothalamus, where it is necessary to monitor the chemical composition of the blood
Homeostatic Imbalances of the Brain Traumatic brain injuries
Concussion—temporary alteration in function
Contusion—permanent damage
Subdural or subarachnoid hemorrhage—may force S g ybrain stem through the foramen magnum, resulting in death
Cerebral edema—swelling of the brain associated with traumatic head injury
Homeostatic Imbalances of the Brain Cerebrovascular accidents (CVAs)(strokes)
Blood circulation is blocked and brain tissue dies, e.g., blockage of a cerebral artery by a blood clot
Typically leads to hemiplegia, or sensory and speed deficits
Transient ischemic attacks (TIAs)—temporary episodes of reversible cerebral ischemia
Tissue plasminogen activator (TPA) is the only approved treatment for stroke
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Homeostatic Imbalances of the Brain Degenerative brain disorders
Alzheimer’s disease (AD): a progressive degenerative disease of the brain that results in dementia
Parkinson’s disease: degeneration of the dopamine-l i f th b t ti ireleasing neurons of the substantia nigra
Huntington’s disease: a fatal hereditary disorder caused by accumulation of the protein huntingtin that leads to degeneration of the basal nuclei and cerebral cortex
The Spinal Cord: Embryonic Development By week 6, there are two clusters of neuroblasts
Alar plate—will become interneurons; axons form white matter of cord
Basal plate—will become motor neurons; axons will grow to effectorsgrow to effectors
Neural crest cells form the dorsal root ganglia sensory neurons; axons grow into the dorsal aspect of the cord
Alar plate:interneurons
Dorsal root ganglion: sensoryneurons from neural crest
Figure 12.28
Whitematter
Neural tubecells
Centralcavity
interneurons
Basal plate:motor neurons
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Spinal Cord
Location Begins at the foramen magnum
Ends as conus medullaris at L1 vertebra
Functions Functions Provides two-way communication to and from the brain
Contains spinal reflex centers
Spinal Cord: Protection
Bone, meninges, and CSF
Cushion of fat and a network of veins in the epidural space between the vertebrae and spinal dura mater
CSF in subarachnoid space
Spinal Cord: Protection
Denticulate ligaments: extensions of pia mater that secure cord to dura mater
Filum terminale: fibrous extension from conus medullaris; anchors the spinal cord to the coccyx; p y
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Cervicalenlargement
Dura andarachnoidmater
Lumbar
Cervicalspinal nerves
Thoracicspinal nerves
Figure 12.29a
LumbarenlargementConusmedullarisCaudaequina
Filumterminale
Lumbarspinal nerves
Sacralspinal nerves
(a) The spinal cord and its nerveroots, with the bony vertebral arches removed. The dura mater and arachnoid mater are cut open and reflected laterally.
Spinal Cord
Spinal nerves 31 pairs
Cervical and lumbar enlargements The nerves serving the upper and lower limbs emerge The nerves serving the upper and lower limbs emerge
here
Cauda equina The collection of nerve roots at the inferior end of the
vertebral canal
Cross-Sectional Anatomy
Two lengthwise grooves divide cord into right and left halves Ventral (anterior) median fissure
Dorsal (posterior) median sulcus (p )
Gray commissure—connects masses of gray matter; encloses central canal
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Epidural space(contains fat)
Pia mater
Spinalmeninges
Arachnoidmater Dura mater
Bone ofvertebra
Subdural space
Subarachnoidspace(contains CSF)
Figure 12.31a(a) Cross section of spinal cord and vertebra
Dorsal rootganglion
Bodyof vertebra
Dorsal funiculus
Dorsal median sulcus
Central canal
Graycommissure Dorsal horn Gray
matterLateral hornVentral horn
Ventral funiculusLateral funiculus
Whitecolumns
Dorsal rootganglion
Spinal nerve
Figure 12.31b
(b) The spinal cord and its meningeal coverings
Ventral medianfissure
Pia mater
Arachnoid mater
Spinal dura mater
Dorsal root(fans out into dorsal rootlets)
Ventral root(derived from severalventral rootlets)
Gray Matter
Dorsal horns—interneurons that receive somatic and visceral sensory input
Ventral horns—somatic motor neurons whose axons exit the cord via ventral roots
Lateral horns (only in thoracic and lumbar regions) –sympathetic neurons
Dorsal root (spinal) gangia—contain cell bodies of sensory neurons
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Somaticsensoryneuron
Dorsal root (sensory)
Dorsal root ganglion
Visceralsensory neuron
Dorsal horn (interneurons)
Figure 12.32
Somaticmotor neuron
Spinal nerve
Ventral root(motor)
Ventral horn(motor neurons)
Visceralmotorneuron
Interneurons receiving input from somatic sensory neurons
Interneurons receiving input from visceral sensory neurons
Visceral motor (autonomic) neurons
Somatic motor neurons
White Matter
Consists mostly of ascending (sensory) and descending (motor) tracts
Transverse tracts (commissural fibers) cross from one side to the other
Tracts are located in three white columns (funiculi on each side—dorsal (posterior), lateral, and ventral (anterior)
Each spinal tract is composed of axons with similar functions
Pathway Generalizations
Pathways decussate (cross over)
Most consist of two or three neurons (a relay)
Most exhibit somatotopy (precise spatial relationships)relationships)
Pathways are paired symmetrically (one on each side of the spinal cord or brain)
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Ascending tracts Descending tracts
Fasciculus gracilisDorsalwhitecolumn
Fasciculus cuneatus
Dorsalspinocerebellar tract
Ventral whitecommissure
Lateralcorticospinal tract
Lateralreticulospinal tract
Rubrospinal
Figure 12.33
Lateralspinothalamic tract
Ventral spinothalamictract
Ventral corticospinaltract
Medialreticulospinal tract
Rubrospinaltract
Vestibulospinal tractTectospinal tract
Ventralspinocerebellartract
Ascending Pathways
Consist of three neurons
First-order neuron Conducts impulses from cutaneous receptors and
proprioceptorsp p p
Branches diffusely as it enters the spinal cord or medulla
Synapses with second-order neuron
Ascending Pathways
Second-order neuron Interneuron
Cell body in dorsal horn of spinal cord or medullary nuclei
Axons extend to thalamus or cerebellum
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Ascending Pathways
Third-order neuron Interneuron
Cell body in thalamus
Axon extends to somatosensory cortexy
Ascending Pathways
Two pathways transmit somatosensory information to the sensory cortex via the thalamus Dorsal column-medial lemniscal pathways
Spinothalamic pathwaysp p y
Spinocerebellar tracts terminate in the cerebellum
Dorsal Column-Medial Lemniscal Pathways Transmit input to the somatosensory cortex for
discriminative touch and vibrations
Composed of the paired fasciculus cuneatus and fasciculus gracilis in the spinal cord and the medial g plemniscus in the brain (medulla to thalamus)
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Primary somatosensory cortex
Axons of third-order neurons
Thalamus
Cerebrum
Midbrain
Cerebellum
PonsM di l l i (t t)
Dorsal spinocerebellartract (axons of second order
Figure 12.34a
Medulla oblongataFasciculus cuneatus(axon of first-order sensory neuron)
Fasciculus gracilis(axon of first-order sensory neuron)
Axon of first-order neuron
Muscle spindle (proprioceptor)
Joint stretch receptor (proprioceptor)Cervical spinal cord
Touch receptor
Medial lemniscus (tract)(axons of second-order neurons)
tract (axons of second-orderneurons)
Nucleus gracilisNucleus cuneatus
Lumbar spinal cord
(a) Spinocerebellarpathway
Dorsal column–mediallemniscal pathway
Anterolateral Pathways
Lateral and ventral spinothalamic tracts
Transmit pain, temperature, and coarse touch impulses within the lateral spinothalamic tract
L t l i th l i t t
Primary somatosensory cortex Axons of third-order
neurons
Thalamus
Cerebrum
Midbrain
Cerebellum
Pons
Figure 12.34b
Axons of first-order neuronsTemperature receptors
Lateral spinothalamic tract(axons of second-order neurons)
Pain receptors
Medulla oblongata
Cervical spinal cord
Lumbar spinal cord
(b) Spinothalamic pathway
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Spinocerebellar Tracts
Ventral and dorsal tracts
Convey information about muscle or tendon stretch to the cerebellum
Descending Pathways and Tracts
Deliver efferent impulses from the brain to the spinal cord Direct pathways—pyramidal tracts
Indirect pathways—all othersp y
Descending Pathways and Tracts
Involve two neurons:1. Upper motor neurons
Pyramidal cells in primary motor cortex
2. Lower motor neurons Ventral horn motor neurons
Innervate skeletal muscles
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The Direct (Pyramidal) System
Impulses from pyramidal neurons in the precentral gyri pass through the pyramidal (corticospinal)l tracts
Axons synapse with interneurons or ventral horn y pmotor neurons
The direct pathway regulates fast and fine (skilled) movements
Primary motor cortexInternal capsule
Cerebralpeduncle
Midbrain
Cerebellum
Cerebrum
Pons
Pyramidal cells(upper motor neurons)
Ventralcorticospinaltract
Figure 12.35a
Medulla oblongata
Cervical spinal cord
Skeletalmuscle
PyramidsDecussation of pyramid
Lateralcorticospinaltract
tract
Lumbar spinal cord
Somatic motor neurons(lower motor neurons)(a) Pyramidal (lateral and ventral corticospinal)
pathways
Indirect (Extrapyramidal) System
Includes the brain stem motor nuclei, and all motor pathways except pyramidal pathways
Also called the multineuronal pathways
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Indirect (Extrapyramidal) System
These pathways are complex and multisynaptic, and regulate: Axial muscles that maintain balance and posture
Muscles controlling coarse movements g
Head, neck, and eye movements that follow objects
Indirect (Extrapyramidal) System
Reticulospinal and vestibulospinal tracts—maintain balance
Rubrospinal tracts—control flexor muscles
Superior colliculi and tectospinal tracts mediate Superior colliculi and tectospinal tracts mediate head movements in response to visual stimuli
Midbrain
Cerebellum
Cerebrum
Red nucleus
Pons
Rubrospinal tract
Figure 12.35b
Medulla oblongata
Cervical spinal cord
Rubrospinal tract(b)
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Spinal Cord Trauma
Functional losses Parasthesias Sensory loss
Paralysis Loss of motor function
Spinal Cord Trauma
Flaccid paralysis—severe damage to the ventral root or ventral horn cells Impulses do not reach muscles; there is no voluntary or
involuntary control of muscles
Muscles atrophy
Spinal Cord Trauma
Spastic paralysis—damage to upper motor neurons of the primary motor cortex Spinal neurons remain intact; muscles are stimulated by
reflex activity
No voluntary control of muscles
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Spinal Cord Trauma
Transection Cross sectioning of the spinal cord at any level
Results in total motor and sensory loss in regions inferior to the cut
Paraplegia—transection between T1 and L1
Quadriplegia—transection in the cervical region
Poliomyelitis
Destruction of the ventral horn motor neurons by the poliovirus
Muscles atrophy
Death may occur due to paralysis of respiratory Death may occur due to paralysis of respiratory muscles or cardiac arrest
Survivors often develop postpolio syndrome many years later, as neurons are lost
Amyotrophic Lateral Sclerosis (ALS)
Also called Lou Gehrig’s disease Involves progressive destruction of ventral horn
motor neurons and fibers of the pyramidal tract Symptoms—loss of the ability to speak, swallow,
and breathe Death typically occurs within five years Linked to glutamate excitotoxicity, attack by the
immune system, or both
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Developmental Aspects of the CNS
CNS is established during the first month of development
Gender-specific areas appear in both brain and spinal cord, depending on presence or absence of fetal testosterone
Maternal exposure to radiation, drugs (e.g., alcohol and opiates), or infection can harm the developing CNS
Smoking decreases oxygen in the blood, which can lead to neuron death and fetal brain damage
Developmental Aspects of the CNS
The hypothalamus is one of the last areas of the CNS to develop
Visual cortex develops slowly over the first 11 weeks
Neuromuscular coordination progresses in superior-to-inferior and proximal-to-distal directions along with myelination
Developmental Aspects of the CNS
Age brings some cognitive declines, but these are not significant in healthy individuals until they reach their 80s
Shrinkage of brain accelerates in old ageg g
Excessive use of alcohol causes signs of senility unrelated to the aging process
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Peripheral Nervous System (PNS)
All neural structures outside the brain Sensory receptors
Peripheral nerves and associated ganglia
Motor endingsM g
Central nervous system (CNS) Peripheral nervous system (PNS)
Motor (efferent) divisionSensory (afferent)division
Figure 13.1
Somatic nervoussystem
Autonomic nervoussystem (ANS)
Sympatheticdivision
Parasympatheticdivision
Sensory Receptors
Specialized to respond to changes in their environment (stimuli)
Activation results in graded potentials that trigger nerve impulsesp
Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain
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Classification of Receptors
Based on: Stimulus type
Location
Structural complexityS p y
Classification by Stimulus Type
Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch
Thermoreceptors—sensitive to changes in temperature Photoreceptors—respond to light energy (e.g., retina)
Ch d h i l ( ll Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry)
Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)
Classification by Location
1. Exteroceptors Respond to stimuli arising outside the body
Receptors in the skin for touch, pressure, pain, and temperature
Most special sense organs
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Classification by Location
2. Interoceptors (visceroceptors) Respond to stimuli arising in internal viscera and
blood vessels
Sensitive to chemical changes, tissue stretch, and temperature changes
Classification by Location
3. Proprioceptors Respond to stretch in skeletal muscles, tendons, joints,
ligaments, and connective tissue coverings of bones and muscles
Inform the brain of one’s movements
Classification by Structural Complexity1. Complex receptors (special sense organs)
Vision, hearing, equilibrium, smell, and taste
2. Simple receptors for general senses: Tactile sensations (touch, pressure, stretch, vibration),
temperature, pain, and muscle sense Unencapsulated (free) or encapsulated dendritic
endings
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Unencapsulated Dendritic Endings
Thermoreceptors Cold receptors (10–40ºC); in superficial dermis
Heat receptors (32–48ºC); in deeper dermis
Unencapsulated Dendritic Endings
Nociceptors Respond to: Pinching
Chemicals from damaged tissue
Temperatures outside the range of thermoreceptors
Capsaicin
Unencapsulated Dendritic Endings
Light touch receptors Tactile (Merkel) discs
Hair follicle receptors
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Table 13.1
Encapsulated Dendritic Endings
All are mechanoreceptors Meissner’s (tactile) corpuscles—discriminative touch
Pacinian (lamellated) corpuscles—deep pressure and vibration
Ruffini endings—deep continuous pressure
Muscle spindles—muscle stretch
Golgi tendon organs—stretch in tendons
Joint kinesthetic receptors—stretch in articular capsules
Table 13.1
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From Sensation to Perception
Survival depends upon sensation and perception
Sensation: the awareness of changes in the internal and external environment
Perception: the conscious interpretation of those Perception: the conscious interpretation of those stimuli
Sensory Integration
Input comes from exteroceptors, proprioceptors, and interoceptors
Input is relayed toward the head, but is processed along the wayg y
Sensory Integration
Levels of neural integration in sensory systems:1. Receptor level—the sensor receptors
2. Circuit level—ascending pathways
3. Perceptual level—neuronal circuits in the cerebral p vcortex
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3
Ci it l l
Cerebellum
Reticularformation
PonsMedulla
Perceptual level (processing incortical sensory centers)
Motorcortex
Somatosensorycortex
Thalamus
Figure 13.2
1
2
Receptor level(sensory receptionand transmissionto CNS)
Circuit level(processing inascending pathways)
Spinalcord
Musclespindle
Jointkinestheticreceptor
Free nerveendings (pain,cold, warmth)
Medulla
Processing at the Receptor Level
Receptors have specificity for stimulus energy
Stimulus must be applied in a receptive field
Transduction occurs Stimulus energy is converted into a graded potential Stimulus energy is converted into a graded potential
called a receptor potential
Processing at the Receptor Level
In general sense receptors, the receptor potential and generator potential are the same thing
stimulus
receptor/generator potential in afferent neuron
action potential at first node of Ranvier
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Processing at the Receptor Level
In special sense organs:stimulus
receptor potential in receptor cell
release of neurotransmitter
generator potential in first-order sensory neuron
action potentials (if threshold is reached)
Adaptation of Sensory Receptors
Adaptation is a change in sensitivity in the presence of a constant stimulus Receptor membranes become less responsive
Receptor potentials decline in frequency or stopp p q y p
Adaptation of Sensory Receptors
Phasic (fast-adapting) receptors signal the beginning or end of a stimulus Examples: receptors for pressure, touch, and smell
Tonic receptors adapt slowly or not at all Tonic receptors adapt slowly or not at all Examples: nociceptors and most proprioceptors
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Processing at the Circuit Level
Pathways of three neurons conduct sensory impulses upward to the appropriate brain regions
First-order neurons Conduct impulses from the receptor level to the second-
order neurons in the CNSorder neurons in the CNS
Second-order neurons Transmit impulses to the thalamus or cerebellum
Third-order neurons Conduct impulses from the thalamus to the somatosensory
cortex (perceptual level)
Processing at the Perceptual Level
Identification of the sensation depends on the specific location of the target neurons in the sensory cortex
Aspects of sensory perception: Perceptual detection—ability to detect a stimulus (requires
summation of impulses)summation of impulses) Magnitude estimation—intensity is coded in the frequency
of impulses Spatial discrimination—identifying the site or pattern of the
stimulus (studied by the two-point discrimination test)
Main Aspects of Sensory Perception
Feature abstraction—identification of more complex aspects and several stimulus properties
Quality discrimination—the ability to identify submodalities of a sensation (e.g., sweet or sour ( g ,tastes)
Pattern recognition—recognition of familiar or significant patterns in stimuli (e.g., the melody in a piece of music)
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Perception of Pain
Warns of actual or impending tissue damage
Stimuli include extreme pressure and temperature, histamine, K+, ATP, acids, and bradykinin
Impulses travel on fibers that release Impulses travel on fibers that release neurotransmitters glutamate and substance P
Some pain impulses are blocked by inhibitory endogenous opioids
Structure of a Nerve
Cordlike organ of the PNS
Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue
Structure of a Nerve
Connective tissue coverings include: Endoneurium—loose connective tissue that encloses
axons and their myelin sheaths
Perineurium—coarse connective tissue that bundles fibers into fascicles
Epineurium—tough fibrous sheath around a nerve
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Fascicle
Epineurium
Perineurium
Endoneurium
AxonMyelin sheath
Bloodvessels
(b)
Figure 13.3b
Classification of Nerves
Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers
Pure sensory (afferent) or motor (efferent) nerves are rare
Types of fibers in mixed nerves: Somatic afferent and somatic efferent
Visceral afferent and visceral efferent
Peripheral nerves classified as cranial or spinal nerves
Ganglia
Contain neuron cell bodies associated with nerves Dorsal root ganglia (sensory, somatic) (Chapter 12)
Autonomic ganglia (motor, visceral) (Chapter 14)
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Regeneration of Nerve Fibers
Mature neurons are amitotic If the soma of a damaged nerve is intact, axon will
regenerate Involves coordinated activity among:
M h d b i Macrophages—remove debris Schwann cells—form regeneration tube and secrete growth
factors Axons—regenerate damaged part
CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration
Endoneurium
Droplets
Schwann cells The axonbecomesf t d t
1
of myelin
Fragmentedaxon Site of nerve damage
fragmented atthe injury site.
Figure 13.4 (1 of 4)
Schwann cell MacrophageMacrophages
clean out thedead a on distal
2
Figure 13.4 (2 of 4)
dead axon distalto the injury.
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Aligning Schwann cellsform regeneration tube
3 Axon sprouts,or filaments,grow through a
Figure 13.4 (3 of 4)Fine axon sproutsor filaments
grow through aregeneration tubeformed bySchwann cells.
Schwann cell Site of newmyelin sheathf ti
4 The axonregenerates anda new myelin
Figure 13.4 (4 of 4)
formation a new myelinsheath forms.
Single enlargingaxon filament
Levels of Motor Control
Segmental level
Projection level
Precommand level
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Feedback
Precommand Level(highest)• Cerebellum and basal
nuclei• Programs and instructions
(modified by feedback)
Projection Level (middle) • Motor cortex (pyramidal
system) and brain stemnuclei (vestibular, red,reticular formation, etc.)
• Convey instructions to
Internalfeedback
Figure 13.13a
Reflex activity Motoroutput
Sensoryinput
(a) Levels of motor control and their interactions
Convey instructions tospinal cord motor neuronsand send a copy of thatinformation to higher levels
Segmental Level (lowest)• Spinal cord• Contains central pattern
generators (CPGs)
Segmental Level
The lowest level of the motor hierarchy
Central pattern generators (CPGs): segmental circuits that activate networks of ventral horn neurons to stimulate specific groups of musclesp g p
Controls locomotion and specific, oft-repeated motor activity
Projection Level
Consists of: Upper motor neurons that direct the direct (pyramidal)
system to produce voluntary skeletal muscle movements Brain stem motor areas that oversee the indirect
(extrapyramidal) system to control reflex and CPG(extrapyramidal) system to control reflex and CPG-controlled motor actions
Projection motor pathways keep higher command levels informed of what is happening
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Precommand Level
Neurons in the cerebellum and basal nuclei Regulate motor activity Precisely start or stop movements Coordinate movements with posture Block unwanted movements Monitor muscle tone Perform unconscious planning and discharge in advance
of willed movements
Precommand Level
Cerebellum Acts on motor pathways through projection areas of the
brain stem
Acts on the motor cortex via the thalamus
Basal nuclei Inhibit various motor centers under resting conditions
Reflexes
Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus
Learned (acquired) reflexes result from practice or repetition, p , Example: driving skills
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Reflex Arc
Components of a reflex arc (neural path)1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent impulses to the CNS
3. Integration center—either monosynaptic or polysynaptic region within the CNS
4. Motor neuron—conducts efferent impulses from the integration center to an effector organ
5. Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting
Receptor
Sensory neuron
Interneuron
Stimulus
Skin
1
2
Figure 13.14
Sensory neuron
Integration center
Motor neuron
Effector
Spinal cord(in cross section)
2
3
4
5
Spinal Reflexes
Spinal somatic reflexes Integration center is in the spinal cord
Effectors are skeletal muscle
Testing of somatic reflexes is important clinically to Testing of somatic reflexes is important clinically to assess the condition of the nervous system
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Stretch and Golgi Tendon Reflexes
For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary Muscle spindles inform the nervous system of the length
of the muscle
Golgi tendon organs inform the brain as to the amount of tension in the muscle and tendons
Muscle Spindles
Composed of 3–10 short intrafusal muscle fibers in a connective tissue capsule
Intrafusal fibers Noncontractile in their central regions (lack Noncontractile in their central regions (lack
myofilaments)
Wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers
Muscle Spindles
Contractile end regions are innervated by gamma () efferent fibers that maintain spindle sensitivity
Note: extrafusal fibers (contractile muscle fibers) are innervated by alpha () efferent fibersy p ( )
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Secondary sensoryendings (type II fiber)
Efferent (motor)fiber to muscle spindle
Primary sensoryendings (type Iafiber)
Muscle spindle
Efferent (motor)fiber to extrafusalmuscle fibers
Extrafusal musclefiber
Figure 13.15
Connectivetissue capsule
Tendon
Sensory fiber
Golgi tendonorgan
Intrafusal musclefibers
Muscle Spindles
Excited in two ways:1. External stretch of muscle and muscle spindle2. Internal stretch of muscle spindle:
Activating the motor neurons stimulates the ends to contract thereby stretching the spindlecontract, thereby stretching the spindle
Stretch causes an increased rate of impulses in Ia fibers
Musclespindle
Intrafusalmuscle fiber
Primarysensory (la)nerve fiberExtrafusalmuscle fiber
Figure 13.16a, b
(a) Unstretchedmuscle. Actionpotentials (APs)are generated ata constant rate inthe associatedsensory (la) fiber.
Time
(b) Stretchedmuscle. Stretching activates the musclespindle, increasingthe rate of APs.
Time
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Muscle Spindles
Contracting the muscle reduces tension on the muscle spindle
Sensitivity would be lost unless the muscle spindle is shortened by impulses in the motor neuronsy p
– coactivation maintains the tension and sensitivity of the spindle during muscle contraction
Figure 13.16c, d
(d) - Coactivation.Both extrafusal andintrafusal musclefibers contract. Muscle spindletension is main-tained and it can still signal changesin length.
Time
(c) Only motorneurons activated.Only the extrafusalmuscle fibers contract. The muscle spindle becomes slack and no APs are fired. It isunable to signal furtherlength changes.
Time
Stretch Reflexes
Maintain muscle tone in large postural muscles
Cause muscle contraction in response to increased muscle length (stretch)
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Stretch Reflexes
How a stretch reflex works: Stretch activates the muscle spindle
Ia sensory neurons synapse directly with motor neurons in the spinal cord
motor neurons cause the stretched muscle to contract
All stretch reflexes are monosynaptic and ipsilateral
Stretch Reflexes
Reciprocal inhibition also occurs - Ia fibers synapse with interneurons that inhibit the motor neurons of antagonistic muscles
Example: In the patellar reflex, the stretched muscle p p ,(quadriceps) contracts and the antagonists (hamstrings) relax
Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist.
When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord.
The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.
The events by which muscle stretch is damped
12
Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist.
When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord.
The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.
The events by which muscle stretch is damped
12
Figure 13.17 (1 of 2)
Efferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch.
Efferent impulses of alpha motorneurons to antagonist muscles arereduced (reciprocal inhibition).
Initial stimulus(muscle stretch)
Cell body ofsensory neuron
Sensoryneuron
Muscle spindleAntagonist muscle
Spinal cord
3a 3bEfferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch.
Efferent impulses of alpha motorneurons to antagonist muscles arereduced (reciprocal inhibition).
Initial stimulus(muscle stretch)
Cell body ofsensory neuron
Sensoryneuron
Muscle spindleAntagonist muscle
Spinal cord
3a 3b
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The patellar (knee-jerk) reflex—a specific example of a stretch reflex
Musclespindle
Quadriceps(extensors)
Hamstrings(flexors)
Patella
Patellarligament
Spinal cord(L2–L4)
Tapping the patellar ligament excitesmuscle spindles in the quadriceps.
Afferent impulses (blue) travel to the
1
2
1
2
3a3b 3b
Figure 13.17 (2 of 2)
The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee.
Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.
The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.
Excitatory synapseInhibitory synapse
+
–
2
3a
3b
Golgi Tendon Reflexes
Polysynaptic reflexes
Help to prevent damage due to excessive stretch
Important for smooth onset and termination of muscle contractionmuscle contraction
Golgi Tendon Reflexes
Produce muscle relaxation (lengthening) in response to tension Contraction or passive stretch activates Golgi tendon organs
Afferent impulses are transmitted to spinal cord
Contracting muscle relaxes and the antagonist contracts (reciprocal activation)
Information transmitted simultaneously to the cerebellum is used to adjust muscle tension
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Quadriceps strongly contracts. Golgi tendon organs are activated.
Afferent fibers synapse with interneurons in the spinal cord.
Interneurons
S i l d
Quadriceps(extensors)
1 2
Figure 13.18
+ Excitatory synapse– Inhibitory synapse
Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension.
Efferent impulses to antagonist muscle cause it to contract.
Spinal cordGolgi
tendonorgan
Hamstrings(flexors)
3a 3b
Flexor and Crossed-Extensor Reflexes Flexor (withdrawal) reflex
Initiated by a painful stimulus
Causes automatic withdrawal of the threatened body part
Ipsilateral and polysynaptic
Flexor and Crossed-Extensor Reflexes Crossed extensor reflex
Occurs with flexor reflexes in weight-bearing limbs to maintain balance
Consists of an ipsilateral flexor reflex and a contralateral extensor reflex The stimulated side is withdrawn (flexed)
The contralateral side is extended
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Afferentfiber
Efferentfibers
Interneurons
Efferentfibers
+ Excitatory synapse– Inhibitory synapse
Figure 13.19
Extensorinhibited
Flexorstimulated
Site of stimulus: a noxiousstimulus causes a flexorreflex on the same side,withdrawing that limb.
Site of reciprocalactivation: At thesame time, theextensor muscleson the oppositeside are activated.
Armmovements
FlexorinhibitedExtensorstimulated
Superficial Reflexes
Elicited by gentle cutaneous stimulation
Depend on upper motor pathways and cord-level reflex arcs
Superficial Reflexes
Plantar reflex Stimulus: stroking lateral aspect of the sole of the foot
Response: downward flexion of the toes
Tests for function of corticospinal tracts p
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Superficial Reflexes
Babinski’s sign Stimulus: as above
Response: dorsiflexion of hallux and fanning of toes
Present in infants due to incomplete myelinationp y
In adults, indicates corticospinal or motor cortex damage
Superficial Reflexes
Abdominal reflexes Cause contraction of abdominal muscles and movement
of the umbilicus in response to stroking of the skin
Vary in intensity from one person to another
Absent when corticospinal tract lesions are present
Developmental Aspects of the PNS
Spinal nerves branch from the developing spinal cord and neural crest cells Supply both motor and sensory fibers to developing
muscles to help direct their maturation
Cranial nerves innervate muscles of the head
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Developmental Aspects of the PNS
Distribution and growth of spinal nerves correlate with the segmented body plan
Sensory receptors atrophy with age and muscle tone lessens due to loss of neurons, decreased ,numbers of synapses per neuron, and slower central processing
Peripheral nerves remain viable throughout life unless subjected to trauma