© 2012 pearson education, inc. an introduction to the nervous system organs of the nervous system...
TRANSCRIPT
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An Introduction to the Nervous System
• Organs of the Nervous System
• Brain and spinal cord (Processing incoming info and
creating motor commands)
• Sensory receptors of sense organs (eyes, ears,
pressure, pain, chemicals, etc.)
• Nerves connect nervous system with other
systems( a nerve is a bundle of axons)
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Anatomical Divisions of the Nervous System• The Central Nervous System (CNS)
• Consists of the spinal cord and brain
• Contains neural tissue, connective tissues, and blood vessels
• Functions of the CNS are to process and coordinate:
• Sensory data from inside and outside body
• Motor commands control activities of peripheral organs (e.g., skeletal muscles)
• Higher functions of brain intelligence, memory, learning, emotion
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Anatomical Divisions of the Nervous System• The Peripheral Nervous System (PNS)
• Includes all neural tissue outside the CNS
• Functions of the PNS
• Deliver sensory information to the CNS
• Carry motor commands to peripheral tissues and systems
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12-1 Divisions of the Nervous System
• The Peripheral Nervous System (PNS)
• Nerves (also called peripheral nerves)
• Bundles of axons with connective tissues and blood
vessels
• Carry sensory information and motor commands in PNS
• Cranial nerves — connect to brain
• Spinal nerves — attach to spinal cord
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12-1 Divisions of the Nervous System
• Functional Divisions of the PNS
• Afferent division (means “entrance”)
• Carries sensory information
• From PNS sensory receptors to CNS
• Efferent division (means “exit”)
• Carries motor commands
• From CNS to PNS muscles and glands
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12-1 Divisions of the Nervous System
• Functional Divisions of the PNS
• Receptors and effectors of afferent division
• Receptors
• Detect changes or respond to stimuli
• Neurons and specialized cells
• Complex sensory organs (e.g., eyes, ears)
• Effectors
• Respond to efferent signals
• Cells and organs
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Functional Divisions of the PNS
• Somatic nervous system (SNS)
• Controls voluntary and involuntary (reflexes)
muscle skeletal contractions
• Autonomic nervous system (ANS)
• Controls subconscious actions, contractions of
smooth muscle and cardiac muscle, and
glandular secretions
• Sympathetic division has a stimulating effect
• Parasympathetic division has a relaxing effect
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Figure 12-1a The Anatomy of a Multipolar Neuron
Dendrites
Perikaryon
NucleusCell body
TelodendriaAxon
This color-coded figureshows the four generalregions of a neuron.
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Figure 12-1b The Anatomy of a Multipolar Neuron
Axolemma
Dendritic branches
Nissl bodies (RERand free ribosomes)
Mitochondrion
Axon hillock
Initial segmentof axon
Golgi apparatus
Neurofilament
Nucleus
Nucleolus
Dendrite
PRESYNAPTIC CELL
Telodendria
Axon
Synapticterminals
See Figure 12–2
POSTSYNAPTICCELL
An understanding of neuronfunction requires knowing itsstructural components.
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12-2 Neurons
• The Structure of Neurons
• The synapse
• Presynaptic cell
• Neuron that sends message
• Postsynaptic cell
• Cell that receives message
• The synaptic cleft
• The small gap that separates the presynaptic
membrane and the postsynaptic membrane
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• Neurotransmitters
• Are chemical messengers
• Are released at presynaptic membrane
• Affect receptors of postsynaptic membrane
• Are broken down by enzymes (like AChE)
• Are reassembled at synaptic terminal
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12-2 Neurons
• Types of Synapses
• Neuromuscular junction
• Synapse between neuron and muscle
• Neuroglandular junction
• Synapse between neuron and gland
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Figure 12-2 The Structure of a Typical Synapse
Telodendrion
Synaptic terminal
Mitochondrion
Synapticvesicles
Presynapticmembrane
Postsynapticmembrane
Synapticcleft
Endoplasmicreticulum
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12-2 Neurons
• Functions of Sensory Neurons
• Monitor internal environment (visceral sensory neurons)
• Monitor effects of external environment (somatic sensory
neurons)
• Structures of Sensory Neurons
• Unipolar
• Cell bodies grouped in sensory ganglia
• Processes (afferent fibers) extend from sensory
receptors to CNS
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12-2 Neurons
• Three Types of Sensory Receptors
1. Interoceptors
• Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive)
• Internal senses (taste, deep pressure, pain)
2. Exteroceptors
• External senses (touch, temperature, pressure)
• Distance senses (sight, smell, hearing)
3. Proprioceptors
• Monitor position and movement (skeletal muscles and joints)
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12-2 Neurons• Motor Neurons
• Carry instructions from CNS to peripheral effectors
• Via efferent fibers (axons)
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12-2 Neurons
• Motor Neurons
• Two major efferent systems
1. Somatic nervous system (SNS)
• Includes all somatic motor neurons that innervate skeletal
muscles
2. Autonomic (visceral) nervous system (ANS)
• Visceral motor neurons innervate all other peripheral (like
internal organs) effectors
• Smooth muscle, cardiac muscle, glands, adipose tissue
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12-2 Neurons
• Interneurons
• Most are located in brain, spinal cord, and autonomic ganglia
• Between sensory and motor neurons
• Are responsible for:
• Distribution of sensory information
• Coordination of motor activity
• Are involved in higher functions
• Memory, planning, learning
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12-6 Axon Diameter and Speed
• Information
• “Information” travels within the nervous system
• As propagated electrical signals (action potentials)
• The most important information (vision, balance,
motor commands)
• Is carried by large-diameter, myelinated axons
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12-4 Transmembrane Potential
• Ion Movements and Electrical Signals
• All plasma (cell) membranes produce electrical
signals by ion movements
• Transmembrane potential is particularly important to
neurons
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12-4 Transmembrane Potential
• Five Main Membrane Processes in Neural
Activities
1. Resting potential
• The transmembrane potential of resting cell
2. Graded potential
• Temporary, localized change in resting potential
• Caused by stimulus
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12-4 Transmembrane Potential
• Five Main Membrane Processes in Neural
Activities
3. Action potential
• Is an electrical impulse
• Produced by graded potential
• Propagates along surface of axon to synapse
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12-4 Transmembrane Potential
• Five Main Membrane Processes in Neural
Activities
4. Synaptic activity
• Releases neurotransmitters at presynaptic membrane
• Produces graded potentials in postsynaptic
membrane
5. Information processing
• Response (integration of stimuli) of postsynaptic cell
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Figure 12-8 An Overview of Neural Activities
Presynaptic neuron
Restingpotential
stimulusproduces
Gradedpotential
mayproduce
Action potential
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Figure 12-8 An Overview of Neural Activities
Action potential
triggers
Syn
aptic
act
ivity
Informationprocessing
Postsynaptic cell
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12-4 Transmembrane Potential
• The Transmembrane Potential
• Three important concepts
1. The extracellular fluid (ECF) and intracellular fluid (cytosol) differ greatly in ionic composition
• Concentration gradient of ions (Na+, K+)
2. Cells have selectively permeable membranes
3. Membrane permeability varies by ion
A&P FLIX Resting Membrane Potential
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Figure 12-9 The Resting Potential is the Transmembrane Potential of an Undisturbed Cell
CYTOSOL
Na+ leakchannel
EXTRACELLULAR FLUID
Sodium–potassiumexchange
pump
0+30
–70
Plasmamembrane
Protein
–30
K+ leakchannel
2 K+
3 Na+
Cl–
ProteinProtein
mV
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12-4 Transmembrane Potential
• Changes in the Transmembrane Potential
• Transmembrane potential rises or falls
• In response to temporary changes in membrane
permeability
• Resulting from opening or closing specific membrane
channels
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12-4 Transmembrane Potential
• Sodium and Potassium Channels
• Membrane permeability to Na+ and K+ determines
transmembrane potential
• They are either passive or active
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12-4 Transmembrane Potential
• Passive Channels (Leak Channels)
• Are always open
• Permeability changes with conditions
• Active Channels (Gated Channels)
• Open and close in response to stimuli
• At resting potential, most gated channels are closed
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12-4 Transmembrane Potential
• Three Classes of Gated Channels
1. Chemically gated channels
2. Voltage-gated channels
3. Mechanically gated channels
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12-4 Transmembrane Potential
• Chemically Gated Channels
• Open in presence of specific chemicals (e.g., ACh) at a binding site
• Found on neuron cell body and dendrites
• Voltage-gated Channels
• Respond to changes in transmembrane potential
• Have activation gates (open) and inactivation gates (close)
• Characteristic of excitable membrane
• Found in neural axons, skeletal muscle sarcolemma, cardiac muscle
• Mechanically Gated Channels
• Respond to membrane distortion
• Found in sensory receptors (touch, pressure, vibration)
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Figure 12-11a Gated Channels
Resting state
Presence of ACh
Bindingsite
ACh
GatedchannelChannel closed
Channel open
A chemically gated Na+ channel thatopens in response to the presence ofACh at a binding site.
Chemically gated channel
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Figure 12-11b Gated Channels
Voltage-gated channel
Channel closed
Channel open
Channel inactivated
+30 mV
–60 mV
–70 mV
Inactivationgate
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Figure 12-11c Gated Channels
Mechanically gated channel
Channel closed
Channel open
Channel closed
Appliedpressure
Pressureremoved
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12-4 Transmembrane Potential
• Transmembrane Potential Exists Across Plasma
Membrane
• Because:
• Cytosol and extracellular fluid have different chemical/ionic
balance
• The plasma membrane is selectively permeable
• Transmembrane Potential
• Changes with plasma membrane permeability
• In response to chemical or physical stimuli
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12-4 Transmembrane Potential
• Graded Potentials
• Also called local potentials
• Changes in transmembrane potential
• That cannot spread far from site of stimulation
• Any stimulus that opens a gated channel
• Produces a graded potential
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12-4 Transmembrane Potential
• Graded Potentials
• The resting state
• Opening sodium channel produces graded potential
• Resting membrane exposed to chemical
• Sodium channel opens
• Sodium ions enter the cell
• Transmembrane potential rises
• Depolarization occurs
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Figure 12-12 Graded Potentials
Initialsegment
Resting State
–70 mV
EXTRA-CELLULARFLUID
CYTOSOL
Resting membrane with closed chemically gated sodium ion channels
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12-4 Transmembrane Potential
• Graded Potentials
• Depolarization
• A shift in transmembrane potential toward 0 mV
• Movement of Na+ through channel
• Produces local current
• Depolarizes nearby plasma membrane (graded potential)
• Change in potential is proportional to stimulus
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Figure 12-12 Graded Potentials
Membrane exposed to chemical that opens the sodium ion channels
Stimulusapplied
here
Stimulation
–65 mV
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Figure 12-12 Graded Potentials
Graded Potential
Spread of sodium ions inside plasma membrane produces a local currentthat depolarizes adjacent portions of the plasma membrane
–60 mV –65 mV –70 mVLocalcurrent
Local current
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12-4 Transmembrane Potential
• Graded Potentials
• Repolarization
• When the stimulus is removed, transmembrane
potential returns to normal
• Hyperpolarization
• Increasing the negativity of the resting potential
• Result of opening a potassium channel
• Opposite effect of opening a sodium channel
• Positive ions move out, not into cell
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12-5 Action Potential
• Action Potentials
• Propagated changes in transmembrane potential
• Affect an entire excitable membrane
• Link graded potentials at cell body with motor end
plate actions
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12-5 Action Potential
• Initiating Action Potential
• All-or-none principle
• If a stimulus exceeds threshold amount
• The action potential is the same
• No matter how large the stimulus
• Action potential is either triggered, or not
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Figure 12-14 Generation of an Action Potential
Resting Potential
–70 mV
KEY
The axolemma contains both voltage-gated sodium channels and voltage-gated potassium channels that areclosed when the membrane is at theresting potential.
= Sodium ion
= Potassium ion
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12-5 Action Potential
• Four Steps in the Generation of Action Potentials
• Step 1: Depolarization to threshold
• Step 2: Activation of Na channels
• Step 3: Inactivation of Na channels and activation
of K channels
• Step 4: Return to normal permeability
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12-5 Action Potential
• Step 1: Depolarization to threshold
• Step 2: Activation of Na channels
• Rapid depolarization
• Na+ ions rush into cytoplasm
• Inner membrane changes from negative to positive
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Figure 12-14 Generation of an Action Potential
–60 mV
KEY
= Sodium ion
= Potassium ion
Localcurrent
Depolarization to Threshold
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Figure 12-14 Generation of an Action Potential
KEY
= Sodium ion
= Potassium ion
+10 mV
Activation of SodiumChannels and RapidDepolarization
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12-5 Action Potential
• Step 3: Inactivation of Na channels and
activation of K channels
• At +30 mV
• Inactivation gates close (Na channel inactivation)
• K channels open
• Repolarization begins
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Figure 12-14 Generation of an Action Potential
KEY
= Sodium ion
= Potassium ion
+30 mV
Inactivation of SodiumChannels and Activationof Potassium Channels
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12-5 Action Potential
• Step 4: Return to normal permeability
• K+ channels begin to close
• When membrane reaches normal resting potential (–70
mV)
• K+ channels finish closing
• Membrane is hyperpolarized to –90 mV
• Transmembrane potential returns to resting level
• Action potential is over
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Figure 12-14 Generation of an Action Potential
KEY
= Sodium ion
= Potassium ion
–90 mV
Closing of PotassiumChannels
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Figure 12-14 Generation of an Action Potential
Graded potential causes threshold
Sodium channels close, voltage-gated potassium channels open
Threshold
Restingpotential
Tra
nsm
emb
ran
e p
ote
nti
al (
mV
)
D E P O L A R I Z A T I O N R E P O L A R I Z A T I O N
Time (msec)
Voltage-gated sodiumion channels open
All channels closed
Cannot respond Responds only to a largerthan normal stimulus
RELATIVE REFRACTORY PERIODABSOLUTE REFRACTORY PERIOD
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12-5 Action Potential
• Propagation of Action Potentials
• Propagation
• Moves action potentials generated in axon hillock
• Along entire length of axon
• Two methods of propagating action potentials
1. Continuous propagation (unmyelinated axons)
2. Saltatory propagation (myelinated axons)
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12-5 Action Potential
• Continuous Propagation
• Of action potentials along an unmyelinated axon
• Affects one segment of axon at a time
• Steps in propagation
• Step 1: Action potential in segment 1
• Depolarizes membrane to +30 mV
• Local current
• Step 2: Depolarizes second segment to threshold
• Second segment develops action potential
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Figure 12-15 Continuous Propagation of an Action Potential along an Unmyelinated Axon
Actionpotential
Extracellular Fluid
Cell membrane Cytosol
As an action potential develops at the initial segment , thetransmembrane potential at this site depolarizes to +30 mV.
–70 mV+30 mV
Na+
–70 mV
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Figure 12-15 Continuous Propagation of an Action Potential along an Unmyelinated Axon
As the sodium ions entering at spread away from theopen voltage-gated channels, a graded depolarizationquickly brings the membrane in segment to threshold.
Local
Graded depolarization
current
–60 mV –70 mV
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12-5 Action Potential
• Continuous Propagation
• Steps in propagation
• Step 3: First segment enters refractory period
• Step 4: Local current depolarizes next segment
• Cycle repeats
• Action potential travels in one direction (1 m/sec)
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Figure 12-15 Continuous Propagation of an Action Potential along an Unmyelinated Axon
An action potential now occurs in segment whilesegment beings repolarization.
Repolarization(refractory) –70 mV+30 mV
Na+
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Figure 12-15 Continuous Propagation of an Action Potential along an Unmyelinated Axon
As the sodium ions entering at segment spread laterally,a graded depolarization quickly brings the membrane insegment to threshold, and the cycle is repeated.
–60 mV
Local current
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12-5 Action Potential
• Saltatory Propagation
• Action potential along myelinated axon
• Faster and uses less energy than continuous
propagation
• Myelin insulates axon, prevents continuous
propagation
• Local current “jumps” from node to node
• Depolarization occurs only at nodes
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Figure 12-16 Saltatory Propagation along a Myelinated Axon
–70 mV+30 mV –70 mV
An action potentialhas occurred at the initial segment .
Extracellular Fluid
Myelinatedinternode
Myelinatedinternode
Myelinatedinternode
Plasma membrane Cytosol
Na+
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Figure 12-16 Saltatory Propagation along a Myelinated Axon
–60 mV –70 mV
Localcurrent
A local currentproduces a gradeddepolarization thatbrings the axolemmaat the next node tothreshold.
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Figure 12-16 Saltatory Propagation along a Myelinated Axon
–70 mV+30 mV
An action potentialdevelops at node .
Na+
Repolarization(refractory)
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Figure 12-16 Saltatory Propagation along a Myelinated Axon
–60 mV
A local currentproduces a gradeddepolarization thatbrings the axolemmaat node to threshold.
Localcurrent
A&P FLIX Propagation of an Action Potential
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12-6 Axon Diameter and Speed
• Axon Diameter and Propagation Speed
• Ion movement is related to cytoplasm concentration
• Axon diameter affects action potential speed
• The larger the diameter, the lower the resistance
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12-7 Synapses
• Synaptic Activity
• Action potentials (nerve impulses)
• Are transmitted from presynaptic neuron
• To postsynaptic neuron (or other postsynaptic cell)
• Across a synapse
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12-7 Synapses
• Two Types of Synapses
1. Electrical synapses
• Direct physical contact between cells
2. Chemical synapses
• Signal transmitted across a gap by chemical
neurotransmitters
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12-7 Synapses
• Electrical Synapses
• Are locked together at gap junctions (connexons)
• Allow ions to pass between cells
• Produce continuous local current and action potential
propagation
• Are found in areas of brain, eye, ciliary ganglia
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12-7 Synapses
• Chemical Synapses
• Are found in most synapses between neurons and all
synapses between neurons and other cells
• Cells not in direct contact
• Action potential may or may not be propagated to
postsynaptic cell, depending on:
• Amount of neurotransmitter released
• Sensitivity of postsynaptic cell
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12-7 Synapses
• Two Classes of Neurotransmitters
1. Excitatory neurotransmitters
• Cause depolarization of postsynaptic membranes
• Promote action potentials
2. Inhibitory neurotransmitters
• Cause hyperpolarization of postsynaptic membranes
• Suppress action potentials
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12-7 Synapses
• The Effect of a Neurotransmitter
• On a postsynaptic membrane
• Depends on the receptor
• Not on the neurotransmitter
• For example, acetylcholine (ACh)
• Usually promotes action potentials
• But inhibits cardiac neuromuscular junctions
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12-7 Synapses
• Cholinergic Synapses
• Any synapse that releases ACh at:
1. All neuromuscular junctions with skeletal muscle
fibers
2. Many synapses in CNS
3. All neuron-to-neuron synapses in PNS
4. All neuromuscular and neuroglandular junctions of
ANS parasympathetic division
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12-7 Synapses
• Events at a Cholinergic Synapse
1. Action potential arrives, depolarizes synaptic terminal
2. Calcium ions enter synaptic terminal, trigger
exocytosis of ACh
3. ACh binds to receptors, depolarizes postsynaptic
membrane
4. ACh removed by AChE
• AChE breaks ACh into acetate and choline
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Figure 12-17 Events in the Functioning of a Cholinergic Synapse
POSTSYNAPTICNEURON
An action potential arrives anddepolarizes the synaptic terminal
Presynaptic neuron
Synaptic vesicles
ER
Action potential
EXTRACELLULARFLUID
Synapticterminal
Initialsegment
AChE
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Figure 12-17 Events in the Functioning of a Cholinergic Synapse
Extracellular Ca2+ enters the synapticterminal, triggering the exocytosis of ACh
Ca2+Ca2+
ACh
Synaptic cleft
Chemically gatedsodium ion channels
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Figure 12-17 Events in the Functioning of a Cholinergic Synapse
Na+Na+ Na+Na+
Na+
ACh binds to receptors and depolarizesthe postsynaptic membrane
Initiation ofaction potentialif threshold isreached at theinitial segment
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Figure 12-17 Events in the Functioning of a Cholinergic Synapse
ACh is removed by AChE
Propagation ofaction potential
(if generated)
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Table 12-4 Synaptic Activity
Mitochondrion
AcetylcholineSynapticvesicle
SYNAPTICTERMINAL
SYNAPTICCLEFT
AChreceptor
(AChE)
AcetylcholinesteraseCholine
Acetate
POSTSYNAPTICMEMBRANE
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12-8 Neurotransmitters and Neuromodulators
• Other Neurotransmitters
• At least 50 neurotransmitters other than ACh,
including:
• Biogenic amines
• Amino acids
• Neuropeptides
• Dissolved gases
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12-8 Neurotransmitters and Neuromodulators
• Important Neurotransmitters
• Other than acetylcholine
• Norepinephrine (NE)
• Dopamine
• Serotonin
• Gamma aminobutyric acid (GABA)
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12-8 Neurotransmitters and Neuromodulators
• Norepinephrine (NE)
• Released by adrenergic synapses
• Excitatory and depolarizing effect
• Widely distributed in brain and portions of ANS
• Dopamine
• A CNS neurotransmitter
• May be excitatory or inhibitory
• Involved in Parkinson’s disease and cocaine use
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12-8 Neurotransmitters and Neuromodulators
• Serotonin
• A CNS neurotransmitter
• Affects attention and emotional states
• Gamma Aminobutyric Acid (GABA)
• Inhibitory effect
• Functions in CNS
• Not well understood
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12-8 Neurotransmitters and Neuromodulators
• Chemical Synapse
• The synaptic terminal releases a neurotransmitter that
binds to the postsynaptic plasma membrane
• Produces temporary, localized change in permeability
or function of postsynaptic cell
• Changes affect cell, depending on nature and number
of stimulated receptors
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12-8 Neurotransmitters and Neuromodulators
• Many Drugs
• Affect nervous system by stimulating receptors that
respond to neurotransmitters
• Can have complex effects on perception, motor
control, and emotional states
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12-8 Neurotransmitters and Neuromodulators
• Neuromodulators
• Other chemicals released by synaptic terminals
• Similar in function to neurotransmitters
• Characteristics of neuromodulators
• Effects are long term, slow to appear
• Responses involve multiple steps, intermediary compounds
• Affect presynaptic membrane, postsynaptic membrane, or
both
• Released alone or with a neurotransmitter
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12-8 Neurotransmitters and Neuromodulators
• Neuropeptides
• Neuromodulators that bind to receptors and activate
enzymes
• Opioids
• Neuromodulators in the CNS
• Bind to the same receptors as opium or morphine
• Relieve pain
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12-9 Information Processing
• Information Processing
• At the simplest level (individual neurons)
• Many dendrites receive neurotransmitter messages
simultaneously
• Some excitatory, some inhibitory
• Net effect on axon hillock determines if action potential
is produced
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12-9 Information Processing
• Postsynaptic Potentials
• Graded potentials developed in a postsynaptic cell
• In response to neurotransmitters
• Two Types of Postsynaptic Potentials
1. Excitatory postsynaptic potential (EPSP)
• Graded depolarization of postsynaptic membrane
2. Inhibitory postsynaptic potential (IPSP)
• Graded hyperpolarization of postsynaptic membrane
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12-9 Information Processing
• Inhibition
• A neuron that receives many IPSPs
• Is inhibited from producing an action potential
• Because the stimulation needed to reach threshold is increased
• Summation
• To trigger an action potential
• One EPSP is not enough
• EPSPs (and IPSPs) combine through summation
1. Temporal summation
2. Spatial summation
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12-9 Information Processing
• Temporal Summation
• Multiple times
• Rapid, repeated stimuli at one synapse
• Spatial Summation
• Multiple locations
• Many stimuli, arrive at multiple synapses
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Figure 12-19a Temporal and Spatial Summation
First stimulus arrives
FIRSTSTIMULUS
Initialsegment
Second stimulus arrives and isadded to the first stimulus
SECONDSTIMULUS
Thresholdreached
Action potential is generated
ACTIONPOTENTIAL
PROPAGATION
Temporal Summation. Temporal summation occurs on a membrane that receives two depolarizing stimuli from thesame source in rapid succession. The effects of the second stimulus are added to those of the first.
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Figure 12-19b Temporal and Spatial Summation
Two stimuli arrive simultaneously Action potential is generated
TWOSIMULTANEOUS
STIMULI
ACTIONPOTENTIAL
PROPAGATION
Thresholdreached
Spatial Summation. Spatial summation occurs when sources of stimulationarrive simultaneously, but at different locations. Local currents spread thedepolarizing effects, and areas of overlap experience the combined effects.
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Figure 12-21a Presynaptic Inhibition and Presynaptic Facilitation
GABArelease
Actionpotentialarrives Inactivation of
calcium channels
4. Reducedeffect onpostsynapticmembrane
2. Less calciumenters
Ca2+
3. Lessneurotransmitterreleased
1. Actionpotentialarrives
Presynaptic inhibition
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Figure 12-21b Presynaptic Inhibition and Presynaptic Facilitation
Ca2+
Ca2+
Actionpotentialarrives
Serotoninrelease Activation of
calcium channels
2. More calciumenters
1. Actionpotentialarrives 3. More
neurotransmitterreleased
4. Increasedeffect onpostsynapticmembrane
Presynaptic facilitation