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Nervous System Functions

• Control• Communication

– Sensory– Integration– Motor

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Organization of Nervous System

PERIPHERAL NERVOUS SYSTEMPNS

Cranial Nerves and Spinal Nerves(Nerve tissue outside of CNS)

CENTRAL NERVOUS SYSTEMCNS

Brain and Spinal Cord

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SOMATIC RECEPTORSSpecial senses,

Proprioceptors, etc.

VISCERAL RECEPTORSChemoreceptors,

Baroreceptors, Mechanoreceptors

SENSORY (AFFERENT) DIVISIONSomatic and Visceral

PERIPHERAL NERVOUS SYSTEMPNS

CENTRAL NERVOUS SYSTEMCNS

Integration and Control

PNS and Sensory Division

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Sensory Division(afferent PNS)

Special senses (such as for sight, smell, and hearing)

Somatic receptors (such as skeletal muscle stretch receptors)

Somatic and visceral (have some receptorsin common such as pressure receptors)

Cranial nerves

Spinal nerves

Spinal cord

Brain

CNS

Visceral receptors (such as baroreceptors, chemoreceptors, mechanoreceptors)

PNS

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PNS and Motor Division

SOMATIC NERVOUS SYSTEMTo skeletal muscles

Voluntary

SympatheticFight or flight response

ParasympatheticRest and repose response

AUTONOMIC NERVOUS SYSTEMCardiac muscle, smooth muscle, and glands (some)

Involuntary

MOTOR (efferent) DIVISIONSomatic and Autonomic

PERIPHERAL NERVOUS SYSTEMPNS

CENTRAL NERVOUS SYSTEMCNS

Integration and Control

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Motor Division(efferent PNS)

Parasympathetic(autonomic, visceral)

Sympathetic(autonomic, visceral)

Somatic(skeletal muscle, voluntary)

Sympathetic(sweat glands, involuntary)

Spinal cord

Brain

CNS

Cranial nerves

Spinal nerves

PNS

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Nerve Tissue

• Nervous tissue forms the nervous system– consists mostly of the (1) brain, (2) spinal

cord, and (3) nerves. • Two fundamental types of cells form the

basis of nervous tissue: • (1) neurons (nerve cells) • (2) neuroglia (supporting cells)

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Neurons• Structure• Functions

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Neurons • There are many types of neurons• Most neurons have

– cell body (soma)– cell processes (axon and dendrites)

• Functions include– Generation and transmission of

electrical events resulting in the inhibition or excitation of postsynaptic neuron (or effector)

• Neuroglial cells– Mostly function in supporting and

insulating the nervous tissue. There are several different types of neuroglial cells.

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Typical Neurons

• Must live (amitotic) and function for the life of the individual.

• Function is the generation and conduction of an electrical event, the nerve impulse.

• Result is the excitation or inhibition of the associated (synapsed) neuron, muscle, or gland.

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Histology of “Nerve Tissue”Label:• Axon• Collaterals• Dendrites• Neuroglia• Nucleus• Soma (body)

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Structure of the Neuron

• Soma (body)– The portion of the cell with houses abundant cytoplasmic

organelles and is associated with the cell processes– Cell bodies are either located within the gray matter of the

CNS (most) or in structures called ganglia of the PNS

• Nucleus– Control center of the cell

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Structure of the Neuron• Dendrites

– The cell processes which function as the receptive portion of the neuron.

– May be modified and described as receptors such as corpuscles, spindles, etc.

– Conduct toward the cell body– Generate electrical information as graded potentials

(not nerve impulses, or action potentials)

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Structure of the Neuron• Axon

– Originate from cell body at region called hillock– Typically axons are called nerve fibers (are thin

and may be several feet in length)– May branch to form collaterals– Usually ends in terminal branches each with

knoblike ending called synaptic knobs, axonal terminals, or boutons.

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Structure of the Neuron• Axon

– Can be functionally divided into three regions (1) generator, (2) conductor, (3) and secretor.

• Generation occurs at its most proximal region called the axon hillock

• Conduction proceeds along the length of the fiber• Secretion (the release of neurotransmitter) occurs upon the

arrival of the nerve impulse at the axon terminals

– Axons are organized into regions called tracts (or columns) of the CNS or into the nerves of the PNS.

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Synapse• Anatomical relationship between neurons,

or neurons and an effector organ, and at which a nerve impulse is transmitted through the action of a neurotransmitter.

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Termination of Neurotransmitter

• Enzymes associated with postsynaptic membrane or present in cleft

• Reuptake by astrocytes into presynapticterminal where degraded by enzymes

• Neurotransmitter diffuses away from synapse

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Structural Classification of Neurons• Unipolar• Bipolar• Multipolar

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Structural Classification• Unipolar neuron

– Has a single process associated with the cell body. – Functions as sensory (afferent) neuron

• Bipolar neuron– Has two processes associated with the cell body– Functions as sensory neuron (locations include retina

and olfactory mucosa)

• Multipolar neuron– Has more than two processes (usually many)

associated with the cell body.– Functions as motor (efferent) neuron

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Classification of Neurons

Unipolar Bipolar Multipolar

Structural Classification(based upon number of processes

associated with cell body)

Sensory (afferent) Motor (efferent) Association

Functional Classification(based upon direction of conduction

in reference to CNS)

Neurons

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Functional Classification of Neurons

• Sensory neurons• Association neurons• Motor neurons

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Sensory Neurons• Transmit impulses generated at their receptors

toward the central nervous system are sensory, or afferent, neurons. They constitute the sensory (afferent) division of the peripheral nervous system.

• Consist of somatic and visceral neurons and are mostly unipolar

Golgi tendon organs(stretch receptors)

Spinal cordSensory neuron

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Types of Receptors

• Exteroceptors– External environment in form of touch, temperature,

pressure, sight, smell, and hearing.• Proprioceptors

– Monitor position and movement of skeletal muscles and joints

• Interoceptors– Monitor systems such as urinary, digestive, respiratory,

cardiovascular – provide pressure, pain, and input into autonomic pathways.

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Motor Neurons• Motor, or efferent, neurons transmit impulses from the

central nervous system to effectors (glands and muscles). They constitute the motor (efferent) division of the peripheral nervous system.

• Consist of somatic and visceral neurons and are mostly multipolar

Neuromuscular junctions

Spinal cordMotor neuron

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Interneurons (association)• Transmit impulses from one neuron to

another. They are located in the central nervous system.

Spinal cord

Association neuron

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Neuroglia of the CNS

• Astrocytes• Microglia• Ependymal• Oligodendrocytes

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Astrocytes (as-tro-cytes)Function in exchange– Star-shaped cells which associates neurons and

their surrounding capillaries.

Microglia (mi-krog-le-ah)Phagocytosis– neural debris and microorganisms\

Ependymal (ep-en-di-mal)Form the lining of ventricles of the brain and central canal of spinal cord.

Oligodendrocytes (ol-i-go-den-dro-site)

Form insulating covering called myelin sheatharound the axons of the CNS

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Neuroglia of the PNS

• Satellite cells• Schwann cells

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Schwann and Satellite Cells• Schwann cells (neurolemmocytes) are

associated with axons of PNS. – May produce myelinated or unmyelinated fibers

• Satellite cells surround neuron cell bodies which are located within ganglia

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Myelination of Axon• All axons of the PNS are associated with

Schwann cells• All axons of the CNS are associated with

oligodendrocytes• Myelin (my-e-lin) is a lipoid substance located

in the membranes of Schwann cells (neurilemmocytes) and oligodendrocytes

• Axons are either MYELINATED ORUNMYELINATED depending upon whether the axon is wrapped or loosely associated with its associated cells

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Schwann Cells –Myelinated fibers in PNS

• Myelinated fiber– The sequential wrapping of each Schwann cell

membrane around the axon produces a myelinatedfiber.

– The space between adjacent Schwann cells is called the node of Ranvier.

– Neurilemma is nucleus and cytoplasm outside of myelin sheath

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Schwann Cells Unmyelinated fibers of PNS

– The loose association of each Schwann cell membrane around the axon produces anunmyelinated fiber.

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Myelinated Fiber

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Histology of PNS Axons

Identify:

Axons

Myelin sheath

Node of Ranvier

Neurilemma

Schwann cell

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CNS AxonsOligodendrocytes (ol-i-go-den-dro-site)

• Form insulating covering called myelin sheath around CNS axons

• Single oligodendrocyte associates with many axons

• Lack neurilemma

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Neurophysiology: Ions and Electrical Signals

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Membrane Potentials • Resting membrane potential

– Potential maintained by neuron.• Graded potential

– Temporary, localized change in resting membrane potential. Diminishes with distance

• Action potential– Electrical impulse that travels along an axon. Propagated and does

not diminish with distance.• Synaptic activity

– Typically involves release of neurotransmitter from axon. Binding to postsynaptic membrane causes graded potential.

• Postsynaptic cell response– Usually the result of an overall change due to the influence of

many graded potentials (many synapses).

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Electrical Terminology• Potential energy

– State of electrical energy as measured by the potential to produce electrical effects

• Voltage (potential)– Electrical measurement used to describe electrical

potential between two points.

• Current– Flow of electrical charge and is due to the electrical

difference (voltage) between two points

• Resistance– Opposition to electrical flow

• Insulators have high resistance• Conductors have low resistance

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Electrical Terminology• How might the following terms

apply to these two batteries?– Potential energy

• Are both the same?

– Voltage• Are both the same?

– Current• Do both produce the same?

– Resistance• Does a battery contain a “resister?”Size AAA Size D

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Electrical Terminology and the Cell Membrane

• How might the following terms apply to the illustrated cell membrane?– Potential energy– Voltage– Current– Resistance Extracellular

Intracellular

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• How might the following terms apply to the illustrated cell membrane?– Potential energy– Voltage– Current– Resistance

Electrical Terminology and the Cell Membrane

Extracellular

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• How might the following terms apply to the illustrated cell membrane?– Potential energy– Voltage– Current– Resistance

Electrical Terminology and the Cell Membrane

Extracellular

Intracellular

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Transmembrane Potentials• Ionic difference between

intracellular and extracellularfluids– Extracellular higher concentration

of Na+ (and Cl-)– Intracellular higher concentration

of K+ and negative proteins.

Net result is potential difference between extracellular and intracellular.– Extracellular is positive (Na+)– Intracellular is negative due to

negative proteins.

Extracellular

Intracellular

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Resting Membrane Potential

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Membrane Potential Changes• If the resting membrane potential is to change

– must be a change in the distribution of positive and/or negative charges; a redistribution of ions –

• Movement of ions can result when ions move through channels which include– Mechanically-gated (regulated) channels

• Open when subjected to a mechanical stimulus – Voltage-gated (regulated) channels

• Open when subjected to an electrical stimulus – Chemically-gated (regulated) channels

• Open when subjected to a specific chemical such as a neurotransmitter or hormone

– Passive (leakage) channels• Ions may leak through channels (or the phospholipid bilayer)

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Mechanical Channels

• Sodium channels (typical) open when subjected to mechanical stimulus

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Channels• Identify regions which are

– Mechanically gated– Electrically gated– Chemically gated

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Graded Potentials• Local response (graded potential at stretch receptor)• Sodium ions move across membrane• Interior becomes less negative (more positive)• Depolarization (changes toward less negative

(positive) voltage– May not reach threshold, thus no effect (action potential)– May reach threshold and produce an action potential

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IPSP and EPSP• EPSP

• Excitatory postsynaptic potential results when interior becomes more positive

• IPSP• Inhibitory post-synaptic

potential results when interior becomes more negative

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Threshold and Action Potentials• Threshold

– Point of depolarization (stimulation) which initiates an effect (action potential)

– In this case the electrically-gated Na+ channels open, (which are adjacent to the active mechanically-gated channels).

– The mechanically gated Na+ channels become inactive

• Action potential– Not local; travels great distance– Involves electrically-gated channels– Propagated along fiber (axon)– All-or-None Principle applies

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Generation of Action Potential1. Resting membrane potential is established2. Depolarization phase

– Increase in sodium ion permeability– Self propagating event

3. Repolarization phase– Decrease in sodium ion permeability– Increase in potassium ion permeability

• Undershoot or after-hyperpolarizationoccurs

• Redistribution of sodium and potassium by ATP driven sodium-potassium pump

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Resting Membrane Potential

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Depolarization as Na+ Moves Inward

• Receptor’s Na+ channels become inactive

• Local current opened adjacent electrically-gated Na+ channels (threshold)

• These channels produce local current

• Adjacent Na+ channels open

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Repolarization as K+ Moves Out• Local current opens

adjacent electrically-gated K+ channels

• K+ moves outward and repolarizationoccurs

• Local currents open adjacent Na+ channels

• Action potential is propagated to adjacent forward section

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Na+ / K+ Pump

• The Na+ / K+ re-establishes the extracellular and intracellular ionic gradients – Pump requires ATP– Na+ is pumped outward– K+ is pumped inward

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Propagation of Action Potential

• Propagation– Refers to the relay of the electrical event, the action

potential, along the axon

• Continuous Propagation– Involves adjacent membrane proteins, typical of

unmyelinated axons

• Saltatory Propagation– Involves propagation from node to node, typical of

myelinated axons

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Conduction Velocity• Myelinated fibers conduct faster than

unmyelinated fibers– Continuous conduction– Saltatory conduction

• Large fibers conduct faster than small fibers– Larger fibers offer less resistance

• What is the approximate range of conduction velocities?

• What is multiple sclerosis (MS)?

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Synaptic Activity– Impulse passes from presynaptic membrane

(typically the neuron’s axon) to postsynaptic membrane. Typical postsynaptic membranes are located on

• Neurons (neuronal synapse)• Muscle cells (neuromuscular synapse)• Glands (neuroglandular synapse)

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Chemical Synapses• Involve a neurotransmitter

– Excitatory neurotransmitters always produce an EPSP (excitatory postsynaptic potential)

– Inhibitory neurotransmitters always produce an IPSP (inhibitory postsynaptic potential)

– The production of an action potential depends upon “reaching threshold.” Thus, it is the property of the receptor not the neurotransmitter. The same neurotransmitter may be inhibitory at one location and excitatory at another location

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Synapse

1. Action potentialarrives

2. Calcium ionchannels open

3. Calcium ions promoteexocytosis of neurotransmitter,calcium ions are quickly removed

4. Neurotransmitter binds to postsynaptic receptors

5. Receptors allow passageof specific type of ions

6. Depending upon ion movementpostsynaptic membrane is either

depolarized (EPSP) or hyperpolarized (IPSP)

7. Neurotransmitter is deactivated by enzymatic action; somecomponents may be reused

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Synapse

A B

The result is “A” or “B”?Which is produced?

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Synapse

A B

The result is “A” or “B”?Which is produced?

A) action potential, B) IPSP, C) EPSP?

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Cholinergic Synapses

• Release acetylcholine– At all neuromuscular juntions– At many CNS synapses– At all neuron-to-neuron synapses in PNS– At all effector sites (muscles and glands) of

parasympathetic nervous system (ANS).

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• Compounds that have a direct effect on membrane potential

• Compound that have an indirect effect on membrane potential or cell activity

• Lipid-soluble gases that effect the inside of the cell

How Neurotransmitters Work

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How Neurotransmitters Work• Direct acting (effect)

– Channel linked receptors result in the opening of ion channels –

– Alter membrane potential of target– Can produce depolarization (sodium ions move

inward) and hyperpolarization (potassium ions move outward)

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Mechanisms of Neurotransmitters

• Indirect acting– Involves G-protein complex– Results in the production of a second

messenger– Second messenger may influence enzymes to

• Activate or inactivate proteins (translation)• Regulate gene activity (transcription)• Regulate membrane ion channels and potentials

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Postsynaptic Potentials• IPSPs (review)• EPSPs (review)• Summation

– The adding together of synaptic potentials (SPs). Could be EPSPs, IPSPs, or both EPSPsand IPSPs.

– Temporal summation– Spatial summation

• Faciliation

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SummationTemporal summation

– Pertaining to time; the quick succession of SPsat a few synapses are summated

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Summation• Spatial summation

– Pertaining to space; many SPs occur over the postsynaptic membrane and are summated

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Nerves• Cordlike organ which conducts impulses

from a part of the central nervous system and another region of the body.– Components of the peripheral nervous system

(PNS) – Include the spinal nerves, the cranial nerves, and

all of their branches.

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Nerve Structure• Consisting of

– parallel axons (fibers) and– their associated Schwann cells (neurilemmocytes)

– enclosed in successive connective tissue wrappings (endoneurium, perineurium, epineurium).

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Nerves• May contain

– only myelinated fibers, – only unmyelinated fibers, – or a combination of both myelinated and unmyelinated

fibers

• Classified as– Sensory (afferent)– Motor (efferent)– Mixed (combination of afferent and efferent)

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Nerve• Define and Identify:

– Nerve– Epineurium– Perineurium– FasicleWhat is the difference

between a nerve and a tract?

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Nerve• Define and Identify

– Perineurium– Endoneurium– Schwann cell– Axon– Myelin sheath– Neurilemma

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Sensory Nerves (fibers)• Typically involve unipolar neurons• Impulse GENERATION begins at a dendrite (receptor)• Flow of information is into the CNS

– Typically to an association neuron.

Golgi tendon organs(stretch receptors)

Spinal cordSensory neuron

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• Includes– Sensory (afferent) neurons– Association neurons– Motor (efferent) neurons

• Response is predictable

Reflex

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Autonomic Regulation

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Motor Division(efferent PNS)

Parasympathetic(autonomic, visceral)

Sympathetic(autonomic, visceral)

Somatic(skeletal muscle, voluntary)

Sympathetic(sweat glands, involuntary)

Spinal cord

Brain

CNS

Cranial nerves

Spinal nerves

PNS

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Autonomic Systems• Sympathetic

– “fight or flight response”– Terminal neurotransmitter is epinephrine (E) or

norepinephrine (NE)• Parasympathetic

– “resting and digesting,” or “rest and repose”– Terminal neurotransmitter is acetylcholine (ACh)

• Organs – May have dual innervations, response is excitation by one

system and inhibition by other system– May have single innervations, response is promoted or

not promoted.