The Nervous System

Functions of the Nervous System

Figure 7.1

Functions of the Nervous SystemSensory input—gathering information

To monitor changes occurring inside and outside the body

Changes = stimuliIntegration

To process and interpret sensory input and decide if action is needed

Functions of the Nervous SystemMotor output

A response to integrated stimuliThe response activates muscles or glands

Organization of the Nervous System

Figure 7.2

Structural Classification of the Nervous SystemCentral nervous system (CNS)

BrainSpinal cord

Peripheral nervous system (PNS)Nerves outside the brain and spinal cord

Spinal nerves Cranial nerves

Functional Classification of the Peripheral Nervous System

Sensory (afferent) divisionNerve fibers that carry information to the

central nervous systemMotor (efferent) division

Nerve fibers that carry impulses away from the central nervous system

Functional Classification ofthe Peripheral Nervous SystemMotor (efferent) division (continued)

Two subdivisions Somatic nervous system = voluntary Autonomic nervous system = involuntary

Neuron Classification

Figure 7.6

Figure 7.4

Neurons= nerve cellsCells specialized to transmit messages

Nervous Tissue: NeuronsCell body

NucleusLarge nucleolus

Processes outside the cell bodyDendrites—conduct impulses toward the cell

bodyAxons—conduct impulses away from the cell

body

Figure 7.4

Nervous Tissue: Neurons

Figure 7.5

Nervous Tissue: NeuronsMyelin sheath—whitish, fatty material

covering axonsSchwann cells—produce myelin sheaths in

jelly roll–like fashionNodes of Ranvier—gaps in myelin sheath

along the axonNeurilemma – exposed layer of cell

membrane of Schwann cells

Nervous Tissue: NeuronsAxons end in axonal terminalsMost cells average about 10,000 axon

terminalsAxon terminals contain vesicles with

neurotransmitters (chemicals that excite or inhibit neurons or effector cells)

Axon terminals are separated from the next neuron by a gapSynaptic cleft—gap between adjacent neuronsSynapse—junction between nerves

Neuron Cell Body LocationMost neuron cell bodies are found in the

central nervous systemWhite matter – condensed areas of

myelinated fibersGray matter—areas of unmyelinated fibers

Ganglia—collections of cell bodies outside the central nervous system

Figure 7.8a

Structural Classification of NeuronsMultipolar neurons—many extensions from

the cell body

Structural Classification of NeuronsBipolar neurons—one axon and one dendrite

Figure 7.8b

Structural Classification of NeuronsUnipolar neurons—have a short single

process leaving the cell body

Figure 7.8c

Nerve ImpulseThe principle way neurons communicate is

by generating and propagating ACTION POTENTIALS (Aps).

Only cells with excitable membranes (like muscle cells and neurons) can generate APs.

An AP is a brief reversal of membrane potential.

In neurons, an AP is called a NERVE IMPULSE and only axons can generate one.

An AP involves the movement of Na+ and K+ ions moving in and out of the neuron, resulting in changes in POLARITY.

All or None ResponseIf the action potential (nerve impulse) starts, it is propagated over the entire axon

There are NO partial impulses. The impulse happens completely or not at all.

Impulses travel faster when fibers have a myelin sheath.

Transmission of a Signal at SynapsesImpulses are able to cross the synapse to

another nerveNeurotransmitter is released from a nerve’s

axon terminalThe dendrite of the next neuron has receptors

that are stimulated by the neurotransmitterAn action potential is started in the dendrite

Transmission of a Signal at Synapses

Figure 7.10

Axonterminal

Vesicles

Synapticcleft

Actionpotentialarrives

Synapse

Axon oftransmittingneuron

Receivingneuron

Neurotrans-mitter is re-leased intosynaptic cleft

Neurotrans-mitter bindsto receptoron receivingneuron’smembrane

Vesiclefuses withplasmamembrane

Synaptic cleftNeurotransmittermolecules

Ion channels Receiving neuron

Transmitting neuron

Receptor

Neurotransmitter

Na+Na+

Neurotransmitterbroken downand released

Ion channel opens Ion channel closes

Transmission of a Signal at Synapses

Figure 7.10, step 1

Axonterminal

Vesicles

Synapticcleft

Actionpotentialarrives

Synapse

Axon oftransmitting

neuron

Receivingneuron

Figure 7.10, step 2

Axonterminal

Vesicles

Synapticcleft

Actionpotentialarrives

Synapse

Axon oftransmittingneuron

Receivingneuron

Vesiclefuses withplasmamembrane

Synaptic cleft

Ion channels Receiving neuron

Transmitting neuron

Figure 7.10, step 3

Axonterminal

Vesicles

Synapticcleft

Actionpotentialarrives

Synapse

Axon oftransmittingneuron

Receivingneuron

Neurotrans-mitter is re-leased intosynaptic cleft

Vesiclefuses withplasmamembrane

Synaptic cleftNeurotransmittermolecules

Ion channels Receiving neuron

Transmitting neuron

Figure 7.10, step 4

Axonterminal

Vesicles

Synapticcleft

Actionpotentialarrives

Synapse

Axon oftransmittingneuron

Receivingneuron

Neurotrans-mitter is re-leased intosynaptic cleft

Neurotrans-mitter bindsto receptoron receivingneuron’smembrane

Vesiclefuses withplasmamembrane

Synaptic cleftNeurotransmittermolecules

Ion channels Receiving neuron

Transmitting neuron

Figure 7.10, step 5

SynapseReceivingneuron

Neurotrans-mitter is re-leased intosynaptic cleft

Neurotrans-mitter bindsto receptoron receivingneuron’smembrane

Vesiclefuses withplasmamembrane

Synaptic cleftNeurotransmittermolecules

Ion channels Receiving neuron

Transmitting neuron

Receptor

Neurotransmitter

Na+

Ion channel opens

Transmission of a Signal at Synapses

Figure 7.10, step 6

Neurotrans-mitter is re-leased intosynaptic cleft

Neurotrans-mitter bindsto receptoron receivingneuron’smembrane

Vesiclefuses withplasmamembrane

Synaptic cleftNeurotransmittermolecules

Ion channels Receiving neuron

Transmitting neuron

Receptor

Neurotransmitter

Na+

Na+

Neurotransmitterbroken downand released

Ion channel opens Ion channel closes

Figure 7.10, step 7

Neurotrans-mitter is re-leased intosynaptic cleft

Neurotrans-mitter bindsto receptoron receivingneuron’smembrane

Vesiclefuses withplasmamembrane

Synaptic cleftNeurotransmittermolecules

Ion channelsReceiving neuron

Transmitting neuron

Receptor

Neurotransmitter

Na+

Na+

Neurotransmitterbroken downand released

Ion channel opens Ion channel closes

Nerve ImpulsesResting neuron

The plasma membrane at rest is polarizedFewer positive ions are inside the cell than

outside the cellDepolarization

A stimulus depolarizes the neuron’s membraneA depolarized membrane allows sodium (Na+)

to flow inside the membraneThe exchange of ions initiates an action

potential in the neuron

Nerve ImpulsesAction potential

If the action potential (nerve impulse) starts, it is propagated over the entire axon

Impulses travel faster when fibers have a myelin sheath

Nerve ImpulsesRepolarization

Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane

The sodium-potassium pump, using ATP, restores the original configuration

Nerve Impulses

Figure 7.9a–b

Nerve Impulses

Figure 7.9c–d

Nerve Impulses

Figure 7.9e–f