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C H A P T E R

Fundamentals of the Nervous System and

Nervous Tissue

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Nervous System

Sensory input

Motor output

Integration

Figure 12.1

Basic Divisions of the Nervous System

Figure 12.2

Brain

Spinalcord

CNS

Nerves

GangliaPNS

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Sensory Input and Motor Output

• Sensory (afferent) signals picked up by sensory receptors• Carried by nerve fibers of PNS to the CNS

• Motor (efferent) signals are carried away from the CNS• Innervate muscles and glands

Types of Sensory and Motor Information

Figure 12.3

Central nervous system (CNS) Peripheral nervous system (PNS)

Parasympatheticdivision

Motor (efferent) divisionSensory (afferent) division

Somatic nervoussystem

Sympathetic division

Autonomic nervoussystem (ANS)

Brain and spinal cordIntegrative and control centers

Cranial nerves and spinal nervesCommunication lines between the CNSand the rest of the body

Conserves energyPromotes house-keeping functionsduring rest

Motor nerve fibersConducts impulses from the CNSto effectors (muscles and glands)

Somatic and visceral sensorynerve fibersConducts impulses fromreceptors to the CNS

Somatic motor(voluntary)Conducts impulsesfrom the CNS toskeletal muscles

Mobilizes body systemsduring activity

Visceral motor(involuntary)Conducts impulsesfrom the CNS tocardiac muscles,smooth muscles,and glands

Somatic sensoryfiber

Visceral sensoryfiber

Motor fiber of somatic nervous system

Skin

Stomach Skeletalmuscle

Heart

BladderParasympathetic motor fiber of ANS

Sympathetic motor fiberof ANS

StructureFunctionSensory (afferent)division of PNS Motor (efferent)division of PNS

Nervous Tissue

• Two main cell types• Neurons (nerve cells) – transmit electrical signals -

excitable• Neuroglial cells (Support cells) – nonexcitable

• Surround and wrap neurons

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Structure of a Typical Large Neuron

Figure 12.4

Dendrites(receptiveregions)

Cell body(biosynthetic centerand receptive region)

Nucleolus

Nucleus

Nissl bodies

Axon(impulse generatingand conductingregion)

Axon hillock

NeurilemmaTerminalbranches

Node of Ranvier

Impulsedirection

Schwann cell(one inter-node)

Axon terminals(secretoryregion)

(b)

(a)

Neurofibril

Chromatophilic(Nissl) bodies

Nuclei ofneuroglialcells

Nucleus withnucleolus

Neuroncell body

Dendrites

Two Neurons Communicating at a Synapse

Figure 12.5a

Axon terminalat synapse

Axon

Presynapticneuron

Postsynapticneuron

Dendrite

(a) Two neurons connected by synapses

Synapse

Some Important Types of Synapses

Figure 12.5b(b) Enlarged view of the synapse

Neurofilament

Postsynaptic dendrite

Axon terminal

NerveimpulsesPresynaptic axon

Microtubule

Mitochondrion

Synapticcleft

Synapticvesicles

Vesicle releasingneurotransmitter

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Classification of Neurons

• Structural classification• Multipolar – possess more than two processes

• Numerous dendrites and one axon• Bipolar – possess two processes

• Rare neurons – found in some special sensory organs• Unipolar (pseudounipolar) – possess one short, single

process• Start as bipolar neurons during development

Neurons Classified by Structure

Neurons Classified by Structure

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Neurons Classified by Function

Functional Classification of Neurons

• Sensory neurons• deliver information from exteroceptors,

interoceptors, or proprioceptors• Motor neurons

• Form the efferent division of the PNS• Interneurons (association neurons)

• Located entirely within the CNS• Distribute sensory input and coordinate motor

output

Supporting Cells

• Six types of supporting cells• Four in the CNS• Two in the PNS

• Provide supportive functions for neurons• Cover nonsynaptic regions of the neurons

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Supporting Cells (Neuroglial Cells) in the CNS

• Four types of neuroglia in the CNS• Ependymal cells

• Related to cerebrospinal fluid• Astrocytes

• Largest and most numerous• Oligodendrocytes

• Myelination of CNS axons• Microglia

• Phagocytic cells

Neuroglia in the CNS

(a) Astrocytes are the most abundant CNS neuroglia.

Capillary

Neuron

Astrocyte

Figure 12.6a

Neuroglia in the CNS

(b) Microglial cells are defensive cells in the CNS.

NeuronMicroglialcell

Figure 12.6b

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Neuroglia in the CNS

Figure 12.6c, d

(d) Oligodendrocytes have processes that form myelinsheaths around CNS nerve fibers.

Nervefibers

Myelin sheath

Process ofoligodendrocyte

Brain orspinal cordtissue

Ependymalcells

Fluid-filled cavity

(c) Ependymal cells line cerebrospinal fluid–filled cavities.

Neuroglia in the PNS

• Satellite cells—surround neuron cell bodies within ganglia

• Schwann cells (neurolemmocytes)—surround axons in the PNS• Form myelin sheath around axons of the PNS

Figure 12.6e

(e) Satellite cells and Schwann cells (which formmyelin) surround neurons in the PNS.

Schwann cells(forming myelin sheath)

Cell body of neuronSatellitecells

Nerve fiber

Myelin Sheaths in the PNS

Axon

Cross section of a myelinated axon (TEM 30,000×)

A Schwann cellenvelops an axon.

The Schwann cell thenrotates around the axon, wrapping its plasma membrane loosely around it in successive layers.

The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath.

(a) Myelinated axon in PNS

Schwann cellcytoplasm

Axon

NeurilemmaMyelinsheath

Schwann cellnucleus

Schwann cell plasma membrane

An axon wrapped with a fatty insulating sheathformed from Schwann cells

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Myelin sheath

Schwann cellcytoplasm

Neurilemma

Figure 12.7a

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Unmyelinated Axons in the PNS

Cross section of unmyelinated axons (TEM 11,000×)

A Schwanncell surrounds multiple axons.

Each axon isencircled by the Schwann cell plasma membrane.

Schwann cell

Axons

Schwann cellnucleus

(b) Unmyelinated axons in PNS

Axons that are not covered with an insulating sheath

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2

Schwann cellNeurilemma

Axons

Figure 12.7b

Structure of a Nerve

Figure 12.8

Blood vessels

Fascicle

Epineurium

Perineurium

Endoneurium

AxonMyelin sheath

(a)

Blood vesselsFascicle

Perineurium

Endoneurium

(b)

Nervefibers

Myelin

Schwann cellnucleus

Node ofRanvier

Axon

(c)

Gray and White Matter in the CNS

Spinalnerve

PNS CNSGray matter

White matter

Hollow central cavity

Sensory (afferent)fiber

Motor (efferent)fiber

Short unmyelinatedinterneuronsCell bodies of interneurons and motor neuronsNeuroglia

Fiber tracts of myelinated and unmyelinated axons

Figure 12.9

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Basic Neuronal Organization of the Nervous System

• Reflex arcs – simple chains of neurons• Explain reflex behaviors• Determine structural plan of the nervous system• Responsible for reflexes

• Rapid, autonomic motor responses

Components of a Reflex Arc

Figure 12.10

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Spinal cord(in cross section)

Interneuron

Stimulus

Skin

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Types of Reflexes

• Monosynaptic reflex – simplest of all reflexes• Just one synapse• The fastest of all reflexes• Example – knee-jerk reflex

• Polysynaptic reflex – more common type of reflex• Most have a single interneuron between the sensory

and motor neuron• Example – withdrawal reflexes

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Types of Reflexes

(a) Monosynaptic stretch reflex

Sensory (stretch) receptor

Sensory (afferent) neuron

Motor (efferent) neuron

Effector organ

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Figure 12.11a

Types of Reflexes

Figure 12.11b

Interneuron

Effector organ

Sensory (afferent) neuron

Motor (efferent) neuron

Sensory receptor

(b) Polysynaptic withdrawal reflex

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Neuronal Circuits

Figure 12.12

(c) Reverberating circuit

(a) Diverging circuit to multiple pathways (b) Converging circuit

Input

Output Output

OutputInput

Input 1

Input 2 Input 3

Output

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Neural Processing

Design of the Nervous System

• Sensory neurons – located dorsally• Cell bodies outside the CNS in sensory ganglia• Central processes enter dorsal aspect of the spinal

cord• Motor neurons – located ventrally

• Axons exit the ventral aspect of the spinal cord• Interneurons – located centrally

• Synapse with sensory neurons

Simplified Design of the Nervous SystemIntegration in gray matter. Multiple interneurons process the nerve impulses to localize the stimulus, identify its source, and plan a response. This complex processing is illustrated here in a simplified manner.

Voluntary motor response.A nonreflexive motor response is initiated in the gray matter and transmitted down a descending fiber in the white matter to stimulate somatic motor neurons.

Figure 12.13 (continued)

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Disorders of the Nervous System

• Multiple sclerosis – common cause of neural disability• Varies widely in intensity among those affected• Cause is incompletely understood• An autoimmune disease

• Immune system attacks the myelin around axons in the CNS

Regeneration of the Peripheral Nerve Fiber

Figure 12.15

The axonbecomes fragmented at the injury site.

EndoneuriumDropletsof myelin

Fragmentedaxon

Schwann cells

Site of nerve damage

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Macrophagesclean out the dead axon distal to the injury.

Schwann cell Macrophage2

Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells.

Fine axon sproutsor filaments

Aligning Schwann cellsform regeneration tube

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The axon regenerates, and a new myelin sheath forms.

Schwann cell

Single enlargingaxon filament

Site of new myelinsheath formation

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