copyright © 2010 pearson education, inc. the nervous system: part c

Copyright © 2010 Pearson Education, Inc.

THE NERVOUS SYSTEM: PART C


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

Motor (efferent) divisionSensory (afferent)division

Somatic nervoussystem

Autonomic nervoussystem (ANS)

Sympatheticdivision

Parasympatheticdivision


Spinal Cord

• Location

• Begins at the foramen magnum

• Ends at L1 vertebra

• Functions

• Provides two-way communication to and from the brain

• Contains spinal reflex centers


Spinal Cord

• Spinal nerves

• 31 pairs

• Cauda equina

• The collection of nerve roots at the inferior end of the vertebral canal


• Bone

• Meninges

• CSF

Spinal Cord Protection

Copyright © 2010 Pearson Education, Inc. Figure 7.18

Dura andarachnoid

mater

Caudaequina

Cervicalspinal nerves

Lumbarspinal nerves

Sacralspinal nerves

Thoracicspinal nerves


Gray Matter

• Dorsal horns—interneurons that receive sensory input

• Ventral horns—somatic motor neurons whose axons exit the cord via ventral roots

• Dorsal root (spinal) ganglia—contain cell bodies of sensory neurons


Somaticsensoryneuron

Dorsal root (sensory)

Dorsal root ganglion

Visceralsensory neuron

Somaticmotor neuron

Spinal nerve

Ventral root(motor)

Ventral horn(motor neurons)

Dorsal horn (interneurons)

Visceralmotorneuron

Interneurons receiving input from somatic sensory neurons

Interneurons receiving input from visceral sensory neurons

Visceral motor (autonomic) neurons

Somatic motor neurons


(b) The spinal cord and its meningeal coverings

Central canal

Pia mater

Arachnoid mater

Spinal dura mater

Dorsal horn Graymatter

Lateral hornVentral hornWhite

matterDorsal rootganglion

Dorsal root

Ventral root

Spinal nerve


White Matter

• Mainly ascending (sensory) and descending (motor) tracts

• Tracts are located in three white columns on each side—posterior, lateral, and anterior columns


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


Structure of a Nerve

• Cordlike organ of the PNS

• Bundle of myelinated and unmyelinated peripheral axons enclosed by CT


Structure of a Nerve

• Connective tissue coverings include:

• Endoneurium—loose CT; encloses axons and their myelin sheaths

• Perineurium—coarse CT ; bundles fibers into fascicles

• Epineurium—tough fibrous sheath around a nerve


Bloodvessels

Fascicle

Epineurium

Perineurium

Endoneurium

AxonMyelin sheath


20

Classifying Nerves

• Mixed nerves carry both sensory and motor fibers

• Afferent (sensory) nerves carry impulses toward CNS

• Efferent (motor) nerves carry impulses away from CNS


23

Spinal Nerves • A spinal nerve is formed where ventral and dorsal roots fuse

• After spinal nerve is formed it divides into dorsal and ventral rami

• Ventral Rami form intercostal nerves (T1-T12) and networks of nerves called plexuses

• Plexuses serve limbs:

• Brachial plexus

• Lumbar plexus

• Sacral plexus


21

Cranial Nerves

• 12 pairs

• Extend from base of brain

• Primarily innervate head and neck (except Vagus N.)


22


• Sensory only

• CN I (Olfactory) -smell

• CN II (Optic) -vision

• CN VIII (Vestibulocochlear) -Hearing and balance

• Remaining cranial nerves are motor or mixed

11

Cranial Nerves


Somatic & Autonomic Nervous System Compared

Somatic Nervous System Autonomic Nervous System

The axon of ONE motor neuron extends all the way to skeletal

muscle

A chain of TWO motor neurons


• Somatic nervous system

• All somatic motor neurons release acetylcholine (ACh)

• ANS

• Preganglionic fibers release acetylcholine (Ach)

• Postganglionic fibers release norepinephrine or ACh at effectors

Somatic & Autonomic Nervous System Compared


Skeletal muscle

Cell bodies in centralnervous system Peripheral nervous system Effect

Effectororgans

ACh

AChSmooth muscle(e.g., in gut),glands, cardiacmuscle

Ganglion

Adrenal medulla Blood vessel

ACh

ACh

ACh

NE

Epinephrine andnorepinephrine

Acetylcholine (ACh) Norepinephrine (NE)

Ganglion

Neuro-transmitterat effector

Single neuron from CNS to effector organs

Two-neuron chain from CNS to effector organs

SO

MA

TIC

NER

VO

US

SYSTE

M

AU

TO

NO

MIC

NER

VO

US

SYS

TEM

PA

RA

SYM

PA

TH

ETIC

SYM

PA

TH

ETIC

Figure 7.24


Divisions of the ANS

1.Sympathetic division

2.Parasympathetic division

• Dual innervation

• Most visceral organs are served by both divisions, but they cause opposite effects


Role of the Sympathetic Division

• Mobilizes the body during activity; “fight-or-flight” system

• Promotes adjustments during exercise, or when threatened

• Blood flow directed to skeletal muscles & heart

• Bronchioles dilate

• Liver releases glucose


Role of the Parasympathetic Division

• Promotes maintenance activities, conserves body energy

• Its activity is illustrated in a person who relaxes, reading, after a meal

• Blood pressure, heart rate, and respiratory rates are low

• GI tract activity is high


Salivaryglands

Eye

Skin*

Heart

Lungs

Liverand gall-bladder

Genitals

Pancreas

Eye

Lungs

Bladder

Liver andgall-bladder

Pancreas

Stomach

Cervical

Sympatheticganglia

Cranial

Lumbar

Thoracic

Genitals

Heart

Salivaryglands

Stomach

Bladder

Adrenalgland

Parasympathetic Sympathetic

Sacral

Brainstem

L1

T1

Figure 7.25


Sympathetic Division

• Preganglionic fibers pass through white rami communicantes and enter sympathetic trunk


Spinal cord

Dorsal root

Ventral root

Sympathetictrunk ganglion

SympathetictrunkVentral ramusof spinal nerve

Gray ramuscommunicansWhite ramuscommunicans

(a) Location of the sympathetic trunk


Sympathetic Trunks and Pathways

• Upon entering a sympathetic trunk ganglion a preganglionic fiber may do one of the following:

1.Synapse with a ganglionic neuron at the same level

2.Ascend or descend the sympathetic trunk to synapse at another level

3.Pass through trunk ganglion and emerge without synapsing


To effector

Blood vessels

Skin (arrectorpili musclesand sweatglands)

Dorsal root ganglionDorsal ramus ofspinal nerve

Dorsal root

Sympathetictrunk ganglion

Lateral horn (visceralmotor zone)

Ventral root

Sympathetic trunk

Gray ramuscommunicansWhite ramuscommunicans

Ventral ramus ofspinal nerve

Synapse at the same level

(b) Three pathways of sympathetic innervation

1


To effector

Blood vessels

Skin (arrectorpili musclesand sweatglands)

Synapse at a higher or lower level


2


Splanchnic nerve

Collateral ganglion

Target organin abdomen

Synapse in a distant collateral ganglionanterior to the vertebral column


3


(b) The spinal cord and its meningeal coverings

Central canal

Pia mater

Arachnoid mater

Spinal dura mater

Dorsal horn Graymatter

Lateral hornVentral hornWhite

matterDorsal rootganglion

Dorsal root(fans out into dorsal rootlets)

Ventral root(derived from severalventral rootlets)

Spinal nerve

copyright © 2010 pearson education, inc. the nervous system: part c

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