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Reading: GAFS 62-88 Objectives: •Familiarize students with the structure and organization of the nervous system

Dr. Morton

G03: ANS, CNS, PNS

Nervous System Definitions and Overview •Structural Divisions

• Central nervous system (CNS)- the portion of the nervous system consisting of the brain and spinal cord. Command center that integrates and processes nervous system information.

• Peripheral nervous system (PNS)- the part of the vertebrate nervous system constituting the nerves outside the central nervous system (nerves and gan-glia)

• Upper motor neuron (UPN)- a motor neuron whose cell body is located in the motor area of the cerebral cortex and whose processes connect with motor nuclei in the brainstem or the anterior horn of the spinal cord.

• Lower motor neuron (LMN)- a motor neuron whose cell body is located in the brainstem or the spinal cord and whose axon innervates skeletal muscle fi-bers. Also called final motor neuron.

• Functional Divisions • Sensory (afferent = back to the CNS)

• General afferent (touch, temperature, pain, etc) • Visceral afferent (viscera = internal organ) • Special afferent (sight, taste, sound)

• Motor (efferent= away from the CNS) • Somatic efferent- innervates skeletal muscles derived from somites

(body wall) • Branchial efferent- innervates skeletal muscles derived from the pha-

ryngeal arches • Visceral efferent (viscera = internal organ)

•Functional Organization •Receive stimuli

•Receptors (e.g., pain, temperature) •Transmit responses

•Conductors (e.g., muscle contraction) •Process input

•Brain (interpretation)

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CNS

PNS Motor Sensory

Touch Pain Temperature Vibration Proprioception

Branchial Efferent

Somatic Efferent

Visceral Efferent

Visceral Afferent

General Afferent

Special Afferent

Skeletal mus-cle of the body wall

Skeletal mus-cle derived from pharyn-geal arches

Return information (reflexes, pain) con-cerning hollow or-gans and blood ves-sels

Sight, Taste, Sound

Smooth mus-cle, glands, and modified cardiac muscle

Sympathetic T1-L2

Parasympathetic CN 3,7,9,10

S2-S4

Organ, Tract, or System Sympathetic Parasympathetic

Pupil •Dilates •Constricts

Skin •Arrector pili muscle contraction •Vessels- vasoconstriction •Sweat glands- sweating

•No effect

Lacrimal and salivary glands •Decreases secretion •Increase secretion

Heart •↑ rate and strength of contrac-tion

•Dilates coronary vessels

•↓ rate and strength of contrac-tion

•Constricts coronary vessels

Lung •Bronchodilation •Bronchoconstriction

Digestive tract •Inhibits peristalsis •Constricts blood vessels

•Stimulates peristalsis

Genital system •Ejaculation •Erection

Suprarenal medulla •Release of adrenaline •No effect

Overview •Fight or Flight (E) •Exercise •Excitement •Emergency

•Rest and Digest (D) •Digestion •Defecation •Diuresis

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CNS •Organization

•Brain and spinal cord •Gray matter

•Contains nerve cells •White matter

•Contains nerve fibers

•Meninges (membranes) •Dura mater (dur = tough, mater =

mother) •Arachnoid mater (arachnoid = spi-

der, i.e., spider web-like) •Pia mater

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CNS (continued) •Spinal Cord

•Located in the vertebral column (spinal canal) •Length

•Embryo: fills the column length •At birth: extends caudally to L2 - L3 •Adult: extends caudally to L1 - L2 •Why is this distinction clinically important?

•Enlargements (2) •Cervical (upper limbs) •Lumbar (lower limbs)

Regions of the Spinal Cord and Verterbral Column

Region Number of Spinal Nerves

Number of Vertebrae

Cervical 8 7

Thoracic 12 12

Lumbar 5 5

Sacral 5 5

Coccygeal 1 4

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CNS (continued) •Spinal cord components

•Dentate (denticulate) ligaments •Conus medullaris •Cauda equina •Filum terminale

PNS •Organization

•Ganglia (cell bodies) •Sensory

•Dorsal root ganglia •Cranial nerve ganglia

•Nerves (fibers) •Sensory and motor •Peripheral nerves (31 pairs)

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PNS (continued) •Typical Spinal Nerve and Its Peripheral Distribution

•Components •Dorsal roots (sensory)

•Cell bodies located in the dorsal root ganglia •Ventral roots (motor)

•Cell bodies located in the CNS •Formed by fusion of segmental ventral and dorsal roots (2-4 mm

long) •Exit intervertebral foramen •Divide into dorsal and ventral primary rami

•Plexuses •Cervical (neck) •Brachial (upper limb) •Lumbar (distal thoracic region, abdomen, pelvis and lower limb) •Sacral (pelvis and lower limb)

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Pathways • Somatic afferent (sensory)

• Sensory fibers carry sensor (afferent information from the tissues of the limbs and back through the ventral and dorsal rami respec-tively, to the dorsal root to the spinal cord (Note: the collection of sensory cell bodies in the dorsal root forming the dorsal root gan-glion (DRG))

• Somatic efferent (motor) • Motor neurons carry so-

matic motor information from the spinal cord to the skeletal muscles of the limbs and back

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Sympathetic system • “Thoracolumbar” nervous system • T1 to L2 • Function: fight, fright, flight

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• Visceral efferents (sympathetic) • Preganglionic fibers from

spinal levels T1-L2 course out the ventral root and exit the spinal nerve through a white rami communicantes to synapse in the paraverte-bral ganglion

• Postganglionic sympathetic neurons exit through a gray rami communicantes to course to the blood vessels, erector pili muscles and sweat glands of the upper and lower limbs

• Visceral efferents (sympathetics) • Preganglionic fibers from spinal

levels T1-L2 course out the ven-tral root and exit the spinal nerve through a white rami communi-cantes to synapse in a higher or lower paravertebral ganglion

• Postganglionic sympathetic neu-rons exit through a gray rami communicantes to course to the blood vessels, erector pili mus-cles and sweat glands of the up-per and lower limbs

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• Visceral efferents (sympathetics) • Preganglionic fibers course through a splanchnic nerve to synapse in a

pre-vertebral ganglion (i.e. celiac ganglion) • Postganglionic fibers go to intestines, kidneys, liver, pancreas, etc.

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• Visceral efferents (sympathetics)

• Preganglionic fibers from spinal levels T1-T4 course up the sympathetic trunk to synapse in a more superior sympathetic ganglion (i.e. superior cervical ganglion)

• Postganglionic fibers go from superior sympathetic ganglion go to heart, lung, eye, lacrimal gland, salivary glands, cranial vessels

• Visceral efferents (sympathetics)

• Preganglionic fibers from spinal levels T12-L2 course down the sympathetic trunk to synapse in a more infe-rior sympathetic ganglion (inferior mesenteric)

• Postganglionic fibers go to colon, rectum, and genitalia

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• Visceral afferents (on sympathetic fibers) • Return information from internal organs and smooth muscle of large blood

vessel walls in the limbs

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Parasympathetic system • “Cranio-caudal” nervous system • Cranial nerves 3, 7, 9, 10; S2-S4 • Function: homeostasis

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Comparison of Pre- versus Postganglionic Neurons of the ANS

Sympathetic Parasympathetic

Short preganglionic fibers Long preganglionic fibers

Long postganglionic fibers Short postganglionic fibers

Ganglia located relatively far from their target organ

Ganglia located on or near their target organ

Sympathetic Parasympathetic

Preganglionic cell bodies: T1 to L2

Preganglionic cell bodies: brainstem and S2-S4

Postganglionic cell bodies: chain or collateral ganglia

Postganglionic cell bodies: cranial or enteric ganglia

Ganglia run entire length of spinal cord

Ganglia are at one end or the other of spinal cord