Divisions of the Nervous System

Peripheral Nervous System

Includes cranial nerves arising from the brain, somatic fibers connecting to the skin and skeletal muscles, autonomic fibers connecting to viscera, and spinal nerves arising from the spinal cord.

Sensory Nerves

Conduct impulses into the brain or spinal cord.

Motor Nerves

Conduct impulses to muscles or glands.

Mixed Nerves

Contain both sensory nerve fibers and motor nerve fibers; most nerves.

General somatic efferent fibers

Carry motor impulses from CNS to skeletal muscles.

General somatic afferent fibers

Carry sensory impulses to CNS from skin and skeletal muscles.

General visceral efferent fibers

Carry motor impulses from CNS to smooth muscles and glands.

General visceral afferent fibers

Carry sensory impulses to CNS from blood vessels and internal organs.

Special somatic efferent fibers

Carry motor impulses from brain to muscles used in chewing, swallowing, speaking, and forming facial expressions.

Special visceral afferent fibers

Carry sensory impulses to brain from olfactory and taste receptors.

Special somatic afferent fibers

Carry sensory impulses to brain from receptors of sight, hearing, and equilibrium.

Cranial Nerve I

Olfactory (I) - sensory fibers transmit impulses associated with smell.

Cranial Nerve II

Optic (II) - sensory fibers transmit impulses associated with vision.

Cranial Nerve III

Oculomotor (III) - primarily motor; motor impulses to muscles that raise eyelids, move the eyes, focus lens, adjust light entering eye.

Cranial Nerve IV

(Trochlear) Cranial Nerve IV is primarily a motor nerve that facilitates movement of the eyes, specifically controlling the muscles that move the eyes downward and medially.

Cranial Nerve V

Trigeminal (V) - mixed; sensory from surface of eyes, tear glands, scalp, forehead, upper eyelids, and motor to muscles of mastication.

Cranial Nerve VI

Abducens (VI) - primarily motor; motor impulses to muscles that move the eyes.

Cranial Nerve VII

Facial (VII) - mixed; sensory from taste receptors, motor to muscles of facial expression.

Cranial Nerve VIII

Vestibulocochlear (VIII) - sensory; vestibular branch from equilibrium receptors of ear.

Cranial Nerve IX

Glossopharyngeal (IX) - mixed; sensory from pharynx, tonsils, tongue, and motor to salivary glands.

Cranial Nerve X

Vagus (X) - mixed; somatic motor to muscles of speech and swallowing, autonomic motor to viscera of thorax and abdomen.

Cranial Nerve XI

Accessory (XI) - primarily motor; motor to muscles of soft palate, pharynx, and larynx.

Cranial Nerve XII

Hypoglossal (XII) - primarily motor; motor to muscles of the tongue.

Spinal Nerves

31 pairs including 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

Dorsal root

Posterior or sensory root; axons of sensory neurons in the dorsal root ganglion.

Dorsal root ganglion

Cell bodies of sensory neurons whose axons conduct impulses inward from peripheral body parts.

Dermatome

An area of skin that the sensory nerve fibers of a particular spinal nerve innervate.

Ventral root

Anterior or motor root.

Ventral root

axons of motor neurons whose cell bodies are in spinal cord

Spinal nerve

union of ventral root and dorsal root

Cervical Plexuses

Nerve plexus - complex networks formed by anterior branches of spinal nerves; fibers of various spinal nerves are sorted and recombined

Cervical Plexus

formed by anterior branches of C1-C4; lies deep in the neck; supply muscles and skin of the neck

C3 - C5

contribute to phrenic nerves

Brachial Plexuses

C5-T1; Lies deep within shoulders

Musculocutaneous nerves

supply muscles of anterior arms and skin of forearms

Ulnar and Median nerves

supply muscles of forearms and hands; supply skin of hands

Radial nerves

supply posterior muscles of arms and skin of forearms and hands

Axillary nerves

supply muscles and skin of anterior, lateral, and posterior arms

Lumbosacral Plexuses

T12 - S5; Extend from lumbar region into pelvic cavity

Obturator nerves

supply motor impulses to adductors of thighs

Femoral nerves

supply motor impulses to muscles of anterior thigh and sensory impulses from skin of thighs and legs

Sciatic nerves

supply muscles and skin of thighs, legs, and feet

Autonomic Nervous System

Functions without conscious effort; Controls visceral activities; Regulates smooth muscle, cardiac muscle, and glands

Efferent fibers

typically lead to ganglia outside CNS

Sympathetic Division

thoracolumbar division - location of preganglionic neurons

Preganglionic fibers

leave spinal nerves through white rami and enter paravertebral ganglia

Sympathetic trunk

made up of paravertebral ganglia and fibers that connect them

Postganglionic fibers

extend from sympathetic ganglia to visceral organs

Parasympathetic Division

craniosacral division - location of preganglionic neurons

Terminal ganglia

Ganglia are near or within various organs

Cholinergic Fibers

release acetylcholine; include preganglionic sympathetic and parasympathetic fibers and postganglionic parasympathetic fibers

Adrenergic Fibers

release norepinephrine; most postganglionic sympathetic fibers

Cholinergic receptors

bind to acetylcholine; include Muscarinic (excitatory, slow) and Nicotinic (excitatory, rapid)

Adrenergic Receptors

bind to epinephrine and norepinephrine; Alpha and Beta both elicit different responses on various effectors

Control of Autonomic Activity

Controlled largely by CNS; Medulla oblongata regulates cardiac, vasomotor and respiratory activities; Hypothalamus regulates visceral functions

Life-Span Changes

Brain cells begin to die before birth; Over average lifetime, brain shrinks 10%; Most cell death occurs in temporal lobes

Aging Effects

By age 90, frontal cortex has lost half its neurons; Number of dendritic branches decreases; Decreased levels of neurotransmitters; Fading memory; Slowed responses and reflexes; Increased risk of falling; Changes in sleep patterns that result in fewer sleeping hours