Divisions of the Nervous System: Cranial and Spinal Nerves, Plexuses, and Reflexes

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Divisions of the Nervous System

The nervous system is divided into central and peripheral components, with the peripheral nervous system (PNS) further subdivided into cranial and spinal nerves. These divisions coordinate sensory input, integration, and motor output throughout the body.

Cranial Nerves

Overview of Cranial Nerves

12 pairs of cranial nerves are associated with the brain.
Two pairs attach to the forebrain; the rest attach to the brain stem.
Most are mixed nerves (sensory and motor), but two pairs are purely sensory.
Numbered I through XII from rostral to caudal.
Mnemonic for names: "Oh, once one takes the anatomy final, very good vacations are heavenly"
Mnemonic for function (sensory, motor, both): "Some say money matters, but my brother boasts big brains matter more"

Location and Function of Cranial Nerves

Cranial Nerve	Name	Type	Main Function
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor	Eye movement, pupil constriction
IV	Trochlear	Motor	Eye movement (superior oblique muscle)
V	Trigeminal	Both	Facial sensation, mastication
VI	Abducens	Motor	Eye movement (lateral rectus muscle)
VII	Facial	Both	Facial expression, taste (anterior 2/3 tongue)
VIII	Vestibulocochlear	Sensory	Hearing, balance
IX	Glossopharyngeal	Both	Taste (posterior 1/3 tongue), swallowing
X	Vagus	Both	Parasympathetic control of heart, lungs, digestive tract
XI	Accessory	Motor	Shoulder and neck muscles
XII	Hypoglossal	Motor	Tongue movement

Selected Cranial Nerves: Details

Olfactory (I): Sensory nerves of smell; fibers run from nasal mucosa to olfactory bulbs, synapse in bulbs, terminate in primary olfactory cortex.
Optic (II): Sensory nerves of vision; arise from retina, pass through optic canals, cross at optic chiasma, continue to thalamus and occipital cortex.
Oculomotor (III): Motor; controls most eye movements, pupil constriction, lens shape.
Trochlear (IV): Motor; innervates superior oblique muscle of eye.
Trigeminal (V): Both; largest cranial nerve, three divisions (ophthalmic, maxillary, mandibular), facial sensation, mastication.
Abducens (VI): Motor; innervates lateral rectus muscle (eye abduction).
Facial (VII): Both; facial expression, taste (anterior 2/3 tongue), parasympathetic to glands.
Vestibulocochlear (VIII): Sensory; hearing and equilibrium.
Glossopharyngeal (IX): Both; taste (posterior 1/3 tongue), swallowing, monitors carotid body and sinus.
Vagus (X): Both; only cranial nerve to extend beyond head/neck, parasympathetic to thoracic and abdominal viscera.
Accessory (XI): Motor; innervates sternocleidomastoid and trapezius muscles.
Hypoglossal (XII): Motor; tongue movement for speech and swallowing.

Spinal Nerves

Overview of Spinal Nerves

31 pairs of spinal nerves, all mixed nerves.
Named for point of issue from spinal cord.
Supply all body parts except head and part of neck.
Distribution:
- 8 pairs cervical (C1–C8)
- 12 pairs thoracic (T1–T12)
- 5 pairs lumbar (L1–L5)
- 5 pairs sacral (S1–S5)
- 1 pair coccygeal (C0)

Spinal Nerve Structure

Each spinal nerve connects to the spinal cord via two roots:
- Ventral roots: Motor (efferent) fibers from ventral horn motor neurons to skeletal muscles.
- Dorsal roots: Sensory (afferent) fibers from sensory neurons in dorsal root ganglia to CNS.
Rami: Branches carrying visceral, somatic motor, and sensory information.
- Ventral rami: Innervate ventral trunk and limbs.
- Dorsal rami: Innervate dorsal trunk.

Innervation of Specific Body Regions

Nerve Plexuses

All ventral rami except T2–T12 form interlacing networks called nerve plexuses (cervical, brachial, lumbar, sacral).
Within plexuses, fibers from different spinal nerves crisscross, so each limb muscle is innervated by more than one spinal nerve (prevents paralysis from single nerve damage).

Cervical Plexus

Formed by ventral rami of C1–C4.
Most branches are cutaneous nerves (innervate skin of neck, ear, back of head, shoulders).
Phrenic nerve: Major motor and sensory nerve of diaphragm (breathing); receives fibers from C3–C5.
Irritation of phrenic nerve causes hiccups.

Brachial Plexus

Formed by ventral rami of C5–C8 and T1 (sometimes C4 and T2).
Gives rise to nerves that innervate the upper limb.

Nerves of the Upper Limb

Axillary: Deltoid, teres minor, skin/joint capsule of shoulder.
Musculocutaneous: Biceps brachii, brachialis, skin of lateral forearm.
Median: Most flexors, forearm pronators, skin of hand; carpal tunnel syndrome affects this nerve.
Ulnar: Flexor carpi ulnaris, hand muscles, skin of medial hand; "funny bone" sensation.
Radial: Extensor muscles, supinators, posterior skin of limb.

Lumbosacral Plexus

Significant overlap between lumbar and sacral plexuses.
Lumbar plexus: Innervates thigh, abdominal wall, psoas muscle.
Sacral plexus: Serves buttock, lower limb, pelvic structures, perineum.
Sciatic nerve: Longest and thickest nerve of the body.

Sciatica

Characterized by stabbing pain along the sciatic nerve.
Causes: injury, disc herniation, injection into buttock.
If nerve is transected, leg cannot be flexed; foot drops into plantar flexion (footdrop).
Recovery is slow and incomplete; depends on location of injury.

Innervation of the Anterolateral Thorax, Abdominal Wall, Back, and Joints

Intercostal nerves (T1–T12): supply muscles of ribs, thorax, abdominal wall.
Back: Dorsal rami innervate skin and muscles in line with emergence from spinal cord.
Joints: Hilton's law: any nerve serving a muscle that moves a joint also innervates the joint and skin over it.

Innervation of the Skin: Dermatomes

Dermatome: Area of skin innervated by cutaneous branches of a single spinal nerve.
All spinal nerves except C1 participate in dermatomes.
Most dermatomes overlap; destruction of a single spinal nerve does not cause complete numbness.

Innervation of Muscle and Glands

Innervation of Skeletal Muscle

Occurs at the neuromuscular junction.
Neurotransmitter acetylcholine (ACh) is released at axon terminal.
ACh binds to receptors, causing movement of Na+ and K+ across membrane, depolarizing muscle cell.
End plate potential spreads, triggering action potential and muscle contraction.

Innervation of Visceral Muscle and Glands

Autonomic motor endings and visceral effectors are simpler than somatic junctions.
Branches form en passant synapses with effector cells via varicosities.
Acetylcholine and norepinephrine act indirectly via second messengers.
Visceral motor responses are slower than somatic responses.

Reflexes

Types of Reflexes

Inborn (intrinsic) reflex: Rapid, involuntary, predictable motor response to stimulus (e.g., posture, visceral activities).
Learned (acquired) reflex: Result from practice or repetition (e.g., driving skills).

Components of a Reflex Arc

Receptor: Site of stimulus action.
Sensory neuron: Transmits afferent impulses to CNS.
Integration center: Mono- or polysynaptic region within CNS.
Motor neuron: Conducts efferent impulses from integration center to effector organ.
Effector: Muscle fiber or gland cell that responds by contracting or secreting.

Spinal Reflexes

Somatic reflexes: Activate skeletal muscle.
Autonomic (visceral) reflexes: Activate smooth/cardiac muscle or glands.
Spinal reflexes occur without direct involvement of higher brain centers.
Testing somatic reflexes is important for assessing nervous system health.
Proprioceptor input includes muscle length (muscle spindles) and tension (tendon organs).

Superficial Reflexes

Elicited by gentle cutaneous stimulation.
Plantar reflex: Tests cord from L4 to S2; abnormal response (Babinski's sign) indicates corticospinal tract damage.
Abdominal reflex: Tests cord from T8 to T12; contraction of abdominal muscles in response to stroking skin.

Discussion and Application

Sensory cranial nerves: Olfactory (I), Optic (II), Vestibulocochlear (VIII).
"Funny bone" sensation: Ulnar nerve is stimulated, causing tingling in the arm.
Reflex arc components: Receptor, sensory neuron, integration center, motor neuron, effector.