Organization of the Nervous System

6 min read Updated Apr 19, 2026

The nervous system is a hierarchy: sensory input flows in, motor output flows out, and everything in between is processing. This section lays out the major divisions you need to memorize for any MCAT neuroscience passage.

CNS and PNS

Central Nervous System (CNS) = brain + spinal cord. Where most processing happens.
Peripheral Nervous System (PNS) = everything else. All 12 cranial nerves and 31 pairs of spinal nerves, plus associated ganglia.

Brain Divisions

The brain forms from three primary vesicles in the embryo:

Forebrain → cerebrum, thalamus, hypothalamus (higher functions)
Midbrain → midbrain (sensory relay, motor coordination)
Hindbrain → pons, medulla, cerebellum (automatic and motor control)

The 12 Cranial Nerves

The 12 cranial nerves emerge directly from the brain or brainstem (not the spinal cord). You should know each nerve’s number, name, function class (sensory, motor, or both), and what it does.

Inferior view of the human brain showing all twelve pairs of cranial nerves emerging from the underside of the brain and brainstem, each labeled with its name and Roman numeral: I Olfactory, II Optic, III Oculomotor, IV Trochlear, V Trigeminal, VI Abducens, VII Facial, VIII Vestibulocochlear, IX Glossopharyngeal, X Vagus, XI Accessory, XII Hypoglossal — The twelve cranial nerves arising from the inferior surface of the brain. Note their anterior-to-posterior emergence order — the same order as their Roman numerals — which is why "On Old Olympus' Towering Tops…" works as a mnemonic. *Credit: OpenStax College via Wikimedia Commons (CC BY 4.0).*

#	Name	Function	What It Does
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor	Most eye movements, pupil constriction, lens shape
IV	Trochlear	Motor	Superior oblique (eye moves down-and-in)
V	Trigeminal	Both	Facial sensation + muscles of chewing
VI	Abducens	Motor	Lateral rectus (eye abducts, looks outward)
VII	Facial	Both	Muscles of facial expression + taste (anterior $\frac{2}{3}$ tongue) + lacrimation, salivation
VIII	Vestibulocochlear	Sensory	Hearing and balance
IX	Glossopharyngeal	Both	Taste (posterior $\frac{1}{3}$ tongue), swallowing, parotid salivation, carotid baroreceptors
X	Vagus	Both	Parasympathetic to heart, lungs, gut; voice (larynx); swallowing
XI	(Spinal) Accessory	Motor	Sternocleidomastoid and trapezius (head turning, shoulder shrug)
XII	Hypoglossal	Motor	Tongue movement

Mnemonic 1: Name Order

“On Old Olympus’ Towering Tops A Finn And German Viewed Some Hops”

Each first letter maps to a nerve in order: Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, (Vestibulo-)Acoustic, Glossopharyngeal, Vagus, Spinal accessory, Hypoglossal.

Mnemonic 2: Function (Sensory / Motor / Both)

“Some Say Marry Money But My Brother Says Big Brains Matter More”

In order I → XII: Sensory, Sensory, Motor, Motor, Both, Motor, Both, Sensory, Both, Both, Motor, Motor.

Clinically High-Yield Nerves

CN II (optic). Tested with visual acuity and visual fields. Afferent limb of the pupillary light reflex.
CN III (oculomotor). Efferent limb of the pupillary light reflex - damage produces a “blown pupil” (dilated, unreactive) and a “down-and-out” eye because only IV and VI still pull.
CN V (trigeminal). Three divisions: V1 ophthalmic, V2 maxillary, V3 mandibular. Corneal reflex afferent.
CN VII (facial). Bell’s palsy affects this nerve and produces unilateral facial droop including the forehead (an upper motor neuron stroke spares the forehead because of bilateral cortical input).
CN X (vagus). The main parasympathetic output to the body. Carries “rest and digest” signals to the heart and gut. Vagal stimulation slows the heart.

The Two Branches of the PNS

Somatic Nervous System

Carries voluntary motor commands to skeletal muscles and brings in sensory information from the skin, joints, and muscles. Under conscious control: you decide to lift your arm; the somatic nervous system makes it happen.

Autonomic Nervous System (ANS)

Controls involuntary functions: heart rate, digestion, pupil size, glandular secretion. Not under conscious control. Two opposing branches:

Sympathetic - “fight or flight.” Prepares the body for action.

Pupils dilate, heart rate increases, respiration deepens.
Digestion slows; blood diverts to skeletal muscle and vital organs.
Sweat glands activate, adrenal medulla releases epinephrine.
Short pre-ganglionic axons near the spinal cord, long post-ganglionic axons to target organs.

Parasympathetic - “rest and digest.” Restores baseline.

Pupils constrict, heart rate slows, salivation increases.
Digestion resumes, blood flow to the viscera increases.
Long pre-ganglionic axons from brainstem or sacral spinal cord; short post-ganglionic axons.

Both branches usually act on the same organs with opposite effects - the balance between them tunes physiology moment-to-moment.

A side-by-side comparison diagram of parasympathetic (green, left) and sympathetic (orange, right) outflows from the spinal cord, with each branch's effects on pupils, salivation, heart rate, bronchi, digestion, pancreas, gallbladder, adrenal medulla, urinary bladder, and genitals labeled. Parasympathetic exits cranially and sacrally; sympathetic exits thoracically and lumbarly via the sympathetic chain — The two branches of the autonomic nervous system, with their target organs and opposite effects. Parasympathetic ("rest and digest") fibers exit at the cranial and sacral levels; sympathetic ("fight or flight") fibers exit thoracolumbarly via the sympathetic chain. *Credit: Geo-Science-International via Wikimedia Commons (CC0).*

Afferent vs Efferent Neurons

Remember the A/E distinction: Afferent = Arrival (sensory, into CNS). Efferent = Exit (motor, out of CNS). Receptors send info through afferent axons; motor commands travel out through efferent axons.

Upper and Lower Motor Neurons

Voluntary muscle control is a two-step chain:

Upper motor neurons (UMNs) originate in the cerebral cortex and travel down to synapse on lower motor neurons in the brainstem or spinal cord. Their axons form the corticospinal tract (to spinal cord) or corticobulbar tract (to brainstem, for head/neck).
Lower motor neurons (LMNs) exit the CNS and synapse directly on skeletal muscle at the neuromuscular junction.

UMNs control LMNs. LMNs actually move muscles.

Motor Neuron Signs

Damage to UMNs and LMNs produces characteristically different clinical pictures:

LMN signs. Weakness, atrophy, fasciculations (involuntary twitches), hypotonia (low muscle tone), hyporeflexia.
UMN signs. Weakness, hyperreflexia (exaggerated stretch reflexes), clonus (rhythmic contraction), hypertonia (increased muscle tone), extensor plantar response (Babinski sign - toes fan upward instead of downward when the sole is stroked).

The logic: without UMN input, the spinal reflex circuits lose their “moderator” and become hyperactive. Without LMN input, the muscle gets no drive at all - it withers and twitches.

The Muscle Stretch Reflex

A reflex is a quick, involuntary response to a stimulus that doesn’t require the brain. The classic knee-jerk is a monosynaptic stretch reflex:

The hammer hits the patellar tendon, stretching the quadriceps.
Stretch receptors in the muscle (muscle spindles) fire afferent signals up to the spinal cord.
The afferent neuron synapses directly onto an LMN that innervates the same muscle.
The LMN fires, contracting the quadriceps - the leg kicks out.

At the same time, a separate inhibitory interneuron silences the antagonist muscle (hamstring) so it doesn’t fight the kick. This is reciprocal inhibition.

Reflexes demonstrate that the spinal cord does meaningful processing on its own - you don’t think “leg, kick.” The body protects itself faster than the brain can decide.

Diagram of the patellar (knee-jerk) reflex arc. A tendon hammer strikes the patellar tendon, stretching the quadriceps. A red sensory (afferent) neuron carries the signal from the muscle spindle through the dorsal root ganglion into the dorsal horn of the spinal cord, where it synapses directly on a green alpha motor neuron exiting the ventral horn back to the quadriceps. A blue inhibitory interneuron simultaneously suppresses the antagonist alpha motor neuron supplying the hamstrings — The monosynaptic patellar (knee-jerk) reflex. The afferent neuron synapses directly on the LMN to the quadriceps (one synapse → muscle contracts), while a separate inhibitory interneuron silences the hamstring — reciprocal inhibition. The whole circuit runs through the spinal cord without involving the brain. *Credit: Amiya Sarkar via Wikimedia Commons (CC BY-SA 4.0).*

Gray and White Matter

Gray matter - cell bodies (somas) of neurons, as well as dendrites and synapses.
White matter - myelinated axons. Looks white because of the fatty myelin.

Distributions differ between brain and spinal cord:

Brain. Gray matter on outside (cortex), white matter inside.
Spinal cord. White matter on outside (tracts going up and down), gray matter on inside (where the synapses happen).

What's the difference between the somatic and autonomic nervous systems?

Click to reveal answer

Somatic: voluntary control of skeletal muscle and conscious sensation. Autonomic: involuntary control of smooth muscle, cardiac muscle, and glands. ANS has sympathetic and parasympathetic branches.

Distinguish upper motor neuron signs from lower motor neuron signs.

Click to reveal answer

UMN: hyperreflexia, clonus, hypertonia, Babinski (up-going toes). LMN: atrophy, fasciculations, hypotonia, hyporeflexia. Weakness is common to both but the other signs distinguish them.

Give the mnemonic for the 12 cranial nerves in order.

Click to reveal answer

"On Old Olympus' Towering Tops A Finn And German Viewed Some Hops." Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, (Vestibulo)Acoustic, Glossopharyngeal, Vagus, Spinal accessory, Hypoglossal.

Which cranial nerves are purely sensory, which are purely motor, and which are both?

Click to reveal answer

"Some Say Marry Money But My Brother Says Big Brains Matter More." Sensory: I, II, VIII. Motor: III, IV, VI, XI, XII. Both: V, VII, IX, X.

Which cranial nerves move the eye, and what does "LR6 SO4 rest3" mean?

Click to reveal answer

Eye movement nerves: CN III (oculomotor), IV (trochlear), VI (abducens). LR6 = Lateral Rectus by CN VI. SO4 = Superior Oblique by CN IV. Rest3 = all remaining eye muscles by CN III.

Where are gray and white matter located in the brain vs. the spinal cord?

Click to reveal answer

Brain: gray outside (cortex), white inside. Spinal cord: white outside (tracts), gray inside. The layout is reversed between the two.