Introduction to Behavioral Neuroscience · Structure and Function of the Nervous System: Cells and Anatomy

The Peripheral Nervous System: PNS

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

The peripheral nervous system (PNS) is the nervous system's interface with the rest of the body: every , , and receptor outside the brain and spinal cord. It has two jobs that mirror each other. Its sensory (afferent) division carries information from the body into the CNS — touch from the skin, stretch from muscles, pressure from blood vessels, light from the eyes. Its motor (efferent) division carries commands out to effectors, splitting into the somatic nervous system (voluntary control of skeletal muscle) and the autonomic nervous system (involuntary regulation of glands, cardiac muscle, and smooth muscle).

Structurally, the PNS is built from nerves — bundles of axons wrapped in connective tissue — and ganglia, clusters of cell bodies sitting outside the CNS. The human body has 12 pairs of cranial nerves emerging from the brain and brainstem and 31 pairs of spinal nerves emerging from the spinal cord (commonly taught as 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal). Because peripheral axons can regenerate to a limited degree (unlike most CNS axons), the PNS heals differently than the CNS — a difference with major clinical consequences.

Why this matters

Most of the sensations and movements you experience daily run through the PNS. When a is compressed (a herniated disc), symptoms appear in the body region that nerve serves — a dermatomal pattern that clinicians use to localize problems. When the somatic system fails, the result is weakness or paralysis of specific muscles; when the autonomic system fails, the results can include abnormal heart rate, blood pressure, sweating, or digestion. function is tested in nearly every neurological exam: pupil reactivity (cranial nerve III), facial movement (VII), hearing (VIII), tongue movement (XII). The PNS is also the site of action for local anesthetics and many peripheral nerve blocks, making it central to pain management.

The college version

Core Concepts

Nerves, ganglia, and their coverings

A nerve is a bundle of axons (each axon belonging to one neuron) held together by connective tissue layers: the endoneurium around each fiber, the perineurium around each fascicle, and the epineurium around the whole nerve. Nerves can be sensory (all afferent), motor (all efferent), or mixed (both) — most spinal nerves are mixed. A ganglion is a swelling along a nerve containing the cell bodies of sensory neurons (dorsal root ganglia) or of autonomic motor neurons (autonomic ganglia). Unlike CNS neurons, most peripheral neurons are wrapped by Schwann cells, which form myelin in the PNS and also guide regeneration after injury.

Cranial nerves: twelve pairs, many jobs

The 12 pairs of cranial nerves emerge directly from the brain and brainstem, bypassing the spinal cord. They are traditionally memorized by Roman numeral (I–XII) and name. Their functions span every modality:

I Olfactory — smell (sensory)
II Optic — vision (sensory)
III Oculomotor — most eye movements, pupil constriction (motor)
IV Trochlear — eye movement (motor)
V Trigeminal — facial sensation, chewing (mixed)
VI Abducens — eye movement (motor)
VII Facial — facial expression, taste from front of tongue (mixed)
VIII Vestibulocochlear — hearing and balance (sensory)
IX Glossopharyngeal — taste, swallowing, throat sensation (mixed)
X Vagus — autonomic control of heart, lungs, gut; voice (mixed)
XI Spinal accessory — neck and shoulder muscles (motor)
XII Hypoglossal — tongue movement (motor)

The mnemonic "On Old Olympus' Towering Top A Finn And German Viewed Some Hops" is a commonly taught memory aid for the order.

Spinal nerves and dermatomes

31 pairs of spinal nerves exit the vertebral column: 8 cervical (C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), and 1 coccygeal (Co1). Each spinal nerve forms from the union of a sensory dorsal root and a motor ventral root, so each nerve carries both sensory and motor fibers. Nerves from adjacent spinal levels innervate overlapping regions of skin called dermatomes — maps of which spinal level serves which patch of skin. Clinically, charts help locate nerve root damage: pain or numbness in a specific band of skin points to a specific spinal level.

After leaving the spine, spinal nerves merge into networks called plexuses (cervical, brachial, lumbar, sacral), where fibers from several levels are rearranged into named peripheral nerves such as the median, ulnar, and sciatic nerves.

The somatic nervous system: voluntary movement

The somatic nervous system consists of motor neurons whose cell bodies sit in the ventral horn of the spinal cord (or in cranial nerve nuclei of the brainstem) and whose axons run through spinal/cranial nerves to skeletal muscle. Each skeletal muscle fiber is controlled by a single motor neuron, and one motor neuron innervates many fibers — the . Somatic motor neurons release acetylcholine at the , which always excites the muscle (there is no "off" signal; relaxation occurs when the motor neuron stops firing). Somatic sensory fibers carry touch, pain, temperature, and proprioception (body position) from skin, muscles, and joints into the CNS.

The autonomic nervous system: automatic regulation

The autonomic nervous system (ANS) controls cardiac muscle, smooth muscle, and glands without conscious effort. Its motor pathway has a distinctive two-neuron chain: a preganglionic neuron in the CNS synapses in an autonomic ganglion onto a postganglionic neuron, which then innervates the target organ.

("fight or flight"): preganglionic fibers arise from thoracic and lumbar spinal levels; the ganglia lie near the spinal cord; the main postganglionic neurotransmitter is norepinephrine (with epinephrine released by the adrenal medulla). Effects: faster heart rate, dilated pupils and airways, inhibited digestion, redirection of blood to muscles.
("rest and digest"): preganglionic fibers arise from the brainstem (via cranial nerves, especially the vagus) and the sacral spinal cord; the ganglia lie near or in the target organ; the neurotransmitter at both synapses is acetylcholine. Effects: slower heart rate, constricted pupils, active digestion and salivation.
Enteric division: the intrinsic neural network of the gastrointestinal tract wall, capable of coordinating digestion with substantial independence from the CNS.

How It Works / Step-by-Step Process

Determine whether the signal is sensory (into CNS) or motor (out of CNS).
If motor, determine the target: skeletal muscle (somatic) or organ/gland (autonomic).
Trace the somatic route: CNS motor neuron → spinal/cranial nerve → skeletal muscle; release acetylcholine at the neuromuscular junction.
Trace the autonomic route: → autonomic ganglion → → organ; note whether the chain is sympathetic (norepinephrine at the organ) or parasympathetic (acetylcholine at the organ).
For clinical localization, use the dermatome or cranial nerve map to match symptoms to spinal level or nerve.

Common Confusions

Do not confuse	With	Difference
A spinal nerve and a spinal root	Nerve vs root	A spinal nerve is formed by the union of a sensory dorsal root and a motor ventral root; the nerve itself is mixed.
Somatic and autonomic motor systems	Voluntary vs involuntary	Somatic → skeletal muscle, consciously controlled; autonomic → organs/glands, automatic.
Sympathetic and parasympathetic neurotransmitters	Norepinephrine vs acetylcholine	Sympathetic postganglionic fibers mostly release norepinephrine; parasympathetic fibers release acetylcholine (as do all preganglionic fibers and somatic motor neurons).
Cranial nerves and spinal nerves	12 vs 31	Cranial nerves emerge from the brain/brainstem (12 pairs); spinal nerves emerge from the spinal cord (31 pairs).
The dorsal root ganglion is in the CNS	PNS vs CNS	The dorsal root ganglion sits outside the spinal cord — it is PNS tissue.
The PNS cannot regenerate	CNS vs PNS healing	Peripheral axons can regenerate with Schwann cell support; most CNS axons cannot — recovery prospects differ.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Your peripheral nervous system is the bundle of cables that connects your brain and spinal cord to the rest of your body. One set of wires carries messages in — "the floor is cold," "my stomach is full." Another set carries orders out — "lift your foot," "digest this food." Some orders are ones you choose (moving your arm), and some happen automatically without asking you (your heart beating faster when you're scared).

Worked example

A person steps on a sharp object. Pain receptors in the foot fire, and sensory axons travel up the leg inside the sciatic nerve — a branch of the sacral plexus — through the dorsal root into the spinal cord. Within the cord, the signal splits: one branch drives a withdrawal reflex (motor neurons activate the muscles that lift the foot), and another ascends to the brain, where it becomes conscious pain.

Meanwhile, the same event triggers an autonomic response: the sympathetic division raises heart rate and blood pressure, pupils dilate, and the adrenal medulla releases epinephrine. The person may not notice any of this — it is the automatic, involuntary side of the PNS. One stimulus, three peripheral systems working together: somatic sensory (pain in), somatic motor (foot lifted), and autonomic (body mobilized). Classifying each leg of the response is exactly the skill this topic teaches.

Key takeaways

PNS = nerves + ganglia + receptors; sensory (afferent) in, motor (efferent) out.
Somatic vs autonomic: somatic controls skeletal muscle voluntarily; autonomic controls organs automatically.
Two-neuron autonomic chain: preganglionic (CNS) → ganglion → postganglionic → target organ.
Sympathetic = norepinephrine, fight-or-flight (thoracolumbar origin); parasympathetic = acetylcholine, rest-and-digest (craniosacral origin).
12 cranial, 31 spinal nerves (8+12+5+5+1); each spinal nerve is formed by a sensory dorsal root + motor ventral root.
Dermatomes map spinal levels to skin regions — the clinical key to localizing nerve-root problems.
Peripheral axons can regenerate (with Schwann cell help); most CNS axons cannot — a key difference in recovery after injury.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

What are the two functional divisions of the PNS, and what does each do?
Show answer
Sensory (afferent) division, carrying information into the CNS, and motor (efferent) division, carrying commands out to muscles and glands.
How many pairs of cranial nerves and spinal nerves are there (give the spinal breakdown)?
Show answer
12 pairs of cranial nerves; 31 pairs of spinal nerves — commonly taught as 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
What is a motor unit?
Show answer
A motor neuron plus all the skeletal muscle fibers it innervates — the basic unit of voluntary muscle control.
Describe the two-neuron chain of the autonomic nervous system, and state which neurotransmitter the sympathetic postganglionic neuron releases at its target.
Show answer
Preganglionic neuron (in CNS) → autonomic ganglion → postganglionic neuron → target organ. The sympathetic postganglionic neuron releases norepinephrine (with epinephrine from the adrenal medulla).
What is a dermatome, and why is it clinically useful?
Show answer
A dermatome is the skin region served by a single spinal nerve level; it helps clinicians localize nerve-root damage from patterns of numbness or pain.
In the stepping-on-a-sharp-object example, which three peripheral systems were involved?
Show answer
Somatic sensory (pain carried inward), somatic motor (withdrawal reflex lifting the foot), and autonomic sympathetic (heart rate, blood pressure, epinephrine mobilization).

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

nerve: A bundle of axons in the PNS wrapped in connective tissue.
ganglion: A cluster of neuron cell bodies outside the CNS.
cranial nerve: One of 12 pairs of nerves emerging from the brain/brainstem.
spinal nerve: One of 31 pairs of nerves formed by dorsal + ventral roots.
dermatome: The skin region served by a single spinal nerve level.
motor unit: One motor neuron plus all the muscle fibers it innervates.
neuromuscular junction: The synapse between a motor neuron and a muscle fiber.
preganglionic neuron: The first neuron in the autonomic chain, originating in the CNS.
postganglionic neuron: The second neuron in the autonomic chain, from ganglion to organ.
sympathetic division: The fight-or-flight autonomic branch (thoracolumbar origin).
parasympathetic division: The rest-and-digest autonomic branch (craniosacral origin).

Sources & references

openstax.org — Introduction Behavioral Neuroscience

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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