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

Organization of the Nervous System

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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 nervous system is not one organ but a set of interconnected parts organized along two axes. The first axis is anatomical: the , made up of the brain and spinal cord, versus the , made up of all the nerves and ganglia outside the brain and spinal cord. The second axis is functional: the nervous system gathers information about the world (sensory input), processes it, and produces motor output. Every behavior is a transaction across these divisions: the PNS brings information in, the CNS integrates it, and the PNS carries commands back out.

The organizational chart matters because it predicts both normal function and patterns of injury. Damage to a specific division produces a characteristic set of deficits, which is how neurologists localize problems. A student who can trace "which division, which direction, which function" for any symptom will find clinical case material far more tractable.

Why this matters

Every later topic in this book assumes this map. Sensory systems (vision, hearing, touch) are PNS-to-CNS pathways; motor control is a CNS-to-PNS pathway; the is a specialized PNS division governing organs you do not consciously control. In clinical settings, the CNS/PNS split explains why spinal cord injury produces paralysis below the injury level while damage in a limb affects only that limb. Understanding the organization also clarifies everyday distinctions: a "nerve" in the arm is PNS tissue, whereas the "nerves" inside the brain are actually CNS tracts.

The college version

Core Concepts

The central nervous system: brain and spinal cord

The CNS is the processing core. It contains the vast majority of neurons (a commonly taught estimate is roughly 86 billion neurons in the human brain, though the exact count is debated and depends on method) plus supporting glial cells. The brain performs perception, thought, emotion, memory, and the coordination of movement; the spinal cord carries signals between the brain and the body and also performs local processing such as reflexes.

Two anatomical distinctions inside the CNS are essential vocabulary:

is made of neuronal cell bodies, dendrites, and synapses — the places where processing happens. In the brain, most gray matter forms the outer cerebral cortex; in the spinal cord, it forms an inner core shaped roughly like a butterfly.
is made of myelinated axons — the cables that carry signals between processing areas. The paler color comes from the lipid-rich myelin.

The peripheral nervous system: nerves and ganglia

The PNS is everything outside the brain and spinal cord: bundles of axons called nerves, clusters of neuron cell bodies called ganglia, and the sensory receptors embedded in tissues. Its two great jobs are to deliver information to the CNS and to deliver commands from the CNS to muscles and glands.

A useful terminology rule: a nerve (PNS) is a bundle of axons; a is the CNS equivalent (a bundle of axons within the brain or spinal cord); a (PNS) is a cluster of cell bodies; a is the CNS equivalent. Students frequently trip over these four words, and they are worth memorizing together.

Functional organization: sensory in, motor out

Functionally, the nervous system is divided into:

Sensory (afferent) division: carries information toward the CNS from receptors. Somatic sensory signals come from skin, muscles, joints; visceral sensory signals come from internal organs.
Motor (efferent) division: carries commands away from the CNS. It splits into the somatic motor system (voluntary control of skeletal muscle) and the autonomic nervous system (involuntary control of glands, cardiac muscle, and smooth muscle).

The autonomic nervous system: three branches

The autonomic nervous system (ANS) regulates functions you rarely think about — heart rate, digestion, pupil size, sweating, blood pressure — through three divisions:

: the "fight-or-flight" system. It mobilizes energy: increases heart rate and blood flow to muscles, dilates pupils and airways, slows digestion. Its neurons generally arise from the thoracic and lumbar spinal cord.
: the "rest-and-digest" system. It conserves and restores energy: slows the heart, stimulates digestion and salivation, constricts pupils. Its neurons generally arise from the brainstem and sacral spinal cord.
: a network of neurons embedded in the walls of the gastrointestinal tract, sometimes called the "second brain," which governs digestion with substantial local autonomy.

Most organs receive input from both sympathetic and parasympathetic divisions, and the balance between them adjusts moment to moment.

How It Works / Step-by-Step Process

Identify the stimulus and the receptors involved (skin, eye, ear, organ).
Trace the sensory pathway: receptors → PNS nerve → spinal cord/brainstem → CNS processing areas.
Identify where processing occurs (spinal reflex circuits, brainstem, cortex).
Trace the motor pathway: CNS → PNS nerve → effector (muscle or gland).
Classify the motor command: somatic (voluntary) or autonomic (sympathetic, parasympathetic, or enteric).

Applying this five-step trace to any behavior — from blinking to breathing to running — reveals which divisions are involved and where a lesion would cause symptoms.

Common Confusions

Do not confuse	With	Difference
A nerve and a tract	Nerve vs tract	A nerve is PNS axonal cable; a tract is the same idea inside the CNS.
A ganglion and a nucleus	Ganglion vs nucleus	Ganglion = PNS cell-body cluster; nucleus = CNS cell-body cluster.
Sympathetic and parasympathetic effects	Fight-or-flight vs rest-and-digest	Sympathetic mobilizes energy (faster heart, dilated pupils, slowed digestion); parasympathetic restores it (slower heart, constricted pupils, active digestion).
The somatic nervous system and the autonomic nervous system	Voluntary vs involuntary	Somatic controls skeletal muscle you can move at will; autonomic regulates organs automatically.
The spinal cord is part of the PNS	CNS	The spinal cord is central nervous system tissue; spinal nerves are PNS tissue.
Autonomic = only sympathetic	Autonomic has three divisions	The ANS includes sympathetic, parasympathetic, and enteric divisions.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of your nervous system as a headquarters (the brain) with a highway (the spinal cord) and a network of side roads (the nerves in your body). Cars carrying messages from your skin, eyes, and stomach drive in to headquarters; cars carrying orders drive out to your muscles. Some orders are urgent reflexes that the highway office handles on its own, and some of your organs run on an automatic pilot you never think about.

Worked example

Imagine you are standing up quickly after sitting for an hour, and you feel briefly lightheaded. Specialized receptors in blood vessels detect a drop in blood pressure and send sensory signals through PNS nerves to the brainstem. The brainstem's autonomic control centers respond by increasing sympathetic output: your heart beats faster and blood vessels constrict, restoring blood pressure to the brain. Within seconds the lightheadedness passes.

Now classify what happened: the sensory leg of this loop was visceral sensory (afferent), carried by PNS nerves into the CNS; the response was autonomic motor (efferent), predominantly sympathetic. No conscious decision was involved — the entire transaction ran on automatic pilot. This is the same organization, in miniature, that governs every behavior in the book: sense in, process, command out.

Key takeaways

Two axes: CNS vs PNS (anatomy); sensory vs motor (function). Always classify a pathway along both.
Gray = processing, white = cabling: gray matter holds cell bodies; white matter holds myelinated axons.
Vocabulary rule: nerve (PNS axon bundle), tract (CNS axon bundle), ganglion (PNS cell-body cluster), nucleus (CNS cell-body cluster).
Motor division splits twice: somatic (voluntary, skeletal muscle) vs autonomic (involuntary); autonomic splits into sympathetic, parasympathetic, and enteric.
Sympathetic = energy out ("fight or flight"), parasympathetic = energy in ("rest and digest").
Afferent in, efferent out — the single most reused directional rule in neuroscience.

Check yourself

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

What are the two anatomical divisions of the nervous system, and what does each contain?
Show answer
The central nervous system (brain and spinal cord) and the peripheral nervous system (nerves, ganglia, and receptors outside the CNS).
What is the difference between gray matter and white matter?
Show answer
Gray matter contains neuronal cell bodies, dendrites, and synapses (processing); white matter contains myelinated axons (signal transmission).
Name the three divisions of the autonomic nervous system and one effect of each.
Show answer
Sympathetic ("fight or flight": increases heart rate, dilates pupils, slows digestion); parasympathetic ("rest and digest": slows heart, constricts pupils, stimulates digestion); enteric (governs gastrointestinal function).
A bundle of axons in the brain is called a _, while a bundle of axons outside the CNS is called a _.
Show answer
Tract; nerve.
In the lightheadedness example, which division of the nervous system restored blood pressure, and was that command somatic or autonomic?
Show answer
The brainstem's autonomic control centers increased sympathetic output, which raised heart rate and constricted blood vessels. The command was autonomic (a division of the motor/efferent system), not somatic.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

central nervous system (CNS): The brain and spinal cord.
peripheral nervous system (PNS): All nerves, ganglia, and receptors outside the brain and spinal cord.
nerve: A bundle of axons in the PNS.
tract: A bundle of axons within the CNS.
ganglion: A cluster of neuron cell bodies in the PNS.
nucleus: A cluster of neuron cell bodies in the CNS.
gray matter: Regions rich in cell bodies, dendrites, and synapses.
white matter: Regions rich in myelinated axons.
somatic nervous system: The division controlling voluntary skeletal muscle and carrying somatic sensation.
autonomic nervous system: The division regulating glands, smooth muscle, and cardiac muscle involuntarily.
sympathetic division: The "fight-or-flight" branch of the autonomic nervous system.
parasympathetic division: The "rest-and-digest" branch of the autonomic nervous system.
enteric division: The neural network within the wall of the gastrointestinal tract.

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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