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

Building a 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

Every thought, movement, memory, and emotion depends on cells that are specialized for signaling. The nervous system is built from two broad families of cells: neurons, which carry electrical and chemical signals, and glial cells (glia), which support, protect, and maintain the environment around neurons. Behavioral neuroscience begins here because behavior is what the nervous system does, and you cannot explain what a system does until you know what it is made of.

A is a single cell with a shape that matches its job. Most neurons have three functional regions: dendrites that receive signals from other cells, a cell body () that maintains the cell and integrates incoming information, and an that conducts an electrical signal (the action potential) over distance to axon terminals, where chemical messengers called neurotransmitters are released onto the next cell. The junction where one neuron passes a signal to another is the . Although neurons come in many shapes, this general plan — receive, integrate, conduct, transmit — explains how a nervous system can gather information, decide, and act.

Why this matters

The neuron–glia distinction organizes almost everything else in this book. Later chapters on neurophysiology explain how neurons signal; chapters on neurochemistry explain which molecules they use; chapters on sensation, movement, and cognition explain what those signals accomplish. Clinically, many conditions are best understood as failures of these basic units: multiple sclerosis involves damage to (a glial product), amyotrophic lateral sclerosis (ALS) involves degeneration of motor neurons, and many neurodevelopmental differences involve how neurons form connections. If you can picture the cells, the symptoms become far easier to reason about than if you only memorize disease names.

The college version

Core Concepts

The neuron: a cell built for signaling

Neurons are electrically excitable: they can change the voltage across their membrane and propagate that change along the axon. The dendrites are branched extensions that act like antennas; most incoming synapses land on them. The soma contains the nucleus and the machinery for making proteins; it also sums the excitatory and inhibitory inputs arriving from dendrites. The axon hillock is the region where the axon leaves the soma; in many neurons it is where the decision to fire an action potential is made, because its membrane has a high density of voltage-gated ion channels.

The axon is a long, often myelinated fiber that conducts the action potential without losing strength — the signal is regenerated as it travels. Axons end in terminal boutons (terminal buttons), which contain vesicles filled with . When the action potential arrives, vesicles fuse with the membrane and release transmitter into the synaptic cleft, the narrow gap between the terminal and the next cell's membrane. Receptors on the receiving cell convert that chemical signal back into an electrical change.

Neurons come in different jobs

Neurons are commonly grouped by direction of information flow:

Sensory () neurons carry information toward the central nervous system from receptors in the skin, muscles, eyes, ears, and internal organs.
Motor () neurons carry commands away from the central nervous system to muscles and glands.
Interneurons connect neurons to other neurons within the central nervous system. They are the most numerous class and perform the processing that turns sensation into perception and decision.

The prefix afferent (arriving) and efferent (exiting) is worth locking in early; it reappears throughout the book.

Glia: the support and maintenance crew

Glial cells do not fire action potentials, but the nervous system cannot function without them:

Astrocytes are star-shaped cells that regulate the chemical environment around neurons, take up excess neurotransmitter, help form the blood–brain barrier, and supply metabolic support. They are the most abundant glial cells in the brain.
Oligodendrocytes (in the central nervous system) and Schwann cells (in the peripheral nervous system) produce myelin, the fatty sheath that wraps axons and dramatically speeds up signal conduction. One oligodendrocyte myelinates segments of several axons; one Schwann cell myelinates a single segment of a single axon.
are the immune cells of the central nervous system. They patrol the brain, remove debris and pathogens, and participate in the pruning of unused synapses during development.
Ependymal cells line the fluid-filled ventricles of the brain and help produce and circulate cerebrospinal fluid.

How the parts work as a system

A useful way to study any behavior is to trace the circuit: a stimulus activates a receptor, a sensory neuron carries the message inward, interneurons process it, a motor neuron carries a command outward, and a muscle or gland responds. This sensor → processor → effector loop is the "unit of behavior" that later chapters build on. The same loop underlies a reflex, a learned skill, and (in vastly expanded form) a thought.

How It Works / Step-by-Step Process

A stimulus (for example, a tap on the knee tendon) activates a receptor.
The receptor triggers a sensory (afferent) neuron, which conducts an electrical signal toward the central nervous system.
Inside the spinal cord, the signal may pass through an interneuron or synapse directly onto a motor neuron.
The motor (efferent) neuron conducts the signal outward to a muscle.
The muscle contracts, producing the response — all without the brain being required, in the case of a reflex.

This same five-step loop, with more interneurons and more parallel pathways, underlies everything from catching a ball to planning a sentence.

Common Confusions

Do not confuse	With	Difference
Neurons are the only important cells in the brain.	Glial cells	Glia outnumber or rival neurons in number and perform essential support, insulation, immune, and maintenance functions.
The synapse is a physical connection like a wire splice.	Synaptic cleft	There is a narrow gap; signals cross it chemically via neurotransmitters, not electrically.
Sensory neurons carry signals out of the CNS.	Motor neurons	Sensory (afferent) neurons carry signals in; motor (efferent) neurons carry commands out.
Myelin is made by neurons.	Myelin made by glia	Oligodendrocytes (CNS) and Schwann cells (PNS) produce myelin; neurons do not.
Dendrites send signals and axons receive them.	Axons send, dendrites receive	Information generally flows dendrite → soma → axon → terminal.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Your nervous system is like a city with two kinds of workers: neurons are the phone lines and couriers who carry messages, and glial cells are the builders and cleaners who keep the roads working and tidy. A message starts at one end of a neuron, travels along the wire, and hops to the next neuron at a tiny gap. Put enough of those lines together, and you get thoughts, feelings, and movements.

Worked example

Consider what happens when you touch a hot stove. Pain receptors in your fingertip activate sensory neurons that carry the message up your arm toward your spinal cord. Within the cord, interneurons pass the signal to motor neurons controlling your arm muscles, and you jerk your hand away — often before you are consciously aware of the heat. Simultaneously, other sensory neurons carry the message up the spinal cord to the brain, where it is registered as pain after the withdrawal has already begun.

The example is worth unpacking: the reflex (withdrawal) is a fast, spinal loop; the pain you feel is a slower, brain-bound loop. Two different circuits, built from the same basic neuron plan, explain why you can react before you can think. Whenever a later chapter describes a behavior, try to ask: which cells are carrying the message, in which direction, and through how many synapses?

Key takeaways

Neuron = receive, integrate, conduct, transmit: dendrites receive, soma integrates, axon conducts, terminals transmit at synapses.
Two cell families: neurons signal; glia (astrocytes, oligodendrocytes, Schwann cells, microglia, ependymal cells) support.
Direction matters: afferent = arriving (sensory, toward CNS); efferent = exiting (motor, away from CNS).
Myelin speeds conduction: made by oligodendrocytes in the CNS and Schwann cells in the PNS; damage to myelin disrupts signaling (as in multiple sclerosis).
The synapse is a gap, not a wire: signals cross it chemically via neurotransmitters.
Interneurons are the majority: most brain neurons connect neuron to neuron, not directly to muscles or senses.

Check yourself

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

List the four functional regions of a typical neuron and the job of each.
Show answer
Dendrites (receive signals), soma (integrate and maintain the cell), axon (conduct the action potential), axon terminals (release neurotransmitter at synapses).
What is the difference between afferent and efferent neurons?
Show answer
Afferent (sensory) neurons carry information toward the central nervous system; efferent (motor) neurons carry commands away from it.
Which glial cells produce myelin in the central nervous system, and which produce it in the peripheral nervous system?
Show answer
Oligodendrocytes in the CNS; Schwann cells in the PNS.
Why is the axon hillock important for signaling?
Show answer
It is the region where the axon leaves the soma and where the decision to fire an action potential is typically made, due to a high density of voltage-gated ion channels.
In the hot-stove example, why does the hand withdraw before the person feels pain?
Show answer
The withdrawal reflex is a fast spinal circuit (receptor → sensory neuron → interneuron/motor neuron → muscle), while conscious pain perception requires the signal to travel up the spinal cord to the brain — a slower route.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

neuron: A cell specialized to receive, integrate, and transmit electrical and chemical signals.
dendrite: Branched extension of a neuron that receives incoming signals.
soma: The cell body containing the nucleus and metabolic machinery.
axon: Long fiber that conducts the action potential away from the soma.
axon terminal: The ending of an axon where neurotransmitter is released.
synapse: The junction where a signal passes from one neuron to the next.
neurotransmitter: A chemical messenger released at synapses.
myelin: Fatty sheath around axons produced by glial cells.
astrocyte: A star-shaped glial cell that supports neurons and regulates their environment.
microglia: Immune cells of the central nervous system.
afferent: Carrying information toward the central nervous system.
efferent: Carrying commands away from the central nervous system.

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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