Anatomy and Physiology 2e · The Nervous System and Nervous Tissue
Basic Structure and Function of the Nervous System
On this page 9 sections
In 30 seconds
The nervous system is the body's master control and communication network. It detects changes in the internal and external environment, processes that information, and sends commands to muscles and glands — all in fractions of a second. While the endocrine system also communicates (with slower chemical signals carried in the blood), the nervous system is built for speed and precision.
Its work is usually summarized as three overlapping functions: sensation (receiving sensory input), integration (processing and interpreting it), and response (motor output that produces movement or glandular activity). The nervous system also continuously regulates homeostasis — heart rate, breathing, blood pressure, digestion, and body temperature — usually without conscious effort.
Why this matters
Everything that follows in this book — brain and spinal cord anatomy, reflexes, sensory and motor pathways, the autonomic nervous system The involuntary motor division for organs, glands, smooth/cardiac muscle Full entry → — builds on the basic map drawn here. In healthcare, almost every patient encounter involves the nervous system: a stroke changes motor output, nerve A bundle of axons in the PNS Full entry → damage changes sensation, and spinal cord injury disrupts both. This map is also the foundation for interpreting neurological exams and for understanding how drugs acting on the autonomic nervous system affect the body.
The college version
Core Concepts
Structural division: CNS and PNS
The nervous system is first divided by location.
- central nervous system (CNS) The brain and spinal cord Full entry →: the brain and spinal cord. This is the integration center where information is processed and decisions are made.
- peripheral nervous system (PNS) Nerves and ganglia outside the CNS Full entry →: all nervous tissue outside the CNS — the nerves (bundles of axons) and ganglia (clusters of neuron cell bodies) that carry signals to and from the CNS. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves (commonly taught reference counts; verify against current texts).
Functional division: sensory and motor
The PNS is then divided by the direction of information flow.
- Sensory (afferent Carrying information toward the CNS Full entry →) division: carries information toward the CNS. The somatic sensory part carries signals from the skin, muscles, bones, and joints (touch, pressure, pain, temperature, and proprioception — the sense of body position). The visceral sensory part carries signals from internal organs, such as stretch in the stomach or blood pressure in vessel walls.
- Motor (efferent Carrying commands away from the CNS Full entry →) division: carries commands away from the CNS to effectors. The somatic motor part is voluntary — it signals skeletal muscle. The autonomic nervous system (ANS) is involuntary — it signals cardiac muscle, smooth muscle, and glands, and is divided into the sympathetic The mobilizing autonomic branch Full entry → (mobilizing, "fight-or-flight") and parasympathetic The restorative autonomic branch Full entry → (restorative, "rest-and-digest") branches.
The cells: neurons and neuroglia
Nervous tissue contains two cell families. Neurons are the excitable cells that generate and transmit electrical signals; they are the functional units of the nervous system. Neuroglia (glial cells) are supporting cells that outnumber neurons and protect, nourish, insulate, and defend them. More detail follows in Nervous Tissue.
The language of the nervous system
- Gray matter: regions rich in neuron cell bodies and dendrites — the outer cortex of the brain and the inner "H" of the spinal cord.
- White matter: regions rich in myelinated axons — the deeper brain and the outer columns of the spinal cord. The myelin gives it a pale appearance.
- Nerve vs. tract: a bundle of axons in the PNS is a nerve; a bundle of axons within the CNS is a tract (or pathway).
- Ganglion vs. nucleus: a cluster of neuron cell bodies in the PNS is a ganglion; a cluster within the CNS is a nucleus.
The reflex arc: the simplest circuit
The basic unit of nervous function is the reflex arc The simplest neural circuit: receptor → sensory neuron → integration center → motor neuron → effector Full entry →: a receptor detects a stimulus → a sensory neuron carries the signal to the CNS → an integration center (often in the spinal cord) processes it → a motor neuron carries a command out → an effector (muscle or gland) responds. Reflexes happen without conscious thought, though the brain is informed at the same time.
How It Works / Step-by-Step Process
Trace what happens when a fly lands on your arm:
- Reception: receptors in your skin detect the light touch and convert it into electrical signals.
- Sensory input (afferent): sensory neurons carry the signals up the spinal nerve into the spinal cord (CNS).
- Integration: interneurons in the spinal cord process the signal and pass it upward toward the brain; in a reflex, the spinal cord itself can generate the command.
- Motor output (efferent): motor neurons carry the command out to skeletal muscle.
- Response: the muscle contracts and you brush the fly away — with the brain notified along the way.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| CNS vs. PNS (structural) | Somatic vs. autonomic (functional) | One is about location, the other about whether the response is voluntary — they are two different axes of classification |
| Afferent and efferent | Which way signals travel | Afferent = toward the CNS (sensory); efferent = away from the CNS (motor) |
| A nerve and a neuron | One axon vs. a bundle of many axons | A "nerve" is a whole bundle of neuronal axons, not a single cell |
| Ganglion and nucleus | PNS vs. CNS location | Both are clusters of cell bodies; the name depends on which division they are in |
| Sympathetic and parasympathetic | Which one "rests" | Sympathetic mobilizes (fight-or-flight); parasympathetic restores (rest-and-digest) |
| Gray matter as "only in the brain" | Location in both brain and cord | Gray matter is on the outside of the brain but on the inside of the spinal cord |
| Reflexes as "brain decisions" | Spinal integration | Reflex arcs often complete in the spinal cord before the brain is involved |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your nervous system is like a super-fast message network. Your eyes, skin, ears, and other sensors send messages to the "command center" (brain and spinal cord). The command center decides what to do and sends messages back to your muscles and organs. When you touch something hot, a message races to your spinal cord, which instantly tells your hand to pull back — even before your brain fully realizes what happened.
Worked example
A student touches a hot pan handle and jerks the hand away before consciously feeling pain. This is a withdrawal reflex: pain receptors fire, sensory neurons carry the signal to the spinal cord, interneurons activate motor neurons, and the arm muscles pull the hand back. The pain sensation reaches the brain slightly later — which is why the withdrawal "beats" conscious awareness. In a neurological exam, a clinician taps a tendon (like the knee) to test the same kind of arc; a brisk or absent response helps localize where a problem may lie.
Key takeaways
- Two structural halves: CNS (brain + spinal cord) and PNS (nerves + ganglia).
- Two functional halves: sensory/afferent (toward CNS) and motor/efferent (away from CNS).
- Somatic = voluntary (skeletal muscle); autonomic = involuntary (smooth muscle, cardiac muscle, glands).
- Autonomic has two branches: sympathetic (mobilizing) and parasympathetic (restorative).
- Vocabulary pairs: nerve (PNS axons) vs. tract (CNS axons); ganglion (PNS cell bodies) vs. nucleus (CNS cell bodies).
- Reflex arcs are the fastest responses because the "decision" happens in the spinal cord without waiting for the brain.
- Neurons signal; neuroglia support — and glia outnumber neurons (commonly taught; verify against current texts).
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Name the two structural divisions of the nervous system and the two functional divisions of the PNS.
Show answer
Structural: CNS (brain and spinal cord) and PNS (nerves and ganglia). Functional: the sensory (afferent) division and the motor (efferent) division.
What is the difference between a nerve and a tract? Between a ganglion and a nucleus?
Show answer
A nerve is a bundle of axons in the PNS; a tract, in the CNS. A ganglion is a cluster of cell bodies in the PNS; a nucleus, in the CNS.
Which division of the motor system is voluntary, and which is involuntary?
Show answer
The somatic motor division is voluntary (skeletal muscle); the autonomic nervous system is involuntary (cardiac muscle, smooth muscle, glands).
List the five components of a reflex arc in order.
Show answer
Receptor → sensory neuron → integration center → motor neuron → effector.
What are the two branches of the autonomic nervous system, and what is each associated with?
Show answer
Sympathetic (mobilizing, "fight-or-flight") and parasympathetic (restorative, "rest-and-digest").
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- central nervous system (CNS)
- The brain and spinal cord
- peripheral nervous system (PNS)
- Nerves and ganglia outside the CNS
- afferent
- Carrying information toward the CNS
- efferent
- Carrying commands away from the CNS
- autonomic nervous system
- The involuntary motor division for organs, glands, smooth/cardiac muscle
- sympathetic
- The mobilizing autonomic branch
- parasympathetic
- The restorative autonomic branch
- nerve
- A bundle of axons in the PNS
- reflex arc
- The simplest neural circuit: receptor → sensory neuron → integration center → motor neuron → effector
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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