The nervous system gets harder than it needs to be because most textbooks introduce twelve subdivisions in a single page. The whole thing is actually just two parts that talk to each other — the central nervous system (CNS) does the processing, and the peripheral nervous system (PNS) carries information to and from it. Once you can place a structure on that map, every other label is just a finer cut. This overview gives you the structure of the nervous system in the order an exam expects it: division by division, then neuron by neuron, then by what the signal is doing.
The Two Big Divisions: CNS and PNS
The CNS is just two organs: the brain and the spinal cord. Everything housed inside the skull and vertebral column counts as central. The CNS integrates incoming signals, makes decisions, and issues commands.
The PNS is every nerve outside that bony case — the cranial nerves (12 pairs emerging from the brain) and the spinal nerves (31 pairs emerging from the spinal cord), plus the ganglia (small clusters of neuron cell bodies) scattered along them. The PNS is the wiring that connects the CNS to skin, muscle, and viscera. A signal from your fingertip to your brain travels through the PNS, into the CNS for processing, and back out through the PNS to a muscle. Nothing happens in only one division.
The Three Functional Types of Neurons
Every signal in the nervous system travels through one of three neuron types, distinguished by the direction they carry information.
- Sensory (afferent) neurons carry signals toward the CNS — from receptors in skin, joints, muscles, and organs. "Afferent" and "arriving" both start with A; that is the mnemonic almost every textbook uses.
- Motor (efferent) neurons carry signals away from the CNS — out to muscles and glands that act on the command. "Efferent" and "exiting" both start with E.
- Interneurons sit entirely inside the CNS and connect sensory neurons to motor neurons, or to other interneurons. They do the processing — the vast majority of neurons in your brain are interneurons.
A simple reflex shows all three at work in order. Touch something hot: a sensory neuron carries the pain signal into the spinal cord, an interneuron in the cord passes it to a motor neuron, and the motor neuron contracts your bicep before the brain has even registered the burn.
Inside the PNS: Somatic vs. Autonomic
The PNS splits again based on what it controls. The somatic nervous system carries voluntary motor commands to skeletal muscle — when you decide to lift your arm, that signal travels along a somatic motor neuron. It also carries the sensory information you are consciously aware of, like touch and proprioception.
The autonomic nervous system (ANS) is the involuntary half. It controls smooth muscle, cardiac muscle, and glands — heart rate, digestion, blood vessel diameter, pupil size, sweat. You do not decide for your stomach to contract; the ANS does it for you.
The ANS itself splits into two opposing branches. The sympathetic branch is the "fight-or-flight" system: it speeds up the heart, dilates pupils, redirects blood to skeletal muscle, and pauses digestion. The parasympathetic branch is "rest-and-digest": it slows the heart, constricts pupils, and turns digestion back on. Most organs receive input from both branches and live somewhere between the two extremes — heart rate at any moment is the balance of sympathetic and parasympathetic tone, not the output of one alone.
How a Signal Actually Moves Through the System
A complete journey ties the divisions together. Suppose you step on a tack:
- A pain receptor in your foot fires; a sensory neuron carries the signal up through a spinal nerve into the dorsal horn of the spinal cord (PNS into CNS).
- Interneurons in the cord do two things: they pass the signal up the cord to the brain so you become aware of the pain, and they trigger a fast withdrawal reflex on the spot.
- A motor neuron in the ventral horn of the spinal cord sends a command back out through a spinal nerve to your leg flexor muscles, and your foot lifts.
- Meanwhile, the sympathetic branch of the ANS — without your asking — bumps your heart rate up a notch.
Every label from the overview shows up in that one event. Sensory and motor are direction; somatic and autonomic are what is controlled; sympathetic and parasympathetic are the autonomic mode. Layering them in that order is the cleanest way to organize the nervous system on paper.
Getting Help
Once the overview clicks, the next step is understanding the signal itself — see action potentials explained. For more nervous-system walkthroughs, browse the full set of Anatomy & Physiology study guides.
Conclusion
A nervous system overview is mostly about getting the divisions in the right order. Start with CNS vs. PNS, then split the PNS into somatic and autonomic, then split the autonomic into sympathetic and parasympathetic. Layer the three neuron types — sensory in, interneurons inside, motor out — on top of that map. Any structure in the system has a place on this chart, and any signal you trace will pass through several of them in a predictable sequence.
