The Human Nervous System: A Visual Guide

Every thought you have, every breath you take without thinking about it, and every time you pull your hand back from something hot, is the same system at work. It is also the one system in the body that is genuinely hard to picture, so this post is mostly illustrations.

How it is organised

The whole thing splits in two. The central nervous system, the brain and the spinal cord, is where signals are processed and decisions are made. The peripheral nervous system is everything else, the nerves that fan out to reach the rest of you.

Organisation chart of the nervous system, splitting into central and peripheral, with the peripheral dividing into somatic and autonomic, and the autonomic into sympathetic, parasympathetic and enteric

The peripheral side splits again. The somatic division handles the things you choose to do, such as picking up a cup. The autonomic division handles the things you never think about, such as your heart rate, your pupils and your digestion.

The central and peripheral nervous systems

The brain sits at the top, protected by the skull. The spinal cord runs down from it inside the vertebral column, roughly 45 cm in an adult, ending around the first or second lumbar vertebra. Below that point the canal carries a bundle of nerve roots rather than cord itself.

Schematic of the human body showing the brain and spinal cord in the centre, with peripheral nerves branching out to the arms and legs

Two sets of nerves leave the CNS:

12 pairs of cranial nerves, which come off the brain and brainstem directly and mostly serve the head and neck. The vagus nerve is the exception, wandering down into the chest and abdomen.
31 pairs of spinal nerves, which leave the spinal cord between the vertebrae and serve everything below the neck.

The neuron

The working unit is the neuron. The adult brain holds somewhere around 86 billion of them, alongside a roughly similar number of glial cells that support, insulate and clean up after them.

Diagram of a neuron showing dendrites, cell body, nucleus, axon covered in myelin sheath with nodes of Ranvier, and axon terminals

A neuron has three working parts:

Dendrites are the branches that receive signals from other neurons. A single neuron may collect input from thousands.
The cell body, or soma, adds all of that input up. If the total is strong enough, it fires.
The axon carries the outgoing signal, sometimes a metre or more, to the terminals where it passes to the next cell.

Most axons are wrapped in myelin, a fatty insulating sheath laid down in segments with small bare gaps between them called nodes of Ranvier. The signal effectively jumps from node to node, which is why a myelinated fibre can carry a signal at up to around 120 metres per second, while an unmyelinated one manages closer to one.

How a signal actually travels

A neuron at rest holds its inside at about -70 mV relative to the outside. When incoming signals push that voltage up to roughly -55 mV, the threshold, the cell fires.

Graph of membrane voltage during an action potential, showing the resting potential, the threshold, a spike to positive voltage, and the return through hyperpolarisation

What follows is an action potential. Sodium rushes in and the voltage shoots up past zero, then potassium flows out and brings it back down, overshooting slightly before settling. The whole spike takes about two milliseconds.

The important detail is that it is all or nothing. A signal below threshold does nothing at all. A signal above it produces the same full-sized spike every time. Strength is not encoded in the size of the spike, it is encoded in how often the neuron fires.

At the end of the axon the electrical signal has to cross a gap of a few tens of nanometres to the next cell. It does this chemically, by releasing neurotransmitters such as glutamate, which excites the next neuron, or GABA, which quietens it. Others you will recognise by name include acetylcholine, dopamine, serotonin and noradrenaline.

Reflexes, or why your hand moves first

Touch something hot and your hand is already moving before you have consciously registered the pain. That is not your brain being fast. It is your brain being skipped.

Diagram of a withdrawal reflex arc, from heat at the hand through a sensory neuron to the spinal cord, across an interneuron, and back out along a motor neuron to the muscle

The loop runs from receptor to sensory neuron to spinal cord to motor neuron to muscle. The spinal cord makes the decision locally and the signal to the brain arrives afterwards, which is why the pain seems to land a moment late.

Fight or flight, rest and digest

The autonomic division has two branches that pull in opposite directions, and both are running all the time. What you feel is the balance between them.

Comparison of sympathetic and parasympathetic effects on the eyes, airways, heart, digestion and adrenal glands

The sympathetic branch leaves the spinal cord between T1 and L2 and prepares you for effort. The parasympathetic branch leaves from the brainstem and the sacral segments S2 to S4, and handles recovery, digestion and repair. Most of its output travels down the vagus nerve.

There is a third branch worth mentioning. The enteric nervous system is a mesh of several hundred million neurons embedded in the wall of the gut, and it can run digestion largely on its own. It is the reason the gut is sometimes called a second brain.

A few numbers worth keeping


Neurons in the brain	around 86 billion
Cranial nerve pairs	12
Spinal nerve pairs	31
Resting membrane potential	about -70 mV
Firing threshold	about -55 mV
Length of an action potential	roughly 2 milliseconds
Fastest signal speed	up to about 120 m/s
Neurons in the gut	several hundred million

This is general background reading, not medical advice. Anything that concerns you about your own health is a conversation for a doctor.