The Neuron

What a neuron is

A neuron is a cell, but a highly specialised one. Like other cells it has a membrane, a nucleus, and the usual internal machinery. What sets it apart is that it is electrically excitable: it can generate and conduct electrical signals, and it is shaped to pass those signals to other cells with precision. In this sense a neuron is less like a general-purpose building block and more like a wire that can also compute, deciding, moment to moment, whether to fire.

The brain is built from these cells in staggering numbers. A careful count puts the figure at roughly 86 billion neurons in the adult human brain, each connecting to hundreds or thousands of others. It is not the number of cells alone that gives the brain its power, but the density and pattern of their connections: the wiring, far more than the parts list, is where the brain's capabilities live.

The structure of a neuron

Although neurons vary widely in shape, most share the same basic plan: a central cell body with branching inputs on one side and a single long output fibre on the other. Information generally flows in one direction, from the dendrites, through the cell body, and out along the axon.

At the far end of the axon, the fibre divides into fine branches ending in swellings called axon terminals. Each terminal comes close to, but does not quite touch, a target cell, leaving a microscopic gap called the synapse. It is across this gap that one neuron passes its message to the next.

Types of neuron

Neurons are classified in two useful ways: by what they do, and by their shape. Both classifications matter, because a neuron's job and its form are closely linked.

By function

By structure

By shape, neurons are grouped by how many projections leave the cell body: multipolar neurons (one axon and many dendrites) are the most common type in the brain; bipolar neurons (one axon and one dendrite) are found in some sensory systems such as the retina; and unipolar or pseudounipolar neurons have a single projection that splits, common among sensory neurons carrying touch and pain.

How a neuron signals

A neuron's whole purpose is to receive signals, decide whether they add up to something worth passing on, and, if so, send a signal of its own. This happens in two stages that use two different kinds of signalling.

1. Gathering input

   Signals arrive at the dendrites and cell body as small changes in membrane voltage. Some push the neuron toward firing (excitatory), others push it away (inhibitory). The cell body continuously adds these up.

2. Reaching threshold

   If the summed signal crosses a critical level, the threshold, the neuron fires. This decision is all-or-nothing: below threshold nothing happens, at or above it the neuron produces a full impulse.

3. The electrical impulse

   Firing generates an action potential, a brief, self-propagating spike of voltage that races down the axon without weakening, carried by the flow of ions across the membrane.

4. Passing the message on

   When the impulse reaches the axon terminals, it triggers the release of neurotransmitters across the synapse to the next cell, converting the electrical signal into a chemical one.

This two-part design, electrical along the axon, chemical across the synapse, is fundamental. The electrical impulse is fast and reliable over distance; the chemical step is slower but far more flexible, letting the brain adjust the strength of each connection. That flexibility is the physical basis of learning.

Myelin and speed

Not all axons carry signals at the same speed, and the difference is largely down to myelin. Where an axon is wrapped in this fatty sheath, the impulse does not travel smoothly along the whole length. Instead it jumps between small gaps in the myelin called nodes of Ranvier, a process called saltatory conduction. Skipping from node to node is far quicker than travelling continuously, so myelinated axons can conduct at over 100 metres per second, while unmyelinated fibres manage only about one.

Neurons by the numbers

These figures are approximate and drawn from careful modern counts rather than the older, often-repeated estimates. The 86 billion figure, for instance, comes from a method that dissolves the brain into a uniform suspension and counts cell nuclei directly, correcting the round "100 billion" number that circulated for decades.

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