The Parts of a Neuron

One cell, with a direction

Most cells in your body are roundish little blobs, more or less the same all over. A [[neuron|neuron]] is the odd one out: it is stretched and shaped like a tiny tree wired to a long thread, and — this is the key idea — it has a direction. A message always enters at one end, runs through the middle, and leaves at the far end, never the other way. Picture a one-way street, or a letter moving through a post office: it comes in the front door, gets sorted, and goes out the back. This built-in one-wayness has a name, [[neuron-polarity|neuron polarity]], and it is the reason a neuron's parts make sense in a fixed order.

So instead of memorizing a diagram of scattered bumps and threads, we'll follow the message itself, in the order it travels: it is caught by the antennae, gathered at the body, decided on at a launch point, carried down a long cable, and handed off at the endings. Five stops. Keep that journey in your head and every part below will simply be the next place the signal arrives.

Dendrites — the receiving antennae

The journey begins at the [[dendrite|dendrites]] — a spreading, branching thicket that fans out from the cell like the bare twigs of a winter tree. (The word even comes from the Greek for *tree*.) Their job is to listen. Each dendrite is studded with spots where other neurons make contact, and from those spots tiny inputs trickle inward. A single neuron may carry thousands of these branches, so it isn't hearing one voice — it's standing in a crowd, picking up a murmur from all sides at once.

Look closer and many dendrites are covered in thousands of tiny bumps, like beads of dew along a twig. These are [[dendritic-spine|dendritic spines]], and each little bump is usually the landing pad for one incoming connection. Spines matter more than their size suggests: they can swell, shrink, sprout, or vanish over hours and days, and that quiet reshaping is one of the physical traces of learning. When you practice something until it sticks, spines are part of what is changing.

Soma and axon hillock — the body and the launch point

All those whispers from the dendrites flow inward to the [[soma|soma]], the cell body — the plump, central hub where the neuron's life-support lives. The soma holds the nucleus (the cell's instruction library) and the busy machinery that builds proteins and burns fuel. Think of it as the neuron's kitchen and headquarters rolled into one: it keeps the whole long, sprawling cell alive and supplied. It is also where the many trickling inputs pool together and add up.

Right where the soma narrows and the long output cable begins sits a small, special cone called the [[axon-hillock|axon hillock]] — and despite being so easy to overlook, it is the decision point of the whole cell. Here the summed-up inputs are weighed against a tipping line. If the total push is strong enough to cross that line, the hillock launches a signal down the cable; if not, nothing happens and the moment passes. It is the neuron's voting booth: every input gets counted, but only the result that clears the threshold ever leaves the building. That launched signal is the famous all-or-nothing pulse, the [[action-potential|action potential]].

Axon and terminals — the cable and the output ends

Once the hillock says yes, the signal travels out along the [[axon|axon]] — the neuron's single long transmitting cable. Where dendrites are short and bushy and many, the axon is usually one slender fiber that can run an astonishing distance: some axons stretch from your lower back all the way to your toe, a single cell more than a meter long. Its one job is to carry the pulse outward, fast and faithfully, from the decision point to wherever the message is needed.

Long cables lose signal, so many axons are wrapped in a fatty insulating sleeve — and that sleeve isn't one smooth tube but a chain of segments with bare little gaps between them. Those gaps are the [[node-of-ranvier|nodes of Ranvier]], and they are where the signal gets refreshed, letting the pulse leap from gap to gap instead of crawling the whole length. For now just notice them in passing: regular pinch-points along the cable that keep a far-traveling signal strong. We'll return to why those gaps speed things up when we meet myelin.

At its far end the axon usually splits into a small spray of fine tips, the [[axon-terminal|axon terminals]] — the neuron's output ends, its mouths. This is where the pulse finally stops traveling as electricity and gets handed off to the next cell: each terminal nearly touches a target, leaving a microscopic gap, and across that gap the message is passed by releasing a chemical messenger. One branching cable can split into many terminals, so a single neuron's one decision can be whispered to dozens of others at once. That handoff junction is the [[synapse|synapse]], the star of the next part of this rung.

  dendrites        soma        axon ──────────────►  terminals
  (catch) ╲        (gather)    (send)  ┊   ┊   ┊      (hand off)
     ╲╲╲   ╲___________O═════╤══════════╪═══╪═══╪═════<
     ╱╱╱   ╱           ▲     │          ┊   ┊   ┊
  spines            hillock  axon    nodes of Ranvier
                    (decide) (cable)  (gaps along axon)

         SIGNAL ALWAYS FLOWS LEFT  ──►  RIGHT

Walk it once yourself

The best way to lock these five parts in is to trace a single signal through them, naming each stop out loud. Here is the journey, start to finish:

1. Caught: Incoming messages land on the dendrites, many of them on tiny spines, like rain on a tree's bare twigs.
2. Gathered: Everything flows inward to the soma, the life-support hub, where the inputs pool and add up.
3. Decided: At the axon hillock, the total is weighed against a tipping line — fire, or stay quiet.
4. Sent: If it fires, the pulse races down the axon, refreshed at each node of Ranvier so it stays strong over distance.
5. Handed off: At the axon terminals, the message crosses to the next cell and the journey starts again in a new neuron.