multipolar, bipolar, and unipolar neurons

Neurons — the signaling cells of the nervous system — come in different shapes, and one of the oldest ways to sort them is simply by counting the wires sticking out of the cell body. The cell body, called the soma, is the rounded central part that holds the nucleus. The wires growing out of it are its processes: thin branches that either gather incoming signals (dendrites) or carry the outgoing signal away (the axon). Count those stalks where they leave the soma, and a neuron falls into one of three classic groups — multipolar, bipolar, or unipolar.

A multipolar neuron has many processes leaving the soma — typically one axon and a whole bush of dendrites — like a tree with one deep root and a wide crown. This is by far the most common type in the brain and spinal cord, well suited to collecting input from thousands of partners at once; the motor neurons that drive your muscles are a familiar example. A bipolar neuron has exactly two processes, one at each end of the soma, like a bead threaded on a string — one branch brings information in, the other sends it on. Bipolar cells are rarer and show up in dedicated sensory pathways, such as the retina of the eye and the parts of the nose and inner ear that handle smell and balance.

A unipolar neuron has just a single process emerging from the soma. In invertebrates many neurons are truly unipolar, but in our own bodies the important version is the pseudounipolar neuron ("pseudo" meaning false): it starts out with two processes in early development that later fuse into one stalk, which then splits like a T a short distance from the cell body. These carry touch, temperature, and pain signals from your skin and joints inward toward the spinal cord, with the soma tucked off to the side rather than sitting in the signal's path. Sorting neurons this way is a description of form, not function — but form hints at the job, since the number and arrangement of branches shapes how a cell gathers and relays its messages.