Neurons and glial cells

An adult human brain holds about 86 billion neurons; a typical neuron has one axon.

A cell built for sending messages

Nervous tissue is made of two kinds of cells: neurons, which carry the signals, and glial cells, which support them. The NIH estimates the human brain at about 86 billion nerve cells, alongside many other types of cells. Every sensation, movement, thought and memory is the result of signals passing through neurons, so it is worth looking closely at how one is put together.

The cell body, or soma, contains the nucleus and organelles, including ribosomes that make proteins. Branching out from it are dendrites, which receive most of the input from other neurons at contact points called synapses. One fibre leaves the cell body and carries the signal away: the axon. A typical neuron has one axon, though exceptions exist. Where the axon leaves the soma there is a narrowing called the axon hillock, followed by the initial segment. Together they form the trigger zone, the place where incoming signals are added up and where an action potential begins.

Many axons are wrapped in myelin, a fatty insulation made by glial cells. Unlike the plastic on an electric wire, myelin has gaps. Each gap is a node of Ranvier, and the wrapped stretch between two nodes is an axon segment. At its far end the axon splits into several branches, the axon terminals, and each branch ends in a small swelling, the synaptic end bulb, which meets the next cell. Axons can be very short, linking two cortex cells less than a hair's width apart, or long enough to run from the brain all the way down the spinal cord.

Three shapes of neuron

Neurons are sorted by how many processes leave the cell body. Multipolar neurons have one axon and two or more dendrites, and most neurons in the body are this type, including the large motor neurons of the spinal cord. Bipolar neurons have one axon and one dendrite on opposite ends of the cell body. They are rare: they are found in the retina of the eye and in the olfactory epithelium, where smells are sensed. Unipolar neurons have a single process that splits into two long branches. In humans these are sensory neurons, and their cell bodies always sit in ganglia. Some multipolar neurons have their own names, such as the pyramid-shaped pyramidal cells of the cortex and the Purkinje cells of the cerebellum.

Glial cells: the support crew

There are six types of glial cells, four in the CNS and two in the PNS. Astrocytes, star-shaped under the microscope, keep the chemical surroundings of neurons in balance, remove excess signalling molecules, react to damage and help form the blood-brain barrier, which keeps many substances in the blood out of the brain. Oligodendrocytes make myelin in the CNS, and one of them can wrap segments of several axons. Microglia are small cells that watch for trouble and can swallow debris and germs. Ependymal cells line the four fluid-filled ventricles of the brain and make cerebrospinal fluid.

In the peripheral nervous system, satellite cells support the cell bodies inside ganglia, and Schwann cells make the myelin. A Schwann cell wraps just one segment of one axon, so a long peripheral axon needs a whole row of them.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.