Many neurons are highly specialized and they differ widely in appearance. All neurons consist of the bulbous center of the cell body, called the soma, a much finer tube-like projection called an axon, and in diametric opposition to the axon a typically shorter and highly branching collection of projections called dendrites. Axon and dendrites typically are only about a micrometer thick or less. The axon of a human motoneuron can be a meter long, however. Neurons join to one another and to other cells through synapses, which connect the axon tip of one cell to a dendrite of another, or less commonly to its axon or soma. Neurons of the cortex in mammals, such as the Purkinje cells, have over 1000 dendrites apiece, enabling connections to tens of thousands of other cells.
Neurons stimulate one another across synapses chemically by the rapid secretion of neurotransmitter molecules. But they are most known for their ability to become "electrically excited" and to convey this excitation along their axons as an impulse, or "action potential." It is the arrival of an action potential at the tip of an axon that triggers the release of neurotransmitter into a synaptic gap. Likewise, the effect of neurotransmitters is to either stimulate or suppress an action potential in the target cell.
The narrow cross-section of axons and dendrites lessens the metabolic expense of carrying action potentials, although fatter axons convey the impulses more rapidly, generally speaking.
Many neurons have insulating sheaths of myelin around their axons, which enable their action potentials to travel faster than in unmyelinated axons of the same diameter (see saltatory conduction under action potential). Formed by glial cells, the myelin sheathing normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier. Neurons and glia make up the two chief cell types of the nervous system.
There are about 100 billion neurons in the human brain.
See also: F wave, Neuroscience