Neuron

Most neurons receive signals via the dendrites and soma and send out signals down the axon. At the majority of synapses, signals cross from the axon of one neuron to a dendrite of another. However, synapses can connect an axon to another axon or a dendrite to another dendrite.

Schematic of an anatomically accurate single pyramidal neuron, the primary excitatory neuron of cerebral cortex, with a synaptic connection from an incoming axon onto a dendritic spine.

Axons and dendrites in the central nervous system are typically only about one micrometer thick, while some in the peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter and often is not much larger than the cell nucleus it contains. The longest axon of a human motor neuron can be over a meter long, reaching from the base of the spine to the toes.

There are different internal structural characteristics between axons and dendrites. Typical axons almost never contain ribosomes, except some in the initial segment. Dendrites contain granular endoplasmic reticulum or ribosomes, in diminishing amounts as the distance from the cell body increases.

Some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are:

Afferent and efferent also refer generally to neurons that, respectively, bring information to or send information from the brain.

A signal propagating down an axon to the cell body and dendrites of the next cell

An autapse is a synapse in which a neuron's axon connects to its own dendrites.

An annotated diagram of the stages of an action potential propagating down an axon including the role of ion concentration and pump and channel proteins.

Beyond electrical and chemical signaling, studies suggest neurons in healthy human brains can also communicate through:

Other receptor types include quickly adapting or phasic receptors, where firing decreases or stops with steady stimulus; examples include skin which, when touched causes neurons to fire, but if the object maintains even pressure, the neurons stop firing. The neurons of the skin and muscles that are responsive to pressure and vibration have filtering accessory structures that aid their function.

The neuron doctrine is the now fundamental idea that neurons are the basic structural and functional units of the nervous system. The theory was put forward by Santiago Ramón y Cajal in the late 19th century. It held that neurons are discrete cells (not connected in a meshwork), acting as metabolically distinct units.