Neuron

Neurons (also called nerve cells) are the primary cells of the nervous system. In vertebrates, they are found in the brain, the spinal cord and in the nerves and ganglia of the peripheral nervous system.

Anatomy

Many highly specialized types of neurons exist, and these differ widely in appearance. Characteristically, neurons are highly asymmetric in shape, consisting of a relatively fat central cell body called the soma, a much finer, cable-like projection called an axon, which may extend tens, hundreds or even tens of thousands of times the diameter of the soma in length; and projecting oppositely from the axon, a short, branching arbor of cellular extensions called the dendrites. Axon and dendrites alike are typically only about a micrometer thick, while the soma is usually about 25 micrometers in diameter and not much larger than than the cell nucleus it contains. An axon of a human motoneuron, meanwhile, can be a meter long.

Connectivity

Neurons join to one another and to other cells through synapses, which connect the axon tip of one cell and 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 rapid secretion of neurotransmitter molecules. They are known most, however, for their ability to undergo electrical excitation and to transmit this excitation along their axons as an impulse, called an "action potential." Arrival of an action potential at the tip of an axon triggers the release of neurotransmitter into the synaptic gap. Arriving neurotransmitters then either stimulate or suppress an action potential in the target cell, depending on the neurotransmitter and its receptor.

Adaptations to carrying action potentials

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. 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.

Neurons of the brain

There are about 100 billion neurons in the human brain.

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