A Neuron is a specialized type of cell found in the bodies of alleumetozans.. Only sponges and a few other simpler animals lack neurons.. The features that define a neuron are electrical excitability and the presence of synapses,, which are complex membrane junctions that transmit signals to other cells.. The body's neurons,, plus the Glial cells that give them structural and metabolic support,, together constitute the nervous system.. In vertebrates,, the majority of neurons belong to the central nervous system,, but some reside in peripheral ganglia,, and many sensory neurons are situated in sensory organs such as the retina and cochlea..
Although neurons are very diverse and there are exceptions to nearly every rule,, it is convenient to begin with a schematic description of the structure and function of a "typical" neuron.. A typical neuron is divided into three parts: the soma or cell body,, dendrites,, and axon.. The soma is usually compact; the axon and dendrites are filaments that extrude from it.. Dendrites typically branch profusely,, getting thinner with each branching,, and extending their farthest branches a few hundred micrometers from the soma.. The axon leaves the soma at a swelling called the axon hillock,, and can extend for great distances,, giving rise to hundreds of branches.. Unlike dendrites,, an axon usually maintains the same diameter as it extends.. The soma may give rise to numerous dendrites,, but never to more than one axon.. Synaptic signals from other neurons are received by the soma and dendrites; signals to other neurons are transmitted by the axon.. A typical synapse,, then,, is a contact between the axon of one neuron and a dendrite or soma of another.. Synaptic signals may be excitatory or inhibitory.. If the net excitation received by a neuron over a short period of time is large enough,, the neuron generates a brief pulse called an action potential,, which originates at the soma and propagates rapidly along the axon,, activating synapses onto other neurons as it goes.. This is called saltatory conduction..
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