Neuron

A neuron is an electrically excitable cell that constitutes the primary computational unit of the nervous system. Its defining functional property is the action potential: a self-propagating electrical signal generated by the coordinated opening and closing of voltage-gated ion channels in the cell membrane, which allows a neuron to transmit information over distances without signal decay.

The neuron's computational role is defined by its inputs (dendrites receiving synaptic signals from other neurons), its integration point (the axon hillock, where incoming signals summate and an action potential is or is not initiated), and its outputs (axon terminals that release neurotransmitters onto downstream neurons or effector cells). This input-integration-output architecture is the physical implementation of a threshold function — a nonlinear gate that transforms weighted sums of inputs into binary or graded outputs. The parallels to artificial neural network units are superficial: real neurons implement temporal coding, rate coding, dendritic computation, and spike-timing-dependent plasticity that artificial nodes do not.

Neurons are not homogeneous. The mammalian cortex alone contains hundreds of identified cell types distinguished by morphology, connectivity, ion channel expression, and function. The distinction between excitatory (glutamatergic) and inhibitory (GABAergic) neurons is the coarsest functional division; within each class, further specialization determines what a neuron encodes and how it participates in circuit dynamics.