A neuron is a specialized cell that receives, integrates, and transmits information within the nervous system. Neurons communicate through electrical changes in their membranes and chemical signals exchanged with other cells. They form circuits underlying sensation, movement, learning, memory, and the regulation of internal bodily functions. In humans, neurons occur in the central nervous system, comprising the brain and spinal cord, and in the peripheral nervous system. Their activity depends on close interaction with glial cells, which support and regulate the environment of nervous tissue. (ncbi.nlm.nih.gov)
Structure
A typical neuron has three principal regions: a cell body, dendrites, and an axon. The cell body, or soma, contains the nucleus and much of the cellular machinery needed to maintain the neuron. Its shape and size vary among neuronal types. Unlike a simple conducting wire, a neuron is a living cell whose specialized regions perform different aspects of information processing. (openstax.org)
Dendrites are usually branching processes that receive signals from other neurons. Their arrangement provides numerous sites for incoming connections. The axon typically carries electrical impulses toward other cells and may branch to reach multiple targets. Some human sensory and motor axons extend approximately a metre. Axon terminals form communication sites called synapses with neurons, muscle cells, or gland cells. These structural divisions describe a common organization rather than an identical shape shared by every neuron. (openstax.org)
Many axons are wrapped in myelin, a multilayered insulating covering produced by glial cells. Oligodendrocytes make myelin in the central nervous system, whereas Schwann cells perform this function in the peripheral nervous system. Interruptions in the covering, called nodes of Ranvier, expose specialized regions of axonal membrane where electrical impulses are regenerated. (openstax.org)
Electrical signaling
The cell membrane separates fluids containing different concentrations of electrically charged ions. Selective membrane permeability and ion pumps maintain these differences, producing a resting membrane potential in which the cell interior is usually negative relative to its surroundings. Pumps consume adenosine triphosphate to maintain the gradients needed for continued signaling. Ion channels allow particular ions to cross the membrane when activated by voltage changes, chemical messengers, or other stimuli. (ncbi.nlm.nih.gov)
An action potential is a brief, regenerative change in membrane voltage. In a typical neuronal action potential, voltage-gated sodium channels open, permitting sodium entry and rapid depolarization. Sodium-channel inactivation and potassium movement out of the cell then promote repolarization. A refractory period temporarily limits the generation of another impulse. Action potentials are approximately all-or-none events: a stronger stimulus does not simply produce a proportionally larger spike. Instead, changes in firing frequency can convey differences in stimulus intensity. (ncbi.nlm.nih.gov)
In unmyelinated axons, action potentials are regenerated along successive membrane regions. In myelinated axons, regeneration occurs chiefly at the nodes of Ranvier, with current spreading between them. This mechanism, called saltatory conduction, increases conduction speed. Axon diameter also influences how rapidly impulses travel. (openstax.org)
Synaptic communication and integration
At a chemical synapse, an arriving action potential opens calcium channels in the presynaptic terminal. Calcium entry triggers the fusion of transmitter-containing vesicles with the membrane. Released neurotransmitters cross the synaptic cleft and bind to receptors on the receiving cell, altering its electrical activity or intracellular signaling. Electrical synapses operate differently: channels called gap junctions permit direct current flow between connected cells. (ncbi.nlm.nih.gov)
Synaptic signals can increase or decrease the likelihood that a receiving neuron will fire. Excitatory and inhibitory inputs interact through synaptic integration. Inputs arriving close together in time can combine through temporal summation; signals from different locations can combine through spatial summation. Consequently, activity in one presynaptic neuron does not necessarily trigger an action potential in its target. A neuron's output reflects the combined effects of its inputs rather than merely repeating each incoming signal. (med.uth.edu)
Types and circuits
Functional classifications distinguish sensory neurons, which convey information about stimuli; motor neurons, which influence muscles or other effectors; and interneurons, which connect neurons within circuits. In the stretch reflex, for example, sensory neurons detecting muscle stretch excite motor neurons supplying that muscle while engaging inhibitory interneurons that reduce activity in opposing muscles. (med.uth.edu)
Structural classifications include multipolar neurons, with multiple dendrites and one axon; bipolar neurons, with two principal processes; and pseudounipolar neurons, whose single process divides into two branches. Pyramidal cells of the cerebral cortex and Purkinje cells of the cerebellum illustrate distinctive neuronal forms. Structure helps determine the inputs a neuron receives and the targets it can influence. (openstax.org)
Development and plasticity
During development, neurons arise from neural precursor cells, differentiate, and migrate to locations where they establish connections. Radial glial fibers can provide scaffolds for migration, while molecular signals help guide cells toward their destinations. Most brain neurons are generated before birth. Many mature neurons are exceptionally long-lived and are not routinely replaced through cell division. (ninds.nih.gov)
Neuronal connections are nevertheless changeable. Neuroplasticity includes changes in synaptic strength that alter how circuits respond to activity and experience. Hebbian theory describes an influential relationship between coordinated neuronal activity and the strengthening of connections. Such modifications provide cellular mechanisms through which circuits can store information without requiring each experience to generate a new neuron. (med.uth.edu)