A neuron, or nerve cell, is the fundamental anatomical and physiological unit of the nervous system, specialized for receiving, processing, and transmitting biological information via electrical and chemical signals. Unlike somatic cells that perform static structural roles, neurons are electrically excitable cells capable of generating rapid shifts in transmembrane electrical voltage. The human central nervous system contains approximately eighty-six billion neurons, interconnected in intricate neural circuits that coordinate voluntary movements, autonomic physiological homeostasis, sensory perception, and complex cognitive thought.
Structurally, a prototypical multipolar neuron consists of three morphological compartments: the cell body (soma or perikaryon), branching dendrites, and a single elongated axon. The soma contains the nucleus, mitochondria, and prominent rough endoplasmic reticulum clusters known as Nissl bodies, which synthesize proteins and neurotransmitters. Dendrites extend outwards as arborized processes that receive incoming biochemical inputs from upstream neurons. When integrated synaptic inputs reach a critical threshold at the axon hillock, the neuron initiates an electrical signal termed an action potential. This nerve impulse travels rapidly down the axon—frequently insulated by lipid-rich myelin sheaths synthesized by Schwann cells in the peripheral nervous system or oligodendrocytes in the central nervous system—propagating toward distal axon terminals.
The propagation of an action potential relies on electrochemical gradients maintained across the axonal lipid bilayer. At rest, the sodium-potassium ATPase pump actively expels three sodium ions (Na+) while importing two potassium ions (K+), establishing a negative resting membrane potential of approximately minus seventy millivolts (−70 mV). Upon reaching threshold voltage (approximately −55 mV), voltage-gated sodium channels open abruptly, permitting an influx of sodium that depolarizes the membrane to plus thirty millivolts (+30 mV). Subsequent channel inactivation and delayed potassium efflux repolarize the cell. In myelinated axons, action potentials jump rapidly between uninsulated gaps called Nodes of Ranvier in a process termed saltatory conduction. At terminal synapses, depolarization triggers calcium influx, causing vesicles to release chemical neurotransmitters across the synaptic cleft.
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A neuron is an electrically excitable cell specialized for generating, conducting, and transmitting biological signals across the nervous system.
The human brain contains approximately 86 billion neurons, supported by an equal or greater number of non-neuronal glial cells (neuroglia).
The structural anatomy of a neuron comprises three primary parts: dendrites (signal reception), the soma/cell body (metabolic integration), and the axon (signal transmission).
Nissl granules (or Nissl bodies) located within the neuronal soma are specialized granules of rough endoplasmic reticulum and ribosomes dedicated to protein synthesis.
The axon hillock is the conical junction between the soma and the axon where graded postsynaptic potentials integrate to trigger an action potential.
The resting membrane potential of an unstimulated neuron is approximately -70 millivolts (mV), with the interior of the cell negatively charged relative to the extracellular fluid.
The sodium-potassium pump (Na+/K+ ATPase) consumes ATP to actively transport 3 Na+ ions out of the neuron in exchange for 2 K+ ions entering the cell.
An action potential operates on the 'all-or-none' principle: if depolarizing stimuli fail to achieve the threshold potential (around -55 mV), no action potential fires.
Depolarization is driven by the rapid opening of voltage-gated sodium (Na+) channels, causing an influx of positively charged sodium ions up to approximately +30 mV.
Repolarization occurs as sodium channels inactivate and voltage-gated potassium (K+) channels open, allowing K+ ions to exit the intracellular space.
The refractory period is a brief interval following an action potential during which the axon membrane cannot generate another impulse, enforcing unidirectional signal propagation.
Myelin is a lipid-rich insulating sheath produced by oligodendrocytes in the Central Nervous System (CNS) and by Schwann cells in the Peripheral Nervous System (PNS).
Nodes of Ranvier are periodic unmyelinated gaps along the axon where high densities of voltage-gated ion channels allow saltatory conduction.
Saltatory conduction enables nerve impulses to jump between Nodes of Ranvier, increasing signal propagation velocity up to 120 metres per second while conserving metabolic energy.
At chemical synapses, action potential arrival depolarizes the presynaptic terminal, triggering the opening of voltage-gated calcium (Ca2+) channels.
Calcium influx drives synaptic vesicle exocytosis, releasing neurotransmitters (such as acetylcholine, dopamine, or GABA) across the 20-nanometre synaptic cleft.
Multiple Sclerosis (MS) is an autoimmune neurodegenerative disorder where the immune system destroys myelin sheaths, impairing action potential conduction.
Spanish neuroanatomist Santiago Ramón y Cajal established the 'Neuron Doctrine' in the late 19th century, proving that neurons are individual discrete cells rather than a continuous web.