Neuron
The fundamental cellular unit of the nervous system. An electrically excitable cell specialized to receive, integrate, and transmit information encoded as electrochemical signals. ~86 billion in the adult human brain, forming ~100–500 trillion synaptic connections — the most complex information-processing structure known to biology. Over 1,000 morphologically distinct subtypes have been identified; all share the Hodgkin-Huxley action potential mechanism.
Overview
The neuron is both a computational unit (integrating hundreds to thousands of simultaneous synaptic inputs across its dendritic tree) and a communication node (transmitting the integrated result as a stream of action potentials over axons ranging from millimeters to over a meter in length). This dual role underpins all nervous-system function, from spinal reflexes to the construction of conscious experience.
Neurons come in extraordinary variety — over 1,000 morphologically and molecularly distinct cell types in the mouse brain alone, with human diversity likely greater. Yet all neurons share the same fundamental architecture and electrical excitability mechanism, first described quantitatively by Hodgkin and Huxley in 1952 — work earning the 1963 Nobel Prize in Physiology or Medicine.
Unlike most somatic cells, the vast majority of neurons in the adult brain are post-mitotic and long-lived — many neurons persist for the entire lifetime of the organism, making their maintenance, protection from injury, and resistance to neurodegeneration of extraordinary biological importance.
Structure — Core Morphological Components
| Compartment | Size / Description | Function |
|---|---|---|
| Soma (cell body) | 5–100 µm diameter; nucleus, rough ER (Nissl bodies), mitochondria, Golgi apparatus | Protein synthesis and metabolic hub; integrates distal dendritic signals; contains the neuronal genome |
| Dendrites | Arborizing processes, hundreds of µm span; surface studded with dendritic spines at excitatory synapses | Input compartment — receive and spatiotemporally integrate synaptic signals via passive + active propagation toward soma |
| Axon | Single long process (up to ~1 m in motor neurons); arises from axon initial segment (AIS) | Output compartment — conducts action potentials from AIS to terminals; AIS has the highest Nav channel density and is the spike initiation zone |
| Myelin sheath | Oligodendrocyte (CNS) or Schwann cell (PNS) wraps; interrupted at nodes of Ranvier every ~1–2 mm | Enables saltatory conduction (10–120 m/s); reduces metabolic cost ~100-fold per conducted impulse |
| Axon terminals (boutons) | 0.5–2 µm presynaptic endings; synaptic vesicles, active zone scaffold, voltage-gated Ca²⁺ channels | Vesicle exocytosis → neurotransmitter release into synaptic cleft; release probability regulated by Ca²⁺ and neuromodulators |
| Dendritic spines | Mushroom-shaped protrusions, ~0.2–2 µm; PSD scaffold with AMPA and NMDA receptors | Compartmentalize Ca²⁺ signaling for individual excitatory synapses; structural substrate of LTP and memory |
Structure — Neuronal Diversity
By morphology
Multipolar — most CNS neurons; multiple dendrites, single axon (pyramidal cells, Purkinje cells). Bipolar — one dendrite + one axon (retinal ganglia, olfactory sensory neurons). Pseudounipolar — single process bifurcating into peripheral and central branches (dorsal root ganglion cells).
By neurotransmitter identity
Glutamatergic (excitatory; ~80% cortical neurons; VGLUT1/2). GABAergic (inhibitory; ~20% cortex; GAD65/67). Dopaminergic (VTA, SNc; tyrosine hydroxylase). Serotonergic (raphe nuclei; TPH2). Cholinergic (basal forebrain; ChAT). Noradrenergic (locus coeruleus; TH+DBH).
Function — Action Potential Propagation
Neurons maintain a resting membrane potential of ~−70 mV (inside negative) sustained by the Na⁺/K⁺-ATPase (3 Na⁺ out / 2 K⁺ in) and selective K⁺ resting conductances (Kir channels). Summed synaptic depolarization reaching threshold (~−55 mV) at the AIS triggers the action potential cycle:
In myelinated axons, current jumps between nodes of Ranvier (spaced ~1–2 mm) — saltatory conduction. Velocity: ~0.5–2 m/s unmyelinated C-fibres vs. 70–120 m/s in Aα motor fibres.
| Axon class | Diameter | Myelinated? | Velocity | Function |
|---|---|---|---|---|
| Aα | 13–20 µm | Yes (thick) | 70–120 m/s | Motor efferent, proprioception (Ia spindle) |
| Aβ | 6–12 µm | Yes | 33–75 m/s | Tactile mechanoreception, pressure |
| Aδ | 1–5 µm | Lightly | 3–30 m/s | Fast (sharp) pain, cold temperature |
| C fibres | 0.2–1.5 µm | No | 0.5–2 m/s | Slow pain, warmth, autonomic post-ganglionic |
Function — Synaptic Integration
A typical cortical pyramidal neuron receives ~10,000 synaptic inputs distributed across its dendritic tree — ~8,000 excitatory (glutamatergic, onto spines) and ~2,000 inhibitory (GABAergic, onto shaft/soma/AIS). The neuron computes a weighted, time-varying sum of these inputs, incorporating active dendritic conductances (voltage-gated Na⁺, Ca²⁺, K⁺) that can amplify or suppress distal signals.
The output is a stream of action potentials whose firing rate (rate code) and precise spike timing relative to network oscillations (temporal code) together encode the neuron's assessment of its inputs. Firing rates range from near-silent (<0.1 Hz in some PFC neurons) to fast-spiking (>100 Hz in PV+ interneurons).
Graded potentials (synaptic PSPs) summate spatially (from different input locations) and temporally (within the integration time window ~5–20 ms) before reaching the AIS. Active backpropagating action potentials (bAPs) into dendrites via Nav1.2 enable coincidence detection for Hebbian plasticity (bAP + EPSP within ~20 ms → NMDA receptor opens → Ca²⁺ influx → LTP).
Lifecycle
Neurogenesis and migration. Most neurons are born during embryonic and early postnatal development in the ventricular and subventricular zones. Radial glial cells serve as neural stem cells; post-mitotic neurons migrate along radial glial scaffolds to their final cortical positions (inside-out lamination in the cortex: deep layers first, superficial last). Adult neurogenesis — well-established in the hippocampal dentate gyrus and olfactory bulb in rodents — remains actively debated in adult humans.
Differentiation, connectivity, and pruning. After migration, neurons extend axons and dendrites guided by molecular cues (netrins, semaphorins, ephrins). Synaptic connections form initially in excess; synaptic pruning (complement-mediated via C1q/C3 tagging, executed by microglial CR3 phagocytosis) eliminates ~50% of connections, sculpting the final wiring by experience-dependent activity during critical/sensitive periods.
Long-term maintenance. Post-mitotic neurons require continuous metabolic supply (O₂ + glucose — no meaningful glycogen reserves), trophic support via BDNF/TrkB, NGF/TrkA, NT-3/TrkC signaling, and axonal transport (kinesin anterograde; dynein retrograde; rates 0.3–400 mm/day). Structural synaptic plasticity (spine growth/retraction, new synapse formation) persists throughout life as the cellular substrate of learning and memory.
Neuronal death in disease. Modes include excitotoxicity (NMDA Ca²⁺ overload in stroke/TBI → calpain, caspase, mitochondrial PTP); apoptosis (Bcl-2/caspase cascade in neurodegeneration); necrosis (acute ischaemia); and aggregation-induced toxicity (amyloid-β, α-synuclein, tau, TDP-43). The adult CNS has very limited regenerative capacity — lost neurons are generally not replaced, unlike PNS axons.
Pathology
Stroke / Ischaemic Excitotoxicity
Energy failure in ischaemic penumbra → glutamate spillover → NMDA receptor Ca²⁺ overload → calpain activation, mitochondrial permeability transition pore opening → neuronal death within minutes to hours. ~87% of strokes ischaemic; CA1 hippocampal neurons (Sommer sector) uniquely vulnerable to brief global ischaemia.
Alzheimer's Disease
Amyloid-β oligomers impair synaptic plasticity and activate complement-mediated synapse pruning; tau neurofibrillary tangles (Braak I–VI) spread trans-synaptically. Entorhinal cortex and CA1 pyramidal neurons lost earliest. ~55 million globally; leading cause of dementia.
Parkinson's Disease
Selective degeneration of SNc dopaminergic neurons; α-synuclein aggregates (Lewy bodies) impair proteasomal and mitochondrial function. Loss of >60% of SNc neurons before motor symptoms emerge (tremor, rigidity, bradykinesia). ~10 million globally.
Epilepsy
E/I imbalance → neuronal hyperexcitability → synchronised burst firing → seizure propagation. ~50 million globally; 30% drug-resistant. Ion channel mutations (Nav1.1 DRAVET, Kv1.1, GABA-A subunits) underlie many genetic epilepsies.
ALS (Amyotrophic Lateral Sclerosis)
Progressive degeneration of upper and lower motor neurons; TDP-43 and SOD1/C9orf72 aggregates impair axonal transport and RNA homeostasis. Median survival 2–5 years post-diagnosis; ~10% familial.
Spinal Cord Injury
Mechanical disruption of axons + secondary excitotoxic and inflammatory cascade → permanent motor/sensory loss below injury. No approved neuroregenerative therapy; nogo-A/LINGO-1 antagonism in trials.
Prion Diseases (CJD)
PrPSc-templated misfolding accumulates in neural tissue → spongiform vacuolation, synaptic loss, neuronal death. Rapidly fatal; no treatment. Tetanospasmin (C. tetani) cleaves VAMP-2 in inhibitory interneurons → spastic paralysis by removing inhibitory control of motor neurons.
Rabies
RABV enters via nicotinic ACh receptor at NMJ → retrograde axonal transport to CNS → encephalitis. Near 100% fatal once symptomatic; post-exposure prophylaxis (vaccine + RIG) nearly 100% effective.
Cross-Atlas Connections
References
- Hodgkin AL, Huxley AF. A quantitative description of membrane current and its application to conduction and excitation in nerve. J Physiol. 1952;117(4):500–544. doi:10.1113/jphysiol.1952.sp004764 · PubMed 12991237
- Azevedo FA, Carvalho LR, Grinberg LT, et al. Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. J Comp Neurol. 2009;513(5):532–541. doi:10.1002/cne.21974 · PubMed 19226510
- Kandel ER, Koester JD, Mack SH, Siegelbaum SA. Principles of Neural Science. 6th ed. McGraw-Hill; 2021. mhprofessional.com
- OpenStax. Anatomy and Physiology 2e. Chapter 12: Introduction to the Nervous System. Rice University; 2022. openstax.org