Cells of the brain

The brain is made of two classes of cells. Neurons carry and process information; glial cells support, protect, insulate and regulate them.

86 billionneurons, on average, in the adult human brain

85 billionnon-neuronal cells, most of them glia

19%of neurons are in the cerebral cortex; most of the rest are in the cerebellum

Cell counts: Azevedo et al., 2009, J Comp Neurol.

Neurons and glia

Neurons are excitable cells that communicate through electrical impulses and chemical synapses. Glial cells do not fire action potentials in the same way, but neurons cannot survive, signal quickly or form stable circuits without them.

Distribution of neurons in the human brain. The cerebral cortex makes up about 82% of the brain’s mass but holds only about 19% of its neurons (about 16 billion). The cerebellum, about 10% of the mass, holds about 80% (about 69 billion), most of them small granule cells. Values for the average adult male human brain.Sources: Azevedo et al., 2009, J Comp Neurol; Herculano-Houzel, 2009, Front Hum Neurosci.

Both classes are diverse. Neurons differ in shape, size, chemical messenger and connections, and each region of the brain has characteristic cell types. Glia include several distinct classes with different functions, from insulating axons to defending the brain against infection.

Anatomy of a neuron

Most neurons share the same basic structure: branches that receive signals, a cell body that integrates them, and a single axon that carries the output to other cells.

DendritesBranches that receive input from other neurons.
Dendritic spinesTiny protrusions where most excitatory synapses form.
Cell body (soma)Contains the nucleus and makes most of the cell’s proteins.
Axon hillockWhere the action potential begins.
Myelin sheathInsulation formed by glial cells around the axon.
Node of RanvierA gap in the myelin where the signal is regenerated.
AxonCarries the signal away from the cell body.
Axon terminalsRelease neurotransmitter onto the next cell.
SynapseThe junction with the receiving neuron.

A signal begins at the axon hillock and jumps from node to node along the myelinated axon to the terminals, where it is passed to the next cell.

How neurons are classified

Neurons can be grouped by their shape, by their role in a circuit, or by the chemical messenger they release. Most neurons can be described in all three ways.

By shape

MultipolarMany dendrites and one axon. The most common type, including pyramidal and motor neurons.
BipolarOne dendrite and one axon, at opposite poles. Found in the retina and olfactory system.
PseudounipolarA single process that splits in two. Typical of sensory neurons.
UnipolarA single process. Common in invertebrates; rare in humans.

By role

Sensory neuronsCarry information from the body and the outside world towards the central nervous system.
Motor neuronsCarry commands from the central nervous system to muscles and glands.
InterneuronsConnect neurons within the brain and spinal cord. The vast majority of neurons belong here.
Projection neuronsSend long axons to distant regions, linking brain areas with one another.

By chemical messenger

GlutamateThe main excitatory transmitter; used by pyramidal and granule cells.
GABA and glycineThe main inhibitory transmitters; used by interneurons, Purkinje cells and medium spiny neurons.
NeuromodulatorsDopamine, serotonin, noradrenaline and acetylcholine, released by small populations whose axons reach large parts of the brain.

Types of neuron

Pyramidal neuron

The principal excitatory neuron of the cerebral cortex, named for its triangular cell body.

Where it is found: Cerebral cortex, especially layers 2/3 and 5; hippocampus (CA1 and CA3) and amygdala.
Morphology: A pyramid-shaped cell body with a single long apical dendrite rising towards the brain surface, a skirt of basal dendrites, and thousands of dendritic spines. One axon leaves the base and can travel to distant regions.
Normal function: Releases glutamate to excite its targets. Pyramidal neurons integrate thousands of inputs and carry the output of the cortex to other cortical areas, the thalamus, the basal ganglia, the brainstem and the spinal cord. Changes in the strength of their synapses underlie learning and memory.
Works with: Local inhibitory interneurons, other pyramidal neurons, thalamic inputs, astrocytes wrapping their synapses, and microglia that prune their spines.
In disease: Synapse and neuron loss in the cortex and hippocampus is central to Alzheimer’s disease. The giant pyramidal (Betz) cells of motor cortex degenerate in amyotrophic lateral sclerosis, and hyperexcitable pyramidal networks generate seizures in epilepsy.

Purkinje cell

One of the largest neurons in the brain, with a flat, fan-shaped dendritic tree.

Where it is found: The Purkinje cell layer of the cerebellar cortex, in a single row.
Morphology: A large, flask-shaped cell body and an enormous dendritic tree that is flattened in one plane, like a fan, densely covered with spines. A single axon descends to the deep cerebellar nuclei.
Normal function: The sole output of the cerebellar cortex. Purkinje cells release GABA to inhibit the deep cerebellar nuclei, refining the timing and accuracy of movement, balance and motor learning.
Works with: Very large numbers of parallel fibres from granule cells, a single climbing fibre from the inferior olive that wraps each cell, local basket and stellate interneurons, and Bergmann glia.
In disease: Purkinje cells are lost in many spinocerebellar ataxias and in cerebellar degeneration associated with chronic alcohol misuse, causing unsteady gait and poor coordination.

Cerebellar granule cell

Tiny, densely packed excitatory neurons that make up most of the brain’s neurons.

Where it is found: The granular layer of the cerebellum; a separate population of granule cells forms the dentate gyrus of the hippocampus.
Morphology: A very small round cell body with three to five short dendrites ending in claw-like tips. The thin axon rises and splits in a T to form a parallel fibre running along the cerebellar folium.
Normal function: Granule cells receive mossy-fibre input carrying information from the cortex, brainstem and spinal cord, and relay it as glutamatergic excitation to many Purkinje cells, expanding and recoding sensory and motor signals.
Works with: Mossy fibres, Golgi cells (which inhibit them), and Purkinje cells and molecular-layer interneurons through their parallel fibres.
In disease: The cerebellum contains most of the brain’s neurons, the great majority of them granule cells. Granule cell loss occurs in some inherited ataxias and following certain toxic and viral injuries.

Inhibitory interneuron

A diverse family of local neurons that inhibit their neighbours and shape the timing of brain activity.

Where it is found: Throughout the cortex, hippocampus and other regions, making up a minority of cortical neurons.
Morphology: Usually a rounded or oval cell body with smooth or sparsely spiny dendrites. Their axons branch densely nearby; basket cells wrap the cell bodies of their targets, and chandelier cells contact the start of their axons.
Normal function: Release GABA to inhibit their targets, balancing excitation, preventing runaway activity and pacing brain rhythms. Subtypes are often identified by markers such as parvalbumin, somatostatin and VIP, each controlling a different part of the target cell.
Works with: Pyramidal neurons, other interneurons, and long-range inputs that recruit them.
In disease: Loss or dysfunction of inhibitory interneurons contributes to epilepsy. Altered parvalbumin interneurons and disturbed gamma rhythms are being studied in schizophrenia and autism.

Medium spiny neuron

The main cell type of the striatum, central to movement, habits and reward.

Where it is found: The striatum of the basal ganglia: the caudate nucleus, putamen and nucleus accumbens.
Morphology: A medium-sized round cell body with radiating dendrites that are densely covered with spines from a short distance beyond the cell body.
Normal function: GABAergic projection neurons forming the great majority of striatal neurons. Two main classes, expressing D1 or D2 dopamine receptors, form the direct and indirect pathways that facilitate or suppress actions.
Works with: Glutamatergic inputs from the cortex and thalamus, dopaminergic inputs from the midbrain, and striatal interneurons.
In disease: Medium spiny neurons are among the first and most severely affected cells in Huntington’s disease. Their altered signalling also contributes to the symptoms of Parkinson’s disease and to addiction.

Dopaminergic neuron

A small population of midbrain neurons whose dopamine influences movement, motivation and learning.

Where it is found: The substantia nigra pars compacta and the ventral tegmental area of the midbrain.
Morphology: A medium-sized, often elongated cell body with a few long dendrites, pigmented by neuromelanin in humans. The thin axon branches enormously, forming many release sites in its targets.
Normal function: Release dopamine broadly in the striatum and frontal cortex. Nigral neurons support the initiation of movement; neurons of the ventral tegmental area signal reward and the difference between expected and actual outcomes, which drives learning.
Works with: Medium spiny neurons, prefrontal cortex, and inputs from many brain regions.
In disease: Progressive loss of substantia nigra dopaminergic neurons causes the motor symptoms of Parkinson’s disease. Dopamine signalling is also involved in addiction and is a target of antipsychotic medicines.

Lower motor neuron

A large neuron whose axon leaves the central nervous system to make muscles contract.

Where it is found: The ventral horn of the spinal cord and the motor nuclei of the brainstem.
Morphology: A very large, multipolar cell body with several thick dendrites and a long, heavily myelinated axon that travels through a peripheral nerve to the muscle.
Normal function: Releases acetylcholine at the neuromuscular junction. Each motor neuron and the muscle fibres it controls form a motor unit, the basic unit of movement.
Works with: Upper motor neurons from the cortex, spinal interneurons, sensory fibres in reflex circuits, Schwann cells along the axon, and muscle fibres.
In disease: Motor neurons degenerate in amyotrophic lateral sclerosis and spinal muscular atrophy, causing weakness and muscle wasting. They were also the target of the poliovirus.

Sensory neuron

Neurons that carry information from the skin, muscles and organs into the spinal cord.

Where it is found: The dorsal root ganglia beside the spinal cord, and the sensory ganglia of the cranial nerves.
Morphology: A round cell body with a single process that divides into two branches: one runs to the skin or organ, the other into the spinal cord. There are no conventional dendrites.
Normal function: Specialised endings detect touch, vibration, temperature, pain and body position and convert them into action potentials. Different subtypes have fibres of different thickness and conduction speed.
Works with: Neurons of the dorsal horn of the spinal cord, satellite glial cells around the cell body, and Schwann cells along the fibres.
In disease: Damage to sensory neurons causes peripheral neuropathy, common in diabetes and after some chemotherapies. Varicella-zoster virus lies dormant in sensory ganglia and can reactivate as shingles.

Bipolar neuron

Neurons with two processes, one at each end, that relay signals in sensory pathways.

Where it is found: The retina, the olfactory epithelium and the inner-ear ganglia.
Morphology: An elongated cell body with one dendrite and one axon extending from opposite poles.
Normal function: In the retina, bipolar cells carry signals from photoreceptors to ganglion cells, already separating responses to light increases and decreases. Olfactory receptor neurons detect odour molecules directly.
Works with: Photoreceptors, horizontal and amacrine cells, and ganglion cells in the retina.
In disease: Bipolar cell signalling is disrupted in inherited retinal degenerations and in congenital stationary night blindness. Loss of smell is common early in Parkinson’s and Alzheimer’s disease.

Types of glia

Astrocyte

Star-shaped cells that support neurons, synapses and blood vessels.

Where it is found: Throughout the brain and spinal cord: protoplasmic astrocytes in grey matter, fibrous astrocytes in white matter.
Morphology: A small cell body with many fine, highly branched processes that occupy their own territory. Some processes form endfeet that cover blood vessels; others wrap synapses.
Normal function: Astrocytes take up excess neurotransmitters, especially glutamate, recycle them for neurons, buffer potassium, supply energy substrates, regulate blood flow and help maintain the blood–brain barrier. Their processes form part of the “tripartite synapse”.
Works with: Synapses, blood vessels, neurons, other astrocytes through gap junctions, microglia and oligodendrocytes.
In disease: Astrocytes become reactive after injury and in diseases such as Alzheimer’s disease. Antibodies against the astrocyte water channel aquaporin-4 cause neuromyelitis optica, and mutations in the astrocyte protein GFAP cause Alexander disease. Many gliomas share features of astrocytes.

Oligodendrocyte

Cells that wrap axons in myelin, the insulation that makes fast signalling possible.

Where it is found: Throughout the central nervous system, most abundant in white matter.
Morphology: A small round cell body with a few processes, each ending in a sheath of compacted membrane wound many times around a segment of axon. A single oligodendrocyte can myelinate segments of dozens of axons.
Normal function: Myelin allows saltatory conduction, speeding signals and saving energy. Oligodendrocytes also provide metabolic support to the axons they wrap. Oligodendrocyte precursor cells persist in the adult brain and can generate new myelin.
Works with: Axons, astrocytes, and precursor cells that replace them.
In disease: In multiple sclerosis, immune attack destroys myelin and oligodendrocytes. Inherited leukodystrophies affect myelin formation, and premature birth can injure developing oligodendrocytes.

Microglia

Motile immune cells that protect the brain and help to shape its circuits.

Where it is found: Throughout the brain and spinal cord, forming a small fraction of all cells.
Morphology: In the healthy brain, a small cell body with long, finely branched processes that constantly extend and retract to survey their surroundings. When activated, they retract their processes and become rounded.
Normal function: Microglia detect infection and injury, remove debris and dying cells, and release signalling molecules. During development and in adulthood they prune unneeded synapses. They originate from early embryonic precursors, not from the blood.
Works with: Neurons and synapses, astrocytes, oligodendrocytes and immune cells entering from the blood.
In disease: Variants in microglial genes such as TREM2 increase the risk of Alzheimer’s disease. Microglial activation is a feature of neurodegeneration, chronic pain and brain injury.

Schwann cell

Cells that myelinate and support nerve fibres outside the brain and spinal cord.

Where it is found: Peripheral nerves throughout the body.
Morphology: Each myelinating Schwann cell wraps a single segment of one axon in a spiral of membrane. Non-myelinating Schwann cells hold several thin axons in pockets of their cytoplasm.
Normal function: Speed conduction in peripheral nerves and, unlike oligodendrocytes, actively support regeneration: after injury they clear debris and form guidance tracks for regrowing axons.
Works with: Peripheral axons, the surrounding connective tissue and macrophages after injury.
In disease: Guillain–Barré syndrome and Charcot–Marie–Tooth disease affect peripheral myelin. Schwannomas, such as vestibular schwannomas, are benign tumours of Schwann cells.

Ependymal cell

Cells that line the ventricles and help move cerebrospinal fluid.

Where it is found: The walls of the ventricles and the central canal of the spinal cord; specialised relatives form the choroid plexus.
Morphology: Cuboidal to columnar cells forming a single layer, with hair-like cilia and microvilli on the surface facing the fluid.
Normal function: The beating of their cilia helps circulate cerebrospinal fluid. Choroid plexus epithelial cells, closely related to ependymal cells, secrete most of the cerebrospinal fluid.
Works with: Cerebrospinal fluid, the underlying brain tissue and astrocytes.
In disease: Defects in ependymal cilia can contribute to hydrocephalus, and ependymomas are tumours that arise from ependymal cells.

Other glial cells

Several further types of glia have specialised roles in particular places.

Where it is found: Radial glia in the developing brain; Bergmann glia in the cerebellum; Müller glia in the retina; satellite glia in peripheral ganglia.
Morphology: Radial glia are long, thin cells spanning the developing brain wall. Bergmann glia send parallel processes through the cerebellar cortex. Müller glia span the full thickness of the retina.
Normal function: Radial glia act both as stem cells that generate neurons and as scaffolds along which young neurons migrate. Bergmann and Müller glia perform astrocyte-like support roles, and satellite glia surround sensory neuron cell bodies.
Works with: Migrating neurons in development, Purkinje cells, retinal neurons and sensory neurons.
In disease: Disrupted radial glial function during development is linked to malformations of cortical development. Satellite glia are thought to contribute to chronic pain.

Interactions between neurons and glia

At each synapse, neurons and several types of glia interact closely.

A signal travels along a myelinated axon to a synapse. Transmitter is released onto the receiving dendrite; the astrocyte wrapping the synapse responds and links the activity to the nearby blood vessel, while a microglial cell surveys the area.

SynapseThe presynaptic terminal releases neurotransmitter onto the receiving dendrite.
Receiving neuronIts dendrite responds to the transmitter, and the signal moves on.
AstrocyteWraps the synapse, clears excess glutamate and couples activity to blood flow.
OligodendrocyteMyelinates segments of the incoming axon so the signal arrives quickly.
MicrogliaSurvey the tissue with moving processes and can prune synapses.
Blood vesselAstrocyte endfeet cover the vessel and help form the blood–brain barrier.

Cells in disease

Many brain disorders begin in, or mainly affect, specific cell types. The vulnerable cells help to explain the symptoms and guide the search for treatments.

Condition	Cells most affected	What happens
Alzheimer’s disease	Pyramidal neurons of cortex and hippocampus; microglia; astrocytes	Synapse and neuron loss with amyloid plaques and tau tangles; microglial and astrocyte reactions
Parkinson’s disease	Dopaminergic neurons of the substantia nigra	Progressive loss of dopamine neurons with Lewy bodies, causing slowness, rigidity and tremor
Huntington’s disease	Medium spiny neurons of the striatum	Inherited degeneration causing involuntary movements and cognitive and psychiatric changes
Amyotrophic lateral sclerosis	Upper and lower motor neurons	Degeneration of motor neurons causing progressive weakness
Multiple sclerosis	Oligodendrocytes and myelin; axons	Immune-mediated loss of myelin, slowing or blocking nerve signals
Epilepsy	Pyramidal neurons and inhibitory interneurons	Imbalance between excitation and inhibition producing seizures
Neuromyelitis optica	Astrocytes	Antibodies against aquaporin-4 damage astrocytes in the optic nerves and spinal cord
Peripheral neuropathy	Sensory neurons and Schwann cells	Damage to peripheral nerves causing numbness, pain or weakness
Gliomas	Glial lineage and precursor cells	Brain tumours that arise from glial lineage cells, including glioblastoma

Last updated 20 October 2026