The Neuron
Structure follows function — every part has a precise role
Neurons are the electrically excitable cells that transmit information throughout the nervous system. The human brain contains approximately 86 billion neurons, each connected to thousands of others through synapses — creating a network of roughly 100 trillion connections. No other structure of comparable complexity exists in the known universe.
The key principle of neuron structure: information flows in one direction. Dendrites receive signals → the cell body integrates them → the axon transmits the output → synaptic terminals deliver it to the next cell. Every structural feature of the neuron reflects and supports this directional information flow.
💡 Glial Cells — The Support Network
Neurons are outnumbered by glial cells approximately 10:1 in the human brain (~860 billion glia). Glia are not passive scaffolding — they actively regulate neuronal function:
Astrocytes: Most abundant glia. Maintain the blood-brain barrier. Regulate extracellular K⁺ and neurotransmitter concentrations. Provide metabolic support (lactate) to neurons. Form glial scars after injury. Participate in synaptic signaling (tripartite synapse).
Oligodendrocytes (CNS) / Schwann cells (PNS): Produce myelin sheaths. One oligodendrocyte myelinates segments of up to 50 different axons. Multiple sclerosis = autoimmune demyelination of CNS oligodendrocytes → slowed/blocked nerve conduction → motor, sensory, and cognitive deficits.
Microglia: The brain's resident immune cells. Derived from macrophage lineage. Constantly survey brain tissue. Activated by injury or infection → phagocytose debris and pathogens. Excessive microglial activation implicated in Alzheimer's disease neuroinflammation.
Ependymal cells: Line the ventricles and central canal. Produce and circulate cerebrospinal fluid (CSF).
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Dendrites — the receivers
Dendrites are branching extensions of the neuron that receive signals from other neurons. They are covered with thousands of dendritic spines — tiny protrusions that are the postsynaptic sites of most excitatory synapses. The greater the number and surface area of dendrites, the more synaptic input a neuron can receive.
Dendritic trees vary enormously in complexity between neuron types. Purkinje cells of the cerebellum have extraordinarily elaborate dendritic arbors — each Purkinje cell receives input from up to 200,000 other neurons. Motor neurons in the spinal cord have simpler dendritic trees. The complexity of the dendritic tree reflects how much integration that neuron needs to perform.
Memory trick: Dendrites = antennas on a radio receiver. More dendrites, more signals received. Information flows INTO the neuron through dendrites.
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Soma (cell body) — the integrator
The soma (cell body) contains the nucleus, rough ER (Nissl substance — for protein synthesis), mitochondria, Golgi apparatus, and all other organelles required for cell function. It is the metabolic center of the neuron and the site where incoming signals from dendrites are integrated.
The axon hillock is a cone-shaped region where the soma transitions into the axon. It is the site of action potential initiation — the location with the highest density of voltage-gated Na⁺ channels in the neuron. All the excitatory and inhibitory inputs from dendrites are summed at the axon hillock. If the membrane potential reaches threshold (~−55 mV) at the axon hillock, an action potential fires.
Memory trick: Soma = the decision-making center. Axon hillock = the trigger. All incoming signals converge here. If the sum reaches threshold, it fires.
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Axon — the transmitter
The axon is a long, thin process that carries action potentials from the soma to the synaptic terminals. Axon length varies enormously — from less than 1 mm (local interneurons) to over 1 meter (motor neurons from spinal cord to foot). Each neuron has only one axon (unlike the multiple dendrites).
Many axons are wrapped in myelin — a fatty insulating sheath produced by oligodendrocytes (CNS) or Schwann cells (PNS). Myelin dramatically increases conduction velocity by enabling saltatory conduction — the action potential 'jumps' from one node of Ranvier (gap in myelin) to the next, rather than traveling continuously along the entire membrane. Myelinated axons conduct at 70–120 m/s; unmyelinated axons at 0.5–2 m/s.
Memory trick: Axon = the output cable. Myelin = the insulation around the cable. Nodes of Ranvier = gaps in the insulation where the signal 'jumps.' Saltatory = Latin for 'jumping.' Myelinated = fast. Unmyelinated = slow.
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Synaptic terminals — the transmitters
At the end of the axon, branching synaptic terminals (also called axon terminals, boutons, or end bulbs) contain synaptic vesicles — membrane-bound packages of neurotransmitter molecules. When an action potential arrives at the terminal, voltage-gated Ca²⁺ channels open → Ca²⁺ flows in → vesicles fuse with the presynaptic membrane → neurotransmitter released into the synaptic cleft → diffuses to postsynaptic receptors on the next neuron.
The synaptic cleft is approximately 20–40 nm wide — tiny enough for neurotransmitter molecules to diffuse across in less than 1 millisecond.
Memory trick: Synaptic terminal = the output end of the neuron. Action potential arrives → Ca²⁺ enters → vesicles release neurotransmitter → signal crosses to next neuron. No Ca²⁺ = no neurotransmitter release.
🔬 Clinical Scenario — Multiple Sclerosis and Demyelination
Multiple sclerosis directly illustrates how myelin function determines neurological function:
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MS pathophysiology. In MS, the immune system attacks myelin (produced by oligodendrocytes in the CNS). Inflammatory plaques form at sites of demyelination → action potential conduction slows or fails at demyelinated segments → symptoms correspond to the location and extent of demyelination. Common plaques form in periventricular white matter, spinal cord, optic nerves, and cerebellum.
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Symptom-anatomy correlation. Optic nerve demyelination → optic neuritis (blurred vision, pain with eye movement). Spinal cord demyelination → weakness, spasticity, sensory loss, bladder dysfunction. Cerebellar demyelination → ataxia, intention tremor, dysarthria. Each symptom directly reflects loss of myelin in the corresponding pathway.
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Relapsing-remitting MS. In early MS, partial remyelination by surviving oligodendrocytes and Schwann cells (at the plaque edges) allows partial recovery → the relapsing-remitting pattern. With repeated attacks, remyelination becomes less complete → cumulative neurological deficit. Neuroprotective and remyelination therapies (anti-LINGO-1 antibodies, opicinumab) aim to enhance remyelination.
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Guillain-Barré syndrome — PNS demyelination. GBS is an acute autoimmune demyelination of peripheral nerves (Schwann cells). Often triggered by infection (Campylobacter jejuni, CMV, Zika virus) → molecular mimicry → antibodies attack gangliosides on Schwann cells → ascending paralysis from feet upward. Most patients recover with IVIG or plasmapheresis, because Schwann cells can remyelinate more effectively than oligodendrocytes.
📌 Exam Application
Neuron structure is tested for component functions, myelin, and glia:
1. Information flow direction: Dendrites (receive) → Soma (integrate) → Axon hillock (trigger) → Axon (conduct) → Synaptic terminals (transmit).
2. Myelin producers: Oligodendrocytes (CNS — one cell myelinates up to 50 axons). Schwann cells (PNS — one cell myelinates one axon segment). MS = CNS demyelination (oligodendrocytes). GBS = PNS demyelination (Schwann cells).
3. Saltatory conduction: Action potential jumps between nodes of Ranvier → faster than continuous conduction. Myelinated = 70–120 m/s. Unmyelinated = 0.5–2 m/s.
4. Synaptic transmission: Action potential → Ca²⁺ entry → vesicle fusion → neurotransmitter release.
5. Glial cell types: Astrocytes (BBB, K⁺ regulation, metabolic support). Oligodendrocytes (CNS myelin). Schwann cells (PNS myelin). Microglia (immune surveillance). Ependymal cells (CSF production).
⚠️ The Most Common Neuron Structure Mistakes
Oligodendrocytes make CNS myelin; Schwann cells make PNS myelin. This distinction is clinically critical. MS attacks oligodendrocytes (CNS). GBS attacks Schwann cells (PNS). Recovery is better in GBS because Schwann cells remyelinate more effectively than oligodendrocytes — and because CNS inflammation is more destructive. Getting these reversed is a very common exam error.
Each neuron has ONE axon but MANY dendrites. Students sometimes confuse axons and dendrites. A neuron can have hundreds of dendrites but always has exactly one axon (which may branch distally). The axon can be extremely long (motor neurons from spinal cord to foot = over 1 meter).
Action potentials are initiated at the AXON HILLOCK — not the soma and not the dendrites. The axon hillock has the highest density of voltage-gated Na⁺ channels in the neuron. Graded potentials from dendrites sum at the axon hillock — if threshold is reached there, an action potential fires. The soma and dendrites do not fire action potentials under normal conditions.
✓ Quick Self-Test
1. List the components of a neuron in order of information flow.
2. What is saltatory conduction and why is it faster than continuous conduction?
3. Which glial cells produce myelin in the CNS and PNS respectively?
4. What triggers neurotransmitter release at the synaptic terminal?
5. What is the clinical significance of demyelination?
Answers:
1. Dendrites (receive input) → Soma/cell body (integrate signals) → Axon hillock (site of action potential initiation) → Axon (conduct action potential) → Synaptic terminals (release neurotransmitter to next cell).
2. Saltatory conduction is the propagation of action potentials by jumping from one node of Ranvier to the next along a myelinated axon, rather than traveling continuously along the entire membrane surface. It is faster because myelin insulates the axon between nodes, forcing current to flow to the next node rather than depolarizing every point of membrane sequentially. Myelinated axons conduct at 70–120 m/s vs 0.5–2 m/s for unmyelinated.
3. CNS myelin: oligodendrocytes (one cell can myelinate segments of up to 50 different axons). PNS myelin: Schwann cells (one Schwann cell myelinates one segment of one axon). Loss of CNS myelin = multiple sclerosis. Loss of PNS myelin = Guillain-Barré syndrome.
4. When an action potential arrives at the synaptic terminal, voltage-gated Ca²⁺ channels open → Ca²⁺ flows into the terminal → Ca²⁺ triggers fusion of synaptic vesicles with the presynaptic membrane → neurotransmitter is released into the synaptic cleft → diffuses to postsynaptic receptors.
5. Demyelination slows or blocks action potential conduction along affected axons. In MS (CNS demyelination), symptoms include optic neuritis, motor weakness, sensory loss, ataxia, and bladder dysfunction — each corresponding to the anatomical location of demyelinated plaques. In GBS (PNS demyelination), ascending paralysis occurs. Remyelination allows partial or complete recovery; repeated demyelination causes cumulative permanent deficits.