Understanding Neuron Structure: A Comprehensive Guide

Neuron: The Functional Unit of the Nervous System

Neurons are specialized for rapid electrochemical signaling. Despite diverse morphologies, all share a common structural blueprint.

Key Components

1. Cell Body (Soma / Perikaryon)

• Contains nucleus (DNA), nucleolus, rough ER (Nissl bodies), Golgi, mitochondria.
• Site of protein synthesis, metabolic integration, decision-making (summation of synaptic inputs).
• Size: 5–100 µm diameter.

2. Dendrites

• Short, highly branched processes receiving synaptic input.
• Covered in dendritic spines — microscopic protrusions hosting excitatory synapses (increases surface area 100x).
• Integrate thousands of EPSPs/IPSPs; passive/active conduction to soma.
• Dendritic morphology defines neuron type (stellate, pyramidal, Purkinje).

3. Axon

• Single, long process (up to 1 m in humans) conducting action potentials away from soma.
• Originates at axon hillock (trigger zone — lowest threshold).
• May branch (collaterals) to innervate multiple targets.
• Axoplasm contains microtubules, neurofilaments, mitochondria, vesicles; axonal transport (kinesin/dynein).

4. Axon Terminal (Presynaptic Terminal)

• Specialized ending with active zones: voltage-gated Ca²⁺ channels, synaptic vesicles (neurotransmitters), release machinery (SNAREs).
• Action potential → Ca²⁺ influx → vesicle fusion → neurotransmitter release into synaptic cleft.
• Forms synapses on dendrites (axodendritic), soma (axosomatic), axons (axoaxonic), muscle (NMJ), glands.

5. Myelin Sheath (Myelinated Neurons)

• CNS: Oligodendrocytes (one cell myelinates multiple axons).
• PNS: Schwann cells (one cell per internode).
• Multilamellar lipid insulation → increases membrane resistance, decreases capacitance.
• Enables saltatory conduction — action potential jumps between nodes of Ranvier (10–100x faster).

6. Nodes of Ranvier

• ~1 µm gaps between myelin segments.
• High density of voltage-gated Na⁺/K⁺ channels.
• Site of action potential regeneration.

Structural Diversity

• Multipolar: 1 axon, many dendrites (motor neurons, interneurons).
• Bipolar: 1 axon, 1 dendrite (retinal, olfactory, vestibular).
• Pseudounipolar: Single process bifurcates → peripheral + central branches (dorsal root ganglion sensory neurons).
• Unipolar: Rare in vertebrates.

Clinical Relevance

• Demyelination (MS, Guillain-Barré) → conduction block, slowed velocity.
• Axonopathy (diabetic neuropathy, chemo-toxicity) → dying-back degeneration.
• Synaptopathies (Alzheimer's, autism, schizophrenia) — spine density/function altered.
• Neurodegeneration: protein aggregates (tau, α-synuclein, TDP-43) disrupt transport, synaptic function.