Structure of neuron — dendrite cell body axon synapse

A neuron (nerve cell) is the structural and functional unit of the nervous system. It is specialised to receive, process, and transmit electrical signals called nerve impulses. Despite their enormous diversity in shape and size, all neurons share the same basic architecture.

Neurons are non-dividing cells in adults — once destroyed, they are usually not replaced.

The cell body (also called soma or perikaryon) is the metabolic centre of the neuron. It contains:

• Nucleus: Controls all cellular activities
• Nissl bodies: Rough ER stacked with ribosomes — site of protein synthesis (no other cells have Nissl bodies; they’re a neuron-specific feature)
• Mitochondria, Golgi apparatus, cytoskeleton — standard organelles

The cell body integrates incoming signals. If the combined signal exceeds a threshold, it fires an action potential.

Dendrites are short, branched projections arising from the cell body. They dramatically increase the surface area available to receive signals.

Function: Receive incoming nerve impulses from other neurons (or from receptors) and carry them toward the cell body.

A single neuron can have hundreds of dendrites — each making synaptic contacts with thousands of other neurons.

The axon is a single, long projection that carries impulses away from the cell body toward target cells (other neurons, muscles, or glands).

• Axons can be extremely long — the sciatic nerve’s motor neurons have axons extending from the spinal cord to the toes (up to 1 metre in humans)
• The point where the axon leaves the cell body is called the axon hillock — the decision point where action potentials are generated

Many axons are wrapped in a myelin sheath — a fatty insulating layer produced by Schwann cells (in the peripheral nervous system). Myelin:

• Insulates the axon, preventing signal leakage
• Greatly speeds up nerve impulse conduction by enabling saltatory conduction

Nodes of Ranvier: Small gaps in the myelin sheath where the axon membrane is exposed. Impulses “jump” from node to node in myelinated fibres — this is saltatory conduction (Latin: saltare = to jump), which is much faster than continuous conduction in unmyelinated fibres.

A synapse is the junction between the axon terminal of one neuron (pre-synaptic neuron) and the dendrite/cell body of the next neuron (post-synaptic neuron).

At a chemical synapse:

1. Action potential reaches the axon terminal (synaptic knob)
2. $\text{Ca}^{2+}$ ions enter, triggering fusion of synaptic vesicles with the membrane
3. Neurotransmitters (e.g., acetylcholine, dopamine, serotonin) are released into the synaptic cleft (~20 nm wide)
4. Neurotransmitters bind to receptors on the post-synaptic membrane
5. This either excites or inhibits the next neuron

The synapse introduces a small time delay (~0.5 ms) and allows signal modulation.