Structure of a neuron

A neuron is microscopic and has three main parts: the cell body, the dendrites and the axon.

The cell body holds cytoplasm with the usual organelles and granular bodies called Nissl's granules.

Dendrites are short, repeatedly branched fibres that project from the cell body; they also contain Nissl's granules and carry impulses towards the cell body.

The axon is a long fibre whose far end branches. Each branch ends in a bulb-like synaptic knob holding synaptic vesicles filled with neurotransmitters.

Axons carry impulses away from the cell body, either to another neuron at a synapse or to a muscle at a neuromuscular junction.

By the number of axons and dendrites: multipolar neurons (one axon, two or more dendrites) are in the cerebral cortex; bipolar neurons (one axon, one dendrite) are in the retina; unipolar neurons (cell body with one axon only) are usually found in the embryonic stage.

Axons are myelinated or non-myelinated. In myelinated fibres, Schwann cells form a myelin sheath round the axon; the gaps between neighbouring sheaths are the nodes of Ranvier. Myelinated fibres are found in spinal and cranial nerves.

In a non-myelinated fibre, a Schwann cell encloses the axon without wrapping a myelin sheath round it; such fibres are common in the autonomous and somatic neural systems.