The neuron

A neuron has three main parts:

• The cell body: holds the nucleus and cytoplasm, with small dark granules called Nissl's granules.
• Dendrites: short fibres that branch again and again, also containing Nissl's granules. They carry impulses towards the cell body.
• The axon: a single long fibre that carries impulses away from the cell body, to the next neuron (at a synapse) or to a muscle (at a neuro-muscular junction). The axon branches at its end, and each branch ends in a bulb-like synaptic knob, filled with small sacs (synaptic vesicles) of chemicals called neurotransmitters.

Neurons are named by how many fibres leave the cell body:

• Multipolar: one axon and two or more dendrites. Found in the cerebral cortex.
• Bipolar: one axon and one dendrite. Found in the retina of the eye.
• Unipolar: a cell body with just one axon. Usually found in the embryo.

Axons are of two types. Myelinated fibres are wrapped by Schwann cells, which form a fatty insulating layer, the myelin sheath, around the axon. The small gaps between neighbouring sheaths are the nodes of Ranvier. These fibres are found in spinal and cranial nerves. Non-myelinated fibres are also enclosed by a Schwann cell, but it does not form a myelin sheath around them. They are common in the autonomic and somatic neural systems.