Neurons

What is a neuron?

• The neuron is the structural and functional unit of nervous tissue.
• Neurons have the least power of regeneration.

Structure of a neuron

The structure of a neuron consists of 3 parts:

• Dendrites/ dendrons
• Cyton (cell body/ soma)
• Axon

Cyton (cell body)

• Contains cytoplasm with typical cell organelles and a nucleus.
• The cyton of neurons has groups of ribosomes and RER called Nissl’s granules.

Dendrites

• Short fibres which branch repeatedly and project out of the cell body.
• They also contain Nissl’s granules.
• They carry impulses towards the cell body.

Axon

• The long fibre of the neuron is called the axon, which can be grey (non-myelinated) or white (myelinated).
• It carries impulses away from the cell body, to a synapse or a neuromuscular junction.
• The cytoplasm of the axon is called the axoplasm, and its cell membrane is called the axolemma.
• The myelin sheath is secreted by Schwann cells (in the peripheral nervous system).
• The gaps between two adjacent myelin sheaths are called nodes of Ranvier.
• The distal end of the axon is branched, and each branch ends in a bulb-like structure called the synaptic knob, which contains synaptic vesicles filled with neurotransmitters.

Myelinated and non-myelinated fibres

Feature	Myelinated fibres	Non-myelinated fibres
Myelin sheath	Present, formed by Schwann cells around the axon	Absent; the axon is enclosed by a Schwann cell that does not form a myelin sheath
Nodes of Ranvier	Present	Absent
Colour	White	Grey
Found in	Spinal and cranial nerves	Autonomous and somatic neural systems

Types of neurons

On the basis of the number of axons and dendrites, neurons are of three types:

• 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 one axon only. Usually found in the embryonic stage.