Parts of a neurone

Although neurones vary in their shape and size they all have three basic parts:

Cell body — this has a central nucleus and is surrounded by cytoplasm and contains standard organelles such as mitochondria and a golgi body. Dendrites — these are highly branched extensions of the nerve cell. These neural extensions receive and transmit stimuli towards the cell body. Axon — this is long, single nerve fibre extending from the cell body. The function of an axon is to transmit impulses away from the cell body.

KEY FACT

A thicker myelinated nerve fibre will enable nervous signals to be transmitted very quickly, such as pain fibres, whereas hot and cold receptor fibres are non-myelinated and their signals are transmitted more slowly.

Dendrites

Nerve cell

Nucleus

Axon

Nucleus in myelin sheath

Synaptic end bulb

Dendrites

Nerve cell

Nucleus

Axon

Nucleus in myelin sheath

Synaptic end bulb

Nodes of Ranvier

Myelin sheath Neurilemma

Nodes of Ranvier

Myelin sheath Neurilemma

Fig 8.1 The structure of a nerve cell

Other parts of a neurone's structure include the following. Myelin sheath

This is a fatty insulating sheath that covers the axon. Its function is to insulate the nerve and accelerate the conduction of nerve impulses along the length of the axon. The myelin sheath is produced by Schwann cells (large flat cells containing a nucleus and cytoplasma) which wrap themselves around the axon in a spiral fashion layer after layer.

Neurilemma

This is a fine delicate membrane that surrounds the axon and consists of a layer of one or more Schwann cells enclosing the myelin sheath. The neurilemma plays an important role in the regeneration of PNS nerve fibres.

Nodes of Ranvier

The myelin sheath has gaps at intervals of 2-3 mm along the length of the axon which are called the nodes of Ranvier. During neural activity impulses jump from one node to another, resulting in an increased rate of conduction.

Synapse

This is the minute gap across which nerve impulses pass from one neurone to the next at the end of a nerve fibre. Reaching a synapse causes the release of a neurotransmitter which diffuses across the gap and triggers an electrical impulse in the next neurone.

Synaptic end bulb/feet

The ends of the axon terminals have bulb-like structures containing sacs called synaptic vesicles that store the transmitters. These are chemicals that facilitate, arouse or inhibit the transmission of impulses between neurones across synapses.

There are three types of neurones:

Sensory/afferent neurones

Receive stimuli from sensory organs and receptors and transmit the impulse to the spinal cord and brain. Sensations transmitted by the sensory neurones include heat, cold, pain, taste, smell, sight and hearing

Motor/efferent neurones

Conduct impulses away from the brain and the spinal cord to muscles and glands in order to stimulate them into carrying out their activities

Association (mixed) neurones

Link sensory and motor neurones, helping to form the complex pathways that enable the brain to interpret incoming sensory messages, decide on what should be done and send out instructions in response along motor pathways to keep the body functioning properly

Cervical Nerve Root Radiculopathy
Fig 8.2 A simple nerve pathway
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