by Caroline Fynn

Created on: January 23, 2011   Last Updated: January 26, 2011

The human nervous system consists of neurones that lie within both the central and peripheral nervous system. The brain and spinal cord make up the central nervous system. The peripheral nervous system comprises of the spinal and cranial nerves, along with the autonomic nervous system.

Neurone (also neuron)

Neurones are commonly known as nerves, each neurone consists of a cell, an axon and/or one or more dendrites.

Cells

Nerve cells are microscopic structures; they vary in shape and size and contain a nucleus. It is the cells that give rise to grey matter within the nervous system.

Axons

The axon forms white matter and is the longest part of the neurone, and of course, neurons vary in size depending on location and function. The larger axons are surrounded by a myelin sheath which is made up of Schwann cells, these run along the length of the axon. Neurones consisting of a myelin sheath are known as myelinated neurones and those without as non-myelinated. The axon makes up the length of the neurone and tails off into a number of branches. The end of each branch forms into tiny bulb like structures known as boutons or presynaptic knobs. The axon is responsible for transporting impulses away from the cells and therefore transmitting impulses through the boutons, on to neighbouring neurones.

Dendrites

Dendrites are fibres that also form white matter; they transport impulses towards and into the cell. Dendrites branch outwards and form a relationship with the axon branches of neighbouring neurones. The gaps between these structures are known as synapses and it is here that impulses travel from one neuron to another.

There are three types of neurones; unipolar neurones consist of one axon projecting from the cell. Bipolar neurones comprise of a cell, one axon and one dendrite. Multipolar neurones include the cell, one axon and many dendrites.

THE CENTRAL NERVOUS SYSTEM

BRAIN

The brain is divided into three basic parts; the cerebrum, brainstem and cerebellum

The Cerebrum

The cerebrum is the largest structure of the brain; the outer layer consists of grey matter and is known as the cerebral cortex. The inner layer is made up of white matter.

The cerebrum is divided into right and left hemispheres by a longitudinal fissure. Both hemispheres are folded into lobes known as the frontal, parietal, temporal and occipital. The frontal lobe is at the front of the brain and is situated above the temporal lobe. The parietal lobe is at the back above the occipital lobe.

The

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