The cell processes of neurons may occur singly within the parenchyma of animal tissues or they may be grouped into bundles to form a nerve. A nerve is thus, an assembly of a large number of cell processes often called the fibres.
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The nerve is surrounded by a loose connective tissue covering, the epineurium due to which it is separated from other body tissues.
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The bundles of individual nerve fibres within the nerve are covered in a relatively strong sheath of connective tissue known as the perineurium.
Inside the bundles are individual nerve fibres, each surrounded by the network of connective tissue making up the endoneurium.
Nerves are supplied with a profusion of blood-vessels. Small arteries and arterioles are present in the epineurium and perineurium and the capillaries in the endoneurium.
Nerves are of different types depending on the functions. Some nerves are motor or efferent nerves and these carry the messages or impulses from the central nervous system to the effector organs.
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Other nerves are sensory or afferent nerves as these carry the messages or impulses from the different parts of the body to the central nervous system. Motor nerves consist of only motor nerve fibres, while the sensory nerves consist of sensory nerve fibres.
There are also nerves which include both motor and sensory nerve fibres and, hence, these are called mixed nerves.
The motor nerves or fibres terminate in organs (for example— in muscles) with, motor endings, while sensory nerves or fibres terminate in organs (for example—in the skin) with sensory endings or receptors.
In this way the central nervous system communicates with all organs and tissues through the nerves.