Education PNS and CNS axons

Peripheral nervous system and central nervous system nerve axons

Mark Baker tells you more about nerves

A nerve that we see in the peripheral nervous system (PNS) or a white-matter ‘tract’ in the central nervous system (CNS) is composed of many axons. The axons are elongated extensions of individual nerve cells, and they are structures specialized for signalling over distance. Let me try to explain.

When you see a peripheral nerve in a limb, from the outside, it is protected by a robust sheath, made of connective tissue. This sheath gives the peripheral nerve a degree of robustness and stretchiness. 

Peripheral nerves are unharmed by being stretched a bit, and to a surprising degree they move or glide past adjacent tissues for example at the carpal tunnel, when we move and bend our limbs, quite normally. Held within that stretchy coating are the nerve axons, either sending signals towards the spinal cord or the brain, in the form of sensory input, or away from the spinal cord or brain to provide some motor function be it muscle contraction, or secretion. 

The same is not true of nerves protected inside the skull and spinal column. 

The nerve fibres within the central nervous system are not covered in a stretchy coat in the same way as in the periphery, and these axons are usually to be found in tracts, notable for their white appearance in dissection, the ‘white matter’ that most people have heard about. These nerve fibres are known to signal over distance by sending nerve impulses from one part of the CNS to another, or also down the spinal cord to control our body movements. 

They are embedded in the three dimensional anatomical structure of the brain and spinal cord. Like the axons in peripheral nerves they are accompanied by nearby blood vessels and capillaries that provide the necessary oxygen and glucose that allow them to function.

The median nerve (in red) is an example of a long peripheral nerve innervating the arm and hand. It is a mixed nerve, with sensory and motor function. It innervates the hand via the carpal tunnel at the wrist. The main stem of the nerve is flexible and moves around quite a lot as we move and bend our arm. Central axons in the brain or spinal cord are not able to move independently of the tissue around them, and are held fast