Saturday, 24 February 2018

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


  1. Prof G, I have PPMS. Had it since the mid-naughties. I am still in my thirties. I am about EDSS 6 or 6.5.

    Should I ask my neurologist for cladribine? Do I stand a chance in the UK NHS? Is it worth getting condescending looks from my MS team yet again?

    1. Re: "Should I ask my neurologist for cladribine?"

      Cladribine is only licensed for highly-active relapsing MS. So it is unlikely to be prescribed. The only drug licensed for PPMS is ocrelizumab and it is not available yet on the NHS. It has to be NICEd first and there is no guarantee that ocrelizumab will be given the greenlight by NICE.

      Another option in ~6-9 months time will be the new phase 3b ocrelizumab in PPMS trial. However, this will be a placebo-controlled trial with a 50% chance of being on placebo.

      I hope this helps.

    2. Make a private appointment with a neurologist. Costs about £200. Get a private prescription for Rituximab. Get administered free on NHS. It will halt progression for short term. However as it's mouse DNA your body will eventually produce antibodies and will cause disease breakthrough. However before that happens Ocrelizumab will of been licenced by Nice.

  2. What is the reason that in MS the meyelin in the CNS is destroyed and not in the PNS?


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