Question

Attachment of myelin to the surface of some neurons occurs via the interactions of transmembrane adhesion molecules expressed on the myelin membrane with receptors on the target axons. Several human peripheral neuropathies are associated with mutations in genes encoding theses myelin-associated proteins that disrupt myelin-axon attachment, (1) Explain the symptoms this loss of peripheral myelination might be expected cause and why, given the function of myelin in the nervous system. (2) Propose a hypothesis to explain why only peripheral and not central nervous system symptoms are observed in these diseases.

Answer 1

Ans 1) A range of symptoms may present-

Parasthesia- ‘pins and needle sensations’ are experienced by the patients, they may describe it as an ant walking on their skin.

Numbness- If myelin loss increases they can have decreased sensations. This is classic in old diabetic patients who sensations in their lower limbs.

Pain- due to the myelin loss going on either inflammation occurs or pain occurs as an abnormal sensation.

Weakness- Due to nerve damage the patients experience motor weakness in their limbs.

Inability to walk- Due to weakness in the limbs.

All these symptoms occur due to loss or destruction of myelin sheath resulting in abnormal sensations-parasthesias, reduced or complete loss of sensations and functions.

Myelin sheath acts as the insulation, nerve impulses travel much faster and efficiently through it. If it’s absent nerve impulses slow down even die out before reaching the target organ.

Loss of myelin associated proteins destabilise the the tertiary structure of myelin, resulting in it’s loss.

Ans 2) There are disorders which primarily affect peripheral nerves. And others which only cause lesions in the CNS.

But in disorders affecting peripheral nerves the demarcation is not very clear. Neuropathy also occurs in the cerebellum- leading to cerebellar ataxia that results in loss of coordination and balance. Other CNS lesions also occur.