In multiple sclerosis(MS), myelin sheath is attacked and damaged. When this happens, there is a decrease in the amount of information reaching the brain from sensory receptors. How and why does a decrease in the amount of information occur?

I know that myelin sheath speeds up the conduction of nerves by which the action potentials can jump from node to node by saltatory conduction by their local circuits; but how when this stops happening action potentials and information can be lost (I believe that it must have not decreased in information but the whole amount of information will reach the brain but slowly because of lower speed of conduction)?

I got more mixed up when I saw on a website that:

The myelin sheath prevents these impulses from escaping from the nerve at the wrong point.

Can please someone tell me how this happens? And how in the absence of myelin sheath or it being damaged, how the nerve impulse can escape from the nerve?




2 Answers 2


As you write in your question, myelin is an insulating sheath that speeds neuronal transmission.

However, the mechanism for transmission in myelinated axons is very different from unmyelinated fibers. In an unmyelinated axon, voltage-gated channels are present along the entire length of the axons; in a myelinated axon, these channels are concentrated and only present at nodes of Ranvier.

The way that myelination speeds up transmission is by not allowing ions to leak out along the distance between nodes of Ranvier. More accurately, myelination reduces the capacitance (see cable theory) of the membrane. However, electrical transmission in neurons is still ultimately driven by ions moving across membranes, so the nodes of Ranvier are the locations where that movement can happen at gaps in the myelin sheath. This is called saltatory conduction.

The fastest possible conduction velocity would be for nodes of Ranvier to be as far apart as possible without the signal from one node failing to reach the next one. Nodes of Ranvier are roughly spaced according to this distance, with a bit of safety margin built in. However, if the integrity of the myelin sheath is damaged, the distance that an action potential can propagate in that damaged stretch of axon may be shorter than the actual distance between nodes of Ranvier. In that case, there is not sufficient depolarization at the next node to trigger voltage-gated channels to open. This causes a "failure" in transmission.


I will try to explain in layman's terms

Myelin sheath is the protective covering which basically works like the rubber insulation of a typical electrical wire.

Now here's how the impulse in neuron network works - I tried to explain in points

  1. In the normal condition, the outer side of neuron has Sodium ion i.e. positive charge and the inside of neuron has negative charge. This phase is called the resting phase and the neuron is said to be polarized

  2. On stimulation, this polarization breaks and the axon membrane (myelin sheath) becomes more permeable to sodium ions. This is called depolarized state

  3. This depolarized spot becomes stimulation for the immediate next spot and it follows so on.

  4. Meanwhile, the previous spot regains its polarity because sodium ions are "pumped" out using energy from ATP

This simply means that damage to myelin sheath will sure cause disturbances in polarization and depolarization, which means improper impulse or may be delay in transmission time.

Hope I could help


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