MS Fatigue is worsened by heat, both ambient and/or body temperature increases; it has a diurnal rhythm, being worse midday and easing at night; its progression mirrors the extend of DeMyelination; it is much worse immediately following an Exacerbation; and it slowly improves as you recover.
Primary Fatigue in Multiple Sclerosis is technically termed, Nerve Fiber Fatigue, and you probably recognize it as: the sudden inability to continue an activity.
Example: You are walking along (however slowly) and know you cannot take another step, suddenly each step requires way too much energy.
The reason is all Neural tissue, especially Neurons, are adversely affected by heat, since Nerve Impulses are dependent upon the efficient production of electricity.
This Conduction Block prevents injury and is only temporary. Normal functioning levels resume after a short rest - your system cools off and recovers - without sustaining permanent injury. (Why hot showers help Spasticity, without harm).
In either case, Sodium channels are redistributed along the Axon and additional ones are formed. Their firing rate (open & close) is also increased, to keep balance with corresponding Potassium channels.
Temperature Sensitivity results from the low safety margin for Conduction, which is due to the Sodium channel response to temperature.
When a Nerve Impulse triggers a Node, essentially all Sodium channels sequentially open and Sodium pours in, depolarizing the Axon. But, the rate Sodium channels close is very temperature dependent.
With an increase in temperature, they close faster, decreasing the time in which current can be produced; thus decreasing total current production, which is one cause of conduction failure.
Cooling has the opposite effect, increasing the time channels are open and improves Saltatory Conduction efficiency.
It is this Nerve Fiber Fatigue that accounts for the problem many MSers have, in walking more than short distances, or performing extended physical activity, and is know to us as heat sensitivity.