Heat sensitivity and Uhthoff’s phenomenon are closely related to conduction block in nerve fibers, particularly in the context of demyelinated axons such as those affected in multiple sclerosis (MS). When nerve fibers lose their myelin sheath, their ability to conduct electrical impulses efficiently is compromised. This demyelination makes the nerves highly sensitive to temperature changes, especially increases in temperature, which can lead to conduction block.
Conduction block occurs when an action potential, the electrical signal traveling along a nerve, fails to propagate past a certain point. In healthy myelinated nerves, the myelin sheath acts as insulation, allowing rapid and efficient signal transmission. However, in demyelinated nerves, the exposed axon segments have altered electrical properties. The ion channels that normally help propagate the signal become dysfunctional or insufficiently distributed, making the nerve vulnerable to disruptions.
Heat sensitivity in this context means that even a small rise in body or environmental temperature can impair nerve conduction. This happens because increased temperature affects the kinetics of ion channels and the electrical properties of the nerve membrane. Specifically, higher temperatures can accelerate the inactivation of sodium channels and increase the leak of potassium ions, both of which reduce the ability of the nerve to generate and propagate action potentials. As a result, the nerve conduction slows down or stops altogether, causing symptoms like weakness, numbness, or visual disturbances.
Uhthoff’s phenomenon is the clinical manifestation of this heat sensitivity. It describes the temporary worsening of neurological symptoms in people with demyelinating diseases when their body temperature rises, such as during exercise, fever, or exposure to hot weather. The underlying mechanism is essentially a temperature-induced conduction block in demyelinated axons. When the temperature increases, the already compromised nerve fibers fail to conduct signals properly, leading to a transient exacerbation of symptoms.
The relationship between heat sensitivity, Uhthoff’s phenomenon, and conduction block can be understood through the lens of nerve physiology and biophysics. Demyelination reduces the safety factor for conduction, meaning the margin by which the nerve can reliably transmit signals is diminished. Heat further lowers this safety factor by altering ion channel function and membrane excitability, pushing the nerve into a state where conduction fails. This failure is reversible if the temperature returns to normal, which explains why symptoms improve once the heat stress is removed.
In summary, heat sensitivity and Uhthoff’s phenomenon are expressions of conduction block caused by temperature-induced dysfunction in demyelinated nerve fibers. The loss of myelin disrupts normal electrical conduction, and increased temperature exacerbates this disruption by affecting ion channel behavior and membrane properties, leading to temporary failure of nerve signal transmission.
