Hi Mr. Larsen, I recently went on a run with my dad on a below-freezing day - which led to a conversation and speculation about why our fingers and toes had gone numb. I was wondering if you could give me a little bit of insight into the cell behavior that occurs when parts of your body go numb. Is it an inactivity only on the part of thermoreceptors? I would think other skin receptors would also be implicated since numbness also seems to preclude a sense of pressure and other forms of touch.
Hi Sabina! Thank you for the question!
There are a number of reasons for experiencing the sensation of numbness including nerve dysfunction, electrolyte imbalances, medications, toxins, and problems with blood flow. However, you asked specifically about numbness secondary to cold exposure.
Physiologically, exposure to cold will cause a shunting of blood (via vasoconstriction) away from the peripheral limbs which is meant to try to maintain a steady core temperature. Lower limb temperatures will stimulate thermoreceptors which results in thermogenesis and the aforementioned vasoconstriction of peripheral vessels. Essentially the peripheral tissues are not being perfused at the same rate that they would be normally.
Thermoreceptors are sensory receptors found on the skin. There is a class of transmembrane proteins called the Transient Receptor Potential family of proteins (TRP) which are agonist activated calcium channels. As the TRP proteins are a family of proteins, each subtype responds to different stimuli. For example, the TRPV1 protein is activated by heat (>43*C), pH >6, and capsaicin. TRPV2 and TRPV3 proteins are activated by heat but not by capsaicin. TRPM1 is known as melsatatin and is a tumor suppressor.
To get to your question, TRPM8 is a protein which is activated by cold (<30*C) and also by menthol. So, when cold or menthol is sensed by the receptor and protein, the channel opens and calcium is allowed to enter the cell which then allows an action potential to propagate and thus, the feeling of cold is experienced.
Hope this helps!