What actually happens when my leg 'falls asleep'?

Question

Most people have experienced the temporary loss of feeling and tingling in their leg resulting from sitting in an abnormal position for a short while. Usually you get a loss of feeling in your leg while it is being compressed/constricted at some point and then the tingling sensation as the pressure is removed. But what is actually happening? I understand the the blood vessels are probably constricted from the pressure, but how does this lead to loss of feeling and later the strange tingling sensation? Are there any other things that extended compression on the leg does to cause this? What exactly are the requirements to get the sensation of a one's leg or other limb falling tingling in that manner?

Answer 1

The numbness and tingling you feel when your foot "falls asleep" is due to interference with the nerves that provide sensory innervation. When you sit in a weird position, you can compress these nerves and they stop sending the signals that our brains interpret as sensation. In some conditions, nerves responsible for sensation are chronically compressed, which can lead to neurological symptoms like numbness and pain. One such condition is sciatica, in which chronic compression of the sciatic nerve leads to recurrent numbness and pain in the lower limb.

After changing positions, restoration of nervous function and normal blood flow can result in a "pins and needles" sensation. You do not typically lose blood flow altogether since the circulatory system has multiple routes by which it can supply blood to a given tissue. So, if one artery is completely compressed, there's usually another one that can ensure a constant blood supply. If there is complete loss of circulation, this would eventually lead to tissue death due to oxygen deprivation.