lkirkpat@udenva.cair.du.edu (lkirkpat) (12/04/88)
This is probably a very dumb question. I'm not even sure if sci.bio is the right place for it, but it's the closest I can come among the newsgroups I read. Here goes: What does it mean, physiologically, when your arm or leg "falls asleep"? Is it a "pinched" nerve, constricted blood circulation, and/or something else? Sorry to litter the net with such drivel, but I've just got to know. Thanks.
dbf@myrias.UUCP (David Ferrier) (12/06/88)
In article <11098@udenva.cair.du.edu> lkirkpat@udenva.UUCP (Lee Kirkpatrick) writes: >What does it mean, physiologically, when your arm or leg "falls >asleep"? Is it a "pinched" nerve, constricted blood circulation, >and/or something else? There are three sensations associated with a limb falling asleep: initial numbness or absense of sensation, difficulty with moving the limb, pins and needles as sensation returns. All are due to a temporary malfunction of the nerves in the limb. The malfunction is a result of a lack of oxygen caused by a diminished blood supply. This decreased blood supply is usually due to constriction of a major artery to the limb by compression, for examply, by sitting the wrong way for a period of time. Nerves cells are constantly doing work to create the electrical potentials required to transmit nerve impulses. To do this work, they require energy. They get this energy by metabolizing at a high rate, that is, through oxidation processes that combine lots of oxygen with other substances. For this reason, nerves are more quickly affected by lack of oxygen than most other tissue and they stop working pretty fast if you cut off or decrease the blood supply by lying on the limb or whatever. When they stop working, it is like having the telephone line down between your brain and the sensors and muscles in the limb. No information gets back to the brain, which is interpreted as the sensation of numbness. Likewise, it is difficult or impossible to move the parts of the limb actuated by muscles which are served by affected nerves. No information gets through to the muscles, so no movement occurs. When the blood supply is restored, the nerves do not recover instantly and while recovering go through a phase of firing off nerve impulses randomly and not due to any real sensation. This random impulse pattern is interpreted by the brain as something happening in the limb, giving the pins and needles feeling. You might reflect on the fact that any part of the body containing nerve cells works this way, including the brain. That is why marital arts choke holds are effective so quickly: they constrict the carotid arteries in the neck, decreasing or even cutting off the supply of blood to the brain, which consists entirely of nerve tissues. In a matter of seconds, the brain's nerve cells stop working properly and the person blacks out. The same phenomenon is the cause of high-g manoeuvre blackouts among jet pilots. They wear g suits to compress the rest of the body and fight the pooling of blood in the lower anatomy which causes the shortage of blood in the brain and the blackouts. On a more prosaic level, this is also why some people get dizzy when they stand up too quickly. It's called postural hypotension; even the slight diminution of blood pressure caused by standing up is enough to affect the function of the never cells in the brain slightly.