You’re sitting cross-legged on the floor playing a game with your child or curled up on the sofa watching a video. But when you try to stand up, you can’t take a step — your foot has “fallen asleep.” So you hop around on one leg, hoping to shake off the prickling sensation in the other.
WHAT CAUSES those pins and needles? Dr. Robert Daroff, a professor of neurology at Case Western Reserve University in Cleveland, explains that when sustained pressure is exerted on a portion of your leg, or any distal limb, one of two things usually happens. In some cases, arteries can become compressed, making them unable to supply local tissues and nerve cells with the nutrients (mainly oxygen and glucose) they need to function properly.
Other times, nerve pathways can become blocked, preventing normal transmission of electrochemical impulses to the brain. Some of the nerves, starved or pinched or both, stop firing while others fire hyperactively. This leads to mixed signals being sent to the brain, where they are interpreted as burning, prickling or tingling feelings. It is these sensations, known medically as paresthesia, that alert you to move your foot.
When you do so, the pressure on your arteries and nerves is released. As nutrient-rich blood flows back into the area and nerve cells start firing more regularly, the “pins and needles” feeling often intensifies until the neurons re-establish their normal transmission of electrochemical impulses.
True, this is an uncomfortable process, but it’s actually beneficial. If we didn’t feel discomfort, we wouldn’t adjust our position — and harm could result. “Permanent damage can occur if blood flow is restricted and nerves are compressed for many hours,” according to Daroff.
In fact, “Saturday night palsy” is a name given to a condition that results from permanent injuries incurred when someone has passed out in an awkward position and is sleeping too deeply to respond to the body’s signals to move.
Do you have “Cosmic Questions” about the universe’s great mysteries — such as the prospects for interstellar travel? Or are you wondering about “Common Curiosities,” such as the reasons we snore and what we can do about it? In either case, e-mail your suggestions to mysteries@msnbc.com, or discuss your questions and answers on the Mysteries of the Universe Bulletin Board. Your topic could become the subject of a future installment of “Mysteries of the Universe.”