Knocking your elbow against a table ledge results in a yelping pain that often feels worse than it should. We blame it on the “funny bone,” but it's neither funny nor a bone. So what's the joke?
The culprit: your ulnar nerve. Located at the elbow, it earns the title of the largest exposed nerve, as it circles around the inside edge of the ulna bone without the usual protective padding of muscle.
A main function of most nerves is to provide a host of sensations, like tickles, temperatures, pain, and vibrations. So when nerves are directly twanged, they scream alarm. The ulnar nerve, layered only between skin and bone as it rounds the elbow, does just what it's made to do when bumped at the right spot.
What's so funny? Some believe “funny” refers to the peculiar quality of the pain, and others believe it may be called the “funny bone” because the elbow connects the ulna bone with the humerus, the upper bone of the arm, which sounds like “humorous.” The ulnar nerve flows to near the tip of the fifth finger and splits the innervation of the fourth finger with another nerve. That's why when the funny bone is whacked, the pain shoots down the arm into the pinky and the adjacent half of the ring finger.
Then there's the arm (or foot) that goes to sleep. Maybe after a heavy sleep, or from just sitting too long, the arm just won't work right, or it starts feeling tingly all over. Often it's due to pressure, even from the body itself when lying in bed, which disrupts the nerve pathways and circulation to that part of the body. Sleeping with arms over the head can limit the flow of blood to the hands, also resulting in numbness.
The tingling sensation is because some nerve cells are transmitting messages and some aren't. The brain gets an erratic mix of signals, so it sends that “pins and needles” sensation, telling you to move the arm around to improve circulation. Movement fires up the nerves, improves the circulation, and typically in short order everything is back to normal.
Keep in mind that occasionally such numbness can mean something else. One in 7 strokes occurs overnight while we sleep. A stroke occurs when the brain does not get enough oxygen, such as from a blocked artery. This may result in numbness or tingling, often on just one side of the body, or difficult speaking, or a drooping face, or a headache. If waking up is accompanied by an arm that is asleep and other stroke symptoms are found, someone should call 911 for emergency help.