HOW COME? Hitting the funny bone

How come it hurts so much when I hit my "funny bone"? asks John Fretto, a student in Brookville, NYBump your shin on the coffee table, and you feel a sharp pain in the front of your leg. Ouch. But bang your inner elbow on your desk, and the pain is searing, buzzing, electrical -- and may extend down your arm and into your fingers. Double ouch.

And just what makes that funny? Some say the dreaded spot is called the funny bone in honor of the humerus bone, which runs from shoulder to elbow. (How humorous.) But the real reason is probably because the feeling we get from hitting it is not only painful, but downright weird.

If the funny bone were actually a bone, the pain we'd feel would be a lot like shin pain. Instead, what we experience is, in a way, worse: pain from a dinged nerve.

The culprit is the ulnar nerve, named after the ulna bone it runs near in the forearm. The ulnar is one of three big nerves (the others are the radian and the median) that run from the spine, down our arms, and to our hands. The ulnar nerve's main function is to control muscles in the forearm and hand, as well as enable our pinkie and ring fingers to feel sensations.

Our body's large nerves are usually protected by bone and muscle. But the ulnar nerve is mostly defenseless. In fact, you can actually (creepily) feel it, by pressing on the grooved space on the inside of your elbow.

(Fast fact: The ulnar nerve runs through several grooves on its way to the fingers: The cubital tunnel, in the elbow, and the ulnar, or Guyan's canal, in the wrist.)

What's not so funny: All that stands between this exquisitely sensitive nerve and a hard table top is a thin layer of fat and skin. Bump your inner elbow on a desk, and the big nerve is slammed between solid bone and solid wood. The result: A starburst of searing, electric pain in your elbow, which shoots down your arm, and into the fourth and fifth fingers of your hand. (Are we having fun yet?)

The pain from your so-called funny bone only lasts a few seconds. But the unprotected ulnar can also become compressed and irritated by everyday activities. Like lying half-propped up on your elbow to read. Keeping your elbow bent for many minutes, holding your phone to your ear. Or sleeping for hours on your bent arm. Chronic elbow pain, or tingling numbness in the pinkie and ring fingers, or along the outside of the hand, can result. Remembering that the ulnar has no armor can help us to protect our funny-bone nerve from long-term injury.