Whatever you were doing, there was no way to anticipate what came next. The back of your elbow smacked against a hard surface and the impact sent a sensation so distinctly disquieting shooting down your arm that it took several moments to regain composure. So, why does hitting this spot on your elbow— often referred to as your funny bone— feel so weird?
Well, to start, your elbow is one of the most complicated joints in your body. It's where your upper arm bone, the humerus, meets the two bones of your forearm. Three main nerves run through your elbow. One of them, the ulnar nerve, is particularly relevant for our purposes. Your ulnar nerve is composed of both sensory and motor nerve fibers. It extends from your spine to the tips of your fourth and fifth fingers, providing them with sensation along with swaths of your forearm and hand.
So, say your little finger gets crushed by a car door. Sensory receptors in your pinky finger's tissues register that unfortunate information and send electrical impulses up the ulnar nerve, which are then processed in your central nervous system. This leads to the perception of acute, sharp pain, followed by aching.
So that's how your ulnar nerve generally works. But your elbow has an anatomical quirk that leaves your ulnar nerve vulnerable to getting activated in a different way. Along almost all of its length, your ulnar nerve is protected, hidden beneath muscle, fat, and bone. Thankfully, this is generally true of your nerves. But when your ulnar nerve reaches your elbow, it runs beneath the bony bump at the bottom of your humerus, named the medial epicondyle, and it passes through a tunnel of ligament and muscle tissue called the cubital tunnel. When you bend your elbow, your ulnar nerve gets flattened against the medial epicondyle and the protective tissue in the area gets displaced, bringing the nerve especially close to the skin's surface. In this position, the ulnar nerve is covered only by skin and cubital tunnel ligament tissue, making it perhaps the longest area of semi-exposed nerve in your body.
So, when the area is struck at the right angle with enough force, something interesting happens. Instead of the sensation simply activating peripheral receptors that send electrical signals along your ulnar nerve, the nerve's fibers are stimulated directly. This is why hitting your so-called funny bone hits different. The exact mechanisms aren't yet fully understood. But the impact alters the ulnar nerve's signaling, perhaps by causing a proliferation of nerve signals, leading to that intense pins-and-needles sensation called transient paresthesia, which radiates down the length of your arm. In rare cases, whacks to this special spot have led to temporary loss of consciousness, perhaps because a hard enough strike could disrupt nerve signaling and blood flow.
In addition to these more immediately distressing moments, regular bent-elbow activities are thought to put pressure on the ulnar nerve over time. Ulnar nerve entrapment is a relatively common condition. It happens when the ulnar nerve becomes compressed and obstructed, most commonly with the cubital tunnel. The result of this chronic ulnar nerve pain can be persistent paresthesia, as well as weakness in the forearm and hand.
In the hopes of preventing such conditions, doctors recommend conscientious elbow usage. That includes stretching your arms and straightening your elbows, and generally taking breaks that help provide regular relief and maximal blood flow to your ulnar nerve.
