Ask Dr. H: Anatomy of a knee-jerk reaction

Question:

I've always found it kind of funny the way my leg jumps when my doctor taps on the knee with his rubber hammer. What causes the knee to jerk like that?

Answer: It's a mighty fast reflex, averaging five one-hundredths of a second between the hammer tap and the start of the leg kick. The biceps tendon, the triceps tendon, and the Achilles tendon all do the same thing. The reflex itself is pretty simple: when your doctor strikes the patellar tendon with his rubber hammer, it causes a momentary tension in the tendon.

The nerve receptors within the quadriceps thigh muscle detect this and send a message to the spinal cord, which in turn relays a signal to the muscle that it needs to move (contract). But since the tension was momentary, the muscle then quickly relaxes when it realizes that it was all just a false alarm. It's all happening so fast that the brain, while informed of what's happening, takes no part in the actual knee-jerk response. Bypassing the brain makes the response quicker.

Amusement aside, we need these lightning-fast reflexes. If you've ever burned yourself touching something very hot, you can appreciate the benefit of fast reflexes. Although many reflexes are protective, some are merely to help coordinate complex events, such as swallowing a piece of food. Doctors look at the strength and speed of a reflex to determine whether there is a neurological problem.

With some folks, I just can't easily get a reflex. It's not that they've got anything wrong with them; they're just too focused on what I'm doing to relax. For others, a decreased reflex may indicate a peripheral nerve problem. If a person has leg or arm weakness and pain along with a decreased reflex, a compressed nerve root may be the cause. Spinal-cord and brain injuries tend to cause exaggerated reflexes. Slow or delayed reflexes suggest a possible underactive thyroid. It's important to look at the reflexes in the context of a complete medical exam.

nolead begins