Have you ever wondered why your leg kicks when the doctor taps your knee with that
little rubber hammer? It's because of your reflex.
What's a Reflex?
A reflex is an involuntary (say: in-VAHL-un-ter-ee), or automatic,
action that your body does in response to something — without you even having
to think about it. You don't decide to kick your leg, it just kicks. There are many
types of reflexes and every healthy person has them. In fact, we're born with most
Reflexes protect your body from things that can harm it. For example, if you put
your hand on a hot stove, a reflex causes you to immediately remove your hand before
a "Hey, this is hot!" message even gets to your brain.
Other protective reflexes are blinking when something flies toward your eyes
or raising your arm if a ball is thrown your way. Even coughing and sneezing
are reflexes. They clear the airways of irritating things.
What's a DTR?
The reflex that the doctor checks by tapping your knee is called the patellar,
or knee-jerk, reflex. It is also known as a deep tendon
reflex (DTR) because the doctor is actually tapping on a tendon called the patellar
(say: puh-TEL-ur) tendon. This tap stretches the tendon and the muscle
in the thigh that connects to it. A message then gets sent to the spinal cord that
the muscle has been stretched.
The spinal cord very quickly sends a message back to the muscle telling it to contract.
The contraction of the muscle causes your lower leg to kick out. You might wonder
why such a reflex exists. This type of reflex is important in keeping your balance.
When you're standing up, gravity might cause your knee to bend slightly, and this
could make you fall if you didn't have the protective DTR to straighten that knee
and keep you standing upright.
A doctor often checks for DTRs to make sure that the nervous
system is working properly. Aside from the knee, they also can be checked along
the outside of the elbows, in the crooks of the arms, and at the wrists and ankles.
So the next time your doctor taps you with that little rubber hammer, say, "Hey
doc, how are my DTRs?"