Health Library

What happens when your healthcare provider taps on your knee with a rubber mallet?
Your leg kicks forward, seemingly on its own. And in a sense, your leg has a mind
of its own — in your spinal cord.

When the mallet hits your knee, it stretches the tendon just below the kneecap. That
causes a signal to travel along a nerve to your spine, where a waiting motor neuron
sends out an automatic command to contract the muscle attached to the tendon. When
the muscle contracts, your leg kicks.

You are born with such “hard-wired” reflexes. Most are located in the spinal cord,
but some are in the motor centers of your brain. They work to protect your body from
injury. And they also form the basis for more complicated physical activities, like
standing, walking, or riding a bike.

Your brain’s role

Your brain gets involved by modifying and fine-tuning reflex actions. For example,
when you trip and fall, reflexes automatically command your hands and arms to reach
out and break your fall. Muscles will contract throughout your body to minimize injury.
But what if you were carrying a priceless object, say, a Ming vase? Would you drop
the vase in order to use your hands to break the fall?

Not necessarily, experts say. Within 10 to 30 milliseconds after tripping, the conscious
motor centers of the brain would take control of the fall, weighing the chances of
breaking the vase versus breaking your neck.

If the object in your hands is important enough to you, you can modify the reflex
action and keep a grip on the vase.

Modifying reaction time

Reflexes do play a role in reaction time. Some people are born with faster reflexes.
Electrical impulses actually travel more quickly through their nerves. But you can
also speed up nerve conduction through practice.

A soccer player, for example, can improve his running or kicking, and in the process,
his knee jerk might get faster. But those kinds of improvements are specific to the
activity. A soccer player’s feet and legs might develop faster nerve conduction than
average. But if that same soccer player were to engage in a contest of finger speed
with a classical pianist, the pianist would win, hands down.

The real key to reaction time is practice. By repeating the same movements, you make
them almost automatic. That’s why professional baseball players can dive to catch
a sizzling line drive. And it’s also why once you learn to ride a bike, you never
forget.

These actions aren’t classical reflexes, but with so much practice, your movements
almost mimic a reflex. They are motor skills that have been etched into your nerves
and brain so that those motor pathways are almost reflexive.

Reflexes and age

Reflexes do slow with age. Physical changes in nerve fibers slow the speed of conduction.
And the parts of the brain involved in motor control lose cells over time. But the
effect of age on reflexes and reaction time varies tremendously from person to person.
You can actually slow down — even reverse — the effects of aging by staying physically
active.

Remember the saying: If you don’t use it, you’ll lose it.