Human Reflexes

These demonstrations show how the nervous system controls involuntary reflexes such as the knee-jerk, pupil response, and blinking.

1. For the knee-jerk reflex: Have a student sit with legs dangling. Gently tap the tendon just below the kneecap with the side of your hand or a soft object. Watch for the leg to kick forward.
2. For the pupillary light reflex: Have a student cover one eye with their hand for about 30 seconds, then quickly remove it while observing the pupil in a mirror or with a partner’s help. Notice the pupil constricting in response to light.
3. For the blink reflex: Move your hand quickly but safely toward a student’s eyes (without touching). Observe the automatic blink.

How Reflexes Work (Knee jerk and eye blink) - Interactive Biology:

• Measure reaction time with the Dropping Ruler Reaction Time experiment.

• Ensure no hard or sharp objects are near the eyes during the blink reflex test.
• Tap the knee tendon gently to avoid discomfort.
• Use caution when dropping the ruler to avoid hitting fingers.

• How is a reflex different from a voluntary movement? (Reflexes are automatic and involve the spinal cord, while voluntary movements require brain input.)
• Why does the knee move when the tendon is tapped? (The tap stretches the tendon, triggering a spinal reflex that contracts the thigh muscle.)
• What does the pupil’s reaction to light show about the nervous system? (It demonstrates automatic control by the brainstem.)
• Why might your reaction time change if you are distracted? (Attention and brain processing affect voluntary reactions but not reflexes.)