Name:________________________
Date:_________________________
Reflex Arc Lab
PURPOSE: To review the characteristics of reflex arcs and reflex behavior and to demonstrate some
of the reflexes that occur in the human body.
DIRECTIONS: Work with a laboratory partner to demonstrate each of the reflexes listed below. It is
important that muscles involved in the reflexes be totally relaxed in order to observe proper
responses. After each demonstration, record your observations in the table provided in Part B of
the laboratory report.
a. Knee-Jerk Reflex: Have your laboratory partner sit on a table (or sturdy chair) with legs
relaxed and hanging freely over the edge without touching the floor. Gently strike your
partner’s patellar ligament (just below the patella) with the blunt side of a rubber
percussion hammer. The normal response is a moderate extension of the leg at the knee
joint. Now repeat on the other leg.
b. Biceps- Jerk Reflex: Have your partner place a bare arm bent about 90 degrees at the elbow
on the table. Press your thumb on the inside of the elbow on the table. Press your thumb
on the inside of the elbow over the tendon of the biceps brachii and gently strike your
thumb with the rubber hammer. Watch the biceps brachii for a response. The response
might be a slight twitch of the muscle or flexion of the forearm at the elbow joint. Now
repeat on the other elbow.
c. Achilles-Jerk Reflex: Have your partner place one leg on the set of a chair so that the
knee faces down and the foot hangs off the edge. Lift your partners pant leg so their
ankle is showing. Lightly tap the hammer on the long Achilles tendon extending up
from the heel. The normal response is movement of the foot. Now repeat on the
other foot.
Reflex Tested
Left Side Observed
Right Side Observed
Effector Muscle
Response
Response
Involved
Knee-Jerk
Biceps-Jerk
Achilles-Jerk
Questions:
1. What is a reflex arc?
2. List the major events that occur in the knee-jerk reflex, from the striking of the patellar
ligament to the resulting response.
3. Were the responses of both the right and left side the same? Should both sides react
the same way?
4. What is the difference between polysynaptic and monosynaptic reflex arcs?