Question

When a physician elicits the knee-jerk reflex by tapping deep tendons in the knee, the normal response is for the lower leg to swing forward. When this happens:
A. Muscles in the back of the lower leg are contracting but those in the front are relaxing.
B. Muscles in the front of the lower leg are contracting but those in the back are relaxing.
C. Muscles in the back of the thigh are contracting but those in the front are relaxing.
D. Muscles in the front of the thigh are contracting but those in the back are relaxing.

Answer 1

Hint: Reflex is an involuntary, spontaneous reaction, response shown by the body to a given stimulus. Knee jerk reflex or patellar reflex is a common example for this. On tapping the patellar tendons, the lower leg kicks forward spontaneously.

Complete Step by Step Answer:
Knee jerk reflex is a complex example to showcase how different systems in a body work together. In this case, when the tendon of the patella is tapped, it gets stretched. This sends an impulse via the sensory neurons to the spinal cord.

The motor neurons associated (L3 and L4) then cause the following activity:
The quadriceps muscles (quadriceps femoris)present in the thigh get contracted while the muscles at the back stay in a relaxed position.

Thus, the correct answer is D: Muscles in the front of the thigh are contracting but those in the back are relaxing.

Note: It is potent for the reflex actions to work efficiently as these protect us from harmful circumstances, in an extremely amount of time. For example: Withdrawal of our hand after touching a hot object. If it were not for the reflex mechanisms, we would have hurt ourselves with some or the other harmful stimulus due to an increased time lag in trying to save ourselves.