During a knee-jerk reflex, a doctor taps the patellar tendon causing a brief stretch of the quadriceps tendon. If the sensory conduction velocity is , the motor conduction velocity is , and the distance from the knee to the spinal cord is , estimate the total time delay from the stretch to muscle contraction. Assume the interneuron conduction speed is negligible.
Answer
The correct answer is — The calculation assumes negligible interneuron delay, which is valid for a simple monosynaptic reflex.
Step-by-step solution
- The time taken by the sensory neuron is distance ÷ velocity = (≈5.7 ms).
- The time taken by the motor neuron is (≈6.7 ms).
- Total time delay = .
- Hence the delay is approximately .
- The calculation assumes negligible interneuron delay, which is valid for a simple monosynaptic reflex.
Common mistake: Adding distances instead of adding times
Concept tested: Conduction velocity and reflex delay
Published · How this solution was prepared and checked
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