The knee jerk reflex is a monosynaptic reflex arc that takes about 32 millisecon

Question

The knee jerk reflex is a monosynaptic reflex arc that takes about 32 milliseconds to occur in humans. This time delay occurs because it takes about 2 millisecond for the receptor to sense stretch and initiate an electrical signal, about 6 milliseconds to conduct the afferent signal to the spinal cord, 1 millisecond for synaptic transmission to the efferent neuron, 10 milliseconds through the efferent neuron, 2 milliseconds for neurotransmission, to the muscle, and 12 milliseconds between the muscle action potential, and the onset o contraction. Imagine that instead of being a monosynaptic reflex arc, the knee-jerk reflex arc was due to a poly synaptic flex arc. How would this affect the time needed to complete the reflex? Provide a quantitative estimate of the change needed for the reflex, if any. What might be an advantage of a polysynaptic arc?

Explanation / Answer

The polysynaptic reflex contains two or more synapses. Therefore, it will take longer reaction time than the monosynaptic reflex because of synaptic delay.

Polysynaptic reflexes provide survival advantage over monosynaptic reflexes. They cause complicated flexion and crossed extension. For example, in withdrawal reflex there is flexion of the stimulated arm and extension of the opposite arm. This takes place involuntarily to ward off the danger.

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