Assertion A: When a flexor muscle is excited, the probability of excitation decreases in the paired extensor muscle.
Reason R: The interneuron that excited a motor neuron to the flexor muscle also inhibited a motor neuron connected to the extensor muscle.
In the light of the above statements, choose the most appropriate answer from the options given below:
Reciprocal Innervation: Flexor and Extensor Muscles
This question concerns the physiological relationship between flexor and extensor muscles, a concept known as reciprocal innervation.
Analysis of Assertion A
Assertion A states that when a flexor muscle is excited, the probability of excitation decreases in the paired extensor muscle. This is a fundamental principle of motor control. For coordinated movement, when one set of muscles contracts (agonist, e.g., flexor), the opposing set (antagonist, e.g., extensor) must relax or be inhibited. Therefore, Assertion A is correct.
Analysis of Reason R
Reason R explains the mechanism behind this phenomenon. It states that the interneuron excited by the flexor muscle's pathway also inhibits the motor neuron connected to the extensor muscle. This specific neural circuit is called reciprocal inhibition. Sensory afferents (like Ia afferents from muscle spindles) or descending pathways activate excitatory interneurons to the agonist's motor neurons and inhibitory interneurons (often Ia inhibitory interneurons) to the antagonist's motor neurons. Thus, Reason R accurately describes the neural basis for reciprocal innervation and is correct.
Evaluating the Explanation
Reason R provides the direct physiological mechanism (inhibition of the extensor motor neuron via an interneuron) that causes the effect described in Assertion A (decreased excitation probability in the extensor muscle). Therefore, Reason R is the correct explanation of Assertion A.
Conclusion
Both Assertion A and Reason R are correct statements, and Reason R provides the accurate explanation for Assertion A.
Final Answer Choice
Based on the analysis, the most appropriate choice is that both A and R are correct, and R is the correct explanation of A.
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