Facilitation of the IA peroneal reflex by volleys in the IA gastrocnemius afferents
During the cycle of normal walking, the ankle is dorsiflexed approximately 0.30 sec before the heel-strike occurs (Murray, 1967). This dorsiflexion (Figure 1,A) begins to lessen only slightly during the last 0.10 sec prior to the heel-strike. As the heel touches the ground, however, the position of the ankle changes abruptly, and in approximately 0.05 sec it reaches the endpoint of its plantar flexion (Murray, 1967), also referred to as extension (Figure 1,B). A gradual shift from this extension to a 90° stance position follows. The latter provides for an adequate support of the body during the time that the other leg is in the swing phase. This sequence of changes in the position of the ankle (from flexed to extended to stance) should produce corresponding changes in the amount of stretch imposed on the ankle extensor and flexor muscles. Consequently, the stretch-sensitive primary endings of the muscle spindle receptors should send afferent volleys over the IA fibers and thus initiate a certain sequence of monosynaptic stretch reflex activity. Therefore, a question was raised as to how the interaction of the IA volleys at the motor neuron pools influences the contraction of those muscles which control the position of the ankle. This interaction was analyzed by studying the effects of a sequential activation of the gastrocnemius and the peroneal reflexes.
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