Central-Peripheral Interactions Influencing the Locomotor Rhythm
Proprioceptive information may act to adjust the timing of retraction and protraction transitions during the step cycle (Grillner and Rossignol, 1978; Duysens and Pearson, 1980). Transitions within the turtle swimming cycle are altered by electrical stimuli applied to the ambiens (knee extensor/femoral adductor) muscle-nerve at different times within the locomotor cycle (Lennard, 1985). A stimulus delivered midway during the power-stroke shortens cycle period and advances the onset time of the next swim cycle. The same muscle-nerve stimulus delivered late in the returnstroke lengthens cycle period and delays the onset of the protraction/retraction transition. In the turtle, muscle spindles are the most likely receptor system mediating these effects on the locomotor rhythm (see discussion of Lennard, 1985). Lennard has hypothesized that an increased ambiens spindle afferent discharge during mid to late powerstroke could signal the completion of the powerstroke and aid in the initiation of the retraction/protraction transition. The additional spindle input to the spinal locomotor network produced by muscle-nerve stimulation would “incorrectly” signal a limb position appropriate for the termination of the powerstroke and advance the transition to the returnstroke.
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