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A low-spinal immobilized turtle displays a fictive scratch reflex in hindlimb muscle nerves in response to mechanical stimulation of specific regions of the shell (Robertson et al., 1985). There are 3 forms of the scratch reflex: the rostral, the pocket, and the caudal; each exhibits rhythmic activation of hindlimb motor neurons. Cutaneous stimulation of the distal hindlimb elicits a fictive flexion reflex that exhibits tonic excitation of hip protractor (flexor) motor neurons and tonic inhibition of knee extensor motor neurons (Stein et al., 1982). In the present study, we describe the motor pattern blends that resulted from transient activation of either the ipsilateral or the contralateral flexion reflex pathway during ongoing scratch motor patterns. Two types of blends were observed: (1) insertions of a flexion reflex synergy into an interrupted scratch cycle and (2) deletions of parts of a scratch cycle. Associated with each type of motor pattern blend was a permanent reset of the ongoing scratch rhythm. The sign of the reset (phase-advance or phase-delay) could be predicted for all forms of the scratch based on the location of the foot stimulus (ipsi- or contralateral) and its timing relative to the hip protractor/retractor cycle. The timing of knee extensor activity within the hip cycle is different for each form of the scratch (Robertson et al., 1985); thus, the sign of the reset cannot be predicted from the timing of the stimulus relative to the knee extensor cycle. These results indicate the importance of the hip rhythm in determining the overall timing of the scratch reflex.(ABSTRACT TRUNCATED AT 250 WORDS)

Interruptions of fictive scratch motor rhythms by activation of cutaneous flexion reflex afferents in the turtle