Central program for scratch reflex in turtle

Abstract

1. A scratch reflex is displayed by a low spinal turtle in response to gentle mechanical stimulation delivered to specific regions of the shell anterior to a hindlimb (Valk-Fai and Crowe, 1978, 1979). The behavior consists of a rhythmic limb movement in which the foot protracts and rubs against the stimulated patch of shell once per cycle. A brief stimulus can elicit a response composed of a small number of cycles or even a fraction of a cycle (Fig. 1); maintained rubbing of the shell can elicit a response consisting of 5–20 cycles (Fig. 2A). 2. The scratch motor program is divisible into an A phase in which IT-KE is active and a B phase in which IT-KE is quiescent and HR-KF is active (Fig. 2B). The A phase is further divisible into an A1 phase in which IT-KE is the only active KE muscle and into an A2 phase in which all three KE muscles are active. 3. Two variations of the scratch program exist (Figs. 3 and 4). These variations indicate that there are flexibilities in the organization of the spinal generator for turtle scratch reflex. 4. The scratch motor program can also be expressed in a preparation immobilized by a neuromuscular blocking agent (Figs. 5 and 6). The responses in this preparation are termed the fictive scratch reflex since they occur in the absence of a real movement. One of the variations in the scratch program which was seen in preparations with a moving limb was also seen in the fictive scratch reflex (Fig. 7). 5. The scratch motor program can also be expressed in preparations with the hindlimb enlargement deafferented via bilateral dorsal rhizotomies (Figs. 8 and 9). 6. These observations establish that the turtle scratch reflex is centrally programmed, i.e., it is not dependent upon phasic timing cues derived from sensory feedback. Even though the reflex is centrally programmed, it is still possible to demonstrate that sensory feedback from a moving limb can modulate the scratch rhythm (Fig. 10 of this paper and Fig. 6 of Valk-Fai and Crowe, 1979).