Motor neuron synaptic potentials during fictive scratch reflex in turtle

1. A scratch reflex motor program, termed the 'fictive' scratch reflex (Stein and Grossman 1980), is displayed by an immobilized low spinal turtle in response to gentle mechanical stimulation delivered to specific regions of the shell. The fictive scratch is a cyclic program; each cycle is divisible into three phases, the A1, the A2 and the B phases (Fig. 1). Rhythmic A1 and A2 activities may be produced even when the B phase is deleted (Fig. 2). 2. Intracellular recordings from hindlimb motor neurons during the fictive scratch reveal that each motor neuron is depolarized to fire action potentials during its active period and is hyperpolarized during at least part of its quiescent period (Figs. 3-5). Moreover, the activation pattern of A1 motor neurons is the inverse of the activation pattern of B motor neurons (Figs. 3 and 5). The synaptic activation patterns of motor neurons during the A1 and A2 phases are preserved even during B phase deletions (Fig. 6). 3. A fictive flexion reflex motor program is produced in this preparation in response to gentle mechanical pressure applied to the dorsum of the foot (Figs. 7 and 8). The synergies observed in fictive flexion reflex differ from those observed during the fictive scratch reflex. 4. These data support a model for turtle of a three phase scratch generator that is asymmetrically arranged. A similar conclusion has been reached in studies of the scratch program generator in the cat (Berkinblit et al. 1978a, b). Our data also indicate that the motor neuron activation pattern of flexion reflex is different from that of scratch reflex. Therefore data obtained from turtle flexion reflex can not be utilized to construct a model of the turtle scratch generator.