GLYCINERGIC INHIBITION CONTRIBUTES TO THE GENERATION OF ROSTRAL SCRATCH MOTOR PATTERNS IN THE TURTLE SPINAL-CORD

Cutaneous stimulation within the rostral scratch receptive field in a low spinal-immobilized turtle elicits a fictive rostral scratch reflex characterized by robust rhythmic motor output from ipsilateral hindli mb muscle nerves and weaker, alternating motor discharge in contralate ral nerves. Simultaneous bilateral stimulation elicits bilateral rostr al scratch motor patterns in which activity on the right and left side s alternates. We investigated the role of glycinergic inhibition in th e generation and coordination of fictive rostral scratch motor pattern s. Glycine (2 or 5 mM) and strychnine (5-50 mu M), a glycine antagonis t, were superfused over the anterior spinal hindlimb enlargement while fictive rostral scratch motor output was recorded bilaterally from hi ndlimb muscle nerves in the form of electroneurograms (ENGs). Although glycine reduced rostral scratch burst frequencies, strychnine tended to increase burst frequency. Strychnine also changed the shape of hip flexor ENG bursts, resulting in more abrupt burst onsets, indicating a n earlier recruitment of motor neurons with large ENG spikes. During b ilateral stimulation, strychnine increased the variability of interlim b phase values (left vs right hip flexor bursts) but did not abolish r ight-left alternation. These results indicate that glycinergic neurons in or near the anterior hindlimb enlargement contribute to the overal l timing of the rostral scratch rhythm and to the recruitment timing o f individual hip flexor motor neurons within each scratch burst. Our d ata also indicate that glycinergic mechanisms contribute to, but are n ot critically important for, maintaining an alternating interlimb coor dination during bilateral scratch motor patterns.