Fig. 1.
Characterization of the effects of muscimol: the acute (a–e) and chronic (f) effects of muscimol on motoneuron bursting in an isolated cord preparation. a, Neurogram recording from the femorotibialis muscle nerve of an episode of spontaneous bursting from a st. 32 chick embryo under control conditions (top trace) and in the presence of 5 × 10−6m muscimol (bottom trace). Note the absence of any bursts. b, Mean time interval ± SEM between spontaneous bursting episodes under control (open bars) and muscimol treatments (dark bars) at st. 30, 32, and 34 (n = 7, 6, and 11, respectively). c, A single stimulus (arrowhead) to the cervical/thoracic spinal cord of a st. 32 chick embryo elicits a bursting episode before (toptrace), but not after (bottom trace), the application of 5 × 10−6m muscimol. d, In the presence of 5 × 10−6mmuscimol, 1/sec stimulation of the motoneurons directly at LS2 elicits two peaks (top trace; single asterisk denotes second peak); the second peak (two asterisks), but not the first, fatigues with 10 Hz stimulation (bottom trace).e, In the presence of 5 × 10−6m muscimol and low Ca2+ Tyrode’s solution, 1/sec stimulation of the motoneurons directly at LS2 elicits one peak (top trace), which does not fatigue at 10 Hz stimulation (bottom trace); st. 32 chick embryo for a, c, d, and e. Calibration bars: c(also applies to a), 1 sec; d, e, 2 msec. Gain for d, e, and g is 5× that ofa–c and f. f, Effect of chronic muscimol treatment as follows: at st. 33 (top traces), no bursting activity occurred spontaneously nor could it be elicited by stimulation; at st. 31(middle traces), no bursting occurred spontaneously or after single stimuli. However, sometimes with multiple stimuli (arrowheads) a very short burst could be elicited, in this case after the fourth stimulus. This very short burst (enclosed in open gray box) is displayed on an expanded time scale at the end of the trace (shaded gray box). The very brief length of these bursts can be seen by comparing this trace with the control inc. At st. 29 (bottom traces) very brief bursts occurred spontaneously (left) and after stimulation of descending input (right).g, Effect of muscimol on synaptic transmission at the NMJ was assessed by recording the compound action potential elicited in a st. 29 sartorius muscle by stimulation (arrowhead) of the LS1 spinal nerve. Under both control conditions and in the presence of 5 × 10−6m muscimol, the sartorius muscle exhibited a vigorous response. However, the addition of 5 × 10−6 mdTC blocked the postsynaptic muscle response. Calibration bar, 5 msec.