MUSCLE RESPONSE TO CHANGING NEURONAL INPUT IN THE LOBSTER (PANULIRUS-INTERRUPTUS) STOMATOGASTRIC SYSTEM - SLOW MUSCLE PROPERTIES CAN TRANSFORM RHYTHMIC INPUT INTO TONIC OUTPUT

Slow, non-twitch muscles are widespread in lower vertebrates and inver tebrates and are often assumed to be primarily involved in posture or slow motor patterns. However, in several preparations, including some well known invertebrate ''model'' preparations, slow muscles are drive n by rapid, rhythmic inputs. The response of slow muscles to such inpu ts is little understood. We are investigating this issue with a slow s tomatogastric muscle (cpv1b) driven by a relatively rapid, rhythmic ne ural pattern. A simple model suggests that as cycle period decreases, slow muscle contractions show increasing intercontraction temporal sum mation and at steady state consist of phasic contractions overlying a tonic contracture. We identify five components of these contractions: total, average, tonic, and phasic amplitudes, and percent phasic (phas ic amplitude divided by total amplitude). cpv1b muscle contractions in duced by spontaneous rhythmic neural input in vitro consist of phasic and tonic components. Nerve stimulation at varying cycle periods and c onstant duty cycle shows that a tonic component is always present, and at short periods the muscle transforms rhythmic input into almost com pletely tonic output. Varying spike frequency, spike number, and cycle period show that frequency codes total, average, and tonic amplitudes , number codes phasic amplitude, and period codes percent phasic. Thes e data suggest that tonic contraction may be a property of slow muscle s driven by rapid, rhythmic input, and in these cases it is necessary to identify the various contraction components and their neural coding . Furthermore, the parameters that code these components are interdepe ndent, and control of slow muscle contraction is thus likely complex.