Oscillatory command input to the motor pattern generators of the crustacean stomatogastric ganglion

Summary

• 1. In the rock lobsterHomarus gammarus, the gastric rhythm controls the chewing movements of three cuticular teeth in the stomach (gastric mill). The rhythmic motor output (Fig. 3) arises from a network of neurones, the gastric pattern generator (GPG), located within the stomatogastric ganglion. In addition our in vitro recordings indicate that in each of the two commissural ganglia there is a gastric interneurone (CG) projecting to the stomatogastric ganglion (Fig. 4).

• 2. The following physiological characteristics serve to identify this commissural gastric neurone (CG): (1) bursting activity phase-locked with the gastric rhythm (Fig. 4), (2) large epsps (usually larger than the decremented axon spikes) in cell body recordings (Fig. 5), (3) modulation of its activity in phase with the pyloric rhythm (Fig. 6) and (4) spontaneous spike inactivation at depolarized levels of membrane potential (Fig. 7).

• 3. Blocking spike conduction between the stomatogastric ganglion and the commissural ganglion demonstrates that CG bursting activity is generated within the commissural ganglion (Fig. 8). Thereby this indicates that this ganglion contains a separate gastric oscillator (the commissural gastric oscillator, CGO) of which CG is an intrinsic element.

• 4. CG itself possesses the capability for endogenous oscillation of its membrane potential (Fig. 9). This property, in association with spike inactivation at depolarized membrane potential levels, results in the ability to generate bursts of spikes either by depolarization or by hyperpolarization (Fig. 10).

• 5. CG has strong excitatory monosynaptic connection with GM neurones of the GPG. These motor neurones are responsible for rhythmic movements of the medial tooth of the gastric mill. Furthermore activation of CG results in sequential firing of other gastric neurones, LG and LPG, responsible for movements of the two lateral teeth (Fig. 11). Due to its intrinsic properties of burstiness and spike generation within a ‘window’ of membrane potential, CG is able to activate its follower motor neurones of the GPG in several different ways (Fig. 13).

• 6. The results suggest that the gastric motor output is organized by both a motoneuronal oscillator (the CPG) and a premotoneuronal oscillator (the CGO). This coupled oscillator organization is similar to that recently described for the pyloric system (Robertson and Moulins 1981c; Moulins and Nagy 1983). Moreover interactions between pyloric and gastric motor outputs appear to be mainly achieved at the level of the premotoneuronal oscillators of the two systems (Figs. 14–16).