Organization of the stomatogastric ganglion of the spiny lobster

Summary

The neurons of the gastric system form two discrete subsets which drive the median and lateral teeth of the gastric mill. These two subsets can each produce alternating contractions of antagonistic muscles when the other subset is not active, but whenever the two are active simultaneously their motor patterns are coordinated.

This coordination is produced by synaptic connections between motor neurons in the two subsets, and by the interneurons common to both subsets. One of the neurons driving the lateral teeth, LGN, inhibits several of the neurons driving the medial tooth—DGN (Fig. 1) and each of the GMs. LGN and the GMs are also electrotonically coupled (Fig. 2). The GMs are electrotonically coupled to all the motor neurons of the lateral teeth subset (Figs. 2, 3, and 9). These synaptic connections are reflected in the spontaneous motor patterns generated by the ganglion. In addition, there are several interactions (Figs. 4–8 and 11b) which may not be the products of direct synaptic connections which nonetheless have been demonstrated experimentally and are reflected in the spontaneous motor patterns (Figs. 14–17).

Two hypothesis about the mechanism generating one of the two types of motor patterns observed are proposed. One uses observations of synaptic depression at particular synapses to explain overlapping bursts of reciprocally inhibitory neurons. The other draws on observations of accommodation and postinhibitory rebound to explain how the repetition rate of the pattern might be determined.