RECRUITMENT OF A PROJECTION NEURON DETERMINES GASTRIC MILL MOTOR PATTERN SELECTION IN THE STOMATOGASTRIC NERVOUS-SYSTEM OF THE CRAB, CANCER-BOREALIS

1. In the isolated stomatogastric nervous system of the crab Cancer bo realis (Fig. 1), the muscarinic agonist oxotremorine elicits several d istinct gastric mill motor patterns from neurons in the stomatogastric ganglion (STG; Fig. 2). Selection of a particular gastric mill rhythm is determined by activation of distinct projection neurons that influ ence gastric mill neurons within the STG. In this paper we identify on e such neuron, called commissural projection neuron 2 (CPN2), whose rh ythmic activity is integral in producing one form of the gastric mill rhythm. 2. There is a CPN2 soma and neuropilar arborization in each co mmissural ganglion (CoG). The CPN2 axon projects through the superior esophageal nerve (son) and the stomatogastric nerve (stn) to influence neurons in the STG (Figs. 3 and 4A). 3. CPN2 activity influences most of the gastric mill neurons in the STG. Specifically, CPN2 excites ga stric mill neurons GM and LG (gastric mill and lateral gastric, respec tively) and inhibits the dorsal gastric (DG), anterior median (AM), me dial gastric (MG), and inferior cardiac (IC) neurons (Figs. 5 and 6). CPN2 also indirectly inhibits gastric mill neurons Int1 and VD (intern euron 1 and ventricular dilator neuron, respectively) through its acti vation of LG. The CPN2 excitatory effects are mediated at least partly via discrete excitatory postsynaptic potentials( EPSPs; Fig. 4B), whe reas its inhibitory effects are produced via smooth hyperpolarizations . 4. Within the CoG, CPN2 receives excitatory synaptic input from the anterior gastric receptor neuron (AGR), a gastric mill proprioceptive sensory neuron (Fig. 7) and inhibitory synaptic input from the gastric mill interneuron, Int1 (Fig. 8). 5. During one form of the gastric mi ll rhythm, CPN2 fires rhythmically in time with the gastric mill motor pattern, whereas it is silent or fires weakly during other gastric mi ll rhythms (Fig. 9). 6. When CPN2 rhythmic activity is suppressed duri ng a CPN2-influenced gastric mill rhythm, the gastric mill rhythm cont inues, but the pattern is altered( Fig. 10). Moreover, transiently sti mulating CPN2 during any ongoing gastric mill motor pattern can reset the timing of that rhythm (Fig. 11). 7. Tonic activity in CPN2 is insu fficient to elicit a gastric mill rhythm (Fig. 12). Phasic activity in CPN2 can elicit a gastric mill rhythm only in preparations in which g astric mill neurons are already in an excited state (Figs. 12 and 13). 8. CPN2 recruitment plays a pivotal role in determining the final for m of the gastric mill rhythm. Moreover, its patterned influence on thi s rhythm, as well as its ability to reset ongoing gastric mill rhythms , shows it to be a member of the underlying neural network that genera tes this motor pattern (Fig. 14).