Command fibres from the supra-oesophageal ganglion to the stomatogastric ganglion inPanulirus argus

Summary

In the higher Decapoda Crustacea a small inferior ventricular nerve (ivn) directly connects the supra-oesophageal ganglion (S.O.G.) with the stomatogastric nervous system. Collision experiments (Fig. 3) in the isolated preparation of the stomatogastric nervous system ofPanulirus argus demonstrated that two fibres (ivn through fibres) pass directly from the ivn to the stomatogastric nerve (sgn). The sgn is considered to be the sole input pathway to the stomatogastric ganglion.

The 28–30 neurones of the stomatogastric ganglion (St. G.) form a motor relay station which directly controls much of the activity of the foregut in these animals. The stomatogastric neurones form two distinct groups. One of these groups controls the activity of the gastric mill (gastric mill cycle) and the other group controls the activity of the pyloric stomach filtering apparatus (pyloric cycle). The mechanism of formation of the pyloric cycle is intrinsic to the St. G. and apparently normal pyloric cycle activity is recorded in the isolated preparation. The situation with regard to the gastric mill cycle is less clear and normal activity was not recorded in our isolated preparations.

Stimulation of the ivn selectively activated the two through fibres in the sgn and produced marked frequency-dependent changes in the output of both groups of stomatogastric ganglion neurones (Table 1). The ivn through fibres are therefore regarded as command fibres. The effects of the two fibres were indistinguishable and additive when both fibres were stimulated simultaneously. Low frequency stimulation of the ivn (Figs. 4, 5) produced increased pyloric cycle frequency with more pronounced activity in the dilator group of neurones (PD, VD, AB). Higher frequency stimulation (5–10 per second) effectively disrupted the pyloric cycle (Figs. 6, 7), and at 20 per second almost all activity ceased. Only the activity of the GM neurones of the gastric cycle was examined. These neurones were relatively unaffected by low frequency stimulation but were progressively inhibited by higher frequency stimulation (Fig. 8) and at 20 per second they were subject to rapid, total and sustained inhibition.

The mechanisms producing these changes were investigated by intracellular recordings from identified neurones of the St. G. In the pyloric cycle at least the PD pacemaker and the VD neurones of the dilator group received direct epsps from the command fibres (Figs. 11–14). The antagonistic pyloric constrictor elements (PY) were subject to increased indirect inhibition from the more active dilator neurones and in some cases there was probably also direct inhibition of these constrictor neurones (Figs. 9, 10). The GM neurones received a possible direct ipsp and at least one indirect effect by an increase in an ongoing ipsp during stimulation (Figs. 15, 16). There was little facilitation of the command ipsps at the frequencies used but at higher frequencies there was a summation which produced strong hyperpolarization. The innervation pattern of the ivn command fibres in the St. G. is summarized in Fig. 17.

The ivn through fibres have very strong direct effects. The weight of present evidence is that in this system most presynaptic fibres have much smaller modulating effects. The function of the ivn through fibres is unknown at this time.