Pattern and reset analysis of the gastric mill rhythm in a spiny lobster, Panulirus interruptus.

The burst pattern of the gastric mill rhythm was studied by varying its cycle period in in vitro preparations comprising the stomatogastric (STG), oesophageal and (paired) commissural ganglia. Reset tests using intracellular polarization of identified STG neurones showed that the CI, LC, GP and GM cells can all strongly affect the cycle period, and therefore apparently play a role in generating the gastric rhythm. Variation in the cycle period could be obtained by: (i) cutting certain input nerves; (ii) relative coordination between the gastric and oesophageal rhythms; or (iii) intracellular polarization of identified STG cells, especially the LC motoneurone. Variation in the cycle period by any of these means showed that the gastric pattern (in such preparations) comprises two basic alternating phases: a variable-duration 'powerstroke' and a constant-duration 'returnstroke'. The powerstroke is taken to include bursts in the LC, GP and GM motoneurones (since they evoke closing of the gastric mill teeth and mastication of food), along with the interburst intervals of the other cells. The durations of all these events co-varies over a large range, as a linear function of the cycle period. The activity level of neurones bursting during the powerstroke is directly proportional to their burst length, and hence appears to be a basic parameter affecting the cycle period. The returnstroke is taken to include bursts in the CP, AM and LG motoneurones (since they evoke opening and resetting of the gastric mill teeth), along with the interburst intervals of the powerstroke cells. All these events tended to assume a fixed duration. The two-part gastric mill pattern can be analogized to other two-part rhythms, e.g. for terrestrial locomotion, in which the load-bearing phase has a variable duration and accounts for most of the variation in the cycle period whereas the unloaded phase tends to assume a constant duration.