Feeding motor programme in Limax. II. Modulation by sensory inputs in intact animals and isolated central nervous systems.

The sources of variability in feeding motor programme (FMP) of the terrestrial slug, Limax maximus, were examined in relation to: (1) changes in load on the feeding apparatus; (2) changes in concentration of attractive food stimuli; and (3) changes in satiety signalled by feedback from the gut. These sources of variability, which affect both timing of the central pattern generator for feeding and probability of occurrence of FMP, were compared in intact animals and in isolated brain preparations. The load on feeding apparatus of intact animals is altered by varying the hardness of their food. An animal will show a higher 'bite cycle' frequency on soft food as compared with hard food. In physiological preparations, weights attached to buccal muscles similarly increase load. Cycle frequency of FMP triggered by food extracts is increased when buccal muscles are unloaded compared with FMP when muscles are loaded. Increasing the chemostimulant concentration of food results in greater numbers of intact animals feeding for longer periods. Increasing the food extract concentration used to trigger FMP in physiological preparations causes similar increases in feeding duration. Intact animals use cues from gut distention to indicate satiation and terminate feeding. Inflation of the crop in physiological preparations causes an early termination of feeding activity, along with decreased FMP cycle frequency.