One of the most basic tasks that animals have to undertake in their everyday business is that of finding and eating their next meal. Charles Horn and his colleagues in Philadelphia and New York have been looking at the system that controls the cycle of biting, swallowing and the possible rejection of food in Aplysia - a purple sea slug that slithers around on the ocean floor devouring pieces of seaweed. A bundle of neurons in the central nervous system, called the central pattern generator, sends out a regular rhythm to the muscles of the radula – a hard, tongue-like structure covered in small teeth that Aplysia uses to feed. Any variability in this rhythm, or in the muscles it controls, has usually been dismissed as experimental error. However, Horn and his colleagues thought that this variation may affect how a hungry Aplysia gets its next meal;they decided to take a closer look.
First they isolated the part of the central nervous system containing the central pattern generator – the buccal ganglia – and the radula muscles from the rest of the animal. They then stimulated the oesophageal nerve, which connects to the buccal ganglia, with regular pulses to activate the central pattern generator to produce cycles of signals that would cause feeding behaviour. They also recorded from two motor neurons that controlled one of the radula muscles to chart the feeding behaviour and, finally,recorded the force generated in this muscle. What the team found surprised them. When they had controlled for sources of error in the experiment, and analysed their results mathematically, the signals generated in the nerves,neurons and muscles were still unexpectedly variable. But would they find the same variability in the freely feeding sea slugs?
Carrying out experiments in feeding Aplysia, the team made recordings from the nerves and radula muscles. They thought that control mechanisms from the rest of the nervous system in an intact animal would kick in and remove some of the variation – not so. The results were always the same: even when Aplysia were feeding on seaweed that was controlled for shape and size, the feeding rhythm was still variable. The team think most of the variability comes from the central pattern generator, and next want to discover the mechanism that generates the variability.
It would seem that Aplysia operate a far from perfect eating system, but Horn suggests that it is this variability that allows the feeding mechanism to function effectively over a very wide range of circumstances,tolerating disturbances and hiccups; a less variable system could fail to cope with problem situations. While this can cause Aplysia to mess up sometimes, missing its mouth or inadvertently spitting out half chewed pieces,in the end this could be the best strategy to ensure that the hungry mollusc gets the most from a meal, while using the least amount of energy per mouthful.
