Conclusions

All three studies in this chapter have described interneurons that are not dedicated to particular behaviours, but which operate as parts of ensembles in the interface between sensory analysis and motor control. Whenever an Aplysia retracts its gill, many interneurons are active no matter whether the movement is a reflex in response to a sensory stimulus or a movement associated with respiratory pumping. A similar conclusion applies to the interneurons involved in local bending movements of the leech because most of them respond to touch anywhere on the surface of their segment. In the locust, both local spiking and non-spiking interneurons have distinct receptive fields in their sensory responses, so are probably more restricted in their patterns of activity. Experiments in the leech and locust involving paired recordings with microelectrodes in order to construct circuit diagrams of the neurons involved have shown that information mostly flows through one or two layers of interneurons rather than directly from sensory to motor neurons. Each interneuron sums inputs from a number of presynaptic sources and distributes outputs onwards to many different targets. Information flows in one direction, with no feedback from the motor neurons to the interneurons. Although the pattern of connections made by interneurons is complex, the movements controlled by each motor neuron are oriented appropriately in response to stimulation of an area of skin.

It is difficult to appreciate how networks of neurons organise well-coordinated behaviour simply be examining a diagram of the connection pattern. Computer modelling can be very useful because factors such as the strengths of different connections can be incorporated, and because an experimenter can trace the way that information flows sequentially through the network. Also, as the study with the leech showed, modelling with computers can test whether a particular network works in a specific way.

There is undoubtedly a continuum in the degree to which interneurons are dedicated to particular behaviours, with some giant interneurons (see Chapter 3) being apparently concerned with just one behaviour. Probably the major advantage in using neurons that are not strongly dedicated to particular behaviours is that the circuits are flexible, so that different activities in which an animal is engaged are properly integrated together. Although we can neatly categorise different activities performed by an animal as different patterns of behaviour, the diffuse nature of the organisation of nervous systems makes it difficult to recognise discrete circuits that are concerned with particular behaviours.

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Essentials of Human Physiology

Essentials of Human Physiology

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