There are very few instances in which complete neuronal pathways can be traced from the level of sense organs all the way to that of motor neurons. Notable exceptions are some startle behaviours like those described in Chapter 3, in which the size of the giant neurons involved makes experimental study relatively easy and in which the links between sensory processing and motor control are short. However, most of an animal's behavioural repertoire is not performed with the same urgency as escape movements. Much sensory analysis, particularly in visual and auditory pathways, involves several different stages, distributed over different regions of a brain. How are different sensory messages identified, and how are appropriate motor programs selected? The type of problem can be illustrated with a simple example. If a fly lands on your cheek, or if the skin of your knee itches, you can move your hand without thinking to those locations to remove the source of annoyance. This might seem like a trivial example of behaviour, but the neuronal mechanisms that allow us to perform such an act are far from being understood. It is relatively straightforward to map the locations of sensory receptors in the skin in an orderly manner within the brain, which generates a somatotopic map. However, it is not simple to generate the correct commands that will generate the correct balance of activation in different muscles of a jointed limb so that its end arrives at a specific location on the body surface. How is a map of position transformed into a series of finely graded muscle contractions? For an electrical engineer, it would be a major challenge to build a robot that could locate an object on its surface and then move the tip of a jointed appendage to the correct location to scratch itself.
This chapter, examines some extremely simple behaviours that involve reflex withdrawal movements of restricted regions of animals' bodies. Different experimental approaches have been adopted to study these behaviours. Microelectrodes are the most widely used tool for studying the activity of neurons, and allow connections between neurons to be characterised in detail. However, only a few neurons can be monitored at any one time by using microelectrodes. There is great interest, therefore, in techniques that allow an investigator to record the activity of many neurons simultaneously. One approach is to use optical methods, and another is to study the way that model networks of neurons work. A common theme that is emerging is that the central nervous system is rather diffusely organised, with relatively few interneurons dedicated to single behaviour patterns.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.