Presynaptic Control of Information Transmission in the Vertebrate Spinal Cord

Despite their different ways of looking at the nervous system, neurophysiologists, neuroanatomists and neurochemists seem to have a similar concern with the regulation of synaptic transmission, in its broadest sense. The problem is to define what do we mean by regulation, and this is not an easy task. In this context we may distinguish two types of mechanisms controlling synaptic transmission. One, involving actions tending to maintain a constant operating level in the system, for example, those mechanisms involved in keeping a constant pool of available transmitter, and second, those mechanisms involving a change in the operating level of the system, such as those involved in learning. Perhaps the first, but not the second mechanism implies the existence of a state detector, a command action and an effector (100). Figure 1 shows some of the steps involved in the generation of impulses in motoneurons by activity in muscle spindles. In principle any procedure interfering with any of these steps will affect synaptic transmission (90). This can be achieved by external influences or can be a consequence of the functioning of the nervous system itself. From the point of view of transmitted information, what seems to be relevant is the change in the state. If there is no change, there will be no information transfer.