Chapter 2 Repetitive Impulse Firing in Motoneurons: Facts and Perspectives

Abstract

Publisher Summary When activating the skeletal muscles, the motoneurons (MNs) practically always deliver more or less prolonged bursts of repetitive firing. Each MN receives thousands of synapses and hundreds of these have to be more or less simultaneously active for the action potential (AP) threshold to be reached. In maintained synaptic activation, the MNs are stimulated by relatively steady postsynaptic currents, produced by the summation of many asynchronous postsynaptic events. Hence, a logical way to proceed is to study the input–output relations of MNs by measuring the relation between an activating steady current and the AP rate. This is usually done by injecting maintained stimulating currents through an intracellular microelectrode. Control experiments have shown that such injected currents have actions very similar to those often seen for maintained postsynaptic currents. The input–output relations of spinal MNs were initially investigated in anesthetized animals. Later, it was found that in nonanesthetized animals, the baseline properties seen under anesthesia are supplemented by additional MN-modulating mechanisms, such as the plateau behavior and other changes of input–output relations. In this chapter, the baseline properties are briefly described. Some comments are also added concerning other aspects, including the MN-modulatory postsynaptic effects.