Neuromodulation of Microcircuits in Motor Systems with Special Reference to Invertebrates

In this chapter we introduce current knowledge on neuromodulation at the various levels of analysis on which neuromodulator action becomes apparent. These span from animal behavior, to biomechanics and musculature, to neural networks, single neurons and, finally, intracellular signal cascades, genes, and protein synthesis. In contrast to neurohormones that are released into the hemolymph, neuromodulators are delivered more precisely at their target tissues. While extrinsic neuromodulation provides an independent release control, intrinsic neuromodulation can represent a more automatic release control. Neuronal networks underlying the generation of rhythmic motor patterns are known to receive a particularly rich supply of neuromodulators, and thus provide excellent case studies, some of which are presented in detail in this article. Neuromodulators play a pivotal role in the reconfiguration of neuronal networks, and thus extend the working range of a given neuronal network enormously. Neuromodulators organize the formation of neuronal networks during development and are involved in shaping the final tuning towards their function in the adult organism. Furthermore, neuromodulators have profound effects on peripheral systems such as muscles. Not only may they change the efficacy of neuromuscular transmission, or the function of glands, but they also regulate the muscular energy metabolism, to name some examples treated here. This chapter aims to identify some “holes” in current knowledge to guide future research. These concern in particular the following issues: (a) How are neuromodulatory neurons themselves part of larger neural networks? (b) How are these neurons activated during the respective behaviors of an organism rather than in isolated systems or in vitro cultures? (c) Finally, much more knowledge is required to gain insight into the particular “functional compartments” of neuromodulators released by neurons. HISTORY AND DEFINITION OF THE TERM