Synaptic central of motoneuronal excitability

Synaptic Control of Motoneuronal Excitability. Physiol. Rev. 80: 767-852, 2 000, -Movement, the fundamental component of behavior and the principal ext rinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons . The processes that determine the firing behavior of motoneurons are there fore important in understanding the transformation of neural activity to mo tor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first prese nt a background description of motoneurons: their development, anatomical o rganization, and membrane properties, both passive and active. Pie then des cribe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect mot oneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate i s the main excitatory, and GABA and glycine are the main inhibitory transmi tters acting through ionotropic receptors. These amino acids signal the pri ncipal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneur onal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on commoneffecters, e .g., leak K+ current, cationic inward current, hyperpolarization-activated inward current, Ca2+ channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimatel y generating the coordinated firing patterns that underlie muscle contracti ons during motor behavior.