A network of non-spiking premotor and spiking interneurons, connected by an array of chemical and electrical synapses, that selectively regulates motor pattern related motoneuron recruitment in a motoneuron activity dependent fashion.

MARIANO J RODRIGUEZ; LIDIA SZCZUPAK

Washington

Congreso; Annual Meeting of the Society for Neuroscience; 2006

The classical notion on the functional structure of motor systems is being revised, shifting from a strictly hierarchical model to one of complex interactions between higher and lower levels. Several evidences indicate that the spinal motor system is an active participant in movement organization, rather than a simple executive level. The effort made to identify spinal interneurons that contribute to this organization takes on serious difficulties in higher organisms. In the leech Hirudo medicinalis, it has been described a network that regulates the functional coupling among segmental motoneurons (Rela & Szczupak, 2003). Stimulation of a motoneuron generates an excitatory signal on a second electrically coupled motoneuron. In addition, the latter receives from the former a polysynaptic inhibitory signal, mediated by a nonspiking neuron (the NS neuron), that counteracts the excitatory signal. This loop depends on NS membrane potential: if NS is depolarized, the inhibition, far from its reversal potential, is enhanced, canceling out the excitation; if NS is hyperpolarized, the coupling excitation predominates. Here, by means of intra and extracellular electrophysiological recordings in isolated nerve cords, we show that this network regulates the coupling between two motoneurons that are strongly modulated during the dorso-ventral undulatory swimming pattern: the L cell (excitor of longitudinal muscles in general) and cell 3 (excitor of only dorsal longitudinal muscles). The uncoupling mechanism only works in the 3 to L direction, in accordance with the fact that every time L is active during a motor pattern cell 3 is active too, while cell 3 is activated in motor patterns in which L is inhibited (e.g. swimming). Moreover, NS is depolarized during the swimming motor pattern, which enhances the inhibitory loop, and further depolarizing NS during the pattern decreases the firing rate of the L cell, which must be inactive in order to execute swimming efficiently. These novel network interactions add one more possibility to the growing body of knowledge about the mechanistic complexity of the motor system.Hirudo medicinalis, it has been described a network that regulates the functional coupling among segmental motoneurons (Rela & Szczupak, 2003). Stimulation of a motoneuron generates an excitatory signal on a second electrically coupled motoneuron. In addition, the latter receives from the former a polysynaptic inhibitory signal, mediated by a nonspiking neuron (the NS neuron), that counteracts the excitatory signal. This loop depends on NS membrane potential: if NS is depolarized, the inhibition, far from its reversal potential, is enhanced, canceling out the excitation; if NS is hyperpolarized, the coupling excitation predominates. Here, by means of intra and extracellular electrophysiological recordings in isolated nerve cords, we show that this network regulates the coupling between two motoneurons that are strongly modulated during the dorso-ventral undulatory swimming pattern: the L cell (excitor of longitudinal muscles in general) and cell 3 (excitor of only dorsal longitudinal muscles). The uncoupling mechanism only works in the 3 to L direction, in accordance with the fact that every time L is active during a motor pattern cell 3 is active too, while cell 3 is activated in motor patterns in which L is inhibited (e.g. swimming). Moreover, NS is depolarized during the swimming motor pattern, which enhances the inhibitory loop, and further depolarizing NS during the pattern decreases the firing rate of the L cell, which must be inactive in order to execute swimming efficiently. These novel network interactions add one more possibility to the growing body of knowledge about the mechanistic complexity of the motor system.