Cellular Bases of Locomotor Behaviour in Lamprey: Coordination and Modulatory Control of the Spinal Circuitry
The rhythmic movements performed during locomotion in vertebrates are controlled by pattern-generating circuits in the spinal cord. The different network units are precisely coordinated to achieve a proper timing of the different muscle groups. Different modulatory systems act upon the circuits in order to keep the motor pattern well-adapted to external demands, or to modify the pattern to produce various forms of locomotor behaviour. The neuronal mechanisms underlying this motor control system are being investigated in the lamprey model system, in which the spinal circuitry has been characterized in considerable detail. Computer simulations are performed in conjunction with experiments in the cellular analysis of these control mechanisms. Rhythmic burst activity in the lamprey segmental network is initiated from the brainstem via activation of both NMDA- and non-NMDA types of glutamate receptors. The rate of bursting activity can be controlled by varying the relative amount of activation of the two types of receptor. Serotonergic and GABAergic systems act to modulate the activity of the spinal circuitry. Serotonin specifically reduces the slow afterhyperpolarization (sAHP) by blocking apamin-sensitive Ca-dependent K-channels, leading to a smaller contribution from sAHP summation in burst termination, and thereby burst prolongation and a lower burst rate. GABA acts both pre- and postsynaptically, via GABAA- and GABAB receptors, giving differential effects on the operation of the circuitry. The intersegmental coordination of the network units along the spinal cord during the undulatory swimming movements can be accounted for by the leading segment having a higher “intrinsic burst frequency” than the other segments, which become entrained to become coordinated with a constant phase lag. The intersegmental coordination can be modified by a local control of intrinsic frequency, resulting in e.g. modifications in body curvature or backwards locomotion. Both the serotonergic and the GAB Aergic systems may exert such a modulatory influence on the intersegmental coordination.
KeywordsReciprocal Inhibition Segmental Network Burst Rate Reticulospinal Neurone Nipecotic Acid
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