Locomotion can be generated in the vertebrate spinal cord by neuronal networks known as central pattern generators (CPGs). However, their action is constantly modified by extrinsic modulatory inputs and sensory feedback to compensate for an ever changing environment. To add questions of locomotor rhythm generation and its modulation in a walking vertebrate, we used an in vitro preparation isolated from a mature aquatic amphibian, the mudpuppy (Necturus maculatus). The preparation consisted of the first five segments of the spinal cord and the forelimb(s) attached by the brachial nerves. During chemically induced locomotion, electromyographic activity was recorded unilaterally or bilaterally from elbow flexor and extensor muscles. Combined use of pharmacological and immunostaining techniques revealed that some of the conventional neurotransmitters (i.e., serotonin, glycine, GABA) prominent in higher and lower vertebrates also affect locomotion in the mudpuppy. While not essential for rhythmogenesis, activation of serotonergic, glycinergic and GABAergic systems play a role in the control of locomotor frequency. In addition, glycine and GABA are important in mediating the reciprocal antagonism that coordinates the locomotor rhythm. The modulatory action of serotonin may be exerted by a well developed spinal serotonergic system.; A subsequent study, combining chemical and techniques with sectioning of the spinal cord revealed that the CPG for forelimb locomotion comprises at least two distinct centers responsible for producing rhythmic elbow flexion mid extension. The two centers can operate independently in the absence of reciprocal inhibitory interconnections between them. Sensory input was shown to interact with the flexion/extension networks to reset the ongoing walking rhythm in a phase-dependent manner. Intracellular recording revealed that the majority of interneurons within flexor and extensor centers were active during walking and were classified into four phasic types in the step cycle. Differential longitudinal distribution along the spinal cord of the four types of interneurons was in agreement with the existence of separate flexor and extensor centers.; Preliminary experiments conducted on a novel two-limb preparation showed that roughly one segment of the spinal cord contains neural circuitry capable of generating walking-like activity that involves both forelimbs. Midsagittal sectioning of this segment did not abolish activity in both disconnected sides indicating that each spinal contains neuronal elements capable of producing rhythmic bursting. However, the resulting bursts were mutually uncorrelated indicating importance of crossed spinal connections for coordination of the two limbs. The results contribute to the growing body of evidence for a common role of neuromodulatory substances in the control of locomotion among vertebrates. The distinctly localized rhythmogenic centers suggest that more complex neuronal networks probably contain small modular pattern generators which can be assembled to meet changing behavioral needs.
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