Previous work using electrical microstimulation has suggested the existence of modules subserving limb posture in the spinal cord. In this study, the question of modular organization was reinvestigated with the more selective method of chemical microstimulation. N-methyl-D-aspartate (NMDA) iontophoresis was applied to 229 sites of the lumbar spinal cord gray while monitoring the isometric force output of the ipsilateral hindlimb at the ankle. A force response was elicited from 69 sites. At 18 of these sites, tonic forces were generated and rhythmic forces at 44. In the case of tonic forces, their directions clustered along four orientations: lateral extension, rostral flexion, adduction, and caudal extension. For the entire set of forces (tonic and rhythmic), the same clusters of orientations were found with the addition of a cluster directed as a flexion toward the body. This distribution of force orientations was quite comparable to that obtained with electrical stimulation at the same sites. The map of tonic responses revealed a topographic organization; each type of force orientation was elicited from sites that grouped together in zones at distinct rostrocaudal and depth locations. In the case of rhythmic sequences of force orientations, some were distinctly more common, whereas others were rarely elicited by NMDA. Mapping of the most common rhythms showed that each was elicited from two or three regions of the cord. These regions were close in location to the tonic regions that produced those forces that represented components specific to that rhythm. There was an additional caudal region from which the different rhythms also could be elicited. Taken together, these results support the concept of a modular organization of the motor system in the frog's spinal cord and delineate the topography of these modules. They also suggest that these modules are used by the circuitry underlying rhythmic pattern generation by the spinal cord.
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