SPINAL-CORD MODULAR ORGANIZATION AND RHYTHM GENERATION - AN NMDA IONTOPHORETIC STUDY IN THE FROG

Previous work using electrical microstimulation has suggested the exis tence of modules subserving limb posture in the spinal cord. In this s tudy, the question of modular organization was reinvestigated with the more selective method of chemical microstimulation. N-methyl-D-aspart ate (NMDA) iontophoresis was applied to 229 sites of the lumbar spinal cord gray while monitoring the isometric force output of the ipsilate ral hindlimb at the ankle. A force response was elicited from 69 sites . At 18 of these sites, tonic forces were generated and rhythmic force s 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 cl uster directed as a flexion toward the body. This distribution of forc e orientations was quite comparable to that obtained with electrical s timulation at the same sites. The map of tonic responses revealed a to pographic organization; each type of force orientation was elicited fr om sites that grouped together in zones at distinct rostrocaudal and d epth locations. In the case of rhythmic sequences of force orientation s, some were distinctly more common, whereas others were rarely elicit ed by NMDA. Mapping of the most common rhythms showed that each was el icited from two or three regions of the cord. These regions were close in location to the tonic regions that produced those forces that repr esented components specific to that rhythm. There was an additional ca udal region from which the different rhythms also could be elicited. T aken together, these results support the concept of a modular organiza tion of the motor system in the frog's spinal cord and delineate the t opography of these modules. They also suggest that these modules are u sed by the circuitry underlying rhythmic pattern generation by the spi nal cord.