Repeatable spatial maps of a few force and joint torque patterns elicited by microstimulation applied throughout the lumbar spinal cord of the spinal frog

Motor learning and construction of novel behavior must be constrained by the spinal motor apparatus and how it supports movement. Recent work supports a modularity of spinal cord function in both mammals and lower vertebrates. We sought to extend these analyses by a complete mapping of the lumbar enlargement not previously attempted. The frog lumbar spinal cord grey matter motor responses were systematically mapped using microstimulation at a fine grain of 200 μm separation mediolaterally and in depth throughout the enlargement. The patterns of force magnitude and direction were noted. Large areas of spinal cord produced very small forces. Some areas produced strong responses. Both the strong and the weak force responses fell into a few classes. In all frogs examined the forces elicited fell into a few (5) classes of directions. These forces were expressed as joint torques by standard means. Forces were observed to comprise both pure hip torques and combined knee/hip torque patterns, but no pure knee torques. The contiguous regions producing these force directions at high magnitudes were arranged in repeating patterns. The directions of the forces elicited were strongly correlated among frogs in specific regions of spinal cord while other regions showed individual variations. The data are consistent with microstimulation recruitment of specific motor responses or force-field primitives in some spinal cord regions in the frog. Similar constraints may exist early in mammalian motor development or after spinal cord injury.