Broadly tuned spinal neurons for each form of fictive scratching in spinalturtles

Behavioral choice can be mediated either by a small number of sharply tuned neurons or by large populations of broadly tuned neurons. This issue can b e conveniently examined in the turtle spinal cord, which generates each of three forms of scratching-rostral, pocket, and caudal-in response to mechan ical stimulation in each of three adjacent regions of the body surface. Pre vious research showed that many propriospinal neurons are broadly tuned to either the rostral scratch region or the pocket scratch region, but respons es to caudal scratch stimulation could not be examined in that reduced prep aration. In the current study, individual spinal neurons were recorded extr acellularly from the gray matter of the turtle spinal cord hindlimb enlarge ment, while sites in the rostral, pocket, and caudal scratch regions were m echanically stimulated. Many neurons were broadly tuned to the caudal scrat ch region; other neurons were broadly tuned to either the pocket scratch or rostral scratch region. All three types were typically found within a sing le animal. These data are consistent with the hypothesis that the turtle sp inal cord relies on large populations of broadly tuned neurons to select ea ch of the three forms of scratching. In addition, neurons that were broadly tuned to each of the scratch regions were typically found in each spinal c ord segment and within the same range of mediolateral and dorsoventral loca tions. Providing that these neurons are related to the selection and genera tion of the three forms of scratching, this would indicate that cells of th is type are not segregated into distinct regions of the spinal cord gray ma tter.