PROPERTIES OF SPINAL MOTONEURONS AND INTERNEURONS IN THE ADULT TURTLE- PROVISIONAL CLASSIFICATION BY CLUSTER-ANALYSIS

The purpose of the present study was to compare, in motoneurons (MNs) vs. interneurons (INs), selected passive, transitional, and active (fi ring) properties, as recorded in slices of lumbosacral spinal cord (SC ) taken from the adult turtle. The cells were provisionally classified on the basis of(1) the presence (in selected INs) or absence (MNs and other INs) of spontaneous discharge, (2) a cluster analysis of select ed properties of the nonspontaneously firing cells, (3) a comparison t o previous data on turtle MNs and INs, and (4) a qualitative compariso n of the results with those reported for other vertebrate species (lam prey, cat). The provisional nomenclature accommodated properties appro priate for solely MNs (Main MN group) vs. nonspontaneously firing INs (Main IN-N) vs, spontaneously firing INs (IN-S) and for neurons with t wo degrees of intermediacy between the Main MN and the Main IN-N group s (Overlap MN, Overlap MN/IN). Morphological reconstructions of additi onal cells, which had been injected with biocytin during the electroph ysiological tests, were shown to provide clear-cut support for the pro visional classification procedure. The values for the measured paramet ers in the 96 tested cells covered the spectrum reported previously ac ross adult vertebrate species and were robust in measurements made on different SC slices up to 5 days after their removal from. the host an imal. The interspecies comparisons permitted the predictions that (1) our Main MN and Overlap MN cells would be analogous to two MN types th at innervate fast-twitch and slow-twitch skeletomotor muscle fibers, r espectively, in the cat, and (2) the MNs in our Overlap MN/IN group pr obably innervate slow (nontwitch, tonic) muscle fibers whose presence has recently been established in the turtle hindlimb. In summary, the results bring out the utility of the SC slice preparation of the turtl e for study of spinal motor mechanisms in adult tetrapod vertebrates, particularly as an adjunct to the in vivo cat, because of the ease wit h which robust measurements can be made of the active properties of bo th MNs and INs. (C) 1998 Wiley-Liss, Inc.