Properties of spinal motoneurons and interneurons in the adult turtle: Provisional classification by cluster analysis

Abstract

The purpose of the present study was to compare, in motoneurons (MNs) vs. interneurons (INs), selected passive, transitional, and active (firing) 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 selected properties of the nonspontaneously firing cells, (3) a comparison to previous data on turtle MNs and INs, and (4) a qualitative comparison of the results with those reported for other vertebrate species (lamprey, cat). The provisional nomenclature accommodated properties appropriate for solely MNs (Main MN group) vs. nonspontaneously firing INs (Main IN-N) vs. spontaneously firing INs (IN-S) and for neurons with two degrees of intermediacy between the Main MN and the Main IN-N groups (Overlap MN, Overlap MN/IN). Morphological reconstructions of additional cells, which had been injected with biocytin during the electrophysiological tests, were shown to provide clear-cut support for the provisional classification procedure. The values for the measured parameters in the 96 tested cells covered the spectrum reported previously across adult vertebrate species and were robust in measurements made on different SC slices up to 5 days after their removal from the host animal. The interspecies comparisons permitted the predictions that (1) our Main MN and Overlap MN cells would be analogous to two MN types that innervate fast-twitch and slow-twitch skeletomotor muscle fibers, respectively, in the cat, and (2) the MNs in our Overlap MN/IN group probably 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 turtle for study of spinal motor mechanisms in adult tetrapod vertebrates, particularly as an adjunct to the in vivo cat, because of the ease with which robust measurements can be made of the active properties of both MNs and INs.

Somato-Dendritic Morphology Predicts Physiology for Neurons That Contribute to Several Kinds of Limb Movements

“…The small ventral horn somata and extensive transverse plane dendrites of T cells appear consistent with either or both populations of turtle ventral horn interneurons, one having plateau potentials and one not, studied in slice preparations (Hounsgaard and Kjaerulff 1992), but inconsistent with plateau-generating (Russo and Hounsgaard 1996b) and burst-generating (Russo and Hounsgaard 1996a) dorsal horn neurons and giant neurons (Fernandez et al 1996). The T cell somato-dendritic orientations, simple dendritic trees, and brief spike afterhyperpolarizations appear more similar to nonspontaneously active than spontaneously active turtle ventral horn interneurons studied in slice preparations (McDonagh et al 1998(McDonagh et al , 2002; most T cells also did not have spontaneous activity under our conditions. Mean soma diameters of T cells were smaller than six turtle ventral horn interneurons from slice preparations studied in detail (McDonagh et al 2002).…”

“…3; see also Alaburda et al 2005;Robertson and Stein 1988). Motoneurons, having unusually large somata and unusually complex dendritic trees (McDonagh et al 1998(McDonagh et al , 2002, may require either simultaneous inputs from a large number of premotor interneurons or inputs from rhythmic premotor interneurons that have especially high peak firing rates, as T cells do. Second, T cells, having dendrites that extend far into the lateral funiculus and ventral funiculus, are positioned to receive and integrate inputs from a wide variety of ascending and descending propriospinal and descending brain sources that are known to modulate hindlimb motor patterns (Drew et al 2004;Grillner and Dubuc 1988;Orlovsky et al 1999) and may make en passant synapses within the white matter (Berkowitz and Stein 1994b).…”

“…Much has been learned about the spinal control of scratching, swimming, and withdrawal from in vivo turtle studies (Stein 2005) and about the morphology and physiology of spinal neurons from in vitro turtle studies (Fernandez et al 1996;Gorman et al 2005;Hounsgaard and Kjaerulff 1992;McDonagh et al 1998McDonagh et al , 2002Russo and Hounsgaard 1996a,b;Stauffer et al 2005). Only recently, however, has intracellular recording of turtle spinal interneurons been applied in vivo, facilitating examination of the morphology and physiology of individual interneurons that contribute to limb movements (Berkowitz 2005).…”

Functional Properties of Motoneurons Derived from Mouse Embryonic Stem Cells

“…These rates are in the appropriate range for developing MNs (Martin-Caraballo and Greer, 1999) as well as adult type slow MNs (Kernell et al, 1983). In contrast, other spinal neurons such as dorsal spinocerebellar tract neurons (cat) (Gustafsson, 1984) or unidentified ventral interneurons (turtle) (McDonagh et al, 1998) fire at much higher frequencies. Another feature of normal MN firing evident in ES cell-derived MNs was spike frequency adaptation.…”

Identification of Multiple Subsets of Ventral Interneurons and Differential Distribution along the Rostrocaudal Axis of the Developing Spinal Cord

“…Multiple distinct IN cell types are present in the adult spinal cord [12], [28], [29], [30], [31] (for review [32], [33]). Many of which are thought to arise from the V0, V1, V2 and V3 cardinal classes, although dorsal embryonic INs are also likely to contribute to the motor circuits (for review [9], [34], [35]).…”