Jennifer C. McDonagh

Motoneurons: A preferred firing range across vertebrate species?

The term “preferred firing range” describes a pattern of human motor unit (MU) unitary discharge during a voluntary contraction in which the profile of the spike‐frequency of the MUs compound action potential is dissociated from the profile of the presumed depolarizing pressure exerted on the units spinal motoneuron (MN). Such a dissociation has recently been attributed by inference to the presence of a plateau potential (PP) in the active MN. This inference is supported by the qualitative similarities between the firing pattern of human MUs during selected types of relatively brief muscle contraction and that of intracellularly stimulated, PP‐generating cat MNs in a decerebrate preparation, and turtle MNs in an in vitro slice of spinal cord. There are also similarities between the stimulus‐response behavior of intracellularly stimulated turtle MNs and human MUs during the elaboration of a slowly rising voluntary contraction. This review emphasizes that there are a variety of open issues concerning the PP. Nonetheless, a rapidly growing body of comparative vertebrate evidence supports the idea that the PP and other forms of non‐linear MN behavior play a major role in the regulation of muscle force, from the lamprey to the human.

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 (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.

Associations between the morphology and physiology of ventral-horn neurons in the adult turtle

This study compared some morphologic and physiological properties of adult turtle spinal motoneurons (MNs) vs. interneurons (INs). Reconstructions were made of 20 biocytin‐stained cells, which had been previously studied physiologically in 2‐mm‐thick slices of lumbosacral spinal cord. The intracellularly measured physiological properties included resting potential, input resistance (RN), threshold (rheobase, IRh), and slope of the stimulus current (I) ‐spike frequency (f) relation. The seven morphologic properties that were quantified for each cell included three indices of somal size (diameter, area, volume), and four of dendritic size: the number of first‐ and last‐order branches, rostrocaudal extent, and Σ individual lengths. Significant differences were shown between all seven morphologic parameters for MNs vs. INs. Despite the small sample size, significant differences were also shown for five of seven parameters for high‐threshold vs. low‐threshold MNs, and three of seven for low‐threshold MNs vs. INs. These latter three parameters were the number of terminal dendritic branches, their rostrocaudal extent, and the Σ dendritic lengths. Linear associations for the MN + IN and the MN samples were stronger between the four dendritic parameters than between soma‐dendritic ones. Exponential associations between morphologic and physiological properties were mostly significant (28 of 30), and their strength was in the order IRh < RN < f/I slope for the MN +IN sample and IRh < RN = f/I slope for the MN sample. There is discussion of the relevance of the above findings to the provisional classification of turtle ventral‐horn neurons on the basis of electrophysiology alone. J. Comp. Neurol. 454:177–191, 2002.

Measurement and nature of firing rate adaptation in turtle spinal neurons.

There is sparse literature on the profile of action potential firing rate (spike-frequency) adaptation of vertebrate spinal motoneurons, with most of the work undertaken on cells of the adult cat and young rat. Here, we provide such information on adult turtle motoneurons and spinal ventral-horn interneurons. We compared adaptation in response to intracellular injection of 30-s, constant-current stimuli into high-threshold versus low-threshold motoneurons and spontaneously firing versus non-spontaneously-firing interneurons. The latter were shown to possess some adaptive properties that differed from those of motoneurons, including a delayed initial adaptation and more predominant reversal of adaptation attributable to plateau potentials. Issues were raised concerning the interpretation of changes in the action potentials’ afterhyperpolarization shape parameters throughout spike-frequency adaptation. No important differences were demonstrated in the adaptation of the two motoneuron and two interneuron groups. Each of these groups, however, was modeled by its own unique combination of action potential shape parameters for the simulation of its 30-s duration of spike-frequency adaptation. Also, for a small sample of the very highest-threshold versus lowest-threshold motoneurons, the former group had significantly more adaptation than the latter. This finding was like that shown previously for cat motoneurons supplying fast- versus slow twitch motor units.