Fatigue of rat hindlimb motor units: biochemical-physiological associations.

Associations between fatigability and biochemical properties within motor unit (MU) types were explored in two hindlimb muscles of the adult rat. Type FF MUs in extensor digitorum longus and type S units in soleus were subjected either to a moderate (type FF) or severe (type S) 6-min, fatigue-inducing stimulation protocol. For both MU types, the range of values for their fatigability was considerably greater than the ranges in the activity levels of three enzymes in the units' constituent muscle fibers (MFs). These enzymes represented major energy-yielding pathways: adenylokinase, for high-energy phosphate metabolism; lactate dehydrogenase, for anaerobic glycolysis; and malate dehydrogenase, for oxidative metabolism. There were also relatively weak associations between the fatigue indices of the MUs and the activity levels of the three enzymes. Thus, this work supports previous conclusions that the force decline exhibited by MUs during electrically evoked contractions depends on both MF biochemistry and other intracellular mechanisms. Electromyographic measurements suggested that these other mechanisms are distal to the intramuscular branches of the axon in type FF units, and distal to excitation-contraction coupling in type S units.

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 (R(N)), threshold (rheobase, I(Rh)), 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 sigma 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 sigma 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 I(Rh) < R(N) < f/I slope for the MN +IN sample and I(Rh) < R(N) = 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.

Effects of excitatory modulation on intrinsic properties of turtle motoneurons.

The purpose of this study was to quantify the effects of excitatory modulation on the intrinsic properties of motoneurons (MNs) in slices of spinal cord taken from the adult turtle. Responses were noted following application of an excitatory modulator: serotonin (5-HT), muscarine, trans-1-amino-1,3-cyclopentane dicarboxylic acid (tACPD), or all three combined. A sample of 44 MNs was divided into 2 groups, on the basis of whether MNs did (28/44) or did not (16/44) demonstrate a nifedipine-sensitive acceleration of discharge during a 2-s, intracellularly injected stimulus pulse. Such acceleration indicates the development of a plateau potential (PP). Excitatory modulation lowered the MNs' resting potential, increased input resistance, decreased rheobase, reduced several afterhyperpolarization values, and shifted the conventional, one-phase stimulus current-spike frequency (I-f) relation to the left. For both MN groups, the relative efficacy of excitatory modulation on both non-PP and PP MNs was generally in the following order: combined application > 5-HT approximately muscarine > tACPD. In many instances, the effects of modulation differed significantly for non-PP versus PP MNs, the most pronounced being in their I-f relation. To describe this difference, it was necessary to measure a two-phase relation. In PP MNs, excitatory modulation considerably increased the slope of the first (initial) phase and flattened the second (later) phase of this relation. The latter result bore similarities to that obtained in a previous study, which addressed MN firing behavior during fictive locomotion of the high-decerebrate cat.

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 MU's compound action potential is dissociated from the profile of the presumed depolarizing pressure exerted on the unit's 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.