Rat α‐ and γ‐motoneuron soma size and succinate dehydrogenase activity are independent of neuromuscular activity level

Abstract

The chronic level of neuromuscular activity, that is, activation and loading, strongly influences the morphological, metabolic, phenotypic, and physiological properties of skeletal muscles. The effects on the innervating motoneurons, however, are less established. We determined and compared the effects of 30 days of decreased activity (induced by a complete mid-thoracic spinal cord transection, ST) or near inactivity (induced by spinal cord isolation, SI) on the soma size and succinate dehydrogenase (SDH) activity of motoneurons innervating a predominantly slow ankle extensor (soleus) and a predominantly fast ankle flexor (tibialis anterior) muscle of adult rats. Soleus and tibialis anterior motoneuron pools were labeled retrogradely using nuclear yellow. The alpha- and gamma-motoneurons were classified based on soma size. Mean number of labeled motoneurons, and mean soma size and SDH activity for both alpha- and gamma-motoneurons were similar in control, ST, and SI rats. Compared to previous reports showing significant decreases in muscle fiber size and adaptations toward a "faster" metabolic profile following ST and SI, the results indicate that, unlike the muscles they innervate, the motoneurons are relatively unresponsive to chronic reductions in neuromuscular activity. The implication of these results is that mean size and SDH activity are independent of the number of action potentials generated by both alpha- and gamma-motoneurons and that even the absence of afferent input to the spinal cord has no influence on size and oxidative metabolic potential of the motoneuron soma.

Specificity of Sensory and Motor Neurons Associated with BL40 and GB30 in the Rat: A Dual Fluorescent Labeling Study

“…According to the size of soma diameter, the labeled motor neurons were divided into two classes: the large one (soma diameter > 25  μ m) or the small one (soma diameter ≤ 25  μ m), belonging to α and γ motor neurons, respectively [19]. The approximate 60 transverse sections from spinal cord were counted on every rat, resulting in a total of 136 AF488-CTB labeled motor neurons (BL40) and 174 AF594-CTB labeled motor neurons (GB30).…”

Fast Kinetics, High-Frequency Oscillations, and Subprimary Firing Range in Adult Mouse Spinal Motoneurons

“…In the study of McHanwell and Biscoe (1981), the equivalent circle diameters of the soma ranged from 19 to 37 µm (mean ~26 µm; 32 µm in the motoneuron we labeled in Figure 1B). For comparison, in cat motoneurons, soma diameters ranged from 40 to 80 µm (mean ~50 µm; Burke et al, 1977; Ulfhake and Kellerth, 1984; Horcholle-Bossavit et al, 1988) whereas they ranged from 20 to 40 µm (mean 30 µm) in rats (Swett et al, 1986; Hashizume et al, 1988; Roy et al, 2007). Finally, Ishihara et al (2001) showed that the mean somatic diameter of soleus motoneurons was 26 µm in mice, 34 µm in rats, and 51 µm in cats.…”

Distribution and Localization of 5-HT1AReceptors in the Rat Lumbar Spinal Cord after Transection and Deafferentation

“…A complete spinal cord transection at the T7–T8 level was performed as described previously (Talmadge et al, 2002). Spinal cord isolation was performed as previously described (Grossman et al, 1998; Roy et al, 2007a,b). Briefly, a partial laminectomy was performed between vertebral levels T7 and ~S1.…”