Glycerolphosphate oxidase and succinate dehydrogenase activities in IIA and IIB fibres of mouse and rabbit tibialis anterior muscles

Immunohistochemical Identification of Muscle Fiber Types in Mice Tibialis Anterior Sections

Mammalian skeletal muscle is a metabolically active tissue that is made up of different types of muscle fibers. These myofibers are made up of important contractile proteins that provide force during contraction of the muscle like actin and myosin. Murine myofibers have been classified into 4 types: Type I, Type IIa, Type IIb and Type IIX. Each muscle fiber has been identified with specific type of MyHC expressed, which in turn gives differential contractility to the muscle. There have been well-known methodologies to identify different myofibers: histochemical myosin ATPase staining which uses the differential ATPase activity between slow and fast fibers, quantification of metabolic enzymes like malate dehydrogenase and lactate dehydrogenase on specific fragments of muscle fibers. The drawback of these techniques is that they cannot differentiate the subtypes of myofibers, for example, Type IIa and Type IIb. They should be used in conjunction with other known histochemical staining techniques. Here, we devise a direct and robust immunohistochemical staining methodology that utilizes the differential expression of MyHC isoforms in different myofibers types, thus efficiently distinguishing the heterogeneity of the muscle fibers. We use antibodies that specifically recognize Type I, Type IIa and Type IIb fibers on serially cut frozen mouse tibialis anterior sections that can be quantified by ImageJ software.

Effects of divergent selection for 8-week body weight on postnatal enzyme activity pattern of 3 fiber types in fast muscles of male broilers (Gallus gallus domesticus)

A divergent selection experiment was conducted for 8-wk BW in chickens. At 3, 6, 9, and 12 wk of age, samples of pectoralis profundus (PP) and biceps femoris (BF) muscles from fast-growing and slow-growing lines were used to estimate the enzyme activities and muscle fiber diameter. Microphotometric measurements made in situ of succinate dehydrogenase (SDH, EC 1.3.99.1) and glycerol-3-phosphate dehydrogenase (GPDH, EC 1.1.99.5) were completed on serial sections of PP and BF muscles from male chickens, in order to examine the ratio of SDH:GPDH activity in single fibers. On the basis of the SDH:GPDH activity ratios, muscle fibers were divided using cluster analysis into 3 populations of different fiber types (O = oxidative, OG = oxidative-glycolytic, and G = glycolytic). Cockerels of the SGL attained an 8.1-fold increase and those of the FGL a 6.8-fold increase in BW at 12 wk compared with that at 3 wk of age. The O, OG, and G type fibers of the BF muscles of the SGL had significantly (P ≤ 0.001) lower SDH:GPDH activity ratios than those of the FGL. A step decrease in the SDH:GPDH activity of O, OG, and G fibers in the PP of both lines occurred, and this differed significantly between SGL and FGL (P ≤ 0.001). Age and line effects influenced the diameter of the 3 fiber types in the BF muscle only. In contrast to this response, all 3 fiber types of the PP muscles reached similar diameters in both lines during the growth process from wk 3 to 12. From the results of this study, we concluded that the activities of metabolic enzymes in skeletal muscle fibers are under the influence of muscle type, age, and selection pressure. Microphotometry is a suitable method for the evaluation of enzyme activity measured in a single muscle fiber. The method enables precise estimation of enzyme activities, especially in muscles composed of populations of different metabolic fiber types.

Selfishness examined: Cooperation in the absence of egoistic incentives

Social dilemmas occur when the pursuit of self-interest by individuals in a group leads to less than optimal collective outcomes for everyone in the group. A critical assumption in the human sciences is that people's choices in such dilemmas are individualistic, selfish, and rational. Hence, cooperation in the support of group welfare will only occur if there are selfish incentives that convert the social dilemma into a nondilemma. In recent years, inclusive fitness theories have lent weight to such traditional views of rational selfishness on Darwinian grounds. To show that cooperation is based on selfish incentives, however, one must provide evidence that people do not cooperate without such incentives. In a series of experimental social dilemmas, subjects were instructed to make single, anonymous choices about whether or not to contribute money for a shared “bonus” that would be provided only if enough other people in the group also contributed their money. Noncontributors cited selfish reasons for their choices; contributors did not. If people are allowed to engage in discussion, they will contribute resources at high rates, frequently on irrational grounds, to promote group welfare. These findings are consistent with previous research on ingroup biasing effects that cannot be explained by “economic man” or “selfish gene” theories. An alternative explanation is that sociality was a primary factor shaping the evolution of Homo sapiens. The cognitive and affective mechanisms underlying such choices evolved under selection pressures on small groups for developing and maintaining group membership and for predicting and controlling the behavior of other group members. This sociality hypothesis organizes previously inexplicable and disparate phenomena in a Darwinian framework and makes novel predictions about human choice.

On the function of muscle and reflex partitioning

Studies have shown that in the mammalian neuromuscular system stretch reflexes are localized within individual muscles. Neuromuscular compartmentalization, the partitioning of sensory output from muscles, and the partitioning of segmental pathways to motor nuclei have also been demonstrated. This evidence indicates that individual motor nuclei and the muscles they innervate are not homogeneous functional units. An analysis of the functional significance of reflex localization and partitioning suggests that segmental control mechanisms are based on subdivisions of motor nuclei–muscle complexes. A partitioned organization of segmental control mechanisms could utilize (1) the potential functional diversity of muscle fiber types, (2) the variety of mechanical actions of individual muscles arising from their distributed origins and insertions, and (3) diverse architectural features such as intramuscular variations in pinnation and complex in-series and in-parallel arrangements of muscle fibers. The differentiated activity observed in some muscles during natural movements also calls for localized segmental control mechanisms. Partitioning may also play a role in mechanical interactions between contracting motor units and in increasing the stability of neuromuscular systems. The functional advantages of reflex localization and partitioning suggest they are probably common features of segmental systems, whose organization reflects the structure and function of their associated neuromuscular systems.