Evidence of incomplete neural control of motor unit properties in cat tibialis anterior after self-reinnervation

Common errors in textbook descriptions of muscle fiber size in nontrained humans

Exercise science and human anatomy and physiology textbooks commonly report that type IIB muscle fibers have the largest cross-sectional area of the three fiber types. These descriptions of muscle fiber sizes do not match with the research literature examining muscle fibers in young adult nontrained humans. For men, most commonly type IIA fibers were significantly larger than other fiber types (six out of 10 cases across six different muscles). For women, either type I, or both I and IIA muscle fibers were usually significantly the largest (five out of six cases across four different muscles). In none of these reports were type IIB fibers significantly larger than both other fiber types. In 27 studies that did not include statistical comparisons of mean fiber sizes across fiber types, in no cases were type IIB or fast glycolytic fibers larger than both type I and IIA, or slow oxidative and fast oxidative glycolytic fibers. The likely reason for mistakes in textbook descriptions of human muscle fiber sizes is that animal data were presented without being labeled as such, and without any warning that there are interspecies differences in muscle fiber properties. Correct knowledge of muscle fiber sizes may facilitate interpreting training and aging adaptations.

Polysialylated neural cell adhesion molecule is necessary for selective targeting of regenerating motor neurons

It is well established that peripheral nerves regenerate after injury. Therefore, incomplete functional recovery usually results from misguided axons rather than a lack of regeneration per se. Despite this knowledge very little is known about the molecular mechanisms regulating axon guidance during regeneration. In the developing neuromuscular system the neural cell adhesion molecule (NCAM) and its polysialic acid (PSA) moiety are essential for proper motor axon guidance. In this study we used a well established model of nerve transection and repair to examine whether NCAM and/or PSA promotes selective regeneration of femoral motor nerves in wild-type and NCAM (-/-) mice. We found that regenerating axons innervating the muscle pathway and, to a lesser extent, cutaneous axons in the sensory pathway reexpress high levels of PSA during the time when the cut axons are crossing the lesion site. Second, we found that motor neurons in wild-type mice preferentially reinnervated muscle pathways, whereas motor neurons in NCAM (-/-) mice reinnervated muscle and cutaneous pathways with equal preference. Preferential regeneration was not observed in wild-type mice when PSA was removed enzymatically from the regenerating nerve, indicating that this form of selective motor axon targeting requires PSA. Finally, transgenic mice were used to show that the number of collateral sprouts, their field of arborization, and the withdrawal of misprojected axons were all attenuated significantly in mice lacking PSA. These results indicate that regenerating motor axons must express polysialylated NCAM, which reduces axon-axon adhesion and enables motor neurons to reinnervate their appropriate muscle targets selectively.

Regional distribution of slow-twitch muscle fibers after reinnervation in adult rat hindlimb muscles

In adult rats, the sciatic nerve was unilaterally sectioned and reunited above the knee. Following a survival time of 21 weeks, five muscles were removed from both lower hindlimbs after determining their intra-limb positions. In each muscle, cryostat sections from seven equidistant proximo-distal levels were stained for myofibrillar ATPase. Intramuscular positions were determined for all slow-twitch type I fibers. Within each muscle, type I fibers were heterogeneously distributed, and the direction of type I fiber accumulation was, on average, almost identical in reinnervated muscles and contralateral controls. Furthermore, as in controls, a proximo-distal decline of type I fiber density was found in reinnervated muscles. Compared to contralateral controls, reinnervated muscles consistently showed a very high number of type I fibers at close interfiber distances, indicating respecification of muscle fiber types by the ingrowing nerve fibers. The results suggest that slow-twitch motor axons preferentially grew back toward the original slow-twitch muscle regions.

Role of motor unit structure in defining function

Motor units, defined as a motoneuron and all of its associated muscle fibers, are the basic functional units of skeletal muscle. Their activity represents the final output of the central nervous system, and their role in motor control has been widely studied. However, there has been relatively little work focused on the mechanical significance of recruiting variable numbers of motor units during different motor tasks. This review focuses on factors ranging from molecular to macroanatomical components that influence the mechanical output of a motor unit in the context of the whole muscle. These factors range from the mechanical properties of different muscle fiber types to the unique morphology of the muscle fibers constituting a motor unit of a given type and to the arrangement of those motor unit fibers in three dimensions within the muscle. We suggest that as a result of the integration of multiple levels of structural and physiological levels of organization, unique mechanical properties of motor units are likely to emerge.

Transmission of forces within mammalian skeletal muscles

Most models of in vivo musculoskeletal function fail to take into account the diversity of force trajectories defined by muscle fiber architecture. It has been shown for many muscles, across species, that muscle fibers commonly end within muscle fascicles without reaching a myotendinous junction, and that many of these fibers show a progressive decline in cross-sectional area along the length of the muscle. The significance of these anatomical observations is that the tapering would seem to preclude forces generated at the largest cross-sectional area of the fibers being transmitted to the sarcomeres toward the ends of the tapered fiber. If all of the forces are transmitted via the sarcomeres arranged in series, those few sarcomeres at the smaller ends of the fibers must tolerate the stress exerted by the more numerous sarcomeres arranged in parallel at the portions of the fiber with larger cross-sectional areas. A logical alternative would be for forces to be transmitted laterally along the length of a fiber to the cell membrane and the extracellular matrix. Such a structural arrangement would permit an alternative force transmission vector and minimize the necessity for a precise level of force to be generated along the entire length of a fiber. There are cytoarchitectural and biochemical data demonstrating the presence of a subcellular network which is appropriately located to transmit forces from the active intracellular contractile elements to the extracellular intramuscular connective tissues. However, to fully comprehend how forces are transmitted from individual cross bridges to the tendon, it will be necessary to understand the interactions of all of the components of the muscle tendon complex from the molecular to the multicellular level. It is insufficient to know the physiology of the individual components in a restricted experimental paradigm and assume that these conditions account for the functional characteristics in vivo. Thus, the challenge is to understand how the sarcomeres and all of the associated structures transmit the forces of the whole muscle to its attachments.

Limited fiber type grouping in self-reinnervation cat tibialis anterior muscles

The percent and distribution patterns of three immunohistochemically identified fiber types within the anterior compartment of the cat tibialis anterior were determined 6 months after denervation and self-reinnervation. After self-reinnervation, mean frequencies of slow (9%) and fast (91%) fibers were similar to those in control (12% and 88%, respectively) muscles. However, a lower proportion of fast-1 (26%) and a higher proportion of fast-2 (65%) fibers were observed in self-reinnervated than control (32% and 56%) muscles. Quantitation of adjacencies between fibers of similar myosin heavy chain (MHC) phenotype, a measure of type grouping, revealed that the frequencies of two slow or two fast-1 fibers being adjacent in self-reinnervated muscles were similar to control. In contrast, the frequency of fast-2/fast-2 fiber adjacencies found in self-reinnervated muscles (45%) was significantly higher than in control muscles (37%). In both groups, the frequency of adjacencies between slow, fast-1, or fast-2 fibers was largely attributable to the number of each fiber type present. These data show that the incidence of grouping within each fiber type present was not altered after 6 months of self-reinnervation. Minimal changes in the spatial distribution of fiber types following self-reinnervation in adults suggests a limited degree of conversion of muscle fibers to a MHC phenotype matching the motoneuron characteristics.

Myosin heavy chain composition of adult feline (Felis catus) limb and diaphragm muscles

The myosin heavy chain (MHC) compositions of adult feline limb and diaphragm muscles were determined. Sodium dodecyl sulfate-polyacrylamide gels (SDS-PAGE) were able to separate three different MHC isoforms. This was in contrast to rat muscles, in which four MHC isoforms were separated by SDS-PAGE. The fastest migrating cat MHC migrated similar to rat type I MHC and labeled in Western blots with a monoclonal antibody (mAb) specific for slow MHC and was categorized as type I. The other two MHC isoforms labeled in Western blots with a mAb specific for fast MHC and were categorized as type II. The slowest migrating fast isoform migrated similar to rat type IIa MHC and labeled with mAb N2.261, specific for types I and IIa; therefore, this MHC was categorized as type IIa. The intermediate migrating cat MHC did not migrate similar to either rat type IIx or type IIb and was not reactive with mAbs N2.261, 35 (specific for rat I, IIa, and IIb MHCs), or F3 (specific for rat IIb MHC). In tissue sections, type IIB fibers (based on myofibrillar ATPase histochemistry) were also unstained with mAbs N2.261 and 35. Therefore, the intermediate migrating cat MHC was categorized as type IIx. Consequently, feline limb and diaphragm muscles were composed of fibers containing type I, IIa, or IIx MHCs. The observations that type I and IIa isoforms, but not IIx, had similar electrophoretic mobilities in the cat and rat and that type IIb was absent from cat limb muscles suggest that there is greater diversity in MHC isoforms IIb and IIx compared to I and IIa in cats compared to rats.

Composition of newly forming motor units in prenatal rat intercostal muscle

We have examined the composition of rat intercostal motor units during the period of late gestation, when most muscle fibres are formed, in order to see the pattern of the contacts initially made between single motoneurons and myotubes. At this early stage, the muscle contains two types of myotubes, primary and secondary myotubes, and a major aim was to see whether individual motoneurons preferentially made contact with a particular myotube type. The technique used to define myotubes contacted by a single motoneuron was anterograde labelling of the neuron, followed by electron microscopic detection of labelled terminals and their postsynaptic targets. We find that prenatal motor units are inhomogeneous with respect to their primary/secondary myotube composition. Most individual motoneurons show many permutations of contact with primary myotubes, secondary myotubes, and undifferentiated cells, including single nerve terminals which contact both primary and secondary myotubes. Our results are interpreted in terms of changes to the composition of both the muscle and of the motor units during the final 5 days of gestation. We demonstrate that motoneurons necessarily make their initial contacts on primary myotubes, but that these are surprisingly sparse. As secondary myotubes appear and become innervated, motor units are at first all similar and all heterogeneous. However, primary myotubes are represented more often in motor units than in the muscle as a whole. This probably reflects the relative densities of polyinnervation of primary vs. secondary myotubes. By embryonic day 20, motor units have become divergent in composition, with some dominated by primary myotubes and others by secondaries. We propose that motoneurons initially establish contacts at random on either myotube type, but then begin to express preference for one type or the other and reorganise their periphery. Refining of motor unit composition towards homogeneity in the postnatal period probably involves other elements, such as mutability of muscle fibre and/or motoneuron characteristics as a function of usage and muscle position, perhaps influenced by sensory feedback mechanisms.

Function-dependent anatomical parameters of rabbit masseter motor units

Rabbit masseter motor units (22) were studied by stimulation of trigeminal motoneurons. We tested the hypotheses that masseter motor units facilitate fine motor control by concentrating fibers in small areas and that the distribution of motor unit fibers depends on the fiber type. The twitch contraction time and the isometric tetanic force were registered. The motor unit fibers were depleted of their glycogen by prolonged stimulation. Serial sections of the entire muscle were stained with the periodic acid Schiff (PAS) and monoclonal antibody stains. The muscle fibers of the motor unit were mapped and identified by four myosin heavy-chain (MHC) isoforms: I, IIA, IID, and cardiac-alpha. In the PAS-stained sections, anatomical parameters of the motor units, affecting the force output, were analyzed: the innervation ratio (IR), motor unit territory area (TA), and relative (R-DENS) and absolute (A-DENS) motor unit fiber densities. The fiber cross-sectional area (F-CSA) was measured for each MHC fiber type. The F-CSA sum of all motor unit fibers, the physiological cross-sectional area (P-CSA), was calculated. The IR ranged between 77 and 720 fibers (mean, 267). The mean TA was 8.71 mm2 (range, 4.45 to 19.58). The mean R-DENS was 10 fibers per 100; the A-DENS was 31 fibers per mm2. Linear correlations were found between the IR and the R-DENS and between the tetanic force and the IR. The F-CSAs showed a stepwise increase in value from type I- to IID-MHC fibers. The mean P-CSA was 0.90 mm2 (range, 0.09 to 2.97). A high linear correlation was noted between the P-CSA and the tetanic force. In conclusion, increase of motor unit size expressed in higher fiber counts and forces is accomplished by increase of the fiber density. Thus, forces can be exerted selectively in restricted regions of the masseter muscle. Differences in fiber orientation due to complex muscle pinnation emphasize the possibility of an accurate muscle performance.

Further evidence of incomplete neural control of muscle properties in cat tibialis anterior motor units

To examine the influence of a motoneuron in maintaining the phenotype of the muscle fibers it innervates, myosin heavy chain (MHC) expression, succinate dehydrogenase (SDH) activity, and cross-sectional area (CSA) of a sample of fibers belonging to a motor unit were studied in the cat tibialis anterior 6 mo after the nerve branches innervating the anterior compartment were cut and sutured near the point of entry into the muscle. The mean, range, and coefficient of variation for the SDH activity and the CSA for both motor unit and non-motor unit fibers for each MHC profile and from each control and each self-reinnervated muscle studied was obtained. Eight motor units were isolated from self-reinnervated muscles using standard ventral root filament testing techniques, tested physiologically, and compared with four motor units from control muscles. Motor units from self-reinnervated muscles could be classified into the same physiological types as those found in control tibialis anterior muscles. The muscle fibers belonging to a unit were depleted of glycogen via repetitive stimulation and identified in periodic acid-Schiff-stained frozen sections. Whereas muscle fibers in control units expressed similar MHCs, each motor unit from self-reinnervated muscles contained a mixture of fiber types. In each motor unit, however, there was a predominance of fibers with the same MHC profile. The relative differences in the mean SDH activities found among fibers of different MHC profiles within a unit after self-reinnervation and those found among fibers in control muscles were similar, i.e., fast-2 < fast-1 < or = slow MHC fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

The significance of motor unit variability in sustaining mechanical output of muscle

Neuromuscular function and fatigue have been studied using a wide variety of preparations. These range from sections of single fibers from which the cell membrane has been removed to whole muscles or groups of muscles acting about a joint in the intact animal. Each type of preparation has its merits and limitations. There is no ideal preparation; rather the question to be answered will determine the most appropriate model in each case and sometimes a combination of approaches will be needed. In particular, it is important to understand how the mechanical output of whole muscle can be sustained to meet the demands of a task and to take into account the organized variability of the constituent motor units.