Development of the masseter muscle and oral behavior in the pig

During mastication the adult pig masseter contracts with a complex pattern involving a wave of electromyographic (EMG) activity moving from the ventro-rostral corner to the dorso-caudal corner. The present study was undertaken to ascertain the ontogeny of that contraction pattern. Anatomical measurements were made on masseters from fetal, infant, and juvenile pigs. EMG activity from different parts of the masseter was recorded along with oral movements in infant and juvenile pigs as they suckled, drank, and chewed on food and non-food objects. The basic arrangement of muscle fibers and tendinous aponeuroses was found to be the same in all ages. The longest and most vertical fibers were found rostrally and ventrally, whereas the shortest and most horizontal fibers were found caudally and dorsally. The length of fasciculi decreased with age, relative to muscle weight. Variance in length among different parts of the muscle increased with age. Fetal masseters were oriented generally more horizontally than the masseters of older animals, except that the dorso-caudal corner, usually the most horizontal portion, is not developed in fetuses. The contraction patterns within the infant masseter were less complex than those of older animals; only the dorso-caudal corner was distinct. The further development of intramuscular differences in activity may be associated with the increasing anatomical complexity of the masseter, which augments its functional capabilities.     

Contour analysis of masticatory jaw movements and muscle activity in Macaca mulatta

Frontal plane mandibular movements during mastication and the associated electromyographic (EMG) activity for left and right superficial masseter, posterior temporalis, anterior temporalis, and anterior belly of the digastric (ABD) were studied for two adult male Macaca mulatta by the new technique of “contour” analysis. Contour analysis allowed graphic and quantitative portrayal of multiple chew cycle patterns of mandibular movement and EMG activity during active mastication. A series of computer programs (ATS, ATSED, ATSXYZ) facilitated the collection, editing and definition, and finally processing of these masticatory data into contour plots. These preliminary data indicated the essential symmetry of mandibular movement patterns, high chew cycle variability inferior to occlusion, multiple centers of intense EMG activity for balancing-side superficial masseter, and no difference between working-side anterior and posterior temporalis EMG patterns. Maximum EMG amplitude was found in the area of buccal phase power stroke (BPS). Maximum EMG amplitude for ABD was located medial and inferior to occlusion; all other muscle maximum amplitudes were buccal and inferior to occlusion. The location of maximum EMG amplitudes for superficial masseter and ABD were closer to occlusion (more superior) during mastication of carrot than were maximum amplitudes during biscuit mastication. The absence of any detectable shift of EMG maximum amplitude location between biscuit and carrot for posterior and anterior temporalis suggested, along with the continuous EMG activity of working-side posterior temporalis, a secondary role for the temporalis (compensation for superficial masseter activity) during active mastication.     

Symphyseal fusion and jaw-adductor muscle force: an EMG study

The purpose of this study is to test various hypotheses about balancing-side jaw muscle recruitment patterns during mastication, with a major focus on testing the hypothesis that symphyseal fusion in anthropoids is due mainly to vertically- and/or transversely-directed jaw muscle forces. Furthermore, as the balancing-side deep masseter has been shown to play an important role in wishboning of the macaque mandibular symphysis, we test the hypothesis that primates possessing a highly mobile mandibular symphysis do not exhibit the balancing-side deep masseter firing pattern that causes wishboning of the anthropoid mandible. Finally, we also test the hypothesis that balancing-side muscle recruitment patterns are importantly related to allometric constraints associated with the evolution of increasing body size. Electromyographic (EMG) activity of the left and right superficial and deep masseters were recorded and analyzed in baboons, macaques, owl monkeys, and thick-tailed galagos. The masseter was chosen for analysis because in the frontal projection its superficial portion exerts force primarily in the vertical (dorsoventral) direction, whereas its deep portion has a relatively larger component of force in the transverse direction. The symphyseal fusion-muscle recruitment hypothesis predicts that unlike anthropoids, galagos develop bite force with relatively little contribution from their balancing-side jaw muscles. Thus, compared to galagos, anthropoids recruit a larger percentage of force from their balancing-side muscles. If true, this means that during forceful mastication, galagos should have working-side/balancing-side (W/B) EMG ratios that are relatively large, whereas anthropoids should have W/B ratios that are relatively small. The EMG data indicate that galagos do indeed have the largest average W/B ratios for both the superficial and deep masseters (2.2 and 4.4, respectively). Among the anthropoids, the average W/B ratios for the superficial and deep masseters are 1.9 and 1.0 for baboons, 1.4 and 1.0 for macaques, and both values are 1.4 for owl monkeys. Of these ratios, however, the only significant difference between thick-tailed galagos and anthropoids are those associated with the deep masseter. Furthermore, the analysis of masseter firing patterns indicates that whereas baboons, macaques and owl monkeys exhibit the deep masseter firing pattern associated with wishboning of the macaque mandibular symphysis, galagos do not exhibit this firing pattern. The allometric constraint-muscle recruitment hypothesis predicts that larger primates must recruit relatively larger amounts of balancing-side muscle force so as to develop equivalent amounts of bite force. Operationally this means that during forceful mastication, the W/B EMG ratios for the superficial and deep masseters should be negatively correlated with body size. Our analysis clearly refutes this hypothesis. As already noted, the average W/B ratios for both the superficial and deep masseter are largest in thick-tailed galagos, and not, as predicted by the allometric constraint hypothesis, in owl monkeys, an anthropoid whose body size is smaller than that of thick-tailed galagos. Our analysis also indicates that owl monkeys have W/B ratios that are small and more similar to those of the much larger-sized baboons and macaques. Thus, both the analysis of the W/B EMG ratios and the muscle firing pattern data support the hypothesis that symphyseal fusion and transversely-directed muscle force in anthropoids are functionally linked. This in turn supports the hypothesis that the evolution of symphyseal fusion in anthropoids is an adaptation to strengthen the symphysis so as to counter increased wishboning stress during forceful unilateral mastication. (ABSTRACT TRUNCATED)     

Evidence for three fast myosin heavy chain isoforms in type II skeletal muscle fibers in the adult llama (Lama glama).

Skeletal muscle fiber types classified on the basis of their content of different myosin heavy chain (MHC) isoforms were analyzed in samples from hindlimb muscles of adult sedentary llamas (Lama glama) by correlating immunohistochemistry with specific anti-MHC monoclonal antibodies, myofibrillar ATPase (mATPase) histochemistry, and quantitative histochemistry of fiber metabolic and size properties. The immunohistochemical technique allowed the separation of four pure (i.e., expressing a unique MHC isoform) muscle fiber types: one slow-twitch (Type I) and three fast-twitch (Type II) phenotypes. The same four major fiber types could be objectively discriminated with two serial sections stained for mATPase after acid (pH 4.5) and alkaline (pH 10.5) preincubations. The three fast-twitch fiber types were tentatively designated as IIA, IIX, and IIB on the basis of the homologies of their immunoreactivities, acid denaturation of their mATPase activity, size, and metabolic properties expressed at the cellular level with the corresponding isoforms of rat and horse muscles. Acid stability of their mATPase activity increased in the rank order IIA>IIX>IIB. The same was true for size and glycolytic capacity, whereas oxidative capacity decreased in the same rank order IIA>IIX>IIB. In addition to these four pure fibers (I, IIA, IIX, and IIB), four other fiber types with hybrid phenotypes containing two (I+IIA, IIAX, and IIXB) or three (IIAXB) MHCs were immunohistochemically delineated. These frequent phenotypes (40% of the semitendinosus muscle fiber composition) had overlapped mATPase staining intensities with their corresponding pure fiber types, so they could not be delineated by mATPase histochemistry. Expression of the three fast adult MHC isoforms was spatially regulated around islets of Type I fibers, with concentric circles of fibers expressing MHC-IIA, then MHC-IIX, and peripherally MHC-IIB. This study demonstrates that three adult fast Type II MHC isoproteins are expressed in skeletal muscle fibers of the llama. The general assumption that the very fast MHC-IIB isoform is expressed only in small mammals can be rejected.     

Muscular design in the equine interosseus muscle

We studied the forelimb interosseus muscle in horses, Equus caballus, to determine the muscular properties inherent in its function. Some authors have speculated that the equine interosseus contains muscle fibers at birth only to undergo loss of these fibers through postnatal ontogeny. We describe the muscle fibers in eight interosseus specimens from adult horses. These fibers were studied histochemically using myosin ATPase studies and immunocytochemically using several antibodies directed against type I and type II myosin heavy chain antibodies. We determined that 95% of the fibers were type I, presumed slow-twitch fibers. All fibers exhibited normal morphological appearance in terms of fiber diameter and cross-sectional area, suggesting that the muscles are undergoing normal cycles of recruitment. SDS-PAGE studies of myosin heavy chain isoforms were consistent with these observations of primarily slow-twitch muscle. Fibers were determined to be approximately 800 microm long when studied using nitric acid digestion protocols. Short fiber length combined with high pinnation angles suggest that the interosseus muscle is able to generate large amounts of force but can produce little work (measured as pulling the distal tendon proximally). While the equine interosseus muscle has undergone a general reduction of muscle content during its evolution, it remains composed of a significant muscular component that likely contributes to forelimb stability and elastic storage of energy during locomotion.     

Fiber type changes in rat skeletal muscle after intense interval training

Female Sprague-Dawley rats were subjected to a ten week training program to determine the influence of intense interval running on the fiber type composition of selected hindlimb muscles; soleus (S), plantaris (P), deep vastus lateralis (DVL), and superficial vastus lateralis (SVL). The muscles of one hindlimb were used for histochemical ATPase analysis to determine the distribution of fiber types and those of the contralateral hindlimb were assayed biochemically for citrate synthase activity (an aerobic marker). Training induced a significant increase in citrate synthase activity in each muscle section. The largest absolute increase occurred in the DVL and the largest relative increase occurred in the SVL. The distribution of fiber types within the S (85% slow-twitch) and SVL (100% fast-twitch) remained unchanged with training. However, significant increases in the percentage of type I (slow-twitch) fibers in both the P (2-fold) and DVL (3-fold) were observed with concomitant decreases in the type II (fast-twitch) population. In addition, training induced significant changes in the fast-twitch subtype populations of the DVL (IIB----IIA). These data suggest exercise-induced fiber type transformations occurring both within the fast-twitch population and between fast-twitch and slow-twitch fibers in certain hindlimb muscles of the rat following a high intensity interval training program.     

Histochemical properties of skeletal muscle fibers in streptozotocin-diabetic rats

Summary The response of rat gastrocnemius muscle fibers to chronic streptozotocin-diabetes was studied. Transverse sections of this muscle from normal and diabetic rats were histochemically assayed for reduced diphosphopyridine nucleotide-diaphorase, myofibrillar adenosine triphosphatase, mitochondrial alpha-glycerophosphate dehydrogenase, beta-hydroxybutyrate dehydrogenase, and alkaline phosphatase activities. Cross-sectional areas of the fiber types were measured, and fiber capillarization and populations estimated. Chemically-induced diabetes appeared to have little effect on the metabolic or morphological properties of slow-twitch fibers. However, a general dedifferentiation occurred in the 2 fast-twitch fiber populations. There was a loss of oxidative potential in the fast-twitch-oxidative-glycolytic fibers, and a significant decrease in size in the fast-twitch-glycolytic fibers. No change in the proportions of slow- and fast-twitch fibers in the muscles of diabetic rats occurred. It is concluded that hypoinsulinism has differential effects on the 3 fiber types in heterogeneous rat skeletal muscle, and that slow-twitch fibers are least affected by the diabetic condition.     

Fiber type changes in rat skeletal muscle after intense interval training

Summary Female Sprague-Dawley rats were subjected to a ten week training program to determine the influence of intense interval running on the fiber type composition of selected hindlimb muscles; soleus (S), plantaris (P), deep vastus lateralis (DVL), and superficial vastus lateralis (SVL). The muscles of one hindlimb were used for histochemical ATPase analysis to determine the distribution of fiber types and those of the contralateral hindlimb were assayed biochemically for citrate synthase activity (an aerobic marker). Training induced a significant increase in citrate synthase activity in each muscle section. The largest absolute increase occurred in the DVL and the largest relative increase occurred in the SVL. The distribution of fiber types within the S (85% slow-twitch) and SVL (100% fast-twitch) remained unchanged with training. However, significant increases in the percentage of type I (slow-twitch) fibers in both the P (2-fold) and DVL (3-fold) were observed with concomitant decreases in the type II (fast-twitch) population. In addition, training induced significant changes in the fast-twitch subtype populations of the DVL (IIBIIA). These data suggest exercise-induced fiber type transformations occurring both within the fast-twitch population and between fast-twitch and slow-twitch fibers in certain hindlimb muscles of the rat following a high intensity interval training program.     

Morphological and histochemical organization of the flexor carpi radialis muscle in the cat.

Most studies concerning the structure and function of skeletal muscle have utilized the hind limb of the experimental animal. However, it has been shown that the number of behavioral tasks performed by the cat's forelimb is greater than that of the hind limb. In addition, the forelimb muscles exhibit a functional complexity not observed in hind-limb musculature. The purpose of this study was to investigate the distribution of fast-twitch and slow-twitch muscle fibers and muscle spindles in the flexor carpi radialis muscle (FCR) and to correlate the distributional patterns in these structures with muscle tendon architecture and muscle function. It was found that the FCR, a wrist flexor, contains 37% slow-twitch fibers and 63% fast-twitch fibers. However, the slow-twitch fibers were concentrated in the deep region located between the tendons of origin and insertion, while the fast-twitch-glycolytic fibers were concentrated more peripherally. Muscle spindles were associated with the slow-twitch region and were never found in the region containing high concentrations of fast-twitch-glycolytic fibers. Fast-twitch-oxidative-glycolytic fibers were uniformly distributed throughout the muscle. It is proposed that the association of muscle spindles with slow-twitch fibers and the differential distribution of muscle fibers into slow-twitch and fast-twitch regions might allow these regions to function independently of one another when called upon to perform complex behavioral tasks.     

Origin of intrafusal muscle fibers in the rat

The expression of several isoforms of myosin heavy chain (MHC) by intrafusal and extrafusal fibers of the rat soleus muscle at different stages of development was compared by immunocytochemistry. The first intrafusal myotube to form, the bag2 fiber, expressed a slow-twitch MHC isoform identical to that expressed by the primary extrafusal myotubes. The second intrafusal myotube to form, the bag1 fiber, expressed a fast-twitch MHC similar to that initially expressed by the secondary extrafusal myotubes. At subsequent stages of development, the equatorial and juxtaequatorial regions of bag2 and bag1 intrafusal myofibers began to express a slow-tonic myosin isoform not expressed by extrafusal fibers, and ceased to express some of the MHC isoforms present initially. Myotubes which eventually matured into chain fibers expressed initially both the slow-twitch and fast-twitch MHC isoforms similar to some secondary extrafusal myotubes. In contrast, adult chain fibers expressed the fast-twitch MHC isoform only. Hence intrafusal myotubes initially expressed no unique MHCs, but rather expressed MHCs similar to those expressed by extrafusal myotubes at the same chronological stage of muscle development. These observations suggest that both intrafusal and extrafusal fibers develop from common pools of bipotential myotubes. Differences in MHC expression observed between intrafusal and extrafusal fibers of rat muscle might then result from a morphogenetic effect of afferent innervation on intrafusal myotubes.     

Contractile function of single muscle fibers after hindlimb suspension

The purpose of this investigation was to determine how muscle atrophy produced by the hindlimb suspension (HS) model alters the contractile function of slow- and fast-twitch single muscle fibers. After 2 wk of HS, small bundles of fibers were isolated from the soleus and the deep and superficial regions of the lateral and medial heads of the gastrocnemius, respectively. The bundles were placed in skinning solution and stored at -20 degrees C until studied. Single fibers were isolated and suspended between a motor arm and force transducer, the functional properties were studied, and subsequently the fiber type was established by myosin heavy chain (MHC) analysis on 1-D sodium dodecyl sulfate polyacrylamide gel electrophoresis. After HS, slow-twitch fibers of the soleus showed a significant reduction in fiber diameter (68 +/- 2 vs. 41 +/- 1 micron) and peak tension (1.37 +/- 0.01 vs. 0.99 +/- 0.06 kg/cm2), whereas the maximal shortening speed (Vmax) increased [1.49 +/- 0.11 vs. 1.92 +/- 0.14 fiber lengths (FL)/s]. A histogram showed two populations of fibers: one with Vmax values identical to control slow-twitch fibers and a second with significantly elevated Vmax values. This latter group frequently contained both slow and fast MHC protein isoforms. The pCa-force relation of the soleus slow-twitch fibers was shifted to the right; consequently, the free Ca2+ required for the onset of tension and for 50% of peak tension was significantly higher after HS. Slow-twitch fibers isolated from the gastrocnemius after HS showed a significant reduction in diameter (67 +/- 4 vs. 44 +/- 3 microns) and peak tension (1.2 +/- 0.06 vs. 0.96 +/- 0.07 kg/cm2), but Vmax was unaltered (1.70 +/- 0.13 vs. 1.65 +/- 0.18 FL/s). Fast-twitch fibers from the red gastrocnemius showed a significant reduction in diameter (59 +/- 2 vs. 49 +/- 3 microns) but no change in peak tension or Vmax. Fast-twitch fibers from the white superficial region of the medial head of the gastrocnemius were unaffected by HS. Collectively, these data suggest that the effects of HS on fiber function depend on the fiber type and location. Both slow-twitch type I and fast-twitch type IIa fibers atrophied; however, only slow-twitch fibers showed a decline in peak tension, and the increase in Vmax was restricted to a subpopulation of slow-twitch soleus fibers.     

Changes in the metabolic profile of the equine gluteus medius as a function of sampling depth

1. Cross sections from the middle of the gluteus medius were removed from 10 adult horses and used to evaluate changes in histochemically determined muscle fiber type and biochemically determined metabolic enzyme activities as a function of sample depth. 2. Muscle fiber types determined using histochemical methods for myosin ATPase (pH 9.4) and succinic dehydrogenase (SDH) activity indicated percent fast-twitch glycolytic (FG) muscle fibers decreased and slow-twitch oxidative (SO) fibers increased as a function of increasing sampling depth. 3. Percent histochemically determined fast-twitch oxidative glycolytic (FOG) fibers decreased slightly only in the deepest region of the gluteus medius. 4. Citrate synthase (CS) enzymatic activity, used as a marker for mitochondrial oxidative potential, increased 2.5-fold in activity per g of muscle protein from 1 to 8 cm sampling depth. 5. 3-hydroxyacyl-CoA dehydrogenase (HAD) enzymatic activity, used as a marker for lipid oxidation potential, increased 3-fold in activity per g of muscle protein when the depth increased from 1 to 8 cm. 6. Phosphorylase (PS) enzymatic activity, used as a marker for potential glycogen utilization, decreased 50% in activity per g of muscle protein when going from 1 to 8 cm. 7. Lactate dehydrogenase (LDH) enzymatic activity, used as a marker for anaerobic glycolytic potential, decreased about 50% in activity as the sampling depth increased from 1 to 8 cm. 8. In summary, the superficial portion of the equine gluteus medius was found to be more glycolytic and less aerobic in its metabolic profile than deeper regions. The muscle became progressively more aerobic and less glycolytic with increasing sampling depth.     

Functional and evolutionary significance of the recruitment and firing patterns of the jaw adductors during chewing in Verreaux's sifaka (Propithecus verreauxi)

Jaw-muscle electromyographic (EMG) patterns indicate that compared with thick-tailed galagos and ring-tailed lemurs, anthropoids recruit more relative EMG from their balancing-side deep masseter, and that this muscle peaks late in the power stroke. These recruitment and firing patterns in anthropoids are thought to cause the mandibular symphysis to wishbone (lateral transverse bending), resulting in relatively high symphyseal stresses. We test the hypothesis that living strepsirrhines with robust, partially fused symphyses have muscle recruitment and firing patterns more similar to anthropoids, unlike those strepsirrhines with highly mobile unfused symphyses. Electromyographic (EMG) activity of the superficial and deep masseter, anterior and posterior temporalis, and medial pterygoid muscles were recorded in four dentally adult Verreaux's sifakas (Propithecus verreauxi). As predicted, we find that sifaka motor patterns are more similar to anthropoids. For example, among sifakas, recruitment levels of the balancing-side (b-s) deep masseter are high, and the b-s deep masseter fires late during the power stroke. As adult sifakas often exhibit nearly complete symphyseal fusion, these data support the hypothesis that the evolution of symphyseal fusion in primates is functionally linked to wishboning. Furthermore, these data provide compelling evidence for the convergent evolution of the wishboning motor patterns in anthropoids and sifakas.     

Origin of intrafusal muscle fibers in the rat

Summary The expression of several isoforms of myosin heavy chain (MHC) by intrafusal and extrafusal fibers of the rat soleus muscle at different stages of development was compared by immunocytochemistry. The first intrafusal myotube to form, the bag₂ fiber, expressed a slow-twitch MHC isoform identical to that expressed by the primary extrafusal myotubes. The second intrafusal myotube to form, the bag₁ fiber, expressed a fast-twitch MHC similar to that initially expressed by the secondary extrafusal myotubes. At subsequent stages of development, the equatorial and juxtaequatorial regions of bag₂ and bag₁ intrafusal myofibers began to express a slow-tonic myosin isoform not expressed by extrafusal fibers, and ceased to express some of the MHC isoforms present initially. Myotubes which eventually matured into chain fibers expressed initially both the slow-twitch and fast-twitch MHC isoforms similar to some secondary extrafusal myotubes. In contrast, adult chain fibers expressed the fast-twitch MHC isoform only. Hence intrafusal myotubes initially expressed no unique MHCs, but rather expressed MHCs similar to those expressed by extrafusal myotubes at the same chronological stage of muscle development. These observations suggest that both intrafusal and extrafusal fibers develop from common pools of bipotential myotubes. Differences in MHC expression observed between intrafusal and extrafusal fibers of rat muscle might then result from a morphogenetic effect of afferent innervation on intrafusal myotubes.     

Preparation of type-specific antimyosin antibodies and determination of their specificity by biochemical and immunohistochemical methods

This paper reports the preparation of specific anti-slow myosin antibodies (anti-I) and anti-fast myosin antibodies (anti-IIA) raised against myosins from sheep and guinea pig masseter muscles. The specificity of the antibodies has been studied by immunodiffusion in agar and by the GEDELISA test using slow-twitch (type I), fast-twitch red (type IIA) and fast-twitch white (type IIB) myofibrils isolated from guinea pig muscles. The principal specificity of the anti-I and anti-IIA antibodies was for the heavy chains of type I and IIA myosins, respectively. A smaller reaction with the corresponding light chains was also detected. Immunohistochemical staining of muscle sections using these antibodies confirmed their fibre type specificity.     

Lactate dehydrogenase isoenzyme spectrum of fast and slow muscles with innervation disorders

A relative content of muscle fibers of various types and the spectrum of lactate dehydrogenase (LDH) isozymes were studied in fast-twitch (extensor digitorum longus) and slow-twitch (soleus) muscles of newborn rats, of those aged 2, 3 weeks and one month and of adult rats after neonatal sciatic denervation and application of 0.5 mM colchicine solution to the sciatic nerve. No muscle fibers of various types were found (from the level of succinate dehydrogenase activity) in one-month-old rats, whereas the control and fast-twitch muscles showed A, B and C types and the slow-twitch one B and C types. The denervation brought about an increase in the content of LDH4 and LDH5 in both the muscles, while colchicine application gave rise to an increase in LDH2 activity, diminution of LDH1 in the fast-twitch muscle and elevation of LDH4 in the slow-twitch one. The data obtained attest to the retardation of muscle differentiation under application of the colchicine-induced blockade of axoplasmic transport.