Fiber type composition of the architecturally distinct regions of human supraspinatus muscle: a cadaveric study.

The human supraspinatus muscle is clinically important as it is frequently injured in older adults and the elderly. We have previously shown that the supraspinatus has a complex architecture with two distinct regions each consisting of three parts. Further we have found dynamic changes in architectural parameters such as fiber bundle length markedly vary between these regions. Fiber types of the supraspinatus have not been thoroughly investigated throughout its volume and are of interest to clinicians treating supraspinatus pathologies. In this study we investigated the distribution of fiber types within the distinct regions and parts of supraspinatus. Samples of supraspinatus were excised from six distinct parts of each muscle from five formalin embalmed specimens (one male, four female; mean age 77±11.1 years) free of tendon pathology. Samples were frozen in liquid nitrogen and then cryosectioned. Serial sections were labeled using immunohistochemical techniques and antibodies against fast or slow myosin heavy chain isoforms. The mean percentage of Type I (slow) fibers ranged from 56.73% to 63.97%. Results demonstrated significant variations in fiber type distribution. The middle part of the anterior region has a significantly greater percentage of Type I fibers compared to that of the posterior. The superficial part of the anterior region has a greater percentage of Type II (fast) fibers compared to the middle and deep parts. Findings aid in highlighting the distinct functions of the anterior and posterior regions, and prompt the need to re-evaluate assessment and treatment techniques established on a limited understanding of the fiber type distribution.

Evolutionary significance of myosin heavy chain heterogeneity in birds.

This article reviews the complexity, expression, genetics, regulation, function, and evolution of the avian myosin heavy chain (MyHC). The majority of pertinent studies thus far published have focussed on domestic chicken and, to a much lesser extent, Japanese quail. Where possible, information available about wild species has also been incorporated into this review. While studies of additional species might modify current interpretations, existing data suggest that some fundamental properties of myosin proteins and genes in birds are unique among higher vertebrates. We compare the characteristics of myosins in birds to those of mammals, and discuss potential molecular mechanisms and evolutionary forces that may explain how avian MyHCs acquired these properties.

Repression of myosin isoforms in developing and denervated skeletal muscle fibers originates near motor endplates.

During development of chicken pectoralis muscle, a neonatal myosin heavy-chain isoform is supplanted progressively by an adult isoform. This expression is under neuronal control. In this study we test the hypothesis that developmental myosin transformations are initiated near the motor endplate of each muscle fiber, thereafter progressing toward the fiber ends. By using immunocytochemical methods, pectoralis muscle from chickens aged 1-115 days after hatching were labeled by antibody against neonatal isoform. Ellipse minor axis and mean optical density of labeled and/or unlabeled fiber profiles from each bird were measured by computer image analysis. Acetylcholinesterase (AChE) activity was demonstrated histochemically. Using serial cross sections, we show that smaller fiber profiles are the tapered ends of larger fiber profiles. The largest fiber profiles (central regions of the fibers) were the first to lose their neonatal myosin during development. Motor endplates were localized by AChE activity to the central regions of the fibers. The pectoralis of mature chickens was denervated for 3, 7, 15, or 21 days. After 2 weeks' denervation, neonatal myosin is first reexpressed in the fiber ends. Dev Dyn 2000;217:50-61.

Persistent expression of developmental myosin heavy chain isoforms in the tapered ends of adult pigeon pectoralis muscle fibres.

We have shown previously that in addition to the adult myosin heavy chain (MyHC) isoform present throughout the length of each fast-twitch glycolytic muscle fibre within the pectoralis of the mature chicken, the neonatal isoform is retained in the tapered ends of these fibres. This work, however, has been the only published report of this phenomenon. Here, we tested the hypothesis that similar to the chicken, the ends of mature pigeon pectoralis muscle fibres contain developmental MyHC isoform(s). A histological stain was used to visualize endomysium to assist in the analysis of transverse sections of pectoralis muscle from four mature pigeons. Immunocytochemical techniques were used to localize MyHC isoform(s) characteristic of pigeon pectoralis development. We show that within mature pigeon pectoralis, the ends of both fast-twitch glycolytic and fast-twitch oxidative-glycolytic fibre types express MyHC isoform(s) characteristic of their earlier development. Thus, we extend our findings on chicken to another species and an additional muscle fibre type. Retention of developmental MyHC isoform(s) within the tapered ends of mature muscle fibres may be more widespread than is currently appreciated.

Expression of myosin heavy chain isoforms during development of domestic pigeon pectoralis muscle.

The pectoralis muscle of birds provides virtually all the power for the downstroke of the wing during flight. In adults it consists almost entirely of FOG (fast-twitch oxidative-glycolytic) and/or FG (fast-twitch glycolytic) fiber types. The aims of this study are to contrast MyHC (myosin heavy chain) transitions occurring within avian FG and FOG fibers during development, and to test the hypothesis that the pectoralis matures before the acquisition of flight. Pectoralis was obtained from pigeons (Columba livia) aged from 13 days in ovo to adult. Monoclonal antibodies generated against chicken MyHC isoforms were used with Western blots and immunocytochemistry. FG and FOG fibers were differentiated using a histochemical method demonstrating NADH (nicotinamide adenine dinucleotide), and "lesser fiber diameters" were quantified. Western blots confirm that the antibodies label pigeon MyHCs. A small number of the fibers are slow type in ovo, but these are quickly restricted in distribution and lost after hatching. In ovo fast-twitch fibers contain a ventricular isoform, and at least two embryonic-neonatal forms (designated E-N103 and E-N165). One week after hatching, fast-twitch fibers can be distinguished by NADH as FG or FOG. At fledging, four weeks after hatching, FG and FOG fibers are smaller than in older birds and E-N103 and E-N165 persist in both fibertypes. E-N103 wanes in all fibers shortly after fledging. E-N165 gradually disappears from FG fibers. Thus, despite pigeons being at adult body mass at fledging, their pectoralis is not fully mature.

Heterogeneity of myosin heavy-chain expression in fast-twitch fiber types of mature avian pectoralis muscle.

The aims of this study are to investigate the diversity of myosin heavy-chain (MyHC) expression among avian fast-twitch fibers, and to test the hypothesis that dissimilar MyHC isoforms are found in each of the principal avian fast-twitch fiber types. MyHCs within the muscle fibers of the pectoralis of 31 species of bird are characterized using immunocytochemical methods. A library of 11 monoclonal antibodies previously produced against chicken MyHCs is used. The specificity of these antibodies for MyHCs in each of the muscles studied is confirmed by Western blots. The results show that avian fast-twitch glycolytic fibers and fast-twitch oxidative-glycolytic fibers can contain different MyHCs. Among the species studied, there is also a conspicuous variety of MyHC isoforms expressed. In addition, the results suggest that two epitopes are restricted to chickens and closely allied gallinaceous birds. There are no apparent correlations between between MyHC epitope and presupposed contractile properties. However, the presence of different isoforms in different fast-twitch fiber types suggests a correlation between isoform and contractile function.

Myosin heavy chain expression within the tapered ends of skeletal muscle fibers.

BACKGROUND: The pectoralis muscle of the chicken contains fast-twitch glycolytic fibers, which during development undergo a transformation in their myosin heavy chain (MyHC) content from embryonic to a neonatal to an adult isoform (Bandman et al., 1990). Little, however, is known of MyHC expression within the ends of these or other muscle fibers. Here we test the hypothesis that the tapered ends of mature skeletal muscle fibers contain a less mature MyHC isoform than that typically found throughout their lengths. METHODS: We apply an ammoniacal silver histological stain for endomysium and monoclonal antibodies against neonatal and adult MyHCs of chicken pectoralis to transverse serial sections of pectoralis from five mature chickens. The "lesser fiber diameters" of populations of fibers from each bird are also measured. RESULTS: Most (approximately 81.8%) of the small (< 12 microns) and none of the larger (> 20 microns) diameter fibers contain the neonatal MyHC. Following these smaller fibers through serial sections, we show that they are the tapered ends of the larger fibers. Whereas neonatal MyHC is restricted to the tapered fiber ends, adult MyHC is present throughout the entire lengths of all fibers. We also demonstrate acetylcholinesterase (AChE) activity at some of these fiber ends. CONCLUSIONS: We postulate that longitudinal growth of myofibrils in adult muscle is characterized by the sequential expression of MyHC isoforms similar to that observed in rapidly growing muscle and that the presence of the neurotransmitter hydrolase AChE at the tapered fiber ends may be related to the retention of neonatal MyHC.

Metabolic capacity of muscle fibers from high-altitude natives.

We evaluate the effects of chronic hypoxia on the metabolic phenotype of the muscle fiber types of humans. The subjects were three Quechua natives residing in the Peruvian Andes at an altitude greater than 3300 m, and three lowlanders from below 700 m. Biopsy specimens were obtained from the vastus lateralis muscles of volunteers. Muscle fibers were identified histochemically as type 1 (oxidative), 2a (oxidative-glycolytic) or 2b (glycolytic). The relative contribution of each fiber type to the total cross-sectional area of each biopsy sample was determined. In individual fibers, the activities of malate dehydrogenase (MDH, citric acid cycle), lactate dehydrogenase (LDH, glycolysis) and adenylokinase (high-energy phosphate) were quantified. The cross-sectional area of the muscle occupied by each fiber type is comparable between Quechuas and lowlanders. Type 1 fibers are the only fiber type to demonstrate statistically significant (P < or = 0.05) differences in enzyme activities between Quechaus and lowlanders. MDH activity is, on average, 19.6% less (P < or = 0.0001) and LDH activity 28.1% more (P < or = 0.0001) in the type 1 fibers of the Quechuas. Chronic hypoxia appears to produce a shift from oxidative to glycolytic metabolism in those fibers which are typically the most aerobic in human muscle.

Metabolic capacity of individual muscle fibers from different anatomic locations.

We studied muscle fibers by quantitative biochemistry to determine whether metabolic capacity varied among fibers of a given type as a function of their anatomic location. Muscles were selected from both contiguous and diverse anatomic regions within the rats studied. The individual fibers, classified into myosin ATPase fiber types by histochemical means, were assessed for fiber diameters and analyzed for the activities of enzymes representing major energy pathways: malate dehydrogenase (MDH, oxidative), lactate dehydrogenase (LDH, glycolytic), and adenylokinase (AK, high-energy phosphate metabolism). We found that neither the average activities of each of the three enzymes nor the fiber diameters varied in Type I or Type IIa fibers selected from superficial to deep portions of the triceps surae of the hindlimb. However, the IIb fibers in the deep region of this muscle group had significantly greater oxidative capacity, less glycolytic capacity, and smaller diameters than the superficially situated IIb fibers. Type IIa fibers in lateral gastrocnemius, extensor digitorum longus, psoas, diaphragm, biceps brachii, superficial masseter, and superior rectus muscles were highly variable in both diameter and enzyme profiles, with a correlation between MDH activity and fiber diameter. Therefore, our results show that both intermuscular and intramuscular metabolic variations exist in muscle fibers of a given type.

Metabolic transitions in rat jaw muscles during postnatal development.

The program of acquisition of adult metabolic phenotypes was studied in three jaw muscles in order to determine the link between muscle metabolism and functional development. During early postnatal stages, there were similar transitions in the masseter, anterior digastric, and internal pterygoid muscles with respect to fiber growth, fiber type composition, and whole muscle energy metabolism. Oxidative capacity, as judged by the activities of the enzymes succinate dehydrogenase (SDH), malate dehydrogenase (MDH), and beta-hydroxyacyl CoA dehydrogenase (beta OAC), rose sharply after birth to reach near maximal levels by 3 weeks. The capacities for glycolytic metabolism represented by lactate dehydrogenase (LDH), and for high-energy phosphate metabolism represented by adenylokinase (AK) and creatine kinase (CK) activities, rose gradually, not reaching peak values until 6 weeks or later. Thus, the maturation of oxidative metabolism preceded that of glycolytic metabolism in the developing jaw muscles. This was documented for individual fibers in the masseter muscle. Differential metabolic maturation among the jaw muscles was evident beyond 3 weeks. All three jaw muscles attained their specific adult fiber-type profile by about 6 weeks. This maturation program differed from that of hindlimb muscles [Nemeth et al., J Neurosci 9:2336-2343, 1989] and diaphragm muscle [Kelly et al., J Neurosci 11:1231-1242, 1991], reflecting their differential energy demands for contractile performance.

Metabolic response to a high-fat diet in neonatal and adult rat muscle.

Neonatal rats were exposed to a high-fat low-carbohydrate diet to determine how substrate availability might affect the metabolic phenotype of muscle. Mixed-fiber homogenates of extensor digitorum longus, soleus, and diaphragm muscles were assayed for beta-hydroxyacyl-CoA dehydrogenase (beta-OAC), succinate dehydrogenase, malate dehydrogenase, lactate dehydrogenase, phosphofructokinase (PFK), adenylokinase, and creatine kinase. The three muscles showed significant increases in enzyme activity for fatty acid oxidation (beta-OAC) in weaned neonatal rats maintained on the high-fat diet compared with normal weaned controls. This effect persisted for 6 wk of the diet. The other consistent metabolic change was a decrease in PFK. Adult animals subjected to the same diet had similar increases in fatty acid oxidation and a fall in PFK after 1 wk, with most of these changes persisting for the 4 wk of the diet. Examination of individual fibers revealed enzyme changes in fibers of all types, but with the largest effect in type IIb fibers. The data indicate that both adult and neonatal muscles are similarly capable of adjusting their energy metabolism in response to dietary factors.

Metabolic and contractile protein expression in developing rat diaphragm muscle.

Progressive changes in myosin isozyme expression and in energy-generating enzyme activities were followed in the diaphragm and, for comparison, in axial and appendicular muscles of rats from 18 d gestation to maturity. Native myosins were characterized by pyrophosphate gel electrophoresis. Myosin heavy-chain (MHC) isozymes were measured with ELISA using monoclonal antibodies and were localized by immunocytochemistry. RNA transcripts for the MHCs were demonstrated on Northern blots and by RNase protection assays. Quantitative activities of malate dehydrogenase (MDH), beta-hydroxyacyl CoA dehydrogenase (beta OAC), 1-phosphofructokinase (PFK), lactate dehydrogenase (LDH), creatine kinase (CK), and adenylokinase (AK) were measured in muscle homogenates and in individual fibers by fluorometric pyridine nucleotide-dependent assays. Compared to limb muscles, expression of neonatal myosin in the diaphragm is precocious. Neonatal MHC mRNA is prominent in the diaphragm at 19 d gestation, and neonatal myosin is the major MHC isoform present at birth. Slow and fast IIa MHCs are also present at birth. Transcripts for IIa MHC are detectable in the diaphragm at 21 d gestation and are upregulated at birth. Comparable signal for IIa MHC mRNA is not found in the gastrocnemius until 10 d postpartum. Adult fast IIb MHC mRNA was detected only as a faint signal at 30-40 d in the diaphragm and then disappeared. Results indicate that a separate phenotype, the IIx type, matures late in diaphragmatic development. The activities of enzymes representing all of the major energy pathways are higher in the fetal diaphragm than in the fetal hindlimb muscles. For example, beta OAC had sixfold higher activity in the diaphragm than in the extensor digitorum longus (EDL) muscle at birth, activity in the diaphragm than in the extensor digitorum longus (EDL) muscle at birth.

Metabolic and contractile uniformity of isolated motor unit fibres of snake muscle.

1. Motor units in the thin transversus abdominis muscle of the garter snake were identified and physiologically characterized in the living state. Motor unit fibres, and fibres chosen randomly to serve as controls, were subsequently excised and subjected to biochemical analyses. 2. The metabolic capacity of fibres was assessed by measuring activities of three enzymes, each representing a different metabolic pathway. The microchemical enzyme assays were performed using enzyme extraction preparations of whole single fibres. 3. Metabolic capacity ranged widely among the muscle's entire fibre population, even among fibres of the same type. In contrast, enzyme activities of twitch fibres belonging to individual motor units were, within analytical error, identical. 4. Twitch contraction times of individual fibres within one motor unit were similar, compared to a wide range of contraction times observed among fibres of the same type but belonging to different motor units. 5. When several motor units were studied in one muscle, a systematic relationship was observed among motor unit tension, enzymatic profile and contraction time. As motor unit tension increased, fibres exhibited greater capacities for glycolytic and high-energy phosphate metabolism, diminished capacity for oxidative metabolism, and faster twitch contraction times. 6. Given the great diversity of metabolic and contractile properties exhibited within the fibre population, the uniformity of such properties within motor units indicates that neural influence dominates over other extrinsic factors present in the microenvironment of the muscle fibres.

Metabolic capacity and myosin expression in single muscle fibres of the garter snake.

1. The transversus abdominis muscle of the garter snake contains fibres of three types: tonic (T), slower twitch (S) and faster twitch (F). Fibre types can be determined by anatomical criteria in living preparations. Individual fibres identified as T, S or F were excised from the muscle and subdivided for two types of biochemical examination. Enzymes of energy metabolism were assayed using quantitative microfluorometric methods. Myosin heavy chain composition was determined by gel electrophoresis. In separate experiments, twitch time-to-peaks of F and S fibres were measured to assess the range of contraction times present within the muscle's twitch fibre population. 2. Metabolic subgroups of fibres were delineated by the relative activities of adenylokinase (AK), lactate dehydrogenase (LDH) and beta-hydroxyacyl-CoA-dehydrogenase (beta OAC). The metabolic subgroups corresponded to the anatomical fibre types. Type F fibres had high levels of enzymes associated with glycolytic (LDH) and high-energy phosphate (AK) metabolism. Type T fibres had high levels of the oxidative enzyme beta OAC. Type S fibres had both types of enzyme activity in intermediate and variable amounts. 3. Three myosin heavy chain isoforms were present in the muscle. Type F and type T fibres each expressed a single isoform, denoted F and T respectively. Type S fibres expressed significant quantities of two isoforms: an isoform unique to this fibre type (denoted S) and the F isoform. 4. Electrophoretic mobility and antibody reactivity of the F myosin heavy chain isoform resembled that of mammalian fast-twitch myosin. By the same criteria, the T isoform resembled mammalian slow-twitch myosin. The S isoform exhibited intermediate characteristics: its antibody reactivity was similar to mammalian fast-twitch myosin, but its electrophoretic mobility was that of mammalian slow-twitch myosin. 5. Based on whole-muscle analysis, two myosin alkali light chains, denoted ALC1 and ALC2, and one myosin regulatory light chain were present. Gel patterns suggested that ALC1 and ALC2 exist as both homodimers and heterodimers. 6. The population of type S fibres within a given muscle exhibited a much wider range of twitch contraction times than did the population of type F fibres. Diversity of contractile properties among type S fibres may result, in part, from differential co-expression of two myosin heavy chain isoforms, together with highly variable ratios of enzymes from two major metabolic pathways. 7. The clear biochemical distinction among fibre types indicates that each type possesses a unique and limited range of physiological properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Myosin heavy chain expression during development and following denervation of fast fibers in the red strip of the chicken pectoralis.

The myosin heavy chain composition of muscle fibers that comprise the red strip of the pectoralis major was determined at different stages of development and following adult denervation. Using a library of characterized monoclonal antibodies we found that slow fibers of the red strip do not react with antibodies to any of the fast myosin heavy chains of the superficial pectoralis. Immunocytochemical analysis of the fast fibers of the adult red strip revealed that they contain the embryonic fast myosin heavy chain rather than the adult pectoral isoform found throughout the adult white pectoralis. This was confirmed using immunoblot analysis of myosin heavy chain peptide maps. We show that during development of the red strip both neonatal and adult myosin heavy chains appear transiently, but then disappear during maturation. Furthermore, while the fibers of the superficial pectoralis reexpress the neonatal isoform as a result of denervation, the fibers of the red strip reexpress the adult isoform. Our data demonstrate a new developmental program of fast myosin heavy chain expression in the chicken and suggest that the heterogeneity of myosin heavy chain expression in adult fast fibers results from repression of specific isoforms by innervation.

Immunocytochemical localization of aldolase in normal, denervated, and dystrophic chicken muscles.

To investigate whether immunocytochemical localization of muscle-specific aldolase can be used for fiber phenotype determination, we produced specific antibodies against the enzyme and studied its distribution in adult chicken skeletal muscles by indirect immunofluorescence microscopy. Monoclonal antibodies against the myosin heavy chains of fast-twitch (MF-14) and slow-tonic (ALD-58) muscle fibers were also used to correlate aldolase levels with the fiber phenotype. The goat anti-aldolase antibody was found to be specific for the A form of aldolase, as evidenced by sodium dodecyl sulfate gel electrophoresis, immunotitration experiments, and immunoblot analysis. The antibody reacted strongly with the fast-twitch myofibers of normal pectoralis and posterior latissimus dorsi muscles; the phenotype of these muscle fibers was confirmed by a positive immunofluorescent reaction after incubation with MF-14 antibody. By contrast, the slow-tonic myofibers of normal anterior latissimus dorsi, which react positively with ALD-58 antibody, reacted weakly with anti-aldolase antibodies. In denervated chicken muscles, reaction to anti-aldolase antibodies was markedly reduced in fast-twitch fibers, although reaction to MF-14 was not diminished. By contrast, in dystrophic muscle, fast-twitch fibers showed reduced reactivity to anti-aldolase and marked to moderate reduction in MF-14 reactivity. Our results show that: (a) in normal muscles, reactivity to anti-aldolase matches the phenotype obtained by using anti-fast or anti-slow myosin heavy chain antibodies, and therefore can serve to identify mature fibers as fast or slow; and (b) in denervated or dystrophic muscles, the intracellular expressions of aldolase and fast-twitch myosin heavy chains are regulated independently.

A survey of tissue-specific isozyme expressions during chicken ontogeny.

The ontogenetic trends in the expression of 25 isozymes in liver, gizzard, heart, and pectoralis muscle of White Leghorn chickens were examined using starch gel electrophoresis. Little change in expression during development was evident in liver S-AAT-A, GPI-A, S-ICDH-A, S-MDH-A and M-MDH-A, in gizzard S-ACON-A, ADH-A, GPI-A, HK-1, HK-3, ME-A PEP-1, and PGM-A, in heart ADH-A, HK-1, HK-3, ME-A, PEP-2, PGM-A, and LDH-A, in pectoralis M-ACON-A, S-ACON-A, ADH-A, HK-1, HK-3, ME-A, PEP-2, and PGM-A, and in liver, gizzard, and heart M-ACON-A, ALD-A, CK-A, G3PDH-A, HK-1, and PGDH-A. Increasing levels of activity were demonstrated in liver ADH-A, ME-A, and PEP-2, in heart M-MDH-A, S-ICDH-A, M-ICDH, and M-AAT-A, and in pectoralis LDH-A, LDH-B, G3PDH-3, ALD-A, CK-A, HK-2, and PGM-B. There was a decrease in the activity of HK-1 in liver and in PEP-1 and PGDH-A in pectoralis muscle throughout development. While CK-C is active in the embryonic pectoralis, CK-A is restricted to later developmental stages. Isozyme expressions in regions of the pectoralis containing fast and slow muscle fibers in 7-month-posthatch individuals were noted and found to be identical. The results underscore the need to use similar developmental stages and tissue samples in comparative electrophoretic studies of birds.

Ultrastructural and cytological changes in the muscle fibers of the pectoralis of the giant Canada goose (Branta canadensis maxima) in disuse atrophy during molt.

Adult male Branta canadensis maxima were collected from a nonmigratory feral population during their premolt, molt and postmolt phases. Lean dry weight of the pectoralis muscle decreased significantly (p less than or equal to 0.0001) during molt, as a result of disuse atrophy. Histochemical analysis revealed that the region of the pectoralis muscle sampled consisted of Red (fast-twitch oxidative-glycolytic) and White (fast-twitch glycolytic) muscle fiber types, in an approximate ratio of 9 to 1. There was no significant (p = 0.1238) difference in the relative percentages of the two fiber types during the three periods of study. There was, however, a significant decrease in mean cross-sectional area of both Red (p less than or equal to 0.0194) and White (p less than or equal to 0.0001) fibers during molt. Red and White fiber areas were strongly correlated with each other during molt (r2 = 0.76, p = 0.0010) and postmolt (r2 = 0.70, p = 0.0052), but not during premolt (r2 = 0.02, p = 0.7626). The latter finding may be related to fiber-type specific hypertrophy in premolt breeding males. Analysis of ultrastructure revealed that there was a significant (p = 0.0003) decrease in the mean myofibrillar cross-sectional area, and a significant increase in both the density (p = 0.0227) and total number (p = 0.0058) of myofibrils within the muscle fibers of the molting birds. These results indicate that the myofibrils split longitudinally during molt-associated disuse atrophy.(ABSTRACT TRUNCATED AT 250 WORDS)

On the histochemical characterization and distribution of fast and slow muscle fibers in certain avian skeletal muscles.

Histochemical techniques based upon the pH sensitivity of myofibrillar adenosine triphosphatase are used to differentiate avian skeletal muscle fibers as either fast or slow. Pars thoracica m. pectoralis (PM) of several avian species, Pars cranialis m. latissimi dorsi (LDCR) of the Japanese quail, and M. tensor propatagialis (TP) of the domestic pigeon are examined. Fast fibers predominate in the PM, and slow fibers in the LDCR. The TP shows marked internal variation in the distribution of muscle fibers. The occurrence of fast and slow muscle fibers, both intra- and inter-muscularly, is correlated with their functional adaptations.

An exceptionally high density of muscle spindles in a slow-tonic pigeon muscle.

Histochemical and histological observations on the tiny wing muscle, M. coracotriceps, of the pigeon revealed a remarkably high density of muscle spindles (14,582 +/- 2,302/g of muscle)--approximately 15 times the highest densities hitherto reported for any muscle. Furthermore, all of the extrafusal fibers of this muscle were of the slow-tonic variety. This unique muscle probably functions as a mechanoreceptor extremely sensitive to changes in its own length.