IIC Fibers Research Articles

ラット内喉頭筋線維の生後変化

The purpose of this study was to analyse the postnatal development of the intrinsic laryngeal muscle fibers using the myosin ATPase stain method. We measured the ratio of type IIc fibers in all of the intrinsic laryngeal muscle fibers in postnatal-day (PND) 1, 3, 7, 14, 21 and mature Sprague-Dawley rats.Type IIC fibers disappear at PND 14 in most intrinsic laryngeal muscle fibers. However, the decreasing rate of the fibers is different for various intrinsic laryngeal muscles. Decreases in type IIc fibers are observed most in the lateral compartment of the thyroarytenoid muscle (1-TA), and then in the posterior cricoarytenoid muscle (PCA), cricothyroid muscle (CT), lateral cricoarytenoid muscle (LCA), interarytenoid muscle (IA) and the medial compartment of the thyroarytenoid muscle (m-TA).These results suggest that the differentiation of the intrinsic laryngeal muscle fibers in rats is nearly accomplished by the end of the second week.

Estudio Histoquímico y Morfométrico de la Población Fibrilar del Müsculo Diafragma. Parte II: Vaca y Cerdo

M-ATPase techniques, at different pH levels prior to incubation, as well as metabolic procedures (alpha-MGPDH and NADH-TR), were used to determine the fibre-type content of the lumbar and costal portions in the bovine and porcine diaphragm. The fibre samples were exposed to image analysis in order to evaluate both the morphometrical and statistical values. On the basis of staining reactions, the fibre types IA, IB and IIC were found in the lumbar portion of the bovine diaphragm; in the costal portion of the same diaphragm, IA, IB, IIA and IIC fibre types were observed. With regard to the porcine diaphragm, the I, IIA, IIB, IIC fibre types were present in both portions of the muscle. Further morphometrical and statistical data concerning all the fibre types are obtained and discussed.

Comparison of the foetal development of muscle in normal and double-muscled cattle

Muscle differentiation was studied in foetal Semitendinosus muscle from normal cattle and those with the 'culard' gene of muscular hypertrophy sampled at 90, 130, 170 and 210 days of foetal life. The different fibre types were characterized by immunohistochemistry with antibodies specific to different isoforms of myosin heavy chains. The isoforms were separated by electrophoresis, identified by immunoblotting and quantified by ELISA. In double-muscled animals, there was a slower rate of differentiation in the first generation of cells, most markedly apparent at 90 days. At 130 days, differentiation was retarded mainly in the second generation, while at 170 days there was no longer any difference between the two animal types in the differentiation of first generation cells, which were totally slow in both. At the same stage however, type IIC fibres in double-muscled animals were much slower in appearing and continued to be so at 210 days, albeit to a lesser extent. These findings show that differentiation of the muscle fibres occurs at a slower rate in double-muscled foetuses particularly during the first two-thirds of foetal life.

Study of the influence of age and weaning on the contractile and metabolic characteristics of bovine muscle

Weaning is an interesting period for the study of the nutritional regulation of muscle energy metabolism, since during this stage the nature of the substrates supplied to the muscle and their energy balance are profoundly changed. The aim of this study was to determine the effect of these modifications on the contractile and metabolic characteristics of bovine muscle. Two similar groups of 7 male Montbéliard calves were used with the same age and weight, and with the same energy intake. One group consisted of milk-fed calves, the other of weaned animals. The latter were progressively weaned over a period between 107 and 128 d. The average age at slaughter in the 2 groups was 170 d. Biopsy specimens of semitendinosus (ST) muscle were taken at the ages of 66 d, 94 d (before the beginning of weaning) and 136 d (at the end of weaning) to follow the evolution of muscle characteristics. Samples of longissimus thoracis (LT) muscle were taken 24 h after slaughter and used to study the changes in protein and DNA content. The proportion and area of the different types of fiber, I (slow, oxidative), IIA (fast, oxido-glycolytic), IIB (fast, glycolytic) and IIC (fast/slow, oxidoglycolytic) were measured by immunohistochemistry and image analysis. The metabolism of the muscles was determined by studying isocitrate dehydrogenase (ICDH, oxidative) and lactate dehydrogenase (LDH, glycolytic) activity. The results obtained between 2 and 6 months of life showed an overall increase in the area of the fibers (I, IIA, IIB and IIC) and a conversion of type IIA fibers into type IIB accompanied by a shift in the energy metabolism towards a glycolytic type. Weaning caused temporary stress, whose main consequences were to decrease overall muscle fiber area and the percentage of type IIB fibers, and increase the proportion of type IIC fibers in weaned animals. These effects may have been due to the nutritional and behavioral disturbances that accompany weaning, because 42 d after the end of weaning there was no difference in the size of ST and LT fibers between the 2 groups whereas the proportion of type IIA fibers was still higher in weaned animals.

Post-natal changes in the biological characteristics of Semitendinosus muscle in male Limousin cattle

Samples of semitendinosus muscle from 45 male Limousin cattle reared under similar conditions were taken at 1, 6 and 12 months of age (biopsy) and at 16 months (at slaughter). The development of biochemical (DNA, protein/DNA), enzymic (lactic dehydrogenase, isocitric dehydrogenase), and histochemical (ATPase, succunic dehydrogenase) characteristics was studied. Muscle fibres were classified according to their contractile and metabolic properties (SO = slow oxidative; FOG = fast oxidative glycolytic; FG = fast glycolytic). DNA accumulation or hyperplasia was observed up to the age of 16 months. The period of growth and development, between 1 and 12 months of age, was characterized by increased glycolytic metabolism, as evidenced by the conversion of FOG fibres into FG fibres, a decrease in ICDH activity and a rise in LDH activity. The period between 12 and 16 months, was characterized by a slowing down of the conversion of FOG fibres into FG fibres and an increase in ICDH activity. The presence of type IIC fibres at the four ages studied is discussed. Of the muscle characteristics studied, LDH activity seemed to be a discriminating factor between animals from an early age. It could thus be used as to predict animal muscle characteristics at birth.

Morphological, histochemical and stereological analysis of the female canine M. urethralis

The M. urethralis was morphologically investigated in ten medium-sized female dogs of different breeds and age, as well as histochemically and stereologically analysed in a homogeneous group of five female beagles. Macroscopically, the muscle was essentially confined to the distal third and most strongly developed in the fourth quarter of the urethra. Here, it surrounded the urethra transversely at the ventral and lateral aspects, passing with its caudal fibres dorsally onto the vagina. The muscle fibres were assembled in groups of different sizes and usually separated by thick connective tissue septae. Based on the myofibrillar actomyosin ATPase (mATPase) reaction, type I and two main subtype II fibres could be differentiated. Type II fibres were, however, indistinguishable by their metabolic enzyme activities since both subclasses displayed oxidative-glycolytic properties. The subtype II fibres containing the more acid-labile mATPase activity were classified as IIA, whereas the other main subtype was designated IIS (subtype) and considered as peculiar to the dog. In addition, the investigation revealed a rare fibre type exhibiting the histochemical profile of IIC fibres. There was no evidence of classical glycolytic IIB fibres. The M. urethralis was composed of 24% type I and 76% type II fibres with an average diameter of 34.9 and 28.5 microns, respectively. Connective tissue constituted 52.8% of the total muscle volume. Due to the predominance of fast twitch II fibres, the urethral muscle is not designed to maintain a high tone over long periods of time. Its primary role is to function rapidly and intermittently guaranteeing urinary continence during stress situations when other continence factors are overburdened.

Myosin isoforms and their subunits in the lungfish Protopterus annectens: Changes during development and the annual cycle

AbstractMyosin isoforms and their light and heavy subunits were analyzed, in a comparison with the profile of myosin ATPase during the development and annual cycle of the African lungfish Protopterus annectens. Axial muscle of the larvae contained three isoforms of larval myosin. In adult animals, larval isomyosins were shown to coexist with small amounts of fast isomyosins in axial muscle. By contrast, fin muscles displayed essentially the fast isomyosins and a slow myosin isoform. No significant differences were noted in myosin content between swimming, cocooning, and water‐readapted animals. Larval and fast myosin isoforms were composed of two different heavy chains, HCI and HCf, respectively, in association with fast light chains. The slow myosin isoform was characterized by a distinct heavy chain, HCs, which was associated with two slow light chains. Analysis of the profile of myosin ATPase revealed only type IIC fibers in the axial muscles of larvae, while the adult muscles were composed of type IIC fibers in the central part and adult type IIA, IIB, and I fibers in the peripheral part. This study shows that P. annectens resembles the neotenic urodelan amphibian Ambystoma mexicanum at the level of the distribution of muscle fiber types and the expression of myosin isoforms. © 1994 Wiley‐Liss, Inc.

Total parenteral nutrition does not improve diaphragm development in premature baboons

We hypothesized that total parenteral nutrition accelerates growth and development of diaphragm muscle (DPH) in prematurely delivered baboons (140 days gestation). For 10 days after delivery by cesarean section, we administered parenteral nutrition containing glucose, electrolytes, and water or total parenteral nutrition containing lipids, amino acids, glucose, vitamins, and electrolytes. After 10 days of care, dorsolateral and ventrolateral (VL) costal DPH were sampled for histochemically determined mean fiber area (MFA) and fiber type percentages. We determined isolated bundle isometric tension (normalized for cross-sectional area), time to peak tension, half-relaxation time, force-frequency relationship, and fatigability. Neither sex nor nutritional treatment affected contractile properties. Differences among sexes and muscle sites, but not among nutritional treatments, were observed for histochemical characteristics. In females, the VL DPH had a lower percentage of type IIo fibers and a greater MFA of type IIc fibers than the dorsolateral DPH and a lower percentage of type IIo fibers and greater MFA of type IIc and IIo fibers than the VL DPH in males. Mean fiber cross-sectional area of VL DPH was significantly greater in females than males. The larger fibers in females than males suggest a stronger DPH in females. Earlier growth of type II fibers in females could contribute to a better outcome for female than male premature infants with hyaline membrane disease.

Expression of myosin isoforms and of desmin, vimentin and titin in Tunisian Duchenne-like autosomal recessive muscular dystrophy

Morphological, morphometrical, histoenzymological, immunocytochemical and biochemical analysis were performed on muscle biopsies taken from patients suffering from tunisian autosomal recessive Duchenne-like muscular dystrophy (TDLMD) selected both by Duchenne-like clinical criteria and by the presence of normal dystrophin. Data were compared to that obtained from DMD biopsies characterized by the absence of dystrophin. The distribution of myosin heavy chain isoforms, desmin, vimentin and titin were determined in type I and type II muscle fibers. The protein pattern appeared to be less affected in TDLMD than in DMD biopsies. The regenerating fibers were mainly but not exclusively type IIC; a noticeable percentage of both type I and type II fibers coexpressed fast and slow MHC isoforms in TDLMD. This percentage was lower than in DMD. The expression of embryonic, fetal, and fast/slow myosin isoforms in type IIC fibers in TDLMD and DMD suggest different fiber type transformations in these two diseases.

Morphologic and morphometric characteristics of limb muscles of the goat

Use of (1) two histochemical reactions (myosin adenosine triphosphatase, mATPase, and nicotinamide adenine dinucleotide dehydrogenase-tetrazolium reductase, NADH-TR), (2) biopsies from deep and superficial parts of muscles, and (3) different indices (percentage population, PP, vs. cross-sectional area proportion, CSAP; and lesser (LD) vs. equivalent diameter (D)) for estimating fibre type composition or size were compared, and data obtained on composition and size of fibre types in 25 limb muscles of goats. Both reactions differentiated types I, IIA and IIB fibres and yielded reproducible but different estimates of fibre composition or size. In the extensor carpi radialis muscle, mATPase sections yielded higher estimates of type I fibre composition (7.2 ± 0.1 vs. 6.5 ± 0.1; P < 0.01) and lower estimates of type IIA fibre diameter (45.3 ± 1.5 vs. 48.4 ± 1.0; P < 0.05) than NADH-TR sections. PP and CSAP yielded different estimates of composition of types I, IIA and IIB fibres (7.2 ± 0.1 vs. 6.6 ± 0.3, 30.2 ± 1.6 vs. 28.4 ± 1.2 and 62.3 ± 1.8 vs. 65.0 ± 1.4, respectively; P < 0.05) as did biopsies from superficial and deep parts of the muscle (6.4 ± 0.2 vs. 35.8 ± 0.3, 26.3 ± 5.9 vs. 54.3 ± 4.2, and 67.4 ± 5.7 vs. 9.9 ±4 .5, respectively, for types I, IIA and IIB fibres; P < 0.001). LD and D also yielded different estimates of fibre diameter. Most limb muscles of goats contained types I, IIA, IIB and few IIC fibres in varying compositions and sizes. Type I fibre composition ranged from 6.4% in the tensor fascia lata to over 99.7% in the soleus muscles, increased with depth into extensor muscles and decreased with depth into flexor muscles, the latter being contrary to the pattern of fibre type stratification in other species. Goat muscles also differed in that mATPase and NADH-TR activities did not exhibit a reciprocal relationship in types IIA and IIB fibres as in many other mammalian species. These differences indicate caution in comparing results between studies that use different methods, and call for specification or standardization of biopsy sites and methods used to study muscle biopsies. They also suggest that goats may differ from other species in the contractile properties of types IIA and IIB muscle fibres.

Pituitary-thyroid axis controls the final differentiation of the dorsal skeletal muscle in urodelan amphibians.

A histoenzymological study of the ATPase activity of myosin in the dorsal axis muscle (dorsalis trunci) was carried out on two species of urodelan amphibians: Pleurodeles waltlii, a euthyroid species with spontaneous metamorphosis and Ambystoma mexicanum, a neotenic hypothyroid species. P. waltlii and A. mexicanum underwent an operation after which cytological analysis of the remaining pituitary were carried out in parallel. The muscle phenotype of urodelan amphibians varies according to the thyroid status of the species. In euthyroid adults, IIA fibers are dominant whereas in hypothyroid adults, IIC fibers are dominant. The number of type IIB (fast) and type I fibers (slow) are similar in both species. Physiological or experimental modulation of the concentration of circulating thyroid hormones results in a modification of the muscle fiber type profile pertaining to the considered species. We found that pituitary (TSH) plays a dominant role in the maturation of type IIC fibers in both species. Moreover, it seems to modulate the development of IIA fibers in P. waltlii and that of IIB fibers in A. mexicanum. Its action is thus species specific. Through partial or total hypophysectomy experiments, we have been able to demonstrate the influence of the hypophysothyroidian axis on the appearance of the adult muscle phenotype during metamorphosis.

Compensatory effects of chronic electrostimulation on unweighted rat soleus muscle

The purpose of this study was to investigate the effects of electrostimulation in counteracting the transformation of the unweighted rat soleus muscle. The stimulation resembled the firing patterns of normal slow motor units and was imposed during hindlimb suspension. For the 10-day hindlimb suspended rats, the transformation of the slow soleus muscle towards a faster type was characterized by a decrease in the time to peak tension and the half-relaxation time of the twitch, a reduction in the P20/P0 index, i.e. the ratio of the subtetanic tension at 20 Hz relative to the tetanic tension, and a decrease in the percentage distributions of type I fibres accompanied by an increase of type IIa and IIc fibres. These changes were prevented by electrostimulation since, for the parameters mentioned above, no significant difference was observed in the soleus of the suspended rats that received electrostimulation when compared with the control rats. Nevertheless, neither the loss of mass nor the decrease in force output in the suspended rats were prevented by electrostimulation. The present results suggest a positive compensation of the suspension-induced alterations in the contractile and histochemical properties of the soleus muscle by means of chronic electrostimulation, which, however, do not prevent atrophy or the loss of contractile force.

Changes in fibre-type composition and myosin heavy-chain IId isoform in rat soleus muscle during recovery period after hindlimb suspension

We examined the changes in myosin heavy-chain (HC) isoforms and fibre-type composition in rat soleus muscle using both myosin adenosine triphosphatase staining and sodium dodecyl sulphate/polyacrylamide gel electrophoresis (SDS-PAGE) analyses during the recovery period after 4 weeks of hindlimb suspension. Although there was no change in type IIc fibres after the suspension, an increase in this type of fibres was observed during the 1- to 4-week recovery period. The increase in type IIc fibres was considered to be due to a shift from type IIa to IIc fibres. The SDS-PAGE analysis revealed the presence of the HC IId isoform, which was not observed in the control muscle, after a 4-week hindlimb suspension. The HC IId isoform gradually decreased over 3 weeks of recovery and disappeared in the 4th week of recovery after the suspension. These results suggest that the hypogravity conditions induced by hindlimb suspension stimulated the synthesis of the HC IId isoform, whereas an increase in mechanical load to the muscle accelerated the degradation of the HC IId isoform and the synthesis of type IIc fibres during the recovery period after hindlimb suspension.

Muscle histochemical and biochemical properties in relation to meat quality during selection for increased lean tissue growth rate in pigs1

The relationship between the histochemical and biochemical properties of muscle and the quality of meat during selection to improving the lean tissue growth rate was studied. Pigs (n = 82) from Generations 2 and 4 were randomly sampled in a selection experiment in which the genotype x protein interaction was studied. Comparisons were made of longissimus muscle (LM) and quadriceps femoris (QF) from Generations 2 and 4, and biceps femoris (BF) in the fourth generation from pigs fed a low- or high-protein diet. A higher total growth rate, lean tissue growth rate, and lean percentage were found in pigs fed the high-protein diet than in pigs fed the low-protein diet. Pigs fed the high-protein diet had a higher glycolytic capacity in all muscles than pigs fed the low-protein diet. When the meat quality traits were compared, pigs fed the high-protein diet had a lower intramuscular fat content, higher shear force value, higher protein extractability, lower light reflectance, and somewhat higher water-holding capacity. With selection, pigs became fatter on both diets. In Generation 4, glycolytic and oxidative capacity was somewhat higher in muscle tissue than in Generation 2. Muscle metabolic profile and meat quality traits differed among muscles (LM, BF, QF) but no pronounced differences were found between generations. No marked changes were observed in Type I, IIA, or IIB fibers, either between diets or between generations. The Type IIC fibers could only be identified in Generation 4.

Morphological and Histochemical Type Differentiation of Muscle Fibres in Newborn Piglets of the Vietnamese Breed

Kaman J.: Morphological and Histochemical Type Differentiation of Muscle Fibres in Newborn Piglets of the Vietnamese Breed. Acta Vet. Brno, 82, 1993: 139-149. A standard histological technique and a histochemical detection of myosin Ca­ ATPase with acid and alkaline pre-incubation were used in a study of 11 topo­ graphically and functionally different muscles in 11 male and female newborn piglets. Both extra- and intrafusal muscle fibres (MFs) of the newborns were fully morphologically and adequately histochemically differentiated. The myosin Ca­ ATPase detection method was used to differentiate between acid stable and alkali unstable muscle fibres of Type I, alkali stable and acid unstable muscle fibres of Type II (in some cases differentiated further into muscle fibres of Type IIA and lIB), and the intermediary muscle fibres of Type IIC with the MS, SS and, sporadically also SM subtypes transforming into Type I fibres. An objective type identification of muscle fibres is only possible when a comparison of myosin Ca-A TPase reactions in serial sections after acid and alkaline pre-incubations are made. The muscles examined were classified into two basic groups. Those with a higher number (2-7) of Type IIC fibres in the primary bundle (in. sternomastoideus, m. trapezius, m. tibialis cranialis) exhibited better morphological, and particularly histochemical, differentiation than others: these were characterized by a smaller number of Type IIC fibres with largely uniform morphological, but different histochemical, characteristics on the surface as well as in deep in the muscle. In view of our findings and because pigs belong to nidifugous mammals, we reject the views suggesting that the myofibrillar hypoplasia of muscle fibres in newborn piglets of meat breeds is a physiological phenomenon. Skeletal muscles, muscle fibre, morphological differentiation, type differentiation, histo­ chemistry, myojibrillar hypoplasia, splayleg, Vietnamese pig, newborn pig

Immobilization‐induced muscle atrophy is not reversed by lengthening the muscle

In clinical practice, repaired tendocalcaneus (Achilles tendon) ruptures are often protected in immobilization casts for 4 weeks in the fully plantar flexed position and for up to another 4 weeks after returning the ankle to joint neutral. Moderate to severe muscle atrophy occurs within 4 weeks of immobilization in plantar flexion, but it is not known if this atrophy is minimized or reversed following restoration of joint neutral position. We tested the hypothesis that the extent of atrophy could be reduced by returning the ankle to joint neutral after 4 weeks of immobilization. Eighteen rabbits were anesthetized, and their right hind-limbs were casted with the knee flexed 90 degrees and the ankle fully plantar flexed. Three animals each were studied after 3, 4, 6, or 8 weeks of immobilization. After 4 weeks of immobilization, the immobilization casts of the remaining six rabbits were modified to return the ankle to joint neutral for another 2 or 4 weeks. For muscle studies, the animals were anesthetized, and the soleus (SOL), plantaris (PLN), and gastrocnemius (GST) muscles were removed and weighed; the SOL and PLN were quick frozen and processed for histochemical fiber typing and fiber cross-sectional area measurement. All three muscles showed significantly reduced muscle weight to body weight ratios after 3 weeks of immobilization. SOL was the most affected, and GST was least affected. There was no significant further atrophy through 8 weeks of immobilization. The atrophy correlated with a significant reduction of mean fiber area (MFA) for Types I, IIo, and IIc fibers in SOL and PLN. In PLN, Type IIg fiber area was not significantly reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle atrophy continues after early cast removal following tendon repair.

We studied soleus (SOL), plantaris (PLN), and gastrocnemius (GST) muscles to determine whether early cast removal minimizes muscle atrophy or permits recovery from atrophy after tendon repair. After right tendocalcaneus (Achilles tendon) was transected and repaired, rabbit right hindlimbs were immobilized with the ankle plantar flexed and the knee flexed to 90 degrees. Rabbits were maintained in the cast and sacrificed at 5, 15, or 21 days postoperatively or the cast was removed on day 5 and the animals sacrificed at day 15 or 21. SOL, PLN, and GST muscles of both limbs were removed and weighed, and then histochemical analyses were performed on SOL and PLN muscles. Immobilization decreased SOL muscle wet weights, mean fiber cross-sectional area, and percentage of Type I fibers and increased the percentage of Type IIc fibers. Ten days after cast removal (i.e., postoperative day 15), SOL muscle atrophy and fiber composition did not differ significantly from continuously immobilized controls. However, 16 days after cast removal (i.e., postoperative day 21), SOL muscle fiber cross-sectional area and fiber composition were near normal, differing significantly from continuously casted controls. At each of the time intervals studied, PLN (containing many glycolytic fibers) did not atrophy as much as SOL (containing mainly oxidative fibers). Our results indicate that 1) early cast removal prevents atrophy of PLN glycolytic fibers, but not oxidative fibers in either PLN or SOL, and 2) early cast removal promotes recovery from atrophy of both oxidative and glycolytic fibers. In spite of the many differences between rabbits and humans, these findings suggest that, although early cast removal may not prevent oxidative muscle fiber atrophy after postoperative immobilization, it may facilitate recovery from atrophy.

Presence of cardiac alpha-myosin correlates with histochemical myosin Ca2+ ATPase activity in rabbit masseter muscle.

A combined enzyme-histochemical (ATPase reactivity) and immunohistochemical study has been performed on sections of rabbit masseter muscle. The majority of the fibres previously designated as type IIC and/or type I according to their ATPase activity were found to contain 'cardiac' alpha-myosin heavy chain in addition to other myosin heavy chains. All alpha-myosin heavy chain-containing fibres reveal ATPase activity after pre-incubation at pH 4.2-4.6 similar to that of the classical type I fibres, while, in that pH range, limb type IIC fibres show intermediate ATPase activity. One group of these fibres reveal ATPase activity after pre-incubation at pH 10.1-10.3 as well, but not at pH 10.4-10.5. These fibres contain exclusively either alpha- or alpha- and I-myosin heavy chains but do not contain the IIA-myosin heavy chain. The second part of the fibres reveals ATPase activity after treatment within the whole alkaline pre-incubation range (pH 10.1-10.5) and these fibres contain alpha-myosin and IIA-myosin but no I-myosin heavy chain. It is concluded that the classical IIC fibre type is not present in the rabbit masseter muscle. Furthermore, ATPase reactivity does not allow us to distinguish fibres on their myosin heavy chain content in rabbit masseter muscle.

A study of glycogen depletion and the fibre-type composition of cat skeleto-fusimotor units.

1. We have used the glycogen-depletion technique, combined with myofibrillar ATPase (mATPase) staining for muscle fibre type, to study the fibre-type composition of four skeleto-fusimotor (beta) units in cat peroneus tertius, namely, one beta dynamic (beta d) unit and three beta static (beta s) units. 2. Depletion of glycogen was observed in serial cross-sections of thirty-four beta-unit extrafusal muscle fibres of various types traced from origin to insertion. No fibre was depleted of glycogen throughout its length; depletion was restricted to a number of zones, usually about five. Oxidative (type I) and oxidative-glycolytic (type IIA) fibres were depleted for a significantly greater proportion of their total length than glycolytic IIB fibres. 3. The fibre-type composition of the beta d unit was determined by tracing its fibres from end to end. The muscle unit consisted of one intrafusal bag1 fibre and ninety-three extrafusal muscle fibres comprising seventy-six type I fibres, eleven IIC fibres, and six fibres that changed from IIC to I during the course of their length (IIC/I fibres). The extrafusal fibre-type composition was thus 81.7% I plus 18.3% IIC and IIC/I. 4. The three beta s units (beta s1, beta s2, beta s3) were all fast-contracting and fatigued rapidly. Identification of their extrafusal fibre types, made in 1 mm2 areas sampled from different parts of each unit, gave mixed compositions as follows: beta s1, IIB + 6.7% IIA; beta s2, IIB + 5.8% IIA; beta s3, IIB + 29.9% IIA. The intrafusal component of each unit included either one or two long chain fibres. 5. In a discussion of the results, the fact that the continuous stimulation of extrafusal muscle fibres does not deplete them of glycogen throughout their length is examined in relation to the work of others who have assumed that it did. With regard to the finding of mixed extrafusal fibre types in the beta units, a distinction is drawn between minimal (around 5%) and moderate mixing. It is suggested that minimal mixing may occur in any motor unit as the outcome of endplate degeneration with foreign replacement, but that moderate mixing indicates an on-going process of conversion from one fibre type to another which in the adult may prove to occur only among beta units.

Histochemical, biochemical, and ultrastructural analyses of single human muscle fibers, with special reference to the C-fiber population.

A muscle biopsy from the vastus lateralis muscle of a strength-trained woman was found to contain an unusual fiber type composition and was analyzed by histochemical, biochemical, and ultrastructural techniques. Special attention was given to the C-fibers, which comprised over 15% of the total fiber number in the biopsy. The mATPase activity of the C-fibers remained stable to varying degrees over the pH range normally used for routine mATPase histochemistry. Although a continuum existed, the C-fibers were histochemically subdivided into three main fiber types: IC, IIC, and IIAC. The IC fibers were histochemically more similar to the Type I, the IIAC were more similar to the Type IIA, and the IIC were darkly stained throughout the pH range. Biochemical analysis revealed that all C-fibers coexpressed myosin heavy chains (MHC) I and IIa in variable ratios. The histochemical staining intensity correlated with the myosin heavy chain composition such that the Type IC fibers contained a greater ratio of MHCI/MHCIIa, the IIAC contained a greater ratio of MHCIIa/MHCI, and the Type IIC contained equal amounts of these two heavy chains. Ultrastructural data of the C-fiber population revealed an oxidative capacity between fiber Types I and IIA and suggested a range of mitochondrial volume percent from highest to lowest such that I greater than IC greater than IIC greater than IIA-C greater than IIA. Under physiological conditions, it appears that the IC fibers represent Type I fibers that additionally express some fast characteristics, whereas the Type IIAC are Type IIA fibers that additionally express some slow characteristics. Fibers expressing a 50:50 mixture of MHCI and MHCIIa (IIC fibers) were rarely found. It is not known whether C-fibers represent a distinct population between the fast- and slow-twitch fibers that is specifically adapted to a particular usage or whether they are transforming fibers in the process of going from fast to slow or slow to fast.