Organization and fine structure of extraocular muscles in Carassius and Rana

Summary 1. Two types of muscle fibers, red (-slow) and white (-twitch), have been described in the extraocular muscles of Carassius and Rana, respectively. 2. Red and white muscle fibers occupy a definite position in particular eye muscles and occur in almost constant numerical relation. 3. The red fibers in the fish extraocular muscles are supposedly slow. The position of the triads is at the level of the A/I junction, whereas that of the white muscle fibers is at the Z line level. 4. In the frog the extraocular muscles consist of two types of muscle fibers, which have morphological features of slow and fast fibers, respectively, the triads being localized at the Z line level.This work has been supported by the Polish Academy of Sciences.Authors express their thanks to Doc. Dr. J. Kawiak for help in densitometrography.     

Anatomy and physiology of the swimming leg musculature in the blue crab,Callinectes sapidus

1) The anatomy and neurophysiology of muscles moving the fifth leg of Callinectes sapidus are described. Innervation of the muscles was studied using intracellular stimulating and recording techniques.

2) The movement of the leg is controlled by four sets of muscles composed of two, three, or four bundles each. The bundles which make up each muscle shared innervation with other muscle bundles within a functional muscle group. Muscle fibers characteristically displayed both “fast” and “slow” responses to stimulation.

3) Possible functional evolution of the neural changes which occurred with the morphological adaptations for swimming in C. sapidus is discussed.     

Anatomy and histochemistry of spread-wing posture in birds. 2. Gliding flight in the California Gull,Larus californicus: A paradox of fast fibers and posture

Gliding flight is a postural activity which requires the wings to be held in a horizontal position to support the weight of the body. Postural behaviors typically utilize isometric contractions in which no change in length takes place. Due to longer actin-myosin interactions, slow contracting muscle fibers represent an economical means for this type of contraction. In specialized soaring birds, such as vultures and pelicans, a deep layer of the pectoralis muscle, composed entirely of slow fibers, is believed to perform this function. Muscles involved in gliding posture were examined in California gulls (Larus californicus) and tested for the presence of slow fibers using myosin ATPase histochemistry and antibodies. Surprisingly small numbers of slow fibers were found in the M. extensor metacarpi radialis, M. coracobrachialis cranialis, and M. coracobrachialis caudalis, which function in wrist extension, wing protraction, and body support, respectively. The low number of slow fibers in these muscles and the absence of slow fibers in muscles associated with wing extension and primary body support suggest that gulls do not require slow fibers for their postural behaviors. Gulls also lack the deep belly to the pectoralis found in other gliding birds. Since bird muscle is highly oxidative, we hypothesize that fast muscle fibers may function to maintain wing position during gliding flight in California gulls. J. Morphol. 233:237–247, 1997. © 1997 Wiley-Liss, Inc.     

Reciprocal neural regulation of extrajunctional acetylcholinesterase and collagen Q in rat muscles—The role of calcineurin signaling

There are two main differences regarding acetylcholinesterase (AChE) expression in the extrajunctional regions of fast and slow rat muscles: (1) the activity of AChE catalytic subunits (G1 form) is much higher in fast than in slow muscles, and (2) the activity of the asymmetric forms of AChE (A₈ and A₁₂) is quite high extrajunctionally in slow muscles but virtually absent in fast muscles. The latter is due to the absence of the expression of AChE-associated collagen Q (ColQ) in the extrajunctional regions of fast muscle fibers, in contrast to its ample expression in slow muscles. We showed that both differences are caused by different neural activation patterns of fast vs. slow muscle fibers, which determine the respective levels of mRNA of both proteins. Whereas the changes in AChE mRNA levels in fast and slow muscles, as well as the levels of ColQ mRNA levels in slow muscles, observed in response to exposing either slow or fast muscles to different muscle activation patterns, are completely reversible, the extrajunctional suppression of ColQ expression in fast muscle fibers seems to be irreversible. Calcineurin signaling pathway in muscles is activated by high-average sarcoplasmic calcium concentration resulting from tonic low-frequency muscle fiber activation pattern, typical for slow muscle fibers, but is inactive in fast muscle fibers, which are activated by infrequent high-frequency bursts of neural impulses. Application to rats of two inhibitors of calcineurin (tacrolimus-FK506 and cyclosporin A) demonstrated that the mRNA levels of both the AChE catalytic subunit and ColQ in the extrajunctional regions of the soleus muscle are regulated by the calcineurin signaling pathway, but in a reciprocal way. Under the conditions of low calcineurin activity, AChE expression is enhanced and that of ColQ is suppressed, and vice versa. Our results also indicated that different, calcineurin-independent regulatory pathways are responsible for the reduction of AChE expression during muscle denervation, and for maintaining high ColQ expression in the neuromuscular junctions of fast muscle fibers.     

Effects of hypothyroidism on myosin heavy chain composition and fibre types of fast skeletal muscles in a small marsupial, Antechinus flavipes

Effects of drug-induced hypothyroidism on myosin heavy chain (MyHC) content and fibre types of fast skeletal muscles were studied in a small marsupial, Antechinus flavipes. SDS-PAGE of MyHCs from the tibialis anterior and gastrocnemius revealed four isoforms, 2B, 2X, 2A and slow, in that order of decreasing abundance. After 5 weeks treatment with methimazole, the functionally fastest 2B MyHC significantly decreased, while 2X, 2A and slow MyHCs increased. Immunohistochemistry using monospecific antibodies to each of the four MyHCs revealed decreased 2b and 2x fibres, and increased 2a and hybrid fibres co-expressing two or three MyHCs. In the normally homogeneously fast superficial regions of these muscles, evenly distributed slow-staining fibres appeared, resembling the distribution of slow primary myotubes in fast muscles during development. Hybrid fibres containing 2A and slow MyHCs were virtually absent. These results are more detailed but broadly similar to the earlier studies on eutherians. We hypothesize that hypothyroidism essentially reverses the effects of thyroid hormone on MyHC gene expression of muscle fibres during myogenesis, which differ according to the developmental origin of the fibre: it induces slow MyHC expression in 2b fibres derived from fast primary myotubes, and shifts fast MyHC expression in fibres of secondary origin towards 2A, but not slow, MyHC.     

The claw closer muscle of Neohelice granulata (Grapsoidea,Varunidae): a morphological and histochemical study

Longo, M.V., Goldemberg, A.L. and Díaz, A.O. 2011. The claw closer muscle of Neohelice granulata (Grapsoidea, Varunidae): a morphological and histochemical study. —Acta Zoologica (Stockholm) 92 : 126–133. The claw closer muscle of Neohelice granulata was studied according to histological, histochemical, and morphometrical criteria. Adult male crabs in intermoult stage were collected from Mar Chiquita Lagoon (Buenos Aires, Argentina). Muscle fibers show evident striations and oval‐elongated nuclei with loose chromatin. The loose connective tissue among muscle fibers consists of cells and fibers embedded in an amorphous substance. Muscle histochemistry reveals two slow fiber types: ‘A’ and ‘B’. Prevailing A fibers are larger, and they usually show, with respect to B type, a weaker reaction to whole techniques. Fibers with short (SS), intermediate (IS), and long sarcomeres (LS) appear in the claw closer muscle, being the LS fibers predominant. Concluding, the histochemical and morphometrical characteristics of the claw closer muscle fibers of N. granulata are indicative of slow fibers. The slow A type (low resistant to fatigue) prevails.     

The antennal motor system of crickets: modulation of muscle contractions by a common inhibitor,DUM neurons,and proctolin

In the crickets Gryllus bimaculatus and Gryllus campestris, the two intrinsic antennal muscles in the scape (first antennal segment) control antennal movements in the horizontal plane. Of the 17 excitatory antennal motoneurons, three motoneurons, two fast and one slow, can be stimulated selectively and their effect on muscle contraction, i.e. antennal movement, measured. Simultaneously, either a common inhibitor (CI) neuron or two DUM neurons can be stimulated and the effect on the slow and/or fast muscle contraction measured. The activity of the common inhibitor affected only slow muscle contractions. It decreased contraction rate, increased relaxation rate and suppressed prolonged muscle tension. This effect was blocked by picrotoxin. DUM neuron stimulation affected both slow and fast contractions. It reduced slow and enhanced fast contractions but in only 10% of the experiments could this effect be detected. DUM neuron activity could be mimicked by octopamine application. Proctolin application enhanced both slow and fast contractions but did not increase muscle tension in the absence of motoneuron activity. The results are discussed in relation to the large variability of possible antennal movements during behaviors.Abbreviations CI common inhibitor neuron - DUM dorsal unpaired median neuron     

Spatial fiber type distribution in normal human muscle Histochemical and tensiomyographical evaluation

The variability of fiber type distribution in nine limb muscles was examined with histochemical and tensiomyographical (TMG) methods in two groups of 15 men aged between 17 and 40 years. The aim of this study was to determine the extent to which the relative occurrence of different fiber types and subtypes varies within human limb muscles in function to depth and to predict fiber type proportions with a non-invasive TMG method.The distribution of different fiber types varied within the muscles, as a function of depth, with a predominance of type 2b fibers at the surface and type 1 fibers in deeper regions of the muscle. For all the analyzed muscles the contraction times measured at stimulus intensity 10% of supramaximal stimulus (10% MS) were significantly (p<0.05) shorter than the contraction times measured at 50% of supramaximal stimulus intensity (50% MS). The Pearson's correlation coefficient between percentage of type 1 muscle fibers measured at the surface of the muscle and contraction time at 10% MS, obtained by TMG was statistically significant (r=0.76,P<0.01). Also the Pearson's correlation coefficient between percentage of type 1 muscle fibers measured in the deep region of the muscle and contraction time at 50% MS obtained by TMG was also statistically significant (r=0.90,P<0.001).These findings suggest that the contraction time obtained by TMG may be useful for non-invasive examining of muscle fiber types spatial distribution in humans.     

Neural regulation of mammalian fast and slow muscle myosins: an electrophoretic analysis.

Mammalian nerves to fast and slow muscles have the remarkable property of changing the speed of contraction of muscles following cross-reinnervation. The biochemical basis of speed transformation is the change in myosin in ATPase activity. This paper provides electrophoretic evidence for structural changes in myosin from cross-reinnervated muscles. A method is described for the separation of intact fast and slow muscle myosins by polyacrylamide gel electrophoresis. This method utilizes the fact that ATP and its analogs prevent the formation of myosin polymers in low ionic strength buffers. In this system, normal fast muscle myosin has a higher electrophoretic mobility than slow muscle myosin. Normal rat soleus myosin has a major slow and a minor fast component due to two populations of muscle fibers. The same muscle cross-reinnervated by a fast muscle nerve shows only the fast component, The normal, homogeneous fast extensor digitorum longus muscle has only the electrophoretically fast myosin, but following cross-reinnervation it shows both fast and slow components. These results suggest that mammalian motor nerves can induce or suppress the expression of genes that code for fast and slow skeletal muscle myosins.     

A comparative study of the membrane structure in different types of muscle fibers in the frog

The muscle membrane of slow and fast fibers in cruralis and iliofibularis muscles and of intermediate fibers in submaxillaris muscle of the frog is studied in freeze-fracture replicas. A comparison of membrane folds, number, size and distribution of caveolae and of intramembrane particles (IMP) is given. In slow muscle fibers, the membrane folds are systematically present at the level of the I zone with a transversal continuity, whereas in fast and intermediate types the membrane folds are small and are randomly distributed. In slow muscle the caveolae are more numerous at the I zone than in the part corresponding to the center of the sarcomere. In fast muscle, small groups of caveolae form linear patterns, and in intermediate fibers the distribution is random. The number of caveolae in slow muscle fibers is two times more than in fast and intermediate fibers. The mean area of caveolae opening is largest in fast and smallest in slow muscle fibers. The number of IMP is significantly different in the three types of fibers, being highest in slow and lowest in intermediate fibers. The different pattern of folds in slow fibers may correspond to the different contractile properties of this fiber type. The presence of double the number of caveolae in slow fibers correlated to the less elaborate T system in this fiber type shows the possibility that slow fibers may be the result of an arrest during development for the performance of a different function. The difference in IMP density in the three muscle fiber types may be interpreted as the difference in their electrical properties.     

Increased effectiveness of a motorneuron after partial denervation of its target muscle in the cricket Teleogryllus oceanicus

Fibers of the metathoracic extensor tibia muscle of the cricket Teleogryllus oceanicus are innervated by a slow excitatory axon (slow fibers), a fast excitatory axon (fast fibers), or by both slow and fast axons (dual fibers). Sectioning metathoracic nerve 5 removes the fast axon input to the muscle but not that of the slow axon. Following such partial denervation, the mechanical responses initiated by the slow axon increase progressively for at least 30 days; twitch tensions reach 5–10 times those of control muscles and tetanic tensions 10–30 times control values. After sectioning nerve 5, resting membrane potentials decrease in those fibers which originally received fast axon input and the input resistance of all fiber types increases, including that of slow fibers which are not innervated through nerve 5. Excitatory junctional potentials (EJPs) initiated by the slow axon become larger following partial denervation, accounting in part for the larger contraction amplitudes. The increased input resistance is adequate to account for the larger EJPs in slow fibers but not for the proportionally greater increase in EJP amplitude in fibers which were formerly dually innervated. The change in EJP amplitude is abrupt in slow fibers and gradual in formerly dual fibers.     

Relating divergence in polychaete musculature to different burrowing behaviors: A study using opheliidae (Annelida)

Divergent morphologies among related species are often correlated with distinct behaviors and habitat uses. Considerable morphological and behavioral differences are found between two major clades within the polychaete family Opheliidae. For instance, Thoracophelia mucronata burrows by peristalsis, whereas Armandia brevis exhibits undulatory burrowing. We investigate the anatomical differences that allow for these distinct burrowing behaviors, then interpret these differences in an evolutionary context using broader phylogenetic (DNA‐based) and morphological analyses of Opheliidae and taxa, such as Scalibregmatidae and Polygordiidae. Histological three‐dimensional‐reconstruction of A. brevis reveals bilateral longitudinal muscle bands as the prominent musculature of the body. Circular muscles are absent; instead oblique muscles act with unilateral contraction of longitudinal muscles to bend the body during undulation. The angle of helical fibers in the cuticle is consistent with the fibers supporting turgidity of the body rather than resisting radial expansion from longitudinal muscle contraction. Circular muscles are present in the anterior of T. mucronata, and they branch away from the body wall to form oblique muscles. Helical fibers in the cuticle are more axially oriented than those in undulatory burrowers, facilitating radial expansion during peristalsis. A transition in musculature accompanies the change in external morphology from the thorax to the abdomen, which has oblique muscles similar to A. brevis. Muscles in the muscular septum, which extends posteriorly to form the injector organ, act in synchrony with the body wall musculature during peristalsis: they contract to push fluid anteriorly and expand the head region following a direct peristaltic wave of the body wall muscles. The septum of A. brevis is much thinner and is presumably used for eversion of a nonmuscular pharynx. Mapping of morphological characters onto the molecular‐based phylogeny shows close links between musculature and behavior, but less correlation with habitat. J. Morphol. 275:548–571, 2014. © 2014 Wiley Periodicals, Inc.     

Reciprocal neural regulation of extrajunctional acetylcholinesterase and collagen Q in rat muscles--the role of calcineurin signaling

There are two main differences regarding acetylcholinesterase (AChE) expression in the extrajunctional regions of fast and slow rat muscles: (1) the activity of AChE catalytic subunits (G1 form) is much higher in fast than in slow muscles, and (2) the activity of the asymmetric forms of AChE (A(8) and A(12)) is quite high extrajunctionally in slow muscles but virtually absent in fast muscles. The latter is due to the absence of the expression of AChE-associated collagen Q (ColQ) in the extrajunctional regions of fast muscle fibers, in contrast to its ample expression in slow muscles. We showed that both differences are caused by different neural activation patterns of fast vs. slow muscle fibers, which determine the respective levels of mRNA of both proteins. Whereas the changes in AChE mRNA levels in fast and slow muscles, as well as the levels of ColQ mRNA levels in slow muscles, observed in response to exposing either slow or fast muscles to different muscle activation patterns, are completely reversible, the extrajunctional suppression of ColQ expression in fast muscle fibers seems to be irreversible. Calcineurin signaling pathway in muscles is activated by high-average sarcoplasmic calcium concentration resulting from tonic low-frequency muscle fiber activation pattern, typical for slow muscle fibers, but is inactive in fast muscle fibers, which are activated by infrequent high-frequency bursts of neural impulses. Application to rats of two inhibitors of calcineurin (tacrolimus-FK506 and cyclosporin A) demonstrated that the mRNA levels of both the AChE catalytic subunit and ColQ in the extrajunctional regions of the soleus muscle are regulated by the calcineurin signaling pathway, but in a reciprocal way. Under the conditions of low calcineurin activity, AChE expression is enhanced and that of ColQ is suppressed, and vice versa. Our results also indicated that different, calcineurin-independent regulatory pathways are responsible for the reduction of AChE expression during muscle denervation, and for maintaining high ColQ expression in the neuromuscular junctions of fast muscle fibers.     

大鼠和家兔生后发育各阶段比目鱼肌纤维的比较

为研究大鼠与家兔骨骼肌各类型肌纤维的数量和二维分布以及生后发育对其影响,取生后2d和2、4、6、8、10周龄(体重10g和32、95、190、280、320g)大鼠及生后2d和2、4、8、12、16、20、24周龄(体重100g和220、400、750、1200、1600、2100、2500g)家兔的比目鱼肌做琥珀酸脱氢酶染色。实验结果表明,大鼠和家兔比目鱼肌纤维被分成Ⅰ型(SO),ⅡX型(FO)和ⅡA型(FOG)3型。使用图像分析系统分析每型肌纤维在生后发育各阶段的相关变化,大鼠和家兔比目鱼肌中：Ⅰ型纤维分布于整块肌肉,其数量随着生后发育而增加。幼体ⅡX型纤维分布在整块肌肉中,其数量随生后发育而减少;ⅡA型分布在肌肉中深层,数量几乎无变化;至成体时只有少量的ⅡX和ⅡA分布在肌表层。整个发育期间未见ⅡB型纤维。ⅡA型纤维直径最大,Ⅰ型中等,而ⅡX型最小。家兔3型肌纤维的平均横切面积比大鼠的大。这些结果表明大鼠和家兔后肢肌各种类型肌纤维的数量比例和分布随生长过程发生改变。     

Denervation and reinnervation of fast and slow muscles. A histochemical study in rats.

A histochemical study, using myosin-adenosine triphosphatase activity at pH 9.4, was conducted in soleus and plantaris muscles of adult rats, after bilateral crushing of the sciatic nerve at the sciatic notch. The changes in fiber diameter and per cent composition of type I and type II fibers plus muscle weights were evaluated along the course of denervation-reinnervation curve at 1, 2, 3, 4 and 6 weeks postnerve crush. The study revealed that in the early denervation phase (up to 2 weeks postcrush) both the slow and fast muscles, soleus and plantaris, resepctively, atrophied similarly in muscle mass. Soleus increased in the number of type II fibers, which may be attributed to "disuse" effect. During the same period, the type I fibers of soleus atrophied as much or slightly more than the type II fibers; whereas the type II fibers of plantaris atrophied significantly more than the type I fibers, reflecting that the process of denervation, in its early stages, may affect the two fiber types differentially in the slow and fast muscles. It was deduced that the type I fibers of plantaris may be essentially different in the slow (soleus) and fast (plantaris) muscles under study. The onset of reinnervation, as determined by the increase in muscle weight and fiber diameter of the major fiber type, occurred in soleus and plantaris at 2 and 3 weeks postcrush, respectively, which confirms the earlier hypotheses that the slow muscles are reinnervated sooner than the fast muscles. It is suggested that the reinnervation of muscle after crush injury may be specific to the muscle type or its predominant fiber type.     

Skeletal muscle satellite cell diversity: satellite cells form fibers of different types in cell culture

Following skeletal muscle injury, new fibers form from resident satellite cells which reestablish the fiber composition of the original muscle. We have used a cell culture system to analyze satellite cells isolated from adult chicken and quail pectoralis major (PM; a fast muscle) and anterior latissimus dorsi (ALD; a slow muscle) to determine if satellite cells isolated from fast or slow muscles produce one or several types of fibers when they form new fibers in vitro in the absence of innervation or a specific extracellular milieu. The types of fibers formed in satellite cell cultures were determined using immunoblotting and immunocytochemistry with monoclonal antibodies specific for avian fast and slow myosin heavy chain (MHC) isoforms. We found that satellite cells were of different types and that fast and slow muscles differed in the percentage of each type they contained. Primary satellite cells isolated from the PM formed only fast fibers, while up to 25% of those isolated from ALD formed fibers that were both fast and slow (fast/slow fibers), the remainder being fast only. Fast/slow fibers formed from chicken satellite cells expressed slow MHC1, while slow MHC2 predominated in fast/slow fibers formed from quail satellite cells. Prolonged primary culture did not alter the relative proportions of fast to fast/slow fibers in high density cultures of either chicken or quail satellite cells. No change in commitment was observed in fibers formed from chicken satellite cell progeny repeatedly subcultured at high density, while fibers formed from subcultured quail satellite cell progeny demonstrated increasing commitment to fast/slow fiber type formation. Quail satellite cells cloned from high density cultures formed colonies that demonstrated a similar change in commitment from fast to fast/slow, as did serially subcloned individual satellite cell progeny, indicating that the observed change from fast to fast/slow differentiation resulted from intrinsic changes within a satellite cell. Thus satellite cells freshly isolated from adult chicken and quail are committed to form fibers of at least two types, satellite cells of these two types are found in different proportions in fast and slow muscles, and repeated cell proliferation of quail satellite cell progeny may alter satellite cell progeny to increasingly form fibers of a single type.     

Identification of sarcolemma-associated antigens with differential distributions on fast and slow skeletal muscle fibers

We have identified three sarcolemma-associated antigens, including two antigens that are differentially distributed on skeletal muscle fibers of the fast, fast/slow, and slow types. Monoclonal antibodies were prepared using partially purified membranes of adult chicken skeletal muscles as immunogens and were used to characterize three antigens associated with the sarcolemma of muscle fibers. Immunofluorescence staining of cryosections of adult and embryonic chicken muscles showed that two of the three antigens differed in expression by fibers depending on developmental age and whether the fibers were of the fast, fast/slow, or slow type. Fiber type was assigned by determining the content of fast and slow myosin heavy chain. MSA-55 was expressed equally by fibers of all types. In contrast, MSA-slow and MSA-140 differed in their expression by muscle fibers depending on fiber type. MSA-slow was detected exclusively at the periphery of fast/slow and slow fibers, but was not detected on fast fibers. MSA-140 was detected on all fibers but fast/slow and slow fibers stained more intensely suggesting that these fiber types contain more MSA-140 than fast fibers. These sarcolemma-associated antigens were developmentally regulated in ovo and in vitro. MSA-55 and MSA-140 were detected on all primary muscle fibers by day 8 in ovo of embryonic development, whereas MSA-slow was first detected on muscle fibers just before hatching. Those antigens expressed by fast fibers (MSA-55 and MSA-140) were expressed only after myoblasts differentiated into myotubes, but were not expressed by fibroblasts in cell culture. Each antigen was also detected in one or more nonskeletal muscle cell types: MSA-55 and MSA-slow in cardiac myocytes and smooth muscle of gizzard (but not vascular structures) and MSA-140 in cardiac myocytes and smooth muscle of vascular structures. MSA-55 was identified as an Mr 55,000, nonglycosylated, detergent-soluble protein, and MSA-140 was an Mr 140,000, cell surface protein. The Mr of MSA-slow could not be determined by immunoblotting or immunoprecipitation techniques. These findings indicate that muscle fibers of different physiological function differ in the components associated with the sarcolemma. While the function of these sarcolemma-associated antigens is unknown, their regulated appearance during development in ovo and as myoblasts differentiate in culture suggests that they may be important in the formation, maturation, and function of fast, fast/slow, and slow muscle fibers.     

Developmental transition of touch response from slow muscle-mediated coilings to fast muscle-mediated burst swimming in zebrafish

It is well known that slow and fast muscles are used for long-term sustained movement and short bursts of activity, respectively, in adult animal behaviors. However, the contribution of the slow and fast muscles in early animal movement has not been thoroughly explored. In wild-type zebrafish embryos, tactile stimulation induces coilings consisting of 1–3 alternating contractions of the trunk and tail at 24 hours postfertilization (hpf) and burst swimming at 48 hpf. But, embryos defective in flightless I homolog (flii), which encodes for an actin-regulating protein, exhibit normal coilings at 24 hpf that is followed by significantly slower burst swimming at 48 hpf. Interestingly, actin fibers are disorganized in mutant fast muscle but not in mutant slow muscle, suggesting that slower swimming at 48 hpf is attributable to defects of the fast muscle tissue. In fact, perturbation of the fast muscle contractions by eliminating Ca²⁺ release only in fast muscle resulted in normal coilings at 24 hpf and slower burst swimming at 48 hpf, just as flii mutants exhibited. In contrast, specific inactivation of slow muscle by knockdown of the slow muscle myosin genes led to complete loss of coilings at 24 hpf, although normal burst swimming was retained by 48 hpf. These findings indicate that coilings at 24 hpf is mediated by slow muscle only, whereas burst swimming at 48 hpf is executed primarily by fast muscle. It is consistent with the fact that differentiation of fast muscle follows that of slow muscle. This is the first direct demonstration that slow and fast muscles have distinct physiologically relevant contribution in early motor development at different stages.