Muscle Fiber Bundles Research Articles

Micromechanical modelling of skeletal muscles: from the single fibre to the whole muscle

The structure of a skeletal muscle is dominated by its hierarchical architecture in which thousands of muscle fibres are arranged within a connective tissue network. The single muscle fibre consists of many force-producing cells, known as sarcomeres, which contribute to the contraction of the whole muscle. There are a lot of questions concerning the optimisation of muscle strength and agility. To answer these questions, numerical testing tools, e.g. in the form of finite element models can be an adequate alternative to standard experimental investigations. The present approach is crucially based on the use of the finite element method. The material behaviour of the muscle is additively split into a so-called active and a passive part. To describe the passive part special unit cells consisting of one tetrahedral element and six truss elements have been derived. Embedded into these unit cells are non-linear truss elements which represent bundles of muscle fibres. Besides the representation of the material model, this contribution focuses on the application to anatomically based 3D problems, as the animal soleus muscle of the rat.

Time to fatigue is increased in mouse muscle at 37°C; the role of iron and reactive oxygen species

Studies exploring the rate of fatigue in isolated muscle at 37 degrees C have produced mixed results. In the present study, muscle fibre bundles from the mouse foot were used to study the effect of temperature on the rate of muscle fatigue. Provided iron was excluded from the solutions, time to fatigue at 37 degrees C was increased compared to 22 degrees C (125 +/- 8% of 22 degrees C fatigue time). In contrast, when iron was present (approximately 1 microM), fatigue was accelerated (68 +/- 10%). Iron can increase reactive oxygen species (ROS), which are believed to accelerate fatigue. The addition of 25-100 microM H(2)O(2) at 22 degrees C reduced time to fatigue to 80-20% of the control, respectively. Iron was added to cultured primary skeletal muscle cells to determine if iron could increase ROS production. Neither iron entry nor ROS production were detected in non-contracting muscle cells. The addition of 8-hydroxyquinoline, which facilitates iron entry, to iron-ascorbic acid solutions caused a rapid rise in intracellular iron and ROS. Our results indicate that time to fatigue in vitro is increased at 37 degrees C relative to 22 degrees C, but the addition of ROS can accelerate fatigue. An increase in muscle iron can accelerate ROS production, which may be important during or following exercise and in haemochromatosis, disuse atrophy and sarcopenia.

Establishment of a Transgenic Sickle-Cell Mouse Model to Study the Pathophysiology of Priapism

IntroductionPriapism is a poorly understood disease process with little information on the etiology and pathophysiology of this erectile disorder. One group of patients with a high prevalence of priapism is men with sickle-cell disease. AimEstablish an in vivo transgenic sickle-cell mouse model to study the pathophysiology of sickle-cell disease-associated priapism. MethodsTransgenic sickle-cell disease mice, expressing human sickle hemoglobin, were utilized. Three groups of mice were used: (i) wild type (WT), (ii) sickle-cell heterozygotes (Hemi), and (ii) sickle-cell homozygotes (Sickle). Two age groups of each cohort of mice were utilized: young adult (4–6 months) and aged (18–22 months). Main Outcome MeasuresHistological (trichrome stain to measure ratio of collagen to smooth muscle), penile hydroxyproline content (collagen content), and transmission electron microscopic analysis of WT, Hemi, and Sickle mice penes, as well as in vivo erectile responses [change in intracavernous pressure (ICP)] to cavernous nerve stimulation (CNS), were determined. The frequency of erectile responses (erections/hour) pre- and poststimulation was also measured in each of the experimental groups. ResultsSickle mice had increased (P < 0.05) collagen to smooth muscle ratio and hydroxyproline content in the penis when compared with WT and Hemi mice penes. Transmission electron microscopy demonstrated thickened smooth muscle cell bundles, disruption of the endothelial lining of the corporal sinusoids, and increased (P < 0.05) caveolae number. Sickle mice had significantly (P < 0.05) higher ICP to CNS and increased (P < 0.05) frequency of erections pre- and post-CNS when compared with WT and Hemi mice erectile responses. Sickle mice did develop ED (change in ICP in response to CNS) with increasing age. ConclusionsThe morphometric changes of the penis and exaggerated in vivo erectile responses support the use of this transgenic sickle-cell disease animal model to study the pathophysiological mechanisms involved in sickle-cell disease-associated priapism. Bivalacqua TJ, Musicki B, Hsu LL, Gladwin MT, Burnett AL, and Champion HC. Establishment of a transgenic sickle-cell mouse model to study the pathophysiology of priapism. J Sex Med 2009;6:2494–2504.

Investigations into Pulsed High-Intensity Focused Ultrasound–Enhanced Delivery: Preliminary Evidence for a Novel Mechanism

Pulsed high-intensity focused ultrasound (HIFU) exposures without ultrasound contrast agents have been used for noninvasively enhancing the delivery of various agents to improve their therapeutic efficacy in a variety of tissue models in a nondestructive manner. Despite the versatility of these exposures, little is known about the mechanisms by which their effects are produced. In this study, pulsed-HIFU exposures were given in the calf muscle of mice, followed by the administration of a variety of fluorophores, both soluble and particulate, by local or systemic injection. In vivo imaging (whole animal and microscopic) was used to quantify observations of increased extravasation and interstitial transport of the fluorophores as a result of the exposures. Histological analysis indicated that the exposures caused some structural alterations such as enlarged gaps between muscle fiber bundles. These effects were consistent with increasing the permeability of the tissues; however, they were found to be transient and reversed themselves gradually within 72 h. Simulations of radiation force–induced displacements and the resulting local shear strain they produced were carried out to potentially explain the manner by which these effects occurred. A better understanding of the mechanisms involved with pulsed HIFU exposures for noninvasively enhancing delivery will facilitate the process for optimizing their use. (E-mail: vfrenkel@cc.nih.gov)

Quality Characteristics of Broiler Chicken Meat on Salt at Different Temperatures

The aim of this investigation was to compare the quality characteristics and muscle structure of broiler chicken meat stored at different temperatures in the retail market in Oman. The meat quality characteristics of broiler breast meat were analysed. Ten samples were randomly selected from each group of fresh, frozen, and chilled chicken meat. Colour L*, a*, b*, pH, expressed juice, cooking loss, sarcomere length, W-B-shear force and muscle structure (using scanning electron microscopy) were determined. Fresh meat samples had significantly (P < 0.05) lower pH values and lightness L* than those of chilled and frozen samples. The chilled meat samples were significantly (P < 0.05) lighter and had lower shear force values than fresh and frozen samples. Frozen samples had significantly (P < 0.05) higher expressed juice and cooking loss values than either fresh or chilled samples. The pH values of fresh, chilled, and frozen breast samples were related to colour and expressed juice. Electron microscopy demonstrated that the changes in the physical properties of the chilled meat were related to breakdown of the muscle fiber bundles. The quality characteristics of broiler meat from different storage temperatures varied significantly.

Transplantation and repair: Combined cell implantation and chondroitinase delivery prevents deterioration of bladder function in rats with complete spinal cord injury

Additional examination. In this study, we report changes in bladder function after a combined treatment that was designed to study axonal regeneration after complete spinal cord injury (SCI) in rats. To report effects on bladder function following the administration of a combined treatment for complete SCI. University of Alberta, Faculty of Rehabilitation Medicine, Edmonton, Canada. Eight rats received Schwann cells in Matrigel-filled guidance channels, olfactory ensheathing glia and chondroitinase ABC at the lesion site following complete thoracic SCI. Controls (n=7) received Matrigel only. Daily bladder examinations were performed. Analysis of bladder size, wall thickness, actin and collagen type III was performed after 14 weeks. Following SCI, both groups regained bladder voiding after 3 weeks. However, 2 weeks later, incontinence was observed in all untreated rats and two treated rats. Post-mortem examination of bladders revealed enlarged bladder sizes. Thicker bladder walls were found in untreated rats, which were composed of disorganized bundles of smooth muscle fibers surrounded by high amounts of collagen (type III). We show that the combined treatment prevents collagen deposition in bladder walls and maintains the rat's ability to void efficiently. Although the mechanism responsible for this improvement is unclear, our study shows that the present combinatory therapy can influence bladder function, thus expanding their utility as a broad reparative approach for SCI.

Effect of oral appliances on genioglossus muscle tonicity seen with diffusion tensor imaging: A pilot study

The purpose of this study was to examine whether the diffusion tensor imaging (DTI) technique can be used as a modality to represent the structural deformation in the in vivo genioglossus (GG) muscle fibers with oral appliances (OAs). Three healthy subjects were recruited for the pilot study. A custom-made OA, which is modified from a tongue retaining device (TRD), was constructed for each subject before the DTI acquisitions. Recordings were made with and without OAs to compare the GG muscle fiber deformation. DTI provided good resolution of tongue muscle fibers in vivo and successful isolation of each muscle fiber bundle. In particular, the GG muscle fiber deformation due to OAs was clearly visualized. This DTI technique may be used not only to identify the individual myoarchitecture, but also to assess muscle fiber deformations in vivo, such as constriction, dilatation, and rotation with OAs. Clinical studies for OSA patients will be the next step.

Probing The Divalent Cation-binding Region Of The Myosin Regulatory Light Chain During Muscle Contraction Using EPR

We are using electron paramagnetic spectroscopy (EPR) to determine whether the myosin light chain domain (LCD) rotates as a rigid unit during muscle contraction. To accomplish this we are making orientational measurements at selected positions in the LCD using site-directed spin labeling of single-cysteine mutants of RLC. In addition, we are using double-cysteine RLC mutants to measure distance changes using dipolar EPR. We created two RLC mutants that contain a single cysteine (I35C or A55C) in the helices of the helix-loop-helix of the rat ventricular RLC (vRLC) divalent cation-binding site. Starting with each single-cysteine mutant, a second cysteine was added in the opposite helix (I35C/L49C and A55C/A32C) for distance measurements. The four RLC mutants were labeled with the spin probe MTSL and separately reconstituted into rabbit psoas muscle fiber bundles. The EPR spectra of the labeled fiber bundles in rigor were sensitive to muscle fiber orientation, for both I35C- and A55C-RLC mutants, indicating that the spin labels were highly oriented at both sites, making them ideal for measuring orientational changes. For both single-cysteine RLC mutants, myosin in rigor showed two populations of oriented spin labels that became substantially disordered in relaxation, while contraction induced order in a small fraction of the spin labels. The EPR spectra of the fibers with double-cysteine RLC mutants showed that the spin labels were less sensitive to muscle orientation, so they are suitable for distance measurements with myofibrillar preparations at low temperatures. This work is supported by NIH grant AR052360 to OR and by the Minnesota Supercomputing Institute.

Measuring Myosin Light Chain Domain Orientation in the Pre-Power Stroke AlF4 States with a Bifunctional Spin Label in Skinned Muscle Fibers

We are using electron paramagnetic resonance and site-directed spin labeling to measure the orientation of the light chain domain of skeletal muscle myosin in pre-power stroke states trapped by the phosphate analog AlF4. Previous work (Kraft et al. (2005) Proc. Natl. Acad. Sci. USA 102:13861-66) has shown via single-fiber X-ray diffraction that AlF4 traps two distinct pre-powerstroke myosin states in activated muscle. The first state (ADP.AlF4-I) produces a weak actin binding, disordered myosin whereas the second state (ADP.AlF4-II) produces a strong binding, stereospecific actomyosin complex. This weak-to-strong transition is a clear indication of the initiation of the power stroke before phosphate release, but it does not reveal the state of the light chain domain (LCD), which undergoes a rotation during the power stroke. We measured the orientation of the light chain domain by exchanging the native regulatory light chain (RLC) of skinned rabbit psoas muscle fiber bundles with a Di-Cys mutant RLC labeled with a bifunctional spin label. Our group has shown previously (Thompson and Naber et al. 2008 Biophys J, in press) that a bifunctional methanethiosulfonate spin label binds rigidly to myosin and reports protein orientation accurately. EPR spectra of oriented fibers show that the LCD produces a distinct orientation from rigor and relaxation in the ADP.AlF4-II state whereas the ADP.AlF4-I state is indistinguishable from relaxed muscle, supporting the hypothesis that the II state represents a state early in the power stroke with a distinct LCD orientation. This work was supported by NIH (AR32961, AR007612). We thank Bernhard Brenner and Theresia Kraft for guidance.

Venom apparatus of the Brazilian tarantula Vitalius dubius Mello-Leitão 1923 (Theraphosidae)

Tarantula venoms are a cocktail of proteins and peptides that have been increasingly studied in recent years. In contrast, less attention has been given to analyzing the structure of the paired cephalic glands that produce the venom. We have used light, electron, and confocal microscopy to study the organization and structure of the venom gland of the Brazilian tarantula Vitalius dubius. The chelicerae are hairy chitinous structures, each with a single curved hollow fang that opens via an orifice on the anterior surface. Internally, each chelicera contains striated muscle fiber bundles that control fang extension and retraction, and a cylindrical conical venom gland surrounded by a thick well-developed layer of obliquely arranged muscle fibers. Light microscopy of longitudinal and transverse sections showed that the gland secretory epithelium consists of a sponge-like network of slender epithelial cell processes with numerous bridges and interconnections that form lacunae containing secretion. This secretory epithelium is supported by a basement membrane containing elastic fibers. The entire epithelial structure of the venom-secreting cells is reinforced by a dense network of F-actin intermediate filaments, as shown by staining with phalloidin. Neural elements (axons and acetylcholinesterase activity) are also associated with the venom gland. Transmission electron microscopy of the epithelium revealed an ultrastructure typical of secretory cells, including abundant rough and smooth endoplasmic reticulum, an extensive Golgi apparatus, and numerous mitochondria.

A Transgenic Mouse Model to Study Glucose Transporter 4myc Regulation in Skeletal Muscle

Skeletal muscle is the major site for dietary glucose disposal, taking up glucose via glucose transporter 4 (GLUT4). Although subcellular fractionation studies demonstrate that insulin increases GLUT4 density in sarcolemma and transverse tubules, fractionation cannot discern GLUT4 vesicle-membrane association from insertion and exofacial exposure. Clonal muscle cultures expressing exofacially tagged GLUT4 have allowed quantification of GLUT4 exposure at the cell surface, its exocytosis, endocytosis, and partner proteins. We hypothesized that transgenic expression of GLUT4myc in skeletal muscles would provide a useful model to investigate GLUT4 biology in vivo. A homozygous mouse colony was generated expressing GLUT4myc driven by the muscle creatine kinase (MCK) promoter. GLUT4 protein levels were about 3-fold higher in hindlimb muscles of MCK-GLUT4myc transgenic mice compared with littermates (P < 0.05). Insulin (12 nm, 30 min) induced a 2.1-fold increase in surface GLUT4myc detected by immunofluorescence of the exofacial myc epitope in nonpermeabilized muscle fiber bundles (P < 0.05). Glucose uptake and surface GLUT4myc levels were 3.5- and 3-fold higher, respectively, in giant membrane vesicles blebbed from hindlimb muscles of insulin-stimulated transgenic mice compared with unstimulated counterparts (P < 0.05). Muscle contraction also elevated both parameters, an effect partially additive to insulin's. GLUT4myc immunoprecipitation with anti-myc antibodies avoids interfering with associated intracellular binding proteins. Tether, containing a UBX domain, for GLUT4 coimmunoprecipitated with GLUT4myc and insulin stimulation significantly decreased such association (P < 0.05). MCK-GLUT4myc transgenic mice are thus useful to quantify exofacial GLUT4 exposure at the sarcolemma and GLUT4 binding partners in skeletal muscle, essential elements in the investigation of muscle GLUT4 regulation in physiological and pathological states in vivo.

Rhabdomyomatous Mesenchymal Hamartoma With Lentiginous Melanocytic Hyperplasia: An Inductive Phenomenon or Component of the Hamartoma?

Rhabdomyomatous mesenchymal hamartoma (RMH) is a rare cutaneous hamartoma characterized by abundant bundles of mature skeletal muscle fibers admixed with other mesenchymal elements. We report a case of RMH in a 7-month-old girl with associated lentiginous melanocytic hyperplasia. The melanocytic hyperplasia was not apparent clinically and was only evident on histological examination. Unlike a smooth muscle hamartoma, melanocytic abnormalities have never been described in the context of RMH. Though the exact significance of this observation is not clear, we speculate that the melanocytic hyperplasia could be related to an inductive phenomenon or constitute a part of the overall hamartomatous process. Careful attention to epidermal changes in future cases of RMH may help in understanding the significance of this association and refine our knowledge of the pathogenesis of RMH.

Soft‐tissue imprints in fossil and Recent cephalopod septa and septum formation

Several soft-tissue imprints and attachment sites have been discovered on the inside of the shell wall and on the apertural side of the septum of various fossil and Recent ectocochleate cephalopods. In addition to the scars of the cephalic retractors, steinkerns of the body chambers of bactritoids and some ammonoids from the Moroccan and the German Emsian (Early Devonian) display various kinds of striations; some of these striations are restricted to the mural part of the septum, some start at the suture and terminate at the anterior limit of the annular elevation. Several of these features were also discovered in specimens of Mesozoic and Recent nautilids. These structures are here interpreted as imprints of muscle fibre bundles of the posterior and especially the septal mantle, blood vessels as well as the septal furrow. Most of these structures were not found in ammonoids younger than Middle Devonian. We suggest that newly formed, not yet mineralized (or only slightly), septa were more tightly stayed between the more numerous lobes and saddles in more strongly folded septa of more derived ammonoids and that the higher tension in these septa did not permit soft-parts to leave imprints on the organic preseptum. It is conceivable that this permitted more derived ammonoids to replace the chamber liquid faster by gas and consequently, new chambers could be used earlier than in other ectocochleate cephalopods, perhaps this process began even prior to mineralization. This would have allowed faster growth rates in derived ammonoids.

Mechanical specialization of the obliquely striated circular mantle muscle fibres of the long-finned squidDoryteuthis pealeii

The centrally located, mitochondria-poor (CMP) and superficially located, mitochondria-rich (SMR) circular muscle fibres in the mantles of some squids provide one of the few known examples of specialization in an obliquely striated muscle. Little is known of the mechanical properties or of the mechanisms and performance consequences of specialization in these fibres. We combined morphological and physiological approaches to study specialization in the SMR and CMP fibres of the long-finned squid Doryteuthis pealeii. The mean thick filament length was 3.12+/-0.56 mum and 1.78+/-0.27 mum for the SMR and CMP fibres, respectively. The cross-sectional areas of the whole fibre and the core of mitochondria were significantly higher in the SMR fibres, but the area occupied by the myofilaments did not differ between the two fibre types. The area of sarcoplasmic reticulum visible in cross sections was significantly higher in CMP fibres than in SMR fibres. In live bundles of muscle fibres partially isolated from the mantle, mean peak isometric stress during tetanus was significantly greater in SMR [335 mN mm(-2) physiological cross section (pcs)] than in CMP (216 mN mm(-2) pcs) fibres. SMR fibres had a lower average twitch:tetanus ratio (SMR=0.073; CMP=0.18) and a twofold lower unloaded maximum shortening velocity at 20 degrees C (SMR=2.4 L(0) s(-1); CMP=5.1 L(0) s(-1)), where L(0) was the preparation length that yielded the highest tetanic force. The structural differences in the two muscle fibre types play a primary role in determining their mechanical properties, and the significant differences in mechanical properties indicate that squid have two muscle gears. A simple model of the mantle shows that a gradient of strain and strain rate exists across the mantle wall, with fibres adjacent to the outer edge of the mantle experiencing 1.3- to 1.4-fold lower strain and strain rate than fibres adjacent to the inner edge of the mantle. The model also predicts that the CMP fibres generate virtually no power for slow jetting while the SMR fibres are too slow to generate power for the escape jets. The transmural differences in strain and strain rate predicted by the model apply to any cylindrical animal that has circumferentially oriented muscle fibres and an internal body cavity.

Metformin Attenuates a Complex I Mediated Increase in Mitochondrial Oxidant Emitting Potential in Skeletal Muscle of Obese Zucker Rats

Metformin (MET) is currently the most popular antidiabetic drug, although its insulin sensitizing mechanism of action is unknown. To test the effects of metformin (MET) on skeletal muscle mitochondrial function, we measured both mitochondrial respiration and H2O2 emission in saponin‐permeabilized red gastrocnemius muscle fiber bundles from lean and obese Zucker (fa/fa) rats that had been treated with MET for 4 wks. MET treatment significantly (p=.03) improved insulin sensitivity (HOMA) in obese, but not lean rats. In permeabilized fibers, ADP‐stimulated state 3, but not basal state 4, respiration rates were greater in obese vs. lean rats. However, MET treatment did not significantly affect permeabilized fibers respiring on palmitoyl‐carnitine (PC), complex I‐ or complex II‐linked substrates. Surprisingly, H2O2 emission induced by reverse electron flow (titration of complex II substrate succinate) was more than 2‐fold higher in obese vs lean rats. MET treatment significantly (p&lt;.05) attenuated succinate‐ and PC‐induced H2O2 emission in both lean and obese rats. These results suggest that 1) obesity is associated with a marked increase in mitochondrial ROS emitting potential in skeletal muscle, and 2) metformin may exert its antidiabetic effects by inhibiting complex I mediated ROS production, thereby decreasing the susceptibility to conditions favoring ROS emission. Support: NIH DK061314 and DK073488

Inorganic Phosphate Binds to the Empty Nucleotide Binding Pocket of Conventional Myosin II

In muscle inorganic phosphate strongly decreases force generation in the presence of millimolar MgATP, whereas phosphate slows shortening velocity only at micromolar MgATP concentrations. It is still controversial whether reduction in shortening velocity by phosphate results from phosphate binding to the nucleotide-free myosin head or from binding of phosphate to an actomyosin-ADP state as postulated for the inhibition of force generation by phosphate. Because most single-molecule studies are performed at micromolar concentrations of MgATP where phosphate effects on movement are rather prominent, clarification of the mechanisms of phosphate inhibition is essential for interpretation of data in which phosphate is used in single molecule studies to probe molecular events of force generation and movement. In in vitro assays we found that inhibition of filament gliding by inorganic phosphate was associated with increased fragmentation of actin filaments. In addition, phosphate did not extend dwell times of Cy3-EDA-ATP (2'(3')-O-[[2-[[6-[2-[3-(1-ethyl-1,3-dihydro-3,3-dimethyl-5-sulfo-2H-indol-2-ylidene)-1-propenyl]-3,3-dimethyl-5-sulfo-3H-indolio]-1-oxohexyl]amino]ethyl]carbamoyl]ATP) but reduced the number of Cy3-signals per field of view, approaching 50% at phosphate concentrations of 1-2 mM. Apparently, inhibition of movement does not result from binding of phosphate to an actomyosin-ADP intermediate as proposed by Hooft and coworkers (Hooft, A. M., Maki, E. J., Cox, K. K., and Baker, J. E. (2007) Biochemistry 46, 3513-3520) but, rather, from forming a strong-binding actomyosin-phosphate intermediate.

Redescription of the tapeworm Monticellia amazonica de Chambrier et Vaucher, 1997 (Cestoda, Proteocephalidea), a parasite of Calophysus macropterus (Siluriformes, Pimelodidae) from the Amazon River

AbstractThe proteocephalidean cestode Monticellia amazonica de Chambrier et Vaucher, 1997 (nom. nov. for Nomimoscolex piracatinga Woodland, 1935), a parasite of Calophysus macmpterus (Lichtenstein, 1819) (Siluriformes, Pimelodidae), is redescribed on the basis of type specimens and recently collected material from the Amazon River near the type localities in Brazil, and from Peru. This insufficiently known taxon is most similar to M. ventrei de Chambrier et Vaucher, 1999 from Pinirampus spp. from Paraguay and Brazil in having almost indistinguishable internal longitudinal musculature formed by a very few (5–15) tiny bundles of muscle fibres. Monticellia amazonica differs from M. ventrei in the shape of proglottids (much more elongate in the former taxon), absence of medially situated vitelline follicles at the ovarian level (present in M. ventrei) and the number of testes (175–233 in M amazonica vs. 222–325 in M ventrei).

‘Dynamic’ transverse mechanical properties of rat skeletal muscle under in vivo compression

Click to increase image sizeClick to decrease image sizeKeywords: musclecompressiontransverse force

An oscillator theory of motor unit recruitment

The phenomenon of systematic recruitment of motor units with increasing demand load is usually explained by the size principle. Though this principle successfully explains the gain-related aspects of muscle force generation, it does not address the need for desynchronization of motor unit activities in order to produce a smooth tension profile at the level of whole muscle, while individual muscle fibers are "twitching." We propose an oscillator model of motor neurons in which a pool of motor neurons fires a bundle of muscle fibers. Although individual muscle fibers have a complicated tension profile, the tension produced by the entire bundle is regulated and follows a command signal accurately. This is shown to be possible because of uncorrelated activity produced by local inhibitory connections among motor neurons. Connections that produce synchronized oscillations result in uncontrolled contractions of the muscle. These results seem to suggest that while synchronized activity indicates pathology and disease, desynchronized activity is the precondition for normal muscle function. Physiological evidence for the proposed theory of motor unit synchronization is presented.

Smooth muscle: regulation by calcium and phosphorylation.

Skinned smooth muscle cell bundles were used to study the mechanisms by which Ca2+ and protein phosphorylation regulate smooth muscle contraction. The sarcolemma of skinned cells is rendered permeable to ions and proteins by chemical treatment, allowing the investigator to control the ionic and protein environment within the cells and to determine the effect of this environment on the activation of contraction and protein phosphorylation. Results from these studies show that there is a strong correlation between myosin light chain phosphorylation and force development in skinned smooth muscle cells. Both protein phosphorylation and the activation of tension are affected in the predicted manner by agents known to affect myosin light chain kinase activity. Phosphorylation or thiophosphorylation of myosin light chains in the absence of Ca2+ is sufficient to fully activate contraction in skinned smooth muscle cells. Subsequent dephosphorylation of the myosin light chains by phosphatase results in muscle relaxation. Activation of contraction in skinned smooth muscle cells requires phosphorylation of myosin light chains by the Ca2+- and calmodulin-dependent myosin light chain kinase.