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PubMed1989

Smooth muscle expresses a cardiac/slow muscle isoform of the Ca2+-transport ATPase in its endoplasmic reticulum.

Smooth muscle expresses in its endoplasmic reticulum an isoform of the Ca2+-transport ATPase that is very similar to or identical with that of the cardiac-muscle/slow-twitch skeletal-muscle form. However, this enzyme differs from that found in fast-twitch skeletal muscle. This conclusion is based on two independent sets of observations, namely immunological observations and phosphorylation experiments. Immunoblot experiments show that two different antibody preparations against the Ca2+-transport ATPase of cardiac-muscle sarcoplasmic reticulum also recognize the endoplasmic-reticulum/sarcoplasmic-reticulum enzyme of the smooth muscle and the slow-twitch skeletal muscle whereas they bind very weakly or not at all to the sarcoplasmic-reticulum Ca2+-transport ATPase of the fast-twitch skeletal muscle. Conversely antibodies directed against the fast-twitch skeletal-muscle isoform of the sarcoplasmic-reticulum Ca2+-transport ATPase do not bind to the cardiac-muscle, smooth-muscle or slow-twitch skeletal-muscle enzymes. The phosphorylated tryptic fragments A and A1 of the sarcoplasmic-reticulum Ca2+-transport ATPases have the same apparent Mr values in cardiac muscle, slow-twitch skeletal muscle and smooth muscle, whereas the corresponding fragments in fast-twitch skeletal muscle have lower apparent Mr values. This analytical procedure is a new and easy technique for discrimination between the isoforms of endoplasmic-reticulum/sarcoplasmic-reticulum Ca2+-transport ATPases.

Animals↗
PubMed1993

The oculomotor system of decapod cephalopods: eye muscles, eye muscle nerves, and the oculomotor neurons in the central nervous system.

Fourteen extraocular eye muscles are described in the decapods Loligo and Sepioteuthis, and thirteen in Sepia; they are supplied by four eye muscle nerves. The main action of most of the muscles is a linear movement of the eyeball, only three muscles produce strong rotations. The arrangement, innervation and action of the decapod eye muscles are compared with those of the seven eye muscles and seven eye muscle nerves in Octopus. The extra muscles in decapods are attached to the anterior and superior faces of the eyes. At least, the anterior muscles, and presumably also the superior muscles, are concerned with convergent eye movements for binocular vision during fixation and capture of prey by the tentacles. The remaining muscles are rather similar in the two cephalopod groups. In decapods, the anterior muscles include conjunctive muscles; these cross the midline and each presumably moves both eyes at the same time during fixation. In the squids Loligo and Sepioteuthis there is an additional superior conjunctive muscle of perhaps similar function. Some of the anterior muscles are associated with a narrow moveable plate, the trochlear cartilage; it is attached to the eyeball by trochlear membranes. Centripetal cobalt fillings showed that all four eye muscle nerves have fibres that originate from somata in the ipsilateral anterior lateral pedal lobe, which is the oculomotor centre. The somata of the individual nerves show different but overlapping distributions. Bundles of small presumably afferent fibres were seen in two of the four nerves. They do not enter the anterior lateral pedal lobe but run to the ventral magnocellular lobe; some afferent fibres enter the brachio-palliovisceral connective and run perhaps as far as the palliovisceral lobe.

Animals↗
PubMed1999

Neurovascular anatomy of the rectus femoris muscle related to functioning muscle transfer.

To describe the intramuscular neurovascular anatomy of the rectus femoris muscle and to evaluate whether the muscle can be split into two functional units, 40 rectus femoris muscle specimens were studied. Ten fresh human cadavers were injected with a mixture of lead oxide, gelatin, and water through the femoral arteries. The rectus femoris muscle with its neurovascular pedicles was dissected out and then radiographed. Computer wire was sutured to each nerve branch in the muscle, and the muscle was radiographed again. Radiographs with and without radiopaque wire were then analyzed. In 10 preserved cadavers, the rectus femoris muscle was dissected out. Note was made of the vessel and nerve to the muscle. All muscles were cut serially into 2-cm cross-sections, and the position and course of the intramuscular tendon were then grossly examined. Three different vascular patterns in 40 rectus femoris muscles were found, based on the number of vascular pedicles and their relative dominance within the muscle. The rectus femoris muscle received either a single vascular pedicle (12.5 percent), a dominant vascular pedicle and one or two minor pedicles (80 percent), or two dominant vascular pedicles (7.5 percent). The rectus femoris was innervated by a large nerve branch from the posterior division of the femoral nerve, and the branch generally divided into two sub-branches before it reached the muscle. Both branches were respectively accompanied by arterial branches to form neurovascular hila. Furthermore, this present study has provided a detailed description of the intramuscular neurovascular territories. Also, the pattern of neurovascular supply of the muscle makes it possible to subdivide the muscle into two functional units for segmental muscle transfer.

Aged↗
PubMed1989

[Anatomical study of the human lingual muscle. Myofibrous organization of the extrinsic muscles].

The organization and distribution of fibre types in the extrinsic human muscles of 34 males and 19 females were determined by Sudan black B staining. 1) According to our observation, the Genio-hyo-glossus consists of two large bundles; and upper bundle and a lower bundle. The Hyo-glossus is thin, flat and the Stlo-glossus is shorter and smaller. 2) These muscles are composed of small bundles of the fibre which could be seen under microscopic. These contain many small fatty cells. 3) The cross-sectional area (mm2) and the total number of the muscle fibres is highest in the Genio-hyo-glossus. The number of fibres per mm2 is highest in of the Hyo-glossus. The cross-sectional size of muscle fibre (micron) in the Stlo-glossus is generally larger than others. 4) Comparison with other muscles as in sucking show the muscle fibre size in the laryngeal muscles to be greatest. The Cross-sectional area and the total number of muscle fibre is greatest in the Genio-hyo-glossus. 5) Three fibre types of muscle could be discerned in terms of their reaction to pigments and cellular: red muscle fibre a positive reacted positively; white muscle fibres has a weak reaction; intermediate muscle fibres reacted intermediately. White muscle fibres are localized in the external layer, in contrast to red and intermediate muscle fibres which are localized in the inner layer of the extrinsic muscles. The percentage distribution of red muscle fibres in the Hyo-glossus and Stlo-glossus is more than Genio-hyo-glossus. These results suggest that Genio-hyo-glossus acts as a power generating force in contrast to the Hyo-glossus and Stlo-glossus which are fatigue-resisting.

Aged↗
PubMed1993

[Anesthesia and laryngeal muscle, especially intrinsic laryngeal muscles].

The intrinsic laryngeal muscles are highly specialized for phonation, respiration and sphincter activity. The muscles are disposed between three unpaired cartilages (the thyroid, cricothyroid and epiglottic) and the paired arytenoid cartilages. These laryngeal muscles, which are striated in character, are bilaterally disposed and can be grouped as adductors and abductors of the glottis and tensors of the vocal cords. The motor neurons for the intrinsic laryngeal muscles are located in the caudal half of the nucleus ambiguous. The intrinsic laryngeal muscles receive motor nerve supply from the external branches of the superior laryngeal and recurrent nerves on each side. The external branch of the superior laryngeal nerve serves in motor innervation of the cricothyroid muscle, while the other laryngeal muscles receive a motor innervation from the recurrent nerve. Compared with limb skeletal muscles, the intrinsic laryngeal muscle have unique morphologic and histochemical characteristics. In general, the intrinsic laryngeal muscle fibers contain greater percentages of histochemically type 1 fibers than limb skeletal muscles. In addition, morphometrical studies of the subneural apparatus at the neuromuscular junctions of the intrinsic laryngeal muscles demonstrate the difference between the morphological features of the nerve endings and the distribution pattern of the motor end-plates of the laryngeal muscles and those of the limb skeletal muscles. Finally, the influence of anesthetic drugs including neuromuscular blocking agents on the intrinsic laryngeal muscles is discussed. The gross anatomical features such as the motor innervation, localization of the neurons for the motor nerve fibers, and arrangement and characteristics of the motor end-plates of the intrinsic laryngeal muscles are described based on a review of literature and our current investigations.

Anesthesia↗
PubMed1995

Critical period in muscle spindle regeneration in grafts of developing rat muscles.

Extensor digitorum longus (EDL) muscles from rats at various intervals after birth were grafted into EDL muscles of adult recipients. Three to twelve months after the operation, host muscles containing the grafts were removed and examined for the presence of muscle spindles in the graft. The aim of the study was to establish when muscle spindles become capable of regeneration during development. Regenerated muscles grafted during the first week after birth were virtually spindleless. Grafts of muscles transplanted 10 and 15 days postnatally contained only 5-8 muscle spindles on average. In contrast, the regenerated grafts originating from muscles of 24- and 28-day-old rats were spindle-rich as in mature muscle grafts; the number of spindles in the transplanted EDL muscles (25.0 +/- 2.3; mean +/- SE) attained values comparable to free standard autografts of these muscles in adult animals. Thus, the critical period after grafting, which also involves the loss of a vascular supply, is considerably longer than the critical period for muscle-spindle survival after nerve injury. Fifteen days after birth, when muscle spindles still survive denervation, only a few regenerated spindles were present in the individual muscle regenerates. We assume that the low resistance of immature spindle capsules to ischaemia accounts for their massive degeneration and abortive spindle regeneration in grafts from 10- to 15-day-old rats.

Adenosine Triphosphatases↗
PubMed2005

Corticotropin releasing factor 2 receptor agonists reduce the denervation-induced loss of rat skeletal muscle mass and force and increase non-atrophying skeletal muscle mass and force.

Of the two corticotropin releasing factor receptors known, corticotrophin releasing factor 2 receptor (CRF2R) is expressed in skeletal muscle. The function of this receptor in skeletal muscle is at present unknown. In order to better understand the role of the CRF2R in skeletal muscle, we treated rats with CRF2R agonists and evaluated the effect of these agents on normal and denervated muscle mass. Rats treated with the non-selective CRFR agonist, sauvagine, did not demonstrate any significant and consistent change in non-denervated and denervated fast twitch [tibialis anterior (TA) or extensor digitorum longus (EDL)] or slow/mixed twitch [medial gastrocnemius (MG) or soleus] fiber muscle mass. In adrenalectomized rats, sauvagine treatment resulted in no significant and consistent change in non-denervated fast or slow/mixed twitch fiber muscles but did cause a significant and consistent increase in denervated fast twitch (TA and EDL) but not slow/mixed twitch muscle mass. Interestingly adrenalectomy had no effect on the degree of muscle atrophy. Rats treated with the CRF2R selective agonist urocortin 2 demonstrated an increase in non-denervated and denervated fast and slow/mix twitch fiber muscle mass. The urocortin 2 induced increase in muscle mass was accompanied by an increase in muscle fiber cross-sectional area and muscle absolute force. These studies demonstrated that activation of the CRF2R decreased the level of skeletal muscle mass, force, and myocyte cross-sectional area loss resulting from sciatic nerve damage and increased the mass, force and myocyte cross-sectional area of normal (non-atrophying) skeletal muscle. In addition, we also observed that removal of the adrenals increased the effectiveness of the non-selective CRFR agonists sauvagine, presumably via the removal of the pro-atrophy influence of adrenal produced corticosteroids. These results demonstrate that pharmacological modulation of the CRF2R may be a viable method to treat skeletal muscle atrophy.

Amphibian Proteins↗
PubMed1982

Molecular properties and functions in vitro of chicken smooth-muscle alpha-actinin in comparison with those of striated-muscle alpha-actinins.

alpha-Actinin purified from chicken gizzard smooth muscle was characterized in comparison with alpha-actinins from chicken striated muscles, or fast-skeletal muscle, slow-skeletal muscle, and cardiac muscle. The gizzard alpha-actinin molecule consisted of two apparently identical subunits with a molecular weight of 100,000 on SDS-polyacrylamide gel electrophoresis, as do striated-muscle alpha-actinins. Its isoelectric points in the presence of urea were similar to the striated-muscle counterparts. Despite these similarities, distinctive amino acid sequences between smooth-muscle alpha-actinin and striated-muscle alpha-actinins were revealed by peptide mapping using limited proteolysis in SDS. Gizzard alpha-actinin was immunologically distinguished from striated-muscle alpha-actinins. Gizzard alpha-actinin formed bundles of gizzard F-actin as well as of skeletal-muscle F-actin, but could not form any cross-bridges between adjacent actin filaments under conditions where skeletal-muscle alpha-actinin could. Temperature-dependent competition between gizzard alpha-actinin and tropomyosin on binding to gizzard thin filaments was demonstrated by electron microscopic observations. Gizzard alpha-actinin promoted Mg2+-ATPase activity of reconstituted skeletal actomyosin, gizzard acto-skeletal myosin, and gizzard actomyosin. This promoting effect was depressed by the addition of gizzard tropomyosin. These findings imply that, despite structural differences between gizzard and striated-muscle alpha-actinin molecules, they function similarly in vitro, and that gizzard alpha-actinin can interact not only with smooth-muscle actin (gamma- and beta-actin) but also with skeletal-muscle actin (alpha-actin).

Actinin↗
PubMed1990

Motoneuron and muscle-unit properties after long-term direct innervation of soleus muscle by medial gastrocnemius nerve in cat.

1. This study addresses the following questions. 1) In a previous experiment, when the combined lateral gastrocnemius-soleus nerve was cross-innervated by the medial gastrocnemius (MG) nerve, was the predominance of slow muscle units in soleus muscle a result of selective routing of slow motor axons into soleus? 2) Is MG-nerve-induced conversion of soleus muscle fibers from slow to fast more complete at very long (18 mo vs. 9-11 mo) postoperative times? 3) Do MG motoneurons that cross-innervate soleus muscle recover their normal membrane electrical properties at very long postoperative times? 2. The proximal portion of approximately one-third of the MG nerve was coapted directly with the distally isolated soleus nerve. The MG muscle remained innervated by the unoperated portion of the MG nerve. At 6, 10, or 18 mos postoperative, motoneuron and/or muscle-unit properties were determined for MG motoneurons innervating MG, soleus, or neither muscle, and for axotomized soleus motoneurons. 3. In the partially denervated MG muscle, the proportions of motor units of each type were normal. This suggests that the population of MG motor axons that had been directed to the soleus nerve also contained a representative distribution of MG motoneuron types. 4. Most motor units (74%) in cross-innervated soleus (Xsoleus) were type S (based on muscle-unit contractile properties), in spite of the soleus nerve's having been cross-connected by approximately 75% fast MG motoneurons. Thus, even at very long postoperative times, slow soleus muscle units resisted conversion by fast MG motoneurons. 5. Thirty-two percent of MG motoneurons that had been cross-connected to soleus nerve elicited no measurable muscle contraction, compared with approximately 10% in previous reinnervation experiments in which the MG nerve was coapted with the MG or lateral gastrocnemius-soleus nerve. Thus MG motoneurons may be disadvantaged in their ability to innervate soleus muscle fibers. 6. It appears that at long postoperative times, those fast MG motoneurons tha had innervated large soleus muscle units had failed to convert those muscle fibers to fast types and had failed also to recover their normal motoneuron electrical properties. Conversion and recovery did occur for fast MG motoneurons that innervated small soleus muscle units and for slow MG motoneurons.

Animals↗
PubMed2002

Changes in mass and performance in rabbit muscles after muscle damage with or without transplantation of primary satellite cells.

Changes in morphology, metabolism, myosin heavy chain gene expression, and functional performances in damaged rabbit muscles with or without transplantation of primary satellite cells were investigated. For this purpose, we damaged bilaterally the fast muscle tibialis anterior (TA) with either 1.5 or 2.6 ml cardiotoxin 10(-5) M injections. Primary cultures of satellite cells were autotransplanted unilaterally 5 days after muscle degeneration. Two months postoperation, the masses of damaged TAs, with or without transplantation, were significantly larger than those of the controls. Furthermore, damaged transplanted muscles weighed significantly more than damaged muscles only. The increase in muscle mass was essentially due to increased fiber size. These results were independent of the quantity of cardiotoxin injected into the muscles. Maximal forces were similar in control and 2.6 ml damaged TAs with or without satellite cell transfer. In contrast, 1.5 ml damaged TAs showed a significant decrease in maximal forces that reached the level of controls after transplantation of satellite cells. Fatigue resistance was similar in control and 1.5 ml damaged TAs independently of satellite cell transfer. Fatigue index was significantly higher in 2.6 ml damaged muscles with or without cell transplantation. These changes could be explained in part by muscle metabolism, which shifted towards oxidative activities, and by gene expression of myosin heavy chain isoforms, which presented an increase in type IIa and a decrease in type I and IIb in all damaged muscles with or without cell transfer. Under our experimental conditions, these results show that muscle damage rather than satellite cell transplantation changes muscle metabolism, myosin heavy chain isoform gene expression, and, to a lesser extent, muscle contractile properties. In contrast, muscle weight and fiber size are increased both by muscle damage and by satellite cell transfer.

Animals↗
PubMed1996

Myosin isoforms and muscle fiber characteristics in equine gluteus medius muscle.

BACKGROUND: To date, four different myosin heavy chain (MyHC) isoforms have been identified in adult skeletal muscle of a number of species: types I, IIa, IIx or IId, and IIb. The aim of this study was to investigate the distribution of various MyHC isoforms in the equine gluteus medius and gluteus profundus muscles in relation with several morphometric variables of muscle fibers. METHODS: Samples from different depths of the gluteus medius muscle (2, 4, 6, and 8 cm) and gluteus profundus muscle of five sedentary horses were examined by MyHC gel electrophoresis, monoclonal antibodies staining against fast, slow and neonatal MyHC isoforms, myosin adenosine triphosphatase (m-ATPase) activity, nicotinamide adenine dinucleotide tetrazolium reductase, alpha-glycerophosphate dehydrogenase, and alpha-amylase-PAS. Data about relative frequencies, sizes, and capillaries of the various histochemical fiber types were collected by morphometry. RESULTS: Three MyHC isoforms were present in the gluteus medius muscle. Two of them comigrated with type I and IIa MyHC isoforms of rat diaphragm (used as a control). The third isoform showed an electrophoretic mobility closer to type IIx than to the IIb MyHC isoform of rat diaphragm. Only two MyHC isoforms (type I and IIa) were detected in the gluteus profundus muscle. In both muscles, type I fibers (high m-ATPase activity at pH 4.5) only reacted with the anti slow-MyHC antibody and both type IIA and IIB fibers (low and moderate m-ATPase activity at pH 4.5, respectively) only reacted with the anti fast-MyHC antibody. No cross-reactivity of fibers positive for both antibodies was found except for the scarce type IIC fibers. Fiber types and capillaries were heterogeneously distributed across the gluteus medius muscle. The deeper regions of this muscle were found to contain a higher percentage of type I fibers, a large number of capillaries and a lower proportion of type IIB fibers compared to the superficial regions of the muscle. The gluteus profundus muscle had more abundant and larger type I fibers than the deepest sampling site of the gluteus medius muscle. CONCLUSIONS: These results show the existence of three different MyHC isoforms in the equine gluteus medius muscle and that fiber types and MyHC isoforms are heterogeneously distributed within this muscle. The distribution of slow-twitch and fast-twitch MyHCs among the fibers determined by immunohistochemistry was in agreement with histochemically identified type I and type II fibers, respectively.

Adenosine Triphosphatases↗
PubMed2003

Ectopic expression of IGF-I and Shh by skeletal muscle inhibits disuse-mediated skeletal muscle atrophy and bone osteopenia in vivo.

The loss of normal weight-bearing activity, which occurs during bed rest, limb immobilization, and spaceflight, stimulates a catabolic response within the musculoskeletal system, which results in a loss of skeletal muscle mass and bone mineral. The mechanism by which loading of muscle and bone is sensed and translated into signals controlling tissue formation remains a major question in the field of musculoskeletal research. In this investigation, we have examined the ability of two potentially anti-atrophic proteins, IGF-I and Shh, to inhibit disuse atrophy within muscle and bone, when electroporated into skeletal muscle. We have found that electroporation and ectopic expression of IGF-I and/or Shh within the gastrocnemius/soleus muscle significantly stimulated muscle fiber hypertrophy and increases in muscle size. In addition, we report that electroporation and ectopic expression of IGF-I and/or Shh within the gastrocnemius/soleus muscle attenuated the lost of muscle fiber area, muscle mass, and muscle mass density that normally occurs during disuse muscle atrophy. Finally, we found that ectopic expression of IGF-I and Shh within the gastrocnemius/soleus muscle inhibits parameters of osteopenia within the tibia and fibula associated with hindlimb unloading. These results support the theory that skeletal muscle can regulate bone maintenance and could offer potentially novel and efficient therapeutic options for attenuating muscle and bone atrophy during aging, illness and spaceflight.

Animals↗
PubMed1995

Trunk muscle strength in and effect of trunk muscle exercises for patients with chronic low back pain. The differences in patients with and without organic lumbar lesions.

STUDY DESIGN: Trunk muscle strength and the effect of trunk muscle exercises on chronic low back pain were investigated in patients with chronic low back pain. OBJECTIVE: Patients with chronic low back pain include those with organic lumbar lesions and those without such lesions. This study analyzed the differences in trunk muscle strength and effect of trunk muscle exercises between these two groups of patients. SUMMARY OF BACKGROUND DATA: Previous studies have reported lower trunk muscle strength in patients with chronic low back pain and higher efficacy of trunk muscle exercises for treating chronic low back pain. However, no analysis have yet been done of differences in these parameters between patients with chronic low back pain with and without organic lumbar lesions. METHODS: One-hundred-twenty-three patients with chronic low back pain and 126 healthy individuals without low back pain (control group) underwent trunk muscle strength evaluation. The patients were further divided into two groups-- those in group 1 had detectable organic lumbar lesions and those in group 2 had no detectable organic lesions. Seventy-two of these patients performed trunk muscle exercises and the correlation between improvement in low back pain and increase in trunk muscle strength was analyzed. RESULTS: Trunk flexor and extensor strengths were significantly lower in both groups. However, the flexor/extensor ratio of maximum torque was significantly higher in group 1 than in the control group, but was not significantly different between group 2 and the control group. Trunk muscle exercises reduced low back pain in both groups, but were more effective in group 2 than in group 1. The degree of correlation between increase in trunk muscle strength and improvement of low back pain also was higher in group 2 than in group 1. CONCLUSION: This difference in flexor/extensor ratio may have been due to reduced extensor strength resulting from neurogenic muscle weakness induced by the organic lumbar lesions in group 1. The exercise-associated increase in trunk muscle strength did not completely eliminate the low back pain induced by the organic lumbar lesions in group 1. However, increasing trunk muscle strength was extremely effective in patients of Group 2, in which decreased trunk muscle strength was a major factor in chronic low back pain.

Adolescent↗
PubMed1979

Synapse formation in intact innervated cutaneous-pectoris muscles of the frog following denervation of the opposite muscle.

1. Denervation of one cutaneous-pectoris muscle of the frog induces the formation of new synapses in the intact innervated muscle on the opposite side. After crushing the motor nerve to the left muscle the incidence of polyneuronal innervation in the right intact muscle increased from an average normal value of 16% to an average value of 27% (Rotshenker & McMahan, 1976).2. The formation of the new synapses in the intact muscle is independent of the presence of denervated muscle fibres or degenerating axons peripheral to the site of axotomy. After removing the left cutaneous-pectoris muscle, the proportion of polyneuronally innervated muscle fibres in the right intact muscle increased to an average value of 34%.3. The number of new synapses formed in one muscle is dependent upon the type of the lesion to the motor nerve to the opposite muscle; 40% of the muscle fibres on the right side were found to be polyneuronally innervated after transecting the motor nerve on the opposite side, as compared to 27% after crushing it.4. The delay with which new synapses are formed on the unoperated side is dependent upon the distance from the spinal cord of the axotomy. New synapses were detected 4-8 weeks after cutting the opposite nerve close to the muscle. Placing the site of axotomy close to the spinal cord shortened the delay and new synapses were detected as early as 9 days after the operation.5. The stimulus for the formation of new synapses by an intact nerve is ineffective if the injured nerve on the contralateral side originates from distant segments of the spinal cord. The pattern of innervation in cutaneous-pectoris muscles was not altered following denervation of distant muscles in the hind limb.6. These results suggest that the signal for sprouting and synapse formation may arise in the damaged nerve cells, central to the site of axotomy, and then be communicated transneuronally within the spinal cord to the intact motoneurones on the opposite side.

Animals↗
PubMed1988

Activation of human muscles at short muscle lengths during maximal static efforts.

1. Human muscle endurance is apparently enhanced during maximal voluntary contractions at short muscle lengths (McKenzie & Gandevia, 1987) but the ability of subjects to activate muscles fully at short lengths has not been established. Therefore this study examined the voluntary capacity to activate muscles fully at control (near resting) lengths and at decreased muscle lengths. Changes in mechanical properties of twitch responses to electrical stimulation of relaxed muscles at short muscle lengths were also documented. The abductor digiti minimi, elbow flexors and tibialis anterior were studied in five subjects. 2. For the three muscle groups, the mean reduction in twitch force between the control and short muscle lengths ranged from 46-51%. AT the short length there was a 9-13% reduction in the contraction time and a 21-27% reduction in the half-relaxation time. Maximal voluntary force declined by 21-49% at the short muscle length. A reduction in muscle length produced a shift to the right of the force-frequency curve as determined by brief trains of electrical stimuli. 3. During maximal efforts single or brief trains of two to four supramaximal stimuli, delivered to the parent nerve or motor point, failed to increase the force at a latency appropriate for onset of a muscle twitch in some but not all attempts. Each subject achieved 'maximal activation' of the muscle in a similar proportion of attempts at the control and short muscle lengths. 4. These results suggest that maximal voluntary activation of motoneurone pools is possible at short muscle lengths and that the central nervous system is able to maintain the discharge of motoneurones close to 'fusion' frequency despite a decrease in the temporal characteristics of the isometric twitch.

Action Potentials↗
PubMed1999

Whole-muscle and motor-unit contractile properties of the styloglossus muscle in rat.

Investigations of whole muscle and motor-unit contractile properties have provided valuable information for our understanding of the spinal cord and extraocular motor systems. However, no previous investigation has examined these properties in an isolated tongue muscle. The purpose of this study was to determine the contractile properties and muscle fiber types of the rat styloglossus muscle. The styloglossus is one of three extrinsic tongue muscles and serves to retract the tongue within the oral cavity. Adult male Sprague-Dawley rats (n = 19) were used in these experiments. The contractile characteristics of the whole styloglossus muscle (n = 9) were measured in response to stimulation of the hypoglossal nerve branch to the muscle. The average twitch tension produced was 3.30 g with a mean twitch contraction time of 13.81 ms. The mean maximum tetanic tension was 19.66 g and occurred at or near the fusion frequency, which averaged 109 Hz. The styloglossus muscle was resistant to fatigue [fatigue index (F. I.) = 0.76]. In separate experiments (n = 7), the contractile characteristics of 37 single motor units were measured in response to extracellular stimulation of hypoglossal motoneurons. The twitch tension generated by styloglossus motor units averaged 35.7 mg, and the mean twitch contraction time was 12.46 ms. The mean fusion frequency was 92 Hz. Maximum tetanic tension averaged 177.8 mg. Styloglossus single motor units were resistant to fatigue (F. I. = 0.74). The sites of stimulation that yielded a contractile response in the styloglossus muscle were consistent with the location of the styloglossus motoneuron pool reported in earlier anatomy studies. Muscle fiber typing was determined in three animals based on the myofibrillar ATPase reaction at pH 9.8, 4.6, and 4.3. The styloglossus muscle was composed of approximately 99% type IIA fibers with a few scattered type I fibers present in the study sample. On the basis of the combined findings of the physiology and histochemistry experiments, the styloglossus muscle appeared to be a homogeneous muscle composed almost exclusively of fast, fatigue-resistant motor units. These properties of the styloglossus muscle and its motor units were compared with findings in other rat skeletal muscles.

Animals↗
PubMed1978

The properties of the extraocular muscles of the frog. III. Morphological, mechanical and pharmacological properties of the isolated retractor bulbi muscle.

Some morphological, physiological, and pharmacological properties of the retractor bulbi muscle of the frog were tested. The enzyme-histochemical investigation shows that the retractor bulbi muscle contains twitch muscle fibres only. Two types of twitch muscle fibres, which are especially different in their diameter and in the content of mitochondria, build the muscle in an irregular arrangement; tonic muscle fibres were not observed. On the average, the isolated retractor bulbi muscle has at room temperature a contraction time of 26 ms, a half-relaxation time of 28 ms, a fusion frequency of 75 stimuli/s, and a twitch-tetanus ratio of 0.28. The fatigability of this muscle is higher than in oculorotatory eye muscles but lower than in skeletal muscles of the frog. An increase of the extracellular K+-concentration elicits in retractor bulbi muscles a quickly transient contracture; the mechanical threshold of the muscle fibres is found in a range between 20 and 25 mM K+ in Ringer solution. Similar short-lasting contractures, which are probably caused by twitch fibres, rich in mitochondria, are also evoked by application of depolarizing drugs like acetylcholine. The properties of the retractor bulbi muscle are compared with those of the sartorius muscle of the frog, which likewise contains twitch muscle fibres only.

Acetylcholine↗
PubMed1998

Acetylcholinesterase mRNA level and synaptic activity in rat muscles depend on nerve-induced pattern of muscle activation.

Acetylcholinesterase (AChE) mRNA levels are severalfold higher in fast rat muscles compared with slow. We hypothesized that AChE mRNA levels and AChE activity in the neuromuscular junction depend on a specific nerve-induced pattern of motor unit activation. Chronic low-frequency stimulation, mimicking the activation pattern in slow muscles, was applied to fast muscles in rats. Molecular forms of AChE were analyzed by velocity sedimentation, and AChE mRNA levels were analyzed by Northern blots. AChE mRNA levels in stimulated fast muscles dropped to 10-20% of control after 1 week and became comparable to those in slow soleus muscles. The activity of the junctional A12 AChE form in 35 d stimulated fast muscles decreased to 56% of control value, reaching that in the soleus muscle. Therefore, synaptic AChE itself depends on the muscle activation pattern. Complete inactivity after denervation also decreased the AChE mRNA level in fast muscles to <10% in 48 hr. In contrast, profuse fibrillations observed in noninnervated immature regenerating muscles maintain AChE mRNA levels at 80% of that in the innervated fast muscles. If protein synthesis was inhibited by cycloheximide, AChE mRNA levels in 3-d-old regenerating muscle, still containing myoblasts, increased approximately twofold. No significant increase after cycloheximide application was observed either in denervated mature fast muscles or in normal slow muscles. Low AChE mRNA levels observed in those muscles are probably not caused by decreased stability of AChE mRNA as demonstrated in myoblasts.

Acetylcholinesterase↗