Junction Of Muscle Research Articles

Enhanced resistance to fatigue and altered calcium handling properties of sarcalumenin knockout mice

Sarcalumenin is a Ca2+-binding protein located in the sarcoplasmic reticulum of striated muscle cells, the physiological function of which has not been fully determined yet. Using sarcalumenin knockout (sar(-/-)) mice, we showed that sar ablation altered store-operated Ca2+ entry (SOCE) and enhanced muscle fatigue resistance. Sar(-/-) mice fatigued less with treadmill exercise, and intact isolated soleus and extensor digitorum longus muscles from sar(-/-) mice were more resistant to intermittent fatiguing stimulation than those from wild-type mice. Enhanced SOCE was observed in the sar(-/-) muscles. Biochemical analysis revealed that sar(-/-) muscles contained significantly elevated expression of mitsugumin 29 (MG29), a synaptophysin-related membrane protein located in the triad junction of skeletal muscle. Because the ablation of mg29 has been shown to cause increased fatigability and dysfunction of SOCE, the enhanced SOCE activity seen in sar(-/-) muscle may be correlated with the increased expression of MG29. Our data suggest that systemic ablation of sarcalumenin caused enhanced resistance to muscle fatigue by compensatory changes in Ca2+ regulatory proteins that effect SOCE.

The Anatomy of the Muscle Insertion (Scleromuscular Junction) of the Lateral and Medial Rectus Muscle in Humans

To analyze the histologic features of the insertion of the medial and the lateral rectus muscles in humans. Postmortem study performed on 49 extraocular muscles from 21 subjects without known ocular disease. All muscles were obtained no longer than 8 hours after death, after consent for autopsy. Thirty-seven lateral recti muscles and 12 medial recti muscles were studied with light microscopy (hematoxylin-eosin and Goldner stains) as well as with enzyme histochemistry and immunohistochemistry, with monoclonal-human tenascin C antibody. Light microscopic studies of muscle insertions of the lateral and the medial rectus muscle demonstrated muscle tissue connecting directly to the sclera without a tendon. These findings were confirmed immunohistochemically with tenascin C-antibody staining. Based on the results of this postmortem study in humans the term "muscle tendon" should be used with caution for the insertional area (scleromuscular junction) of the lateral and medial extraocular muscles. Light microscopy, histochemistry, and immunohistochemistry demonstrate that the tissue at the scleromuscular junction contains striated muscle with minimal connective (tendinous) tissue connecting to the sclera. To the best of the authors' knowledge, this is the first study in which enzyme histochemistry and immunohistochemistry have been used to investigate the anatomy of the insertional area (muscle-tendon-sclera junction) of the extraocular muscles in humans.

Expression of muscarinic acetylcholine receptors (M1-, M2-, M3- and M4-type) in the neuromuscular junction of the newborn and adult rat.

Using intracellular recording and immunohistochemistry, we studied the presynaptic muscarinic autoreceptor subtypes controlling ACh release in the neuromuscular junctions of the newborn (3-6 days postnatal) and adult (30-40 days) rat. In the Levator auris longus muscles of both newborn and adult rats, acetylcholine release was modified by the M1-receptor selective antagonists pirenzepine (10 microM) and MT-7 (100 nM) and by the M2-receptor selective antagonists methoctramine (1 microM) and AF-DX 116 (10 microM). The M4-receptor selective antagonists tropicamide (1 microM) and MT-3 (100 nM) can also modify the neurotransmitter release in certain synapses of the newborn muscles. The neurotransmitter release was not altered by the M3-receptor selective antagonist 4-DAMP (1 microM) in the adult or newborn rats. However, we directly demonstrate by immunocytochemistry the presence of these receptors in the motor endplates and conclude that M1-, M2-, M3- and M4-type muscarinic receptors are present in all the neuromuscular junctions of the rat muscle both in newborn and adult animals. These receptors may be located in the perisynaptic glial cell as well as at the nerve terminals.

Chicken gizzard filamin, retina filamin and cgABP260 are respectively, smooth muscle‐, non‐muscle‐ and pan‐muscle‐type isoforms: Distribution and localization in muscles

We determined the full cDNA sequences of chicken gizzard filamin and cgABP260 (chicken gizzard actin-binding protein 260). The primary and secondary structures predicted by these sequences were similar to those of chicken retina filamin and human filamins. Like mammals, chickens have 3 filamin isoforms. Comparison of their amino acid sequences indicated that gizzard filamin, retina filamin, and cgABP260 were the counterparts of human FLNa (filamin a), b, and c, respectively. Antibodies against the actin-binding domain (ABD) of these 3 filamin isoforms were raised in rabbits. Using immunoabsorption and affinity chromatography, we prepared the monospecific antibody against the ABD of each filamin. In immunoblotting, the antibody against the gizzard filamin ABD detected a single band in gizzard, but not in striated muscles or brain. In brain, only the antibody against the retina filamin ABD produced a strong single band. The antibody against the cgABP260 ABD detected a single peptide band in smooth, skeletal, and cardiac muscle. In immunofluorescence microscopy of muscular tissues using these antibodies, the antibody against the gizzard filamin ABD only stained smooth muscle cells, and the antibody against the retina filamin ABD strongly stained endothelial cells of blood vessels and weakly stained cells in connective tissue. The antibody against the cgABP260 ABD stained the Z-lines and myotendinous junctions of breast muscle, the Z-lines and intercalated disks of cardiac muscle, and dense plaques of smooth muscle. These findings indicate that chicken gizzard filamin, retina filamin, and cgABP260 are, respectively, smooth muscle-type, non-muscle-type, and pan-muscle-type filamin isoforms.

Neuromuscular function and transmission

Neuromuscular transmission occurs across the tiny 50 nm gap between an axon terminal and a skeletal muscle fibre at the end-plate region. An axon with its many terminal branches and connected muscle fibres forms a motor unit. A single axon terminal contains thousands of vesicles of the chemical transmitter acetylcholine. An axonal action potential increases axon terminal permeability to calcium and the resulting increase of free calcium ions above the resting concentration causes about 100 of the vesicles to release their transmitter into the gap. The end-plate region is folded to increase its surface area to about 3000 square micrometres. The acetylcholine released reacts with acetylcholine receptors situated at the crests of these folds. When the receptor has bound two acetylcholine molecules it changes its configuration to form a cation pore that allows sodium and potassium ions to move in and out of the muscle end-plate region. The combined activation of 30,000,000 receptors occurring at one end-plate causes the muscle membrane to depolarize at the muscle junction to about −10 mV, forming a localized end-plate potential. Acetylcholine is rapidly destroyed by the enzyme acetylcholinesterase, which allows the receptors and membrane of the end-plate to recover and respond to further neuronal action potentials. Since the localized end-plate potential is well in excess of the threshold stimulus required to generate a muscle action potential, as long as acetylcholine is being released by presynaptic action potentials, then postsynaptic muscle action potentials will be produced and result in a maintained contraction.

PET Detection of Solitary Distant Skeletal Muscle Metastasis of Esophageal Adenocarcinoma

A 67-year-old man with progressive dysphagia was recently diagnosed with a gastroesophageal junction adenocarcinoma. Contrast-enhanced CT scans of the abdomen and pelvis reported a large GE junction tumor without evidence of metastatic disease. FDG positron emission tomography revealed intense tracer accumulation in the soft tissue mass at the GE junction consistent with the primary neoplasm. In addition, PET scan also identified a solitary focus of intense FDG accumulation at the musculotendinous junction of the right gluteus minimus muscle. Subsequent MRI demonstrated mild enhancement and could not differentiate between tumor versus inflammation. Needle biopsy was performed and confirmed metastatic esophageal adenocarcinoma. A case of skeletal muscle metastases from late-stage (IV) gastroesophageal adenocarcinoma was previously reported. However, a solitary metastasis to a distant skeletal muscle without evidence of other lymphatic and hematogenous metastasis is quite unusual. The case supports the previous report that PET is superior in detecting distant metastases for initial staging of esophageal carcinoma over CT.

Morphology of the atriovenous junction and extensions of right atrial muscle towards the superior vena cava. A descriptive postmortem study. Implications for catheter ablation

Morphology of the atriovenous junction and extensions of right atrial muscle towards the superior vena cava. A descriptive postmortem study. Implications for catheter ablation

Influence of Hindlimb Suspension and Reloading on the IGF-I System and Collagen in Muscle and Myotendinous Junction

Influence of Hindlimb Suspension and Reloading on the IGF-I System and Collagen in Muscle and Myotendinous Junction

1199: An Ultrastructural Study of Detrusor Muscle Junctions in Patients with Poorly Compliant Bladders

1199: An Ultrastructural Study of Detrusor Muscle Junctions in Patients with Poorly Compliant Bladders

Anatomical variations of the flexor hallucis longus muscle and the consequences for tendon transfer. A cadaver study

The aim of this study was to evaluate the occurrence of anatomical variations of the musculotendinous junction of the flexor hallucis longus muscle. Eighty cadaver specimens preserved according to Thiel's method were assessed. Following careful dissection, the distance between the musculotendinous junction and the bone-cartilage border of the distal tibia was determined. Three typical anatomical variations were found: (1) a long lateral and shorter medial muscle belly; (2) equal-length medial and lateral muscle bellies; (3) a long medial and shorter lateral muscle belly. As a special variation in two cases only one lateral muscle belly was found. When planning a flexor hallucis longus transfer, preoperative magnetic resonance tomography (MRT) should be carried out in order to guarantee sufficient coverage of an existing soft tissue defect. These anatomical characteristics have also to be considered when interpreting ultrasonic or MRT findings in this region.

Reduced intercellular coupling leads to paradoxical propagation across the Purkinje-ventricular junction and aberrant myocardial activation

Ventricular tachycardia is a common heart rhythm disorder and a frequent cause of sudden cardiac death. Aberrant cell-cell coupling through gap junction channels, a process termed gap junction remodeling, is observed in many of the major forms of human heart disease and is associated with increased arrhythmic risk in both humans and in animal models. Genetically engineered mice with cardiac-restricted knockout of Connexin43, the major cardiac gap junctional protein, uniformly develop sudden cardiac death, although a detailed electrophysiological understanding of their profound arrhythmic propensity is unclear. Using voltage-sensitive dyes and high resolution optical mapping techniques, we found that uncoupling of the ventricular myocardium results in ectopic sites of ventricular activation. Our data indicate that this behavior reflects alterations in source-sink relationships and paradoxical conduction across normally quiescent Purkinje-ventricular muscle junctions. The aberrant activation profiles are associated with wavefront collisions, which in the setting of slow conduction may account for the highly arrhythmogenic behavior of Connexin43-deficient hearts. Thus, the extent of gap junction remodeling in diseased myocardium is a critical determinant of cardiac excitation patterns and arrhythmia susceptibility.

Ultrasound-guided musculocutaneous nerve block: A description of a novel technique

Localizing the musculocutaneous nerve for neural blockade is crucial to providing surgical anesthesia for the distal forearm. We present a novel approach for localizing and anesthetizing the musculocutaneous nerve. Ten patients underwent successful ultrasound-guided musculocutaneous nerve blocks. In this technique, either a 10-MHz or a 12-MHz linear probe was placed at the junction of the pectoralis major muscle and the biceps muscle such that the axillary artery was visualized in cross section. The probe was moved towards the biceps muscle until the musculocutaneous nerve was visualized lying between the coracobrachialis and biceps muscles. A 22-gauge, 50-mm b-bevel needle was inserted under direct vision until the needle was adjacent to the nerve. Local anesthetic was then injected, which generated surgical anesthetic conditions in all patients. Ultrasound can facilitate the localization and local anesthetic block of the musculocutaneous nerve.

Changes in Electrogenesis in the Motor Nerve Terminal with Long-Lasting High-Frequency Activation as a Factor of the Control of Transmitter Release

Using the technique of extracellular recording from the region of the neuromuscular junction in the cutaneous-sternal muscle in the frog under conditions of a reduced concentration of Ca2+ in the surrounding milieu, we demonstrated that long-lasting (10 min) rhythmic stimulation of the motor nerve with a frequency of 10 sec− 1 leads to a gradual increase in the evoked transmitter release. These changes are accompanied by a decrease in the amplitude of electrical responses of the nerve terminal (NT) and by a retardation of its second phase, as well as by a diminution of the third phase. Under conditions of long-lasting (5 min) stimulation with a frequency of 50 sec−1, we observed a two-phase change in the intensity of transmitter release: on the 2nd min, the initial rise was replaced by inhibition. Modifications of the response of the NT with different stimulation frequencies were qualitatively similar, but with a frequency of 10 sec−1 they were clearly expressed. Mathematical simulation of ion currents in the NT demonstrated that voltage-dependent potassium and sodium channels are inactivated in the course of long-lasting high-frequency excitation; the shape of the action potential is modified with changes in the rate of such inactivation. This leads to either an increase or a decrease of the inward calcium current. We conclude that the change in electrogenesis in the NT with long-lasting high-frequency activation of neuromuscular junctions exerts a significant influence on the dynamics of transmitter release.

Agrin becomes concentrated at neuroeffector junctions in developing rodent urinary bladder

The formation of somatic neuromuscular junctions in skeletal muscle is regulated by an extracellular matrix protein called agrin. Here, we have examined the expression and localization of agrin during development of the rodent urinary bladder, as a first step to examining its possible role at autonomic neuroeffector junctions in smooth muscle. We have found that agrin is expressed on the surface of developing smooth muscle cells and in the basement membrane underlying the urothelium. More importantly, agrin is progressively concentrated at parasympathetic varicosities during postnatal development and is present at virtually all junctions in mature muscle. Reverse transcription/polymerase chain reaction analysis has shown that pelvic ganglion neurons that innervate the bladder express LN/z8 agrin, whereas bladder smooth muscle expresses LN/z- agrin. Together, these results demonstrate that nerve and/or muscle agrin becomes localized at cholinergic parasympathetic varicosities in smooth muscle, where it could play a role in the maturation of the neuroeffector junction.

Effects of surface electrical stimulation on the muscle–tendon junction of spastic gastrocnemius in stroke patients

Purpose: The purpose of this study was to explore the effects of spasticity suppression by surface electrical stimulation (ES) on the muscle–tendon junction of spastic gastrocnemius muscles in stroke.Methods: Twenty-four neurologically stable stroke patients (aged 41 – 69 years, 12 – 35 months post-stroke), with spasticity graded 2 or 3 on the modified Ashworth scale, were recruited and divided into two groups. In the ES group, each patient received 20 min of surface ES once daily, 6 days per week for 1 month. In the control group, ES was used with stimulation intensity kept at zero. To evaluate the therapeutic effect, the modified Ashworth scale, Fmax/Mmax ratio, H-reflex latency, H-reflex recovery curve, and the 10-m walking time were tested before and after the 1-month treatment.Results: In the ES group, the modified Ashworth Scale showed a trend toward reduced spasticity after 1 month of treatment. The Fmax/Mmax ratio decreased from 8.10% ± 4.84% to 4.00% ± 1.36%; the H-reflex latency increased from 28.87 ± 2.45 ms to 29.40 ± 2.57 ms; the H-reflex recovery curves indicated a downward shift; and the 10-metre walking time significantly decreased after ES. In the control group, none of the measures showed a statistically significant change.Conclusions: In the study, we demonstrated a way to suppress spasticity at a metameric site and to increase walking speed effectively by applying surface ES on the muscle–tendon junction of spastic gastrocnemius muscles.

Utrophin is lacking at the neuromuscular junctions in the extraocular muscles of normal cat: artefact or true?

Extraocular muscles (EOM) are typically spared in Duchenne muscular dystrophy. We hypothesized that this might be due to different patterns of utrophin expression. The expression of utrophin was examined in EOM of normal cats using immunohistochemical methods and Western blot. For detecting acetylcholine receptors (AChR), we used alpha-bungarotoxin. Surprisingly, alpha-bungarotoxin failed to stain the AChR and no expression of utrophin could be detected at the neuromuscular junctions. Our study could indicate that the expression of utrophin is dependent on the structure of the AChR.

Inhomogeneous mechanical behavior of the human supraspinatus tendon under uniaxial loading

Disorders of the rotator cuff, particularly tears of the rotator cuff tendons, cause significant shoulder disability. Among numerous factors thought to be responsible for the initiation and progression of supraspinatus tears are those related to the tendon’s biomechanical properties. We hypothesized that in supraspinatus tendons subjected to tensile loading a strain gradient (difference) exists between the articular and bursal tendon surfaces, that regional strain differences exist on each of these two tendon surfaces, and that tendon surface strains vary with glenohumeral abduction. To test these hypotheses, the intrinsic inhomogeneous deformational characteristics of the articular and bursal surfaces of eight intact human cadaveric supraspinatus tendons were studied at three glenohumeral abduction angles using a novel multiple strain measuring system which simultaneously recorded surface marker displacements on two opposing soft tissue surfaces. Under applied tensile loads, the articular surface exhibited greater strain at 22° (7.4 ± 2.6% vs. 1.3 ± 0.7%, p = 0.0002) and 63° (6.4 ± 1.6% vs. 2.7 ± 1.2%, p = 0.0001) whereas the bursal surface exhibited greater strain at 90° (7.6 ± 2.8% vs. 4.9 ± 0.4%, p = 0.013). At all abduction angles, insertion strains were higher than those of the mid-tendon and tendon–muscle junction regions. The existence of inhomogeneous surface strains in the intact supraspinatus tendon demonstrates that intratendinous shear occurs within the tendon. The higher strain on the articular side of the tendon, especially at the insertion region, suggests a propensity for tears to initiate in the articular tendinous zone.

Location of the Neuromuscular Junction of the Human Masseter Muscle Estimated from the Low Frequency Component of the Surface Electromyogram

The position of the neuromuscular junction of the human masseter muscle has estimated from the low frequency component of the surface electromyogram. Monopolar surface electromyograms were recorded in response to clenching from eight sites with the reference electrode placed on the tip of the nose in six healthy male subjects. Component of the slow wave was separated from the raw recordings using digital filter, and the difference of the polarity and magnitude with the sites was examined. The base line of the recordings deflected either negatively or positively depending on the recording sites. These deflections coincided with bursts of muscular action potentials. Deflection of the slow wave component was largely negative over the inferior masseter close to the mandibular ramus but shifted to positive over the superior part. The greatest magnitude of the muscular action potential coincided with the most negative deflection of the slow wave. The phase of the muscular action potential burst also reversed near the site of the polarity change of the slow wave. These findings suggest that the slow wave, as detected by this method, originates mainly from the synaptic potential at the meuromuscular junction and that this is located on the inferior part of the muscle close to the mandibular ramus.

Composite tissue transplantation in rats: Fusion of donor muscle to the recipient site

Little information currently exists on the repair of muscular tissue at the site of an amputation stump. This study examined the healing process of muscular tissue following composite limb transplantation using transgenic rat models. Methods Green fluorescent protein (GFP) transgenic rats were used to study the process of connection of donor muscle with the recipient. A DsRed2/GFP double-reporter transgenic rat and an NCre transgenic rat were used to study cell fusion. These rats have the unique characteristic of changing red fluorescence to green fluorescence by Cre/LoxP recombination when cell-to-cell fusion occurs between the two transgenic strains. Orthotopic hind limb transplantation was performed in two combinations: GFP transgenic rat to Wild Wistar rat and DsRed2/GFP transgenic rat to NCre transgenic rat. Results We observed extension of donor-derived GFP + myofibers into recipient site a few weeks after limb transplantation. A histologic study of the DsRed2/GFP transgenic rat to the NCre transgenic rat combination showed that red myofibers of the DsRed2/GFP rat were partly replaced by green myofibers as a result of Cre-mediated recombination. PCR analysis detected both the recombined transgene (330 bp) and the nonrecombined gene (1420 bp) in muscle around the junction. These findings indicate that the muscles sutured between the amputation stumps fused with each other and that donor–recipient hybrid cells were formed at the muscle junction following limb transplantation. Conclusions This basic information shows muscle fusion between donor and recipient at the site of composite tissue transplant using newly established transgenic rats.

Calcium inflow-dependent protein kinase C activity is involved in the modulation of transmitter release in the neuromuscular junction of the adult rat

Using intracellular recording, we studied how protein kinase C activity affected miniature endplate potentials (MEPPs) and evoked endplate potentials (EPPs) in the neuromuscular junctions of the levator auris longus muscle from adult rats. The protein kinase C activator phorbol 12-myristate 13-acetate (PMA, 10 nM) increased the quantal content by approximately 150% (P<0.05). On the other hand, the quantal content decreased by approximately 40% (P<0.05) for all the protein kinase C inhibitors tested (Calphostin-C, 10 microM; Chelerythrine, 1 microM; Staurosporine, 200 nM). These changes in acetylcholine release were maintained at plateau for 1 to 7 h. Moreover, none of the protein kinase C activators or inhibitors used could modify the spontaneous MEPP mean size (P>0.05). We reduced the calcium influx in nerve terminals using the P/Q-type channel blocker omega-Aga-IVA(100 nM) or with 5 mM magnesium in physiological solution. In neither situation was the quantal content modified by PMA or by CaC. However, when high Ca2+ (5 mM) was added to a preparation that was previously blocked with omega-Aga-IVA, PMA and CaC had their full effect. We conclude that under physiological conditions PKC is dependent on the calcium inflow through the P/Q-type voltage-dependent calcium channels during evoked activity and works near the maximum rate at normal external calcium concentration.