Differences in neuromuscular junctions between intrinsic muscles of the forepaw and biceps muscles in rats

Motor endplates of the interossei muscles become destabilized, whereas those of the biceps muscles remain stable in a rat model of obstetric brachial plexus palsy. However, it is unclear whether the morphology of the motor endplates of the interossei muscles is different from that of the biceps muscles in normal rat. We hypothesized that the motor endplates in the interossei muscles have specific characteristics different from those in the biceps muscles. The motor endplates were labeled with α-bungarotoxin and synaptophysin. The cross-sectional areas of the muscle fibers, the morphologies of the motor endplates, and the absolute and normalized areas (corrected by muscle fiber diameter) of the motor endplates of the interossei muscles and the biceps muscles were compared in rats at 1, 3, and 5 weeks after birth. The cross-sectional area of the interossei muscles and biceps muscle fibers were found to have increased gradually at 1, 3, and 5 weeks, but that of the biceps muscles was larger than that of the interossei muscles. The motor endplates of the interossei muscles and the biceps muscles gradually develop from crescent to pretzel shape after birth, and those of the interossei muscles have a smaller area. At 1, 3, and 5 weeks postnatally, the area of postnatal normalized motor endplates of the interossei muscles was much smaller than that of the biceps muscles. A better understanding of the morphological differences of the motor endplates between the interossei muscles and the biceps muscles may help to understand their physiological and pathological changes.

Anatomical analysis of the motor endplate zones of the suprascapular nerve to the infraspinatus muscle and its clinical significance in managing pain disorder

Myofascial pain syndrome caused by myofascial trigger points is a musculoskeletal disorder commonly encountered in clinical practice. The infraspinatus muscle is the region most frequently involved in the myofascial pain syndrome in the scapular region. The characteristics of the myofascial trigger points are that they can be found constantly in the motor endplate zone. However, localizing myofascial trigger points within the motor endplate zone and establishing an accurate injection site of the infraspinatus muscle has been challenging because the anatomical position of the motor endplate zone of the infraspinatus muscle is yet to be described. Therefore, this cadaveric study aimed to scrutinize the motor endplate zone of the infraspinatus muscle, propose potential myofascial trigger points within the muscle, and recommend therapeutic injection sites. Twenty specimens of the infraspinatus muscle for nerve staining and 10 fresh frozen cadavers for evaluation of the injection were used in this study. The number of nerve branches penetrating the infraspinatus muscle and their entry locations were analyzed and photographed. Modified Sihler's staining was performed to examine the motor endplate regions of the infraspinatus muscle. The nerve entry points were mostly observed in the center of the muscle belly. The motor endplate was distributed equally throughout the infraspinatus muscle, but the motor endplate zone was primarily identified in the B area, which is approximately 20-40% proximal to the infraspinatus muscle. The second-most common occurrence of the motor endplate zone was observed in the center of the muscle. These detailed anatomical data would be very helpful in predicting potential pain sites and establishing safe and effective injection treatment using botulinum neurotoxin, steroids, or lidocaine to alleviate the pain disorder of the infraspinatus muscle.

Preoperative Muscle Biopsy to Assess Motor End Plate Integrity as a Predictor for Successful Nerve Transfer: A Case Report

CASE

A 60-year-old right-hand-dominant man was referred for persistent right deltoid weakness, lateral shoulder numbness, and severe functional deficit 3 months after undergoing proximal humerus open reduction and internal fixation with plate and fibular strut allograft. Deltoid muscle biopsy demonstrated motor end plate (MEP) degeneration. After partial radial-to-axillary nerve transfer, repeat deltoid muscle biopsy revealed successful regeneration of MEPs with reinnervation of deltoid confirmed with postnerve transfer electromyography.

CONCLUSION

Selective nerve transfer can successfully rescue a denervated target muscle from further degeneration by restoration of healthy MEPs.

Motor end-plate analysis to diagnose immune-mediated myasthenia gravis in seronegative patients

This study aimed to evaluate the diagnostic usefulness of motor end-plate (MEP) analysis along with clustered acetylcholine receptor (AChR) antibody (Ab) assays in patients with myasthenia-like symptoms but negative routine AChR and muscle-specific kinase (MuSK) Ab tests. MEP analysis of muscle biopsies of the biceps brachii was performed in 20 patients to try to differentiate between those with or without immune-mediated myasthenia gravis (MG). Using a quantitative method, complement C3 deposition and AChR densities in MEPs were examined. Independently, cell-based assays were used to detect serum clustered-AChR Abs. Only five of 20 patients had complement deposition at MEPs; four of these patients had reduced AChR densities similar to those in patients with typical AChR Ab positive MG, and distinct from those in the remaining 15 patients. Two of the four serum samples from these patients had clustered-AChR Abs. All complement-positive patients were considered as having immune-mediated MG and improved with appropriate treatments; although one patient presented with MG 3 years later, the remaining patients had other diagnoses during over 10 years of follow-up. These results suggest the usefulness of MEP analysis of muscle biopsies in diagnosing immune-mediated MG in seronegative patients with myasthenia-like symptoms but, due to the invasiveness of the muscle biopsy procedure, clustered AChR Abs should, if possible, be tested first.

Opposite Regulation of Homer Signal at the NMJ Postsynaptic Micro Domain between Slow- and Fast-Twitch Muscles in an Experimentally Induced Autoimmune Myasthenia Gravis (EAMG) Mouse Model

Accelerated postsynaptic remodelling and disturbance of neuromuscular transmission are common features of autoimmune neurodegenerative diseases. Homer protein isoform expression, crosslinking activity and neuromuscular subcellular localisation are studied in mouse hind limb muscles of an experimentally induced autoimmune model of Myasthenia Gravis (EAMG) and correlated to motor end plate integrity. Soleus (SOL), extensor digitorum longus (EDL) and gastrocnemius (GAS) skeletal muscles are investigated. nAChR membrane clusters were studied to monitor neuromuscular junction (NMJ) integrity. Fibre-type cross-sectional area (CSA) analysis is carried out in order to determine the extent of muscle atrophy. Our findings clearly showed that crosslinking activity of Homer long forms (Homer 1b/c and Homer2a/b) are decreased in slow-twitch and increased in fast-twitch muscle of EAMG whereas the short form of Homer that disrupts Homer crosslinking (Homer1a) is upregulated in slow-twitch muscle only. Densitometry analysis showed a 125% increase in Homer protein expression in EDL, and a 45% decrease in SOL of EAMG mice. In contrast, nAChR fluorescence pixel intensity decreased in endplates of EAMG mice, more distinct in type-I dominant SOL muscle. Morphometric CSA of EAMG vs. control (CTR) revealed a significant reduction in EDL but not in GAS and SOL. Taken together, these results indicate that postsynaptic Homer signalling is impaired in slow-twitch SOL muscle from EAMG mice and provide compelling evidence suggesting a functional coupling between Homer and nAChR, underscoring the key role of Homer in skeletal muscle neurophysiology.

Spatial Analysis of Multichannel Surface EMG in Hemiplegic Stroke

We investigated spatial activation patterns of upper extremity muscles during isometric force generation in both intact persons and in hemispheric stroke survivors. We used a 128-channel surface electromyogram (EMG) grid to record the electrical activity of biceps brachii muscles during these contractions. EMG data were processed to develop 2-D root mean square (RMS) maps of muscle activity. Our objective was to determine whether motor impairments following stroke were associated with changes in the muscle activity maps and in the spatial distribution of muscular activation. We found that, for a given subject, spatial patterns in muscle activity maps were consistent across all measured contraction levels differing only the RMS EMG. However, the maps from opposite arms (stroke-affected versus non-affected) of stroke survivors were significantly different from each other, especially when compared with the differences observed intact participants. Our analyses revealed that chronic stroke altered the size and location of the active region in these maps. The former is potentially related to disruption of fiber and tissue structure, possibly linked to factors such as extracellular fat accumulation, connective tissue infiltration, muscle fiber atrophy, fiber shortening, and fiber loss. Changes in spatial patterns in muscle activity maps may also be linked to a shift in the location of the innervation zone or the endplate region of muscles. Furthermore, the textural analysis of EMG activity maps showed a larger pixel-to-pixel variability in stroke-affected muscles. Alterations in the muscle activity maps were also related to functional impairment (estimated using Fugl-Meyer score) and to the degree of spasticity (estimated using the modified Ashworth scale). Overall, our investigation revealed that the muscle architecture and morphology were significantly altered in the chronic stroke.

Botulinum toxin type A injections in the psoas muscle of children with cerebral palsy: muscle atrophy after motor end plate-targeted injections

MEP targeting during BoNT-A injections has been demonstrated to improve outcome. Two injection techniques of the psoas muscle - proximal MEP targeting versus a widely used more distal injection technique - are compared using muscle volume assessment by digital MRI segmentation as outcome measure.

METHOD

7 spastic diplegic children received injections in both psoas muscles: two different injection techniques randomly in 5 patients, in 2 patients bilateral MEP targeting. MRI images of the psoas were taken before, after 2 months and in 3 patients after 6 months.

RESULTS

Average post injection volume (in relation to pre-injection volume) for the MEP targeted muscles (9) is 79.5% versus 107.8% in the 5 distal injected psoas muscles (p=0.0033). In all 5 asymmetric injected patients the MEP targeted psoas had a larger volume reduction than the more distal injected psoas muscle. This atrophy remains even 6 months after the injection. This is the first study were a longitudinal follow-up by MRI demonstrates muscle atrophy after BoNT-A in children with CP. Injections in the MEP zone of the muscle, which is the more proximal part of the psoas muscle, cause atrophy in contrary to more distal injections were this atrophy is not observed.

Determination of slow-tonic MyHC immunoreactivity is an important step in the evaluation of muscle spindles in porcine extraocular muscles

We have tested our hypothesis suggesting (i) that for the reliable determination and counting of muscle spindles (Msp) at the light microscopy level in extraocular muscles (EOM), analysis of the spindle specific myosin heavy chain (MyHC) immunoreactivity of intrafusal fibers, especially after staining with anti-slow-tonic MyHC antibodies, is the most convenient tool, (ii) that the number of Msp determined by the slow-tonic MyHC immunoreactivity of intrafusal fibers in EOM is much lower than that based on histological examination and (iii) that the previously reported numbers of Msp based on histological examination of EOM could be overestimated. In order to determine the number and distribution of Msp and to analyze the MyHC isoform immunoreactivity of intrafusal fibers in porcine EOM, paraffin sections of three 9-month-old pig medial (MR) and lateral rectus (LR), levator palpebrae (LP) and retractor bulbi (RB) muscles were stained histologically or using specific monoclonal antibodies (mAbs) against MyHC isoforms. Msp in recti and LP muscles studied by immunocytochemistry contained nuclear bag (NB) fiber(s) reacting with mAbs against slow-tonic, slow-twitch, alpha-cardiac and neonatal MyHCs, but not with the mAb against fast-twitch MyHC, which, on the contrary, stained nuclear chain (NC) fibers. Based on determination of spindle specific slow-tonic MyHC isoform immunoreactivity we have found 72 Msp in the MR and 68 Msp in the LR and 12 Msp in LP muscles, which was only 62, 55 and 32% of the Msp total counts according to histological examination, respectively. In the RB muscle, we have even found only 15 spindle-like-structures composed of encapsulated thin muscle fibers, which possessed only a reaction with anti-fast-twitch MyHC mAb, but lacked slow-tonic, slow-twitch or alpha-cardiac MyHCs immunoreactivity. Our analysis of porcine EOM confirmed the above suggestions, demonstrating, for the first time in the pig, the presence of "false Msp" mimicking encapsulated muscle fibers on histological sections that lack spindle specific MyHC immunoreactivity. In analogy with other muscles we suggest that "false Msp" are not innervated by sensory axons and therefore do not contribute to the physiological sensation of the muscle length changes. Our results thus show that the reliable identification of functionally effective Msp in EOM must involve immunohistochemical analysis of spindle specific MyHC isoforms of intrafusal fibers, as "false" spindles appearing on histologically stained sections as encapsulated muscle fibers could be regarded as "true" Msp and thus increase the spindle number counts in earlier studies.

Extruded lumbar osseous endplate causing long-term radiculopathy in an adult: an endoscopic excision

In this report, we described an adult case that had a long-term radiculopathy due to an extruded osseous endplate of the lumbar spine at the L5-S1 intervertebral disc level. The osseous material inside the extruded material was not absorbed, and it had continued compressing the nerve root for one year. Endoscopically, the bony fragment was successfully removed. After the surgery, the patient's symptom disappeared, and neurological deficits became normalized. In conclusion, we propose that surgical intervention should be taken into account for the treatment of HNP, when the extruded material contains bony fragment such as osseous endplate.

Neurovascular alignment in adult mouse skeletal muscles

OBJECTIVE

Muscle blood flow increases with motor unit recruitment. The physical relationships between somatic motor nerves, which control muscle fiber contraction, and arterioles, which control microvascular perfusion, are unexplored. The authors tested the hypothesis that motor axons align with arterioles in adult skeletal muscle.

METHODS

Transgenic mice (C57BL/6 background, n = 5; 10 months of age) expressing yellow fluorescent protein in all motor nerves underwent vascular casting (Microfil). Excised epitrochlearis, gracilis, gluteus maximus, and spinotrapezius muscles were imaged at 380x and 760x and a computer-integrated tracing system (Neurolucida) was used to acquire 3-dimensional digital renderings of entire arteriolar and neural networks within each muscle.

RESULTS

Arteriolar networks were typically approximately 3-fold longer than neural networks. Nerves coursed with arterioles until terminating at motor endplates. Across muscles, proximity analyses revealed that approximately 75% of total nerve length (9.8-48.8 mm) lay within 200 microm of the nearest arteriole (diameters of 15-60 microm).

CONCLUSIONS

Somatic motor nerves and arterioles align closely within adult mammalian skeletal muscle. Understanding the signals governing neurovascular alignment may hold important clues for the advancement of tissue engineering and regeneration.

Dehydration affects synaptic transmission at flexor muscle in acute lead-treated mice

The effect of 24 hrs. water deprivation on spontaneous and evoked transmitter release was studied at flexor nerve terminals of control and lead-treated male C57BL mice. Miniature endplate potentials (MEPPs) and endplate potentials (EPPs) were recorded intracellularly from urethane-anesthetized (2 mg/g, i.p.) control and lead exposed mice in both hydrated and dehydrated conditions. Exposure to lead was made by i.p. injection of lead acetate (1.0 mg/kg) dissolved in a 5% glucose solution 24 hrs. prior to the experiment. Unimodal and bimodal MEPP frequencies decreased with dehydration, while small mode MEPPs remained unchanged and large mode MEPPs increased in frequency. EPP amplitude and quantal content were unchanged by dehydration. Lead treatment by itself reduced the frequency of unimodal and bimodal MEPPs but had no effect on the amplitude of EPPs or of quantal content. However a combination of dehydration and acute lead treatment reduced the frequency of unimodal, bimodal and large mode MEPPs and significantly reduced both EPP amplitude and quantal content. Dehydration apparently reveals an underlying neurotoxic action of lead at the neuromuscular junction. This raises a health concern that people subjected to both lead pollution and dehydration are at greater risk to lead poisoning of the neuromuscular junction.

The fine structure of muscle spindles in the lumbrical muscles of the rat

Lumbrical muscles of young rats were fixed with OsO₄ and embedded in methacrylate for electron microscopy. The spindle capsule was found to be continuous with and similar in structure to the sheath of Henle surrounding the nerves supplying the spindle. The capsule consists of several closely applied concentric cytoplasmic sheets. Each sheet is about 1,000 A thick and has no fenestrations. Many caveolae and vesicles in the cytoplasm suggest active transport through the sheets. The periaxial space fluid contains much solid material. It is suggested that the capsule and periaxial space regulate internal chemical environment. The interfibrillar structures are less evident in the polar regions of intrafusal fibres than in extrafusal fibres. Simple motor end-plates occur on the polar regions of intrafusal fibres. In the myotube region of the intrafusal fibre a peripheral zone of myofibrils surrounds a cytoplasmic core containing nuclei, mitochondria, Golgi bodies, reticulum, and a few lipid-like granules. Naked sensory endings lie on the myotube "in parallel" with the underlying myofilaments. Naked processes of the primary sensory ending deeply indent the muscle plasma membrane and the underlying wisps of myofilament in the nuclear bag region. The plasma membranes of sensory nerve ending and intrafusal muscle fibre are about 200 A apart.

Observations on the fine structure and cytochemistry of mouse and human intercostal neuromuscular junctions

The fine structure of the mouse and human intercostal muscle neuromuscular junction was studied after brief fixation in a new formol-sucrose fixative. This primary formalin fixation was followed by brief postosmication in buffered 1 per cent osmium tetroxide. Muscle blocks were embedded in methacrylate or Epon 812 epoxy resin. Marked similarities between mouse and human motor end-plates were observed. Neuromuscular junctions from both mouse and human intercostal muscle showed synaptic vesicles, primary and secondary synaptic clefts, and layered differentiation of the amorphous surface material (ASM) present on the surface of the Schwann cell plasma membrane and on the muscle surface membrane in the region of the neuromuscular junction. An attempt to stain the ASM with lead was unsuccessful. Observations on thick and thin plastic-embedded sections stained by PAS after diastase digestion showed that the ASM within the subneural apparatus is PAS positive. Alcian blue stained the endoneurium and perineurium of peripheral nerve bundles and portions of the end-plates. The similarity of the PAS-positive ASM to other basement membranes described in other sites is discussed and its possible physiologic significance within the subsynaptic apparatus is considered.

Studies on the repetitive discharges evoked in motor nerve and skeletal muscle after injection of anticholinesterase drugs

Repetitive discharges recorded from the ventral root and from the gastrocnemius muscle in response to single motor nerve shocks applied close to the muscle after injection of edrophonium, neostigmine or ambenonium were studied in cats anaesthetized with chloralose. Two closely spaced volleys with an interval of 1 to 5 msec between them produced more repetitive firing than did a single shock. With longer intervals, the repetitive firing was not potentiated by the second volley. All frequencies of tetanic stimulation depressed the repetitive firing and, for successive stimuli to produce a degree of repetitive firing equivalent to the first, it was necessary to stimulate at frequencies below 2 shocks/sec. With stimulation frequencies higher than 100 shocks/sec, repetitive firing did not occur unless the duration of the tetanus was shorter than about 30 msec when slight repetition followed the last stimulus of the train. With stimulation frequencies of 100 down to 20 shocks/sec, repetitive firing was produced only by the first volley of the tetanus. Subsequent nerve action potentials of the tetanus occurring during the repetitive firing in the nerve following the first volley were partially extinguished by collision with the back discharge. This effect contributed to the waning tetanus, which is characteristic of treatment with an anticholinesterase, but the main depression of tetanic contractions appeared to be a consequence of depolarization block through accumulating acetylcholine. Tubocurarine and benzoquinonium reversed the initial "extinction" phase of the depressed tetani by abolishing the repetitive discharge in the nerve and in larger doses reversed the secondary depressant phase presumably by reducing the excessive end-plate depolarization. The results are discussed in relation to the hypothesis that anticholinesterases may effect transmission by acting at three sites at the neuromuscular junction-on acetylcholinesterase, at the motor nerve ending and at the motor end-plate-and that reaction at any one site may be augmented by the production of reverberating activity across the junction.

Triethylcholine compared with other substances affecting neuromuscular transmission

Triethylcholine (triethyl-2-hydroxyethyl ammonium) has been compared, in its actions on neuromuscular transmission, with the motor end-plate blocking drugs tubocurarine and decamethonium, with the anticholinesterase neostigmine, and with the closely related drug tetraethylammonium. The experiments were carried out on conscious rabbits and mice, on the tibialis anterior muscle of cats under chloralose anaesthesia and on the isolated phrenic nerve-diaphragm preparation of the rat. Anticholinesterase activity was determined manometrically using the Warburg apparatus. Triethylcholine possessed a slight curare-like action, but this effect was shown to be too weak and transient to contribute to the slowly developing and long-lasting transmission failure which occurs selectively in frequently excited nervemuscle preparations and in exercised conscious animals. It was confirmed that the site of the blocking action of triethylcholine was pre-junctional. Triethylcholine often produced a slight potentiation of the contractions before blocking them. This effect was not due to a depolarizing or an anticholinesterase action, and it was concluded that the slight initial facilitating action of triethylcholine on neuromuscular transmission was due to an increase in the quantity of acetylcholine released by the nerve impulse. Tetraethylammonium was much more powerful than triethylcholine in this respect. The pre-junctional transmission failure produced by triethylcholine could not be explained simply on the basis that an initial excessive release led to exhaustion of transmitter.

Fiber apoptosis in developing rat muscles is regulated by activity, neuregulin

Motor neurons are thought to critically regulate the survival of a subset of developing muscle fibers. In muscles that develop in the absence of innervation, primary muscle fibers appear to form in normal numbers, but their long-term survival is thought to require innervation-induced suprathreshold activity. Here I examine interactions between motor neurons and muscle fibers in newborn rats. I report that a small fraction of muscle fibers in developing muscles undergo apoptosis. Many, if not all, of these fibers appear to have lost innervation owing to the retraction of nerve terminals. That the apoptosis is initiated by the loss of functional innervation is suggested by a severalfold increase in the magnitude of fiber apoptosis following denervation and muscle paralysis. Finally, both naturally occurring and denervation-induced muscle fiber apoptosis can be prevented by exogenous administration of neuregulin. These results argue that the survival of all or a subset of developing muscle fibers is ultimately regulated by neuregulin. Furthermore, these results suggest a model in which innervation-induced suprathreshold activity indirectly regulates muscle fiber survival by modulating the levels of neuregulin at developing neuromuscular junctions.

A histomorphometric analysis of the cross-facial nerve graft in the treatment of facial paralysis

One of the most unsettling sequela of facial paralysis (FP) is the loss of the blink reflex, leading to both a functional and aesthetic deformity. A successful method of treating FP and, in particular, loss of eye-sphincter function, is the use of the cross-facial nerve graft (CFNG) to reinnervate the previously denervated orbicularis oculi muscle. The present study examined the histomorphometric aspects of the entire CFNG, with respect to axon diameter and myelin area. The axon profile of the CFNG had a positive correlation with motor end-plate counts and electrophysiologic recordings. These results should help in further understanding the number of motor axons needed to restore adequate function to the paralyzed eye sphincter, and establish more rational reconstructive procedures.

FM1-43 dye ultrastructural localization in and release from frog motor nerve terminals

Previous work has shown that the fluorescent styryl dye FM1-43 stains nerve terminals in an activity-dependent fashion. This dye appears to label the membranes of recycled synaptic vesicles by being trapped during endocytosis. Stained terminals can subsequently be destained by repeating nerve stimulation in the absence of dye; the destaining evidently reflects escape of dye into the bathing medium from membranes of exocytosing synaptic vesicles. In the present study we tested two key aspects of this interpretation of FM1-43 behavior, namely: (i) that the dye is localized in synaptic vesicles, and (ii) that it is actually released into the bathing medium during destaining. To accomplish this, we first photolyzed the internalized dye in the presence of diaminobenzidine. This created an electron-dense reaction product that could be visualized in the electron microscope. Reaction product was confined to synaptic vesicles, as predicted. Second, using spectrofluorometry, we quantified the release of dye liberated into the medium from tubocurarine-treated nerve-muscle preparations. Nerve stimulation increased the amount of FM1-43 released, and we estimate that normally a stained synaptic vesicle contains a few hundred molecules of the dye. The key to the successful detection of released FM1-43 was to add the micelle-forming detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), which increased FM1-43 quantum yield by more than two orders of magnitude.

Leukemia inhibitory factor influences the timing of programmed synapses withdrawal from neonatal muscles

We show that leukemia inhibitory factor (LIF) plays a physiological role in the programmed withdrawal of synapses from neonatal muscles. First, LIF mRNA is present in embryonic skeletal muscle and is developmentally regulated. We detect high levels of LIF mRNA at embryonic day 17 (E17) in mouse hind leg muscles. The content of LIF mRNA falls 10-fold between E17 and birth and then remains low in the neonate and adult. The decrease in LIF mRNA in skeletal muscle coincides with the end of secondary myogenesis and the completion of the adult number of myofibers. Second, treatment of the mouse tensor fascia latae (TFL), a superficial muscle of the hind leg, with LIF from birth (100 ng/day), transiently delays the withdrawal of excess inputs from polyneuronally innervated myofibers by approximately 3 days. The midpoint of the process is shifted from 7.5 +/- 10.2 +/- 0.6 days of age. LIF treatment delays synapse withdrawal by altering its timing without an appreciable effect on its rate. Third, in mice homozygous for a disruption of the LIF gene, the midpoint in the reduction of multiply innervated TFL myofibers occurs 1 day earlier, at 6.5 +/- 0.5 days of age. Muscle fiber number is unchanged in LIF null mice. Treatment with LIF does not alter the rate of neonatal growth, the number of muscle fibers in the TFL, or the reappearance of inputs that have been eliminated. Instead, LIF appears to delay maturation of the motor unit by transiently delaying the onset of synapse withdrawal. We hypothesize that this is a necessary component of a selective process that will operate simultaneously and equally on multiple, competing motor units.

Mutation of a new sodium channel gene, Scn8a, in the mouse mutant 'motor endplate disease'

The mouse neurological mutant 'motor endplate disease' (med) is characterized by early onset progressive paralysis of the hind limbs, severe muscle atrophy, degeneration of Purkinje cells and juvenile lethality. We have isolated a voltage-gated sodium channel gene, Scn8a, from the flanking region of a transgene-induced allele of med. Scn8a is expressed in brain and spinal cord but not in skeletal muscle or heart, and encodes a predicted protein of 1,732 amino acids. An intragenic deletion at the transgene insertion site results in loss of expression. Scn8a is closely related to other sodium channel alpha subunits, with greatest similarity to a brain transcript from the pufferfish Fugu rubripes. The human homologue, SCN8A, maps to chromosome 12q13 and is a candidate gene for inherited neurodegenerative disease.

Comparison of neostigmine-induced recovery with spontaneous recovery from mivacurium-induced neuromuscular block

In 24 ASA I-II adults anaesthetized with thiopentone, fentanyl and nitrous oxide in oxygen, we studied neuromuscular transmission with isometric adductor pollicis monitoring. Patients received mivacurium 0.2 mg kg-1 followed by an infusion lasting at least 60 min and adjusted to maintain twitch height at 1-5%. After termination of the mivacurium infusion, when twitch height spontaneously regained 25% of its control value, the patients were allocated to two groups of 12 patients each. In group NEO patients received neostigmine 40 micrograms kg-1 and atropine 15 micrograms kg-1 and in group SPO neuromuscular transmission was allowed to recover spontaneously. Twitch height was measured every 10 s and train-of-four (TOF) (2 Hz) every 3 min. After 15 min, residual force after tetanic stimulation (50 and 100 Hz, 5-s duration (RF50HZ, RF100HZ), 1 min apart) were recorded sequentially. At 15 min, mean TOF ratio was greater in group NEO (0.94 (SEM 0.01)) than in group SPO (0.87 (0.02)) (P < 0.01). All patients in group NEO recovered to a TOF ratio greater than 0.7 after 6 min compared with 15 min in group SPO (P < 0.005). A TOF ratio greater than 0.9 was observed in all patients in group NEO compared with only six in group SPO (P < 0.025). Nevertheless, RF50HZ and RF100HZ did not differ significantly (0.92 (0.01) (group NEO), 0.91 (0.01) (group SPO) and 0.83 (0.02) (group NEO), 0.78 (0.03) (group SPO), respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Quantifying how location and dose of botulinum toxin injections affect muscle paralysis

Despite the widespread use of botulinum toxin to treat muscle dystonias, no method exists to quantify muscle paralysis in either human or nonhuman models. In this study we examined how the location, dose, and volume of botulinum injection affects paralysis in the rat tibialis anterior muscle. Paralysis was quantified by electrically stimulating the nerve to the tibialis anterior and then staining sections of the muscle for glycogen. The areas of glycogen-containing fibers represented regions of botulinum action. The results showed that the most important injection technique is to inject botulinum directly into the motor endplate region of a muscle. Injections only 0.5 cm from the motor endplate resulted in a 50% decrease in paralysis. Increases in dose increased paralysis, however, some of that increase was simply due to the increased volume of injection. Thus, delivering toxin in small volumes near the MEP band of a muscle should produce the most effective paralysis.

Congenital disorders of neuromuscular transmission

Pathology underlying congenital forms of myasthenia gravis has been delineated through microscopic and electrophysiologic studies over the past 15 years. Differentiation from the immune-mediated disorder is crucial because therapy appropriate for acquired myasthenia may be harmful to patients with congenital disease.

Electrophysiological and morphological study of polyneuronal innervation in the cutaneous pectoris muscle of adult frog (Rana esculenta)

The incidence of polyneuronal innervation in the cutaneous pectoris muscle of the adult frog, Rana esculenta, was determined quantitatively using electrophysiological and morphological techniques. The mean percentages of multiple innervated endplates obtained with both techniques from a series of 19 muscles examined at all times of the year were in very good correlation: 30.7% (196/639 endplates) by electrophysiology and 30.5% (478/1569 endplates) by morphology. In nine muscles examined during the period from December to March the mean percentages, 36.8% (110/299) by electrophysiology and 38.6% (281/727) by morphology, were significantly higher than those obtained for 10 muscles investigated during the period from May to November, 25.3% (86/340) and 23.4% (197/842) with both techniques respectively. The higher incidence of collateral sprouted branches detected at polyinnervated endplate sites in muscles of winter frogs might be related to these seasonal variations. Most of the 1688 fibres from 26 muscles examined throughout the year exhibited one centrally located endplate. However, around 11% of them were found to be innervated at two separate endplate sites. Muscle fibres exhibiting this type of innervation were invariably the largest fibres in each muscle tested, having an apparent diameter greater than 48 micron. The distance between the endplates of these fibres represented between 10 and 30% of their total length. No significant seasonal variations were observed in the incidence of these dually innervated fibres. In conclusion, both electrophysiological and morphological results show that the normal incidence of polyneuronal innervation in the cutaneous pectoris muscle of adult Rana esculenta is affected by seasonal related factors which influence the nodal sprouting activity. Moreover, they show that a dual pattern of innervation is a common feature in large fibres of this muscle.

The response to partial denervation

The cranial belly of the anterior gracilis muscle of the rat has two discrete motor endplate zones. A proximal zone is innervated by short branches of the obturator nerve, and a distal zone is innervated by (usually) two longer branches. Each muscle fiber is innervated at a single motor endplate although a substantial number lie within both endplate zones. In addition, motor units are divided between the two zones. In order to dissociate the role of the denervated endplate from that of the denervated muscle fiber in the promotion of motoneuron sprouting, the distal endplate zone in this model was denervated and the response at the proximal zone was studied. Comparisons were made with partial denervation of the muscle by division of the L4 ventral ramus and with partial denervation of the distal endplate zone. Denervation of the distal endplate zone produced profuse terminal sprouting at the proximal zone whereas division of L4 predominantly produced nodal sprouting at both zones. Partial denervation of the distal zone resulted in nodal sprouts in that zone and again mainly terminal sprouts at the proximal zone. The repeated association of terminal sprouting with division of the motor axons supplying the distal zone together with the knowledge that motor units are distributed between the two zones led to the conclusion that the terminal sprouting was stimulated by the reduction in size of motor units rather than by the presence of denervated muscle fibers in the vicinity of the endplates.

Na channel distribution in vertebrate skeletal muscle

The loose patch voltage clamp has been used to map Na current density along the length of snake and rat skeletal muscle fibers. Na currents have been recorded from (a) endplate membrane exposed by removal of the nerve terminal, (b) membrane near the endplate, (c) extrajunctional membrane far from both the endplate and the tendon, and (d) membrane near the tendon. Na current densities recorded directly on the endplate were extremely high, exceeding 400 mA/cm2 in some patches. The membrane adjacent to the endplate has a current density about fivefold lower than that of the endplate, but about fivefold higher than the membrane 100-200 micron from the endplate. Small local variations in Na current density are recorded in extrajunctional membrane. A sharp decrease in Na current density occurs over the last few hundred micrometers from the tendon. We tested the ability of tetrodotoxin to block Na current in regions close to and far from the endplate and found no evidence for toxin-resistant channels in either region. There was also no obvious difference in the kinetics of Na current in the two regions. On the basis of the Na current densities measured with the loose patch clamp, we conclude that Na channels are abundant in the endplate and near-endplate membrane and are sparse close to the tendon. The current density at the endplate is two to three orders of magnitude higher than at the tendon.

Three-dimensional computer reconstruction of the neuromuscular junction distribution in the human posterior cricoarytenoid muscle

Recent studies have indicated that in a denervated muscle highly specific interactions occur between regenerating axons and the basal lamina of the myofiber precisely at the original synaptic sites. Since these findings suggest that synapse regeneration is facilitated at the original synaptic sites, a knowledge of the distribution of neuromuscular junctions in the human posterior cricoarytenoid (PCA) muscle may guide reinnervation procedures in this muscle. We, therefore, have used histochemical and computer graphics techniques to reconstruct the three-dimensional distribution of neuromuscular junctions in the human PCA taken fresh at autopsy. The results indicated a more localized pattern of neuromuscular junctions as compared to the human thyroarytenoid muscle. The results will be discussed with respect to their implications for procedures for reinnervation of the posterior cricoarytenoid muscle.

A cerebellar abnormality in the mouse with motor end-plate disease

The murine mutant with motor end-plate disease (med) exhibits a progressive weakness which is due to a functional denervation of skeletal muscle. It is inherited as an autosomal recessive trait and has two alleles. One of these is the jolting (medjo) mouse which also suffers from a disturbance of locomotion. Although the jolting animal was originally thought to suffer from a disorder of neuromuscular transmission, the organization of the peripheral nervous system is indistinguishable from normal and neuromuscular transmission is unimpaired. It is now clear that abnormalities in the cerebellum are responsible for the gait problems of the jolting mouse, and a preliminary morphological and neurophysiological examination of the mouse with motor end-plate disease has shown that it also has cerebellar abnormalities. Axonal spheroids may be seen on cerebellar Purkinje cell axons in the med mouse, and the character of extra-cellular recordings obtained from the Purkinje cells is abnormal.

Topographical localization of motor endplates in cryosections of whole human muscles

The distribution of motor endplates in biceps brachii, tibialis anterior, and sartorius muscles from human adults was studied by staining longitudinal cryosections of whole muscle for cholinesterase. A special freezing technique was used to prevent the muscle from cracking before sectioning on a heavy cryostat microtome. The results from a large number of cryosections from biceps brachii and tibialis anterior muscles were analyzed by a computer and the topographical distributions of endplates in different views of the muscles were reconstructed. In the biceps brachii muscle, the endplates formed a fairly distinct, slightly V-shaped band through the middle of the two heads. In the tibialis anterior muscle, the majority of the endplates were superficially distributed along the whole muscle. In the longitudinal sections from the middle part of the muscle, they gave the pattern of a parabola with its apex at the proximal end of the muscle. In the sartorius muscle, the endplates were scattered throughout the muscle and no endplate band was observed. The findings are in accordance with results obtained 30 years ago in investigations of muscle from small children and stillborn infants.

The splenius capitis muscle of the rat, architecture and histochemistry, afferent and efferent innervation as compared with that of the quadriceps muscle

The splenius muscle of the rat was investigated with regard to its structure and innervation. The latter was compared with that of the quadriceps muscle. The results can be summarized as follows: The splenius muscles of both sides form a bipennate muscle plate connecting the occipital bone with the spinous process of the second thoracic vertebra. The lateral parts of both muscles are attached directly to this prominent bony process, whereas the medial parts end in a median raphe which forms a tendinous cranial extension of the second thoracic vertebra. This tendinous extension, showing no connection to the cervical vertebrae, serves also for the attachment of acromio-trapezius muscle fibers. The lateral part of the splenius muscle is divided into two parts by a tendinous intersection. The splenius muscle consists mainly of fast twitch fibers: 55% were characterized as IIB and 40% as IIA fibers by histochemical demonstration of myosin ATPase-activity. A high content of muscle spindles--57 spindles per gram of muscle tissue--was found. Comparing several aspects of the innervation of the splenius to that of the quadriceps muscle, the following results could be obtained: The ratio of motor end plate size to muscle fiber volume is significantly higher in the splenius than in the quadriceps muscle. As demonstrated by transganglionary HRP-transport, the main part of labeled splenius afferents to the spinal cord terminates in the central cervical nucleus. Quadriceps afferents, entering the lower thoracic and upper lumbar segments, mainly end in the area of Clarke's column. Several labeled fibers descend to the sixth lumbar and first sacral segments, where they terminate in the area of Stilling's nucleus. A group of primary afferents from both muscles--most probably III- and IV-afferents--projects to the dorsal laminae of the dorsal horn; terminals from the splenius are accumulated in the lateral parts of these laminae, whereas those of the quadriceps are more concentrated in the medial areas. Within the brain stem, most afferents from the splenius terminate in the external cuneate nucleus. Most of the quadriceps afferents course to the gracile nucleus. Terminals from both muscle nerves were found in the area of the spinal vestibular nucleus. In conclusion, the most conspicuous results were: 1) Besides the segmental projection to the dorsal horn there is an almost exclusive projection of splenius primary afferents to relay nuclei to the cerebellum. 2) The relatively high ratio: end plate size/muscle fiber volume, which is characteristic of finely adjusting muscles.(ABSTRACT TRUNCATED AT 400 WORDS)

Paranodal dysmyelination and increase in tetrodotoxin binding sites in the sciatic nerve of the motor end-plate disease (med/med) mouse during postnatal development

Motor end-plate disease (med), in the mouse, is a hereditary neuromuscular defect, caused by a single gene mutation and characterized by a progressive muscle weakness. +Med/+med mice die 21-23 days after birth and the neurobiological abnormalities already reported are nerve terminal sprouting and swelling and neurotransmission failures. We studied +med/+med mice at preclinical (9-11 days after birth) as well as at clinically recognized stages of the disease. The nonmyelinated gaps of the nodes of Ranvier in the +med/+med sciatic nerve are found to be significantly widened in +med/+med animals compared to control littermates, even in the preclinical stage, although the nodes of Ranvier are not yet ultrastructurally mature. The maximal binding capacity for [3H]ethylene-diamine tetrodotoxin, expressed in femtomoles per milligram of protein, is significantly increased in +med/+med sciatic nerves. Thus, Na+ channels, which are known to be located mainly at the nodes of Ranvier in normal myelinated axons, are increased in number in +med/+med mice even before the disease becomes clinically established. Both the ultrastructural and biochemical developmental abnormalities of the node of Ranvier rapidly approach their maximal expression as the behavioral signs develop. Such nerve abnormalities may be closely related to the physiological impairment of nerve impulse conduction which leads to the pathophysiological expression of motor end-plate disease.

Modification of helper and suppressor/cytotoxic lymphocyte subsets in mice with motor end-plate disease

Motor end-plate disease (Med) in mice is associated with complex immunological abnormalities which are shared by the heterozygous +/MedJ mice, which exhibit no or mild clinical manifestations, and by MedJ/MedJ mice which die from this neuromuscular disorder. In the present paper we extend our immunological data with the study of splenic lymphocyte subsets with Lyt monoclonal antibodies. Both MedJ/MedJ and +/MedJ 14-18 day old mice have high Lyt1+/Lyt2+ ratios, with higher Lyt1+ and reduced Lyt2+ lymphocyte pools as compared to normal mice. This correlates with the low suppressive function previously described, but is unexpected in view of the low helper function as measured by the response to SRBC immunization. Adult +/MedJ mice recovered normal T lymphocyte subset levels, while the small group of MedJ/MedJ mice that escapes death but continues to suffer from the neuromuscular illness maintains high Lyt1+/Lyt2+ ratios.

The multiple molecular forms of acetylcholinesterase in "motor end-plate disease" in the mouse (medJ and med allelic forms): sensitivity of the 10 S form to partial or total loss of muscle activity

Motor end-plate disease in the mouse is a mutation, lethal at the time of weaning. Two alleles exist, med and medJ, with medJ/medJ surviving slightly longer. The multiple molecular forms of acetylcholinesterase show an abnormal developmental pattern during the course of the disease. A decrease in the 10 S AChE proportion to total AChE activity is the major change in gastrocnemius muscle. Similar AChE changes occur after total short-term denervation, tenotomy, and in other genetic diseases. Thus it appears that AChE is modified in med/med muscle as the result of a partial or total loss of muscle activity.

Intense ultraterminal sprouting from motor nerves and ultrastructural aspects of the neuromuscular junction and non-junctional sarcolemma of the soleus (slow-twitch) muscle in motor endplate disease in the mouse

Motor end-plate disease (med) in the mouse is an hereditary defect of the neuromuscular system, with partial functional denervation and muscle inactivity in late stages of the disease. Motor end-plate disease is characterized by an intense ultraterminal sprouting of the motor nerves from swollen nerve terminal branches in the soleus muscle. At the ultrastructural level, the neuromuscular junctions extend to very wide territories, often outside the original motor end-plate, in regions where the nerve sprouts are in simple apposition to the muscle fiber, with no secondary synaptic folds. The nerve terminals are rich in neurofilaments and poor in synaptic vesicles. Freeze fracture analysis of the pre-synaptic and post-synaptic membrane specializations fails to reveal any important structural alteration which could suggest a defect in acetylcholine release or in muscle membrane excitability. However, the non-junctional sarcolemmal specializations (the so-called 'square arrays') are found with a frequency slightly higher than in normal muscle. The nerve abnormalities at the neuromuscular junction may be either a consequence of muscle inactivity or the morphological expression of some primary nerve abnormality. Further studies of the soleus muscle at early stages of the disease may provide evidence in favor of either possibility.

Impairment of T lymphocyte functions in mice with motor end-plate disease

The present paper reports complex immunological anomalies associated with motor end-plate disease (Med) in mice. Motor end-plate disease is a severe neuromuscular disorder which leads to death (around the 25th of life) in the Medj/Medj mutant, while the heterozygotes quickly recover from mild manifestations. Medj/Medj and Medj/ + mice share some of the immunological aberrations: reduced PFC response to SRBC in 14-16 day old mice, with reduced suppressor cell function and precocious maturation of the cytotoxic response to allogeneic cells in 21-23 day old mice. The diminished PFC response is corrected in adult Medj/ + mice but persists in the small group of Medj/Medj which escape death and which were studied between the 6th and 16th week of life. In addition, the thymus and spleen of Medj/Medj mice are greatly reduced in size, a symptom which appears with the onset of the clinical disease. Also, a reduction in the NK activity in the small group of older, surviving mice was noted. T and B lymphocyte proportions and the proliferative responses to T cell mitogens were not impaired in 14-16 day old mice. The role of these abnormalities in the pathogenesis of the disease is not known. Since some of these anomalies are shared by Medj/Medj and Medj/ +, the latter of which present no or mild and transient neurological manifestations, there is no clear link between the immunological and neuromuscular disorders.