Changes in myosin expression in denervated laryngeal muscle

Developmental, Physiological and Phylogenetic Perspectives on the Expression and Regulation of Myosin Heavy Chains in Craniofacial Muscles

This review deals with the developmental origins of extraocular, jaw and laryngeal muscles, the expression, regulation and functional significance of sarcomeric myosin heavy chains (MyHCs) that they express and changes in MyHC expression during phylogeny. Myogenic progenitors from the mesoderm in the prechordal plate and branchial arches specify craniofacial muscle allotypes with different repertoires for MyHC expression. To cope with very complex eye movements, extraocular muscles (EOMs) express 11 MyHCs, ranging from the superfast extraocular MyHC to the slowest, non-muscle MyHC IIB (nmMyH IIB). They have distinct global and orbital layers, singly- and multiply-innervated fibres, longitudinal MyHC variations, and palisade endings that mediate axon reflexes. Jaw-closing muscles express the high-force masticatory MyHC and cardiac or limb MyHCs depending on the appropriateness for the acquisition and mastication of food. Laryngeal muscles express extraocular and limb muscle MyHCs but shift toward expressing slower MyHCs in large animals. During postnatal development, MyHC expression of craniofacial muscles is subject to neural and hormonal modulation. The primary and secondary myotubes of developing EOMs are postulated to induce, via different retrogradely transported neurotrophins, the rich diversity of neural impulse patterns that regulate the specific MyHCs that they express. Thyroid hormone shifts MyHC 2A toward 2B in jaw muscles, laryngeal muscles and possibly extraocular muscles. This review highlights the fact that the pattern of myosin expression in mammalian craniofacial muscles is principally influenced by the complex interplay of cell lineages, neural impulse patterns, thyroid and other hormones, functional demands and body mass. In these respects, craniofacial muscles are similar to limb muscles, but they differ radically in the types of cell lineage and the nature of their functional demands.

Botulinum Toxin Intervention in Cerebral Palsy-Induced Spasticity Management: Projected and Contradictory Effects on Skeletal Muscles

Spasticity, following the neurological disorder of cerebral palsy (CP), describes a pathological condition, the central feature of which is involuntary and prolonged muscle contraction. The persistent resistance of spastic muscles to stretching is often followed by structural and mechanical changes in musculature. This leads to functional limitations at the respective joint. Focal injection of botulinum toxin type-A (BTX-A) is effectively used to manage spasticity and improve the quality of life of the patients. By blocking acetylcholine release at the neuromuscular junction and causing temporary muscle paralysis, BTX-A aims to reduce spasticity and hereby improve joint function. However, recent studies have indicated some contradictory effects such as increased muscle stiffness or a narrower range of active force production. The potential of these toxin- and atrophy-related alterations in worsening the condition of spastic muscles that are already subjected to changes should be further investigated and quantified. By focusing on the effects of BTX-A on muscle biomechanics and overall function in children with CP, this review deals with which of these goals have been achieved and to what extent, and what can await us in the future.

Volumetric measures of the paralyzed vocal fold using computerized tomography; its clinical implication

OBJECTIVE

To report the volumetric measures of the paralyzed vocal fold in patients undergoing injection laryngoplasty.

MATERIAL AND METHOD

All the medical records of patients with unilateral vocal fold paralysis who had high resolution computerized tomography scan of the neck and chest prior to injection laryngoplasty between October 2015 and May 2018 were included. Volumetric evaluation of the vocal folds was performed by measuring the vocal fold height using coronal images and the vocal fold length and width using axial images.

RESULTS

A total of 21 patients divided into 13 males and 8 females were identified. The mean age was 56.66 ± 20.94 years. The mean volume of the paralyzed vocal fold was significantly smaller than that of the non-paralyzed vocal fold (p < .05). Similarly, the mean length and height of the paralyzed vocal folds were smaller than those of the non-paralyzed vocal folds (p < .05).

CONCLUSION

Volumetric measurements of the paralyzed vocal fold in comparison to the normal vocal fold in a group of 21 patients with unilateral vocal fold paralysis shows the presence of significant difference between the normal and affected site, and the presence of large inter-subject variation. Information on the volume difference between the two vocal folds may be used to better estimate the amount that needs to be injected in medialization procedures.

Functional electrical stimulation following nerve injury in a large animal model

INTRODUCTION

Controversy exists over the effects of functional electrical stimulation (FES) on reinnervation. We hypothesized that intramuscular FES would not delay reinnervation after recurrent laryngeal nerve (RLn) axonotmesis.

METHODS

RLn cryo-injury and electrode implantation in ipsilateral posterior cricoarytenoid muscle (PCA) were performed in horses. PCA was stimulated for 20 weeks in eight animals; seven served as controls. Reinnervation was monitored through muscle response to hypercapnia, electrical stimulation and exercise. Ultimately, muscle fiber type proportions and minimum fiber diameters, and RLn axon number and degree of myelination were determined.

RESULTS

Laryngeal function returned to normal in both groups within 22 weeks. FES improved muscle strength and geometry, and induced increased type I:II fiber proportion (p = 0.038) in the stimulated PCA. FES showed no deleterious effects on reinnervation.

DISCUSSION

Intramuscular electrical stimulation did not delay PCA reinnervation after axonotmesis. FES can represent a supportive treatment to promote laryngeal functional recovery after RLn injury. Muscle Nerve 59:717-725, 2019.

Early effect of Botox-A injection into the masseter muscle of rats: functional and histological evaluation

Background

The purpose of this study was to evaluate the change of food intake after different dosages of botulinum toxin A (BTX) injection in the animal model. Additionally, the dimensional and histological change at 14 days after BTX injection was also evaluated.

Methods

The comparative study was performed using the BTX injection model in rats (n = 5 for each group). Group 1 was the saline-injected group. Group 2 was the 5-unit BTX-injection group to each masseter muscle. Group 3 was the 10-unit BTX-injection group to each masseter muscle. Food intake rates and body weight were checked daily before and after BTX injection until 10 days. All animals were sacrificed at 14 days after BTX injection, and the specimens underwent hematoxylin and eosin stain and immunohistochemical staining for myosin type II (MYH2).

Results

The recovery of food intake in groups 2 and 3 decreased significantly compared with group 1 from day 2 to day 7 and day 9 after injection (p < 0.05). The BTX-treated masseter muscles were significantly smaller than those in group 1 (p = 0.015). The immunohistochemical findings demonstrated that the expression of MYH2 was significantly higher in group 3 compared to groups 1 and 2 (p < 0.001).

Conclusions

BTX injection to the masseter muscle in rats demonstrated short food-intake-rate reduction with recovery until 10 days after injection. The thickness of the masseter muscle and MYH2 expression were significantly changed according to the injected dose of BTX.

Influence of injection of Chinese botulinum toxin type A on the histomorphology and myosin heavy chain composition of rat gastrocnemius muscles

BACKGROUND AND OBJECTIVE

Botulinum toxin type A (BoNT/A) is a metalloprotease that blocks synaptic transmission via the cleavage of a synaptosomal-associated protein of 25 kDa (SNAP-25). It has gained widespread use as a treatment for cerebral palsy and skeletal muscle hypertrophy. In China, Chinese botulinum toxin type A (CBTX-A), a type of BoNT/A, is in widespread clinical use. However, the changes in the morphological and biochemical properties of treated muscles and in remote muscles from the CBTX-A injection site are relatively unknown. Therefore, we investigated the changes in histomorphology and myosin heavy chain (MyHC) isoform composition and distribution in rat gastrocnemius muscles after intramuscular injection of CBTX-A.

METHODS

The weakness of the injected muscles was assessed periodically to identify their functional deficiency. Muscle slices were stained with hematoxylin-eosin (HE) and adenosine triphosphatase (ATPase). MyHC isoform composition was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) to uncover changes in morphological and biochemical properties.

RESULTS

Our findings demonstrate that following injection of CBTX-A 5 U into rat gastrocnemius muscles, shifts in MyHC isoform composition emerged on the third day after injection and peaked in the fourth week. The composition remained distinctly different from that of the control group after the twelfth week. More specifically, there was a decrease in the proportion of the type IIb isoform and an increase in the proportions of type IIx, type IIa, and type I isoforms.

CONCLUSIONS

Data revealed that CBTX-A led to a shift in MyHC composition towards slower isoforms and that the MyHC composition remained far from normal six months after a single injection. However, no noticeable remote muscle weakness was induced.

Tacrolimus enhances the recovery of normal laryngeal muscle fibre distribution after reinnervation

OBJECTIVES

To assess the recovery of various muscle fibre types in the posterior cricoarytenoid muscle after laryngeal reinnervation in the rat, and to determine the influence of tacrolimus on this process.

METHODS

Four groups of rats underwent resection and anastomosis of the left vagus nerve, and were administered either tacrolimus at a low dose or an immunosuppressive dose, or cyclosporin A at a low dose or an immunosuppressive dose. A fifth group received surgery alone, and a sixth group received neither surgery nor drug treatment (healthy group). Muscles were removed for immunohistochemical analysis 45 days after surgery.

RESULTS

There was no difference in the proportion of types 1, 2a and 2b muscle fibres, comparing the immunosuppressive tacrolimus group and the healthy group, whereas there were fewer type 1 fibres in the group receiving surgery alone, compared with the healthy group (7 vs 12.1 per cent, respectively; p = 0.0303).

CONCLUSION

Tacrolimus enhanced the recovery of normal laryngeal muscle fibres after reinnervation in the rat, indicating a possible role in laryngeal transplantation.

Chronic stimulation-induced changes in the rodent thyroarytenoid muscle

PURPOSE

Therapies for certain voice disorders purport principles of skeletal muscle rehabilitation to increase muscle mass, strength, and endurance. However, applicability of limb muscle rehabilitation to the laryngeal muscles has not been tested. In this study, the authors examined the feasibility of the rat thyroarytenoid muscle to remodel as a consequence of increased activity instantiated through chronic electrical stimulation.

METHOD

Twenty adult Sprague-Dawley rats (Rattus norvegicus), assigned to a 1-week or 2-week stimulation group, were implanted with a nerve cuff electrode placed around the right recurrent laryngeal nerve and were fitted with a head connector. All animals were placed under anesthesia twice a day for 1 hr each time. Following the training, rats were killed, and thyroarytenoid muscles were isolated for histology and immunohistochemistry.

RESULTS

Mean muscle fiber area decreased, neuromuscular junction density increased, mitochondrial content increased qualitatively, and glycogen-positive fibers increased, demonstrating exercise-induced changes similar to those seen in limb muscles after endurance training.

CONCLUSION

Rat thyroarytenoid muscles are capable of remodeling in response to chronic electrical stimulation.

Laryngeal transplantation in minipigs: vascular, myologic and functional outcomes

There is no effective way of replacing all the functions of the larynx in those requiring laryngectomy. Regenerative medicine offers promise, but cannot presently deliver implants with functioning neuromuscular units. A single well-documented laryngeal transplant in man was a qualified success, but more information is required before clinical trials may be proposed. We studied the early response of the larynx to laryngeal transplantation between 17 pairs of NIH minipigs full matched at the MHC2 locus. Following iterative technical improvements, pigs had good swallowing and a patent airway at 1 week. No significant changes in mucosal blood flux were observed compared with pre-operative measurements. Changes in muscle morphology and fibre phenotype were observed in transplant muscles retrieved after 7 days: the levels of fast and slow myosin heavy chain (MyHC) protein were reduced and embryonic MyHC was up regulated consistent with denervation induced atrophy. At 1 week laryngeal transplantation can result in good swallowing, and is not associated with clinical evidence of ischemia-reperfusion injury in MHC-matched pigs.

Immunohistochemical analysis of the effects of cross-innervation of murine thyroarytenoid and sternohyoid muscles

This work uses cross-innervation of respiratory muscles of different developmental origins to probe myogenic and neurogenic mechanisms regulating their fiber types. The thyroarytenoid (TA) originates from the sixth branchial arch, whereas the sternohyoid (SH) is derived from somitic mesoderm. Immunohistochemical analysis using highly specific monoclonal antibodies to myosin heavy chain (MyHC) isoforms reveals that normal rat SH comprises slow, 2a, 2x, and 2b fibers, as in limb fast muscles, whereas the external division of the TA has only 2b/eo fibers coexpressing 2B and extraocular (EO) MyHCs. Twelve weeks after cross-innervation with the recurrent laryngeal nerve, the SH retained slow and 2a fibers, greatly increased the proportion of 2x fibers, and their 2b fibers failed to express EO MyHC. In the cross-innervated TA, the SH nerve failed to induce slow and 2A MyHC expression and failed to suppress EO MyHC expression in 2b/eo fibers. However, 2x fibers amounting to 4.2% appeared de novo in the external division of the TA. We conclude that although MyHC gene expression in these muscles can be modulated by neural activity, the patterns of response to altered innervation are largely myogenically determined, thus supporting the idea that SH and TA differ in muscle allotype.

Quantitative PCR analysis of laryngeal muscle fiber types

Voice and swallowing dysfunction as a result of recurrent laryngeal nerve paralysis can be improved with vocal fold injections or laryngeal framework surgery. However, denervation atrophy can cause late-term clinical failure. A major determinant of skeletal muscle physiology is myosin heavy chain (MyHC) expression, and previous protein analyses have shown changes in laryngeal muscle fiber MyHC isoform with denervation. RNA analyses in this setting have not been performed, and understanding RNA levels will allow interventions better designed to reverse processes such as denervation in the future. Total RNA was extracted from bilateral rat thyroarytenoid (TA), posterior cricoarytenoid (PCA), and cricothyroid (CT) muscles in rats. Primers were designed using published MyHC isoform sequences. SYBR Green real-time reverse transcription-polymerase chain reaction (SYBR-RT-PCR) was used for quantification. The electropherogram showed a clear separation of total RNA to 28S and 18S subunits. Melting curves illustrated single peaks for all type MyHC primers. All MyHC isoforms were identified in all muscles with various degrees of expression. Quantitative PCR is a sensitive method to detect MyHC isoforms in laryngeal muscle. Isoform expression using mRNA analysis was similar to previous analyses but showed some important differences. This technique can be used to quantitatively assess response to interventions targeted to maintain muscle bulk after denervation.

LEARNING OUTCOMES

(1) Readers will be able to describe the relationship between myosin heavy chain expression and muscle contractile properties. (2) Readers will be able to separate myosin heavy chain isoforms into slow and fast twitch phenotypes. (3) Readers will be able to describe differential muscle isoform expression between different laryngeal muscles. (4) Readers will be able to compare this study to other modalities of determining muscle fiber type.

Effects of long-term denervation on the rat thyroarytenoid muscle

OBJECTIVES/HYPOTHESIS

To determine the effects of long-term denervation on the rat thyroarytenoid (TA) muscle and neuromuscular junctions.

STUDY DESIGN

A quantitative histologic assessment of the TA muscle after long-term denervation.

METHODS

Thirty Wistar rats were euthanized 10, 18, 26, 42, and 58 weeks after left recurrent laryngeal nerve resection. The areas of the entire muscle and individual muscle fibers were evaluated using hematoxylin-eosin staining, and neuromuscular junctions were detected by immunohistochemistry. Changes after denervation were evaluated by comparing the treated (T) and untreated (U) sides (T/U ratio). The ratio of the number of nerve terminals (NTs) to that of acetylcholine receptors (AChRs) (NT/AChR ratio) was also assessed.

RESULTS

The average T/U ratio for the entire muscle area of the denervation groups ranged between 61.1% and 72.5% and did not differ significantly. Similarly, the T/U ratios for the individual muscle fiber area ranged between 45.0% and 51.9%, and the differences were not significant. The T/U ratio of AChRs at 58 weeks (35.3 +/- 20.2%) was significantly lower than that at 10 weeks (76.3 +/- 9.0%; P < .01). The NT/AChR ratios ranged between 30.3% and 35.6% and did not differ significantly among the denervation groups.

CONCLUSIONS

The entire TA muscle area, individual muscle fiber area, and NT/AChR ratio did not decrease with long-term denervation. Thus, the TA muscle may retain an ability to receive regenerating nerve axons. However, the ability of the TA muscle to receive nerve axons may deteriorate after an excessively long denervation period because the T/U ratio of AChRs decreased with long-term denervation.

Quantification of myosin heavy chain RNA in human laryngeal muscles: differential expression in the vertical and horizontal posterior cricoarytenoid and thyroarytenoid

BACKGROUND

Human laryngeal muscles are composed of fibers that express type I, IIA, and IIX myosin heavy chains (MyHC), but the presence and quantity of atypical myosins such as perinatal, extraocular, IIB, and alpha (cardiac) remain in question. These characteristics have been determined by biochemical or immunohistologic tissue sampling but with no complementary evidence of gene expression at the molecular level. The distribution of myosin, the main motor protein, in relation to structure-function relationships in this specialized muscle group will be important for understanding laryngeal function in both health and disease.

OBJECTIVES

We determined the quantity of MyHC genes expressed in human posterior cricoarytenoid (PCA) and thyroarytenoid (TA) muscle using real-time quantitative reverse-transcriptase polymerase chain reaction in a large number of samples taken from laryngectomy subjects. The PCA muscle was divided into vertical (V) and horizontal (H) portions for analysis.

RESULTS AND CONCLUSIONS

No extraocular or IIB myosin gene message is present in PCA or TA, but IIB is expressed in human extraocular muscle. Low but detectable amounts of perinatal and alpha gene message are present in both of the intrinsic laryngeal muscles. In H- and V-PCA, MyHC gene amounts were beta greater than IIA greater than IIX, but amounts of fast myosin RNA were greater in V-PCA. In TA, the order was beta greater than IIX greater than IIA. The profiles of RNA determined here indicate that, in humans, neither PCA nor TA intrinsic laryngeal muscles express unique very fast-contracting MyHCs but instead may rely on differential synthesis and use of beta, IIA, and IIX isoforms to perform their specialized contractile functions.

Bioengineered nerve regeneration and muscle reinnervation

The peripheral nervous system has the intrinsic capacity to regenerate but the reinnervation of muscles is often suboptimal and results in limited recovery of function. Injuries to nerves that innervate complex organs such as the larynx are particularly difficult to treat. The many functions of the larynx have evolved through the intricate neural regulation of highly specialized laryngeal muscles. In this review, we examine the responses of nerves and muscles to injury, focusing on changes in the expression of neurotrophic factors, and highlight differences between the skeletal limb and laryngeal muscle systems. We also describe how artificial nerve conduits have become a useful tool for delivery of neurotrophic factors as therapeutic agents to promote peripheral nerve repair and might eventually be useful in the treatment of laryngeal nerve injury.

Myosin heavy chain composition in normal and atrophic equine laryngeal muscle

The myosin heavy chain (MHC) composition of a given muscle determines the contractile properties and, therefore, the fiber type distribution of the muscle. MHC isoform expression in the laryngeal muscle is modulated by neural input and function, and it represents the cellular level changes that occur with denervation and reinnervation of skeletal muscle. The objective of this study was to evaluate the pattern of MHC isoform expression in laryngeal muscle harvested from normal cadavers and cadavers with naturally occurring left laryngeal hemiplegia secondary to recurrent laryngeal neuropathy. Left and right thyroarytenoideus (TA) and cricoarytenoideus dorsalis (CAD) were obtained from 7 horses affected with left-sided intrinsic laryngeal muscle atrophy and from 2 normal horses. Frozen sections were evaluated histologically for degree of atrophy and fiber type composition. MHC isoform expression was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of muscle protein. Histologic atrophy was seen in all atrophic muscles and some right-sided muscles of 3 affected horses, as well as the left TA of 1 normal horse. Fiber type grouping or loss of type I muscle fibers was observed in the left-sided laryngeal muscles in all but 1 affected horse, as well as in the right muscles of 2 affected horses, and the left TA of 1 normal horse. SDS-PAGE showed 2 bands corresponding to the type I and type IIB myosin isoforms in the CAD and TA of the 2 normal horses. Affected horses demonstrated a trend toward increased expression of the type IIB isoform and decreased expression of the type I isoform in atrophic muscles. This study confirmed the presence of histologic abnormalities in grossly normal equine laryngeal muscle, and it demonstrated an increased expression of type IIB MHC with a concurrent decreased expression of type I MHC in affected muscles. Evaluation of muscle fiber changes at the cellular level under denervated and reinnervated conditions may aid in assessing future strategies for reinnervation or regeneration of atrophic laryngeal muscle.

Proteomic profiling of rat thyroarytenoid muscle

PURPOSE

Proteomic methodologies offer promise in elucidating the systemwide cellular and molecular processes that characterize normal and diseased thyroarytenoid (TA) muscle. This study examined methodological issues central to the application of 2-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (2D SDS-PAGE) to the study of the TA muscle proteome using a rat model.

METHOD

2D SDS-PAGE was performed using 4 chemically skinned rat TA muscle samples. Gel images were analyzed and compared. Protein spot detection and matching were performed using computational image analysis algorithms only and computational image analysis followed by visual inspection and manual error correction. A synthetic master gel, constructed to control for uninteresting biological variation and technical artifact due to differences in protein loading and staining, was evaluated against its constituent gels.

RESULTS

Manual error correction resulted in a consistent increase in the number of protein spots detected (between 5.8% and 40.9%) and matched (from 25.8% to 70.8%) across all gels. Sensitivity and specificity of the automatic (computational) spot detection procedure, evaluated against the manual correction procedure, were 74.1% and 97.9%, respectively. Evaluation of protein quantitation parameter values revealed statistically significant differences (p < .0001) in optical density, area, and volume for matched protein spots across gels. The synthetic master gel successfully compensated for these intergel differences.

CONCLUSIONS

Valid and reliable proteomic data are dependant on well-controlled manageable variability and well-defined unmanageable variability. Manual correction of spot detection and matching errors and the use of a synthetic master gel appear to be useful strategies in addressing these issues. With these issues accounted for, 2D SDS-PAGE may be applied to quantitative experimental comparisons of normal and disease conditions affecting voice, speech, and swallowing function.

Proteomic analysis of rat laryngeal muscle following denervation

Laryngeal muscle atrophy induced by nerve injury is a major factor contributing to the disabling symptoms associated with laryngeal paralysis. Alterations of global proteins in rat laryngeal muscle following denervation were, therefore, studied using proteomic techniques. Twenty-eight adult Sprague-Dawley rats were divided into normal control and denervated groups. The thyroarytenoid (TA) muscle was excised 60 days after right recurrent laryngeal nerve was resected. Protein separation and identification were preformed using 2-DE and MALDI-MS with database search. Forty-four proteins were found to have significant alteration in expression level after denervation. The majority of these proteins (57%), most of them associated with energy metabolism, cellular proliferation and differentiation, signal transduction and stress reaction, were decreased levels of expression in denervated TA muscle. The remaining 43% of the proteins, most of them involved with protein degradation, immunoreactivity, injury repair, contraction, and microtubular formation, were found to have increased levels of expression. The protein modification sites by phosphorylation were detected in 22% of the identified proteins that presented multiple-spot patterns on 2-D gel. Significant changes in protein expression in denervated laryngeal muscle may provide potential therapeutic strategies for the treatment of laryngeal paralysis.

Botulinum neurotoxin type A causes shifts in myosin heavy chain composition in muscle

Botulinum neurotoxin type A has gained widespread use for treatment of a host of neuromuscular conditions. However, the potential effect of this toxin has on the histological and biochemical properties of skeletal muscle remains largely unexplored. The purpose of this study was to characterize the myosin heavy chain (MHC) distribution of adult rat skeletal muscle treated with botulinum neurotoxin type. Varying doses of the toxin were injected into the triceps surae muscle group of one hind limb. Force production was assessed periodically to access the functional deficit incurred. After 10 weeks, animals were sacrificed, muscles removed, and MHC composition determined. Body weight, muscle weight and force of the injected leg were significantly reduced in all groups, while loss of muscle weight and force in the contralateral leg was variable. In the injected plantaris and gastrocnemius muscles, type I MHC increased approximately 100%, while type IIa/x decreased approximately 50%. In the contralateral gastrocnemius, types I and IIa/x MHC increased approximately 100%, while type IIb decreased approximately 45%. These data suggest that botulinum neurotoxin causes shifts in MHC composition in injected and contralateral muscles that are contrary to those seen with denervation and similar to those seen with aging.

Effect of deficits in laryngeal sensation on laryngeal muscle biochemistry

Swallowing deficits in elderly people are significant clinical problems and may be associated with impaired pharyngolaryngeal sensation. However, the extent to which sensory innervation affects the motor system is unclear. Our purpose was to examine differences in biochemical properties of laryngeal muscles following sensory nerve ablation. We used sodium dodecyl sulfate-polyacrylamide gel electrophoresis to evaluate laryngeal muscles of young and old Fischer 344/Brown Norway rats, and rats that underwent sensory ablation via bilateral section of the superior laryngeal nerve, internal branch (SLNi), or mixed sensory-motor nerve ablation via left-sided recurrent laryngeal nerve (RLN) section. In lateral thyroarytenoid muscle, a reduction was found in the proportion of the most rapidly contracting myosin heavy chain isoform (type 2B) with SLNi section, RLN section, and aging. Section of the SLNi did not alter the proportion of any myosin heavy chain isoform within the lateral cricoarytenoid or posterior cricoarytenoid muscles, but RLN section resulted in a reduction in the proportion of type 2B. Accordingly, alteration in biochemical properties of the lateral thyroarytenoid muscle alone was demonstrated following sensory ablation. We conclude that sensory changes may affect properties of laryngeal muscles, and may thus have an impact on motor control during critical functions, such as airway protection during swallowing.

Laryngeal muscle fibre types

The internal laryngeal muscles have evolved to subserve the highly specialized functions of airways protection, respiration, and phonation. Their contractile properties, histochemistry, biochemical properties, myosin heavy chain (MyHC) expression and their regulation by nerves and hormones are reviewed and compared with limb muscle fibres. Cricothyroid, the vocal cord tensor, is limb-like in MyHC composition and fibre type properties, while the vocal fold abductor and adductors are allotypically different, with capacity for expressing an isoform of MyHC that is kinetically faster than the fastest limb MyHC. In rats and rabbits the faster isoform is the extraocular (EO) MyHC, while in carnivores, it is the IIB MyHC. These adaptations enable the abductor and adductor muscles to remain always faster than the cricothyroid as the latter changes in speed during evolution to match changing metabolic and respiratory rates in relation to scaling with body mass. Such phylogenetic plasticity is vital to the airways protection and respiratory functions of these muscles. The posterior cricoarythenoid, the abductor muscle, is tonically driven during expiration, and consequently has a slower fibre type profile than the principal adductor, the thyroarythenoid. The human thyroarythenoid appears not to express EO or IIB MyHC significantly, but is unique in expressing the slow-tonic MyHC. The concepts of allotype and phylogenetic plasticity help to explain differences in fibre type between limb and laryngeal muscles and between homologous laryngeal muscles in different species. Laryngeal muscle fibres exhibit physiological plasticity as do limb muscles, being subject to neural and hormonal modulation.

Fibrillation potential amplitude to quantitatively assess denervation muscle atrophy

Denervated muscle fibers exhibit spontaneous, repetitive single muscle fiber discharges and display fibrillation potentials detectable by electromyography. To explore the changing pattern of fibrillation potential amplitude after peripheral nerve injury and its relationship to the degree of muscle atrophy, fibrillation potential amplitudes were recorded on completely denervated biceps brachii of 173 patients with brachial plexus injury. Biceps brachii biopsies were taken at the same sites as the electromyogram recordings in 63 patients. The biopsies were analyzed by ATPase staining and the cross-sectional areas of fast and slow-twitch fibers were calculated. We found that the fibrillation potential amplitude and the cross-sectional areas of denervated muscle decay over time (P < 0.05), and both correlate negatively with denervation time (P < 0.01-0.05) within the first 15 months. The fibrillation potential amplitude correlates positively with both type I and II fiber cross-sectional areas (P < 0.0005-0.01). Our results show that fibrillation potential amplitude is closely correlated with muscle fiber size during the first 15 months after nerve injury, and it may therefore serve as a convenient index to evaluate quantitatively the degree of atrophy of denervated muscles. Electromyographic studies thus may help in designing treatment strategies.