Immunohistochemical characterization of human masseter muscle spindles

A Tutorial on Skeletal Muscle Metabolism and the Role of Blood Lactate: Implications for Speech Production

PURPOSE

The purpose of this tutorial is threefold: (a) present relevant exercise science literature on skeletal muscle metabolism and synthesize the limited available research on metabolism of the adult human speech musculature in an effort to elucidate the role of metabolism in speech production; (b) introduce a well-studied metabolic serum biomarker in exercise science, lactate, and the potential usefulness of investigating this metabolite, through a well-established exercise science methodology, to better understand metabolism of the musculature involved in voice production; and (c) discuss exercise physiology considerations for future voice science research that seeks to investigate blood lactate and metabolism in voice physiology in an ecologically valid manner.

METHOD

This tutorial begins with relevant exercise science literature on the basic cellular processes of muscle contraction that require energy and the metabolic mechanisms that regenerate the energy required for task execution. The tutorial next synthesizes the available research investigating metabolism of the adult human speech musculature. This is followed by the authors proposing a hypothesis of speech metabolism based on the voice science literature and the application of well-studied exercise science principles of muscle physiology. The tutorial concludes with a discussion and the potential usefulness of lactate in investigations to better understand the metabolism of the musculature involved in vocal demand tasks.

CONCLUSION

The role of metabolism during speech (respiratory, laryngeal, and articulatory) is an understudied yet critical aspect of speech physiology that warrants further study to better understand the metabolic systems that are used to meet vocal demands.

Stretch Receptor and Somatic Dysfunction: A Narrative Review

From its founding by Andrew Taylor Still, MD, DO, through the work of many contributors, one of the cornerstones of osteopathic medicine has been its ability to aid health by promoting neuromuscular homeostasis. As part of the understanding of osteopathic medicine since the time of Still, the proper functioning of stretch receptor organs (SROs) of skeletal muscle have been recognized as having a central role in this homeostasis. In doing so, the complexities of these numerous and vital sensors are described, including recent findings regarding their structure, function, and the nature of their neural connections. In their homeostatic role, SROs conduct information centrally for integration in proprioceptive and autonomic reflexes. By virtue of their integral role in muscle reflexes, they are putatively involved in somatic dysfunction and segmental facilitation. In reviewing some well-established knowledge regarding the SRO and introducing more recent scientific findings, an attempt is made to offer insights on how this knowledge may be applied to better understand somatic dysfunction.

Electrophysiological and Morphological Properties of α and γ Motoneurons in the Rat Trigeminal Motor Nucleus

The muscle contraction during voluntary movement is regulated by activities of α- and γ-motoneurons (αMNs and γMNs, respectively). The tension of jaw-closing muscles can be finely tuned over a wide range. This excellent function is likely to be achieved by the specific populations of αMNs innervating jaw-closing muscles. Indeed, we have recently demonstrated that in the rat dorsolateral trigeminal motor nucleus (dl-TMN), the size distribution of αMNs was bimodal and the population of smaller αMNs showed a size distribution similar to that of γMNs, by immunohistochemically identifying αMNs and γMNs based on the expressions of estrogen-related receptor gamma (Err3) and neuronal DNA binding protein NeuN together with ChAT. This finding suggests the presence of αMNs as small as γMNs. However, differences in the electrophysiological membrane properties between αMNs and γMNs remain unknown also in the dl-TMN. Therefore, in the present study, we studied the electrophysiological membrane properties of MNs in the dl-TMN of infant rats at postnatal days 7–12 together with their morphological properties using whole-cell current-clamp recordings followed by immunohistochemical staining with an anti-NeuN and anti-ChAT antibodies. We found that the ChAT-positive and NeuN-positive αMNs were divided into two subclasses: the first one had a larger cell body and displayed a 4-aminopyridine (4-AP)-sensitive current while the second one had a smaller cell body and displayed a less prominent 4-AP-sensitive current and a low-threshold spike, suitable for their orderly recruitment. We finally found that γMNs showing ChAT-positive and NeuN-negative immunoreactivities had smaller cell bodies and displayed an afterdepolarization mediated by flufenamate-sensitive cation current. It is suggested that these electrophysiological and morphological features of MNs in the dl-TMN are well correlated with the precise control of occlusion.

A novel path to chronic proprioceptive disability with oxaliplatin: Distortion of sensory encoding

Persistent neurotoxic side effects of oxaliplatin (OX) chemotherapy, including sensory ataxia, limit the efficacy of treatment and significantly diminish patient quality of life. The common explanation for neurotoxicity is neuropathy, however the degree of neuropathy varies greatly among patients and appears insufficient in some cases to fully account for disability. We recently identified an additional mechanism that might contribute to sensory ataxia following OX treatment. In the present study, we tested whether that mechanism, selective modification of sensory signaling by muscle proprioceptors might result in behavioral deficits in rats. OX was administered once per week for seven weeks (cumulative dose i.p. 70mg/kg) to adult female Wistar rats. Throughout and for three weeks following treatment, behavioral analysis was performed daily on OX and sham control rats. Compared to controls, OX rats demonstrated errors in placing their hind feet securely and/or correctly during a horizontal ladder rung task. These behavioral deficits occurred together with modification of proprioceptor signaling that eliminated sensory encoding of static muscle position while having little effect on encoding of dynamic changes in muscle length. Selective inability to sustain repetitive firing in response to static muscle stretch led us to hypothesize that OX treatment impairs specific ionic currents, possibly the persistent inward Na currents (NaPIC) that are known to support repetitive firing during static stimulation in several neuron types, including the class of large diameter dorsal root ganglion cells that includes muscle proprioceptors. We tested this hypothesis by determining whether the chronic effects of OX on the firing behavior of muscle proprioceptors in vivo were mimicked by acute injection of NaPIC antagonists. Both riluzole and phenytoin, each having multiple drug actions but having only antagonist action on NaPIC in common, reproduced selective modification of proprioceptor signaling observed in OX rats. Taken together, these findings lead us to propose that OX chemotherapy contributes to movement disability by modifying sensory encoding, possibly via a chronic neurotoxic effect on NaPIC in the sensory terminals of muscle proprioceptors.

Expression of Unique and Developmental Myosin Heavy Chain Isoforms in Adult Human Digastric Muscle

Digastric muscle (DGM) is a powerful jaw-opening muscle that participates in chewing, swallowing, breathing, and speech. For better understanding of its contractile properties, five pairs of adult human DGMs were obtained from autopsies and processed with immunocytochemistry and/or immunoblotting. Monoclonal antibodies against α-cardiac, slow tonic, neonatal, and embryonic myosin heavy chain (MHC) isoforms were employed to determine whether the DGM fibers contain these MHC isoforms, which have previously been demonstrated in restricted specialized craniocervical skeletal muscles but have not been reported in normal adult human trunk and limb muscles. The results showed expression of all these MHC isoforms in adult human DGMs. About half of the fibers reacted positively to the antibody specific for the α-cardiac MHC isoform in DGMs, and the number of these fibers decreased with age. Slow tonic MHC isoform containing fibers accounted for 19% of the total fiber population. Both the α-cardiac and slow tonic MHC isoforms were found to coexist mainly with the slow twitch MHC isoform in a fiber. A few DGM fibers expressed the embryonic or neonatal MHC isoform. The findings suggest that human DGM fibers may be specialized to facilitate performance of complex motor behaviors in the upper airway and digestive tract. (J Histochem Cytochem 52:851–859, 2004)

Fiber Content and Myosin Heavy Chain Composition of Muscle Spindles in Aged Human Biceps Brachii

The present study investigated potential age-related changes in human muscle spindles with respect to the intrafusal fiber-type content and myosin heavy chain (MyHC) composition in biceps brachii muscle. The total number of intrafusal fibers per spindle decreased significantly with aging, due to a significant reduction in the number of nuclear chain fibers. Nuclear chain fibers in old spindles were short and some showed novel expression of MyHC α-cardiac. The expression of MyHC α-cardiac in bag1and bag2fibers was greatly decreased in the A region. The expression of slow MyHC was increased in nuclear bag1fibers and that of fetal MyHC decreased in bag2fibers whereas the patterns of distribution of the remaining MyHC isoforms were generally not affected by aging. We conclude that aging appears to have an important impact on muscle spindle composition. These changes in muscle spindle phenotype may reflect an age-related deterioration in sensory and motor innervation and are likely to have an impact in motor control in the elderly.

Proprioceptive ability at the lips and jaw measured using the same psychophysical discrimination task

In the human face, the muscles and joints that generate movement have different properties. Whereas the jaw is a conventional condyle joint, the facial musculature has neither distinct origin nor insertion points, and the muscles do not contain muscle spindle proprioceptors. This current study aims to compare the proprioceptive ability at the orofacial muscles with that of the temporomandibular joint (TMJ) in 21 neuro-typical people aged between 18 and 65 years. A novel psychophysical task was devised for use with both structures that involved a fixed 30.5 mm start separation followed by closure onto stimuli of 5, 6, 7, 8 mm diameter. The mean proprioceptive score when using the lips was 0.84 compared to 0.79 at the jaw (p < 0.001), and response error was lower by 0.1 mm. The greater accuracy in discrimination of lip movement is significant because, unlike the muscles controlling the TMJ, the orbicularis oris muscle controlling the lips inserts on to connective tissue and other muscle, and contains no muscle spindles, implying a different more effective, proprioceptive mechanism. Additionally, unlike the lack of correlation previously observed between joints in the upper and lower limbs, at the face the scores from performing the task with the two different structures were significantly correlated (r = 0.5, p = 0.018). These data extend the understanding of proprioception being correlated for the same left and right joints and correlated within the same structure (e.g. ankle dorsiflexion and inversion), to include use-dependant proprioception, with performance in different structures being correlated through extended coordinated use. At the lips and jaw, it is likely that this arises from extensive coordinated use. This informs clinical assessment and suggests a potential for coordinated post-injury training of the lips and jaw, as well as having the potential to predict premorbid function via measurement of the uninjured structure, when monitoring progress and setting clinical rehabilitation goals.

Fibre typing of intrafusal fibres

The first descriptions of muscle spindles with intrafusal fibres containing striated myofibrils and nervous elements were given approximately 150 years ago. It took, however, another 100 years to establish the presence of two types of intrafusal muscle fibres: nuclear bag and nuclear chain fibres. The present paper highlights primarily the contribution of Robert Banks in fibre typing of intrafusal fibres: the confirmation of the principle of two types of nuclear bag fibres in mammalian spindles and the variation in occurrence of a dense M-band along the fibres. Furthermore, this paper summarizes how studies from the Umeå University group (Laboratory of Muscle Biology in the Department of Integrative Medical Biology) on fibre typing and the structure and composition of M-bands have contributed to the current understanding of muscle spindle complexity in adult humans as well as to muscle spindle development and effects of ageing. The variable molecular composition of the intrafusal sarcomeres with respect to myosin heavy chains and M-band proteins gives new perspectives on the role of the intrafusal myofibrils as stretch-activated sensors influencing tension/stiffness and signalling to nuclei.

Developmental myosins: expression patterns and functional significance

Developing skeletal muscles express unique myosin isoforms, including embryonic and neonatal myosin heavy chains, coded by the myosin heavy chain 3 (MYH3) and MYH8 genes, respectively, and myosin light chain 1 embryonic/atrial, encoded by the myosin light chain 4 (MYL4) gene. These myosin isoforms are transiently expressed during embryonic and fetal development and disappear shortly after birth when adult fast and slow myosins become prevalent. However, developmental myosins persist throughout adult stages in specialized muscles, such as the extraocular and jaw-closing muscles, and in the intrafusal fibers of the muscle spindles. These myosins are re-expressed during muscle regeneration and provide a specific marker of regenerating fibers in the pathologic skeletal muscle. Mutations in MYH3 or MYH8 are responsible for distal arthrogryposis syndromes, characterized by congenital joint contractures and orofacial dysmorphisms, supporting the importance of muscle contractile activity and body movements in joint development and in shaping the form of the face during fetal development. The biochemical and biophysical properties of developmental myosins have only partially been defined, and their functional significance is not yet clear. One possibility is that these myosins are specialized in contracting against low loads, and thus, they may be adapted to the prenatal environment, when fetal muscles contract against a very low load compared to postnatal muscles.

Jaw-opening accuracy is not affected by masseter muscle vibration in healthy men

There is a functional integration between the jaw and neck regions with head extension-flexion movements during jaw-opening/closing tasks. We recently reported that trigeminal nociceptive input by injection of hypertonic saline into the masseter muscle altered this integrated jaw-neck function during jaw-opening/closing tasks. Thus, in jaw-opening to a predefined position, the head-neck component increased during pain. Previous studies have indicated that muscle spindle stimulation by vibration of the masseter muscle may influence jaw movement amplitudes, but the possible effect on the integrated jaw-neck function is unknown. The aim of this study was to investigate the effect of masseter muscle vibration on jaw-head movements during a continuous jaw-opening/closing task to a target position. Sixteen healthy men performed two trials without vibration (Control) and two trials with bilateral masseter muscle vibration (Vibration). Movements of the mandible and the head were registered with a wireless three-dimensional optoelectronic recording system. Differences in jaw-opening and head movement amplitudes between Control and Vibration, as well as achievement of the predefined jaw-opening target position, were analysed with Wilcoxon's matched pairs test. No significant group effects from vibration were found for jaw or head movement amplitudes, or in the achievement of the target jaw-opening position. A covariation between the jaw and head movement amplitudes was observed. The results imply a high stability for the jaw motor system in a target jaw-opening task and that this task was achieved with the head-neck and jaw working as an integrated system.

Intrafusal myosin heavy chain expression of human masseter and biceps muscles at young age shows fundamental similarities but also marked differences

Muscle spindles are skeletal muscle mechanoreceptors that provide proprioceptive information to the central nervous system. The human adult masseter muscle has greater number, larger and more complex muscle spindles than the adult biceps. For a better knowledge of muscle diversity and physiological properties, this study examined the myosin heavy chain (MyHC) expression of muscle spindle intrafusal fibres in the human young masseter and young biceps muscles by using a panel of monoclonal antibodies (mAbs) against different MyHC isoforms. Eight MyHC isoforms were detected in both muscles-slow-tonic, I, IIa, IIx, foetal, embryonic, α-cardiac and an isoform not previously reported in intrafusal fibres, termed IIx'. Individual fibres co-expressed 2-6 isoforms. MyHC-slow tonic separated bag1, AS-bag1 and bag2 fibres from chain fibres. Typically, bag fibres also expressed MyHC-I and α-cardiac, whereas chain fibres expressed IIa and foetal. In the young masseter 98 % of bag1 showed MyHC-α cardiac versus 30 % in the young biceps, 35 % of bag2 showed MyHC-IIx' versus none in biceps, 17 % of the chain fibres showed MyHC-I versus 61 % in the biceps. In conclusion, the result showed fundamental similarities in intrafusal MyHC expression between young masseter and biceps, but also marked differences implying muscle-specific proprioceptive control, probably related to diverse evolutionary and developmental origins. Finding of similarities in MyHC expression between young and adult masseter and biceps muscle spindles, respectively, in accordance with previously reported similarities in mATPase fibre type composition suggest early maturation of muscle spindles, preceding extrafusal fibres in growth and maturation.

Jaw muscle spindle afferents coordinate multiple orofacial motoneurons via common premotor neurons in rats: an electrophysiological and anatomical study

Jaw muscle spindle afferents (JMSA) in the mesencephalic trigeminal nucleus (Vme) project to the parvocellular reticular nucleus (PCRt) and dorsomedial spinal trigeminal nucleus (dm-Vsp). A number of premotor neurons that project to the trigeminal motor nucleus (Vmo), facial nucleus (VII) and hypoglossal nucleus (XII) are also located in the PCRt and dm-Vsp. In this study, we examined whether these premotor neurons serve as common relay pool for relaying JMSA to multiple orofacial motoneurons. JMSA inputs to the PCRt and dm-Vsp neurons were verified by recording extracellular responses to electrical stimulation of the caudal Vme or masseter nerve, mechanical stimulation of jaw muscles and jaw opening. After recording, biocytin in recording electrode was inotophorized into recording sites. Biocytin-Iabeled fibers traveled to the Vmo, VII, XII, and the nucleus ambiguus (Amb). Labeled boutons were seen in close apposition with Nissl-stained motoneurons in the Vmo, VII, XII and Amb. In addition, an anterograde tracer (biotinylated dextran amine) was iontophorized into the caudal Vme, and a retrograde tracer (Cholera toxin B subunit) was delivered into either the VII or Xll to identify VII and XII premotor neurons that receive JMSA input. Contacts between labeled Vme neuronal boutons and premotor neurons were observed in the PCRt and adjacent dm-Vsp. Confocal microscopic observations confirmed close contacts between Vme boutons and VII and XII premotor neurons. This study provides evidence that JMSA may coordinate activities of multiple orofacial motor nuclei, including Vmo, VII, XII and Amb in the brainstem via a common premotor neuron pool.

Illusion caused by vibration of muscle spindles reveals an involvement of muscle spindle inputs in regulating isometric contraction of masseter muscles

Spindle Ia afferents may be differentially involved in voluntary isometric contraction, depending on the pattern of synaptic connections in spindle reflex pathways. We investigated how isometric contraction of masseter muscles is regulated through the activity of their muscle spindles that contain the largest number of intrafusal fibers among skeletal muscle spindles by examining the effects of vibration of muscle spindles on the voluntary isometric contraction. Subjects were instructed to hold the jaw at resting position by counteracting ramp loads applied on lower molar teeth. In response to the increasing-ramp load, the root mean square (RMS) of masseter EMG activity almost linearly increased under no vibration, while displaying a steep linear increase followed by a slower increase under vibration. The regression line of the relationship between the load and RMS was significantly steeper under vibration than under no vibration, suggesting that the subjects overestimated the ramp load and excessively counteracted it as reflected in the emergence of bite pressure. In response to the decreasing-ramp load applied following the increasing one, the RMS hardly decreased under vibration unlike under no vibration, leading to a generation of bite pressure even after the offset of the negative-ramp load until the vibration was ceased. Thus the subjects overestimated the increasing rate of the load while underestimating the decreasing rate of the load, due to the vibration-induced illusion of jaw opening. These observations suggest that spindle Ia/II inputs play crucial roles both in estimating the load and in controlling the isometric contraction of masseter muscles in the jaw-closed position.

Differences in fibre type composition between human masseter and biceps muscles in young and adults reveal unique masseter fibre type growth pattern

The human jaw system is different from those of other primates, carnivores, ruminants, and rodents in temporomandibular joint and muscle anatomy. In adults, jaw muscles also differ markedly from limb and trunk muscles in composition and distribution of fibre types. It can be assumed that age-related changes between young age to adulthood in terms of craniofacial growth, teeth eruption, and improvement of jaw functions are paralleled by alterations also in composition and distribution of jaw muscle fibre types. To address this question, we have examined the fibre type composition of the human masseter, a jaw closing muscle, at young age. For comparison, the young biceps brachii was examined. The results were compared with previous data for adult masseter and biceps muscles. Young masseter and biceps were similar in that type I fibres outnumbered other fibre types and were of the same diameter. However, they differed in composition of other fibre types. Young masseter contained fibre types I, IM, IIC, IIAB, IIB, and scarce IIA, with regional differences, whereas young biceps showed types I, IIA, IIAB, and few IIB. Young masseter differed from young biceps also by smaller type II fibre diameter and by containing fetal MyHC. In addition, the masseter and biceps differed in age-related changes of composition and distribution of fibre types between young age and adulthood. We conclude that the human masseter is specialized in fibre types already at young age and shows a unique fibre type growth pattern, in concordance with being a separate allotype of muscle.

Muscle spindle composition and distribution in human young masseter and biceps brachii muscles reveal early growth and maturation

Significant changes in extrafusal fiber type composition take place in the human masseter muscle from young age, 3-7 years, to adulthood, in parallel with jaw-face skeleton growth, changes of dentitions and improvement of jaw functions. As motor and sensory control systems of muscles are interlinked, also the intrafusal fiber population, that is, muscle spindles, should undergo age-related changes in fiber type appearance. To test this hypothesis, we examined muscle spindles in the young masseter muscle and compared the result with previous data on adult masseter spindles. Also muscle spindles in the young biceps brachii muscle were examined. The result showed that muscle spindle composition and distribution were alike in young and adult masseter. As for the adult masseter, young masseter contained exceptionally large muscle spindles, and with the highest spindle density and most complex spindles found in the deep masseter portion. Hence, contrary to our hypothesis, masseter spindles do not undergo major morphological changes between young age and adulthood. Also in the biceps, young spindles were alike adult spindles. Taken together, the results showed that human masseter and biceps muscle spindles are morphologically mature already at young age. We conclude that muscle spindles in the human young masseter and biceps precede the extrafusal fiber population in growth and maturation. This in turn suggests early reflex control and proprioceptive demands in learning and maturation of jaw motor skills. Similarly, well-developed muscle spindles in young biceps reflect early need of reflex control in learning and performing arm motor behavior.

Recruitment of masseter motoneurons by the presumed spindle Ia inputs

The slow-closing phase of the mastication cycle plays a major role in the mastication of foods. However, the neuronal mechanism underlying the slow-closing phase remains unknown. The isometric contraction of jaw-closing muscles is developed through the recruitment of jaw-closing motoneurons during the slow-closing phase. It is well established that motor units are recruited depending on the order of sizes or input resistances (IRs) of motoneurons, which is known as the size principle. Two-pore-domain acid-sensitive K(+) (TASK1/3) channels are recently found to be the molecular correlates of the IR, and also found to be expressed in the masseter motoneurons. Here, we addressed the question whether spindle Ia inputs onto masseter motoneurons can induce the orderly recruitment of motoneurons in slice preparations of the rat brain using voltage-sensitive dye imaging and whole-cell patch-clamp methods. Voltage-sensitive dye imaging revealed the recruitment of many motoneurons in the whole nucleus of masseter in response to repetitive stimulation of the presumed spindle Ia inputs. Dual whole-cell recordings obtained from two adjacent motoneurons revealed the IR-ordered recruitment of motoneurons in response to repetitive stimulation of the presumed spindle Ia inputs. Thus, Ia inputs are likely to play a crucial role in the orderly recruitment of motoneurons of the trigeminal motor nucleus, which would be progressed during the slow-closing phase of the mastication cycle. Possible involvements of TASK channels in the orderly recruitment are discussed.

Reflex control of human jaw muscles

The aim of this review is to discuss what is known about the reflex control of the human masticatory system and to propose a method for standardized investigation. Literature regarding the current knowledge of activation of jaw muscles, receptors involved in the feedback control, and reflex pathways is discussed. The reflexes are discussed under the headings of the stimulation conditions. This was deliberately done to remind the reader that under each stimulation condition, several receptor systems are activated, and that it is not yet possible to stimulate only one afferent system in isolation in human mastication experiments. To achieve a method for uniform investigation, we need to set a method for stimulation of the afferent pathway under study with minimal simultaneous activation of other receptor systems. This stimulation should also be done in an efficient and reproducible way. To substantiate our conviction to standardize the stimulus type and parameters, we discuss the advantages and disadvantages of mechanical and electrical stimuli. For mechanical stimulus to be delivered in a reproducible way, the following precautions are suggested: The stimulus delivery system (often a probe attached to a vibrator) should be brought into secure contact with the area of stimulation. To minimize the slack between the probe, the area to be stimulated should be taken up by the application of pre-load, and the delivered force should be recorded in series. Electrical stimulus has advantages in that it can be delivered in a reproducible way, though its physiological relevance can be questioned. It is also necessary to standardize the method for recording and analyzing the responses of the motoneurons to the stimulation. For that, a new technique is introduced, and its advantages over the currently used methods are discussed. The new method can illustrate the synaptic potential that is induced in the motoneurons without the errors that are unavoidable in the current techniques. We believe that once stimulation, recording, and analysis methods are standardized, it will be possible to bring out the real "wiring diagram" that operates in conscious human subjects.

Adult human upper esophageal sphincter contains specialized muscle fibers expressing unusual myosin heavy chain isoforms

The functional upper esophageal sphincter (UES) is composed of the cricopharyngeus muscle (CP), the most inferior part of the inferior pharyngeal constrictor (iIPC), and the upper esophagus (UE). This sphincter is collapsed and exhibits sustained muscle activity in the resting state; it only relaxes and opens during swallowing, vomiting, and belching. The tonic contractile properties of the UES suggest that the skeletal muscle fibers in this sphincter differ from those in the limb and trunk muscles. In this study, myosin heavy chain (MHC) composition in the adult human UES muscles obtained from autopsies was investigated using immunocytochemical and immunoblotting techniques. Results showed that the adult human UES muscle fibers expressed unusual MHC isoforms such as slow-tonic (MHC-ton), alpha-cardiac (MHC-alpha), neonatal (MHC-neo), and embryonic (MHC-emb), which coexisted with the major MHCs (i.e., MHCI, IIa, and IIx). MHC-ton and MHC-alpha were coexpressed predominantly with slow-type I MHC isoform, whereas MHC-neo and MHC-emb coexisted mainly with fast-type IIa MHC. A slow inner layer (SIL) and a fast outer layer (FOL) in the iIPC and CP were identified immunocytochemically. MHC-ton- and MHC-alpha-containing fibers were concentrated mainly in the SIL, whereas MHC-neo- and MHC-emb-containing fibers were distributed primarily to the FOL. Identification of the specialized muscle fibers and their distribution patterns in the adult human UES is valuable for a better understanding of the physiological and pathophysiological behaviors of the sphincter.

Intrinsic properties of the adult human mylohyoid muscle: neural organization, fiber-type distribution, and myosin heavy chain expression

The mylohyoid (MH) muscle plays a critical role in chewing, swallowing, respiration, and phonation. The present study was designed to test the hypothesis that the functional properties of the MH are reflected by its intrinsic specializations, including the neural organization, fiber-type distribution, and myosin heavy chain (MHC) expression. Adult human MH muscles were investigated to determine the nerve supply pattern using Sihler's stain, banding pattern and types of motor endplates using acetylcholinesterase (AChE) staining and silver impregnation, and muscle fiber type and MHC composition using immunocytochemical and immunoblotting techniques. The adult human MH was found to have the following neuromuscular specializations. First, the muscle was innervated by several branches of the MH nerve derived from the mandibular division of the trigeminal nerve. Each of the nerve branches supplied a distinct region of the muscle, forming a segmental innervation pattern. Second, the MH had a single motor endplate band which was located in the middle of the muscle length. Both en plaque and en grappe types of motor endplates were identified on the MH muscle fibers. Finally, the adult human MH fibers expressed unusual MHC isoforms (i.e., slow-tonic, alpha-cardiac, embryonic, and neonatal) which coexisted with the major MHC isoforms (i.e., slow type I, fast type IIa, and fast type IIx), thus forming various major/unusual (or m/u) MHC hybrid fiber types. The m/u hybrid fibers (84% of the total fiber population) were the predominant fiber types in the adult MH muscle. Determination of the neuromuscular specializations of the MH is helpful for better understanding of the muscle functions and for development of strategies to treat MH-related upper airway disorders.

Adult human mylohyoid muscle fibers express slow-tonic, alpha-cardiac, and developmental myosin heavy-chain isoforms

Some adult cranial muscles have been reported to contain unusual myosin heavy-chain (MHC) isoforms (i.e., slow-tonic, alpha-cardiac, embryonic, and neonatal), which exhibit distinct contractile properties. In this study, adult human mylohyoid (MH) muscles obtained from autopsies were investigated to detect the unusual MHC isoforms. For comparison, the biceps brachii and masseter muscles of the same subjects were also examined. Serial cross-sections from the muscles studied were incubated with a panel of isoform-specific anti-MHC monoclonal antibodies that distinguish major and unusual MHC isoforms. On average, the slow type I and fast type II MHC-containing fibers in the MH muscle accounted for 54% and 46% of the fibers, respectively. In contrast to limb and trunk muscles, the adult human MH muscle was characterized by a large proportion of hybrid fibers (85%) and a small percentage of pure fibers (15%; P < 0.01). Of the fast fiber types, the proportion of the type IIa MHC-containing fibers (92%) was much greater than that of the type IIx MHC-containing fibers (8%; P < 0.01). Our data demonstrated that the adult human MH fibers expressed the unusual MHC isoforms that were also identified in the masseter, but not in the biceps brachii. These isoforms were demonstrated by immunocytochemistry and confirmed by electrophoretic immunoblotting. Fiber-to-fiber comparisons showed that the unusual MHC isoforms were coexpressed with the major MHC isoforms (i.e., MHCI, IIa, and IIx), thus forming various major/unusual (or m/u) MHC hybrid fiber types. Interestingly, the unusual MHC isoforms were expressed in a fiber type-specific manner. The slow-tonic and alpha-cardiac MHC isoforms were coexpressed predominantly with slow type I MHC isoform, whereas the developmental MHC isoforms (i.e., embryonic and neonatal) coexisted primarily with fast type IIa MHC isoform. There were no MH fibers that expressed exclusively unusual MHC isoforms. Approximately 81% of the slow type I MHC-containing fibers expressed slow-tonic and alpha-cardiac MHC isoforms, whereas 80% of the fast type IIa MHC-containing fibers expressed neonatal MHC isoform. The m/u hybrid fibers (82% of the total fiber population) were found to constitute the predominant fiber types in the adult human MH muscle. At least seven m/u MHC hybrid fiber types were identified in the adult human MH muscle. The most common m/u hybrid fiber types were found to be the MHCI/slow-tonic/alpha-cardiac and MHCIIa/neonatal, which accounted for 39% and 33% of the total fiber population, respectively. The multiplicity of MHC isoforms in the adult MH fibers is believed to be related to embryonic origin, innervation pattern, and unique functional requirements.

Deranged jaw-neck motor control in whiplash-associated disorders

Recent findings of simultaneous and well coordinated head-neck movements during single as well as rhythmic jaw opening-closing tasks has led to the conclusion that 'functional jaw movements' are the result of activation of jaw as well as neck muscles, leading to simultaneous movements in the temporomandibular, atlanto-occipital and cervical spine joints. It can therefore be assumed that disease or injury to any of these joint systems would disturb natural jaw function. To test this hypothesis, amplitudes, temporal coordination, and spatiotemporal consistency of concomitant mandibular and head-neck movements during single maximal jaw opening-closing tasks were analysed in 25 individuals suffering from whiplash-associated disorders (WAD) using optoelectronic movement recording technique. In addition, the relative durations for which the head position was equal to, leading ahead of, or lagging behind the mandibular position during the entire jaw opening-closing cycle were determined. Compared with healthy individuals, the WAD group showed smaller amplitudes, and changed temporal coordination between mandibular and head-neck movements. No divergence from healthy individuals was found for the spatiotemporal consistency or for the analysis during the entire jaw opening-closing cycle. These findings in the WAD group of a 'faulty', but yet consistent, jaw-neck behavior may reflect a basic importance of linked control of the jaw and neck sensory-motor systems. In conclusion, the present results suggest that neck injury is associated with deranged control of mandibular and head-neck movements during jaw opening-closing tasks, and therefore might compromise natural jaw function.

Staining of human thyroarytenoid muscle with myosin antibodies reveals some unique extrafusal fibers, but no muscle spindles

This study describes the myosin composition of extrafusal and intrafusal muscle fibers found in the human thyroarytenoid (TA) and sternohyoid (control) muscles. We sought to determine the presence of muscle spindles in the TA muscle, and to identify unusual extrafusal fiber types, using the commonly accepted approach of tissue staining with myosin isoform specific antibodies. Extrafusal fibers are organized into motor units, which subsequently produce muscle movement, whereas intrafusal fibers compose muscle spindles, the primary stretch receptor that provides afferent (feed back) information to the nervous system for regulation of motor unit length and tonicity. Immunohistochemical identification of muscle spindles was confirmed in sternohyoid, but not in TA samples; however, some extrafusal fibers contained tonic myosin. These results indicate that human TA muscle functions similar to some mammalian extraocular muscle, performing unloaded (non-weight bearing) contractions without afferent information from native muscle spindles.

Muscle spindles in the deep muscles of the human neck: a morphological and immunocytochemical study

Muscle spindle density is extremely high in the deep muscles of the human neck. However, there is a paucity of information regarding the morphology and immunoreactivity of these muscle spindles. The objective of this study was to investigate the intrafusal fiber content and to assess the myosin heavy chain (MyHC) composition of muscle spindles from human deep neck muscles. In addition to the conventional spindles containing bag(1), bag(2), and chain fibers (b(1)b(2)c spindle), we observed a number of spindles lacking bag(1) (b(2)c spindle) or bag(2) (b(1)c spindle) fibers. Both bag(1) and bag(2) fibers contained slow tonic MyHCs along their entire fiber length and MyHCI, MyHCIIa, embryonic, and alpha-cardiac MyHC isoforms along a variable length of the fibers. Fetal MyHC was present in bag(2) fibers but not in bag(1) fibers. Nuclear chain fibers contained MyHCIIa, embryonic, and fetal isoforms with regional variations. We also compared the present data with our previous results obtained from muscle spindles in human biceps brachii and the first lumbrical muscles. The allotment of numbers of intrafusal fibers and the MyHC composition showed some muscle-related differences, suggesting functional specialization in the control of movement among different human muscles.

Abnormal palatopharyngeal muscle morphology in sleep-disordered breathing

The aim of the present study was to investigate whether histopathological changes can be detected in two soft palate muscles, the palatopharyngeus and the uvula, in 11 patients with long duration of sleep-disordered breathing (SDB). Muscle samples were collected from patients undergoing uvulo-palatopharyngoplasty (UPPP). Reference samples from the corresponding areas were obtained at autopsy from five previously healthy subjects. Muscle morphology, fibre type and myosin heavy chain (MyHC) compositions were analysed with enzyme-histochemical, immunohistochemical and biochemical techniques. The muscle samples from the patients, and especially those from the palatopharyngeus, showed several morphological abnormalities. The most striking findings were (i) increased amount of connective tissue, (ii) abnormal variability in fibre size, (iii) increased proportion of small-sized fibres, (iv) alterations in fibre type and MyHC compositions, (v) increased frequency of fibres containing developmental MyHC isoforms. Our findings point towards a pathological process of denervation and degeneration in the patient samples. Conclusively, the morphological abnormalities suggest a neuromuscular disorder of the soft palate in SDB patients.

Myosin heavy chain composition of muscle spindles in human biceps brachii

Data on the myosin heavy chain (MyHC) composition of human muscle spindles are scarce in spite of the well-known correlation between MyHC composition and functional properties of skeletal muscle fibers. The MyHC composition of intrafusal fibers from 36 spindles of human biceps brachii muscle was studied in detail by immunocytochemistry with a large battery of antibodies. The MyHC content of isolated muscle spindles was assessed with SDS-PAGE and immunoblots. Four major MyHC isoforms (MyHCI, IIa, embryonic, and intrafusal) were detected with SDS-PAGE. Immunocytochemistry revealed very complex staining patterns for each intrafusal fiber type. The bag(1) fibers contained slow tonic MyHC along their entire fiber length and MyHCI, alpha-cardiac, embryonic, and fetal isoforms along a variable part of their length. The bag(2) fibers contained MyHC slow tonic, I, alpha-cardiac, embryonic, and fetal isoforms with regional variations. Chain fibers contained MyHCIIa, embryonic, and fetal isoforms throughout the fiber, and MyHCIIx at least in the juxtaequatorial region. Virtually each muscle spindle had a different allotment of numbers of bag(1), bag(2) and chain fibers. Taken together, the complexity in intrafusal fiber content and MyHC composition observed indicate that each muscle spindle in the human biceps has a unique identity.

Morphological characterization of the levator veli palatini muscle in children born with cleft palates

OBJECTIVE

The aim of this study was to analyze, morphologically and biochemically, one of the soft palate muscles, the levator veli palatini (LVP), in children born with cleft palate.

SUBJECTS AND METHODS

Biopsies were obtained from nine male and three female infants in connection with the early surgical repair of the hard and soft palate. Samples from five adult normal LVP muscles were used for comparison. The muscle morphology, fiber type and myosin heavy chain (MyHC) compositions, capillary supply, and content of muscle spindles were analyzed with different enzyme-histochemical, immunohistochemical, and biochemical techniques.

RESULTS

Compared with the normal adult subjects, the LVP muscle from the infantile subjects with cleft had a smaller mean fiber diameter, a larger variability in fiber size and form, a higher proportion of type II fibers, a higher amount of fast MyHCs, and a lower density of capillaries. No muscle spindles were observed. Moreover, one-third of the biopsies from the infantile subjects with cleft LVP either lacked muscle tissue or contained only a small amount.

CONCLUSIONS

The LVP muscle from children with cleft palate has a different morphology, compared with the normal adult muscle. The differences might be related to different stages in maturation of the muscles, changes in functional demands with growth and age, or a consequence of the cleft. The lack of contractile tissue in some of the cleft biopsies offers one possible explanation to a persistent postsurgical velopharyngeal insufficiency in some patients, despite a successful surgical repair.

Human skeletal muscle fibres: molecular and functional diversity

Contractile and energetic properties of human skeletal muscle have been studied for many years in vivo in the body. It has been, however, difficult to identify the specific role of muscle fibres in modulating muscle performance. Recently it has become possible to dissect short segments of single human muscle fibres from biopsy samples and make them work in nearly physiologic conditions in vitro. At the same time, the development of molecular biology has provided a wealth of information on muscle proteins and their genes and new techniques have allowed analysis of the protein isoform composition of the same fibre segments used for functional studies. In this way the histological identification of three main human muscle fibre types (I, IIA and IIX, previously called IIB) has been followed by a precise description of molecular composition and functional and biochemical properties. It has become apparent that the expression of different protein isoforms and therefore the existence of distinct muscle fibre phenotypes is one of the main determinants of the muscle performance in vivo. The present review will first describe the mechanisms through which molecular diversity is generated and how fibre types can be identified on the basis of structural and functional characteristics. Then the molecular and functional diversity will be examined with regard to (1) the myofibrillar apparatus; (2) the sarcolemma and the sarcoplasmic reticulum; and (3) the metabolic systems devoted to producing ATP. The last section of the review will discuss the advantage that fibre diversity can offer in optimizing muscle contractile performance.

Intermuscular and intramuscular differences in myosin heavy chain composition of the human masticatory muscles

Among and within the human masticatory muscles a large number of anatomical differences exists indicating that different muscles and muscle portions are specialized for certain functions. In the present study we investigated whether such a specialization is also reflected by intermuscular and intramuscular differences in fibre type composition and fibre cross-sectional area. Fibre type compositions and fibre cross-sectional areas of masticatory muscles were determined in eight cadavers using monoclonal antibodies against myosin heavy chain (MyHC). The temporalis, masseter and pterygoid muscles could be characterized by a relatively large number of fibres containing more than one MyHC isoform (hybrid fibres). In these muscles a large number of fibres expressed MyHC-I, MyHC-fetal and MyHC-cardiac alpha. Furthermore, in these muscles type I fibres had larger cross-sectional areas than type II fibres. In contrast, the mylohyoid, geniohyoid and digastric muscle were characterized by less hybrid fibres, and by less fibres expressing MyHC-I, MyHC-fetal, and MyHC-cardiac alpha, and by more fibres expressing MyHC-IIA; the cross-sectional areas of type I and type II fibres in these muscles did not differ significantly. Compared to the masseter and pterygoid muscles, the temporalis had significantly larger fibres and a notably different fibre type composition. The mylohyoid, geniohyoid, and digastric muscles did not differ significantly in their MyHC composition and fibre cross-sectional areas. Also intramuscular differences in fibre type composition were present, i.e., a regionally higher proportion of MyHC type I fibres was found in the anterior temporalis, the deep masseter, and the anterior medial pterygoid muscle portions; furthermore, significant differences were found between the bellies of the digastric.

Characterisation of human soft palate muscles with respect to fibre types, myosins and capillary supply

Four human soft palate muscles, and palatopharyngeus, the uvula, the levator and tensor veli palatini were examined using enzyme-histochemical, immunohistochemical and biochemical methods and compared with human limb and facial muscles. Our results showed that each palate muscle had a distinct morphological identity and that they generally shared more similarities with facial than limb muscles. The palatopharyngeus and uvula muscles contained 2 of the highest proportions of type II fibres ever reported for human muscles. In contrast, the levator and tensor veli palatini muscles contained predominantly type I fibres. A fetal myosin heavy chain isoform (MyHC), not usually found in normal adult limb muscles, was present in a small number of fibres in all palate muscles. The mean muscle fibre diameter was smaller than in limb muscles and the individual and intramuscular variability in diameter and shape was considerable. All palate muscles had a high capillary density and an unusually high mitochondrial enzyme activity in the type II fibres, in comparison with limb muscles. No ordinary muscle spindles were observed. The fibre type and MyHC composition indicate that the palatopharyngeus and uvula muscles are functionally involved in quick movements whereas the levator and tensor veli palatini muscles perform slower and more continuous contractions. The high aerobic capacity and the rich capillarisation suggest that the palate muscles are relatively fatigue resistant. Absence of ordinary muscle spindles indicates a special proprioceptive control system. The special morphology of the palate muscles may be partly related to the unique anatomy with only one skeletal insertion, a feature consistent with muscle work at low load and tension and which may influence the cytoarchitecture of these muscles. Other important factors determining the special morphological characteristics might be specific functional requirements, distinct embryological origin and phylogenetic factors.

Co-ordinated mandibular and head-neck movements during rhythmic jaw activities in man

Recent observations in man of concomitant mandibular and head movements during single maximal jaw-opening/-closing tasks suggest a close functional relationship between the mandibular and the head-neck motor systems. This study was aimed at further testing of the hypothesis of a functional integration between the human jaw and neck regions. Spatiotemporal characteristics of mandibular and associated head movements were evaluated for 3 different modes of rhythmic jaw activities: self-paced continuous maximal jaw-opening/-closing movements, paced continuous maximal jaw-opening/-closing movements at 50 cycles/minute, and unilateral chewing. Mandibular and head-neck movements were simultaneously recorded in 12 healthy young adults, by means of a wireless opto-electronic system for 3-D movement recordings, with retro-reflective markers attached to the lower (mandible) and upper (head) incisors. The results showed that rhythmic mandibular movements were paralleled by head movements. An initial change in head position (head extension) was seen at the start of the first jaw-movement cycle, and this adjusted head position was retained during the following cycles. In addition to this prevailing head extension, the maximal jaw-opening/-closing cycles were paralleled by head extension-flexion movements, and in general the start of these head movements preceded the start of the mandibular movements. The results support the idea of a functional relationship between the temporomandibular and the cranio-cervical neuromuscular systems. We therefore suggest a new concept for human jaw function, in which "functional jaw movements" are the result of activation of jaw as well as neck muscles, leading to simultaneous movements in the temporomandibular, atlanto-occipital, and cervical spine joints.

Myosin heavy chain composition of the human lateral pterygoid and digastric muscles in young adults and elderly

The myosin heavy chain (MyHC) content in different parts of, two jaw opening muscle, the human lateral pterygoid and the digastric muscles of five young adult and five elderly subjects (mean age 22 and 73 years, respectively) was determined, using gel electrophoresis and immunohistochemical methods. The lateral pterygoid of both young and elderly contained predominantly slow MyHC, and fast A MyHC was the major fast isoform. In contrast, the digastric was composed of slow, fast A and fast X MyHCs in about equal proportions in both age groups. About half of the lateral pterygoid fibres contained mixtures of slow and fast MyHCs, often together with alpha-cardiac MyHC. In the digastric, co-existence of slow and fast MyHCs was rare, and alpha-cardiac MyHC was lacking. On the other hand, co-expression of fast A and fast X MyHCs was found more often in the digastric than in the lateral pterygoid. In both age groups about half of the digastric IIB fibres contained solely fast X MyHC. In the lateral pterygoid, type IIB fibres with pure fast X MyHC was found in only one subject. The lateral pterygoid in elderly showed a significant amount of fibres with solely fast A MyHC, which were occasionally found in young adults. In the digastric, no significant differences were found between young and elderly, although the muscles of elderly contained lower mean value of slow MyHC, as compared to that of young muscles. It is concluded that the lateral pterygoid and the digastric muscles differ not only in the MyHC composition but also in modifications of the MyHC phenotypes during aging, suggesting that they have separate roles in jaw opening function.

Oro-facial muscles: internal structure, function and ageing

Structure and function are reviewed in the masticatory muscles and in the muscles of the lower face and tongue. The enormous strength of jaw closure is in large part due to the pinnated arrangement of the muscle fibres in the masseter. This muscle, like other masticatory muscles, is unusual in that the cell bodies of the muscle spindle afferents lie in the brain stem rather than in an external ganglion; spindles are absent in the lower facial muscles. Although few data are available, the numbers of motor units in the masticatory muscles, and probably in the lower facial muscles also, appear to be much greater than in limb muscles. The motor units in the facial and tongue muscles are largely composed of histochemical type II ('fast-twitch') fibres, but in the masticatory muscles there are substantial numbers of fibres intermediate between type I ('slow twitch') and type II, and fibre type grouping is present. In comparison with limb muscles, there is little information on ageing changes in oro-facial muscles. The masticatory muscles do, however, show some atrophy and loss of X-ray density, while motor unit twitches are prolonged. Strength is reduced in the tongue and masticatory muscles. It is known that limb muscle properties are largely governed by their innervation, both through the pattern and amount of impulse activity, and the delivery of trophic messengers; the situation for oro-facial muscles is unclear. The structural and functional differences between the two types of muscle indicate the need for conducting ageing studies on the oro-facial muscles, rather than relying on extrapolations from limb muscles.

Slow tonic muscle fibers in the thyroarytenoid muscles of human vocal folds; a possible specialization for speech

Most of the sounds of human speech are produced by vibration of the vocal folds, yet the biomechanics and control of these vibrations are poorly understood. In this study the muscle within the vocal fold, the thyroarytenoid muscle (TA), was examined for the presence and distribution of slow tonic muscle fibers (STF), a rare muscle fiber type with unique contraction properties. Nine human TAs were frozen and serially sectioned in the frontal plane. The presence and distribution pattern of STF in each TA were examined by immunofluorescence microscopy using the monoclonal antibodies (mAb) ALD-19 and ALD-58 which react with the slow tonic myosin heavy chain (MyHC) isoform. In addition, TA muscle samples from adjacent frozen sections were also examined for slow tonic MyHC isoform by electrophoretic immunoblotting. STF were detected in all nine TAs and the presence of slow tonic MyHC isoform was confirmed in the immunoblots. The STF were distributed predominantly in the medial aspect of the TA, a distinct muscle compartment called the vocalis which is the vibrating part of the vocal fold. STF do not contract with a twitch like most muscle fibers, instead, their contractions are prolonged, stable, precisely controlled, and fatigue resistant. The human voice is characterized by a stable sound with a wide frequency spectrum that can be precisely modulated and the STF may contribute to this ability. At present, the evidence suggests that STF are not presented in the vocal folds of other mammals (including other primates), therefore STF may be a unique human specialization for speech.

Distribution of cholinergic neurons in cell group K of the rabbit brainstem

The cell bodies of efferent neurons supplying the masseter and digastric muscles of the rabbit are located in two brainstem nuclei: the trigeminal motor nucleus and cell group k. The latter also contains neurons innervating muscles of the middle ear and Eustachian tube, as well as neurons that project to the cerebellum and the oculomotor complex. As part of an attempt to identify the functional subpopulations within the three cell divisions (kl-k3) that make up cell group k, we have investigated the distribution of neurons containing choline acetyltransferase, because these are likely to be motoneurons. Five rabbits anaesthetized with sodium pentobarbital (90 mg/kg, i.v.) were used in this study. They were perfused with 4% paraformaldehyde and 0.1% glutaraldehyde in phosphate buffer (0.1 M, pH 7.4). Two animals were used for preliminary studies. In the other three cases, serial Vibratome coronal sections of the brainstem were cut at 50 microm and two series of alternating sections were collected. The first was stained with a monoclonal antibody (code AB8, Incstar) directed against choline acetyltransferase, using the avidin-biotin-peroxidase method. The other was stained with Cresyl Violet. Cell counts and three-dimensional reconstructions were made for both series to determine positions and ratios of cholinergic and non-cholinergic neurons within the trigeminal motor nucleus and the subdivisions of cell group k. The results showed that the numbers of choline acetyltransferase- and Nissl-stained neurons within the trigeminal motor nucleus were almost identical. In cell group k, significantly fewer choline acetyltransferase-stained cells were counted in all three animals (ratios of choline acetyltransferase/Nissl=0.53-0.71). In addition, the distribution of cholinergic neurons was not uniform throughout cell group k. Subdivisions kl and k3 contained proportionately fewer choline acetyltransferase-positive cells (ratios of choline acetyltransferase/Nissl=0.23-0.64) than did k2 (ratios choline acetyltransferase/ Nissl=0.75-0.88). Within each subdivision, there were significant differences in the spatial coordinates of Nissl- and choline acetyltransferase-positive neurons. We conclude that cell group k contains at least two populations of neurons which are unevenly distributed between and within the three subdivisions. While the majority of neurons in subgroup k2 contain choline acetyltransferase and thus are likely to be motoneurons, more than half of the neurons in subgroups k1 and k3 are not cholinergic. It remains to be determined whether these are the neurons that project to the cerebellum and to other CNS regions.

Opposite changes in myosin heavy chain composition of human masseter and biceps brachii muscles during aging

The myosin heavy chain (MyHC) content in functionally different parts of the human masseter muscle of six elderly and five young adult subjects (mean age 74 and 22 years, respectively) was determined, using gel electrophoresis. The MyHC composition of the old masseter was also studied by enzyme- and immunohistochemical methods and compared with previous data for young adults. For comparison, the biceps brachii muscle of the same subjects was also analysed. The old masseter contained smaller amounts of slow and larger amounts of fast and fetal MyHCs. These differences were region-dependent and were more pronounced in the superficial portion. There was also a larger proportion of "hybrid" fibres, containing two to four MyHC isoforms (42%), compared with the young adult masseter (23%). No such differences were observed between old and young biceps. In contrast to the masseter, the old biceps contained more slow MyHC and less fast MyHC. This investigation demonstrates that the aging process in human skeletal muscle is accompanied by a modification in the muscle phenotype which is both muscle and region specific; a transformation towards a fast and fetal phenotype concomitant with an increased number of fibres with a mixture of different MyHC isoforms in the masseter; and an opposite shift towards a slower phenotype in the biceps brachii. The results might reflect differences between jaw and limb muscles in genetic programs and adaptive responses to changed functional demands following aging.

Concomitant mandibular and head-neck movements during jaw opening-closing in man

To test the hypothesis of a functional relationship between the human mandibular and cranio-cervical motor systems, head-neck movements during voluntary mandibular movements were studied in 10 healthy young adults, using a wireless optoelectronic system for three-dimensional (3D) movement recording. The subjects, unaware of the underlying aim of the study, were instructed to perform maximal jaw opening-closing tasks at fast and slow speed. Movements were quantified as 3D movement amplitudes. A consistent finding in all subjects was parallel and coordinated head-neck movements during both fast and slow jaw opening-closing tasks. Jaw opening was always accompanied by head-neck extension and jaw closing by head-neck flexion. Combined movement and electromyographic recordings showed concomitant neck muscle activity during head-neck movements, indicative of an active repositioning of the head. No differences in 3D movement amplitudes could be seen with respect to speed. The head movement was 50% of the mandibular movement during jaw opening, but significantly smaller (30-40%), during the jaw closing phase. In repeated tests, the 3D movement amplitudes of the concomitant head movements were less variable during slow jaw movement and during the jaw opening phase, than during fast and jaw closing movements, suggesting speed- and phase-related differences in the mechanisms controlling the integrated mandibular and head-neck motor acts. The present results give further support to the concept of a functional trigeminocervical coupling during jaw activities in man.

Skeletal fiber types and spindle distribution in limb and jaw muscles of the adult and neonatal opossum, Monodelphis domestica

The South American opossum, Monodelphis domestica, is very immature at birth, and we wished to assess its potential for studies of jaw muscle development. Given the lack of prior information about any Monodelphis fiber types or spindles, our study aimed to identify for the first time fiber types in both adult and neonatal muscles and the location of spindles in the jaw muscles. Fiber types were identified in frozen sections of adult and 6-day-old jaw and limb muscles by using myosin ATPase and metabolic enzyme histochemistry and by immunostaining for myosin isoforms. The distribution of fiber types and muscle spindles throughout the jaw-closer muscles was identified by immunostaining of sections of methacarnoy-fixed, wax-embedded heads. Most muscles contained one slow (type I) and two fast fiber types (equivalent to types IIA and IIX), which were similar to those in eutherian muscle, and an additional (non-IIB) fast type. In jaw-closer muscles, the main extrafusal fiber type was IIM (characteristic of these muscles in some eutherians), and almost all spindles were concentrated in four restricted areas: one in masseter and three in temporalis. Six-day neonatal muscles were very immature, but future spindle-rich areas were revealed by immunostaining and corresponded in position to the adult areas. Extrafusal and spindle fiber types in Monodelphis share many similarities with eutherian mammalian muscle. This finding, along with the immaturity of myosin isoform expression observed 6 days postnatally, indicates that Monodelphis could provide a valuable model for studying early developmental events in the jaw-closer muscles and their spindles.

Adverse changes in fibre type composition of the human masseter versus biceps brachii muscle during aging

The fibre composition of functionally different regions, the superficial and the deep portions, of the human masseter and the biceps brachii muscles of six elderly subjects (mean age 74 years) was studied by morphological and enzyme-histochemical methods. When compared to previous data for young adults, the masseter muscle from elderly subjects showed a significant decrease in the proportion of type I fibres and an increase of type IM and II fibre types, concomitant with muscle fibre atrophy. In the old biceps, there were no changes in the type I fibre proportion and fibre diameter, but a significant decrease in the type IIB fibre content. The present results of changes in fibre composition, in addition to previous findings of functional deterioration of the old masseter, probably reflect a combination of age related degenerative and adaptive processes within the trigeminal neuromuscular system. When compared with previous findings in young adult masseter, it can be concluded that the human masseter muscle differs from limb muscles both in fibre composition and in structural response of the muscle during aging. These differences between the masseter and the biceps brachii muscle suggest muscle specific alterations during aging probably due to differences in genetical control, functional performance and nerve and hormonal influences.

Fetal myosin heavy chain increases in human masseter muscle during aging

Biochemical, immunohistochemical and molecular biological methods were used to detect fetal myosin heavy chain (MyHC) in the human masseter of elderly and young subjects. Samples from the elderly subjects contained larger amounts of fetal MyHC than those of young adults. Only a very small amount of embryonic MyHC could be detected in both age groups. Embryonic and fetal MyHCs were never detected in the control adult orofacial, limb and trunk muscles. Polymerase chain reaction (PCR) analysis revealed the presence of fetal mRNA sequences in elderly and young masseter muscles. We conclude that fetal MyHC is present in the human masseter throughout the life span and that there is an increase in the relative amount of this protein with age.

Expression of myosin heavy chain isoforms and myogenesis of intrafusal fibres in rat muscle spindles

This review concerns the pattern of expression and regulation of myosin heavy chain (MHC) isoforms in intrafusal fibres of rat muscle spindles detected by immunocytochemistry. The three types of intrafusal fibres--nuclear bag1, nuclear bag2, and nuclear chain fibres--are unique in co-expressing several MHCs including special isoforms such as slow tonic and alpha cardiac-like MHC and isoforms typical of muscle development, such as embryonic and neonatal MHC. The distinct intrafusal fibre types appear sequentially during rat hind limb development, the nuclear bag2 precursors being first identifiable at 17-18 days in utero as the only primary myotubes expressing slow tonic MHC. Sensory innervation is required for the expression of "spindle-specific" MHC isoforms. Motor innervation contributes to the diversity in distribution of the different MHCs along the length of the nuclear bag fibres. It is suggested that unique populations of myoblasts are destined to become intrafusal fibres during development in the rat hind limb muscles and that the regional heterogeneity in MHC expression is related both to sensory and motor innervation and to the properties of the myoblast lineages. These distinct features make intrafusal fibres an attractive in situ model for investigating myogenesis, myofibrillogenesis, and the mechanisms regulating MHC expression.

Differences in myosin composition between human oro-facial, masticatory and limb muscles: enzyme-, immunohisto- and biochemical studies

Immunohistochemistry was used to determine the myosin composition of defined fibre types of three embryologically different adult muscles, the oro-facial, masseter and limb muscles. In addition, the myosin composition in whole muscle specimens was analysed with biochemical methods. Both similarities and differences between muscles in the content of myosin heavy chains and myosin light chains were found. Nevertheless, each muscle had its own distinct identity. Our results indicated the presence of a previously undetected fast myosin heavy chain isoform in the oro-facial type II fibre population, tentatively termed 'fast F'. The masseter contained aberrant myosin isoforms, such as foetal myosin heavy chain and alpha-cardiac myosin heavy chain and unique combinations of myosin heavy chain isoforms which were not found in the limb or oro-facial muscles. The type IM and IIC fibres coexpressed slow and fast A myosin heavy chains in the oro-facial and limb muscles but slow and a fast B like myosin heavy chain in the masseter. While single oro-facial and limb muscle fibres contained one or two myosin heavy chain types, single masseter fibres coexpressed up to four different myosin heavy chain isoforms. Describing the fibres according to their expression of myosin heavy chain isozymes, up to five fibre types could be distinguished in the oro-facial and limb muscles and eight in the masseter. Oro-facial and limb muscles expressed five myosin light chains, MLC1S, MLC2S, MLC1F, MLC2F and MLC3F, and the masseter four, MLC1S, MLC2S, MLC1F, and, in addition, an embryonic myosin light chain, MLC1emb, which is usually not present in normal adult skeletal muscle. These results probably reflect the way the muscles have evolved to meet the specialized functional requirements imposed upon them and are in agreement with the previously proposed concept that jaw and limb muscles belong to two distinct allotypes.