Motor unit territories and fiber types in rabbit masseter muscle

Do skeletal muscle motor units and microvascular units align to help match blood flow to metabolic demand?

PURPOSE

We explore the motor unit recruitment and control of perfusion of microvascular units in skeletal muscle to determine whether they coordinate to match blood flow to metabolic demand.

METHODS

The PubMed database was searched for historical, current and relevant literature.

RESULTS

A microvascular, or capillary unit consists of 2-20 individual capillaries. Individual capillaries within a capillary unit cannot increase perfusion independently of other capillaries within the unit. Capillary units perfuse a short segment of approx. 12 muscle fibres located beside each other. Motor units consist of muscle fibres that can be dispersed widely within the muscle volume. During a contraction, where not all motor units are recruited, muscle fibre contraction will result in increased perfusion of associated capillaries as well as all capillaries within that capillary unit. Perfusion of the entire capillary unit will result in an increased blood flow delivery to muscle fibres associated with active motor unit plus approximately 11 other inactive muscle fibres within the same region. This will result in an overperfusion of the muscle resulting in blood flow in excess of the muscle fibre needs.

CONCLUSIONS

Given the architecture of the capillary units and the dispersed nature of muscle fibres within a motor unit, during submaximal contractions, where not all motor units are recruited, there will be a greater perfusion to the muscle than that predicted by the number of active muscle fibres. Such overperfusion brings into question if blood flow and metabolic demand are as tightly matched as previously assumed.

Motor units in the human medial gastrocnemius muscle are not spatially localized or functionally grouped

KEY POINTS

Human medial gastrocnemius (MG) motor units (MUs) are thought to occupy small muscle territories or regions, with low-threshold units preferentially located distally. We used intramuscular recordings to measure the territory of muscle fibres from MG MUs and determine whether these MUs are grouped by recruitment threshold or joint action (ankle plantar flexion and knee flexion). The territory of MUs from the MG muscle varied from somewhat localized to highly distributed, with approximately half the MUs spanning at least half the length and width of the muscle. There was also no evidence of regional muscle activity based on MU recruitment thresholds or joint action. The CNS does not have the means to selectively activate regions of the MG muscle based on task requirements.

ABSTRACT

Human medial gastrocnemius (MG) motor units (MUs) are thought to occupy small muscle territories, with low-threshold units preferentially located distally. In this study, subjects (n = 8) performed ramped and sustained isometric contractions (ankle plantar flexion and knee flexion; range: ∼1-40% maximal voluntary contraction) and we measured MU territory size with spike-triggered averages from fine-wire electrodes inserted along the length (seven electrodes) or across the width (five electrodes) of the MG muscle. Of 69 MUs identified along the length of the muscle, 32 spanned at least half the muscle length (≥ 6.9 cm), 11 of which spanned all recording sites (13.6-17.9 cm). Distal fibres had smaller pennation angles (P < 0.05), which were accompanied by larger territories in MUs with fibres located distally (P < 0.05). There was no distal-to-proximal pattern of muscle activation in ramp contraction (P = 0.93). Of 36 MUs identified across the width of the muscle, 24 spanned at least half the muscle width (≥ 4.0 cm), 13 of which spanned all recording sites (8.0-10.8 cm). MUs were not localized (length or width) based on recruitment threshold or contraction type, nor was there a relationship between MU territory size and recruitment threshold (Spearman's rho = -0.20 and 0.13, P > 0.18). MUs in the human MG have larger territories than previously reported and are not localized based on recruitment threshold or joint action. This indicates that the CNS does not have the means to selectively activate regions of the MG muscle based on task requirements.

Tenomodulin regulated the compartments of embryonic and early postnatal mouse masseter muscle

The masseter muscle (MM) is a complex tendinous laminar structure during development; however, the stage of the laminar structure formation is unknown. Tenomodulin (TeM) is a useful marker of tendons and has an anti-angiogenic cysteine-rich C-terminal domain. Therefore, we analyzed mRNA of TeM and angiogenesis markers (CD31 and vascular endothelial growth factor (VEGF)) and performed in situ hybridization for the TeM genes in MM from on embryonic day 12.5 (E12.5) to postnatal day 5 (P5). The TeM expression is at first detectable in the middle region of the mesenchymal connective tissue in the MM at E 12.5. The expression domains of the TeM during development typically include the middle region of the MM, particularly surrounding the vascular regions. The level of TeM mRNA in the MM increased from E12.5 to E17.5 and decreased after birth. In contrast, the levels of CD31 and VEGF mRNAs were almost constant from E12.5 to E18.5 and then low from birth onward. Therefore, the development of the laminar tendinous structure in the middle region between superficial and deeper regions of the MM first occurs during the process of tendon formation at embryonic day 12.5. In our study of MM development, the laminar structure regulating TeM also prevents vascular invasion during the formation of compartment of the MM. The tendon may relate to the components of muscle mass of MM.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Mammalian masticatory muscles: homology, nomenclature, and diversification

There is a deep and rich literature of comparative studies of jaw muscles in mammals but no recent analyses employ modern phylogenetic techniques to better understand evolutionary changes that have occurred in these muscles. In order to fully develop and utilize the Feeding Experiments End-user Database (FEED), we are constructing a comprehensive ontology of mammalian jaw muscles. This process has led to a careful consideration of nomenclature and homologies of the muscles and their constituent parts. Precise determinations of muscle attachments have shown that muscles with similar names are not necessarily homologous. Using new anatomical descriptions derived from the literature, we defined character states for the jaw muscles in diverse mammalian species. We then mapped those characters onto a recent phylogeny of mammals with the aid of the Mesquite software package. Our data further elucidate how muscle groups associated with the feeding apparatus differ and have become highly specialized in certain mammalian orders, such as Rodentia, while remaining conserved in other orders. We believe that careful naming of muscles and statistical analyses of their distributions among mammals, in association with the FEED database, will lead to new, significant insights into the functional, structural, and evolutionary morphology of the jaw muscles.

Sprouting capacity of lumbar motoneurons in normal and hemisected spinal cords of the rat

Nerve sprouting to reinnervate partially denervated muscles is important in several disease and injury states. To examine the effectiveness of sprouting of active and inactive motor units (MUs) and the basis for a limit to sprouting, one of three rat lumbar spinal roots was cut under normal conditions and when the spinal cord was hemisected at T12. Muscle and MU isometric contractile forces were recorded and muscle fibres in glycogen-depleted single muscle units enumerated 23 to 380 days after surgery. Enlargement of intact MUs by sprouting was effective in compensating for up to 80% loss of innervation. For injuries that removed >70-80% of the intact MUs, muscle contractile force and weight dropped sharply. For partial denervation of <70%, all MUs increased contractile force by the same factor in both normally active muscles and muscles whose activity was reduced by T12 hemisection. Direct measurements of MU size by counting glycogen-depleted muscle fibres in physiologically and histochemically defined muscle units, provided direct evidence for a limit in MU size, whether or not the activity of the muscles was reduced by spinal cord hemisection. Analysis of spatial distribution of muscle fibres within the outer boundaries of the muscle unit demonstrated a progressive increase in fibres within the territory to the limit of sprouting when most of the muscle unit fibres were adjacent to each other. We conclude that the upper limit of MU enlargement may be explained by the reinnervation of denervated muscle fibres by axon sprouts within the spatial territory of the muscle unit, formerly distributed in a mosaic pattern.

Lamination of the masticatory muscles in the Phascolarctos cinereus (Koala) according to their innervations

The masticatory muscles are usually classified into four groups: masseter, temporalis, lateral pterygoid and medial pterygoid. The communicating muscle bundle between the temporalis and masseter called the zygomaticomandibular muscle exists. The laminations within these muscles are commonly separated by aponeuroses. Nerves control the action of muscles, so improved understanding about innervation patterns in the masticatory muscles is important in the consideration of muscle function. In this study, we focus on the relationships between the nerves supply and the lamination of masticatory muscles in Phascolarctos cinereus (Koala). The masseter muscle consists of superficial and deep muscle layers. The superficial muscle layer of the masseter muscle is divided into rostro-lateral and caudo-internal nerve layers. The deep muscle layer of the masseter muscle is divided into rostral, rostro-lateral, medial and caudo-internal nerve layers. The nerves that innervate the zygomaticomandibular muscle are distributed to the lateral area of the coronoid process. The temporalis muscle was divided into internal layer of the coronoid process, a lateral layer of the coronoid process and a posterior layer by the nerve distribution pattern. The medial pterygoid muscle divided into rostro-internal, medial and caudo-lateral nerve layers.

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.

Developmental and functional considerations of masseter muscle partitioning

The masseter muscle participates in a wide variety of activities including mastication, swallowing and speech. The functional demands for accurate mandibular positioning and generation of forces during incising or a power stroke require a diverse set of forces that are determined by the innate muscle form. The complex internal tendon architecture subdivides the masseter into multiple partitions that can be further subdivided into neuromuscular compartments representing small motor unit territories. Individual masseter compartments have unique biomechanical properties that, when activated individually or in groups, can generate a wide range of sagittal and off-sagittal torques about the temporomandibular joint. The myosin heavy chain (MyHC) fibre-type distribution in the adult masseter is sexually dimorphic and is influenced by hormones such as testosterone. These testosterone-dependent changes cause a phenotype switch from slower to faster fibre-types in the male. The development of the complex organization of the masseter muscle, the MyHC fibre-type message and protein expression, and the formation of endplates appear to be pre-programmed and not under control of the muscle nerve. However, secondary myotube generation and endplate maturation are nerve dependent. The delayed development of the masseter muscle compared with the facial, tongue and jaw-opening muscles may be related to the delayed functional requirements for chewing. In summary, masseter muscle form is pre-programmed prior to birth while muscle fibre contractile characteristics are refined postnatally in response to functional requirements. The motor control mechanisms that are required to coordinate the activation of discrete functional elements of this muscle remain to be determined.

Transneuronal tracing of vestibulo-trigeminal pathways innervating the masseter muscle in the rat

Previous studies reported that the activity of trigeminal motoneurons innervating masseter muscles is modulated by vestibular inputs. We performed the present study to provide an anatomical substrate for these physiological observations. The transynaptic retrograde tracer pseudorabies virus-Bartha was injected into multiple sites of the lower third of the superficial layer of the masseter muscle in rats, a subset of which underwent a sympathectomy prior to virus injections, and the animals were euthanized 24-120 h later. Labeled masseteric motoneurons were first found in the ipsilateral trigeminal motor nucleus following a 24-h postinoculation period; subsequent to 72-h survival times, the number of infected motoneurons increased, and at > or =96 h many of these cells showed signs of cytopathic changes. Following 72-h survival times, a few transynaptically labeled neurons appeared bilaterally in the medial vestibular nucleus (MVe) and the caudal prepositus hypoglossi (PH) and in the ipsilateral spinal vestibular nucleus (SpVe). At survival times of 96-120 h, labeled neurons were consistently observed bilaterally in all vestibular nuclei (VN), although the highest concentration of infected cells was located in the caudal part of the MVe, the SpVe, and the caudal portion of PH. The distribution and density of labeling in the VN and PH were similar in sympathectomized and nonsympathectomized rats. These anatomical data provide the first direct evidence that neurons in the VN and PH project bilaterally to populations of motoneurons innervating the lower third of the superficial layer of the masseter muscle. The MVe, PH, and SpVe appear to play a predominant integrative role in producing vestibulo-trigeminal responses.

Somatotopic Organization of Perioral Musculature Innervation within the Pig Facial Motor Nucleus

The orbicularis oris and buccinator muscles of mammals form an important subset of the facial musculature, the perioral muscles. In many taxa, these muscles form a robust muscular hydrostat capable of highly manipulative fine motor movements, likely accompanied by a specialized pattern of innervation. We conducted a retrograde nerve-tracing study of cranial nerve (CN) VII in pigs (Sus scrofa) to: (1) map the motor neuron pool distributions of the superior and inferior orbicularis oris, and the buccinator, to test the hypothesis that perioral muscle motor neuron pools exhibit a somatotopic organization within the facial motor nucleus; and (2) test the hypothesis that portions of the superior orbicularis oris (SOO) motor neuron pool also exhibit a somatotopic organization, reflecting a potential compartmentalization of function of the rostral, middle, and caudal segments of this muscle. Cresyl violet histological staining showed that the pig facial motor nucleus was comprised of 7 well-defined subnuclei. Neuroanatomical tracers injected into these perioral muscles transported to the motor neuron pools of the lateral 4 of the 7 subnuclei of the facial motor nucleus. The motor neuron pools of the perioral muscles were generally segregated from motoneurons innervating other facial muscles of the rostrum. However, motor neuron pools were not confined to single nuclei but instead spanned across 3-4 subnuclei. Perioral muscle motor neuron pools overlapped but were organized somatotopically. Motor neuron pools of portions of the SOO overlapped greatly with each other but exhibited a crude somatotopy within the SOO motor neuron pool. The large and somatotopically organized SOO motor neuron pool in pigs suggests that the upper lip might be more richly innervated than the other perioral muscles and functionally divided.

Differential activation of neuromuscular compartments in the rabbit masseter muscle during different oral behaviors

The rabbit masseter muscle is composed of multiple anatomical partitions that produce different mechanical actions. The purpose of this study was to test the hypothesis that these compartments are differentially activated during the performance of different oral behaviors. Rhythmic activation of the masticatory muscles was elicited by stimulating the cortical masticatory area (CMA) while recording forces generated at the incisors in three dimensions with the mandible immobilized. Torques about the right temporomandibular joint (TMJ) were calculated using these forces recorded during isometric function. A set of 1-15 unique rhythmic behaviors was identified for each rabbit using torque phase plot patterns. Electromyographic recordings were made at nine different compartments in the right masseter, two compartments in the left masseter, two regions in the right digastric, and single locations in the left digastric and right and left medial pterygoid muscles. In activation cycles producing similar mechanical actions, activity patterns at the 16 recording sites were clustered into three to six groups using principal component analysis (PCA). To test for similarities in the activation of masseter compartments, pair-wise comparisons of the PCA assignment for the nine masseter compartments were conducted and frequencies of common assignment were compiled for each unique rhythmic behavior for each rabbit. Masseter muscle compartments were found to vary significantly in their PCA from the expected distribution of 100% common principal component (PC) assignment (i.e., similar activation pattern). This finding is consistent with the independent activation of masseter compartments.

Spatial distribution of myosin heavy-chain isoforms in mouse masseter

There is a paucity of information regarding the anatomy and muscle fiber phenotype of the masseter. The objective of this study was to characterize the distribution of each myosin heavy-chain (MyHC) isoform within different anatomical regions of male and female mouse masseters. Masseters from male and female CD-1 mice (2-4 months old) were examined for description of the anatomical partitioning of muscle fibers and endplate distribution. The spatial distribution of MyHC isoforms--embryonic, neonatal, slow, alpha-cardiac, IIa, and IIb--was determined within the defined masseter partitions by means of Western blot analysis and immunofluorescent localization. Types IIa, IIx, and IIb were the predominant MyHC isoforms observed. Distinct differences in the spatial distribution of these MyHC isoforms were found between muscle regions and varied between sexes. The regionalization of muscle fiber types in the mouse masseter is consistent with the functional compartmentalization of the masseter observed in other species.

Role of motor unit structure in defining function

Motor units, defined as a motoneuron and all of its associated muscle fibers, are the basic functional units of skeletal muscle. Their activity represents the final output of the central nervous system, and their role in motor control has been widely studied. However, there has been relatively little work focused on the mechanical significance of recruiting variable numbers of motor units during different motor tasks. This review focuses on factors ranging from molecular to macroanatomical components that influence the mechanical output of a motor unit in the context of the whole muscle. These factors range from the mechanical properties of different muscle fiber types to the unique morphology of the muscle fibers constituting a motor unit of a given type and to the arrangement of those motor unit fibers in three dimensions within the muscle. We suggest that as a result of the integration of multiple levels of structural and physiological levels of organization, unique mechanical properties of motor units are likely to emerge.

Muscle fibre types in the suprahyoid muscles of the rat

Five muscle fibre types (I, IIc, IIa, IIx and IIb) were found in the suprahyoid muscles (mylohyoid, geniohyoid, and the anterior and posterior bellies of the digastric) of the rat using immuno and enzyme histochemical techniques. More than 90% of fibres in the muscles examined were fast contracting fibres (types IIa, IIx and IIb). The geniohyoid and the anterior belly of the digastric had the greatest number of IIb fibres, whilst the mylohyoid was almost exclusively formed by aerobic fibres. The posterior belly of the digastric contained a greater percentage of aerobic fibres (83.4%) than the anterior belly (67.8%). With the exception of the geniohyoid, the percentage of type I and IIc fibres, which have slow myosin heavy chain (MHCbeta), was relatively high and greater than has been previously reported in the jaw-closing muscles of the rat, such as the superficial masseter. The geniohyoid and mylohyoid exhibited a mosaic fibre type distribution, without any apparent regionalisation, although in the later MHCbeta-containing fibres (types I and IIc) were primarily located in the rostral 2/3 region. In contrast, the anterior and posterior bellies of the digastric revealed a clear regionalisation. In the anterior belly of the digastric 2 regions were observed: both a central region, which was almost exclusively formed by aerobic fibres and where all of the type I and IIc fibres were located, and a peripheral region, where type IIb fibres predominated. The posterior belly of the digastric showed a deep aerobic region which was greater in size and where type I and IIc fibres were confined, and a superficial region, where primarily type IIx and IIb fibres were observed.

EMG power spectrum and motor unit characteristics in the masseter muscle of the rabbit

Masticatory muscles contain a large variety of motor units with different physiological and morphological properties. In this study, we tested the hypothesis that a relationship exists between the mechanical and myo-electric properties of single motor units in the masseter muscle of the rabbit. It was expected that faster-contracting motor units, which usually have a relatively large number of fibers with large diameters, should have faster action potentials with larger amplitudes than slower motor units. Single motor units were stimulated. A two-dimensional force transducer registered mechanical parameters of the units. EMG electrodes were used to determine amplitude and frequency parameters of the action potentials of the same units. The results showed that faster-contracting motor units indeed produced action potentials with higher conduction velocities. However, faster motor units had no significant larger amplitude of the action potential. Small but significant positive correlations were found between the tetanic peak force and the amplitude of the action potentials. Little difference was found among the various frequency and amplitude parameters, respectively, making them equally suitable to describe the action potential. Surprisingly, a negative correlation between the amplitude and frequency parameters of the action potential was found, which may result from variability in arrival times of action potentials at the electrode site. Regional differences in the frequency parameters were found between the anterior and posterior parts of the superficial masseter.

Skeletal muscle function and fibre types: the relationship between occlusal function and the phenotype of jaw-closing muscles in human

Mammalian skeletal muscle cells are composed of repeated sarcomeric units containing thick and thin filaments of myosin and actin, respectively. Excitation of the myosin ATPase enzyme is possible only with presence of Mg-ATP and Ca(2+). Skeletal muscle fibres may be classified into several types according to the isoform of myosin they contain. Nine isoforms of myosin heavy chain are known to exist in mammalian skeletal muscle including type I, IIA, IIB, IIX, IIM, alpha, neonatal, embryonic, and extra-ocular. Healthy adult human limb skeletal muscle contains type I, IIA, IIB, and IIX myosin heavy chains. The jaw-closing muscles of most carnivores and primates have tissue-specific expression of the type IIM or 'type II masticatory' myosin heavy chain. Adult human jaw-closing muscles, however, do not contain IIM myosin. Rather, they express type I, IIA, IIX (as in human limb muscle), and myosins typically expressed in developing or cardiac muscle. The morphology of human jaw-closing muscle fibres is also unusual in that the type II fibres are of smaller diameter that type I fibres, except in cases of increased function and hypertrophy. This paper describes the relationship of fibre types and motor unit function to changes in human occlusion and masticatory activity. Refereed Scientific Paper

Force vectors of single motor units in a multipennate muscle

The masseter muscle of the rabbit has a complex architectural design. Restricted motor unit territories in the muscle provide an anatomic basis for accurate control of the force vector through selective activation. In addition, the muscle shows regional differences in fiber type composition. The main objective of the present study was to measure the force vectors of single motor units within the rabbit masseter muscle by a direct mechanical approach to test the hypothesis that: (1) motor units within the masseter muscle are capable of generating different force vectors; and (2) different motor unit types are distributed heterogeneously throughout the muscle. We used a force transducer, capable of measuring both the magnitude and the position of the line of action of a force in a single plane. Motor units in the masseter muscle showed a large range of twitch contraction times and force magnitudes. There was also a large variation in the direction and moment arm of the lines of action. The variation of the lines of action was (almost) as large as the range of fiber directions found inside the muscle. Largest forces, with relatively slow contraction velocities, were produced by motor units in the anterior masseter. Smaller forces and fastest twitch contractions were produced by motor units in the posterior deep masseter. In addition, motor units in the anterior masseter showed more variability in force production than in the posterior masseter. Our results support the idea that the masseter muscle is divided into functionally different parts.

Evidence that trigeminal brainstem interneurons form subpopulations to produce different forms of mastication in the rabbit

To determine how trigeminal brainstem interneurons pattern different forms of rhythmical jaw movements, four types of motor patterns were induced by electrical stimulation within the cortical masticatory areas of rabbits. After these were recorded, animals were paralyzed and fictive motor output was recorded with an extracellular microelectrode in the trigeminal motor nucleus. A second electrode was used to record from interneurons within the lateral part of the parvocellular reticular formation (Rpc-alpha, n = 28) and gamma- subnucleus of the oral nucleus of the spinal trigeminal tract (NVspo-gamma, n = 68). Both of these areas contain many interneurons projecting to the trigeminal motor nucleus. The basic characteristics of the four movement types evoked before paralysis were similar to those seen after the neuromuscular blockade, although cycle duration was significantly decreased for all patterns. Interneurons showed three types of firing pattern: 54% were inactive, 42% were rhythmically active, and 4% had a tonic firing pattern. Neurons within the first two categories were intermingled in Rpc-alpha and NVspo-gamma: 48% of rhythmic neurons were active during one movement type, 35% were active during two, and 13% were active during three or four patterns. Most units fired during either the middle of the masseter burst or interburst phases during fictive movements evoked from the left caudal cortex. In contrast, there were no tendencies toward a preferred coupling of interneuron activity to any particular phase of the cycle during stimulation of other cortical sites. It was concluded that the premotoneurons that form the final commands to trigeminal motoneurons are organized into subpopulations according to movement pattern.

Different phenotypes among slow/beta myosin heavy chain-containing fibres of rabbit masseter muscle: a novel type of diversity in adult muscle

Difference in the phenotype of different mammalian muscle fibres are usually attributed to differences in the expression of the product of different myosin heavy chain (MyHC) genes, which are known as isoforms. We studied differences in phenotype among fibres containing a single MyHC isoform (slow/beta) of the masseter muscle of adult rabbits. Four different monoclonal antibodies to slow/beta MyHC were used to stain serial sections from muscles in males and females. All antibodies recognize a single band on immunoblots and stain the same set of fibres in rabbit postcranial muscles. However, differential staining was observed in the masseter muscles. Antibody BA-D5 reacts with the most fibres, antibody A4.951 reacts with a subset of these fibres, and antibody A4.840 reacts with a subset of these fibres, and antibody A4.840 reacts with a subset of A4.951-positive fibres. Antibody S58 reacts only with an even smaller subset of fibres. Even though differential staining using four antibodies might allow for the expression of as many as 15 different staining patterns, or patterns, or phenotypes, only four were observed on > 99% of over 30 000 fibres studied. In females, nearly 40% of the fibres stain exclusively with antibody BA-D5, while in males, fewer than 8% of the fibres express this phenotype. The proportions of fibres of the other phenotypes do not differ so strikingly with gender. We conclude that an epitope diversity exists among muscle fibres in the adult rabbit masseter and that it is not necessarily a consequence of differences in gene expression. We feel that it is a regulated process and that, at least for some phenotypes, this regulation may be hormonally influenced.

Specificity and development of innervation pattern of Xenopus pectoralis muscle

Multielectrode recordings were used to identify and measure the axonal inputs to each end plate on contiguous surface fibers covering about 25% of the Xenopus pectoralis muscle in mature and developing animals. The mature innervation pattern was remarkably precise. Individual axons tended to innervate fibers of similar input resistance (R(in)) in compact motor units restricted to only a portion of the region studied. Motor units comprising fibers of similar R(in) overlapped mainly near their borders. Most fibers had two end plates. In more than 80% of these fibers, both end plates received input from the same axon. In 57%, this was the only input to both end plates. This implies a powerful mechanism for excluding or eliminating inputs from other axons. About 16% of the mature junctions showed focal polyneuronal innervation, with the weaker end plate potential component often less than 1 mV in noncurarized preparation. However, we have no evidence that the weaker inputs were being eliminated. During development, motor units became more compact, which was associated with synapse elimination; but from the earliest times studied, soon after metamorphosis when many fibers were adding second end plates, a majority of those that had two end plates were innervated at both sites by the same axon.

A new transducer system for direct motor unit force measurement

A new transducer was developed for in situ measurement of the force vector in a complex muscle. The transducer measures the magnitude, and the line of action of a force in a single plane. The dynamic range of the transducer is 0-5 N. This range includes the small forces developed by an active motor unit and the relatively large passive force of a whole muscle. In this study we present the details of the transducer design and specifications, and describe its application in the measurement of motor unit forces of the rabbit masseter muscle.

A method for the study of single jaw adductor muscle motor units in the rat

Single motor units of the superficial masseter muscle were stimulated by impaling axons in the third division of the trigeminal nerve. Glass micropipette electrodes were driven into the nerve as it runs along the floor of the cranial cavity while passing small current pulses through the electrode to activate axons. Stable penetrations were maintained for as long as 45 min and twitch tensions were recorded for motor units that generated as little as 159 mg (1.6 mN) of force within a superficial masseter muscle capable of producing 20 g of twitch force. These are the first measurements of single motor-unit forces measured directly from the tendon of a whole jaw adductor muscle.

Unloaded shortening velocities of rabbit masseter muscle fibres expressing skeletal or alpha-cardiac myosin heavy chains

1. Some rabbit masseter fibres express the alpha-cardiac myosin heavy chain (MHC). To compare the biochemical and physiological properties of these fibres with other skeletal fibre types, we examined the histochemical and immunohistochemical staining characteristics, maximum velocity of shortening (V(zero)) and MHC isoform content of fibres from rabbit masseter and soleus muscles. 2. The fibre-type composition of muscle sections was determined with MHC antibodies and myofibrillar ATPase histochemistry. Fibres we designated 'type alpha-cardiac' were different from type I and type II fibres in that they stained positively with the alpha-cardiac MHC antibody and they maintained. ATPase reactivity after acid and alkali pre-incubations. Samples of superficial masseter contained a few type I fibres, with the majority of fibres classified as either type IIA or type alpha-cardiac. Soleus samples contained type I, IIA and IIC fibres. 3. The V(zero) of chemically skinned fibres was determined by the slack-test method. Each fibre was subsequently characterized as type I, IIA, IIC or alpha-cardiac from MHC identification using gel electrophoresis (SDS-PAGE). In masseter fibres the V(zero) values were (in muscle lengths s-1): type I, 0.54 +/- 0.05 (mean +/- S.D., n = 3); type IIA, 1.23 +/- 0.34 (n = 27); type alpha-cardiac, 0.78 +/- 0.08 (n = 9). In soleus fibres V(zero) values were: type I, 0.55 +/- 0.06 (n = 14); type IIA, 0.89 +/- 0.04 (n = 8); type IIC, 0.73 (n = 2). 4. We conclude that the rabbit masseter muscle contains an 'alpha-cardiac' fibre type that is distinct from other skeletal fibres. This fibre type expresses only the alpha-cardiac MHC, has unusual myofibrillar ATPase reactivity and has a V(zero) intermediate between type I and type II fibres.

Functional somatotopic organisation of motoneurons supplying the rabbit masseter muscle

The localisation within the trigeminal motor nucleus of motoneurons supplying different regions of the rabbit masseter muscle was investigated to test the hypothesis that muscle regions with different motor tasks are controlled from different subregions of the motor nucleus. Motoneurons were labeled retrogradely with horseradish peroxidase, applied surgically to small sections of the masseter in 22 animals, and also by applying this tracer to the cut masseteric nerve. After sacrifice, the labeled muscle sections were mapped. The distribution of labeled motoneurons within the nucleus was described and compared for the muscle regions. The motoneurons for the masseter muscle are confined to the dorsal and lateral sections of the motor nucleus, along its full rostrocaudal extent. Within this subnucleus, the motoneurons for the superficial masseter occupy the dorsolateral portion, the motoneurons for the deep masseter the dorsomedial portion. The anatomical and functional subdivision of the deep masseter into an anterior and posterior portion appeared to be matched by a separation of the motoneurons for these portions in the rostrocaudal direction along the nucleus. The separation of the motoneurons for the anterior and posterior deep masseter is not complete; the territories in the motor nucleus overlap each other for about 50%. The well-established differentiation in motor tasks between the masseter portions during feeding is thus clearly reflected in a separation of motoneurons, making possible differentiation of descending or afferent input to the separate regions in the nucleus.

Function-dependent anatomical parameters of rabbit masseter motor units

Rabbit masseter motor units (22) were studied by stimulation of trigeminal motoneurons. We tested the hypotheses that masseter motor units facilitate fine motor control by concentrating fibers in small areas and that the distribution of motor unit fibers depends on the fiber type. The twitch contraction time and the isometric tetanic force were registered. The motor unit fibers were depleted of their glycogen by prolonged stimulation. Serial sections of the entire muscle were stained with the periodic acid Schiff (PAS) and monoclonal antibody stains. The muscle fibers of the motor unit were mapped and identified by four myosin heavy-chain (MHC) isoforms: I, IIA, IID, and cardiac-alpha. In the PAS-stained sections, anatomical parameters of the motor units, affecting the force output, were analyzed: the innervation ratio (IR), motor unit territory area (TA), and relative (R-DENS) and absolute (A-DENS) motor unit fiber densities. The fiber cross-sectional area (F-CSA) was measured for each MHC fiber type. The F-CSA sum of all motor unit fibers, the physiological cross-sectional area (P-CSA), was calculated. The IR ranged between 77 and 720 fibers (mean, 267). The mean TA was 8.71 mm2 (range, 4.45 to 19.58). The mean R-DENS was 10 fibers per 100; the A-DENS was 31 fibers per mm2. Linear correlations were found between the IR and the R-DENS and between the tetanic force and the IR. The F-CSAs showed a stepwise increase in value from type I- to IID-MHC fibers. The mean P-CSA was 0.90 mm2 (range, 0.09 to 2.97). A high linear correlation was noted between the P-CSA and the tetanic force. In conclusion, increase of motor unit size expressed in higher fiber counts and forces is accomplished by increase of the fiber density. Thus, forces can be exerted selectively in restricted regions of the masseter muscle. Differences in fiber orientation due to complex muscle pinnation emphasize the possibility of an accurate muscle performance.

Further evidence of incomplete neural control of muscle properties in cat tibialis anterior motor units

To examine the influence of a motoneuron in maintaining the phenotype of the muscle fibers it innervates, myosin heavy chain (MHC) expression, succinate dehydrogenase (SDH) activity, and cross-sectional area (CSA) of a sample of fibers belonging to a motor unit were studied in the cat tibialis anterior 6 mo after the nerve branches innervating the anterior compartment were cut and sutured near the point of entry into the muscle. The mean, range, and coefficient of variation for the SDH activity and the CSA for both motor unit and non-motor unit fibers for each MHC profile and from each control and each self-reinnervated muscle studied was obtained. Eight motor units were isolated from self-reinnervated muscles using standard ventral root filament testing techniques, tested physiologically, and compared with four motor units from control muscles. Motor units from self-reinnervated muscles could be classified into the same physiological types as those found in control tibialis anterior muscles. The muscle fibers belonging to a unit were depleted of glycogen via repetitive stimulation and identified in periodic acid-Schiff-stained frozen sections. Whereas muscle fibers in control units expressed similar MHCs, each motor unit from self-reinnervated muscles contained a mixture of fiber types. In each motor unit, however, there was a predominance of fibers with the same MHC profile. The relative differences in the mean SDH activities found among fibers of different MHC profiles within a unit after self-reinnervation and those found among fibers in control muscles were similar, i.e., fast-2 < fast-1 < or = slow MHC fibers.(ABSTRACT TRUNCATED AT 250 WORDS)