Examination of intrafascicular muscle fiber terminations: implications for tension delivery in series-fibered muscles

A Conceptual Exploration of Hamstring Muscle-Tendon Functioning during the Late-Swing Phase of Sprinting: The Importance of Evidence-Based Hamstring Training Frameworks

An eccentrically lengthening, energy-absorbing, brake-driven model of hamstring function during the late-swing phase of sprinting has been widely touted within the existing literature. In contrast, an isometrically contracting, spring-driven model of hamstring function has recently been proposed. This theory has gained substantial traction within the applied sporting world, influencing understandings of hamstring function while sprinting, as well as the development and adoption of certain types of hamstring-specific exercises. Across the animal kingdom, both spring- and motor-driven muscle-tendon unit (MTU) functioning are frequently observed, with both models of locomotive functioning commonly utilising some degree of active muscle lengthening to draw upon force enhancement mechanisms. However, a method to accurately assess hamstring muscle-tendon functioning when sprinting does not exist. Accordingly, the aims of this review article are three-fold: (1) to comprehensively explore current terminology, theories and models surrounding muscle-tendon functioning during locomotion, (2) to relate these models to potential hamstring function when sprinting by examining a variety of hamstring-specific research and (3) to highlight the importance of developing and utilising evidence-based frameworks to guide hamstring training in athletes required to sprint. Due to the intensity of movement, large musculotendinous stretches and high mechanical loads experienced in the hamstrings when sprinting, it is anticipated that the hamstring MTUs adopt a model of functioning that has some reliance upon active muscle lengthening and muscle actuators during this particular task. However, each individual hamstring MTU is expected to adopt various combinations of spring-, brake- and motor-driven functioning when sprinting, in accordance with their architectural arrangement and activation patterns. Muscle function is intricate and dependent upon complex interactions between musculoskeletal kinematics and kinetics, muscle activation patterns and the neuromechanical regulation of tensions and stiffness, and loads applied by the environment, among other important variables. Accordingly, hamstring function when sprinting is anticipated to be unique to this particular activity. It is therefore proposed that the adoption of hamstring-specific exercises should not be founded on unvalidated claims of replicating hamstring function when sprinting, as has been suggested in the literature. Adaptive benefits may potentially be derived from a range of hamstring-specific exercises that vary in the stimuli they provide. Therefore, a more rigorous approach is to select hamstring-specific exercises based on thoroughly constructed evidence-based frameworks surrounding the specific stimulus provided by the exercise, the accompanying adaptations elicited by the exercise, and the effects of these adaptations on hamstring functioning and injury risk mitigation when sprinting.

The influence of longitudinal muscle fascicle growth on mechanical function

Skeletal muscle has the remarkable ability to remodel and adapt, such as the increase in serial sarcomere number (SSN) or fascicle length (FL) observed after overstretching a muscle. This type of remodelling is termed longitudinal muscle fascicle growth, and its impact on biomechanical function has been of interest since the 1960s due to its clinical applications in muscle strain injury, muscle spasticity, and sarcopenia. Despite simplified hypotheses on how longitudinal muscle fascicle growth might influence mechanical function, existing literature presents conflicting results partly due to a breadth of methodologies. The purpose of this review is to outline what is currently known about the influence of longitudinal muscle fascicle growth on mechanical function and suggest future directions to address current knowledge gaps and methodological limitations. Various interventions indicate longitudinal muscle fascicle growth can increase the optimal muscle length for active force, but whether the whole force-length relationship widens has been less investigated. Future research should also explore the ability for longitudinal fascicle growth to broaden the torque-angle relationship's plateau region, and the relation to increased force during shortening. Without a concurrent increase in intramuscular collagen, longitudinal muscle fascicle growth also reduces passive tension at long muscle lengths; further research is required to understand whether this translates to increased joint range of motion. Lastly, some evidence suggests longitudinal fascicle growth can increase maximum shortening velocity and peak isotonic power, however, there has yet to be direct assessment of these measures in a neurologically intact model of longitudinal muscle fascicle growth.

Human Frontalis Muscle Innervation and Morphology

Due to its clinical importance and due to a suggestion regarding the afferent innervation, the microscopic appearance of the frontalis muscle was investigated.

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.

Identifying the Structural Adaptations that Drive the Mechanical Load-Induced Growth of Skeletal Muscle: A Scoping Review

The maintenance of skeletal muscle mass plays a critical role in health and quality of life. One of the most potent regulators of skeletal muscle mass is mechanical loading, and numerous studies have led to a reasonably clear understanding of the macroscopic and microscopic changes that occur when the mechanical environment is altered. For instance, an increase in mechanical loading induces a growth response that is mediated, at least in part, by an increase in the cross-sectional area of the myofibers (i.e., myofiber hypertrophy). However, very little is known about the ultrastructural adaptations that drive this response. Even the most basic questions, such as whether mechanical load-induced myofiber hypertrophy is mediated by an increase in the size of the pre-existing myofibrils and/or an increase in the number myofibrils, have not been resolved. In this review, we thoroughly summarize what is currently known about the macroscopic, microscopic and ultrastructural changes that drive mechanical load-induced growth and highlight the critical gaps in knowledge that need to be filled.

Multiscale modeling of the neuromuscular system: Coupling neurophysiology and skeletal muscle mechanics

Mathematical models and computer simulations have the great potential to substantially increase our understanding of the biophysical behavior of the neuromuscular system. This, however, requires detailed multiscale, and multiphysics models. Once validated, such models allow systematic in silico investigations that are not necessarily feasible within experiments and, therefore, have the ability to provide valuable insights into the complex interrelations within the healthy system and for pathological conditions. Most of the existing models focus on individual parts of the neuromuscular system and do not consider the neuromuscular system as an integrated physiological system. Hence, the aim of this advanced review is to facilitate the prospective development of detailed biophysical models of the entire neuromuscular system. For this purpose, this review is subdivided into three parts. The first part introduces the key anatomical and physiological aspects of the healthy neuromuscular system necessary for modeling the neuromuscular system. The second part provides an overview on state-of-the-art modeling approaches representing all major components of the neuromuscular system on different time and length scales. Within the last part, a specific multiscale neuromuscular system model is introduced. The integrated system model combines existing models of the motor neuron pool, of the sensory system and of a multiscale model describing the mechanical behavior of skeletal muscles. Since many sub-models are based on strictly biophysical modeling approaches, it closely represents the underlying physiological system and thus could be employed as starting point for further improvements and future developments. This article is categorized under: Physiology > Mammalian Physiology in Health and Disease Analytical and Computational Methods > Computational Methods Models of Systems Properties and Processes > Organ, Tissue, and Physiological Models.

Extracellular matrix remodelling induced by alternating electrical and mechanical stimulations increases the contraction of engineered skeletal muscle tissues

Engineered skeletal muscles are inferior to natural muscles in terms of contractile force, hampering their potential use in practical applications. One major limitation is that the extracellular matrix (ECM) not only impedes the contraction but also ineffectively transmits the forces generated by myotubes to the load. In the present study, ECM remodelling improves contractile force in a short time, and a coordinated, combined electrical and mechanical stimulation induces the desired ECM remodelling. Notably, the application of single and combined stimulations to the engineered muscles remodels the structure of their ECM networks, which determines the mechanical properties of the ECM. Myotubes in the tissues are connected in parallel and in series to the ECM. The stiffness of the parallel ECM must be low not to impede contraction, while the stiffness of the serial ECM must be high to transmit the forces to the load. Both the experimental results and the mechanistic model suggest that the combined stimulation through coordination reorients the ECM fibres in such a way that the parallel ECM stiffness is reduced, while the serial ECM stiffness is increased. In particular, 3 and 20 minutes of alternating electrical and mechanical stimulations increase the force by 18% and 31%, respectively.

Age-related remodelling of the myotendinous junction in the mouse soleus muscle

The age-related loss of muscle mass and function predominantly affect muscles of the lower limbs and have largely been associated with decline in muscle fibre size and number, although the exact mechanisms underlying these losses are poorly understood. In addition, consistent reports that the loss of muscle strength exceeds that which can be explained by declines in muscle mass has widened the search for causes of sarcopenia to include supporting tissues such as the extracellular matrix and tendons. Although the changes to both muscle and tendon with age are well characterised, little work has focused on the interface between these two tissues, the myotendinous junction (MTJ). Given the crucial role for this structure in force transfer between muscle and tendon, we asked whether the myotendinous junction underwent structural changes with age in lower limb muscle. We used whole muscle to assess gross muscle and tendon morphology, and immunohistochemistry to determine fibre and MTJ profile number in young (6 months), middle aged (18 months) and elderly (24 months) C57BL/6 female mice. MTJ length was quantified using serial cross sections of the soleus muscle. We found an apparent 3.5-fold increase in MTJ profiles per cross section with no increase in fibre number in old mice, and found this to be a result of a doubling in length of the MTJ region with age. This coincided with an increase in proximal tendon length (31%), as well as an increase in collagen deposition between 6 and 24-months of age consistent with an expansion of the fibre termination area. These findings uncover a previously undescribed effect of ageing on the MTJ and open up new lines of investigation into the role of this structure in the age-related loss of muscle function.

A DIC Based Technique to Measure the Contraction of a Skeletal Muscle Engineered Tissue

Tissue engineering is a multidisciplinary science based on the application of engineering approaches to biologic tissue formation. Engineered tissue internal organization represents a key aspect to increase biofunctionality before transplant and, as regarding skeletal muscles, the potential of generating contractile forces is dependent on the internal fiber organization and is reflected by some macroscopic parameters, such as the spontaneous contraction. Here we propose the application of digital image correlation (DIC) as an independent tool for an accurate and noninvasive measurement of engineered muscle tissue spontaneous contraction. To validate the proposed technique we referred to the X-MET, a promising 3-dimensional model of skeletal muscle. The images acquired through a high speed camera were correlated with a custom-made algorithm and the longitudinal strain predictions were employed for measuring the spontaneous contraction. The spontaneous contraction reference values were obtained by studying the force response. The relative error between the spontaneous contraction frequencies computed in both ways was always lower than 0.15%. In conclusion, the use of a DIC based system allows for an accurate and noninvasive measurement of biological tissues' spontaneous contraction, in addition to the measurement of tissue strain field on any desired region of interest during electrical stimulation.

A multiscale chemo-electro-mechanical skeletal muscle model to analyze muscle contraction and force generation for different muscle fiber arrangements

The presented chemo-electro-mechanical skeletal muscle model relies on a continuum-mechanical formulation describing the muscle's deformation and force generation on the macroscopic muscle level. Unlike other three-dimensional models, the description of the activation-induced behavior of the mechanical model is entirely based on chemo-electro-mechanical principles on the microscopic sarcomere level. Yet, the multiscale model reproduces key characteristics of skeletal muscles such as experimental force-length and force-velocity data on the macroscopic whole muscle level. The paper presents the methodological approaches required to obtain such a multiscale model, and demonstrates the feasibility of using such a model to analyze differences in the mechanical behavior of parallel-fibered muscles, in which the muscle fibers either span the entire length of the fascicles or terminate intrafascicularly. The presented results reveal that muscles, in which the fibers span the entire length of the fascicles, show lower peak forces, more dispersed twitches and fusion of twitches at lower stimulation frequencies. In detail, the model predicted twitch rise times of 38.2 and 17.2 ms for a 12 cm long muscle, in which the fibers span the entire length of the fascicles and with twelve fiber compartments in series, respectively. Further, the twelve-compartment model predicted peak twitch forces that were 19% higher than in the single-compartment model. The analysis of sarcomere lengths during fixed-end single twitch contractions at optimal length predicts rather small sarcomere length changes. The observed lengths range from 75 to 111% of the optimal sarcomere length, which corresponds to a region with maximum filament overlap. This result suggests that stability issues resulting from activation-induced stretches of non-activated sarcomeres are unlikely in muscles with passive forces appearing at short muscle length.

Physiology and metabolism of tissue-engineered skeletal muscle

Skeletal muscle is a major target for tissue engineering, given its relative size in the body, fraction of cardiac output that passes through muscle beds, as well as its key role in energy metabolism and diabetes, and the need for therapies for muscle diseases such as muscular dystrophy and sarcopenia. To date, most studies with tissue-engineered skeletal muscle have utilized murine and rat cell sources. On the other hand, successful engineering of functional human muscle would enable different applications including improved methods for preclinical testing of drugs and therapies. Some of the requirements for engineering functional skeletal muscle include expression of adult forms of muscle proteins, comparable contractile forces to those produced by native muscle, and physiological force-length and force-frequency relations. This review discusses the various strategies and challenges associated with these requirements, specific applications with cultured human myoblasts, and future directions.

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.

Laminin-211 in skeletal muscle function

A chain is no stronger than its weakest link is an old idiom that holds true for muscle biology. As the name implies, skeletal muscle's main function is to move the bones. However, for a muscle to transmit force and withstand the stress that contractions give rise to, it relies on a chain of proteins attaching the cytoskeleton of the muscle fiber to the surrounding extracellular matrix. The importance of this attachment is illustrated by a large number of muscular dystrophies caused by interruption of the cytoskeletal-extracellular matrix interaction. One of the major components of the extracellular matrix is laminin, a heterotrimeric glycoprotein and a major constituent of the basement membrane. It has become increasingly apparent that laminins are involved in a multitude of biological functions, including cell adhesion, differentiation, proliferation, migration and survival. This review will focus on the importance of laminin-211 for normal skeletal muscle function.

[Muscles and connective tissue: histology]

Here, we give some comments about the DVD movies "Muscle Attitudes" from Endovivo productions, the movies up lighting some loss in the attention given to studies on the connective tissue, and especially them into muscles. The main characteristics of the different components in the intra-muscular connective tissue (perimysium, endomysium, epimysium) are shown here with special references to their ordered architecture and special references to their spatial distributions. This connective tissue is abundant into the muscles and is in continuity with the muscles in vicinity, with their tendons and their sheath, sticking the whole on skin. This connective tissue has also very abundant connections on the muscles fibres. It is then assumed that the connective tissue sticks every organs or cells of the locomotion system. Considering the elastic properties of the collagen fibres which are the most abundant component of connective tissue, it is possible to up light a panel of connective tissue associated functions such as the transmission of muscle contractions or the regulation of protein and energetic muscles metabolism.

Structure and function of the skeletal muscle extracellular matrix

The skeletal muscle extracellular matrix (ECM) plays an important role in muscle fiber force transmission, maintenance, and repair. In both injured and diseased states, ECM adapts dramatically, a property that has clinical manifestations and alters muscle function. Here we review the structure, composition, and mechanical properties of skeletal muscle ECM; describe the cells that contribute to the maintenance of the ECM; and, finally, overview changes that occur with pathology. New scanning electron micrographs of ECM structure are also presented with hypotheses about ECM structure–function relationships. Detailed structure–function relationships of the ECM have yet to be defined and, as a result, we propose areas for future study.

Constraints for control of the human hand

More than 30 muscles drive the hand to perform a multitude of essential dextrous tasks. Here we consider new views on the evolution of hand structure and on peripheral and central constraints for independent control of the digits of the hand. The human hand is widely assumed to have evolved from hands like those of African apes, yet recent studies have shown that our hands and those of the earliest hominids are very similar and unlike those of living apes. Understanding the limits of hand function may come from investigation of our last common ancestor with the great apes, rather than the great apes themselves. In the periphery, movement across the full range of joint space can be limited by mechanical linkages among the extrinsic muscles. Further, peripheral limits occur when the hand adopts some positions in which the contraction of muscles fails to move the joints on which they usually act; there is muscle 'disengagement' and functional paralysis for some actions. Surprisingly, the central nervous system drives the hand seamlessly through this landscape of mechanical limits. Central constraints on control of the individual digits include the spillover of neural drive to neighbouring muscles and their 'compartments', and the inability to make maximal muscle forces when multiple digits contract strongly which produces a force deficit. The pattern of these latter constraints correlates with amounts of daily use of each digit and favours enslaved extension to lift fingers from an object but selective flexion of fingers to contact it.

Fine structure of myotendinous junction between the anterior belly of the digastric muscle and intermediate tendon in adults rats

This study analyzed the ultrastructural characteristics of the myotendinous junction (MTJ) between anterior belly of digastrics muscle and the intermediate tendon in adult rats. Six male Wistar rats were used and were anesthetized with an overdose of urethane and sacrificed by intracardiac perfusion with modified Karnovsky solution, postfixed in 1% osmium tetroxide, dehydrated in increasing series of alcohols and embedded in Spurr resin for transmission electron microscopic analysis. Ultrastructural analysis showed conical shape of the fiber extremity in MTJ region, highlighting the presence of numerous mitochondria arranged in groups in the subsarcolemmal and intermyofibrillary regions. Atypical MTJ characteristics were seen interspersed with bundles of collagen fibers. Classic characteristics such as finger-like processes by means of sarcoplasmic projections were observed among interdigitations. Terminals and periphericals bundles of myofibrils showed close relationship with the adjacent muscle fiber's endomysium through lateral junctions. In the distal portion, it was observed that the communication region of microtendons forming the intermediate tendon of digastric muscle, and it can highlight the columns disposition of tenocytes. In conclusion, the MTJ ultrastructure between the anterior belly of digastric muscle and intermediate tendon of adult rats showed classical morphologic descriptions and presented an atypical region revealed by the subspecialization between the myofibrils bundles and collagen fibers in the MTJ region.

A mathematical model of force transmission from intrafascicularly terminating muscle fibers

Many long skeletal muscles are comprised of fibers that terminate intrafascicularly. Force from terminating fibers can be transmitted through shear within the endomysium that surrounds fibers or through tension within the endomysium that extends from fibers to the tendon; however, it is unclear which pathway dominates in force transmission from terminating fibers. The purpose of this work was to develop mathematical models to (i) compare the efficacy of lateral (through shear) and longitudinal (through tension) force transmission in intrafascicularly terminating fibers, and (ii) determine how force transmission is affected by variations in the structure and properties of fibers and the endomysium. The models demonstrated that even though the amount of force that can be transmitted from an intrafascicularly terminating fiber is dependent on fiber resting length (the unstretched length at which passive stress is zero), endomysium shear modulus, and fiber volume fraction (the fraction of the muscle cross-sectional area that is occupied by fibers), fibers that have values of resting length, shear modulus, and volume fraction within physiologic ranges can transmit nearly all of their peak isometric force laterally through shearing of the endomysium. By contrast, the models predicted only limited force transmission ability through tension within the endomysium that extends from the fiber to the tendon. Moreover, when fiber volume fraction decreases to unhealthy ranges (less than 50%), the force-transmitting potential of terminating fibers through shearing of the endomysium decreases significantly. The models presented here support the hypothesis that lateral force transmission through shearing of the endomysium is an effective mode of force transmission in terminating fibers.

Motor nucleus activity fails to predict extraocular muscle forces in ocular convergence

For a given eye position, firing rates of abducens neurons (ABNs) generally (Mays et al. 1984), and lateral rectus (LR) motoneurons (MNs) in particular (Gamlin et al. 1989a), are higher in converged gaze than when convergence is relaxed, whereas LR and medial rectus (MR) muscle forces are slightly lower (Miller et al. 2002). Here, we confirm this finding for ABNs, report a similarly paradoxical finding for neurons in the MR subdivision of the oculomotor nucleus (MRNs), and, for the first time, simultaneously confirm the opposing sides of these paradoxes by recording physiological LR and MR forces. Four trained rhesus monkeys with binocular eye coils and custom muscle force transducers on the horizontal recti of one eye fixated near and far targets, making conjugate saccades and symmetric and asymmetric vergence movements of 16-27°. Consistent with earlier findings, we found in 44 ABNs that the slope of the rate-position relationship for symmetric vergence (k(V)) was lower than that for conjugate movement (k(C)) at distance, i.e., mean k(V)/k(C) = 0.50, which implies stronger LR innervation in convergence. We also found in 39 MRNs that mean k(V)/k(C) = 1.53, implying stronger MR innervation in convergence as well. Despite there being stronger innervation in convergence at a given eye position, we found both LR and MR muscle forces to be slightly lower in convergence, -0.40 and -0.20 g, respectively. We conclude that the relationship of ensemble MN activity to total oculorotary muscle force is different in converged gaze than when convergence is relaxed. We conjecture that LRMNs with k(V) < k(C) and MRMNs with k(V) > k(C) innervate muscle fibers that are weak, have mechanical coupling that attenuates their effective oculorotary force, or serve some nonoculorotary, regulatory function.

The role of extracellular matrix composition in structure and function of bioengineered skeletal muscle

One of the obstacles to the potential clinical utility of bioengineered skeletal muscle is its limited force generation capacity. Since engineered muscle, unlike most native muscle tissue, is composed of relatively short myofibers, we hypothesized that, its force production and transmission would be profoundly influenced by cell-matrix interactions. To test this hypothesis, we systematically varied the matrix protein type (collagen I/fibrin/Matrigel) and concentration in engineered, hydrogel-based neonatal rat skeletal muscle bundles and assessed the resulting tissue structure, generation of contractile force, and intracellular Ca(2+) handling. After two weeks of culture, the muscle bundles consisted of highly aligned and cross-striated myofibers and exhibited standard force-length and force-frequency relationships achieving tetanus at 40 Hz. The use of 2 mg/ml fibrin (control) yielded isometric tetanus amplitude of 1.4 ± 0.3 mN as compared to 0.9 ± 0.4 mN measured in collagen I-based bundles. Higher fibrin and Matrigel concentrations synergistically yielded further increase in active force generation to 2.8 ± 0.5 mN without significantly affecting passive mechanical properties, tetanus-to-twitch ratio, and twitch kinetics. Optimized matrix composition yielded significant cellular hypertrophy (protein/DNA ratio = 11.4 ± 4.1 vs. 6.5 ± 1.9 μg/μg in control) and a prolonged Ca(2+) transient half-width (Ca(50) = 232.8 ± 33.3 vs. 101.7 ± 19.8 ms). The use of growth-factor-reduced Matrigel, instead of standard Matrigel did not alter the obtained results suggesting enhanced cell-matrix interactions rather than growth factor supplementation as an underlying cause for the measured increase in contractile force. In summary, biomaterial-based manipulation of cell-matrix interactions represents an important target for improving contractile force generation in engineered skeletal muscle.

The innervation and organization of motor units in a series-fibered human muscle: the brachioradialis

We studied the innervation and organization of motor units in the brachioradialis muscle of 25 normal human subjects. We recorded intramuscular EMG signals at points separated by 15 mm along the proximodistal muscle axis during moderate isometric contractions, identified from 27 to 61 (mean 39) individual motor units per subject using EMG decomposition, and estimated the locations of the endplates and distal muscle/tendon junctions from the motor-unit action potential (MUAP) propagation patterns and terminal standing waves. In three subjects all the motor units were innervated in a single endplate zone. In the other 22 subjects, the motor units were innervated in 3-6 (mean 4) distinct endplate zones separated by 15-55 mm along the proximodistal axis. One-third of the motor units had fibers innervated in more than one zone. The more distally innervated motor units had distinct terminal waves indicating tendonous termination, while the more proximal motor units lacked terminal waves, indicating intrafascicular termination. Analysis of blocked MUAP components revealed that 19% of the motor units had at least one doubly innervated fiber, i.e., a fiber innervated in two different endplate zones by two different motoneurons, and thus belonging to two different motor units. These results are consistent with the brachioradialis muscle having a series-fibered architecture consisting of multiple, overlapping bands of muscle fibers in most individuals and a simple parallel-fibered architecture in some individuals.

Investigation of neuromuscular abnormalities in neurotrophin-3-deficient mice

Neurotrophin-3 (NT-3) is a trophic factor that is essential for the normal development and maintenance of proprioceptive sensory neurons and is widely implicated as an important modulator of synaptic function and development. We have previously found that animals lacking NT-3 have a number of structural abnormalities in peripheral nerves and skeletal muscles. Here we investigated whether haploinsufficiency-induced reduction in NT-3 resulted in impaired neuromuscular performance and synaptic function. Motor nerve terminal function was tested by monitoring the uptake/release of the fluorescent membrane dye FM1-43 by the electrophysiological examination of synaptic transmission and electron microscopic determination of synaptic vesicle density at the presynaptic active zone. We investigated skeletal muscle form and function by measuring force in response to both nerve-mediated and direct muscle stimulation and by quantification of fiber number and area from transverse sections. Synaptic transmission was not markedly different between the two groups, although the uptake and release of FM1-43 were impaired in mature NT-3-deficient mice but not in immature mice. The electron microscopic examination of mature nerve terminals showed no genotype-dependent variation in the number of synaptic vesicles near the active zone. NT-3(+/-) mice had normal soleus muscle fiber numbers but their fibers had smaller cross-sectional areas and were more densely-packed than wild-type littermates. Moreover, the muscles of adult NT-3-deficient animals were weaker than those of wild-type animals to both nerve and direct muscle stimulation. The results indicate that a reduction in NT-3 availability during development impairs motor nerve terminal maturation and synaptic vesicle recycling and leads to a reduction in muscle fiber diameter.

No oculomotor plant, no final common path

The assumption that there is an oculomotor plant, a fixed relationship between motoneuron firing rate and eye position, is disproved by brainstem recording studies showing that this relationship depends on which supernuclear subsystem determines firing rate. But it remains possible that there is a final common path (FCP), a fixed relationship between firing rate and muscle force. But then, brainstem recording studies predict that lateral rectus (LR) forces (and probably medial rectus (MR) forces, as well) will be higher in converged than in unconverged gaze for a given eye position. We recently measured these forces and found that they are slightly lower in convergence, disproving the FCP hypothesis. Thus, even the relationship between motoneuron firing rate and muscle force is under supernuclear control. What peripheral oculomotor articulations could vary the relationship of firing rate to muscle force?: (1) Actively movable EOM pulleys could alter oculorotary muscle force for a given oculorotory innervation by altering muscle lengths. (2) 'Outer' motoneurons may function as gamma efferents in conjunction with palisade endings and non-twitch global EOM fibers. (3) Complex nonlinear interactions likely arise among both parallel and serially connected muscle fibers.

Preliminary observations on the microarchitecture of the human abdominal muscles

Precise knowledge of muscle architecture and innervation patterns is essential for the development of accurate clinical and biomechanical models. Although the gross anatomy of the human abdominal muscles has been investigated, the finer details of their microanatomy are not well described. Fascicles were systematically sampled from each of the human abdominal muscles, and small fiber bundles from selected fascicles stained with acetylcholinesterase to determine the location of motor endplate bands, myomyonal junctions, and myotendinous junctions. Statistical analysis was used to ascertain the association between fascicular length and number of endplate bands. The number of endplate bands along a fascicle was variable between different portions of each muscle, but was strongly correlated with fascicular length (r = 0.814). In fascicles less than 50 millimeters (mm) in length, only a single endplate band was generally present, while multiple endplate bands (usually two or three) were found in fascicles longer than 50 mm. The presence of myomyonal junctions throughout the longer (>50 mm) fascicles verified that they were composed of short, intrafascicularly terminating fibers, while shorter fascicles comprised fibers spanning the entire fascicular length. This preliminary study provides evidence that multiple endplate bands are contained in some regions of the abdominal muscles, an arrangement that differs from most human appendicular muscles. It is not clear whether the variations in the described fine architectural features reflect regional differences in muscle function.

Electrophysiological evidence of doubly innervated branched muscle fibers in the human brachioradialis muscle

OBJECTIVE

Motor-unit action potentials (MUAPs) with unstable satellite (late-latency) components are found in EMG signals from the brachioradialis muscles of normal subjects. We analyzed the morphology and blocking behavior of these MUAPs to determine their anatomical origin.

METHODS

EMG signals were recorded from the brachioradialis muscles of 5 normal subjects during moderate-level isometric contractions. MUAP waveforms, discharge patterns, and blocking were determined using computer-aided EMG decomposition.

RESULTS

Twelve MUAPs with unstable satellite potentials were detected, always two together in the same signal. Each MUAP also had a second unstable component associated with its main spike. The blocking behavior of the unstable components depended on how close together the two MUAPs were when they discharged.

CONCLUSIONS

The latencies and blocking behavior indicate that the unstable components came from branched muscle fibers innervated by two different motoneurons. The satellite potentials were due to action potentials that traveled to the branching point along one branch and back along the other. The blockings were due to action-potential collisions when both motoneurons discharged close together in time.

SIGNIFICANCE

Animal studies suggest that branched muscle fibers may be a normal characteristic of series-fibered muscles. This study adds to our understanding of these muscles in humans.

Neuromuscular organization of the superior longitudinalis muscle in the human tongue. 1. Motor endplate morphology and muscle fiber architecture

Proper tongue function is essential for respiration and mastication, yet we lack basic information on the anatomical organization underlying human tongue movement. Here we use microdissection, acetylcholinesterase histochemistry, silver staining of nerves, alpha bungarotoxin binding and immunohistochemistry to describe muscle fiber architecture and motor endplate (MEP) distribution of the human superior longitudinalis muscle (SL). The human SL extends from tongue base to tongue tip and is composed of fiber bundles that range from 2.8 to 15.7 mm in length. Individual muscle fibers of the SL range from 1.2 to 17.3 mm in length (1.3-18.2% of muscle length). Seventy-one percent of SL fibers have blunt-blunt terminations; the remainder have blunt-taper terminations. Multiple MEPs are present along SL length and dual MEPs are present on some muscle fibers. These data demonstrate that the human SL is a muscle of "in-series" design. We suggest that SL motor units are organized to innervate specific regions of the tongue body and that activation of SL motor units according to anteroposterior location is one strategy employed by the nervous system to control tongue shape and tongue movement.

Muscle fiber and motor unit behavior in the longest human skeletal muscle

The sartorius muscle is the longest muscle in the human body. It is strap-like, up to 600 mm in length, and contains five to seven neurovascular compartments, each with a neuromuscular endplate zone. Some of its fibers terminate intrafascicularly, whereas others may run the full length of the muscle. To assess the location and timing of activation within motor units of this long muscle, we recorded electromyographic potentials from multiple intramuscular electrodes along sartorius muscle during steady voluntary contraction and analyzed their activity with spike-triggered averaging from a needle electrode inserted near the proximal end of the muscle. Approximately 30% of sartorius motor units included muscle fibers that ran the full length of the muscle, conducting action potentials at 3.9 +/- 0.1 m/s. Most motor units were innervated within a single muscle endplate zone that was not necessarily near the midpoint of the fiber. As a consequence, action potentials reached the distal end of a unit as late as 100 ms after initiation at an endplate zone. Thus, contractile activity is not synchronized along the length of single sartorius fibers. We postulate that lateral transmission of force from fiber to endomysium and a wide distribution of motor unit endplates along the muscle are critical for the efficient transmission of force from sarcomere to tendon and for the prevention of muscle injury caused by overextension of inactive regions of muscle fibers.

Structural organization of the perimysium in bovine skeletal muscle: Junctional plates and associated intracellular subdomains

We analyzed the structural features of the perimysium collagen network in bovine Flexor carpi radialis muscle using various sample preparation methods and microscopy techniques. We first observed by scanning electron microscopy that perimysium formed a regular network of collagen fibers with three hierarchical levels including (i) a loose lattice of large interwoven fibers ramified in (ii) numerous collagen plexi attaching together adjacent myofibers at the level of (iii) specific structures that we call perimysial junctional plates. Second, we looked more closely at the intracellular organization underneath each plate using transmission electron microscopy, immunohistochemistry, and a three-dimensional reconstruction from serial sections. We observed the accumulation of myonuclei arranged in clusters surrounded by a high density of subsarcolemmal mitochondria and the proximity of capillary branches. Third, we analyzed the distribution of these perimysial junctional plates, subsarcolemmal mitochondria, and myonuclei clusters along the myofibers using a statistical analysis of the distances between these structures. This revealed a global colocalization and the existence of adhesion domains between endomysium and perimysium. Taken together, our observations give a better description of the perimysium organization in skeletal muscle, and provide evidence that perimysial junctional plates with associated intracellular subdomains may participate in the lateral transmission of contractile forces as well as mechanosensing.

Hamstring Muscles: Architecture and Innervation

Knowledge of the anatomical organization of the hamstring muscles is necessary to understand their functions, and to assist in the development of accurate clinical and biomechanical models. The hamstring muscles were examined by dissection in six embalmed human lower limbs with the purpose of clarifying their gross morphology. In addition to obtaining evidence for or against anatomical partitioning (as based on muscle architecture and pattern of innervation), data pertaining to architectural parameters such as fascicular length, volume, physiological cross-sectional area, and tendon length were collected. For each muscle, relatively consistent patterns of innervation were identified between specimens, and each was unique with respect to anatomical organization. On the basis of muscle architecture, three regions were identified within semimembranosus. However, this was not completely congruent with the pattern of innervation, as a primary nerve branch supplied only two regions, with the third region receiving a secondary branch. Semitendinosus comprised two distinct partitions arranged in series that were divided by a tendinous inscription. A singular muscle nerve or a primary nerve branch innervated each partition. In the biceps femoris long head the two regions were supplied via a primary nerve branch which divided into two primary branches or split into a series of branches. Being the only muscle to cross a single joint, biceps femoris short head consisted of two distinct regions demarcated by fiber direction, with each innervated by a separate muscle nerve. Architecturally, each muscle differed with respect to parameters such as physiological cross-sectional area, fascicular length and volume, but generally all partitions within an individual muscle were similar in fascicular length. The long proximal and distal tendons of these muscles extended into the muscle bellies thereby forming elongated musculotendinous junctions.

Development of a mammalian series-fibered muscle

This study examines the processes by which multiply innervated, serially fibered mammalian muscles are constructed during development. We previously reported that primary myotubes of such a muscle, the guinea pig sternomastoid muscle, span from tendon to tendon and are innervated at each of the muscle's four innervation zones. Secondary myotubes form later, in association with each point of innervation (Duxson and Sheard, Dev. Dyn., 1995; 204:391-405). We now describe the further growth and development of the muscle. Secondary myotubes initially insert onto and grow along the primary myotube. However, as they reach a critical length, they encounter other secondary myotubes growing from serially adjacent innervation zones and may transfer their attachment(s) to these serially positioned secondary myotubes. Other secondary myotubes maintain attachment at one or both ends to their primary myotube. Thus, an interconnected network of primary and secondary myotubes is formed. Patterns of reactivity for cell adhesion molecules suggest that early attachment points between myotubes are the embryonic precursors of adult myomyonal junctions, characterized by the expression of alpha7Bbeta1 integrin. Finally, the results show that secondary myotubes positioned near a tendon are generally longer than those lying in the mid belly of the muscle, and we suggest that the environment surrounding the tendinous zone may somehow stimulate myotube growth.

Increased jitter and blocking in normal muscles due to doubly innervated muscle fibers

Increased jitter and intermittent impulse blocking in electromyographic (EMG) signals are considered evidence of transmission abnormality and are not usually associated with normal muscle. However, motor unit action potentials (MUAPs) that exhibit increased jitter and blocking have recently been shown to occur in the brachioradialis muscles of neurologically healthy subjects. The jitter and blocking result from collisions, refractoriness, and conduction-velocity variability in long muscle fibers that are innervated by two different motoneurons at widely separated endplates. We analyzed MUAPs obtained by decomposing EMG signals from the brachioradialis muscles of four normal subjects. The rate of blocking of some MUAP components was as high as 28%, the jitter between some components exceeded 300 micros (mean consecutive difference), and the mean incidence of irregular MUAPs was 14%. These values would be considered abnormal in many other muscles. Jitter from doubly innervated fibers can be distinguished from other types of pathological jitter because one component exhibits amplitude variability. Clinical neurophysiologists should be aware that increased jitter and blocking do not necessarily indicate pathology in brachioradialis and perhaps other long, parallel-fibered muscles.

Intramuscular force transmission

The architectural form of skeletal muscle, the pattern of activity/usage between neighbouring fibres, and the pathways for lateral and lengthwise tension delivery are all of interest in understanding muscle function and dysfunction. We have attempted to contribute to understanding of intramuscular force transmission by investigating the functional relationships between coactive motor units, and by examining the detailed molecular and morphological features at sites of tension transfer. We found that tension delivery is modulated by interaction between active and inactive fibres, that many muscle fibre terminations feature structural coupling between fibres, and that sites of tension delivery feature a variety of proteins including acetylcholinesterase, NCAM, dystrophin and two splice variants of the alpha7 integrins. We conclude that structural and molecular pathways exist to deliver force within, along, and between muscle fibres, and that the quality/quantity of tension delivered from any single fibre is at least partly a consequence of whether its neighbouring fibres are synchronously coactive.

Musculoskeletal mechanics: a foundation of motor physiology

The design of the musculoskeletal system has always been a major consideration in the interpretation of experiments on the motor system. However, as motor physiology progresses toward a more comprehensive picture of motor behaviour, the study of the musculoskeletal system has of necessity, and of interest, come to depend more and more on the quantitative methods of biomechanics. Biomechanical studies have led to new hypotheses about the design of the motor system and biomechanical considerations have provided important tests of existing hypotheses concerning the neural control of movement. These hypotheses include global issues such as redundancy and encoded variables as well as specific hypotheses such as Stiffness Regulation, Selective Recruitment and the concept of Flexor Reflex Afferents.

Muscle regeneration in amphibians and mammals: passing the torch

Skeletal muscle in both amphibians and mammals possesses a high regenerative capacity. In amphibians, a muscle can regenerate in two distinct ways: as a tissue component of an entire regenerating limb (epimorphic regeneration) or as an isolated entity (tissue regeneration). In the absence of epimorphic regenerative ability, mammals can regenerate muscles only by the tissue mode. This review focuses principally on the regeneration of entire muscles and covers what is known and what remains to be elucidated about fundamental mechanisms underlying muscle regeneration at this level.

Adaptations in skeletal muscle disuse or decreased-use atrophy

Those factors that seem to play some role in inducing adaptations of skeletal muscle in vivo are discussed. The role of myogenesis in maintaining and repairing muscle during atrophic and hypertrophic states is discussed, including pointing out that the modulation of myonuclear number is one means of adapting to varying chronic levels of neuromuscular activity. Finally, we point out the potential consequences of muscle atrophy on the control of movement and the susceptibility to fatigue.

High frequency oscillations in respiratory networks: functionally significant or phenomenological?

Inspiratory activities, whether recorded from medullary neurons, motoneurons or motor nerves, feature prominent oscillations in high (50-120 Hz) and medium (15-50 Hz) frequency ranges. These oscillations have been extensively characterized and are considered signatures of respiratory network activity. Their functional significance, however, if any, remains unknown. Here we review the literature describing the nature and origin of these oscillations as well as their modulation during development and by mechanoreceptive and chemoreceptive feedback, respiratory- and non-respiratory-related behaviors, temperature and anesthesia. We then consider the potential significance of these oscillations for respiratory network function by drawing on analyses of distributed motor and sensory networks of the cortex where current interest in oscillatory activity, and the synchronization of neural discharge that can result, is based on the increased efficacy with which synchronous inputs influence neuronal output, and the role that synchronous activity may play in information coding. We speculate that synchronized oscillations at the network level help coordinate activity in distributed rhythm and pattern generating systems and at the muscle level enhance force development. Data most strongly support that oscillatory synaptic inputs play an important role in controlling timing and pattern of action potential output.

Different modes of hypertrophy in skeletal muscle fibers

Skeletal muscles display a remarkable diversity in their arrangement of fibers into fascicles and in their patterns of innervation, depending on functional requirements and species differences. Most human muscle fascicles, despite their great length, consist of fibers that extend continuously from one tendon to the other with a single nerve endplate band. Other mammalian muscles have multiple endplate bands and fibers that do not insert into both tendons but terminate intrafascicularly. We investigated whether these alternate structural features may dictate different modes of cell hypertrophy in two mouse gracilis muscles, in response to expression of a muscle-specific insulin-like growth factor (IGF)-1 transgene (mIGF-1) or to chronic exercise. Both hypertrophic stimuli independently activated GATA-2 expression and increased muscle cross-sectional area in both muscle types, with additive effects in exercising myosin light chain/mIGF transgenic mice, but without increasing fiber number. In singly innervated gracilis posterior muscle, hypertrophy was characterized by a greater average diameter of individual fibers, and centralized nuclei. In contrast, hypertrophic gracilis anterior muscle, which is multiply innervated, contained longer muscle fibers, with no increase in average diameter, or in centralized nuclei. Different modes of muscle hypertrophy in domestic and laboratory animals have important implications for building appropriate models of human neuromuscular disease.

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 length affects the architecture and pattern of innervation differently in leg muscles of mouse, guinea pig, and rabbit compared to those of human and monkey muscles

The innervation pattern and fascicular anatomy of muscles of different lengths in mouse, guinea pig, rabbit, macaque monkey and human legs were analyzed. Neuromuscular junctions, muscle tendon junctions and ends of intrafascicularly terminating fibers were stained for acetylcholinesterase, and fascicle lengths measured. A high correlation between increasing fascicle length and increasing number of neuromuscular junctions was found, with non-primate (mouse, guinea pig, rabbit) and primate (macaque monkey, human) muscles forming two discrete groups. In non-primates, muscles with a single endplate band, fascicles were always shorter than 35 mm, fixing the limit of fiber length served by one neuromuscular junction. Muscles with fascicles longer than this had multiple discrete bands of motor endplates crossing their width at regular intervals. An increase in muscle length across or within species corresponded to an equivalent, standard increase of 10-12 mm fascicle length per motor endplate band. All human and monkey leg muscles, with the exception of gracilis and sartorius, were singly innervated and all muscle fibers ran the full distance from tendon to tendon. Singly innervated primate muscle fibers were up to 140 mm long whereas the mean distance between endplate bands in the two multiply innervated muscles was also considerably greater than in non-primates. These data indicate that allometric effects of increasing fascicle length, are distinct in common laboratory animals and two primates, when architecture and pattern of innervation are compared.