Alan J. Sokoloff

Neuromuscular Organization of the Superior Longitudinalis Muscle in the Human Tongue

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.

Orderly Recruitment Among Motoneurons Supplying Different Muscles

Virtually all movements involve the recruitment of motor units from multiple muscles. Given the functional diversity of motor units (motoneurons and the muscle fibers they supply), the effective production of specific movements undoubtedly depends upon some principle(s) to organize the ensemble of active motor units. The principle acting to organize the recruitment of motor units within muscles is the size principle, whereby the first motor units to be recruited have the smallest values for axonal conduction velocity and contractile force, and are the slowest to contract and fatigue. Here we consider the possibility that the size principle applies in the recruitment of motor units across muscles, i.e., that regardless of their muscles of origin, active motor units are recruited in rank order, for example, from low to high conduction velocity. The benefits of orderly recruitment across muscles could be similar to the acknowledged advantages of orderly recruitment within muscles. One benefit is that the neural process involved in organizing active motor units would be simplified. In a muscle-based scheme, the size principle would organize only those motor units within individual muscles, leaving the nervous system with the additional task of coordinating the relative activities of motor units from different muscles. By contrast, in an ensemble-based scheme, orderly recruitment of all motor units according to the size principle would automatically coordinate motor units both within and across motor nuclei. Another potential benefit is the provision for movements with smooth trajectory, the result of interleaving the divergent torque contributions made by motor units from muscles that differ in their orientations about joints. Otherwise, if order were restricted within muscles, the torque trajectory of a joint would change unevenly as participating muscles begin contracting at different times and grade activity at different rates. These considerations support speculation that motor units recruited from co-contracting muscles are collectively recruited according to the size principle.

Limited expression of slow tonic myosin heavy chain in human cranial muscles

Recent reports of slow tonic myosin heavy chain (MHCst) in human masticatory and laryngeal muscles suggest that MHCst may have a wider distribution in humans than previously thought. Because of the novelty of this finding, we sought to confirm the presence of MHCst in human masticatory and laryngeal muscles by reacting tissue from these muscles and controls from extraocular, intrafusal, cardiac, appendicular, and developmental muscle with antibodies (Abs) ALD‐58 and S46, considered highly specific for MHCst. At Ab dilutions producing minimal reaction to muscle fibers positive for MHCI, only extraocular, intrafusal, and fetal tongue tissue reacted with Ab S46 had strong immunoreaction in an appreciable number of muscle fibers. In immunoblots, Ab S46, but not Ab ALD‐58, labeled adult extraocular muscles; no other muscles were labeled with either Ab. We conclude that, in humans, Ab S46 has greater specificity for MHCst than does Ab ALD‐58. We suggest that reports of MHCst in human masticatory and laryngeal muscles reflect false‐positive identification of MHCst due to cross‐reactivity of Ab ALD‐58 with another MHC isoform. Muscle Nerve, 2007

Orderly recruitment tested across muscle boundaries.

Publisher Summary The movements and postures of limb and body segments are achieved through partial activation of multiple muscles. In the cat, multiple muscles are coactive during a variety of movements, such as treadmill locomotion, landing, paw shake, head turning, and in humans, during movements such as hand grip and elbow flexion. There are tens to hundreds of heteronymous motor units belonging to different muscles, which are recruited together during discrete motor actions. This chapter discusses whether recruitment is orderly among heteronymous motor units that are activated together. Recruitment order can be assessed in relation to the motoneuron portion of the motor unit. Alpha motoneurons within a motor nucleus (the collection of motoneurons supplying a single muscle) are progressively recruited from slow to fast axonal conduction velocity. Recruitment that is rank-ordered by these properties is commonly referred to as “recruitment in order” by the size principle. The chapter describes the recruitment order with respect to either a muscle and its motor units or a motor nucleus and its motoneurons or motor axons.

Changes in growth-related kinases in head, neck and limb muscles with age.

Sarcopenia coincides with declines in several systemic processes that signal through the MAP kinase and Akt-mTOR-p70S6k cascades typically associated with muscle growth. Effects of aging on these pathways have primarily been examined in limb muscles, which experience substantial activity and neural changes in addition to systemic hormonal and metabolic changes. Head and neck muscles are reported to undergo reduced sarcopenia and disuse with age relative to limb muscles, suggesting muscle activity may contribute to maintaining mass with age. However many head and neck muscles derive from embryonic branchial arches, rather than the somites from which limb muscles originate, suggesting that developmental origin may be important. This study compares the expression and phosphorylation of MAP kinase and mTOR networks in head, neck, tongue, and limb muscles from 8- and 26-month old F344 rats to test the hypothesis that physical activity and developmental origin contribute to preservation of muscle mass with age. Phosphorylation of p38 was exaggerated in aged branchial arch muscles. Phosphorylation of ERK and p70S6k T421/S424 declined with age only in the biceps brachii. Expression of p70S6k declined in all head and neck, tongue and limb muscles although no change in phosphorylation of p70S6k on T389 could be resolved. A systemic change that results in a loss of p70S6k protein expression may reduce the capacity to respond to acute hypertrophic stimuli, while the exaggerated p38 signaling in branchial arch muscles may reflect more active muscle remodeling.

Sarcomeric Myosin Expression in the Tongue Body of Humans, Macaques and Rats

Expression of developmental and unconventional myosin heavy chain (MHC) isoforms in some adult head and neck muscles is thought to reflect specific contractile demands of muscle fibers active during kinematically complex movements. Mammalian tongue muscles are active during oromotor behaviors that encompass a wide range of tongue movement speeds and tongue shape changes (e.g. respiration, oral transport, swallowing, rejection), but the extent to which tongue muscles express developmental and unconventional MHC is not known. Quantitative PCR was used to determine the mRNA content of conventional MHC-beta, MHC-2a, MHC-2b and MHC-2x, the developmental isoforms embryonic MHC and neonatal MHC and the unconventional isoforms atrial/cardiac-α MHC (MHC-alpha), extraocular MHC, masseter MHC and slow tonic MHC in tongue body muscles of the rat, macaque and human. In all species, conventional MHC isoforms predominate. MHC-2b and MHC-2x account for 98% of total MHC mRNA in the rat. MHC-2a, MHC-2x and MHC-beta account for 94% of total MHC mRNA in humans and 96% of total MHC mRNA in macaque. With the exception of MHC-alpha in humans (5%), developmental and unconventional MHC mRNA represents less than 0.3% of total MHC mRNA. We conclude that in these species, there is limited expression of developmental and unconventional MHC and that diversity of tongue body muscle fiber contractile properties is achieved primarily by MHC-beta, MHC-2a, MHC-2x and MHC-2b. Whether expression of MHC-alpha mRNA in tongue is unique to humans or present in other hominoids awaits further investigation.

Tongue implant for assistive technologies: Test of migration, tissue reactivity and impact on tongue function.

OBJECTIVE The Tongue Drive System (TDS) is a new wearable assistive technology (AT), developed to translate voluntary tongue movements to user-defined computer commands by tracking the position of a titanium-encased magnetic tracer (Ti-Mag) implanted into the tongue. TDS application, however, is constrained by limited information on biological consequence and safety of device implantation into the tongue body. Here we implant a stainless-steel pellet in the rat tongue and assay pellet migration, tongue lick function, and tongue histology to test the safety and biocompatibility of unanchored tongue implants. DESIGN Water consumption, weight and lick behavior were measured before and for >24days after implantation of a stainless-steel spherical pellet (0.5mm) into the anterior tongue body of twelve adult male rats. X-rays were obtained weekly to assess pellet migration. Pellet location and tissue reaction to implantation were determined by post-mortem dissection and histology of the anterior tongue. RESULTS By dissection pellets were distributed across the transverse plane of the tongue. Measures of water consumption, weight, and lick behavior were unchanged by implantation except for a decrease in consumption immediately post-implantation in some animals. By X-ray, there was no migration of the implant, a finding supported by pellet encapsulation demonstrated histologically. Measures of lick behavior were minimally impacted by implantation. CONCLUSION A smooth spherical stainless-steel implant in the anterior tongue of the rat does not migrate, is encapsulated and does not substantially impact lick behavior. These findings support the implantation of small tracers in the anterior tongue in humans for operating wearable assistive technologies.

Tongue Structure and Function

The mammalian tongue is essential for normal respiration, swallowing, oral transport, emesis, coughing and, in humans, speech production. To achieve these behaviors, tongue musculature produces myriad changes in tongue shape and in concert with other head and neck structures a wide range of tongue movement speeds. Head and neck muscles are often described as having unconventional kinematic and mechanical demands. They may be required to apply prolonged, continuous force, as the activation of genioglossus to maintain airway patency, and they may be required to change force very rapidly, as the extraocular muscles during saccade. In this chapter, we describe the neuromuscular specialization that facilitates tongue behavior, and contrast this with typical limb function, in which the muscles undergo cyclical motion during relatively infrequent behaviors.

Ageing and muscular dystrophy differentially affect murine pharyngeal muscles in a region‐dependent manner

Millions of elderly individuals have dysphagia, a debilitating and life‐threatening condition in which the ability to swallow is impaired. Several muscles surround the three regions of the pharynx, which are essential for proper swallowing, yet the effects of ageing and disease on these muscles are not well understood. We demonstrate that the fibre size of murine pharyngeal muscles is differentially affected by ageing and muscular dystrophy depending on their location within the pharynx. Using a mouse model of an age‐associated dysphagic disease (oculopharyngeal muscular dystrophy), we show that overexpression of wild‐type polyadenylate binding nuclear protein 1 in muscle tissue prevents age‐related dysphagia and age‐related muscle atrophy of laryngopharyngeal muscles. These results demonstrate that mice are an excellent model for studying mechanisms of ageing and disease on pharyngeal muscle physiology, and such studies could lead to new therapies for individuals with dysphagia.

Absence of developmental and unconventional myosin heavy chain in human suprahyoid muscles.

Introduction: Contradictory reports of the myosin heavy chain (MHC) composition of adult human suprahyoid muscles leave unresolved the extent to which these muscles express developmental and unconventional MHC. Methods: By immunohistochemistry, separation sodium dodecyl sulfate‐polyacrylamide gel electrophoresis (SDS‐PAGE)‐Coomassie, separation SDS‐PAGE‐Western blot, and mRNA PCR, we tested for conventional MHCI, MHCIIA, MHCIIX, developmental MHC embryonic and MHC neonatal, and unconventional MHC alpha‐cardiac, MHC extraocular, and MHC slow tonic in adult human anterior digastric (AD), geniohyoid (GH), and mylohyoid (MH) muscles. Results: By separation SDS‐PAGE‐Coomassie and Western blot, only conventional MHC are present. By immunohistochemistry all muscle fibers are positive for MHCI, MHCIIA, or MHCIIX, and fewer than 4 fibers/mm2 are positive for developmental or unconventional MHC. By PCR, mRNA of MHCI and MHCIIA dominate, with sporadically detectable MHC alpha‐cardiac and without detectable mRNA of other developmental and unconventional MHC. Conclusions: We conclude that human suprahyoid muscles AD, GH, and MH are composed almost exclusively of conventional MHC isoforms. Muscle Nerve 49:534–544, 2014