Observational study on the occurrence of muscle spindles in human digastric and mylohyoideus muscles

Biomechanical and Cortical Control of Tongue Movements During Chewing and Swallowing

Tongue function is vital for chewing and swallowing and lingual dysfunction is often associated with dysphagia. Better treatment of dysphagia depends on a better understanding of hyolingual morphology, biomechanics, and neural control in humans and animal models. Recent research has revealed significant variation among animal models in morphology of the hyoid chain and suprahyoid muscles which may be associated with variation in swallowing mechanisms. The recent deployment of XROMM (X-ray Reconstruction of Moving Morphology) to quantify 3D hyolingual kinematics has revealed new details on flexion and roll of the tongue during chewing in animal models, movements similar to those used by humans. XROMM-based studies of swallowing in macaques have falsified traditional hypotheses of mechanisms of tongue base retraction during swallowing, and literature review suggests that other animal models may employ a diversity of mechanisms of tongue base retraction. There is variation among animal models in distribution of hyolingual proprioceptors but how that might be related to lingual mechanics is unknown. In macaque monkeys, tongue kinematics-shape and movement-are strongly encoded in neural activity in orofacial primary motor cortex, giving optimism for development of brain-machine interfaces for assisting recovery of lingual function after stroke. However, more research on hyolingual biomechanics and control is needed for technologies interfacing the nervous system with the hyolingual apparatus to become a reality.

Distribution and length of muscle spindles and their 3D visualisation in the medial gastrocnemius of male and female rats

The spatial distribution of the medial gastrocnemius muscle spindles of 10 male and 10 female rats was analysed under a light microscope, and for the first time, visualised using a 3D model of the muscle. Serial cross-sections of the medial gastrocnemius muscles were separated into 10 divisions along with the proximo-distal axis. The muscle spindles of the rat medial gastrocnemius were predominantly distributed on the proximo-medial divisions of the muscle. There were no sex-related differences in the distribution of the studied receptors. A single division contained an average of 2.71 receptors for animals of both sexes. Moreover, the calculated lengths of male and female rat muscle spindles were comparable, and average lengths did not significantly differ (3.30 ± 1.47 mm for male and 3.26 ± 1.32 mm for female rats). Therefore, the present results fill gaps in recent observations concerning similarities in muscle spindle numbers between male and female animals, despite significant differences in muscle mass and size.

Distribution Heterogeneity of Muscle Spindles Across Skeletal Muscles of Lower Extremities in C57BL/6 Mice

Muscle spindles, an important proprioceptor scattered in the skeletal muscle, participate in maintaining muscle tension and the fine regulation of random movement. Although muscle spindles exist in all skeletal muscles, explanations about the distribution and morphology of muscle spindles remain lacking for the indetermination of spindle location across muscles. In this study, traditional time-consuming histochemical technology was utilized to determine the muscle spindle anatomical and morphological characteristics in the lower extremity skeletal muscle in C57BL/6 mice. The relative distance from spindles to nerve-entry points varied from muscles in the ventral-dorsal direction, in which spindles in the lateral of gastrocnemius were not considered to be close to its nerve-entry point. In the longitudinal pattern, the domain with the highest abundance of spindles corresponded to the nerve-entry point, excluding the tibialis anterior. Spindles are mainly concentrated at the middle and rostral domain in all muscles. The results suggest a heterogeneity of the distribution of spindles in different muscles, but the distribution trend generally follows the location pattern of the nerve-entry point. Histochemical staining revealed that the spindle did not have a symmetrical structure along the equator, and this result does not agree with previous findings. Exploring the distribution and structural characteristics of muscle spindles in skeletal muscle can provide some anatomical basis for the study of muscle spindles at the molecular level and treatment of exercise-related diseases and provide a comprehensive understanding of muscle spindle morphology.

Pathophysiological mechanisms of oromandibular dystonia

Oromandibular dystonia (OMD) is a rare form of focal idiopathic dystonia. OMD was clinically identified at the beginning of the 20th century, and the main clinical features have been progressively described over the years. However, OMD has several peculiarities that still remain unexplained, including the high rate of oral trauma, which is often related to the onset of motor symptoms. The purpose of this paper was to formulate a hypothesis regarding the pathophysiology of OMD, starting from the neuroanatomical basis of the masticatory and facial systems and highlighting the features that differentiate this condition from other forms of focal idiopathic dystonia. We provide a brief review of the clinical and etiological features of OMD as well as neurophysiological and neuroimaging findings obtained from studies in patients with OMD. We discuss possible pathophysiological mechanisms underlying OMD and suggest that abnormalities in sensory input processing may play a prominent role in OMD pathophysiology, possibly triggering a cascade of events that results in sensorimotor cortex network dysfunction. Finally, we identify open questions that future studies should address, including the effect of abnormal sensory input processing and oral trauma on the peculiar neurophysiological abnormalities observed in OMD.

Jaw-neck motor strategy during jaw-opening with resistance load

BACKGROUND

The jaw and neck motor systems have a close functional integration but the effect of resistance load to the mandible during jaw opening on the jaw-neck integration is not known.

OBJECTIVES

To evaluate the effect of resistance load compared to no load on integrated jaw and neck motor function in individuals free from pain and dysfunction in the jaw and neck regions.

METHODS

Jaw and head movements during continuous jaw opening were recorded with an optoelectronic system (MacReflex^® ) in 26 pain-free individuals (14 women, 12 men, mean age 22 years). Jaw opening was performed with and without resistance load (1600 g) to the mandible. The relationship between jaw movement amplitude, head movement amplitude, head/jaw ratio (quotient of head and jaw movement amplitude) and resistance load were modelled using linear mixed-model analysis. A p-value <.05 was considered statistically significant.

RESULTS

The expected head/jaw ratio mean was increased by 0.05 (95% CI: 0.03, 0.08, p < .001) with resistance load as compared to no load. This corresponds to an increase in expected mean by 55.6%. With resistance load, expected mean head movement amplitude increased by 1.4 mm (95% CI: 0.2, 2.5, p = .018), and expected mean jaw movement amplitude decreased by 3.7 mm (95% CI: -7.0, -0.5, p = .025).

CONCLUSION

There is a compensation and adaptation of integrated jaw-neck motor function with an altered jaw-neck motor strategy during jaw opening with resistance load compared to no load. The head/jaw ratio demonstrates increased proportional involvement of the neck during increased load on the jaw system.

Generating intrafusal skeletal muscle fibres in vitro: Current state of the art and future challenges

Intrafusal fibres are a specialised cell population in skeletal muscle, found within the muscle spindle. These fibres have a mechano-sensory capacity, forming part of the monosynaptic stretch-reflex arc, a key component responsible for proprioceptive function. Impairment of proprioception and associated dysfunction of the muscle spindle is linked with many neuromuscular diseases. Research to-date has largely been undertaken in vivo or using ex vivo preparations. These studies have provided a foundation for our understanding of muscle spindle physiology, however, the cellular and molecular mechanisms which underpin physiological changes are yet to be fully elucidated. Therefrom, the use of in vitro models has been proposed, whereby intrafusal fibres can be generated de novo. Although there has been progress, it is predominantly a developing and evolving area of research. This narrative review presents the current state of art in this area and proposes the direction of future work, with the aim of providing novel pre-clinical and clinical applications.

The hyoid bone: an overview

The hyoid bone is a small horseshoe-shaped bone located between the mandible and the shoulder girdle. It is classified as a sesamoid bone which means it is a freely floating bone. However, it is anything but freely floating. The hyoid bone is a vestigial structure and is found as part of the tongue in lower animal forms. The hyoid bone is attached to the base of the skull, the mandible, the tongue, the larynx, and the scapular belt. Even though the hyoid bone is an important structure, it has not received the attention that it deserves. It participates in the function of speech, respiration, mastication, and swallowing, as well as maintaining the patency of the airway between the oropharynx and the tracheal rings. Knowledge of the anatomy and physiology of the hyoid is necessary for recognition of the clinical presentation of related disorders and syndromes.

Modification of Masticatory Rhythmicity Leading to the Initiation of the Swallowing Reflex in Humans

Modification of movements by proprioceptive feedback during mastication has an important role in shifting from the oral to the pharyngeal phase of swallowing. The aim of this study was to investigate the kinetics of masticatory muscles throughout a sequence of oropharyngeal swallowing and to present a hypothetical model of the involvement of the nervous system in the transition from mastication to the swallowing reflex. Surface electromyographic signals were recorded from the jaw-closing masseter muscles and the jaw-opening suprahyoid muscle group when a piece of bread (3-5 g) was ingested. Participants were not provided any additional instruction regarding how to chew and swallow. In the final stage of mastication, compared with other stages of mastication, the duration between sequential peak times of rhythmic activity of the masseter muscles was prolonged. Electromyography revealed no significant change in the suprahyoid muscle group. Accordingly, contraction of the jaw-closing muscles and the jaw-opening muscles altered from out-of-phase to in-phase. We have presented a hypothetical model based on the results of the present study, in which mastication shifts to the swallowing reflex when feed-forward inputs from rhythm generators for the jaw-closing and the jaw-opening muscles converge onto an assumed "convertor" neuron group concurrently. This model should contribute to understanding the pathophysiology of dysphagia.