Effect of aging on tongue protrusion forces in rats

Timeline of hypoglossal motor neuron death and intrinsic tongue muscle denervation in high-copy number SOD1G93A mice

In amyotrophic lateral sclerosis (ALS) postmortem tissue and the SOD1 mouse model at mid-disease, death of hypoglossal motor neurons (XII MNs) is evident. These XII MNs innervate the intrinsic and extrinsic tongue muscles, and despite their importance in many oral and lingual motor behaviours that are affected by ALS (e.g., swallowing, speech, and respiratory functions), little is known about the timing and extent of tongue muscle denervation. Here in the well-characterised SOD1G93A (high-copy) mouse model, we evaluated XII MN numbers and intrinsic tongue muscle innervation using standard histopathological approaches, which included stereological evaluation of Nissl-stained brainstem, and the presynaptic and postsynaptic evaluation of neuromuscular junctions (NMJs), using synapsin, neurofilament, and α-bungarotoxin immunolabelling, at presymptomatic, onset, mid-disease, and endstage timepoints. We found that reduction in XII MN size at onset preceded reduced XII MN survival, while the denervation of tongue muscle did not appear until the endstage. Our study suggests that denervation-induced weakness may not be the most pertinent feature of orolingual deficits in ALS. Efforts to preserve oral and respiratory functions of XII MNs are incredibly important if we are to influence patient outcomes.

Sarcopenia of the longitudinal tongue muscles in rats

The tongue is a muscular hydrostat, with lingual movements occurring during breathing, chewing, swallowing, vocalization, vomiting, coughing and grooming/sexual activities. In the elderly, reduced lingual dysfunction and weakness contribute to increased risks of obstructive sleep apnea and aspiration pneumonia. In Fischer 344 (F344) rats, a validated model of aging, hypoglossal motor neuron death is apparent, although there is no information regarding tongue strength. The intrinsic tongue muscles, the superior and inferior longitudinal, transversalis and verticalis exist in an interdigitated state. Recently, we established a method to measure the specific force of individual intrinsic tongue muscle, accounting for the tissue bulk that is not in the direction of uniaxial force. In the longitudinal muscles of 6- (n = 10), 18- (n = 9) and 24-month-old (n = 12) female and male F344 rats, we assessed specific force, fatigability, fiber type dependent cross-sectional area (CSA) and overall CSA. Muscle force and fatigue was assessed ex vivo using platinum plate simulation electrodes. Tongue muscles were frozen in melting isopentane, and transverse sections cut at 10 µm. Muscle fiber type was classified based on immunoreactivity to myosin heavy chain (MyHC) isoform antibodies. In H&E stained muscle, CSA and uniaxial muscle contributions to total tongue bulk was assessed. We observed a robust ∼30% loss of longitudinal specific force, with reductions in overall longitudinal muscle fiber CSA and specific atrophy of type IIx/IIb fibers. It will be important to investigate the mechanistic underpinnings of hypoglossal motor neuron death and tongue muscle weakness to eventually provide therapies for age-associated lingual dysfunctions.

Age-related sex differences in tongue strength and muscle morphometry in a rat model

OBJECTIVE

To investigate potential effects of sex on voluntary tongue strength, evoked twitch and tetanic tension, speed of contraction, and muscle fiber cross-sectional area in the muscles of the rat tongue. Additionally, we aimed to determine whether estrous cycle stage impacts any of the dependent variables as a pilot investigation into the use of female rats in a model of tongue exercise and aging.

DESIGN

Fischer 344-Brown Norway male and female rats in two age groups (16 middle-aged, 16 young-adult) were trained to use a tongue force operandum. Tongue muscle contraction, myosin heavy chain (MyHC) composition, and cross section area of the genioglossus and styloglossus muscles were examined. Vaginal lavage determined estrous cycle stage of the female rats daily.

RESULTS

The female group had significantly lower evoked twitch and tetanic tension, longer contraction times, and a smaller proportion of MyHC type IIa and MyHC type IIx in the styloglossus muscle. There was no significant sex effect in maximal voluntary tongue force (MVTF) despite a significant weight difference between the male and female groups. There were no significant age or sex effects in the genioglossus. Estrous cycle stage did not have a significant effect on any of the dependent variables.

CONCLUSIONS

Sex and age both have a significant effect on tongue muscle structure and physiology. While the female group showed reduced contraction speed and maximal twitch and tetanic tension relative to the male group, differences in muscle morphology appeared to vary by muscle.

Effects of Age and Lingual-Palatal Pressure Levels on Submental Surface Electromyography Measures

Submental surface electromyography (sEMG) may provide information about the lingual-palatal pressure (LPP) during swallowing. However, the extent to which changes in age and LPP levels are reflected in different sEMG measures is unclear. This study aimed to understand the effects of age and different levels of submaximal LPPs on submental sEMG peak, average, and integrated values in healthy adults. Ninety community-dwelling healthy participants were categorized into three age groups (young: 20-39 years, middle age: 40-59 years, older: 60 ≥ years). sEMG and LPP measurements were collected concurrently. After placing the sEMG electrodes, the maximal isometric LPP was established using the Iowa oral performance instrument (IOPI) on the anterior tongue. sEMG recordings were conducted for three submaximal LPP levels, including 40%, 60%, and 80% of the maximum LPP. Two-way repeated measure ANOVAs were conducted to find the effects of age and varying LPP levels on mean sEMG peak, average, and integrated measures. A significant age by LPP level interaction was identified for sEMG peak [F (4, 172) = 4.116, p < 0.007, ηp2 = 0.087], sEMG average [F (4, 170) = 5.049, p < 0.001, ηp2 = 0.106], and sEMG integrated values [F (4, 170) = 4.179, p < 0.003, ηp2 = 0.090]. Post hoc testing demonstrated that, in general, sEMG values significantly increased with rising LPP levels, primarily for younger and middle-aged adults. Furthermore, at 80% of maximum LPP, older adults generated less sEMG average and integrated values than middle age (only significant for sEMG integrated value) and younger adults. Likewise, max LPP was lower in older adults compared with young and middle-aged groups. Submental sEMG has the potential to be incorporated into a skill-based lingual exercise paradigm to improve tongue function during swallowing. Moreover, submental sEMG can characterize some age-related subclinical changes in swallowing.

Loss of larger hypoglossal motor neurons in aged Fischer 344 rats

The intrinsic (longitudinal, transversalis and verticalis) and extrinsic (genioglossus, styloglossus, hyoglossus and geniohyoid) tongue muscles are innervated by hypoglossal motor neurons (MNs). Tongue muscle activations occur during many behaviors: maintaining upper airway patency, chewing, swallowing, vocalization, vomiting, coughing, sneezing and grooming/sexual activities. In the tongues of the elderly, reduced oral motor function and strength contribute to increased risk of obstructive sleep apnoea. Tongue muscle atrophy and weakness is also described in rats, yet hypoglossal MN numbers are unknown. In young (6-months, n=10) and old (24-months, n=8) female and male Fischer 344 (F344) rats, stereological assessment of hypoglossal MN numbers and surface areas were performed on 16µm Nissl-stained brainstem cryosections. We observed a robust loss of ~15% of hypoglossal MNs and a modest ~8% reduction in their surface areas with age. In the larger size tertile of hypoglossal MNs, age-associated loss of hypoglossal MNs approached ~30% These findings uncover a potential neurogenic locus of pathology for age-associated tongue dysfunctions.

Histological and contractile changes in the genioglossus muscle after nasal obstruction in growing rats

The aim of the study was to address the genioglossus muscle physiological and histological changes after unilateral nasal obstruction in growing rats. Fifty-four 6-day-old male Wistar albino rats were randomly divided into control (n = 27) and experimental (n = 27) groups. Unilateral nasal obstruction was performed at 8 days old. Contractile properties of the genioglossus whole muscle were measured at 5-, 7- and 9-week-old, including the twitch and tetanic forces, contraction time, half-decay time, and fatigue index. The histological characteristics of the genioglossus were also evaluated at 5-, 7- and 9-week-old, analyzing the myosin heavy chain composition of the slow, fast, IIa and IIb muscle fiber type, by measuring the number, rate, diameter and cross-sectional area. The maximal twitch force, and tetanic force at 60 Hz and 80 Hz force was significantly increased at all ages after nasal obstruction. The fatigue index was decreased at 5 weeks-old after nasal obstruction. The diameter and cross-sectional area of the fast, IIa and IIb muscle fiber types were increased at 7 and 9 weeks after nasal obstruction, while only the diameter of IIa type and cross-sectional area of IIb type were increased at 5 weeks-old after nasal obstruction. Nasal obstruction during growth affects the whole genioglossus muscle contractile properties and histological characteristics, increasing its force, the diameter and area of its muscle fibers. These changes in the genioglossus muscle may affect the normal growth, development and function of the craniofacial complex.

Impact of Limb Phenotype on Tongue Denervation Atrophy, Dysphagia Penetrance, and Survival Time in a Mouse Model of ALS

Current treatments for dysphagia in ALS do not target the underlying tongue weakness and denervation atrophy that is prevalent in spinal and bulbar ALS cases. To address this clinical gap, we studied the low copy number SOD1-G93A (LCN-SOD1) mouse model of ALS to quantify the impact of limb phenotype on tongue denervation atrophy, dysphagia penetrance, and survival time in preparation for future treatment-based studies. Two male LCN-SOD1 breeders and 125 offspring were followed for limb phenotype inheritance, of which 52 (30 LCN-SOD1 and 22 wild-type/WT, both sexes) underwent characterization of dysphagia penetrance (via videofluoroscopic swallow study; VFSS) and survival time at disease end-stage (15-20% body weight loss). From these, 16 mice (8/genotype) underwent postmortem histological analysis of the genioglossus for evidence of denervation atrophy. Results revealed that both breeders displayed a mixed (hindlimb and forelimb) ALS phenotype and sired equal proportions of hindlimb vs. mixed phenotype offspring. Dysphagia penetrance was complete for mixed (100%) versus incomplete for hindlimb (64%) phenotype mice; yet survival times were similar. Regardless of limb phenotype, LCN-SOD1 mice had significantly smaller genioglossus myofibers and more centralized myonuclei compared to WT mice (p < 0.05). These biomarkers of denervation atrophy were significantly correlated with VFSS metrics (lick and swallow rates, p < 0.05) but not survival time. In conclusion, both LCN-SOD1 phenotypes had significant tongue denervation atrophy, even hindlimb phenotype mice without dysphagia. This finding recapitulates human ALS, providing robust rationale for using this preclinical model to explore targeted treatments for tongue denervation atrophy and ensuing dysphagia.

A Strength Endurance Exercise Paradigm Mitigates Deficits in Hypoglossal-Tongue Axis Function, Strength, and Structure in a Rodent Model of Hypoglossal Motor Neuron Degeneration

The tongue plays a crucial role in the swallowing process, and impairment can lead to dysphagia, particularly in motor neuron diseases (MNDs) resulting in hypoglossal-tongue axis degeneration (e.g., amyotrophic lateral sclerosis and progressive bulbar palsy). This study utilized our previously established inducible rodent model of dysphagia due to targeted degeneration of the hypoglossal-tongue axis. This model was created by injecting cholera toxin B conjugated to saporin (CTB-SAP) into the genioglossus muscle of the tongue base for retrograde transport to the hypoglossal (XII) nucleus via the hypoglossal nerve, which provides the sole motor control of the tongue. Our goal was to investigate the effect of high-repetition/low-resistance tongue exercise on tongue function, strength, and structure in four groups of male rats: (1) control + sham exercise (n = 13); (2) control + exercise (n = 10); (3) CTB-SAP + sham exercise (n = 13); and (4) CTB-SAP + exercise (n = 12). For each group, a custom spout with adjustable lick force requirement for fluid access was placed in the home cage overnight on days 4 and 6 post-tongue injection. For the two sham exercise groups, the lick force requirement was negligible. For the two exercise groups, the lick force requirement was set to ∼40% greater than the maximum voluntary lick force for individual rats. Following exercise exposure, we evaluated the effect on hypoglossal-tongue axis function (via videofluoroscopy), strength (via force-lickometer), and structure [via Magnetic Resonance Imaging (MRI) of the brainstem and tongue in a subset of rats]. Results showed that sham-exercised CTB-SAP rats had significant deficits in lick rate, swallow timing, and lick force. In exercised CTB-SAP rats, lick rate and lick force were preserved; however, swallow timing deficits persisted. MRI revealed corresponding degenerative changes in the hypoglossal-tongue axis that were mitigated by tongue exercise. These collective findings suggest that high-repetition/low-resistance tongue exercise in our model is a safe and effective treatment to prevent/diminish signs of hypoglossal-tongue axis degeneration. The next step is to leverage our rat model to optimize exercise dosing parameters and investigate corresponding treatment mechanisms of action for future translation to MND clinical trials.

Assays of Tongue Force, Timing, and Dynamics in Rat and Mouse Models

Communication and swallowing are highly complex sensorimotor events that are tightly linked to respiration and vital to health and well-being. The tongue is a complex organ, often described as a muscular hydrostat, that is crucial for maintaining airway patency, preparing and safely transporting food/liquid, and rapidly changing position and shape for speech. As with any complex behavior, tongue function can be compromised with aging, diseases/conditions, trauma, or as a pharmacologic side effect. As such, modeling lingual function and dysfunction for basic and translational research is paramount; understanding how the nervous system controls tongue function for complex behavior is foundational to this work. Non-invasive access to tongue tissues and kinematics during awake behavior has been historically challenging, creating a critical need to measure tongue function in model systems. Germane to this field of study are the instruments and assays of licking/lapping and drinking, including tongue force and timing measures, many of which were designed or modified by Dr. Stephen C. Fowler. The focus of this paper is to review some of the important contributions of measuring tongue behaviors in awake rats and mice and how these have been modified by other researchers to advance translational science.

Age-Related Functional Reserve Decline Is Not Seen in Pharyngeal Swallowing Pressures

Purpose Functional reserve represents the difference between an individual's ability to produce a maximum output function and the ability to perform a functional task. Several studies have documented an age-related decrease in functional reserve with oral tongue pressure generation. Whether this pattern is seen in pharyngeal swallowing pressures is unknown. The aim of this study was to investigate pharyngeal functional reserve using high-resolution manometry during normal-effort and effortful swallows. Method Pharyngeal high-resolution manometry was performed on 38 younger healthy individuals (≤ 40 years) and 18 older healthy individuals (≥ 60 years) during normal-effort and effortful water swallows. Pressure metrics included maximum pressure in the velopharynx, tongue base, and hypopharynx, as well as pharyngeal contractile integral and minimum pressure in the upper esophageal sphincter (UES). Repeated-measures analysis of variance was used to determine the effects of swallow task, age, and pharyngeal region on pressure generation. Results Maximum pharyngeal pressures and pharyngeal contractile integral were significantly increased during the effortful swallows compared to normal-effort swallows (p < .001), but there were no interactions between task and age in pharyngeal pressures. In the UES, minimum pressures were significantly elevated in older individuals during effortful swallows compared to normal-effort swallows (p = .007) but did not follow a pattern consistent with reduced functional reserve. Conclusions Healthy individuals increase pharyngeal driving pressures during effortful swallows, without an age-related reduction in the magnitude of pressure increase. Thus, this study did not find evidence for an age-related reduction in pharyngeal functional reserve. The preserved ability to increase pharyngeal pressures during effortful swallowing in aging may support the use of behavioral swallowing interventions in older individuals without neuromuscular conditions. Supplemental Material https://doi.org/10.23641/asha.16606709.

Tongue muscle contractile, fatigue, and fiber type properties in rats

The intrinsic and extrinsic tongue muscles manipulate the position and shape of the tongue and are activated during many oral and respiratory behaviors. In the present study, in 6-mo-old Fischer 344 rats, we examined mechanical and fatigue properties of tongue muscles in relation to their fiber type composition. In an ex vivo preparation, isometric force and fatigue was assessed by direct muscle stimulation. Tongue muscles were frozen in melting isopentane and transverse sections cut at 10 µm. In hematoxylin-eosin (H&E)-stained muscle sections, the relative fractions of muscle versus extracellular matrix were determined. Muscle fibers were classified as type I, IIa and IIx, and/or IIb based on immunoreactivity to specific myosin heavy chain isoform antibodies. Cross-sectional areas (CSAs) and proportions of different fiber types were used to calculate their relative contribution to total muscle CSAs. We found that the superior and inferior longitudinal intrinsic muscles (4.4 N/cm²) and genioglossus muscle (3.0 N/cm²) generated the greatest maximum isometric force compared with the transversalis muscle (0.9 N/cm²). The longitudinal muscles and the transversalis muscle displayed greater fatigue during repetitive stimulation consistent with the greater relative contribution of type IIx and/or IIb fibers. By contrast, the genioglossus, comprising a higher proportion of type I and IIa fibers, was more fatigue resistant. This study advances our understanding of the force, fatigue, and fiber type-specific properties of individual tongue musculature. The assessments and approach provide a readily accessible muscular readout for scenarios where motor control dysfunction or tongue weakness is evident.NEW & NOTEWORTHY For the individual tongue muscles, relatively little quantification of uniaxial force, fatigue, and fiber type-specific properties has been documented. Here, we assessed uniaxial-specific force generation, fatigability, and muscle fiber type-specific properties in the superior and inferior longitudinal muscles, the transversalis, and the genioglossus in Fischer 344 rats. The longitudinal muscles produced the greatest isometric tetanic-specific forces. The genioglossus was more fatigue resistant and comprised higher proportions of I and IIa fibers.

Central activation deficits contribute to post stroke lingual weakness in a rat model

Lingual weakness frequently occurs after stroke and is associated with deficits in speaking and swallowing. Chronic weakness after stroke has been attributed to both impaired central activation of target muscles and reduced force-generating capacity within muscles. How these factors contribute to lingual weakness is not known. We hypothesized that lingual weakness due to middle cerebral artery occlusion (MCAO) would manifest as reduced muscle force capacity and reduced muscle activation. Rats were randomized into MCAO or sham surgery groups. Maximum volitional tongue forces were quantified 8 wk after surgery. Hypoglossal nerve stimulation was used to assess maximum stimulated force, muscle twitch properties, and force-frequency response. The central activation ratio was determined by maximum volitional/maximum stimulated force. Genioglossus muscle fiber type properties and neuromuscular junction innervation were assessed. Maximum volitional force and the central activation ratio were significantly reduced with MCAO. Maximum stimulated force was not significantly different. No significant differences were found for muscle twitch properties, unilateral contractile properties, muscle fiber type percentages, or fiber size. However, the twitch/tetanus ratio was significantly increased in the MCAO group relative to sham. A small but significant increase in denervated neuromuscular junctions (NMJs) and fiber-type grouping occurred in the contralesional genioglossus. Results suggest that the primary cause of chronic lingual weakness after stroke is impaired muscle activation rather than a deficit of force-generating capacity in lingual muscles. Increased fiber type grouping and denervated NMJs in the contralesional genioglossus suggest that partial reinnervation of muscle fibers may have preserved force-generating capacity, but not optimal activation patterns.NEW & NOTEWORTHY Despite significant reductions in maximum volitional forces, the intrinsic force-generating capacity of the protrusive lingual muscles was not reduced with unilateral cerebral ischemia. Small yet significant increases in denervated NMJs and fiber-type grouping of the contralesional genioglossus suggest that the muscle underwent denervation and reinnervation. Together these results suggest that spontaneous neuromuscular plasticity was sufficient to prevent atrophy, yet central activation deficits remain and contribute to chronic lingual weakness after stroke.

Myogenic marker expression as a function of age and exercise-based therapy in the tongue

Degeneration of tongue muscles with aging may contribute to swallowing deficits observed in elderly people. However, the capacity for tongue muscle stem cells (SCs) to regenerate and repair the aged tongue and improve tongue strength following tongue exercise (a current clinical treatment) has never been examined. We found that the expression of regenerative, myogenic markers were impaired with age and may be related to increased expression of senescent marker p16INK4a. Tongue strength increased in young adult and old rats following exercise and was related to the expression of Pax7, MyoD, myogenin, and p16INK4a. Our study also suggests that strengthening of tongue muscles via clinical rehabilitation strategies also increased the expression of SC regenerative markers in the tongue throughout the exercise duration.

Decreased Tongue Volume Post Radiation

OBJECTIVES

To evaluate volume changes within the tongue post chemoradiation therapy (CRT).

STUDY DESIGN

Retrospective review.

SETTING

Academic Medical Center.

SUBJECTS AND METHODS

Subjects included 19 patients that received CRT as the primary treatment for tonsillar or hypopharynx squamous cell carcinoma. Tongue volumes were calculated by three raters from thin slice computed tomography images collected before treatment and up to 29 months post-CRT. Body mass index (BMI) was also collected at each time point.

RESULTS

Inter-rater reliability was high with an ICC of 0.849 (95% CI = 0.773, 0.905). Linear mixed effects modeling showed a mean decrease of 0.45 cm³ (standard error of the mean [SEM] = 0.11) in tongue volume per month post-CRT (P < .001). However, the addition of BMI to the model was significant (χ² (4) = 25.0, P < .001), indicating that BMI was a strong predictor of tongue volume, with a mean decrease of 1.75 cm³ (SEM = 0.49) in tongue volume per unit decrease in BMI (P < .001) and reducing the post-CRT effect on tongue volume decrease per month to 0.23 cm³ (P = .02). BMI significantly (P < .001) decreased by 0.11 units (SEM = 0.02) per month post radiation.

CONCLUSION

Tongue dysfunction and decreased tongue strength are significant contributors to the dysphagia that patients experience after receiving CRT. In this study, both tongue volume and BMI decreased post-CRT; therefore, BMI could potentially be used as a predictor of tongue volume post-CRT.

Tongue Force Training Induces Plasticity of the Lingual Motor Cortex in Young Adult and Aged Rats

Tongue exercise programs are used clinically for dysphagia in aged individuals and have been shown to improve lingual strength. However, the neural mechanisms of age-related decline in swallowing function and its association with lingual strength are not well understood. Using an established rat model of aging and tongue exercise, we hypothesized that the motor cortex of aged rats would have a smaller lingual motor map area than young adult rats and would increase in size as a function of tongue exercise. Over 8 weeks, rats either underwent a progressive resistance tongue exercise program (TE), learned the task but did not exercise (trained controls, TC), or were naïve untrained controls (UC). Cortical motor map areas for tongue and jaw were determined using intracortical microstimulation (ICMS). Rats in the TE and TC groups had a significantly larger motor cortex region for the tongue than the UC group. Lingual cortical motor area was not correlated with protrusive tongue force gains and did not differ significantly with age. These results suggest that learning a novel tongue force skill was sufficient to induce plasticity of the lingual motor cortex yet increasing tongue strength with progressive resistance exercise did not significantly expand the lingual motor area beyond the gains that occurred through the skilled learning component.

An intensive swallowing exercise protocol for improving swallowing physiology in older adults with radiographically confirmed dysphagia

Purpose

The aim of this study was to investigate improvements in swallowing function and physiology in a series of healthy older adults with radiographically confirmed dysphagia, following completion of an exercise-based swallowing intervention.

Patients and methods

Nine otherwise healthy older adults (six females, mean age =75.3, SD =5.3) had confirmed impairments in swallowing safety and/or efficiency on a modified barium swallow study. Each participant completed an 8-week swallowing treatment protocol including effortful swallows, Mendelsohn maneuvers, tongue-hold swallows, supraglottic swallows, Shaker exercises and effortful pitch glides. Treatment sessions were conducted once per week with additional daily home practice. Penetration–Aspiration Scale and the Modified Barium Swallowing Impairment Profile (MBSImP) were scored in a blind and randomized fashion to examine changes to swallowing function and physiology from baseline to post-treatment.

Results

There were significant improvements in swallowing physiology as represented by improved oral and pharyngeal composite scores of the MBSImP. Specific components to demonstrate statistical improvement included initiation of the pharyngeal swallow, laryngeal elevation and pharyngeal residue. There was a nonsignificant reduction in median PAS scores.

Conclusion

Swallowing physiology can be improved using this standardized high-intensity exercise protocol in healthy adults with evidence of dysphagia. Future research is needed to examine the individual potential of each exercise in isolation and to determine ideal dose and frequency. Studies on various etiological groups are warranted.

Age-related alterations in swallowing biomechanics

BACKGROUND

Aging rodent models allow for the discovery of underlying mechanisms of cranial muscle dysfunction. Methods are needed to allow quantification of complex, multivariate biomechanical movements during swallowing. Videofluoroscopic swallow studies (VSS) are the standard of care in assessment of swallowing disorders in patients and validated quantitative, kinematic, and morphometric analysis methods have been developed. Our purpose was to adapt validated morphometric techniques to the rodent to computationally analyze swallowing dysfunction in the aging rodent.

METHODS

VSS, quantitative analyses (bolus area, bolus velocity, mastication rate) and a rodent specific multivariate, morphometric computational analysis of swallowing biomechanics were performed on 20 swallows from 5 young adult and 5 old Fischer 344/Brown Norway rats. Eight anatomical landmarks were used to track the relative change in position of skeletal levers (cranial base, vertebral column, mandible) and soft tissue landmarks (upper esophageal sphincter, base of tongue).

RESULTS

Bolus area significantly increased and mastication rate significantly decreased with age. Aging accounted for 77.1% of the variance in swallow biomechanics, and 18.7% of the variance was associated with swallow phase (oral vs pharyngeal). Post hoc analyses identified age-related alterations in tongue base retraction, mastication, and head posture during the swallow.

CONCLUSION

Geometric morphometric analysis of rodent swallows suggests that swallow biomechanics are altered with age. When used in combination with biological assays of age-related adaptations in neuromuscular systems, this multivariate analysis may increase our understanding of underlying musculoskeletal dysfunction that contributes to swallowing disorders with aging.

Effect of neuromuscular electrical stimulation frequency on muscles of the tongue

INTRODUCTION

Neuromuscular electrical stimulation (NMES) for the treatment of swallowing disorders is delivered at a variety of stimulation frequencies. We examined the effects of stimulation frequency on tongue muscle plasticity in an aging rat model.

METHODS

Eighty-six young, middle-aged, and old rats were assigned to either bilateral hypoglossal nerve stimulation at 10 or 100 Hz (5 days/week, 8 weeks), sham, or no-implantation conditions. Muscle contractile properties and myosin heavy chain (MyHC) composition were determined for hyoglossus (HG) and styloglossus (SG) muscles.

RESULTS

Eight weeks of 100-Hz stimulation resulted in the greatest changes in muscle contractile function with significantly longer contraction and half-decay times, the greatest reduction in fatigue, and a transition toward slowly contracting, fatigue-resistant MyHC isoforms.

DISCUSSION

NMES at 100-Hz induced considerable changes in contractile and phenotypic profiles of HG and SG muscles, suggesting higher frequency NMES may yield a greater therapeutic effect. Muscle Nerve, 2018.

Volumetric Changes to the Pharynx in Healthy Aging: Consequence for Pharyngeal Swallow Mechanics and Function

Pharyngeal lumen volume is prone to increase as a consequence of pharyngeal muscle atrophy in aging. Yet, the impact of this on swallowing mechanics and function is poorly understood. We examined the relationship between pharyngeal volume and pharyngeal swallowing mechanics and function in a sample of healthy community-dwelling seniors. Data were collected from 44 healthy seniors (21 male, mean age = 76.9, SD = 7.1). Each participant swallowed 9 boluses of barium (3 × 5 ml thin, 3 × 20 ml thin, 3 × 5 ml nectar). Pharyngeal shortening, pharyngeal constriction, pyriform sinus and vallecular residue were quantified from lateral view videofluorosopic swallowing studies. Pharyngeal lumen volume was captured during an oral breathing task with acoustic pharyngometry. In addition, within-participant measures of strength and anthropometrics were collected. Four linear mixed effects regression models were run to study the relationship between pharyngeal volume and pharyngeal constriction, pharyngeal shortening, pyriform sinus residue, and vallecular residue while controlling for bolus condition, age, sex, and posterior tongue strength. Increasing pharyngeal lumen volume was significantly related to worse constriction and vallecular residue. In general, larger and thicker boluses resulted in worse pharyngeal constriction and residue. Pharyngeal shortening was only significantly related to posterior tongue strength. Our work establishes the utility of acoustic pharyngometry to monitor pharyngeal lumen volume. Increasing pharyngeal lumen volume appears to impact both pharyngeal swallowing mechanics and function in a sample of healthy, functional seniors.

Alterations of intrinsic tongue muscle properties with aging

INTRODUCTION

Age-related decline in the intrinsic lingual musculature could contribute to swallowing disorders, yet the effects of age on these muscles is unknown. We hypothesized there is reduced muscle fiber size and shifts to slower myosin heavy chain (MyHC) fiber types with age.

METHODS

Intrinsic lingual muscles were sampled from 8 young adult (9 months) and 8 old (32 months) Fischer 344/Brown Norway rats. Fiber size and MyHC were determined by fluorescent immunohistochemistry.

RESULTS

Age was associated with a reduced number of rapidly contracting muscle fibers, and more slowly contracting fibers. Decreased fiber size was found only in the transverse and verticalis muscles.

DISCUSSION

Shifts in muscle composition from faster to slower MyHC fiber types may contribute to age-related changes in swallowing duration. Decreasing muscle fiber size in the protrusive transverse and verticalis muscles may contribute to reductions in maximum isometric tongue pressure found with age. Differences among regions and muscles may be associated with different functional demands. Muscle Nerve 56: E119-E125, 2017.

Effect of unilateral nasal obstruction on tongue protrusion forces in growing rats

Mouth breathing caused by nasal obstruction affects the normal growth and development of craniofacial structures, including changes in the orofacial muscles. Tongue muscles play an important role in patency of the pharyngeal airway, and changes in the breathing pattern may influence tongue function. The purpose of this study was to evaluate the effect of unilateral nasal obstruction during growth on contractile properties of the tongue-protruding muscles. Sixty 6-day-old male Wistar albino rats were divided randomly into control ( n = 30) and experimental ( n = 30) groups. Rats in the experimental group underwent a unilateral nasal obstruction after cauterization of the external nostril at the age of 8 days, and muscle contractile characteristics were measured at 5, 7, and 9 wk of age. The specific parameters measured were twitch force, contraction time, half-decay time, tetanic force, and fatigue index. Repeated-measures multivariate analysis of variance was used for intergroup and intragroup statistical comparisons. Twitch contraction force and half-decay time were significantly increased in the experimental group at all ages. Tetanic forces at 60 and 80 Hz were significantly higher in the experimental group at all ages. The fatigue index was decreased significantly in the experimental group at the age of 5 wk. These results suggest that early unilateral nasal obstruction may increase the contraction force of the tongue-protruding muscles and prolong the duration of muscle contraction, which may influence the shape and development of the craniofacial complex.

Age-Related Changes in Pharyngeal Lumen Size: A Retrospective MRI Analysis

Age-related loss of muscle bulk and strength (sarcopenia) is often cited as a potential mechanism underlying age-related changes in swallowing. Our goal was to explore this phenomenon in the pharynx, specifically, by measuring pharyngeal wall thickness and pharyngeal lumen area in a sample of young versus older women. MRI scans of the neck were retrospectively reviewed from 60 women equally stratified into three age groups (20s, 60s, 70+). Four de-identified slices were extracted per scan for randomized, blinded analysis: one mid-sagittal and three axial slices were selected at the anterior inferior border of C2 and C3, and at the pit of the vallecula. Pixel-based measures of pharyngeal wall thickness and pharyngeal lumen area were completed using ImageJ and then converted to metric units. Measures of pharyngeal wall thickness and pharyngeal lumen area were compared between age groups with one-way ANOVAs using Sidak adjustments for post-hoc pairwise comparisons. A significant main effect for age was observed across all variables whereby pharyngeal wall thickness decreased and pharyngeal lumen area increased with advancing age. Pairwise comparisons revealed significant differences between 20s versus 70+ for all variables and 20s versus 60s for all variables except those measured at C2. Effect sizes ranged from 0.54 to 1.34. Consistent with existing sacropenia literature, the pharyngeal muscles appear to atrophy with age and consequently, the size of the pharyngeal lumen increases.

Effects of aging on evoked retrusive tongue actions

OBJECTIVE

Tongue strength, timing, and coordination deficits may underlie age-related swallowing function. Retrusive tongue actions are likely important in retrograde bolus transport. However, age-related changes in retrusive tongue muscle contractile properties have not been identified in animal studies. Because previous studies employed whole hypoglossal nerve stimulation that activated both protrusive and retrusive tongue muscles, co-contraction may have masked retrusive muscle force decrements. The hypotheses of this study were: (1) retrusive tongue muscle contraction forces would be diminished and temporal characteristics prolonged in old rats when lateral nerves were selectively activated, and (2) greater muscle contractile forces with selective lateral branch stimulation would be found relative to whole hypoglossal nerve stimulation.

DESIGN

Nineteen Fischer 344/Brown Norway rats (9 old, 10 young adult) underwent tongue muscle contractile property recording elicited by: (1) bilateral whole hypoglossal nerve stimulation, and (2) selective lateral branch stimulation. Twitch contraction time (CT), half-decay time, maximal twitch and tetanic forces, and a fatigue index were measured.

RESULTS

For whole nerve stimulation, CT was significantly longer in the old group. No significant age group differences were found with selective lateral nerve stimulation. Significantly reduced twitch forces (old group only), increased tetanic forces and significantly less fatigue were found with selective lateral nerve stimulation than with whole hypoglossal stimulation.

CONCLUSIONS

Retrusive tongue forces are not impaired in old rats. Deficits observed in swallowing with aging may be due to other factors such as inadequate bolus propulsive forces, mediated by protrusive tongue muscles, or timing/coordination of muscle actions.

Effects of age and radiation treatment on function of extrinsic tongue muscles

Background

Radiation treatment for head and neck cancer often results in difficulty swallowing. Muscle weakness and fibrosis have been identified clinically as possible etiologies for swallowing problems following radiation. Aging may compound the effects of radiation on swallowing because radiation-induced damage to muscles and other tissues critical for the oropharyngeal swallow is overlaid on a declining sensorimotor system. However, there have been no investigations of the manner in which aging and radiation treatment effects combine to impact tongue muscles, which are critical effectors of the oropharyngeal swallow.

Methods

Thirty-seven male Fisher 344/Brown Norway rats were divided into four groups; young adults (9 month old), old (32 months old), young radiation (9 months), and old radiation (32 months old). Two fractions of 11 Gy on consecutive days was delivered by external beam radiation to the ventral side of the rat’s body over the anterior portion (20 X 30 mm area) of the anterior digastric muscle. Two-way analysis of variance (ANOVA) was used to examine the effects of age and radiation and their interaction on muscle contractile properties. Post-hoc testing was completed using Fisher’s least significant differences (LSD).

Results

Radiation was associated with a significant decrease in tongue force production and reduced speed of tongue muscle contraction. However, radiation treatment did not lead to muscle atrophy and fibrosis formation in the GG muscle. Radiation treatment did not exacerbate atrophic changes observed with aging, or lead to additional fibrosis formation in the GG muscle from that observed in the other groups.

Conclusions

The purpose of this research was to determine the effect of radiation on muscles of the tongue and to determine whether aging altered the extent of radiation injury to tongue muscles. Radiation was associated with a significant decrease in tongue force production and reduced speed of tongue muscle contraction, and the reduction in the speed of tongue muscle contraction was exacerbated in the aged-rat tongue. This work provides a foundation for future investigations of treatments for concurrent effects of aging and radiation on muscles of the tongue and swallowing.

Differential effects of amphetamine and GBR-12909 on orolingual motor function in young vs aged F344/BN rats

RATIONALE

Orolingual motor deficits, such as dysarthria and dysphagia, contribute to increased morbidity and mortality in the elderly. In preclinical studies, we and others have reported age-related decreases in tongue motility in both F344 and F344/BN rats. The fact that these deficits are associated with nigrostriatal dopamine (DA), tissue measures suggest that increasing dopamine function might normalize tongue motility.

OBJECTIVE

The purpose of the current study was to determine whether two indirect dopamine agonists with locomotor-enhancing effects, d-amphetamine (amphetamine; 1 and 2 mg/kg) and GBR-12909 (5, 10, and 20 mg/kg), can improve tongue motility in aged F344/BN rats.

METHODS

Young (6 months) and aged (30 months) F344/BN rats licked water from an isometric force disc so that tongue motility (licks/second) and tongue force could be measured as a function of age and drug dose.

RESULTS

Consistent with our previous studies, tongue force was greater and tongue motility was lower in the aged group. Tongue motility was increased by amphetamine but not by GBR-12909. Amphetamine decreased peak tongue force, primarily in the young group. GBR-12909 did not affect tongue force. GBR-12909 increased the number of licks/session in the young group but not in the aged group, while amphetamine increased this measure in both groups.

CONCLUSION

These results demonstrate differential effects of these drugs on orolingual motor function and suggest that blocking DA uptake is insufficient to increase tongue motility in aging.

Videofluorographic assessment of deglutitive behaviors in a rat model of aging and Parkinson disease

Dysphagia is commonly associated with aging and Parkinson disease and can have a significant impact on a person's quality of life. In some cases, dysphagia may be life-threatening. Animal models may be used to study underlying mechanisms of dysphagia, but paradigms that allow adequate imaging of the swallow in combination with measurement of physiological variables have not been forthcoming. To begin development of methods that allow this, we used videofluorography to record the deglutition behaviors of 22 Fisher 344/Brown Norway rats in young adult (9 months old), old (32 months old), and parkinsonian (unilateral lesion to the medial forebrain bundle) groups. We hypothesized that the old and parkinsonian rats would manifest deficits in deglutition behaviors analogous to those found in human clinical populations. Our results supported our hypothesis in that the old group demonstrated reductions in bolus transport speeds and mastication rate while the parkinsonian rats showed impairments in oral processing. Interpretation of these results should consider the particular animal model, lesion type, and videofluorographic protocol used in this work. Future studies will link swallow imaging data of this kind with physiological and anatomical data in a manner not possible with human participants.

Differential effects of targeted tongue exercise and treadmill running on aging tongue muscle structure and contractile properties

Age-associated changes in tongue muscle structure and strength may contribute to dysphagia in elderly people. Tongue exercise is a current treatment option. We hypothesized that targeted tongue exercise and nontargeted exercise that activates tongue muscles as a consequence of increased respiratory drive, such as treadmill running, are associated with different patterns of tongue muscle contraction and genioglossus (GG) muscle biochemistry. Thirty-one young adult, 34 middle-aged, and 37 old Fischer 344/Brown Norway rats received either targeted tongue exercise, treadmill running, or no exercise (5 days/wk for 8 wk). Protrusive tongue muscle contractile properties and myosin heavy chain (MHC) composition in the GG were examined at the end of 8 wk across groups. Significant age effects were found for maximal twitch and tetanic tension (greatest in young adult rats), MHCIIb (highest proportion in young adult rats), MHCIIx (highest proportion in middle-aged and old rats), and MHCI (highest proportion in old rats). The targeted tongue exercise group had the greatest maximal twitch tension and the highest proportion of MHCI. The treadmill running group had the shortest half-decay time, the lowest proportion of MHCIIa, and the highest proportion of MHCIIb. Fatigue was significantly less in the young adult treadmill running group and the old targeted tongue exercise group than in other groups. Thus, tongue muscle structure and contractile properties were affected by both targeted tongue exercise and treadmill running, but in different ways. Studies geared toward optimizing dose and manner of providing targeted and generalized tongue exercise may lead to alternative tongue exercise delivery strategies.

Tongue muscle plasticity following hypoglossal nerve stimulation in aged rats

INTRODUCTION

Age-related decreases in tongue muscle mass and strength have been reported. It may be possible to prevent age-related tongue muscle changes using neuromuscular electrical stimulation (NMES). Our hypothesis was that alterations in muscle contractile properties and myosin heavy chain composition would be found after NMES.

METHODS

Fifty-four young, middle-aged, and old 344/Brown Norway rats were included in this study. Twenty-four rats underwent bilateral electrical stimulation of the hypoglossal nerves for 8 weeks and were compared with control or sham rats. Muscle contractile properties and myosin heavy chain (MHC) in the genioglossus (GG), styloglossus (SG), and hyoglossus (HG) muscles were examined.

RESULTS

Compared with unstimulated control rats, we found reduced muscle fatigue, increased contraction and half-decay times, and increased twitch and tetanic tension. Increased type I MHC was found, except for in GG in old and middle-aged rats.

CONCLUSION

Transitions in tongue muscle contractile properties and phenotype were found after NMES.

Comparison of Morphometric Aspects of Light and Electron Microscopy of the Hypoglossal Nerve between Young and Aged Male Wistar Rats

OBJECTIVE

Age-related changes occur in many different systems of the body. Many elderly people show dysphagia and dysphonia. This research was conducted to evaluate quantitatively the morphometrical changes of the hypoglossal nerve resulting from the aging process in young and aged rats.

MATERIALS AND METHODS

Through an experimental study ten male wistar rats (4 months: 5 rats, 24 months: 5 rats) were selected randomly from a colony of wistars in the UWC. After a fixation process and preparation of samples of the cervical portion of the hypoglossal nerve of these rats, light and electron microscopic imaging were performed. These images were evaluated according to the numbers and size of myelinated nerve fibers, nucleoli of Schwann cells, myelin sheath thickness, axon diameter, and g ratio. All data were analyzed by Mann-Whitney, a non-parametric statistical test.

RESULTS

In light microscope, numbers of myelinated nerve fibers, the mean entire nerve perimeters, the mean entire nerve areas and the mean entire nerve diameters in young and aged rats' were not significantly different between the two groups. In electron microscope, numbers of myelinated axons, numbers of Schwann cell nucleoli and the mean g ratios of myelinated axon to Schwann cell in young and aged rats were not significantly different. The myelinated fiber diameters, the myelin sheath thicknesses, myelinated axon diameters and the mean g ratio of axon diameter to myelinated fiber diameter in young and aged fibers were significantly different.

CONCLUSION

The mean g ratio of myelinated nerve fibers of peripheral nerves stabilizes at the level of 0.6 after maturation and persists without major change during adulthood. This ratio of axon diameter to fiber diameter (0.6) is optimum for normal conduction velocity of neural impulses. Our study indicated that the g ratio of myelinated nerve fiber of the hypoglossal nerve decreased prominently in aged rats and can be a cause of impairment in nerve function in old age. Thus, prospective studies concerning electrophysiological and conductive properties of the peripheral nerve could be useful to clarify further the effects of aging on peripheral nerves.

Upper aerodigestive tract neurofunctional mechanisms: lifelong evolution and exercise

The transformation of the upper aerodigestive tract--oral cavity, pharynx, and larynx--serves the functions of eating, speaking, and breathing during sleeping and waking hours. These life-sustaining functions may be produced by a central neural sensorimotor system that shares certain neuroanatomic networks while maintaining separate neural functional systems and network structures. Current understanding of development, maturation, underlying neural correlates, and integrative factors are discussed in light of currently available imaging modalities and recently emerging interventions. Exercise and an array of additional treatments together seem to provide promising translational pathways for evidence-based innovation, novel habilitation, and rehabilitation strategies and delay, or even prevent neuromuscular decline cross-cutting functions and supporting quality of life throughout increasingly enduring lifespans.

Effects of electrical stimulation on neuromuscular junction morphology in the aging rat tongue

Alterations in neuromuscular junction (NMJ) structure in cranial muscles may contribute to age-related deficits in critical sensorimotor actions such as swallowing. Neuromuscular electrical stimulation (NMES) is used in swallowing therapy, but it is unclear how NMJ structure is affected or if NMJ morphology is best measured in two or three dimensions. Two- and three-dimensional measurements of NMJ morphology in the genioglossus muscle were compared in rats that had undergone 8 weeks of hypoglossal nerve stimulation vs. untreated controls. The relationship between motor endplate volume and nerve terminal volume had a mean positive slope in 90% of the young adult controls, but it was positive in only 50% of the old controls; 89% of NMES old rats had a positive slope. NMJ measurements were more accurate when measured in three dimensions. In the NMJ, aging and NMES are associated with changes in the pre- and post-synaptic relationship.

Effect of tongue exercise on protrusive force and muscle fiber area in aging rats

PURPOSE

Age-related changes in tongue function may contribute to dysphagia in elderly people. The authors' purpose was to investigate whether aged rats that have undergone tongue exercise would manifest increased protrusive tongue forces and increased genioglossus (GG) muscle fiber cross-sectional areas.

METHOD

Forty-eight young adult, middle-aged, and old Fischer 344/Brown Norway rats received 8 weeks of tongue exercise. Protrusive tongue forces were measured before and after exercise. GG muscle fiber cross-sectional area was measured in exercised rats and was compared with cross-sectional areas in a no-exercise control group.

RESULTS

A significant increase in maximum tongue force was found following exercise in all age groups. In addition, a trend for increased GG muscle fiber cross-sectional area and a significant increase in variability of GG muscle fiber cross-sectional area was identified postexercise.

CONCLUSION

The findings of this study have implications for treatment of elderly persons with dysphagia using tongue exercise programs. Specifically, increases in tongue force that occur following 8 weeks of progressive resistance tongue exercise may be accompanied by alterations in tongue muscle fiber morphology. These changes may provide greater strength and endurance for goal-oriented actions associated with the oropharyngeal swallow and should be investigated in future research.

The effects of aging on hypoglossal motoneurons in rats

Aging can result in a loss of neuronal cell bodies and a decrease in neuronal size in some regions of the brain and spinal cord. Motoneuron loss in the spinal cord is thought to contribute to the progressive decline in muscle mass and strength that occurs with age (sarcopenia). Swallowing disorders represent a large clinical problem in elderly persons; however, age-related alterations in cranial motoneurons that innervate muscles involved in swallowing have been understudied. We aimed to determine if age-related alterations occurred in the hypoglossal nucleus in the brainstem. If present, these changes might help explain alterations at the neuromuscular junction and changes in the contractile properties of tongue muscle that have been reported in older rats. We hypothesized that with increasing age there would be a loss of motoneurons and a reduction in neuronal size and the number of primary dendrites associated with each hypoglossal motoneuron. Neurons in the hypoglossal nucleus were visualized with the neuronal marker NeuN in young (9-10 months), middle-aged (24-25 months), and old (32-33 months) male F344/BN rats. Hypoglossal motoneurons were retrograde-labeled with injections of Cholera Toxin beta into the genioglossus muscle of the tongue and visualized using immunocytochemistry. Results indicated that the number of primary dendrites of hypoglossal motoneurons decreased significantly with age, while no age-associated changes were found in the number or size of hypoglossal motoneurons. Loss of primary dendrites could reduce the number of synaptic inputs and thereby impair function.

Differences in age-related alterations in muscle contraction properties in rat tongue and hindlimb

PURPOSE

Because of differences in muscle architecture and biomechanics, the purpose of this study was to determine whether muscle contractile properties of rat hindlimb and tongue were differentially affected by aging.

METHOD

Deep peroneal and hypoglossal nerves were stimulated in 6 young and 7 old Fischer 344-Brown Norway rats to allow recording of muscle contractile properties of tongue and extensor digitorum longus (EDL) muscle in the hindlimb. In the same animals, the following measurements were made: (a) twitch contraction time (CT; in milliseconds), (b) half decay time (HDT; in milliseconds), (c) maximum twitch force (in grams), (d) tetanic force, and (e) fatigue index determined from repetitive stimulation of the muscles.

RESULTS

No significant differences were observed in young versus old groups in retrusive tongue forces, whereas a significant (p < .05) decrement in EDL tetanic forces was found in old rats. Slower CT in old rats was observed only in the tongue. Old and young groups were not significantly different in fatigue index or HDT for tongue or EDL.

CONCLUSIONS

Old animals generated equivalent maximum tongue forces with stimulation, but they were slower in achieving these forces than young animals. Limb and cranial muscles were not affected equally by aging. As such, information derived from limb muscle studies may not easily generalize to the cranial motor system.