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PubMedID: 17350586 The performance of tongue muscles in various feeding behaviours is not well defined. This study was undertaken to examine the role of the intrinsic and extrinsic tongue muscles during natural drinking, food ingestion and chewing. Ten 12-week-old Yucatan miniature pigs (5 in each gender) were used. Under anesthesia, fine-wire electrodes were inserted into three intrinsic (verticalis and transversus [V/T]; superior and inferior longitudinalis [SL and IL]) and two extrinsic (genioglossus [GG] and styloglossus [SG]) tongue muscles and two jaw muscles (masseter [MA] and anterior digastricus [DI]). Electromyogram (EMG) and jaw movement were recorded and synchronized when pigs were drinking water, ingesting and chewing food freely. Chewing frequency (CF), onset of activation, burst duration and integrated activity (IEMG) were assessed quantitatively, and EMG activities during drinking and ingestion were examined qualitatively. Results indicate that during chewing, the V/T and GG had one phase of activity starting at early jaw opening, and the V/T activity lasted through late of jaw closing. The SL, IL and SG had double phases with the first starting at jaw opening and the second at late jaw closing phases. The three intrinsic tongue muscles and the SG were active during 35-48% of the chewing cycle. IEMG values of the SL, IL and SG of both sides were significantly greater compared to the other muscles (p < 0.05-0.01). Both the SL and the IL showed significantly higher activities in the contralateral than ipsilateral sides (p < 0.05). The timing sequences of both extrinsic and intrinsic muscles were similar between ingestion and chewing, but amplitudes of the GG and IL were greatly enhanced and those of the MA and SL were reduced during ingestion. The simultaneous activation of the MA, GG and V/T were seen during drinking, along with major activity in the GG and V/T. These results suggested that the majority of activity in the intrinsic and extrinsic tongue muscles occurred during jaw opening and the occlusal phases of chewing. The activity of the GG and IL played a major role during ingestion, whereas simultaneous activation of jaw, extrinsic and intrinsic tongue muscles and major activity in the GG and V/T occurred during drinking. © 2007 Elsevier Ltd. All rights reserved. National Institute of Dental and Craniofacial Research The authors would like to thank Dr. Sue Herring for her invaluable advice and critical comments, Dr. Katherine Rafferty for her comments, help with experiments and English editing, and Ms. Xian-Qin Bai for her general lab assistance. Thanks also go to two anonymous reviewers and the Associate Editor for their constructive comments and suggestions on earlier versions of the manuscript. This study was supported by the grant R01DE15659 from NIDCR to Dr. Liu.

The distribution of substance P (SP)-containing synaptic terminals in the hypoglossal nucleus (XII) of adult rats was examined by retrograde peroxidase labelling and immunocytochemistry. From the location of peroxidase injections into the tongue and of labelled neurones in the ventral lamina of XII, motor neurones that supply intrinsic vertical, longitudinal and transverse fibres as well as the extrinsic muscle genioglossus appear to have been labelled. SP-containing terminals were found making contact, and sometimes dual synapses, with unlabelled neuronal dendrites but not with retrogradely labelled somata or dendrites. These findings suggest that SP terminals may contact dendrites of interneurones or of neurones supplying other extrinsic muscles located in the anterior part of the tongue. Dual SP-containing synapses between XII motor neurones may be the means by which tongue muscle fibres are recruited and their function synchronized.

Rhythmical jaw movements can be evoked by intracortical microstimulation within four physiologically defined regions, one of which is the primary face somatosensory cortex (face SI). It has been proposed that these regions may be involved in the selection and/or control of masticatory patterns generated at the brainstem level. The aim here was to determine if mastication is affected by reversible, cooling-induced inactivation of the face SI. Two cranial chambers were chronically implanted in two monkeys (Macaca fascicularis) to allow access bilaterally to the face SI. A thermode was placed on the dura or pia overlying each SI that had been shown with micro-electrode recordings to receive intraoral inputs. A hot or cold alcohol-water solution was pumped through the thermodes while the monkey chewed a small piece of apple or a sultana during precool (thermode temperature, 37 degree C), cool (2-4 degrees C), and postcool (37 degrees C) conditions. Electromyographic (EMG) activity was recorded intramuscularly from the masseter, genioglossus, and anterior digastric. Cooling of SI impaired rhythmical jaw and tongue movements and EMG activity associated with mastication in one monkey (H5), and modified the pattern of EMG activity in the other (H6). The total masticatory time (i.e., time taken for chewing and manipulation of the bolus before swallowing) was increased. This was due principally to an increase in the oral transport time (i.e., time taken for manipulation of bolus after chewing and before swallowing: monkey H6, control, 2.7 sec; cool, 5.2 sec, p < 0.05); the bolus was manipulated by the tongue during this period before swallowing. Within the chewing time (i.e., time during which chewing occurred), cooling resulted in a significant increase in anterior digastric muscle duration, a significant delay in the onset of masseter EMG activity, and a significant increase in the variance of genioglossus EMG duration. The data support the view that the face SI plays a part in modulating the central pattern generator for mastication.

This study sought to characterise the electromyographic (EMG) activity patterns of orofacial-muscles during trained tongue-protrusion and biting tasks in two awake monkeys (Macaca fascicularis). Chronic or acute EMG electrodes were placed in the anterior digastric (DIG), genioglossus (GG), masseter (MASS), platysma (PLAT), zygomaticus major (ZYGO), orbicularis oris superior (OOS), and orbicularis oris inferior (OOI) muscles and their EMG activity as well as the force signals of the tongue-protrusion and biting tasks were recorded. A total of 327 tongue-protrusion task trials and a total of 210 biting-task trials were successfully completed in several recording sessions and the EMG patterns were generally consistent between the different sessions. For the tongue task, the mean onset time of increase in GG activity significantly (p0.0001) led the mean onset time of increase in the force. The DIG, GG, and OOI (and also the OOS in one of the monkeys) showed a significant (p0.0001) increase in mean EMG amplitude during the holding phase, but the GG in both monkeys had the highest mean EMG amplitude ratio (MAR), i.e. the mean EMG amplitude during the holding or dynamic phase divided by the mean EMG amplitude during the pre-trial period. A similar EMG pattern was documented for different directions of the tongue-protrusion task (right, symmetrical, and left) and changes in the levels of EMG activities occurred in GG and OOI as the direction of the tongue-protrusion task changed from left to right. The task at different forces was associated with no apparent change in MAR for the holding phase for each muscle recorded. However, during the dynamic phase, only the GG showed a significant increase in EMG activity as the forces were increased. For the biting task, the mean onset times of the MASS activity and force were not significantly different. The MASS and ZYGO muscles (and the PLAT in one of the monkeys) showed a significant increase in mean EMG amplitude during the holding phase compared with the pre-trial period, and the MASS showed the highest MAR. It was also the only muscle showing a significant increase in the EMG activity when the bite-force level was increased. These findings reveal that certain orofacial muscles are selectively recruited during the two different orofacial tasks.(ABSTRACT TRUNCATED AT 400 WORDS)

Genioglossus (GG) is the most important pharyngeal dilator muscle in maintaining upper airway (UA) patency in human; therefore, its dysfunction plays an important role in pathogenesis of sleep-related breathing disorder. Recently, the expression of estrogen receptors (ERs) on mRNA and protein level has been evidenced in GG muscle; however, the cellular localization of two subtypes of ER in GG myoblasts remains unclear. The present study was designed to clarify the expression and cellular distribution of ERs in rat GG muscle-derived satellite cells (MDSCs) and further probe the effect of ERs expression on regulation of intracellular Ca(2+). The immunocytochemistry revealed positive staining for both ERalpha and ERbeta in nuclei and cytoplasm of GG MDSCs. Noticeably, positive signals for ERalpha and ERbeta were comparable in cytoplasm, whereas the positive staining of ERalpha in nuclear was obviously strong than that of ERbeta. More intriguingly, by using Fluo 4-AM as a fluorescent Ca(2+) indicator and 17beta-estradiol (E2) as a stimulant, we observed that the level of intracellular Ca(2+) was not affected by E2 application, which implied that Ca(2+) signaling may not be involved in ER-mediated estrogenic effects on GG MDSCs. Taken together, the present study clearly indicates the differential cellular localization of ERs in rat GG MDSCs; moreover, ER-mediated estrogenic effect in rat GG MDSCs bears no relationship to intracellular Ca(2+) mobilization. In addition, the GG MDSCs express both ERalpha and ERbeta and therefore, provide a suitable and convenient in vitro cell model for investigating the molecular mechanisms of estrogenic effects on rat GG muscle.

PubMedID: 17210116 This study was undertaken to address the lack of information about tongue functional deformation in relation to jaw movement and muscle activity. Dimensional changes in tongue anterior and base widths, body length and base thickness were measured using six ultrasonic crystals implanted into the tongue in eight Yucatan minipigs. Jaw movements were captured on videotape and digitized, and electromyography (EMG) of tongue intrinsic (verticalis/transversus [V/T], superior and inferior longitudinalis [SL, IL]), extrinsic (genioglossus and styloglossus [GG, SG) and jaw (masseter and digastricus [MA, DI]) muscles were recorded. Signals from these three sources were synchronized. Tongue dimensions showed stereotyped and rhythmic changes during chewing cycles, with the largest changes in the body length and base thickness of the contralateral (non-working) side. The anterior tongue widened during jaw opening while the tongue base widened and thickened during jaw closing. The body lengthening accompanied base widening and ended at early power stroke, while base thickening lasted through most of the power stroke. Significant associations were found between changes of anterior width, body length and base thickness and integrated EMGs of VT, SL, SG, GG, MA and DI, but not IL. Thus, majority of tongue dimensional expansions occur during jaw closing. Intrinsic tongue muscle activities are not correlated more with tongue dimensional changes than are extrinsic tongue and jaw muscle activities. © 2006 Elsevier Ltd. All rights reserved. National Institute of Dental and Craniofacial Research The authors would like to thank Dr. Susan Herring for her invaluable helps and comments. This study was supported by grant R01DE15659 from NIDCR (Z.J.L.).

Mammals suckle from a nipple during the early neonatal period to obtain nourishment. The genioglossus muscle helps position and move the tongue for efficient suckling. The purpose of this study was to examine the contractile properties and myosin heavy chain (MHC) phenotype of the genioglossus following an early period of artificial rearing, which reduced nutritive suckling. Beginning at 3 days of age, rats were fed via gastric cannula until postnatal day 14 (P14). At P14, artificially reared rat pups were either allowed to grow to postnatal day 42 (P42) or anaesthetised and prepared for experimentation. Comparisons were made between artificially reared and dam reared groups at P14 and P42. At P14 maximum tetanic tension and fatigue index were lower in the artificially reared group than the dam reared group. By P42, artificially reared rats had a higher fatigue index and lower percentage of MHCIIa than dam reared rats. The artificial rearing technique employed in this study was adequate to produce chronic changes in fatigue resistance and MHC distribution in genioglossus muscle of rat; the changes observed here may be similar to changes that occur in premature human infants requiring early artificial feedings.

To examine whether short-term exogenous activation of a tongue muscle induced a phenotypic shift from a fast to a slow fibre-type, and thus assess a potential therapeutic avenue to protect against obstructive sleep apnoea (OSA).New Zealand White rabbit genioglossus (GG) muscle, characteristically a fast muscle, was continuously stimulated at a frequency attributed to slow muscle (10Hz, 3V DC pulses) using an implanted micro-circuit for 7 days. Changes in muscle fibre types and aerobic capacity were assessed between stimulated and un-stimulated (control) groups using immunohistochemistry and electrophoresis for myosin heavy chain (MHC) and assayed for citrate synthase.Compared to the un-stimulated control group, stimulated GG muscles had more (approximately 13%) type I MHC (slow-twitch) content; a proportional decrease in type II MHC (fast-twitch) isoform also occurred in the stimulated GG muscle (P0.05). Electrophoresis analysis on whole muscle and single fibre MHC showed an increased type I expression in the stimulated GG muscle (P0.01). A commensurate rise in citrate synthase activity, indicating a change in aerobic capacity, was also observed in the stimulated GG muscles.Together, these results demonstrate a successful alteration in tongue muscle characteristics using exogenous electrical stimulation and perhaps a potential therapeutic application for OSA.

The purpose of this investigation was to identify the myosin heavy chain (MHC) phenotype and fibre diameters of hypoglossal innervated extrinsic tongue muscles in rhesus monkey.Genioglossus, styloglossus and hyoglossus muscle samples obtained from three female rhesus monkeys were analysed for MHC isoforms via gel electrophoresis and stained with MHC antibodies to measure least mean diameters.MHC phenotypes were consistent for all three muscles. Each muscle was predominantly composed of MHC type IIa and I. All three isoforms were significantly different from each other in fibre diameter for styloglossus and genioglossus (IIbIIa and IIxI; P0.001). For hyoglossus, the MHC type II isoforms had larger diameters than the MHC type I isoform (P0.001).While the extrinsic tongue muscle MHC and/or muscle fibre type composition may be different between mammalian species, there are consistent similarities between the intrinsic and extrinsic tongue muscles. We suggest this is necessary for the highly coordinated activities performed by the tongue such as mastication, respiration and swallowing. The differences in fibre diameters among MHC isoforms suggest a large force gradation, which would be consistent with the coordination of these activities. The similarities among primates in MHC and/or muscle fibre composition as well as similar cortical inputs to the hypoglossal nucleus, suggest that we could expect to see similar MHC phenotype for extrinsic tongue muscles in human.

It is known that intracortical microstimulation (ICMS) of the lateral pericentral cortex can evoke masticatory movements and swallowing in awake monkeys. The aim was to determine if the ability of monkeys to carry out mastication is affected by reversible bilateral cold block of the ICMS-defined cortical masticatory area/swallow cortex. A cranial chamber was implanted bilaterally in two monkeys and a warm or cold alcohol-water solution was pumped through thermodes placed bilaterally on the dura overlying the ICMS-defined cortical masticatory area/swallow cortex while monkeys chewed standardised amounts of fruit during pre-cool (thermode temperature, 37 degrees C), cool (0-4 degrees C), and post-cool (37 degrees C) trials. Electromyographic (EMG) activity was recorded from masseter, genioglossus, anterior digastric, geniohyoid and thyrohyoid or perilaryngeal muscles. Vertical and horizontal jaw movements were recorded with a photodiode position transducer, which monitored movements of a light-emitting diode fixed to the mandible. Each masticatory period was divided into a food-preparatory phase, a rhythmic chewing phase and a preswallow phase. Both monkeys could readily accept and ingest the foodstuffs during pre-cool and post-cool trials. In contrast, cold block was associated with masticatory deficits, reflected in both monkeys as impaired food intake or manipulation and difficulty in carrying out a sequence of masticatory cycles, alterations in of the food-preparatory phase, and alterations in masticatory-related EMG patterns of the jaw and tongue muscles. The cold block-induced changes included significant (P

Changes in electromyographic discharge of 18 craniofacial muscles in young monkeys were assessed during the first 6 months of adaptation to nasal airway restriction. Thirteen experimental animals were paired with 13 controls for statistical analysis (Sign test, α < 0.05). The adapting craniofacial muscles were assessed for rhythmicity correlated with the respiratory pattern, and increase in spontaneous, background discharge defined as tonic activity. The following muscles were rhythmically active in normal monkeys: genioglossus (5 per cent of pooled observations from 13 animals), dorsal fibres of tongue (3 per cent), levator labii superiores (10 per cent), elevator of upper lip (13 per cent), dilator naris (68 per cent). In the experimental monkeys, significantly increased rhythmicity was demonstrated in: genioglossus (37 per cent, p = 0.001), dorsal tongue fibres (39 per cent, p = 0.004), lip elevator (43 per cent, p = 0.001). Rhythmic discharges were also induced in: geniohyoid (14 per cent, p = 0.031). digastric (7 per cent, p = 0.031), anterior temporalis (14 per cent, p = 0.031), lateral pterygoid (10 per cent, p = 0.031). Early adaptation to oral respiration rhythmically recruited muscles of tongue, upper lip-nasal region, and the mandibular depressors. Muscles of upper lip and nares showed maximum discharge at the start of inspiration; mandibular muscles exhibited a slow rise to maximum similar to that of the diaphragm. Muscles tonically active in the pooled observations from the controls were: geniohyoid (45 per cent), digastric (19 per cent), genioglossus (57 per cent), lip elevator (26 per cent), caninus (37 per cent), mentalis (63 per cent), anterior temporalis (53 per cent). Tonicity did not alter significantly in these 7 muscles, but was induced in several craniofacial muscles: the dorsal fibres of tongue (35 per cent, p = 0.031), superior orbicularis oris (25 per cent, p = 0.05), inferior orbicularis oris (32 per cent, p = 0.035), medial pterygoid (19 per cent, p = 0.031), lateral pterygoid (14 per cent, p = 0.008). Tonic activity increased by oral respiration was prevalent in tongue muscles and orbicularis oris.

Some of the factors involved in the control of genioglossus (GG) and masseter muscle activity were investigated in nine conscious human adults. The GG was active during inspiration and swallowing, and voluntary or passive openings of the jaw produced a definite enhancement of the GG activity. Electrical stimuli delivered to the tooth, tongue tip, anterior palate, upper lip and wrist, as well as tactile stimuli delivered to the maxillary central incisor and premolar teeth, depressed ongoing activity in the masseter and GG for 10–20 ms. Occasionally, a period of excitation preceded this silent period and sometimes later periods of excitation and suppression were noted. These effects may be related to mechanisms of protection and coordination of the oral-facial musculature.

Algesic chemicals (7 per cent NaCl, KCl and histamine) applied to this joint of anaesthetized cats evoked reflexes in the ipsilateral anterior digastric, temporalis and genioglossus muscles. Whereas the application of isotonic saline was only briefly and weakly effective, and only consistently so in the genioglossus, a single application of each chemical could produce a sustained increase in electromyographic activity of all the muscles. The excitatory effects usually lasted 30 s or more, and the onset latency and latency to peak activity were usually less than 10 s and 20 s, respectively. These reflex excitatory effects and their temporal characteristics are consistent with recent findings of the effects of these algesic chemicals on trigeminal brainstem nociceptive neurones, and provide support for concepts of temporomandibular joint pain and dysfunction that are based on reflexly induced increases in masticatory muscle activity.

Most authors consider the tongue active in secondary palate closure; some consider it passive; but specific information is wanting. We therefore studied the Holtzman rat foetal tongue by direct observation, histologically, under direct and neural stimulation, and when curarized. The tongue has differentiated into myoblasts (longitudinalis and verticalis) and young fibres (transversus, hyoglossus, genioglossus) four hours prior to closure. At this age it responds to both direct electrical and XII nerve nucleus stimulation. Curarization at high levels neither prevents closure nor abolishes foetal reflexes. The tongue myoneural apparatus is functional at palate closure.

A study of provoked animals in vivo, of dissection of adult animals and of serial sections of 6 foetuses suggested that the mechanism of drawing the normally intra-nasally situated larynx into the mouth in vocalization is as follows: the joint contraction of the genioglossus, mylohyoid and geniohyoid muscles is suggested as the agent which induces the forward movement of the hyoid. The sternohyoid and sternothyroid, contracting simultaneously with the palato-pharyngeus muscle, could assure the return of the larynx to its original position inside the nasal cavity. A well-developed thyrohyoid muscle would be responsible for the displacement of the larynx as a result of the forces exerted on the hyoid.