Stability of the nasopharynx in horses reportedly is influenced by intrinsic and extrinsic factors.1,2 Most of the information about stability of the nasopharynx has been obtained through studies1–6 in which investigators evaluated the effect of a specific dysfunction through denervation or muscle resection-transection. Currently, knowledge is more advanced about intrinsic factors than extrinsic factors. The tensor veli palatini confers stability to the rostral aspect of the soft palate to stabilize the rostral and ventral aspects of the nasopharynx.3 The palatinus and palatopharyngeus muscles stabilize the caudal aspect of the nasopharynx, specifically the caudal half of the soft palate, and dysfunction of these muscles is associated with DDSP.4 The caudal stylopharyngeus muscles stabilize the roof of the nasopharynx.5 The hyoepiglottic muscle confers stability to the most caudal aspect of the nasopharynx by preventing epiglottic retroversion.6
Few investigations have been conducted to evaluate extrinsic factors relating to pharyngeal stability, but it is known that resection of the thyrohyoideus muscles leads to exercise-induced DDSP in some horses.1 The thyrohyoideus muscle extends from the thyroid cartilage to the thyrohyoid bone and is believed to be the most important muscle affecting elevation of the larynx as it moves the larynx rostrally.7,8 It has been proposed that laryngeal elevation enhances stability of the soft palate during exercise by moving the larynx rostrally and dorsally in relation to the hyoid apparatus.1 Another finding supporting the importance of the role of extrinsic structures is that transpharyngeal pressure (an estimate of increased resistance) increases in horses after partial myectomy of the sternothyroideus and sternohyoideus muscles.9
Information about the activity of many relevant muscles of the extrathoracic airway in horses is limited because studies have focused more on activity of the intrinsic musculature. In 2 studies, investigators reported that intrinsic inspiratory activity of the hyoepiglotticus muscle increases during exercise10 and that the stylopharyngeus muscle has inspiratory-related activity and tonic activity that increases with an exercise intensity.5
The study reported here focused predominantly on muscles extrinsic to the nasopharynx to provide additional information about their contribution in horses during exercise. The hypoglossal nerve innervates the genioglossus and geniohyoideus (suprahyoid or rostral hyoid) muscles.7 Contraction of the genioglossus, an extrinsic muscle of the tongue, causes a reduction in the dorsoventral diameter of the tongue, whereas the coordinated action of the genioglossus and geniohyoideus muscles causes the tongue to protrude, tenses the pharyngeal walls, and rostrally displaces the basihyoid bone to enlarge the diameter of the nasopharynx.7,11 However, computed tomographic analysis of horses with rostral displacement of the tongue manually induced to simulate rostral movement of the basihyoid bone and protrusion of the tongue yielded no measurable difference in nasopharyngeal diameter.12 The caudal hyoid (or infrahyoid) muscles include the sternothyroideus, sternohyoideus, and omohyoideus muscles, which all receive motor innervation from the first and second cervical nerves.7 Contraction of these muscles, which originate from the sternal manubrium and medial aspect of the shoulder fascia, results in caudal traction on the basihyoid bone and larynx. In humans and dogs, the opposing forces of the rostral and caudohyoid muscles sum to cause ventral displacement of the hyoid apparatus and increase the nasopharyngeal diameter.13 The hyoepiglottic and thyrohyoid muscles are innervated by the hypoglossal nerve.7 Hyoepiglottic contraction causes the epiglottic cartilage to be pulled ventrally, which increases the dorsoventral diameter of the rima glottis.14,15 Activity of the thyrohyoid muscle is predominantly linked with elevation of the larynx during swallowing.7,8,16
In humans, the suprahyoid and infrahyoid muscles are activated during the respiratory cycle via ongoing mechanoreceptor reflexes.17,18 Increasingly negative intraluminal pharyngeal pressures appear to recruit the pharyngeal muscles to stabilize the nasopharynx immediately before diaphragmatic and intercostal muscle activity.19–21 Activity of these rostral (suprahyoid) and caudal (infrahyoid) hyoid muscles has not been investigated in horses during exercise. We hypothesized that the genioglossus, geniohyoideus, hyoepiglotticus, omohyoideus, sternohyoideus, sternothyroideus, and thyrohyoideus muscles have activity correlated with intensity of exercise and in phase with inspiration in exercising horses.
Dorsal displacement of the soft palate
Mean electrical activity
Model BMA 830, CWE Inc, Ardmore, Pa.
Model TA11, Gould Electronics, Cleveland, Ohio.
Model 4000A, Vetter Inc, Reedsburg, Pa.
LCVR 100, Celesco transducers, Chatsworth, Calif.
Endevco model 220C, San Juan, Capistrano, Calif.
Labview, National Instruments, Austin, Tex.
PROC MIXED, SAS, version 9.1, SAS Institute Inc, Cary, NC.
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