Localization of brainstem motoneurons involved in dewlap extension in the lizard, Anolis equestris

Abstract

Dewlap extension is a characteristic component of inter- and intraspecific displays of Anolis lizards. Dewlap extension is accomplished by the contraction of ceratohyoid muscles associated with the hyoid apparatus. Retrograde transport of horseradish peroxidase (HRP) was used to investigate the brainstem origins of efferent fibers to the ceratohyoid muscles in Anolis equestris. Following application of HRP to the ceratohyoid muscle or to its nerve supply on one side of the throat, large polygonal motoneurons were found ipsilaterally in the vagal (Amb X) and glossopharyngeal (Amb IX) parts of nucleus ambiguus. Labelled neurons were more abundant and more heavily labelled in Amb X than in Amb IX. In addition, small spindle-shaped cells were labelled ipsilaterally in three parasympathetic nuclei that innervate glandular structures in the pharyngeal floor. HRP injections of the larynx labelled cells in many of the same locations, including Amb X and Amb IX. Thus, the nucleus ambiguus in Anolis contains motoneurons for supply of striated muscles in the hyoid (i.e. ceratohyoid muscles) and the larynx.