Innervation of the syrinx of the zebra finch (Taeniopygia guttata)

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Beyond its outstanding beauty, the singing of songbirds has attracted the attention of scientists as a complex behavior learned by imitation (Marler, 2004). As a central component of social interactions, and particularly of courtship, this behavior has presumably been critical in speciation events throughout the history of songbirds (Grant & Grant, 2010). Furthermore, the learning of song by songbirds parallels to a certain extent the learning of speech by humans (Doupe & Kuhl, 1999; Marler, 1970). Much of the scientific interest in songbirds has focused on the central neural circuits involved in the learning and motor control of singing (Schmidt & Wild, 2014; Wild, 2004), and important advances have been made in recent years in our understanding of the mechanisms of sound production in the avian vocal organ, the syrinx, and its peripheral control (Elemans, 2014; Fee, Shraiman, Pesaran, & Mitra, 1998; Goller & Cooper, 2004; Goller & Larsen, 1997; Suthers & Zollinger, 2004). However, the presence of somatosensory feedback from the syrinx and its possible role in singing has received little attention (Bottjer & Arnold, 1982), a situation that could be alleviated by a more detailed description of syringeal innervation than presently available.
The syrinx is a specialisation of the junction between the trachea and the bronchi (Figure 1a–f). Its structure is highly variable among species, and has been extensively used as a character for phylogenetic analyses (Ames, 1971; King, 1989; Prum, 1992). In songbirds, the tracheo‐ and bronchio‐syringeal skeleton is highly modified. In the zebra finch, four to six of the caudal most tracheal rings and the first pair of bronchial rings are fused into a structure called the tympanum, and the rostral bronchial semi‐rings are flattened (Düring et al., 2013). The luminal side of the syringeal skeleton is covered with tissues rich in collagen, elastic fibers, and glycosaminoglycans (Riede & Goller, 2010). These tissues form two pairs of labia at the level of the bronchial modified semi‐rings, which produce sound as the expiratory airflow causes them to vibrate (Goller & Larsen, 1997). Outside the syringeal skeleton, there are several pairs of muscles that can originate and insert on the trachea, tympanum, bronchial semi‐rings and bronchial cartilages, and are able to independently modify the tension and degree of adduction of the two pairs of labia, thus modulating the acoustic frequencies produced on each side. Such muscles, with both origin and insertion within the syrinx, are called intrinsic syringeal muscles (King, 1989). Other muscles, connecting the syrinx to other structures such as the sternum or the larynx, are called the extrinsic syringeal muscles, and in songbirds they are thought to stabilise the organ during song production (Düring et al., 2013; Goller & Suthers, 1996).
The syringeal muscles are innervated by motoneurons in the tracheosyringeal part of the hypoglossal motor nucleus (nXII ts), which receives a direct projection from the telencephalic nucleus robustus of the arcopallium, the premotor forebrain nucleus of the song system (Nottebohm, Stokes, & Leonard, 1976). The syringeal motoneurons send their axons to the syrinx through the tracheosyringeal hypoglossal nerve (ts nerve). Through this nerve also travel afferent fibers of cell bodies located in the jugular vagal ganglion, the central projections of which project to the trigeminal sensory complex together with lingual hypoglossal afferents (Bottjer & Arnold, 1982; Faunes & Wild, 2017a; Wild, 1981). However, the sensory endings of the syringeal receptors have not yet been described, and their specific trigeminal targets have not been distinguished from the lingual afferent terminal fields.
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