Contribution of the dorsal branch of the accessory nerve to the innervation of the trapezius muscle in the pig – a retrograde tracing study Agnieszka Dudek, 1,2 Waldemar Sienkiewicz 2 and Jerzy Kaleczyc 2 1 Faculty of Veterinary Medicine, Depart- ment of Animal Anatomy, Wroclaw Uni- versity of Environmental and Life Sciences, Kozuchowska 1/3, 51-631, Wroclaw, Poland; 2 Faculty of Veterinary Medicine, Department of Animal Anatomy, Univer- sity of Warmia and Mazury, Oczapowski- ego 13, 10-719, Olsztyn, Poland Keywords: dorsal branch of accessory nerve, Fast blue, pig, retrograde tracing, trapezius muscle Accepted for publication: 11 August 2014 Abstract Dudek, A., Sienkiewicz, W. and Kaleczyc, J. 2014. Contribution of the dorsal branch of the accessory nerve to the innervation of the trapezius muscle in the pig – a retro- grade tracing study —Acta Zoologica (Stockholm) 00: 000–000. The study was aimed to establish the contribution of the dorsal branch of the accessory nerve (DBXI) to the innervation of the porcine trapezius muscle (TRAP). Combined retrograde tracing using fluorescent tracer Fast blue (FB) and surgical denervation procedure (excision of DBXI segment) were applied. FB+ neurons supplying the cervical (c-TRAP) and thoracic part (th-TRAP) of TRAP were localized in following nerve centres: the ipsilateral ventral horn of the grey matter of cervical neuromers, ipsilateral spinal ganglia and bilateral sym- pathetic chain ganglia. After the excision of DBXI segment, no FB+ motoneu- rons supplying c-TRAP were found while the mean number of those supplying th-TRAP was significantly decreased. A slight decrease in average numbers of sensory and autonomic neurons implemented in the innervation of both parts of TRAP was also observed. This study has revealed that in the pig DBXI is the only motor pathway to c-TRAP while the vast majority of motoneurons supply- ing porcine th-TRAP send their axons via DBXI. Agnieszka Dudek, Faculty of Veterinary Medicine, Department of Animal Anat- omy, University of Warmia and Mazury, Oczapowskiego 13, 10-719 Olsztyn, Poland. E-mail: agnieszka.dudek@uwm.edu.pl Introduction In animals, the trapezius muscle (TRAP) is the major suspen- sory muscle of the shoulder girdle and joins the thoracic limb to the trunk. This muscle in cooperation with other muscles of the suspensory group is responsible for coordinating the movement of the limb, trunk, neck and head, but the most important function it has is to maintain the body balance. The porcine TRAP is divided into two parts: the cervical part (c-TRAP) and the thoracic part (th-TRAP) which plays different but complementary roles in its function. The cervical part originates from the nuchal crest (superior nuchal line) and inserts to the cranial border of the spine of the scapula, and helps swing the ventral angle of the scapula forward thus advancing the limb. The thoracic part takes origin from the spinal processes of the first ten thoracic vertebrae and inserts to the caudal border of the spine of the scapula (Klimov and Akajewski 1960), and rotates the scapula backward. Both parts acting together raise the scapula against the trunk and fix the shoulder. There are some controversies surrounding the innervation of the TRAP. Anatomists consider TRAP as unusual with regard to its innervation due to two main reasons: the first deals with the origin of this muscle – the trapezius derives from the 6th branchial arch mesoderm and is innervated by the accessory nerve, which is described as providing special visceral efferent fibres (SVE) and general somatic efferents (GSE). It is known that SVE originate from the nucleus ambiguous (Amb) and run with vagus nerve fibres via the internal branch of the accessory nerve to muscles of palate, pharynx and larynx. GSE originate from the spinal accessory nucleus (SAN) and run via the external branch of XI and innervate the trapezius and sternocleidomastoid derivates (Singh 2004). On the other hand, some researchers believe that all the fibres running via XI nerve must be SVE in nature (De Lahunta 1983). Such possibility is also mentioned in the © 2014 The Royal Swedish Academy of Sciences 1 Acta Zoologica (Stockholm) doi: 10.1111/azo.12098