The Sensory Field and Repeatability of the Cutaneous Trunci Muscle Reflex of the Dog

The cutaneous trunci muscle (CTM) is a thin but extensive sheet of skeletal muscle that covers most of the dorsal and lateral walls of the abdomen and thorax. It originates in the caudal gluteal region and runs cranially and ventrally to insert in the axillary region, where it is associated with the latissimus dorsi muscle and the caudal border of the deep pectoral muscle.1 It is mainly composed of type II (fast, glycolytic) fibers, and does not contain muscle spindles. Contraction of the CTM occurs as a reflex and the skin twitch is a defensive mechanism to remove foreign bodies or insects, or to increase heat production.2, 3 It may also be involved in other functions such as forced expiration, vomiting, coughing, and defecation4 and was originally described in the cat as the viscero-pannicular reflex in response to gall bladder insufflations.5 The CTM reflex, also sometimes called panniculus reflex because the CTM was formerly called the Panniculus Carnosus muscle,6 is classically elicited by applying mechanical or thermal sensory stimuli to the skin.2 The afferent input to the CTM reflex is provided by the cutaneous branches of spinal nerves that supply the skin of the thorax and abdomen.7 They enter the dorsolateral fasciculus and synapse with interneurons in the gray matter that project bilaterally and ascend in the fasciculus proprius to the motor nucleus in the cervical enlargement.2, 4, 8 In the dog, the cutaneous trunci motor nucleus is located in the ventral and ventrolateral nuclei of the ventral horn of the spinal cord primarily in the eighth cervical (C8) and first thoracic (T1) spinal cord segments, and occasionally in the C7 and T2 segments of the spinal cord.9 Motor innervation is provided by the lateral thoracic nerve that comprises a part of the brachial plexus.6, 10 The CTM reflex is commonly used to localize thoracolumbar spinal lesions in dogs but there is limited data available describing this reflex in normal dogs.10–12 Our aims for this prospective descriptive study are to describe the sensory field, and contraction strength and latency of the CTM reflex in normal dogs, and to evaluate its repeatability in myelopathic dogs.