The cutaneus trunci muscles form a subdermal bilateral sheet over the trunk and abdomen in animals that have difficulty scratching the chest wall.1 These muscles cover the thoracic walls caudal to the triceps brachii muscles, extend dorsally typically to within 5 cm of the dorsal midline, and extend ventrally toward the flank. The fibers of the muscles are oriented in a caudoventral direction and are diffusely attached to the skin and fascia.2–4 Cutaneus trunci muscles have a fascial band that unites with the tendons of the latissimus dorsi and teres major muscles, by which it attaches to the teres major tuberosity of the humerus and the glenohumeral joint capsule.4
The cutaneus trunci muscle reflex involves tactile stimulation of the skin that causes reflexive contraction of the cutaneus trunci muscle resulting in a localized skin twitch (response) that has the effect of removing irritants from the skin in areas of the body that are difficult for animals to scratch. The afferent cutaneous nerves of the cutaneus trunci muscle reflex are segmentally arranged and enter the spinal cord at the levels of T1-L3. The efferent part of the reflex is via the lateral thoracic nerve that exits the spinal cord at the levels of C8 and T1.5 Because of the segmental arrangement of the afferent part, the cutaneus trunci muscle reflex has clinical applications in the detection and localization of spinal cord lesions. The cutaneus trunci muscle reflex seems to be resistant to fatigue and habituation.1,6 Persistent stimulation of the cutaneus trunci muscle reflex may be a factor in failure of horses to habituate to wearing tack, which is manifest as behavioral signs of discomfort when the tack is secured. These signs, typically referred to as girthiness, include aggressive facial expressions and attempts to bite or kick the handler.a
Most striated muscles are attached to bones. Concentric contraction results in shortening of muscle fibers that approximates the bones. By contrast, contraction of the cutaneus trunci muscle applies tension to the skin as a result of its diffuse cutaneous attachments. In rats and guinea pigs, the cutaneus trunci muscle reflex results in a skin twitch that is localized approximately 2 cm rostral to the point of tactile stimulation7 and has a biphasic pattern.1 In horses, it is uncertain whether the cutaneus trunci muscle reflex is localized to the area of cutaneous stimulation, which would cause site-specific skin deformation, or whether it is a generalized response of the entire muscle that moves the skin toward the fixed point of attachment on the humerus.
The purpose of the study reported here was to localize and quantify skin deformation during the cutaneus trunci muscle reflex in response to skin stimulation at specific anatomic locations on the thoracic walls of horses. Our hypothesis was that skin deformation during the cutaneus trunci muscle reflex would have a predictable spatial relationship to the site of stimulation. This information would have application in determination of the optimal sites at which to observe the response and to measure nerve conduction velocities in the cutaneus trunci muscle reflex.
van Iwaarden A. A descriptive study on the anatomy and sensitivity of M. cutaneus trunci in horses in relation to girth pressure. MS thesis, University of Edinburgh, Edinburgh, Scotland, 2011.
Motion Analysis Corp, Santa Rosa, Calif.
Eagle Digital Cameras, Motion Analysis Corp, Santa Rosa, Calif.
Cortex software, Motion Analysis Corp, Santa Rosa, Calif.
Scala, Programming Methods Laboratory of EPFL, Lausanne, Switzerland.
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