A 45-year-old man comes to the clinic because of difficulty turning his head to the left and raising his right shoulder after excision of a lymph node from the right posterior triangle of the neck one week ago. He has no difficulty swallowing, speaking, tasting food, or moving his face. Physical examination shows weakness with right shoulder shrug and weakness turning the head to the left against resistance. Sensation over the neck, shoulder, and upper extremity is intact. Pupillary size and sweating are normal bilaterally. The damaged nerve fibers most directly responsible for this patient’s weakness normally innervate which of the following target tissues?
The correct answer is:
C) Skeletal muscle
This patient has injury to the right spinal accessory nerve during a posterior triangle lymph node excision. The spinal accessory nerve innervates the sternocleidomastoid and trapezius muscles, causing weakness with contralateral head rotation and ipsilateral shoulder shrug when injured. These muscles are skeletal muscles under voluntary motor control, so the injured fibers are somatic efferent fibers. The intact sensation, normal facial movement, preserved taste, and absence of autonomic findings help distinguish this lesion from sensory, branchial motor, special sensory, or autonomic pathway injury.
Answer choice A: Cardiac muscle, is incorrect. Cardiac muscle is regulated by autonomic visceral efferent fibers, including sympathetic fibers that increase heart rate and contractility and parasympathetic vagal fibers that decrease heart rate. Injury to the spinal accessory nerve does not directly affect cardiac muscle.
Answer choice B: Glands, is incorrect. Secretomotor innervation to glands is mediated by autonomic fibers, such as parasympathetic fibers to the lacrimal, salivary, and nasal mucosal glands or sympathetic fibers to sweat glands. This patient has a voluntary motor deficit involving the trapezius and sternocleidomastoid rather than impaired lacrimation, salivation, or sweating.
Answer choice D: Smooth muscle, is incorrect. Smooth muscle is innervated by autonomic visceral efferent fibers and is found in structures such as blood vessels, bronchi, gastrointestinal tract, bladder, and iris. A lesion affecting smooth muscle innervation would cause autonomic findings such as altered pupillary size, vascular tone, or visceral motility rather than isolated shoulder shrug and head-turning weakness.
Answer choice E: Taste receptors, is incorrect. Taste is carried by special visceral afferent fibers from cranial nerves VII, IX, and X. Injury to these fibers would cause gustatory deficits, not weakness of the sternocleidomastoid and trapezius muscles.
Key Learning Point
Somatic efferent fibers innervate skeletal muscle, whereas visceral efferent fibers provide autonomic motor innervation to cardiac muscle, smooth muscle, and glands.
