A 65-year-old man reports multiple episodes of lightheadedness while buttoning a tight shirt collar. During 2 episodes, he passed out briefly but sustained no injuries. His blood pressure was 70/40 mm Hg and pulse was 45/min during one of the episodes. Past medical history is significant for hypertension and diet-controlled diabetes mellitus. The patient is a lifetime nonsmoker and drinks alcohol on social occasions. On physical examination, his blood pressure is 125/72 mm Hg and pulse is 76/min without orthostatic changes. Stimulation of afferent sensory fibers in which of the following nerves is most likely responsible for this patient's symptoms?

Question

A 65-year-old man reports multiple episodes of lightheadedness while buttoning a tight shirt collar.  During 2 episodes, he passed out briefly but sustained no injuries.  His blood pressure was 70/40 mm Hg and pulse was 45/min during one of the episodes.  Past medical history is significant for hypertension and diet-controlled diabetes mellitus.  The patient is a lifetime nonsmoker and drinks alcohol on social occasions.  On physical examination, his blood pressure is 125/72 mm Hg and pulse is 76/min without orthostatic changes.  Stimulation of afferent sensory fibers in which of the following nerves is most likely responsible for this patient's symptoms?

Quizplus · Accepted Answer

11ec160b_3c8e_de67_9249_b3915a887227_MD0001_00 The patient's history is suggestive of carotid sinus hypersensitivity, triggered by pressure on the carotid sinus by a tight shirt collar. The carotid sinus baroreceptors are important in blood pressure control and use arterial wall stretch as an indicator of systemic blood pressure. The carotid sinus is a dilation of the internal carotid artery located just above the bifurcation of the common carotid artery. The carotid sinus reflex has an afferent limb that arises from the baroreceptors in the carotid sinus and travels to the medullary centers via the Hering nerve, a branch of the glossopharyngeal nerve (CN IX). The efferent limb of the carotid sinus carries parasympathetic impulses via the vagus nerve (CN X). Carotid sinus pressure or massage stimulates the baroreceptors and increases the firing rate from the carotid sinus, leading to an increase in parasympathetic output and withdrawal of sympathetic output to the heart and peripheral vasculature. The result is decreased blood pressure (via peripheral vasodilation) and decreased cardiac output (decreased contractility/stroke volume and heart rate). In sensitive individuals, this response can cause severe bradycardia, hypotension, and sometimes syncope. (Choices A, C, and D) The accessory (CN XI), hypoglossal (CN XII), and trigeminal (CN V) nerves do not transmit baroreceptor signals. The accessory nerve controls the sternocleidomastoid and trapezius muscles, the hypoglossal nerve primarily controls the muscles of the tongue, and the trigeminal nerve mediates facial sensation and also controls the muscles of mastication (chewing). (Choice E) The vagus nerve (CN X) acts as the afferent limb for the nerve fibers originating from the aortic arch baroreceptors and as an efferent limb for the carotid sinus reflex. However, a tight collar would stimulate carotid sinus (not aortic arch) baroreceptor afferents carried by the glossopharyngeal nerve. Educational objective: The carotid sinus is a dilation of the internal carotid artery located just above the bifurcation of the common carotid artery. The carotid sinus reflex has an afferent limb that arises from the baroreceptors in the carotid sinus and travels to the vagal nucleus and medullary centers via the glossopharyngeal nerve (CN IX); the efferent limb carries parasympathetic impulses via the vagus nerve (CN X).

A 65-Year-Old Man Reports Multiple Episodes of Lightheadedness While Buttoning