WBR0573

Author	[[PageAuthor::Rim Halaby, M.D. [1], Alison Leibowitz [2] (Reviewed by Alison Leibowitz)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::A 68-year-old male, with a history of chronic stable angina and hypertension, presents to the ER following 3 hours of violent involuntary movements of his right extremities. The patient explains that these attacks began suddenly and have since been repetitive and uncontrollable. Upon examination, you observe non-rhythmic, continuous, violent flinging motions of the right arm and leg. Although a cranial CT scan is negative, you still suspects an underlying ischemic stroke. Which of the following receptors is an important component of the pathway most likely affected in this patient?]]
Answer A	AnswerA::α-adrenergic receptors
Answer A Explanation	[[AnswerAExp::α-adrenergic receptors, α1 and α2. α1-receptors have several functions, such as vasoconstriction, pupillary dilation, and bladder sphincter muscle contraction. α2-receptors are responsible for decreasing sympathetic outflow, insulin release, and lipolysis. α-adrenergic receptors are not involved in modulation of skeletal muscle function.]]
Answer B	AnswerB::5-HT2 receptors
Answer B Explanation	AnswerBExp::5-HT2 receptors, not part of the inhibitory pathway of the basal ganglia, mediate many of the central and peripheral physiologic functions of serotonin.
Answer C	AnswerC::Nicotinic receptors
Answer C Explanation	[[AnswerCExp::Nicotinic acetylcholine receptors, transmitter gated ion channels, bind acetylcholine and are responsible for the postsynaptic action potential transmission at the neuromuscular junction. Although muscarinic acetylcholine receptors may be involved in the basal ganglia motor circuit, nicotinic receptors have no documented role.]]
Answer D	AnswerD::Dopamine D1 receptors
Answer D Explanation	[[AnswerDExp::Dopamine D1 receptors are involved in the excitatory or direct pathway in the basal ganglia. A decrease in D1 receptor activity or a defect in the excitatory pathway would lead to a decrease in motion. An imbalance favoring the D1 excitatory pathway over the inhibitory D2 pathway would lead to increased movement, which manifests in chorea, athetosis, or hemiballismus.]]
Answer E	AnswerE::Dopamine D2 receptors
Answer E Explanation	[[AnswerEExp::Dopamine D2 receptors are involved in the inhibitory or indirect pathway, leading to a net decrease in motion. Any defect in the inhibitory pathway, such as a subthalamic nucleus lesion, would lead to a shift of the balance toward the excitatory pathway, leading to increased movement, which is demonstrated in hemiballismus, chorea, and athetosis.]]
Right Answer	RightAnswer::E
Explanation	[[Explanation::Hemiballismus, a rare hyperkinetic disorder, is characterized by involuntary movements of large amplitude involving two ipsilateral limbs. The movements are often repetitive, irregular, and poorly-patterned. They arise from an insult to the contralateral subthalamic nucleus, resulting in an imbalance between the excitatory and inhibitory pathways of the basal ganglia. The subthalamic nuclei, of the inhibitory pathway, are significant for modulating movement activated by dopamine binding to D2 receptors. The inhibitory pathway involves disinhibition of the subthalamic nucleus by the globus pallidus externus (GPe), allowing the subthalamic nuclei to activate the globus pallidus internus (GPi), which inhibits the thalamus and decreases movement. Any process focal to the subthalamic nuclei or its connections may cause hemiballismus. In elderly patients, the most common cause of Hemiballismus is a vascular lesion, such as an infarct. Young patients usually have underlying an infection or inflammatory process. Illustrated below is an image depicting the direct and indirect pathways in the basal ganglia: Educational Objective: Hemiballismus frequently leads to involuntary flinging of extremities contralateral to a subthalamic nucleus lesion in the basal ganglia. The subthalamic nuclei, of the inhibitory pathway, are significant for modulating movement activated by dopamine binding to D2 receptors. References: Shannon KM. Hemiballismus. Curr Treat Options Neurol. 2005;7(3):203-210. Ristic A, Marinkovic J, Dragasevic N, Stanisavljevic D, Kostic V. Long-term prognosis of vascular hemiballismus. Stroke. 2002;33(8):2109-11.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Hemiballismus, WBRKeyword::Basal ganglia, WBRKeyword::Dopamine, WBRKeyword::Dopamine receptors, WBRKeyword::Inhibitory pathway, WBRKeyword::Excitatory pathway, WBRKeyword::hormones, WBRKeyword::neuroscience, WBRKeyword::neurology
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