WBR0937

Author	PageAuthor::William J Gibson (Reviewed by Serge Korjian)
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Anatomy
Sub Category	SubCategory::Head and Neck, SubCategory::Musculoskeletal/Rheumatology
Prompt	[[Prompt::An anatomy PhD student is performing an experiment to determine the muscles involved in mouth opening and clenching. He records electromyographic (EMG) activity from the inferior and superior heads of muscle A. Little EMG activity was noted in the inferior head during clenching of the mandible. However, strong activity was recorded from the inferior head during opening of the jaw. Which of the following does muscle A most likely represent?]]
Answer A	AnswerA::Masseter
Answer A Explanation	AnswerAExp::The masseter is one of the muscles of mastication, but it is responsible for closing the jaw.
Answer B	AnswerB::Lateral pterygoid
Answer B Explanation	AnswerBExp::The lateral pterygoid is the only muscle of mastication with two heads, and the only muscle responsible for opening the jaw.
Answer C	AnswerC::Medial pterygoid
Answer C Explanation	AnswerCExp::The medial pterygoid is responsible for closing the jaw.
Answer D	AnswerD::Temporalis
Answer D Explanation	AnswerDExp::The temporalis is a large muscle on the lateral edge of the skull responsible for closing the jaw.
Answer E	AnswerE::Mylohyoid
Answer E Explanation	AnswerEExp::The mylohyoid muscle is not classically considered a muscle of mastication. It serves to elevates the hyoid and the tongue during swallowing and speaking.
Right Answer	RightAnswer::B
Explanation	[[Explanation::The human body contains four muscles of mastication (chewing): the masseter, the medial pterygoid, the lateral pterygoid and the temporalis. Of these muscles, only the lateral pterygoid (pictured below) is responsible for opening the jaw. The temporomandibular joint plays a major role in mastication as it permits movement of the jaw in all planes. The muscles of mastication originate on the skull and insert into the mandible, thereby allowing for jaw movements during contraction. All of the muscles of mastication are innervated by the third division of the trigeminal nerve (V3). A simple way to remember the function of the lateral pterygoid is L' - Lowers, all the rest of the muscles (the Ms are used for closing the jaw. Ms Munch) Educational Objective: The lateral pterygoid is the only muscle of mastication responsible for opening the jaw. References: Widalm SE, Lillie JH, Ash MM. Anatomical and electromyographic studies of the lateral pterygoid muscle. J Oral Rehabil. 1987;14(5):429-446. Image: OpenStax College. Anatomy & Physiology, Connexions Web site.http://cnx.org/content/m46484/latest/, Jun 19, 2013.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Anatomy, WBRKeyword::Facial, WBRKeyword::Muscle, WBRKeyword::Head
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::