When the jaw is opened
**Core Concept**
The question pertains to the anatomical and physiological mechanisms governing jaw movement, specifically the action of opening the jaw. The primary structures involved are the muscles of mastication, the temporomandibular joint (TMJ), and the associated nerves.
**Why the Correct Answer is Right**
When the jaw is opened, the primary muscle responsible for this movement is the digastric muscle. The digastric muscle consists of two bellies: anterior and posterior. The anterior belly is innervated by the mylohyoid nerve, a branch of the mandibular division of the trigeminal nerve (CN V3), while the posterior belly is innervated by the facial nerve (CN VII). As the digastric muscle contracts, it pulls the hyoid bone downward and the mandible upward, thus opening the jaw. This movement is also facilitated by the lateral pterygoid muscle, which is innervated by the masseteric nerve, a branch of the anterior division of the mandibular nerve (CN V3).
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the action of opening the jaw.
**Option B:** This option is incorrect as the masseter muscle is primarily responsible for closing the jaw, not opening it.
**Option C:** This option is incorrect as the medial pterygoid muscle also assists in closing the jaw, not opening it.
**Clinical Pearl / High-Yield Fact**
To remember the primary muscles involved in jaw movement, use the mnemonic "Digastric Lateral Medial, Closing the Jaw": digastric, lateral pterygoid, medial pterygoid, and masseter.
**Correct Answer:** D.