Opening of jaw is done by –
**Core Concept**
The jaw opening movement is a complex process involving the coordinated action of multiple muscles and bones. It requires the simultaneous depression of the mandible (lower jawbone) and the protrusion of the maxilla (upper jawbone).
**Why the Correct Answer is Right**
The primary muscles responsible for jaw opening are the digastric muscle, the medial pterygoid muscle, and the lateral pterygoid muscle. The lateral pterygoid muscle, specifically its superior head, plays a crucial role in the depression of the mandible. As the lateral pterygoid muscle contracts, it pulls the condyle of the mandible downward, thereby opening the jaw. This movement is facilitated by the joint between the mandible and the temporal bone, known as the temporomandibular joint (TMJ).
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the masseter muscle is primarily responsible for jaw closing, not opening.
**Option B:** This option is incorrect because the temporalis muscle is also involved in jaw closing, with its primary action being elevation of the mandible.
**Option C:** This option is incorrect because the sternocleidomastoid muscle is involved in rotating the head and neck, not opening the jaw.
**Clinical Pearl / High-Yield Fact**
The lateral pterygoid muscle is often the primary muscle involved in the opening of the jaw, and its dysfunction can lead to temporomandibular joint disorders (TMDs). TMDs are characterized by pain and limited mobility in the jaw, and can have significant impacts on a patient's quality of life.
**Correct Answer: C. Lateral pterygoid muscle.**