Mentalis muscle contraction causes the lower lip to
**Core Concept**
The mentalis muscle is a small, paired muscle within the chin area, innervated by the mental nerve, a branch of the inferior alveolar nerve (CN V3). It plays a crucial role in facial expressions, particularly in movements of the lower lip and chin.
**Why the Correct Answer is Right**
The mentalis muscle contracts to depress the lower lip, which is essential for various facial expressions, including a pouting motion or to form words. This action is mediated by the muscle's attachment to the skin and the mandible (lower jawbone). The contraction of the mentalis muscle helps to evert the lower lip, making it more prominent and visible.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the mentalis muscle does not contract to retract the lower lip. Retraction of the lower lip is primarily achieved by the depressor labii inferioris muscle.
**Option B:** This option is incorrect because the mentalis muscle does not contract to protrude the lower lip. Protrusion of the lower lip is primarily achieved by the depressor anguli oris muscle.
**Option C:** This option is incorrect because the mentalis muscle does not contract to elevate the lower lip. Elevation of the lower lip is primarily achieved by the zygomaticus major muscle.
**Clinical Pearl / High-Yield Fact**
The mentalis muscle is often involved in various facial expressions, including a "pouting" motion, and is also affected in conditions such as Bell's palsy, where facial weakness can lead to abnormal muscle contractions.
**Correct Answer: D. Depress the lower lip.**