What is the primary action of the frontalis muscle?

• A. Elevates the eyebrows and wrinkles the forehead
• B. Depresses the mandible to open the mouth
• C. Retracts the angle of the mouth during smiling
• D. Closes the eyelids by blinking or squinting

Answer: (A)

Verified step by step guidance

1. Identify the anatomical location of the frontalis muscle: it is part of the occipitofrontalis muscle located on the forehead.
2. Understand the function of the frontalis muscle by considering its attachments and fiber direction; it connects to the skin of the forehead and scalp.
3. Recall that muscle actions are determined by the direction of muscle fibers and their points of attachment; since the frontalis muscle runs vertically on the forehead, contraction will pull the skin upward.
4. Recognize that pulling the skin of the forehead upward results in elevating the eyebrows and creating horizontal wrinkles on the forehead.
5. Conclude that the primary action of the frontalis muscle is to elevate the eyebrows and wrinkle the forehead, distinguishing it from muscles involved in jaw movement, smiling, or eyelid closure.