Which cranial nerve is assessed when a patient is asked to smile and frown?

The facial nerve, known as cranial nerve VII, is primarily responsible for the movement of the muscles of facial expression. When a patient is asked to smile and frown, the facial nerve is being assessed as it controls the muscles involved in these specific movements. During smiling, the facial nerve activates the muscles in the cheeks and around the mouth, while frowning involves different muscle groups in the forehead and lower face.

The optic nerve (CN II) is involved in vision and would not contribute to facial expressions. The vagus nerve (CN X) has functions related to autonomic control and does not directly affect facial movements. The hypoglossal nerve (CN XII) primarily governs the movements of the tongue and is not involved in facial expressions. Therefore, evaluating a patient's ability to smile and frown directly tests the integrity and function of the facial nerve.