The inability to move the muscles that control smiling, blinking, and other facial movements is a symptom of a condition known as facial nerve paralysis. This condition can make it difficult for a person to communicate how they are feeling. Facial paralysis affects only one side of the face the vast majority of the time.

The facial nerve is responsible for supplying the facial muscles with their respective motor branches.

Therefore, in order to test this nerve, the doctor will ask the patient to furrow their brows (raise their brows), close their eyes and keep them closed against resistance, puff out their cheeks, and show their teeth.