Final answer:
To smile, the zygomaticus major and orbicularis oris muscles contract to pull the mouth upward and shape the mouth, respectively. Other muscles like the buccinator and risorius also play roles in adjusting the nuances of the smile. The facial feedback hypothesis further suggests that these expressions can even influence our emotions.
Step-by-step explanation:
To smile, several muscles of facial expression must contract. The primary muscles involved are the zygomaticus major which pulls the corners of the mouth upward and the orbicularis oris which can shape the mouth for different expressions.
In addition, the risorius may assist in smiling by pulling the lips sideways, and the levator labii superioris can raise the upper lip to create a more complete smile.
Although not directly responsible for smiling, the buccinator muscle compresses the cheek which could affect the nuances of a smile, by adjusting the tension and contour of the cheeks.
These muscles are all controlled by the facial nerve, which is the seventh cranial nerve. Directed tests for facial nerve function can measure the strength and coordination of these muscles, as seen in the video examination where the patient is asked to perform actions such as squeezing their eyes shut against resistance.
The concept that our facial expressions can affect our emotional state is known as the facial feedback hypothesis. This intriguing idea suggests that smiling may indeed lead to feelings of happiness, highlighting the interplay between the muscles of facial expression and our emotions.