Final answer:
Muscles for smiling include the zygomaticus major and minor, while flaring the nostrils involves the nasalis muscle. These are controlled by the facial nerve, with corticobulbar innervation being primarily contralateral for the lower face. Neurological exams assess these muscles' strength and symmetry.
Step-by-step explanation:
The muscles used to smile are primarily the zygomaticus major and minor muscles which elevate the corners of the mouth. To flare the nostrils, the nasalis muscle is used. These muscles, among others, are controlled by the facial nerve. The facial nerve (cranial nerve VII) is responsible for the motor innervation of facial expression muscles. The corticobulbar innervation of these muscles allows voluntary control over these expressions. Notably, the innervation to the lower part of the face is mainly contralateral, meaning that each cerebral hemisphere controls the facial muscles on the opposite side of the face.
In a neurological examination, an examiner may ask a patient to perform facial movements, such as squeezing their eyes shut, to assess the strength and symmetry of the facial nerves. If the initial attempt shows weakness, the examiner might ask for a second attempt to confirm the finding or to see if there is any improvement, indicating a potential fatigue factor in the muscle strength.