How Smiles Control Us All

The neuronal signals for smiles usually start in the cortex of our brain. From there they travel to the deeper part, the brainstem -- which, in terms of evolution, also happens to be one of the oldest parts of our brain. From there, a nerve that's large enough to be visible to the naked eye, called the seventh cranial nerve, carries the signal in front of the ear to the more central part of the face, where it reaches the smile muscle.

Source: How Smiles Control Us All - Eric Finzi - The Atlantic