Human Head and Eye muscles

Epicranius (frontalis), Levator labii superioris, Orbicularis oculi, Orbicularis oris, Buccinator, Masseter, Zygomaticus, Temporalis 1 3 2 7 6 5 6 4

Orbicularis oculi Epicranius Temporalis Levator labii superioris Zygomaticus Buccinator Orbicularis oris Masseter

Buccinator Massetor

What muscles were damaged?

Smiling The French neurologist, Duchenne du Boulogne, writing in the nineteenth century, made a fundamental discovery about the nature of smiling, a discovery that was forgotten for a hundred years. Duchenne electrically stimulated different positions on the face of a patient who had no pain sensation, and photographed the resulting expressions. By this means he learned how the different facial muscles produced different expressions. He showed that the zygomatic major muscle, which runs from the cheek bone down to the lip corners, pulls those lip corners upwards into a smiling shape (Fig. 1). Importantly, Duchenne noted that the man in this picture did not really look happy. He told him a joke, and noted that, when the man smiled spontaneously, it involved not just the zygomatic major, but part of a second muscle, the orbicularis oculi, which encircles the eye (Fig. 2). Duchenne noted that this is a muscle that most people cannot contract voluntarily, and so it ‘unmasks the false friend’.

Smiling In 1982, Paul Ekman resurrected Duchenne's distinction to explain why people often smile when they are not happy. As Duchenne had noted, Ekman found that people who smile when they are not feeling enjoyment do not show activity of the muscle around the eyes, just the lip muscle. Many studies now support Duchenne's distinction between these two types of smiling — what scientists now call, in honour of Duchenne, Duchenne smiles, or D-smiles for short (zygomatic major outer part, and orbicularis oculi) and non D-smiles ( zygomatic major only). For example, 5-month-old infants show D-smiles when approached by their mother, non D-smile when approached by a stranger. In adults the D-smile is accompanied by the pattern of brain activity found with enjoyment, but that brain activity pattern is not found when the non D-smile is shown. Happily married couples show D-smiles when they meet at the end of the day, while unhappily married couples show non D-smiles.

Smiling Non-Duchenne smile Duchenne smile

Smiling It is not always easy to distinguish these two types of smiles. If the lips are pulled only slightly or moderately by the zygomatic major muscle, it is easy to see whether the eye muscle is involved, for it will produce crow's feet wrinkles and bagging of the skin under the eyes. Those signs are absent in a slight to moderate non D-smile. However, when the smile is very broad, the lip pulling itself will produce those changes in the face and it is necessary to look elsewhere. Only in the D-Smile will the eyebrows move down ever so slightly. Instead of signalling genuine enjoyment, non D-smiles serve many different social functions. They may indicate agreement, they show a person is willing to go along with something, even something unpleasant (grin and bear it), and they may also be used to send a false message of enjoyment when none is felt. Research has shown that most people do not notice the difference between D-smiles and non D-smiles, and it is hard not to reciprocate a smile, even a non-D smile.

Superior, Inferior, Medial, Lateral rectus, Superior, Inferior oblique

Superior rectus Superior oblique Lateral rectus Medial rectus Inferior Oblique

Superior oblique Superior rectus Inferior rectus Lateral rectus

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NOTE: This is NOT the model in our lab. This is a RIGHT eye. Superior rectus Lateral rectus Medial rectus Inferior Oblique Inferior rectus

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Superior rectus Superior Oblique Lateral rectus Medial rectus

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Superior rectus Lateral rectus Inferior Oblique

NOTE: This is the model in our lab. This is a left eye. Medial rectus Lateral rectus Inferior oblique

NOTE: This is the model in our lab. This is a left eye. Lateral rectus Inferior oblique