Muscles that are affected by Bell’s palsy:
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Occipitofrontalis (frontal belly) — draws scalp anteriorly, raises eyebrows, and wrinkles skin of forehead horizontally as in look of surprise.
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Orbicularis oculi — closes eyelids.
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Orbicularis oris — closes and protrudes lips, as in kissing; compresses lips against teeth; and shapes lips during speech
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Buccinator — presses cheeks against teeth and lips, as in whistling, blowing, and sucking; draws the corner of mouth laterally; and assists in mastication (chewing) by keeping food between the teeth (and not between teeth and cheeks).
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Platysma muscles — depresses mandible (against resistance); draws outer part of lower lip inferiorly and posteriorly as in pouting
Mechanism of muscle weakness:
Bell’s palsy is an unexplained episode of facial muscle weakness or paralysis that is caused by damage to the facial nerve (7th cranial nerve). It begins suddenly and worsens over 48 hours. It is preceded by pain on the side of the face, particularly within the ear, temple, and mastoid area. It causes your face to droop on one side or, in rare cases, both sides of the face. You may also have an asymmetrical smile and/or an eyelid that won't close.
Other symptoms:
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Difficulty in speaking and eating
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Food is retained in the upper and lower buccal
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Buccinator weakens that results in the folding of labial
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Eyes cannot be closed; watering of eye
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Dropping of the mouth that may cause drooling of saliva
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Eyebrows on the affected side won’t rise
References:
Johns Hopkins Medicine. (2021, August 8). Bell's palsy. Retrieved from https://www.hopkinsmedicine.org/health/conditions-and-diseases/bells-palsy
Singh, P. (2016, November 3). Muscles of facial expressions. [Presentation slide]. Retrieved from https://www.slideshare.net/DrPraveenkumarSingh1/muscles-of-facial-expressions-68138031
Tortora, G. & Derrickson, B. (2014). Principles of Anatomy and Physiology (14th ed.). New Jersey, NJ: Wiley