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Platysma
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Action: It draws the outer part of the lower lip inferiorly and posteriorly; depresses the mandible and wrinkles the neck.
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Orbicularis oculi
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Action: It closes the eye (blinking and squinting).
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Orbicularis oris
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Action: Closes and protrudes lips, as in kissing; compresses lips against teeth; and shapes lips during speech.
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Buccinator
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Action: Presses cheeks against teeth and lips, as in whistling, blowing, and sucking; draws 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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Occipitofrontalis
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Action: It draws the scalp anteriorly, raises eyebrows, and wrinkles the skin of the forehead, as in look of surprise.
Mechanism of the muscle weakness: As was stated by the National Institute of Neurological Disorders and Strokes (n.d.), it involves the facial nerve being inflamed which causes the paralysis of one side of the face of an individual. The said paralysis suddenly appears and Bell’s palsy is known to be the most common cause of facial paralysis. Ultimately, the specific cause for this is unknown.
Other symptoms aside for muscle weakness:
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Drooping eyebrow and mouth
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Drooling from one side of the mouth
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Difficulty closing an eyelid
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Excessive tearing in one eye
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Problems with taste
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Low tolerance for loud noises
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Problems in eating and drinking
Reference
Bell’s Palsy. (n.d.). National Institute of Neurological Disorders and Stroke. https://www.ninds.nih.gov/health-information/disorders/bells-palsy
Tortora, G. J., & Derrickson, B. (2017). Principles of Anatomy and Physiology (15th ed.). John Wiley & Sons, Inc.