Platysma: action and cervical branch of the facial nerve
Platysma is a thin sheet of muscle under the neck skin that tightens (tenses) it, supplied by the cervical (neck) branch of the facial nerve.
Remember
Platysma tenses the skin of the neck and helps to pull the lower jaw down, and the cervical branch of the facial nerve supplies it, so isolated loss of neck skin tension points to that branch.
Mnemonic
To Zanzibar By Motor Car
- T Temporal
- Z Zygomatic
- B Buccal
- M Marginal mandibular
- C Cervical
The phrase lists the branches from top to bottom, and the last letter is the neck branch.
The platysma is a very thin muscle lying just under the skin at the front of the neck, in the superficial fascia (the loose connective tissue layer directly under the skin). It runs from the upper chest, over the collarbone, up to the lower border of the mandible (the lower jaw bone) and the lower face.
When it contracts, it tenses the skin of the neck and helps to depress the lower jaw, which simply means pulling it downwards. It also draws the lower lip and the skin of the lower face down, as in a grimace. Textbooks differ a little over the size of its effect on the lip and jaw, but jaw depression with tensing of the neck skin is the standard teaching.
It is a muscle of facial expression (one that moves skin and soft tissue rather than bone with great force), not a chewing muscle. Its nerve supply is the cervical branch of the facial nerve, the lowest of the five facial branches. That branch runs below the mandible, deep to platysma, so a superficial neck operation can injure it on its own.
The other branches control the rest of the face: temporal (brow), zygomatic (eyelid), buccal (cheek) and marginal mandibular (lower lip and mouth corner). So isolated loss of neck skin tension with normal forehead, eyelid, cheek and mouth movement points to the cervical branch. The marginal mandibular branch runs close to it and is the classic confusion. A drooping lower lip alone is less specific, because cervical branch injury can look like marginal mandibular injury.
Key facts
- Shape and position
- a very thin sheet in the superficial fascia, just under the skin at the front of the neck.
- Action
- tenses the skin of the neck and helps to lower the jaw.
- Nerve supply
- the cervical branch of the facial nerve, the lowest of the five facial branches.
- Why it can be injured alone
- the cervical branch runs below the mandible, deep to platysma, so a superficial neck operation can injure it on its own.
- Pattern
- isolated loss of neck skin tension with normal forehead, eyelid, cheek and mouth movement points to the cervical branch.
- Lower lip droop alone
- less specific, because cervical branch injury can look like marginal mandibular injury.
Exam traps
- The mandibular division of the trigeminal nerve does not supply platysma, because it supplies the muscles of mastication (chewing), while platysma takes its supply from the facial nerve.
- The marginal mandibular branch does not explain loss of neck skin tension, because it supplies the muscles that move the lower lip and the corner of the mouth; it runs close to the cervical branch, which is why the two are confused.
- The transverse cervical nerve does not move the neck skin, because it is sensory, so its injury causes numbness rather than weakness of a muscle.
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