An extract on #muscles
The facial muscles are subcutaneous (just under the skin) muscles that control facial expression. They generally originate from the surface of the skull bone (rarely the fascia), and insert on the skin of the face. When they contract, the skin moves. These muscles also cause wrinkles at right angles to the muscles action line. Use of these muscles is noted during an extraoral examination, assuring function of the nerve to these muscles.
The facial muscles are innervated by facial nerve (cranial nerve VII), with each nerve serving one side of the face. In contrast, the nearby masticatory muscles are innervated by the mandibular nerve, a branch of the trigeminal nerve (cranial nerve V).
The facial muscles include:
Depressor septi nasi muscle
Orbicularis oculi muscle
Corrugator supercilii muscle
Depressor supercilii muscle
Auricular muscles (anterior, superior and posterior)
Orbicularis oris muscle
Depressor anguli oris muscle
Zygomaticus major muscle
Zygomaticus minor muscle
Levator labii superioris
Levator labii superioris alaeque nasi muscle
Depressor labii inferioris muscle
Levator anguli oris
The platysma is innervated by the facial nerve. Although it is mostly in the neck and can be grouped with the neck muscles by location, it can be considered a muscle of facial expression due to its common innervation.
The stylohyoid muscle, stapedius and posterior belly of the digastric muscle are also innervated by the facial nerve, but are not considered muscles of facial expression.
An inability to form facial expressions on one side of the face may be the first sign of damage to the nerve of these muscles. Damage to the facial nerve results in facial paralysis of the muscles of facial expression on the involved side. Paralysis is the loss of voluntary muscle action; the facial nerve has become damaged permanently or temporarily. This damage can occur with a stroke (cerebrovascular accident or CVA), Bell palsy, or parotid salivary gland cancer (malignant neoplasm) because the facial nerve travels through the gland. The parotid gland can also be damaged permanently by surgery or temporarily by trauma. These situations of paralysis not only inhibit facial expression but also seriously impair the patients ability to speak, either permanently or temporarily.
Since only a small part of the eye called the fovea provides sharp vision, the eye must move to follow a target. Eye movements must be precise and fast. This is seen in scenarios like reading, where the reader must shift gaze constantly. Although under voluntary control, most eye movement is accomplished without conscious effort. Precisely how the integration between voluntary and involuntary control of the eye occurs is a subject of continuing research. It is known, however, that the vestibulo-ocular reflex plays an important role in the involuntary movement of the eye.