Facial Nerve - Main Trunk
Figure 24. Cranial Nerve VII - Facial Nerve (main trunk)
The remaining portion of the facial nerve is mainly a somatic motor nerve with a minor somatic sensory component. The sensory component carries sensation from a small area of the pharynx and another small area of skin around the ear. Having noted the small sensory contribution, we will follow the somatic motor fibers. It must be mentioned that the first somatic efferent branch of the facial nerve is given off in the temporal bone. This nerve carries somatic motor fibers to the stapedius muscle which is located in the middle ear. The remainder of the nerve passes through the stylomastoid foramen into the facial portion of the head.
From here, there is a branch serving muscle in the immediate vicinity named the posterior auricular which carries both somatic sensory fibers as mentioned earlier to the area around the ear and motor fibers to the muscles around the ear as the name implies. These are in the muscles of facial expression group of muscles, as they move soft tissue rather than a joint.
The next branch given off runs to muscles that are not in the group of muscles of facial expression. This short branch goes to the posterior belly of the digastric muscle. The anterior belly is connected to the mandibular nerve which makes this muscle more interesting than ones with single innervation. There is also a short nerve that goes to a suprahyoid muscle, the stylohyoid, which is also anatomically in the path of the nerve as it exits the stylomastoid foramen.
From there the facial nerve enters the parotid gland and even while it is surrounded by the gland, it does not innervate it. Within the gland it splits into five terminal branches that innervate all of the numerous muscles of facial expression except the ones innervated by the aforementioned posterior auricular nerve. These branches are labeled by the area where they travel. These can be clearly seen in Figure 24 and from inferior to superior are the cervical, marginal mandibular, buccal, zygomatic and temporal. Confusion can sometimes result between the buccal branch of the mandibular nerve and this buccal branch of the facial nerve. The mandibular nerve branch is only sensory and the facial nerve branch is only motor. So just remember if you are doing a buccal nerve block it would logically be the sensory one. However, if during any anesthetic administration one injects into the capsule of the parotid gland the patient will experience transient Bell’s palsy as that affliction is caused by lack of function in the facial nerve serving the muscles of facial expression.