Transcription
Welcome back to Anatomy dissected by complete. Anatomy. Here's another cranial nerve for you. This time, it's cranial nerve 7: the facial nerve.
As usual, we'll have a look at the function and course of this nerve and apply that knowledge as we consider facial paralysis. The facial nerve is primarily known for supplying the muscles of facial expression. It also has a supplementary role in receiving taste sensation from part of the tongue, and in the distribution of autonomic innervation to the salivary and lacrimal glands. Okay, let's get stuck into the anatomy.
The facial nerve emerges from the lower margin of the pons where it meets the medulla oblongata below. It emerges as two distinct routes: The motor route, often called the facial motor route (or the facial nerve proper), and the intermediate nerve. The motor root contains motor neurons. The intermediate nerve carries sensory and parasympathetic fibers. These two nerves remain distinct as they enter the internal acoustic meatus in the petrous part of the temporal bone. The intermediate nerve and the motor route merge just within the temporal bone. Here the bone has been faded so we can see what's going on inside. The two merged nerves are referred to as the facial nerve.
Just inside the temporal bone is the geniculate ganglion. This small swelling. It's located on the first of three 90-degree turns of the facial nerve. The greater petrosal nerve branches from the ganglion inside the bone and travels anteriorly to the pterygopalatine ganglion to supply the lacrimal, nasal, palatine, and pharyngeal glands with parasympathetic innervation.
The complex course of the facial nerve within the facial canal can be appreciated in this posterior view. Inside the bone, just after the second 90-degree turn, a small branch passes anteriorly into the middle ear, which we can appreciate in this lateral view. This is chorda tympani. It is so named because it runs behind the tympanic membrane. It exits on the anterior side of the temporal bone to join the lingual nerve where it picks up special sensory information from the anterior 2/3 of the tongue. Additionally, chorda tympani carries parasympathetic innervation to the submandibular and sublingual glands via the lingual nerve.
The rest of the facial nerve carries purely motor innervation. The facial nerve takes one last intracranial turn before it emerges from the stylomastoid foramen, a small hole located between the styloid process and the mastoid process of the temporal bone. It's not visible here, but before it emerges, the facial nerve gives a small branch to the stapedius muscle within the middle ear.
Following its exit from the cranium, two small motor branches emerge: medially is a branch that will provide innervation to the stylohyoid muscle, and laterally a nerve innervating the posterior belly of digastric muscle. There is a larger motor branch that bifurcates from the terminal branch and passes posteriorly. This is the posterior auricular nerve. It supplies the occipital belly of occipitofrontalis and auricularis muscles.
The terminal part of the facial nerve enters the posterior edge of the parotid gland and travels between the deep and superficial divisions of this gland. As it traverses the gland, it divides into five branches. Here's a mnemonic to help you remember the names of these branches: They are the temporal nerve, the zygomatic nerve, the buccal nerve, mandibular nerve, and cervical nerve. Each of these branches supply a subset of the muscles of facial expression around the eye, cheeks, mouth, and into the neck.
The most common cause of facial nerve palsy is Bell's palsy, which is idiopathic, meaning the cause is unknown. It results in unilateral dysfunction of the facial nerve, most obviously exhibited in paralysis of the facial muscles. Diagnosis of the condition is based on appearance and excluding other causes of paralysis. Recovery is usually spontaneous within three weeks.
And that's it! Another cranial nerve to tick off the list. Hopefully now you have a better understanding of the facial nerve, including its course through the temporal bone and parotid gland, as well as its autonomic and somatic functions in the face, tongue, and glands. Keep in touch by subscribing to our channel for more top-class anatomy content!