It is a beautiful day to discuss another cranial nerve. We started with the Olfactory nerve (Cranial Nerve 1), and moved to the optic nerve (Cranial Nerve 2), then to the Oculomotor nerve (The Cranial Nerve 3), the Trochlear nerve (Cranial Nerve 4) wasn't left out, We decided to do a long and extensive post on the Trigeminal Nerve (Cranial Nerve 5), and in my last post, I discussed the Abducens Nerve (Cranial Nerve 6). Still on the Cranial Nerve Series, today, I will be discussing the Facial Nerve (Cranial Nerve 7).
Neuroanatomy, Function, and Clinical Correlation of the Facial Nerve
In my last post, Neurology Explained - The Abducens Nerve (The Sixth Cranial Nerve), I explained where to find the abducens nerve nucleus, and while explaining, I stated that the nucleus was above the nucleus of the cranial nerve 7 which is the Facial nerve. So, the Nucleus of the Facial Nerve (Cranial Nerve 7) is located in the pons of the Midbrain. I hope you remember that the nucleus of the nerve is the origin of the nerve, but the 7th cranial nerve has more than 1 nucleus, meaning it has several origins but which are in the Midbrain. The facial nerve motor nucleus is found in the Pons of the midbrain, and its Axon/fiber [Special visceral Efferent Fiber] (SVE) goes round the 6th cranial nerve (I mentioned this while explaining the 6th cranial nerve). While moving around the 6th cranial nerve, it creates a depression in the 4th ventricle (facial colliculus), then move anteriorly over the 6th cranial nerve to the Pons medulla junction.. The Fiber moves into the Internal Acoustic Meatus, and moves in through the facial canal, where they continue through the posterior wall of the tympanic cavity, the tympanic cavity branches into the ear canal where it innervates the stapedius muscle thereby preventing the Stapes (ear ossicles) from tapping the oval window during hearing process thereby preventing hypersensitivity to sound. The facial nerve innervates the ossicles of the ear , while the rest exits from the stylomastoid foramen to release branches which are the Temporal nerve branch, the zygomatic nerve branch, Maginal mandibular nerve branch, the cervical nerve branch, and Bucal nerve branch.
The Temporal branch supplies the occipiatal frontalis muscle which is found in the scalp, from the eyebrown to the superior region of the occipital bone, the Corrugator supercilii muscle which is located at the end of the eyebrown medially, and the Orbicularis Oculi located in the palpebral.. The zygomatic nerve branch, supplies the zygomaticus minor muscle, Zygomaticus major muscle, Orbicularis Oculi, the Levator Labii Superioris muscle, and the Levator Alaeque nasi muscle., . The Buccal nerve branch supplies the buccinator muscles, risorius, and orbicularis oris.. The Marginal Mandibular branch supplies the facial muscles; Depressor anguli oris, depressor labii inferioris, and mentalis muscles.. The Cervical branch innervates or supplies the platysma muscle. . Asides from the nerve branches of the facial nerve, there is another nerve branch that is facial nerve branch, which supplies the Digastric posterior belly muscle, and the Stylohyoid muscle
(You can read more on the ear ossicles in my post The Anatomy, and Physiology of the Ear.)
The Special visceral Efferent (SVE) provides innervation to the motors of the pharyngeal arches 1 (trigeminal nerve), 2 (facial nerve), 3 (glossopharyngeal nerve), 4 (superior laryngeal branch of the vagus), and 6 (recurrent laryngeal branch of the vagus)., .
Another facial nerve nucleus, in the dorsolateral portion of the pontine tegmentum (Pons), is the Superior salivary nucleus, and its nerve General Visceral Efferent fiber passes through the pons medulla junction with the nerve of the facial motor nucleus.. The General Visceral Efferent fibers go into the facial canal, then interiorly through the posterior canaliculus then to the petrotympanic fissure. The General Visceral Efferent, along with the Lingual nerve, and the Special Visceral Afferent runs together towards the face, but then, the General Visceral Efferent goes to the submandibular ganglion, and then supplies the Sublingual Salivary gland, and Submandibular salivary gland. . It is also important to state that from the facial canal, the General Visceral Efferent fibers branches towards hiatus of greater petrosal nerve to become the Greater Petrosal Nerve which keeps moving towards the middle middle cranial fossa to the foramen lacerum then towards the pterygoid (vidian) canal. From the pterygoid (vidian) canal, it reaches the pterygopalatine ganglion, where the Postganglionic parasympathetic fibers proceed moves to the pterygopalatine fossa. It then moves through the internal orbital fissure where it proceed to provide secretory and vasomotor innervation to the lacrimal gland to produce lacrimal fluid,as well as the nasal gland which produce nasal gland secretion, and the palatine glands to produce palatine secretion..
While discussing the Facial Nerve Pathway, it is important to talk about the General Somatic Afferent Fibers, which has its nucleus in the brain stem known as the Spinal nucleus of the trigeminal nerve (cranial nerve V). It is responsible for touch, temperature, proprioception, pressure, and pain. The fibers supply the tympanic cavity, part of the external acoustic meatus, the tympanic membrane, and skin of the ear. The GSA fibers known as the auricular nerve moves from the supplied regions to the Geniculate Ganglion after which its nerves moves to the Internal Acoustic Meatus, then to the Spinal nucleus..
Still in the Midbrain is the Nucleus Tractus Solitarius receives the sensory signals using the Special Visceral Afferent (SVA) fibers for taste in the anterior two-third of the tongue. It also receives sensory inputs using the General Visceral Afferent fiber (GVA) from the soft pallet in the mouth. The general Visceral afferent (GVA) supplies the Dorsal Nucleus Vagus. From the Soft pallet, the General Visceral Afferent fibers carry sensory sensations to the geniculate ganglion, then to the internal acoustic meatus, and take the sensation to the Nucleus Tractus Solitarius and the Dorsal Nucleus Vagus. From the tongue, the taste bud sends the taste sensation to the Special Visceral Afferent Fibers, which runs with the lingual nerve (trigeminal nerve), and the General Visceral Efferent fiber into the petrotympanic fissure, then to the posterior canaliculus, the General Visceral Afferent fiber, and the Special Visceral Afferent, together goes into the facial bony canal, then into the Internal acoustic meatus, after which the Special Visceral Afferent Fibers reaches its nucleus, the Nucleus Tractus Solitarius., , .
Clinical correlation/situation that could affect this nerve either as a result of tumors in the canal or palsy along the nerve could be disturbing. Facial nerve palsy such as Bell's palsy which is as a result of a Herpes Simplex virus causing facial muscle weakness or paralysis, is usually as a result of the compression og the 7th cranial nerve. These could lead to inability to identify taste, touch, pain, sensation, of the facial nerve regions, as well as neuragia.
Conclusion
The Facial nerve has its origin in the Pons of the Midbrain, with nuclei which includes The main motor nucleus, the parasympathetic nuclei (Superior salivary nucleus), and the sensory nucleus. The facial nerves pocesses Special visceral Efferent (SVE) fibers, General Somatic Afferent (GSA) Fibers, General Visceral Efferent (GVE), Special visceral Afferent (SVA), which supplies virtually all the muscles of the face that is responsible for facial expression, muscle of the ear (stapedius muscle) which prevents the stapes from continuously tapping the oval window and the tympanic membrane of the ear, and it supplies the lacrimal gland, nasal gland, and the palatine gland, it innervates muscles of the anterior two-third of the tongue for taste.
Image 1 || Wikimedia Commons|| Schematic drawing of the facial nerve
Image 2 || Wikimedia Commons