Facial Nerve Definition
One of the most complicated cranial nerves is the facial nerve (CN VII). It’s a paired (left and right) mixed nerve that’s separated into sections depending on where it’s located (intracranial, intratemporal, and extratemporal). Multiple facial muscles, salivary and tear glands, and various sensory surfaces of the tongue are all controlled by CN VII.
Facial Nerve Anatomy
The architecture of the facial nerve is complex; it is a mixed cranial nerve (CN VII) that serves numerous tasks. The cranial nerve is divided into three portions in most textbooks: intracranial, intratemporal (inside the temporal bone), and extracranial.
Intracranial
The brainstem is where the intracranial facial nerve originates. It is made up of neurons that come from two different roots: a motor root and a sensory (and parasympathetic) root. The paired CN VII nerve is formed when the fibres in these roots merge. There are two of them: right and left. You can see how this nerve is separated into two different roots in the picture.
The intermediate (or sensory) root is thicker than the motor root (or motor portion). It is directly related to higher motor neurons that originate in the frontal lobe’s main motor cortex. The top motor neurons arrive in the face nucleus, a group of neurons.
The facial nucleus is divided into two parts: dorsal (back) and ventral (front). This will come in handy later when we look at facial nerve injury.
Both hemispheres of the brain provide information to the dorsal side of the facial nucleus. Both hemispheres govern facial expression in the upper face since the dorsal side is related to the top of the face.
The ventral sides govern the lower half of the face; the right hemisphere sends information to the left ventral component of the facial nucleus, and vice versa.
The face nucleus is where the motor root develops. Motor fibres arise at a place between the olive and the inferior peduncle after passing through the pons.
The medulla oblongata is where the sensory root ends. It’s vital to remember that efferent motor routes connect the central nervous system with muscle fibres, whereas afferent sensory pathways connect the sensory organs with the central nervous system.
Do you have problems distinguishing between efferent and afferent? They are identical (or rather, sensory afferent and motor efferent).
The facial nerve’s sensory root transports descending parasympathetic fibres from the brainstem’s salivary gland control centre (the salivatory nucleus) to the salivary glands. This root also contains ascending sensory fibres that carry data back to the brainstem. The parotid gland is not innervated by CN VII, despite the fact that it flows through it.
The intracranial component of the facial nerve refers to the motor and sensory parts of the facial nerve that are located within this area of the skull.
Intratemporal
The facial nerve leaves the intracranial section via the temporal bone’s internal auditory meatus. It travels along the facial canal, a groove on the interior of the temporal bone.
This mixed nerve is known as the infratemporal section of the facial nerve until it passes through the stylomastoid foramen.
The bodies (somas) of the sensory neurons of the facial nerve group form the geniculate ganglion after passing through the internal auditory meatus. A ganglion is a collection of cells with axons in peripheral tissues that perceive sensory signals including taste, touch, smell, and sound.
Past the geniculate ganglion, facial nerve parasympathetic fibres separate to form the larger petrosal nerve, the first facial nerve branch. Further down, you’ll find a list of facial nerve branches and their roles.
The stapedius muscle is innervated by a set of motor fibres on the second branch.
The chorda tympani is the intratemporal part’s third and final branch. This is the final time the facial nerve’s sensory nerves are used. All branches have motor effects after the chorda tympani.
The stylomastoid foramen is where the facial nerve leaves the skull.
Extracranial
The extracranial facial nerve is located outside of the skull. This section is referred to as the “motor root” since all extracranial branches are motor nerves. The graphic below shows the locations of these CN VII branches.
After the stylomastoid foramen, the extracranial facial nerve branches are:
- The Auricular nerve from behind
- The digastric muscle’s nerve
- The stylohyoid muscle’s nerve
The motor root of the facial nerve penetrates the parotid gland beyond this point, but it does not innervate it. The gland’s last five branches stretch out over the face:
- Temporal limb
- Branch zygomatic
- Buccal artery
- Mandibular marginal branch
- Cervical artery
Facial Nerve Function
Understanding which nerves cause which effects is necessary for facial nerve function. It’s critical to distinguish between the face nerve’s parasympathetic, sensory, and motor activities.
The autonomic nervous system includes the parasympathetic nervous system. The submandibular and sublingual salivary glands are innervated by facial nerve parasympathetic fibres, which urge them to release saliva. The modulation of the lacrimal glands for tear generation is the third and last parasympathetic function.
The sensory activities of CN VII create a perception of taste via innervating the tongue. Facial skin sensations are controlled by the trigeminal nerve rather than the facial nerve.
By far the most common are facial nerve motor functions. Face expression, communication, and the swallowing reflex all rely on these nerve branches.
The Facial Nerve Branches
- The Greater Petrosal Nerve
The larger superficial petrosal nerve is another name for it. The intermediate (sensory) root contains parasympathetic nerve fibres. The greater petrosal nerve controls blood flow to the nasal mucosa and facilitates taste perception in the front two-thirds of the tongue in conjunction with other (non-facial) nerves. The innervation of the tear-producing lacrimal glands is another common task.
- Nerve to Stapedius
The stapedius muscle is the smallest muscle in the body and is found in the inner ear. It stabilises the scapula. The stapes may move excessively if this branch is injured; even soft noises might seem quite loud (hyperacusis).
- Chorda Tympani
The chorda tympani branch is a sensory nerve that runs from the mucous membrane of the tongue to the brainstem. Special sensory afferent fibres innervate the taste buds in the front two-thirds of the tongue, and some parasympathetic fibres pass on to the submandibular and sublingual salivary glands.
- Posterior Auricular Nerve
The posterior auricular nerve innervates the occipitalis muscle, occipitofrontalis muscle, and muscles surrounding (and of) the ear, as well as non-facial nerve branches. Many of these tiny muscles contribute to the formation of auricular folds, which dampen human hearing.
- Nerve to the Digastric Muscle
The paired digastric muscle’s posterior belly is innervated by the facial nerve. The digastric muscle is a loop of muscle beneath the jaw that is joined by a tendon to two bands of muscle (bellies). Both bellies are required for complicated jaw movement as well as the lifting of the tongue bone while swallowing.
- Nerve to the Stylohyoid Muscle
The stylohyoid muscle joins the tongue bone (hyoid bone) to the base of the skull and is located near the digastric muscle’s posterior belly. It is one of the muscles that helps in swallowing.
- Temporal Branch
The frontalis muscle of the forehead, the orbicularis oculi muscle that shuts the eyelid, and the corrugator supercilii muscle that brings the eyebrows toward the nose are all innervated by this branch.
- Zygomatic Branch
This is another nerve that controls the left or right eyelid’s orbicularis oculi. The ocular muscles’ innervation is exceedingly complicated. Drooping eyelids are a typical indication of facial nerve injury.
- Buccal Branch
The orbicularis orbis, which rings the mouth, the buccinator muscle, which lies between the maxilla and mandible and maintains food near to the teeth during chewing, and the zygomaticus muscle are all controlled by the buccal branch of the facial nerve. If you want to smile, the zygomaticus muscle is necessary.
- Marginal Mandibular Branch
The paired mentalis muscle at the centre of the chin raises the skin around it and allows us to pout.
- Cervical Branch
The cervical branch is the last branch of the facial nerve’s motor root. The platysma muscle, which extends from the corners of the mouth to just above the collar bones, is connected to the cervical region. It is necessary for a lower facial expression and pulls the mouth corners downward.
Facial Nerve Damage
Any linked innervated structure may be paralysed, hyposensitive, or hypersensitive as a result of facial nerve injury.
The location of the injury is crucial; intracranial lesions might induce symptoms throughout the nerve’s entire length. Palsy is a phrase that is often used to describe facial nerve diseases. Muscle weakness, loss of feeling, uncontrolled movement, and paralysis are all symptoms of palsy.
Bell’s palsy, skull base or ear tumours, infection, inflammation, stroke, and congenital defects may all cause facial nerve issues. Given the prevalence of facial nerve problems, most hospitals with a neurological department will give patients detailed patient information on diagnosis and therapy. Physiotherapy may be required to recover muscular range of motion and strength.
Bell’s palsy develops quickly, with discomfort behind the ear as the first symptom. This leads to facial paralysis and, in some cases, a loss of taste sense. Patients with diabetes are more prone to having this condition. While the specific origin of Bell’s palsy is unknown, Lyme disease and viral herpes are suspected in certain instances. Bell’s palsy is a condition caused by CN VII injury. A single structure is seldom affected by a stroke or other reasons.
Ischemic stroke is the most prevalent cause of facial nerve injury. One side of the lower face is frequently affected. Remember that the top face is controlled by both hemispheres of the brain, but the lower face is exclusively controlled by the contralateral hemisphere.
If a blood clot in the brain prevents oxygen from reaching the motor cortex of the left hemisphere, right-sided paralysis of the face, mouth, and neck muscles may occur. This is because the lower motor neurons have been damaged. In terms of the upper facial muscles, the opposite hemisphere will try to take up the injured side’s duty.
The muscles of the upper and lower face will be impacted if the brainstem is compromised. This is because the higher motor neurons are damaged.
References
- Prashant MC. (2018). Oral and Maxillofacial Surgery: The Art and Science. Chennai, Notion Press.
- Koshi R. (2018). Cunningham’s Manual of Practical Anatomy Volume Three, Head, Neck and Brain, 16th Oxford, Oxford Unversity Press.
- Burchiel KJ, Raslan AM. (2019). Functional Neurosurgery and Neuromodulation. St Louis, Elsevier.
