The Anatomy of the Facial Nerve

The facial nerve and its branches regulate a number of functions of the mouth and face. Most of its divisions stimulate muscles that allow eyelids to open and close, as well as facial movements. This nerve also mediates the production of tears and saliva and perception of taste in the tongue and receives some sensory input from the face as well. The facial nerve is the seventh of the 12 cranial nerves.

Bell’s palsy is the most common medical problem involving the seventh cranial nerve. It is an impairment of the function of the facial nerve that causes weakness of one side of the face. Bell’s palsy is usually a temporary condition and is not considered a threat to overall health. It does, however, manifest with symptoms similar to those of more serious medical problems, such as a stroke and multiple sclerosis, so be sure to seek urgent medical attention if you develop facial weakness. 

Anatomy 

The facial nerve has a complex anatomy. It is one of the longest cranial nerves, extending from the brainstem to the terminal (end) branches, which are located throughout the face. Several structures of the facial nerve—described as nuclei, segments, and branches—produce the four components of facial nerve function.

The facial nerve has:

• six major sections (described as segments) along the pathway from the brainstem to the terminal branches in the face
• divisions and subdivisions (also called branches), which are small nerves in and around the face that merge along the segments into the main facial nerve 
• three nuclei, which are the areas in the brainstem that transmit messages between the facial nerve and motor, sensory, and other areas of the brain 
• four components, which can be thought of as four functional categories

Structure 

The brainstem nuclei of the facial nerve are part of the central nervous system, while the facial nerve itself is a peripheral nerve. The facial nerve nuclei in the brainstem are called the motor nerve nucleus, the superior salivary nucleus, and the nucleus of the tractus solitarius. 

As the facial nerve emerges from the brainstem, it divides into smaller branches that travel towards the muscles and glands in the face.

Six of the facial nerve branches control facial movement. The temporal nerve controls the frontalis muscle. The zygomatic nerve controls the orbicularis oculi. The buccal nerve controls the buccinator and orbucularis oris muscles. The mandibular nerve controls the mentalis muscle. The cervical nerve controls the platysma, and the posterior auricular nerve controls the occipitalis muscle. 

Location

The facial nerve emerges at the pontine level of the brainstem. The brainstem is the part of the brain that is continuous with the spinal cord. It includes three sections, the medulla (right above the spinal cord), the pons (above the medulla), and the midbrain, (which is the uppermost region of the brainstem).

The first segment of the facial nerve, the intracranial (cisternal) segment, travels within the skull and divides into several branches. The nerve continues in the internal auditory canal near the ear as the meatal segment. As it travels near the structures of the ear, the labyrinthine segment is the shortest segment.

The tympanic segment runs through the bones of the middle ear. The mastoid segment, which follows, gives three branches, and the extratemporal segment branches into the nerves that control the face. 

Anatomic Variations

Minor differences in the structure and location of the branches of the facial nerve are not usually noticeable. These differences may be picked up when detailed imaging studies are performed for a medical reason.

Variations can be challenging when it comes to surgical procedures that involve the facial nerve. Pre-operative planning involves imaging, typically with contrast dye, to identify the anatomy of the nerve and detect variations ahead of time.

Function 

The four components of the facial nerve include motor, sensory, taste, and parasympathetic function. 

Motor

Most of the branches of the facial nerve are motor branches that stimulate the movement of the facial muscles. These muscles include:

• the stapedius muscle in the ear, which controls the vibration of a bone in the ear to help moderate hearing
• the stylohyoid muscle in the neck, which is involved with swallowing
• the posterior belly of the digastric muscle, which is involved with movements of chewing, swallowing, talking, and breathing 
• the muscles of facial expression are controlled by the facial nerve
• the frontalis muscle moves the forehead and eyebrows
• the orbiculus oculi, which controls the muscles of the eyelids
• the buccinator muscle, which moves the mouth and cheek
• the orbicularis oris, which controls movements of the mouth and lips
• the platysma, which is a large muscle in the neck that controls movements of the neck and jaw
• the occipitalis muscle, which is located in the back of the head and moves the scalp skin posteriorly.

The motor branches of the facial nerve activate muscles to move by releasing acetylcholine, a neurotransmitter that binds to the surface of muscle cells.

Sensory 

The facial nerve detects sensation from the small area behind the ear described as the auricle. The sensory information from this area is transmitted through the facial nerve to the brainstem, then to the thalamus in the brain, and eventually to the cerebral cortex, where the brain can integrate and make sense of the sensation.

Taste 

Taste sensation from the front of the tongue and the upper and lower part of the mouth is detected by the chorda tympani, a small nerve that travels from the tongue towards the brain to meet the facial nerve. 

Parasympathetic 

Nerve branches of the facial nerve simulate glands in the face. The lacrimal glands are located around the eyes. These glands regularly produce small amounts of tears to lubricate and protect the eyes. They also produce more tears when a person cries.

The submandibular glands near the mouth produce saliva to lubricate the inside of the mouth and to help with breaking down food. And the sublingual glands in the lower part of the mouth produce saliva as well. Parasympathetic branches of the facial nerve also stimulate mucous glands of the nose.

The parasympathetic function of the facial nerve is closely linked with the hypothalamus, which is an area of the brain that detects emotions and mediates certain survival functions, such as digestion.

Control of the Facial Nerve From the Brain

The function of the facial nerve is interesting because some motor branches of the facial nerve have bilateral (both sides) control from the brain and some only have unilateral (from one side) control from the brain. Nerve stimulation of muscle movement originates from the motor area of the cerebral cortex and crosses to the other side of the brain in the brainstem before transmitting impulses to the facial nerve.

The lower branches of the facial nerve that supply muscles in the lower two-thirds of the face are controlled by messages from only one side of the brain (the contralateral or opposite side). The upper branches of the facial nerve, which control the upper part of the face, receive messages from both sides of the brain.

The fascinating thing about this redundancy is that if the facial nerve can’t function properly due to a problem in the brain, the muscles of the forehead can still move.

On the other hand, if the facial nerve itself or its nuclei in the brainstem are damaged or impaired, then the entire side of the face becomes weak—resulting in more substantial and visibly obvious impairment of facial movement.

The sensation mediated by the facial nerve is detected on the opposite side of the brain because these messages also cross over in the brainstem. The crossing over of nerve impulses is not as influential when it comes to mediating functions of taste and glandular secretions.

Associated Conditions 

There are a few medical conditions that result in decreased function of the facial nerve. The symptoms typically involve weakness of the face because most branches of the facial nerve control facial movement. However, impairment of any of the other three components of facial nerve function can occur as well.

Bell’s Palsy

The most common condition affecting this cranial nerve, Bell’s palsy, can occur without a known cause (idiopathic) or as the result of a viral infection. The symptoms are usually very noticeable, with complete or partial paralysis of one side of the face, including the forehead. Bell’s palsy can be very distressing, often causing trouble speaking or even chewing. One eye can become dry, red, and irritated because it can be difficult to shut the eyelid. Tear production can be impaired as well.

It usually improves on its own, but it is best to seek medical attention because facial nerve weakness can occur as a result of other causes, including tumors or serious infections.

Stroke

A stroke is an interruption of blood flow in the brain. A stroke does not directly injure the facial nerve, but it can cause the decreased function of the facial nerve due to a lack of signals from the cerebral cortex, causing weakness in the contralateral lower portion of the face.

In some cases, a stroke can affect the brainstem and damage the nuclei where the facial nerve originates from. In those cases, the entire side of the face will get weak, like with Bell's palsy. However, other neurological signs will be present, such as double vision and problems with coordination or weakness, due to damage of additional structures nearby.

The effects are not as noticeable as the effects of Bell’s palsy because the forehead is still able to move due to its bilateral facial nerve control from the brain. However, a stroke is a serious condition requiring urgent medical attention. If you have had a stroke, this is also a sign that you could be at risk of having a heart attack or another stroke, so it is important to follow up with your healthcare provider to address the prevention of vascular disease.

Ramsay Hunt Syndrome

Shingles, which is a reactivation of herpes zoster (the chickenpox virus) can affect any nerve, including the facial nerve. Shingles affecting the facial nerve is described as Ramsay Hunt syndrome. The symptoms of Ramsay Hunt syndrome include face weakness and a skin rash on one side of the face. This condition can also cause decreased sensation, dizziness, or hearing loss. Ramsay Hunt syndrome can resolve on its own, but any of its effects may be permanent in severe cases. 

Guillain-Barre Syndrome (GBS)

Also described as acute demyelinating polyneuropathy, or acute ascending polyneuropathy, this is a disease of the peripheral nerves that can affect both facial nerves at the same time. GBS usually begins in the feet, causing weakness. It may rapidly worsen, causing ascending (climbing) weakness in the legs, breathing muscles, arms, and face.

This syndrome can begin suddenly and it may be fatal due to the weakness of the breathing muscles. If you develop GBS, you can recover if you are treated with respiratory support—which may require a mechanical ventilator—until the condition resolves.

Aneurysm

An out-pouching of a blood vessel can compress the facial nerve or one of its branches and may cause facial weakness or hemifacial spasm.

Infection

An infection such as Lyme disease can preferentially attack one or both facial nerves, causing temporary or long-term symptoms. If the infection is identified, antibiotics can help hasten recovery and may prevent permanent damage and dysfunction of the nerve.

Traumatic Injury

Head or face trauma can injure the facial nerve or one of its branches, potentially causing weakness of some muscles of the face, as well as the impaired function of any of the components of the facial nerve. 

Tumor

A brain tumor or metastatic (spreading) cancer from the body can compress or invade the facial nerve nuclei or a segment or branch of the facial nerve, causing impaired function, typically on one side of the face. A large tumor can compress both nuclei in the brainstem, however, producing symptoms on both sides of the face.

Rehabilitation

If you have had any type of facial nerve disease or injury, recovery includes physical therapy, which can help your face and mouth muscles regain at least some of their strength. The extent of recovery depends on the type and severity of the damage, how much of the nerve was involved, and the type of disease.

If you have a tumor or some other mass putting pressure on the facial nerve, surgical removal of the mass may help alleviate the nerve impairment. Facial nerve reconstruction and grafting are methods that have been used to repair a damaged facial nerve.

These procedures may improve motor function for some facial nerve injuries. Techniques include stereotactic and minimally invasive procedures, as well as more extensive open procedures.