The Anatomy of the Trigeminal Nerve

The trigeminal nerve, also called the fifth cranial nerve, mediates sensations of the face and eye as well as many of the muscle movements involved in chewing. It is the largest of the twelve cranial nerves, and like the others, it is a peripheral nerve that originates in the brainstem.

The trigeminal nerve is most commonly associated with trigeminal neuralgia, a condition characterized by severe facial pain. Since it is large and has several divisions, the trigeminal nerve or its branches can also be affected by a number of medical conditions including infections, trauma, and compression from tumors or blood vessels.

Anatomy 

Everyone has two trigeminal nerves—a right trigeminal nerve and a left trigeminal nerve—and they are exactly the same in size and appearance. The trigeminal nerve is composed of several main branches, which include a motor nerve and three sensory nerves.

Structure

The three sensory nerve branches of the trigeminal nerve—the ophthalmic nerve, the maxillary nerve, and the mandibular nerve—converge in the trigeminal nerve at an area called the trigeminal ganglion to bring sensory information into the brain. The motor nerve branch of the trigeminal nerve is smaller than the sensory branches and exits from the brainstem through the root of the trigeminal nerve.

Location

The trigeminal nerve roots and ganglion, like those of other cranial nerves, are located right outside the brainstem. The brainstem is the lower part of the brain that serves as the physical connection between the spinal cord and the cerebral cortex of the brain. All 12 cranial nerves (12 in each side) emerge from the brainstem. The trigeminal nerve ganglion is located outside the pons of the brainstem, which is below the midbrain (the upper part of the brainstem) and above the medulla (the lower part of the brainstem).

The sensory input is received in these small nerve branches, which send their messages to the main sensory branches of the trigeminal nerve, then the trigeminal nerve root. The motor branch travels to the lower part of the head, face, mouth, and jaw to control mastication (chewing).

The branches of the trigeminal nerves travel along the pathways listed below.

Ophthalmic

The frontal nerve, the lacrimal nerve, and the nasociliary nerves converge in the ophthalmic nerve. These nerves and their small branches are located in and around the eye, forehead, nose, and scalp. The ophthalmic nerve enters into the skull through a small opening called the superior orbital fissure before it converges in the main branch of the trigeminal nerve. The region of the face that transmits sensation through the ophthalmic nerve is described as V1.

Maxillary

There are 14 small sensory nerves that converge to form the maxillary nerve. The sensory nerve endings are located in the scalp, the forehead, cheeks, nose, upper part of the mouth, and the gums and teeth. These nerves converge into four larger nerve branches—the middle meningeal nerve, the zygomatic nerve, the pterygopalatine nerve, and the posterior superior alveolar nerve—which converge to form the maxillary branch of the trigeminal nerve.

The maxillary nerve enters into the skull through an opening called the foramen rotundum. The maxillary nerve detects sensation in the middle part of the face, and this sensory area is often described as V2.

Mandibular

A nerve that receives input from nine branches, the mandibular nerve is largely sensory, but it has motor components as well. The nerve branches that detect sensation mediated by the mandibular nerve are located in the outer part of the ear, the mouth, tongue, jaw, lip, teeth, and chin. The mandibular nerve detects sensation in the lower part of the face, an area described as V3. 

Motor Branch

The motor branch of the trigeminal nerve travels from the pons to ipsilateral (on the same side) muscles in the jaw. These muscles are the temporalis, masseter, the medial and lateral pterygoids, the mylohyoid, the tensor tympani, the tensor vali palatini, and the anterior belly of the digastric muscle. 

Anatomical Variations

The structure and location of the trigeminal nerve and its branches are generally consistent from one person to another, but rare anatomical variations have been observed.

Divisions and merging of nerve branches can occur more distally (closer to the skin) or more proximally (closer to the nerve root in the brain) than expected. These variants are not generally associated with any clinical problem or symptoms, but they can present challenges during surgical procedures.

Function

The trigeminal nerve is one of a few nerves in the body that has both sensory and motor functions. The right and left trigeminal nerves each provide ipsilateral motor innervation and receive ipsilateral sensory input.

This means that sensation travels from the right side of the face to the right trigeminal nerve (likewise for the left side) and that motor function travels from the right trigeminal nerve to the muscles on the right side of the head and face (likewise for the left side). The function of the right and left trigeminal nerves is symmetrical.

Motor Function

The motor branch of the trigeminal nerve supplies several muscles, including the temporalis, masseter, the medial and lateral pterygoids, the mylohyoid, the tensor tympani, and the tensor vali palatini. These muscles are located in the jaw and their coordinated movement controls chewing.

The command for motor function of the trigeminal nerve comes from the cerebral cortex, which sends signals down to the pons in the brainstem. These commands are then carried out by the motor branch of the trigeminal nerve.

Sensory Function

The trigeminal nerve is responsible for carrying most of the sensation of the face to the brain.

• Ophthalmic nerve: This nerve detects and carries sensory input from the scalp, forehead, upper eyelid, eye, the outside and inside of the nose, and the sinuses.
• Maxillary nerve: This nerve receives sensation from the forehead, lower eyelid, sinuses, cheeks, middle part of the nose, nasopharynx, upper lips, upper teeth and gums, and the roof of the mouth.
• Mandibular nerve: The mandibular nerve receives sensation from the outer part of the ear, cheek, lower teeth, tongue, mouth, lower lips, and chin.

Associated Conditions

A condition called trigeminal neuralgia is the most common problem associated with the trigeminal nerve. There are also several other medical problems that can involve the trigeminal nerve or its branches.

Trigeminal Neuralgia

A condition that causes pain corresponding to trigeminal nerve sensory distribution on one side of the face, trigeminal neuralgia causes symptoms in either the V1, V2, or V3 regions or in a combination of these regions.

It can occur without any specific cause, and sometimes it can be triggered by an injury or inflammation of the trigeminal nerve. This condition often causes pain that is severe in intensity. Medications used for pain management include antidepressants and anticonvulsants, both of which are frequently used for nerve pain.

Surgical transection (cutting) of the nerve is an option when pain is persistent despite medical therapy. Surgical resection of the whole nerve of one of its branches results in loss of sensation, and may also cause muscle weakness. Trigeminal neuralgia is often difficult to manage, and interestingly, it can also resolve on its own without an explainable reason.

Head Trauma

A traumatic injury can cause damage to the trigeminal nerve. Symptoms correspond to the affected branch. Traumatic injury to the head and face can cause swelling or bleeding near the trigeminal nerve or its branches, impairing function of the nerve. If you have recent trauma affecting your trigeminal nerve, you may experience substantial improvement or even complete improvement once the swelling resolves.

Tumor

A brain tumor or a metastatic tumor that spreads to the brain, face, or neck can compress the trigeminal nerve or any of its branches, causing sensory loss, paresthesias (unusual sensations like tingling), pain, or weakness. Surgery, chemotherapy, or radiation treatment can reduce the impact of the tumor on the nerve if treatment is started before permanent nerve damage occurs. Sometimes, however, the nerve itself can be transected or damaged during surgical removal of a tumor.

Infection

An infection of the brain (encephalitis) or the meninges (the layers of covering that surround and protect the brain) can spread to the trigeminal nerve or any of its branches. Unlike the other conditions, an infection can involve both trigeminal nerves or it can infect branches on both sides.

Treatment with antibiotics and anti-inflammatory medication, if started in a timely manner, can prevent permanent deficits of the trigeminal nerve in the setting of infection. 

Cluster Headache

A recurrent pain syndrome characterized by one-sided head pain and eye pain, a cluster headache can also cause redness, photophobia, and changes in the size of the pupils. It is often considered a variant of migraine and may be caused by dysfunction of the ophthalmic branch of the trigeminal nerve.

Rehabilitation

Recovery and treatment of trigeminal nerve disease or injury depend on the condition itself. Managing the cause can help hasten recovery and prevent permanent nerve damage.

Techniques aimed at rehabilitation of damaged trigeminal nerves have not usually been found successful. However, training the nerve with a method of intermittent sensory stimulation may improve some nerve function, especially among people who have a decreased sensitivity of the nasal region.