The lacrimal nerve represents a branch of the ophthalmic nerve and sensitively innervates the conjunctiva (tunica conjunctiva) and parts of the eyelid. It is connected to the facial nerve and the zygomatic nerve. The latter gives fibers to the lacrimal nerve via a communication branch, which influence the lacrimal gland.
What is the lacrimal nerve?
In German, the lacrimal nerve is also called the tear nerve because it is found in the area of the lacrimal gland (glandula lacrimalis). The lacrimal nerve embodies one of the main branches that branch off from the ophthalmic nerve. This is also known as the ocular nerve and has a total of four such main branches:
- Ramus tentorii
- Frontal nerve
- Lacrimal nerve
- Nasociliary nerve
The ophthalmic nerve originates from the triplet nerve (trigeminal nerve), which is the fifth cranial nerve that provides a direct connection to the brain. Unlike other nerves, the cranial nerves do not leave the skull through the spinal cord, but through various openings and channels in the bone. A total of twelve cranial nerves extend from the head into the neck region and, in some cases, into the chest and abdomen.
Anatomy and structure
The lacrimal nerve branches off from the ophthalmic nerve in the middle cranial fossa (fossa cranii media). From there, it runs to the eye, passing through the superior orbital fissure (fissura orbitalis superior), which is located at the sphenoid bone. Once at the orbit, the lacrimal nerve moves toward the lacrimal gland and divides into many branches. The individual branches are very thin and in this way cover a wider area. Instead of already ending there, the lacrimal nerve extends upward from the corner of the eye and finally reaches the upper eyelid through the orbital septum (ligamentum palpebrale). Only there do the ends of the nerve fibers extend. The individual fibers of the lacrimal nerve represent the elongated extensions of the neurons and arise from the respective axon hillock. The nerve fibers of the lacrimal nerve are sensitive fibers that transmit sensations from the conjunctiva as well as from the skin, which is located at the side of the eye, to the brain. However, efferent fibers are added from the pterygopalatine ganglion.
Function and Tasks
The fibers of the lacrimal nerve are sensitive: they pick up sensations from the area surrounding the lacrimal gland and transmit the perceived stimuli as electrical impulses to the central nervous system. Furthermore, the lacrimal nerve travels to the upper eyelid. There, it also receives sensitive signals from the corner of the eye in the lateral area of the eyelid – in addition, the lacrimal nerve picks up information from the conjunctiva. The electrical signal moves erratically across the nerve fiber because it is surrounded by myelin sheaths. At regular intervals between two such insulations is a so-called Ranvier’s cord ring. Here, the fiber depolarizes, thereby forming the action potential anew at each lacing ring. This type of transmission is particularly fast. At various points, the lacrimal nerve maintains connections to other nerves. These include the ramus zygomaticotemporalis, which is a branch of the zygomatic nerve. This innervates the skin regions at the temporal bone and zygomatic bone. Part of the zygomatic nerve participates in the wing palatal ganglion (pterygopalatine ganglion), which is located at the base of the skull. From this ganglion, nerve fibers of the ramus zygomaticotemporalis extend to the lacrimal nerve and unite with it. This part of the nerve pathway consists of efferent fibers that influence the lacrimal gland. On the upper eyelid, there is a second connection of the lacrimal nerve to another nerve. In this case, it is parts of the facial nerve or seventh cranial nerve. Medicine also refers to it as the facial nerve. It carries not only sensitive fibers like the lacrimal nerve, but also sensory, motor and parasympathetic.
Diseases
The lacrimal nerve forms a branch of the ophthalmic nerve, which in turn belongs to the trigeminal nerve. Damage to this cranial nerve that also affects the lacrimal nerve can lead to sensory disturbances in the affected area. Because the trigeminal nerve includes more than just sensory fibers, paralysis is also possible. Zoster ophthalmicus is an infectious disease that can occur as a variant of shingles (herpes zoster).The infection is due to a virus called varicose zoster. Shingles typically presents as characteristic skin lesions accompanied by pain and paresthesias. In zoster ophthalmicus, the virus affects the ophthalmic nerve, from which the lacrimal nerve branches. The eyes water, redden, and are very sensitive to light. In addition, the eye may swell. After some delay, vesicles form, burst open and subsequently develop crusts. In some cases, sufferers go blind when the cornea becomes scarred in them. Zoster ophthalmicus often occurs in people who have a weakened immune system and are 40-60 years old. Another disease that can affect the lacrimal nerve is trigeminal neuralgia. It manifests as severe pain in the face that comes on very suddenly. The neuralgia may be limited to individual branches of the trigeminal nerve. Its exact causes are not yet known. Some physicians suspect that a disturbance of the nerve in the cerebellopontine angle (between the cerebellum and brainstem) may be responsible for trigeminal neuralgia. The facial nerve and vestibulocochlear nerve (cranial nerve VIII) also cross this angle.
