The ophthalmic nerve is the ocular branch of the trigeminal nerve and as such is involved in trigeminal perception. Because of its location in the human head, it primarily receives sensory stimuli from the ocular region. Functional impairment may be a consequence of various neurological and inflammatory diseases.
What is the ophthalmic nerve?
Part of the larger trigeminal nerve, the ophthalmic nerve is one of three branches and in turn branches further into smaller nerves. Alternatively, medicine also knows it by its trivial name, the ophthalmic nerve: with the help of numerous branches, the ophthalmic nerve collects sensory signals from the eye area and transmits them to relevant processing centers located in the brain and spinal cord. While other cranial nerves each transmit only stimuli of a specific modality (vision, hearing, smell, etc.), the fibers of the ophthalmic nerve are considered general somatosensory; they are responsible for general body sensations, including pressure and pain. In the human nervous system, pain is partly due to very strong stimulation or inadequate stimulation of other sensory cells. In addition, there are specific pain receptors, which medicine also calls nociceptors. Free nerve endings register not only pressure and temperature but also chemical substances that are potentially harmful.
Anatomy and structure
The ophthalmic nerve divides into different branches and in this way helps to cover a larger field. The total of four branches of the ophthalmic nerve, in turn, also branch into finer nerves. The ramus tentorius or ramus meningeus recurrens provides a connection to the dura mater in the cranial cavity. The second branch of the ophthalmic nerve is the frontal nerve, which passes the eye muscles and leads to the orbit. The structure of the frontal nerve is divided into two parts and consists of the supraorbital nerve (“nerve above the orbit”) and the supratrochlear nerve (“nerve above the cartilage“). Next to the external eye muscles is the lacrimal nerve (“nervus lacrimalis”). The fourth and final branch is represented by the nasolingual nerve (nasociliary nerve) with connections to the median eye, conjunctiva and cornea, as well as to the lacrimal ducts and nasal cavity. The nasociliary nerve also does not run in one strand, but separates into the ethmoidal nerve, infratrochlear nerve, and long ciliary nerve.
Function and Tasks
Transmitting and combining signals is the function of the ophthalmic nerve. It has no sensory cells of its own and is not in direct contact with any, which is why humans are not usually aware of its function. Exceptions are unpleasant temperature, pain and pressure stimuli, which can pass through the ophthalmic nerve. Signal transmission within the nerve takes place predominantly with the aid of electrical conduction. For this purpose, the nerve cell generates an electrical impulse that travels as an action potential over the found-like end of the neuron. The nerve fibers or axons of the cells in the ophthalmic nerve are longer than those of most nerve cells; as a result, the nerve relies on only a few connections. Different branches of the ophthalmic nerve perform different tasks in this context. The ramus tentorius innervates the dura mater, one of the meninges; irritation primarily causes pain, thereby warning the body against excessive pressure on the skull, which damages the sensitive body part. The frontal nerve with its two branches, the supraorbital nerve and the supratrochlear nerve, connects the eyelid and the region towards the nose to the sensory nervous system. The supraorbital nerve runs along the upper edge of the orbit just under the skin, where it forms the first trigeminal pressure point. With a total of three trigeminal pressure points on each side of the face, physicians can determine whether and, if so, where lesions or functional limitations of the trigeminal nerve are present. The lacrimal nerve has two important functions: Its sympathetic and parasympathetic nerve fibers signal the lacrimal gland to secrete fluid. In this process, the command originates from the spinal cord. In addition, the lacrimal nerve receives sensory information and transmits it to the brain. Various tissues are connected to the nasociliary nerve; it receives sensory stimuli from the eye membranes as well as the lacrimal ducts and nasal cavity.
Diseases
Numerous nerve diseases can directly or indirectly affect the ophthalmic nerve. Affected individuals feel the consequences either as decreased sensory function in the affected regions or they suffer from (often painful) perceptions that arise in the nervous system even though no triggering stimulus is present. Peripheral and central lesions can limit or completely stop the functioning of the ophthalmic nerve. A peripheral lesion is localized to the nerve itself and may occur as a result of injury, for example. Symptomatically, this clinical picture is expressed by a lack of sensitivity in the affected facial region; in the case of the ophthalmic nerve, affected persons no longer perceive general sensory stimuli from the eye region. If only individual branches of the ophthalmic nerve are damaged, the sensory loss is correspondingly limited to smaller areas. In contrast, central lesions affect larger sections, since in this case the nerve core in the brainstem is damaged. Tumors in the myelin sheaths can also be considered as a cause of symptoms. Doctors refer to them as schwannomas and remove and/or irradiate them to treat them. Pressure pain on the first trigeminal pressure point at the upper edge of the orbit may indicate other causes; sinusitis, meningitis, increased incranial or intracranial pressure, swelling, and other pathologic abnormalities may irritate the ophthalmic nerve, causing an appropriate sensory response. Therapeutic measures in all cases depend on both the specific cause and individual factors.