Cranial nerves arise directly from the brain. Of these, most are located in the brain stem. The job of the cranial nerves is to form the center of the nervous system in the head, neck and trunk.
What are cranial nerves?
Through both halves of the body run twelve cranial nerves , which perform a specific function. Each nerve is assigned a number according to the Roman numeral system. Eleven of the twelve nerves share the common feature that they arise from the brain or do not leave it at all. The accessorius nerve (XI) is an exception. It arises from the spinal cord, but is still classified as a cranial nerve. In terms of expression, cranial nerves are comparable to peripheral nerves. Peripheral nerves are responsible for supplying the body. Deviations occur with the first two cranial nerves, the olfactory nerve (I) and the optic nerve (II). They are direct projections of the cerebrum. Cranial nerves also have the property of being paired. They may contain somatomotor and autonomic fibers and thus cover a different range of tasks. Somatomotor fibers enable conscious voluntary movement. In contrast, vegetative fibers are needed for unconscious automatic responses.
Anatomy and structure
In anatomical structure, a cranial nerve consists of a bundle of nerve fibers. This is sheathed by connective tissue and protected from the forces of the environment. The purpose of nerves is to transmit impulses over longer distances. Along a nerve fiber, information is transmitted from nerve cell to nerve cell. A nerve cell in turn consists of a dendrite, which is designed to pick up stimuli from the environment via a branch system. The recorded signals are transmitted to the cell body and the axon hillock lying on top of it. If a sufficient intensity is reached, the transmission of information can continue. For this purpose, the signals are transported along the axon as electrical impulses. Finally, the signal reaches a synapse. Here, information transmission to the next nerve cell takes place. This process is repeated until the end of the nerve fiber is reached. Then the cranial nerves have reached their destination or they branch off into peripheral nerves. Taken as a whole, the cranial nerves form an important part of the central nervous system.
Function and tasks
Each cranial nerve performs a distinct task. For example, the first, referred to as the olfactory nerve (I), has the function of communicating possible olfactory sensations through the nose. Accordingly, it is also called the olfactory nerve. Thanks to the optic nerve (II), vision is possible with the eyes. The optic nerve transmits the recorded images to the brain. In its function, the optic nerve is complemented by the oculomotor nerve (III), the trochlear nerve (IV) and the abducens nerve (VI). The three nerves are responsible for carrying out eye movements and controlling eye muscles. The trigeminal nerve (V) also plays an important role. Its main function is to transmit stimuli from the head area to the brain. In addition, it is responsible for controlling the muscles of mastication. It consists of three powerful nerve branches and is therefore called the triplet nerve. The facial nerve (VII) is responsible for coordinating facial expressions. In addition, it enables the perception of tastes. We owe the sense of hearing and balance to the vestibulocochlear nerve (VIII). It runs between the inner ear and the brain. Muscles of the throat are controlled by the glossopharyngeal nerve (IX). Furthermore, it is responsible for the swallowing reflex. The vagus nerve (X) regulates the heart rate and supplies the larynx. It also supports the secretion of gastric acid. The muscles of the neck and throat are controlled by the accessorius nerve (XI). Lastly, much of the tongue musculature is accessed via the hypoglossal nerve (XII). Among other things, sticking out the tongue and swallowing are among the most important tasks.
Diseases
Diseases of the cranial nerves can be attributed to various failures and impairments of the nervous system. Depending on the cranial nerve affected, various complaints are conceivable. For example, failure of the olfactory nerve (I) is associated with attenuation of the sense of smell. This is referred to as anosmia. Anosmia can occur in the event of a skull base fracture.A disturbance of the optic nerve (II) leads to a loss of the visual field. This can be caused by high pressure inside the skull. The visual field restriction is technically referred to as quadrant anopsia. Restriction of eye movements occurs when the oculomotor nerve (III) is obstructed. Then dilatation of the pupils and paralysis of the eyes sets in. When the trochlear nerve (IV) or the abducens nerve (VI) fails, patients also complain of a decrease in visual acuity and the perception of double images. Sensory disturbances in the head are especially a problem when the trigeminal nerve (V) is damaged. This is accompanied by hearing difficulties called hypacusis. Paralysis of the face also occurs when the facial nerve (VII) is disturbed. In addition, there is a weakening of the sensation of taste. Failures in the vestibulocochlear nerve (VIII) lead to hearing loss, in the glossopharyngeal nerve (IX) they lead to dysphagia, and in the vagus nerve (X) hoarseness is the result of a disorder. After ENT surgery, damage to the accessorius nerve (XI) can occur. A tilted position of the head is then the consequence. Speech disorders and swallowing difficulties occur when the functionality of the hypoglossal nerve (XII) is restricted. Often, sticking out the tongue is then no longer possible.
