If we say the name of hypoglossal nerve, it may not sound to many. However, if we talk about the vagus nerve or the trigeminal nerve, they may be better known. The three nerves belong to the twelve cranial nerves, also known as the twelve cranial nerves. The name of cranial nerves is because each nerve is double and they are found both on one side and the other of the brainstem, where most are located.
Where are the cranial nerves?
Most of them are located in the brainstem, specifically in the midbrain, the bridge and in the medulla. The first two cranial nerves are the only ones that are located outside the brainstem. Specifically, the olfactory nerve (I) and the optic nerve (II).
The oculomotor (III) and the trocreal (IV) nerves leave from the midbrain. On the bridge are the trigeminal (V), the abducens or external ocular motor (VI), the facial (VII) and the vestibulocochlear or auditory (VIII). Finally, the glossopharyngeal nerve (IX), the vagus (X), the accessory (XI) and the hypoglossal (XII), are found in the spinal bulb.
Cranial nerve function
The cranial nerves have a motor and sensory and mixed function. Some of them belong to the Somatic Nervous System and others to the Autonomous Nervous System. On the one hand, several of them transport sensory information to the Central Nervous System (sensory function), and others do it in the opposite direction, from the CNS to different areas of the body (motor function). Others combine both functions, both motor and sensory.
Its function is sensory. Its axons end in the olfactory bulb. It is composed of axons of neurons whose dendrites and cell bodies are in the olfactory mucosa.
The optic nerve is formed by axons of retinal cells that transport visual information from the eyes to the brain. The two optic nerves converge in the optic chiasma. At this point, some fibers of each nerve cross to the opposite side. From here, through the optical tract, visual information is transported to the thalamus. The function of this nerve is sensory.
Oculomotor (III) and trochlear (IV)
Its functions are motor. They act on the musculature of the eye. The axons start from the midbrain and have autonomic nervous system fibers that innervate the smooth muscles of the eyeball, specifically, the constrictor muscles of the iris and the ciliary musculature that is responsible for controlling the shape of the iris.
This nerve does place on the bridge. It is a motor and sensory nerve. It has three sensory branches that facilitate sensory information of different points of the face, tongue and mouth. Motor fibers innervate the jaw muscles that control chewing.
Abducens or External Eye Motor (VI)
It is a motor muscle that controls the straight muscle of the eye. When activated, it is accompanied in a coordinated manner with the oculomotor and trochlear nerves.
The facial nerve is mixed. On the one hand, It has a somatic motor component that reverses the muscles responsible for facial expression. It also reverses fibers of the autonomic nervous system that are directed towards the tear and salivary glands. On the other hand, sensory fibers take information from the taste buds in the front of the tongue and are involved in the sense of taste.
Vestibulomotor or auditory (VIII)
Sensory nerve with two distinct branches. The cochlear branch innervates the cochlea, inserted into the hearing organ. The vestibular branch carries information from the vestibular apparatus (balance organ).
Located in the medulla oblongata. It is a sensory and motor nerve with somatic and visceral components. The main function of the glossopharyngeal nerve is the collection of information from the mucous membranes of the pharyngeal region and the posterior third of the tongue. Somatic motor innervation is carried out in the striated muscles of the pharynx and visceral over the parotid gland.
It reaches stucts of the head such as the pharynx, larynx and trachea; and to trunk structures such as the heart, lungs and digestive system. It is a sensory and motor nerve that intervenes in a large number of somatic and visceral functions. On the one hand, it reaches the striated musculature of the palate, pharynx and larynx in order to control swallowing. It also collects sensory information from a large part of the viscera of the abdomen and thorax. Its role on the autonomic nervous system stands out in aspects such as in the case of heart rate, gastric secretion and intestinal peristalism.
It is a motor nerve. It is considered an accessory nerve of the vagus nerve since its fibers pass through branches of the latter such as those of the pharynx and larynx, thoracic and abdominal viscera. On the other hand, it also reaches the shoulder and neck to control their movements.
It is a motor nerve that controls the tongue musculature.
Abril, A., Ambriosio, E., Rosario de Blas, M., Caminero, A., García, C., Pablo, J. and Sandoval, E. (2005). Biological basis of behavior. Madrid: Sanz and Torres.