Cranial nerves refer to the nerves that emerge from the brain directly. This is in contrast to the spinal nerves that come from spinal cord segments. There are 12 pairs of cranial nerves in human and only the both the first and the second cranial nerve pairs originate from cerebrum. All the other pairs originate from the brainstem. Other than the cranial nerve II, all the other cranial pairs belong to the peripheral nervous system (PNS). The cranial nerve ganglia come from the central nervous system (CNS) (Patestas and Gartner).
The seventh pair of cranial nerves is the facial nerve. This is made up of two parts, which are nervus intermedius and the facial nerve proper. The facial nerve proper acts as the motor root of the facial nerve and is made up of axons of branchiomotor neurons with cell bodies that stay in the facial nucleus. In each of the nucleus, there are subnuclei each of which innervate special or a group of muscles. On the other hand, the nervus intermedius, which is also known as sensory root, is made up of axons of the secretomotor parasympathetic neurons. The cell bodies of these axons stay in the superior salivatory nucleus and innervate the muscles of facial. The facial nerve is involved in innervating the muscles that are involved in facial expression, stylohyoid and digastrics muscles. The facial nerves are also involved in receiving the special taste sense from the major interior tongue, as well as providing innervations of the secremotor to the salivary and lactrimal glands (Patestas and Gartner).
The cranial nerve pair number 8 is the vestibulocochlear nerve. The nerve is made up of two discrete and distinguished nerves that are enclosed within a single connective tissue sheath. These nerves are the vestibular nerve that is involved in balance and position sense and the cochlear nerve, which is involved in hearing. The two nerves transmit the (concerned with hearing). Both nerves transmit Special somatic afferent information from the peripheral ciliated mechanoreceptors that are specialized for their function and are also called hair cells. The vestibular nerve is not complex but is just a cranial nerve that is involved in sending first order neuron processes to the synapses that are directly in the cerebellum. The vestibulocochlear nerve functions in sensing rotation, sound and gravity. Gravity is important for movement and balance. The vestibular branch is involved in carrying the impulses that are necessary for equilibrium while the cochlear branch is involved in carrying impulses that are necessary for hearing (Patestas and Gartner).
The glossopharyngeal nerve is the ninth cranial nerve pair and makes one of the smallest among the 12 pairs and is made up of five functional components. These components are SVA (special visceral afferent) for tasting, the GVA (general visceral afferent) sensation that is from the adjacent pharyngeal wall, the posterior 1/3 of the tongue, as well as the carotid sinus, which is a baroreceptor, GSA (general somatic afferent sensation) that comes from the external ear, branchiomotor or SVE (special visceral efferent) innervation to the stylopharyngeus muscle, and the innervations of the GVE (general visceral efferent) to the parotid gland. The nerve exits from the brainstem mainly as a group made of rootlets that are posterior to the olive that is in the dorsolateral sulcus. The pair is involved in receiving tastes coming from the tongue (posterior 1/3), providing the innervations of the secretomotor to the parotid gland, as well as in innervating stylopharyngeus motors. There are also some sensations that are relayed from the palatine tonsils to the brain (Patestas and Gartner).
The vagus nerve is the tenth pair of the cranial nerves and is a large and extensively distributed in the body. Although a cranial nerve, the innervations by the nerve are not limited to the structures that are in the head, but the innervations extend into other parts such as the neck, abdomen and thorax. There are five functional components that are carried by the vagus nerve, GVE, SVE, GSA, GVA and SVA. There is a group of fine rootlets surface that are located in the medulla inside the dorsolateral sulcus, which is inferior to the ninth cranial nerve and superior to the accessory nerve of the spine. These rootlets join forming two distinct bundles, which are a larger superior and a smaller inferior that collectively make the vagus nerve. The spinal accessory nerve is joined by the inferior bundle and the two moves together for a short distance before moving in different ways.
After the joining of the smaller vagal bundle to the main trunk, the vagus then moves out of the cranial vault through the jugular foramen. Although jugular foramen is superior to the vagus nerve, the vagus nerve shows two swellings, which are the inferior or nodose and the superior or jugular ganglia. The vagus nerve is involved in supplying most pharyngeal and laryngeal muscles with the branchiomotor innervations. The nerve also functions to offer parasympathetic fibers to almost all abdominal and thoracic viscera. It is also involved in receiving special taste sense from the epiglottis. Other functions include sensing the aortic blood pressure, slowing down the heart rate and stimulating the organs of digestion (Patestas and Gartner).
The 11th pair of cranial nerve is the spinal accessory nerve. The nerve is involved in innervations supply to the trapezius, sternocleidomastoid, as well as numerous intrinsic laryngeal muscles. Spinal accessory nerve gets its fibers from the nucleus of the spinal accessory that reside in the ventral horns posterolateral aspect of spinal cord levels C2–C5 (or C6) of the cervix. The delicate rootlets that emerge from the lateral funiculus surface of the spinal cord meet and gather together forming the spinal accessory nerve. The nerve trunk then ascends entering through the foramen magnum to the cranial vault and move on the medulla’s lateral aspect. Here, they join the aberrant vagal fibers when they are moving out from the medulla. The pair is involved in controlling the trapezius and sternocleidomastoid muscles. It also has functions that overlap those of the vagus nerve, as well as controlling swallowing movement (Crimando).
The hypoglossal nerve is the last pair of the cranial nerves and provides the muscles of the tongue with motor innervations. The hypoglossal nucleus, which is a cell column located in the medulla, is the location for the GSE lower motoneurons cell bodies of the hypoglossal nerve. The nucleus is usually located on the ventral side of the floor of the fourth ventricle forming a triangular elevation called the hypoglossal trigone. The nerve cell bodies of the pair give rise to the axons that flow ventrally to come out on the medulla’s ventral surface as a sequence of tiny rootlets. This separates the olive and the pyramid. A collection of the rootlets is the one that forms the hypoglossal nerve. The pair is involved in providing motor innervations to the tongue muscles, as well as other glossal muscles. The hypoglossal nerve is also important bolus formation during swallowing and in speech articulation (Patestas and Gartner).
In conclusion, cranial nerves pairs have distinctive structures and are thus involved in a wide range of activities. For the pairs from number seven to twelve, the functions range from facial expression control, hearing, tasting, and heart rate control to the control of the tongue movement.
Works Cited
Crimando, J. Cranial Nerves: Review Info. 2013. Online. 20 July 2013. <http://www.gwc.maricopa.edu/class/bio201/cn/cranial.htm>.
Patestas, M. A. and L. P. Gartner. A Textbook of Neuroanatomy. New Jersey: Blackwell Publishing, 2007. Print.
