Cranial nerves refer to nerves that emerge directly from the brain. They relay information between the brain and other body parts, basically to and from neck and head regions. They are different from spinal nerves which originate from the spinal cord. Cranial nerves originate from the nervous system. Every cranial nerve is usually paired and found on both sides. Below is a list of 12 cranial nerves and their functions (Albert, 2012).
Olfactory Nerve
Just as its name suggests, the olfactory nerve relates with the sense of smell. The receptors of olfactory nerve lay in the mucosa of nasal cavity i.e. the upper part. Afferent/sensory fibres travel to the olfactory bulb after they pass various openings in the cribriform plate. It is usually the shortest nerve. It, therefore, doesn’t get into the brain stem. It’s also known as the CN1, and it’s the first of the 12 cranial nerve located within the head. Its function is to relay sensory data to the brain. It’s responsible for the sense of smell. When airborne particles and chemicals get into the nasal cavity, they interact with the neural receptors.
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Optic Nerve
The optic nerve is at the back of the eye. It’s commonly referred as cranial nerve II. Its function is to relay visual information from the retina to the vision centre of the brain. The visual information gets transferred via electrical impulses. It contains of nerve cells or ganglionic cells. It has more than one million nerve fibres. The blind spot in humans is as a result of an absence of photoreceptors or photosensitive cells in the retina where the optic nerve gets out of the eye. Glaucoma is a common illness that affects the optic nerve. The disease results from the presence of high intraocular pressure (high pressure in the vitreous fluid). The high pressure ends up compressing the optic nerve leading to the death of the cells. Despite the optic nerve being of the eye, is a part of the nervous system.
Oculomotor Nerve
It’s the third nerve among the 12 cranial nerves. The Oculomotor Nerve is responsible for the eyelid and the eyeball movement. It involves two separate components each with a different function. One of them is the somatic motor component. The somatic motor is engaged in the supply of motor (movement) fibres to the upper eyelid's elevator palpebral superiors and the four extraocular muscles in the eye.it also control muscles that enable fixation by the eye and visual tracking. Visual tracking refers to the eye’s ability to follow an object as it moves across the field of vision. On the other hand, fixation refers to the capability to focus on a stationary object. The second component is the visceral motor. It controls parasympathetic innervation i.e. nerves related to involuntary actions of the constrictor papillae and the ciliary muscles. It thus helps in pupillary light reflexes and accommodation. Accommodation refers to the ability of the eye to keep an object in focus as the distance of the object from the eye varies. Pupillary light reflexes apply to an automatic variation of light which gets into the eye, ensuring that the light is sufficient to see but not too bright. Even though the abducens and the trochlear nerves are also responsible for the eye movement, the oculomotor nerve is responsible for most of the eye movement (Neil, 2008).
Trochlear Nerve
It’s also referred as the cranial nerve IV. It’s the only cranial nerve that originates from the brain dorsally. That type of emergence gives it the longest pathway. Its passes via the superior orbital fissure and provides movement or motor function. It connects the annular tendon and serves the superior oblique eye muscle. It thus processes brain signals that enable the eye to move up, down and outwards. Any damage to this nerve compromises ability uses the superior oblique eye muscle. It’s the muscle that moves the eyeball. The double vision which is also known as diplopia occurs due to problems in the muscle or the nerve. Any complications from these tissues result in compromised ability to walk especially down stairs.
Trigeminal Nerve
It’s responsible for transmission of sensations from the face to the brain. It has three branches which include the maxillary, mandibular and ophthalmic. Each of these branches connects nerves from the brain to the various parts of the face. It transmits sensations from the teeth, face, nasal cavity and mouth. It also controls the muscles that get involved during chewing. Trigeminal neuralgia arises when a blood vessel subjects the nerve at the base of the brain to pressure. It is treatable by medical or surgical means (Harper, 2014).
Abducens Nerve
Abducent nerve is the sixth cranial nerve CN6. It controls the lateral rectus muscle of the eye which is responsible for moving the eye sideways away from the nose. It exits the brainstem and runs across the facial nerve. It passes through the cavernous sinus, subarachnoid space then eventually through the orbital fissure. CN6 is susceptible to some clinical conditions. If it gets injured, double vision arises. The eye thus remains pulled in towards the nose since medial rectus muscle doesn’t work in opposition. Meningitis infections can also affect the CN6. Diabetic neuropathy is also very common and occurs due to blood sugar.
Facial nerve
It’s also known as the CN7. It has two primary functions. The first involves the conveyance of sensory information from the tongue and interior part of the mouth. It specifically serves two-thirds of the tip of the tongue. CN7 extends from the brain stem at the medulla and the pons. It innervates facial muscles thus controlling contraction and facial expressions. It splits into several branches along its course. The bigger petrosal nerve serves the nasal cavity and the lacrimal gland as well as the frontal, ethmoid sinuses, sphenoid and maxillary. One of its branches provides motor signals to the stapedius muscle found in the inner ear. The chords tympani branch serves the submandibular glands (glands located on the mouth floor) and sublingual glands which are a major salivary gland. Chorda tympani is also responsible for conveyance of taste sensations from the tongue tip. Common problems involving CN7 include paralysis due to viral infections (Quian, 2015).
Vestibulocochlear Nerve
It’s a sensory nerve which conducts two special senses i.e. balance (vestibular) and hearing (audition). The receptor cells for these reasons stay in the membranous labyrinth embedded in the petrous portion of the temporal bone. The bony labyrinth has two specialized organs which include vestibular apparatus and the cochlea. The cochlear has a connection to three bony ossicles that transduce sound into fluid movement in the cochlea. It ultimately causes hair cells movements that activate the auditory part of the vestibulocochlear nerve. The vestibular apparatus senses changes in head position about the gravity. Motion results in fluid vibration leading to hair cell displacement which activities the vestibular part of CN8
Glossopharyngeal Nerve
It’s related to the pharynx and the tongue. CN9 provides parasympathetic innervation to the parotid gland. These fibres come from the inferior salivatory nucleus of CN9. They later travel with the tympanic nerve to the middle part of the ear. They then exit the ear and go down and synapse at the otic ganglion. These fibres later hitch hike on the auriculotemporal nerve to the parotid gland. They have a strange motor effect at the parotid gland.
Vagus Nerve
It arises from its four nuclei which are in the medulla. It then passes through the jugular foreman as it descends into the neck and abdomen. Unlike other cranial nerves, it innervates many organs in the abdomen and thorax. That is why it used to be known as the pneumogastric nerve. The CN10 has both afferent and efferent fibres. However, most of them are afferent fibres which are involved in carrying sensory information from district organs. The CN10 also has a parasympathetic effect on many organs like the GIT and the heart. It has many branches some of which include Thoracic cardiac branches.
Pharyngeal nerve , Superior laryngeal nerve, anterior vagal trunk, etc. supplies parasympathetic branches to many organs and helps in control of skeletal muscles.
Accessory Nerve
It is the 11 one on the cranial series. Unlike all the other nerves, it has two parts. The first one is cranial. It arises from spinal part and medulla. The spinal part ascends through the foramen and joins cranial part to make accessory nerve. It supplies trapezius and sternocleidomastoid muscles. The sternocleidomastoid muscle is responsible for rotating and tilting the head. On the other hand, trapezius muscle has some actions on the scapula. One of these measures is the abduction of the arm and shoulder elevation (Matthew, 2012).
Hypoglossal Nerve
It is the last one, and it relates to the tongue just as the name suggests. It passes below the tongue. Its function is to supply motor (efferent) fibres to all the muscles found on the tongue. It only excludes the palatoglossal muscle.
References
Albert, D. (2012). Dorland's Illustrated Medical Dictionary. (32nd ed.). Philadelphia, PA: Saunders/Elsevier . ISBN 978-1-4160-6257-8
Harper, D. (2014) " Trigeminal Nerve". Online Etymology Dictionary .
Matthew, C. (2012). " The naming of the cranial nerves: A historical review". Clinical Anatomy. 27 (1): 14–19. doi:10.1002/ca.22345.
Neil, R. (2008). " Overview of cranial nerves and cranial nerve nuclei". Gray's anatomy: the anatomical basis of clinical practice (40th ed.). [Edinburgh]: Churchill Livingstone/Elsevier . ISBN 978-0-443-06684-9.
Quian, C. (2015). The Clinical Anatomy of the Cranial Nerves: The Nerves of "On Olympus Towering Top". Ames, Iowa: Wiley-Blackwell . ISBN 978-1-118-49201-7.
