The cranial nerves describe the pathways by which the brain analyses, controls and manages information received from structures mainly in the head and neck.
They leave the cranial cavity through certain openings, fissures or canals and are distributed to structures in the body.
The cranial nerves are a series of 12 pairs of nerves that extend from the brain and medulla oblongata and branch into different parts of the head, neck and chest. These nerves are responsible for controlling and coordinating a variety of sensory and motor functions, as well as transmitting information to and from the brain.
In this article, we examine the anatomy and function of each cranial nerve.
Nerve I – olfactory
This transmits sensations and information from the nasal mucosa and reactions to the presence of noxious substances at this level.
Trauma or certain pathologies such as sinusitis or rhinitis can damage or irreversibly damage the nerve’s ability to transmit information to the higher centers of analysis.
The olfactory nerve is responsible for the sense of smell and extends from the nasal mucosa to the olfactory bulb. This nerve is unique among cranial nerves in that it is directly connected to the brain without passing through the thalamus.
Nerve II – optic
This is the nerve of the visual pathways and damage to it can cause temporary, permanent, unilateral or complete loss of vision, depending on the factor causing the damage to the optic pathways. Tumors found at this level can also compress nerve structures and their anterior or posterior location can cause everything from embarrassment to loss of vision.
The optic nerve is responsible for vision and extends from the eyeball to the optic chiasm. This nerve is formed from an axon of ganglion cells in the retina, which come together to form the optic nerve.
Nerve III – oculomotor
Responsible for the movement of the eyeballs. Damage to this pair of nerves can cause squinting or drooping of the eyelid, medically called palpebral ptosis. Asymmetries in the eye region may be caused by malfunction of these nerves.
The oculomotor nerve controls eye movement and extends from the midbrain to the eye muscles. This nerve controls several muscles of the eyeball, including the superior rectus muscle, inferior rectus muscle, medial rectus muscle and the levator pollicis muscle of the upper eyelid.
Nerve IV – trochlear or patellar
The trochlear nerve controls eye movement and extends from the midbrain to the superior oblique muscle. This nerve controls eye movement upwards and inwards.
Nerve V – trigeminal
Is responsible for transmitting tactile, thermal and painful sensation from the skin. Thus, this nerve also picks up information from the lips, conjunctivae of the eye and the muscles involved in chewing.
The trigeminal nerve is responsible for facial sensation and controls the masticatory muscles. This nerve extends from the cerebral pons and branches into three branches: the ophthalmic branch, the maxillary branch and the mandibular branch.
Nerve VI – abducens
The abducens nerve controls eye movement and extends from the cerebral pons to the lateral rectus muscle. This nerve controls the movement of the eye laterally
Nerves IV and VI can cause convergent and divergent squint respectively when injured. Convergent strabismus occurs when the patient looks with crossed eyes, and divergent strabismus occurs when the patient looks with one or both eyes to the outside, lateral orbit
Nerve VII- facial nerve
This pair of nerves pick up motor information from the forehead, allowing the forehead to wrinkle, but also from the eye when squeezing the eye.
They also transmit lip movements in the whistle and sensory information from the anterior portion of the tongue.
The facial nerve is responsible for controlling facial muscles and taste and extends from the cerebral pons to the facial muscles and salivary glands. This nerve also controls taste in the anterior part of the tongue.
Nerve VIII – vestibulocochlear or acoustic-vestibular
Is responsible for maintaining the patient’s balance. Any damage to this nerve results in problems maintaining posture when closing the eyes.
The vestibulocochlear nerve is responsible for hearing and balance and extends from the midbrain and the cerebral pons to the inner ear. This nerve divides into two branches: the vestibular branch and the cochlear branch.
Nerve IX – glossopharyngeal
This nerves the parotid and pharynx. The sensation of nausea is diminished or lost when this nerve is affected.
The glossopharyngeal nerve is responsible for taste and sensation behind the tongue and control of the muscles in the throat and extends from the medulla oblongata to the throat and inner ear. This nerve is involved in the control of reflexes such as the vomiting and swallowing response.
Nerve X – vagus nerve
Extends to both the head and neck as well as the abdomen and chest. Deficiency of the laryngeal muscles causes changes in the sounds shaped by the vocal cords. In the chest, malfunction of this nerve can lead to cardiac arrhythmias.
The vagus nerve is responsible for controlling involuntary functions such as breathing, heartbeat and digestion, as well as controlling reflexes such as coughing and sneezing. This nerve extends from the medulla oblongata to the internal organs in the neck, chest and abdomen.
Nerve XI – spinal accessory nerve
This is also responsible for maintaining posture by innervating back muscles such as the trapezius.
The accessory nerve controls neck and shoulder movements and extends from the medulla oblongata to the muscles in the neck and shoulder. This nerve consists of two branches: the spinal branch and the cranial branch.
Nerve XII – hypoglossal
It is responsible for innervating the muscles of the tongue and damage to it causes tongue deviations. In neuralgia of this nerve, we have burning sensation of the tongue, it stings when spices are ingested and at the base of the throat, we have painful sensation.
The hypoglossal nerve controls tongue movements and extends from the medulla oblongata to the tongue muscles. This nerve is important for speaking, eating and other activities involving tongue movement.
Some nerve disorders are part of syndromes, which are complex conditions that are treated in a multidisciplinary way.
The treatment, course and prognosis in the various nerve disorders depend on the nature of the complaint that led to the damage to the nerve or nerves concerned.
Conclusion
Cranial nerves are essential for the control and coordination of a variety of sensory and motor functions in the head, neck and chest.
Any injury or condition affecting the cranial nerves can lead to a variety of symptoms and dysfunctions.
Understanding the anatomy and function of each cranial nerve can help diagnose and treat conditions associated with these nerves.
