There are four cranial nerves with primarily motor function. Link to Corticobulbar. CN IV, Trochlear, innervates the superior oblique muscle of the eyeball.
CN VI, Abducens, innervates the lateral rectus muscle of the eyeball and abducts the eye. CN XI, Accessory, innervates the trapezius muscle in the back and shoulders.
Five cranial nerves have mixed sensory, motor and parasympathetic function. These five will be discussed in detail here. The occulomotor nerve exits the brain at the midbrain in the middle of the cerebral peduncle.
As such it is compressed whenever there is swelling of the brain and herniation through the tentorium. This results in a dilated pupil which is unreactive to light. A clinical phenomenon known as a "blown" pupil. This signals dire consequences and brain death unless the condition is quickly treated.
The occulomotor nerve supplies four of the six eyeball muscles. Link to corticobulbar. It also carries Parasympathetic input to the ciliary ganglion to constrict pupil. Link to Autonomic. The numbering starts with those most superior and anterior and progresses posteriorly and inferiorly.
The names usually correspond to either the function or the structure of the nerve, hence, the Optic nerve is involved with vision and the Trigeminal nerve has 3 branches. These nerves can carry both sensory and motor information, just as we have seen with spinal nerves. We can subdivide the sensory information further into 1 special senses and 2 general senses.
The motor information can also be subdivided into 1 somatic motor and 2 parasympathetic. In contrast to the spinal nerves, however, not all cranial nerves carry both sensory and motor information. In fact, three cranial nerves carry purely sensory information and four cranial nerves carry almost entirely somatic motor information.
The remaining cranial nerves carry some combination of sensory, somatic motor and parasympathetic information. Because of the clinical importance of the cranial nerves we will discuss further the major functions of each one and some of the common symptoms observed when the nerves are damaged. Lesion: Blindness on affected side and loss of pupillary light reflex Described Later. Major Function: Somatic Motor to four of the six extrinsic muscles that move the eye.
ANS Innervation: Parasympathetic to sphincter pupillae muscle for constriction of the pupil. Lesion: Eye deviation causing double vision, pupil dilation and loss of pupillary light reflex.
Lesion: Loss of sensation in face and forehead or increased sensitivity to pain known as Trigeminal neuralgia. Described Later.
Also, muscle weakness of the muscles of mastication. Lesion: Facial paralysis often called Facial or Bell's palsy. Decreased salivation and lacrimation tearing. This nerve is composed of fibers from two branches: the vestibular nerve and the cochlear nerve, each with specific functions.
Major Function: Sensory - Vestibular branch senses balance. Cochlear branch if for hearing. Lesion: Vestibular-If only the vestibular branch is damaged it would result in loss of balance and dizziness vertigo. Cochlear-If only the cochlear branch is damaged it would result in loss of hearing. If the lesion occurs after the two branches converge then you could have a combination of the above symptoms.
Major Function: Motor - Somatic Motor to throat muscles involved in swallowing and speech and Sensory - Taste from posterior tongue. Also, sensory from throat, thoracic and abdominal organs. It exits the medulla oblongata behind the olivary bodies, together with the vagus nerve and the accessory nerve. In the area of the posterior cranial fossa, the sensory fibers exit the cranial cavity and pass between the internal carotid artery and the internal jugular vein until they laterally reach the root of the tongue.
The motor fibers pass through the jugular foramen from the nuclei of the medulla. The vagus nerve is the parasympathetic main nerve that is responsible for large parts of the body and innervates almost all of the organs of the thorax and the abdomen. The vagus nerve has the longest course of all CNs. After exiting the medulla oblongata, the vagus nerve passes through the skull base.
Near the carotid artery, it passes to the intrapleural space in the neck, where it branches off to the base of the heart to the atria and the hilum of the lung. Sensory parts of the vagus nerve originate from the skin of the outer ear. They can cause a cough and nausea if they receive a certain stimulus cotton bud. A few sensory parts originate in the epiglottis and the pharynx; proprioceptors originate in the muscles of the neck and pharynx and in chemoreceptors in the carotid glomus near the aortic arch.
Further branches pass to the recurrent laryngeal nerve, which passes back to the laryngeal cartilage to supply the vocal folds. In addition, there are axons of viscerosensory receptors of thoracic and abdominal organs. Additionally, parasympathetic fibers extend from the nuclei of the medulla to the lung and the heart. The pressure receptors at the carotid glomus contain fibers of the vagus nerve that are necessary for blood pressure regulation.
The vagus gives rise to a network of branches around the esophagus to enter the abdomen at the esophageal hiatus. The glands of the gastrointestinal tract, the smooth musculature of the respiratory tract , the stomach, the esophagus, the gallbladder, the small intestine, and most parts of the large intestine are supplied by the parasympathetic axons. The skeletal muscles of the inner and outer neck are innervated by somatomotor neurons.
The accessory nerve is the main motor, mixed CN, which emerges from the brainstem and the spinal cord. The cranial root arises from the nuclei of the medulla oblongata. It passes through the jugular foramen and supplies the musculature of the pharynx, the larynx, and the velum to enable the swallowing process.
The spinal root contains mixed, but mainly motor, axons. The motor axons pass through the foramen magnum and exit the jugular foramen together with the cranial fibers. Motor impulses are transmitted via the spinal root to the sternocleidomastoid muscle and the trapezius muscle, which control the movement of the head. Sensory axons originate from the proprioceptors of the muscles. They supply the motor neurons and end in the medulla oblongata. The hypoglossal nerve is a mixed CN with mainly motor functions.
The somatomotor axons passing through the hypoglossal canal arise from a nucleus of the medulla oblongata. However, the sensory part consists of proprioceptor axons of the tongue muscles and ends in the medulla oblongata. Anbarasu, A. Cranial nerves.
Oxford Medicine Online. Fisch, A. Cranial nerves 3, 4, 6, Leblanc, A. The twelve pairs of cranial nerves. The Cranial Nerves , — Mumenthaler, M. Neurological diagnosis of caudal cranial nerve lesions.
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