First know about the cranial nerve:
Cranial nerves are the nerves that emerge directly from the brain, in contrast to spinal nerves . 10 of 12 of the cranial nerves originate in the brainstem. Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck. Spinal nerves emerge sequentially from the spinal cord with the spinal nerve closest to the head emerging in the space above the first cervical vertebra. The cranial nerves, however, emerge from the central nervous system above this level. Each cranial nerve is paired and is present on both sides. Depending on definition in humans there are twelve or thirteen cranial nerves pairs, which are assigned Roman numerals I–XII, sometimes also including cranial nerve zero. The numbering of the cranial nerves is based on the order in which they emerge from the brain, front to back .
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Anatomy
Most typically, humans are considered to have twelve pairs of cranial nerves . They are: the olfactory nerve, the optic nerve, oculomotor nerve, trochlear nerve, trigeminal nerve, abducens nerve, facial nerve, vestibulocochlear nerve, glossopharyngeal nerve, vagus nerve, accessory nerve, and hypoglossal nerve ., which is very small and may or may not be functional in humans and the vagus nerve is named for its wandering course . Cranial nerves are numbered based on their rostral-caudal position, Cranial nerves have paths within and outside of the skull. The paths within the skull are called "intracranial" and the paths outside the skull are called "extracranial". There are many holes in the skull called "foramina" by which the nerves can exit the skull. All cranial nerves are paired, which means that they occur on both the right and left sides of the body. The muscle, skin, or additional function supplied by a nerve on the same side of the body as the side it originates from, is referred to an ipsilateral function. If the function is on the opposite side to the origin of the nerve, this is known as a contralateral function.
Intracranial course
Nuclei :
The cell bodies of many of the neurons of most of the cranial nerves are contained in one or more nuclei in the brainstem. These nuclei are important relative to cranial nerve dysfunction because damage to these nuclei such as from a stroke or trauma can mimic damage to one or more branches of a cranial nerve. In terms of specific cranial nerve nuclei, the midbrain of the brainstem has the nuclei of the oculomotor nerve and trochlear nerve ; the pons has the nuclei of the trigeminal nerve, abducens nerve, facial nerve and vestibulocochlear nerve ; and the medulla has the nuclei of the glossopharyngeal nerve, vagus nerve, accessory nerve and hypoglossal nerve . The fibers of these cranial nerves exit the brainstem from these nuclei. The vagus nerve provides sensory and autonomic motor innervation to structures in the neck and also to most of the organs in the chest and abdomen.
Vision :
The optic nerve transmits visual information. Lesions may also lead to inability to open the eye due to paralysis of the levator palpebrae muscle. Individuals suffering from a lesion to the oculomotor nerve may compensate by tilting their heads to alleviate symptoms due to paralysis of one or more of the eye muscles it controls. and trigeminal zoster.
Facial expression:
Lesions of the facial nerve may manifest as facial palsy. This is where a person is unable to move the muscles on one or both sides of their face. A very common and generally temporary facial palsy is known as Bell's palsy. Bell's Palsy is the result of an idiopathic, unilateral lower motor neuron lesion of the facial nerve and is characterized by an inability to move the ipsilateral muscles of facial expression, including elevation of the eyebrow and furrowing of the forehead. Patients with Bell's palsy often have a drooping mouth on the affected side and often have trouble chewing because the buccinator muscle is affected. The glossopharyngeal nerve also provides parasympathetic innervation to the parotid gland.
Vagus nerve :
Loss of function of the vagus nerve will lead to a loss of parasympathetic innervation to a very large number of structures. Major effects of damage to the vagus nerve may include a rise in blood pressure and heart rate. Isolated dysfunction of only the vagus nerve is rare, but can be diagnosed by a hoarse voice, due to dysfunction of one of its branches, the recurrent laryngeal nerve. A cranial nerve exam begins with observation of the patient because some cranial nerve lesions may affect the symmetry of the eyes or face. The visual fields are tested for nerve lesions or nystagmus via an analysis of specific eye movements. The sensation of the face is tested, and patients are asked to perform different facial movements, such as puffing out of the cheeks. Hearing is checked by voice and tuning forks. The position of the patient's uvula is examined because asymmetry in the position could indicate a lesion of the glossopharyngeal nerve. After the ability of the patient to use their shoulder to assess the accessory nerve, and the patient's tongue function is assessed by observing various tongue movements. A loss of functionality of a single cranial nerve may sometimes be the first symptom of an intracranial or skull base cancer. An increase in intracranial pressure may lead to impairment of the optic nerves due to compression of the surrounding veins and capillaries, causing swelling of the eyeball . A cancer, such as an optic glioma, may also impact the optic nerve . A pituitary tumour may compress the optic tracts or the optic chiasm of the optic nerve, leading to visual field loss. A pituitary tumour may also extend into the cavernous sinus, compressing the oculuomotor nerve, trochlear nerve and abducens nerve, leading to double-vision and strabismus. These nerves may also be affected by herniation of the temporal lobes of the brain through the falx cerebri.
Inflammation :
Inflammation resulting from infection may impair the function of any of the cranial nerves. Inflammation of the facial nerve may result in Bell's palsy. Multiple sclerosis, an inflammatory process that may produce a loss of the myelin sheathes which surround the cranial nerves, may cause a variety of shifting symptoms affecting multiple cranial nerves. Much later, in 1664, English anatomist Sir Thomas Willis suggested that there were actually 9 pairs of nerves. Finally, in 1778, German anatomist Samuel Soemmering named the 12 pairs of nerves that are generally accepted today.
Other animals:
Cranial nerves are also present in other vertebrates. Other amniotes have cranial nerves similar to those of humans. In anamniotes, the accessory nerve and hypoglossal nerve do not exist, with the accessory nerve being an integral part of the vagus nerve ; the hypoglossal nerve is represented by a variable number of spinal nerves emerging from vertebral segments fused into the occiput. These two nerves only became discrete nerves in the ancestors of amniotes .
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