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Assessment Of Muscles Innervated By Cranial Nerves

The oculomotor nerve and its associated cranial nerve nuclei exist within the midbrain. The midbrain develops from the mesencephalon. Neuroblasts from the basal plates develop into the tegmentum. The tegmentum includes cranial nerves III and IV, Edinger-Westphal nuclei, oculomotor nuclei, trochlear nuclei, red nuclei, and reticular nuclei.

Assessment of Muscles Innervated by Cranial Nerves

Cranial nerve III has somatic and autonomic functions. Somatic nerves are homologous with ventral roots of spinal nerves. They originate from the basal plate and innervates the superior rectus, inferior rectus, medial rectus, and inferior oblique muscles. These muscles derive from the first preoptic myotome.

Most research about the development of parasympathetic nerves and the ciliary ganglion has been on chickens. The parasympathetic nerve supply of the oculomotor nerves develops from the caudal midbrain and rostral hindbrain neural crest cells. Some cells may also originate from the ectodermal placode caudal to the eye.[3][4] These cells migrate ventrolaterally and rostrally toward the optic vesicle. As the neurons in the ciliary ganglion differentiate, they extend axons to innervate the blood vessels of the choroid plexus as well as the striated muscles of the iris and ciliary body.[5]

The microvascular disease can affect the vasa vasorum. In these instances, the fibers innervating the upper eyelid and most of the extraocular muscles become ischemic. Compression of the oculomotor nerve can also occur via an aneurysm, intracranial neoplasm, or uncal herniation. These ischemic events can affect the outer pia blood vessels supplying the fibers to the pupil and lens and eventually the inner vasa vasorum.

Intraoperative neurophysiological monitoring (IOM) has established itself as one of the paths by which modern neurosurgery can improve surgical results while minimizing morbidity. IOM consists of both monitoring (continuous "on-line" assessment of the functional integrity of neural pathways) and mapping (functional identification and preservation of anatomically ambiguous nervous tissue) techniques. In posterior-fossa and brainstem surgery, mapping techniques can be used to identify - and therefore preserve - cranial nerves, their motor nuclei and corticospinal or corticobulbar pathways. Similarly, free-running electromyography (EMG) and muscle motor-evoked potential (mMEP) monitoring can continuously assess the functional integrity of these pathways during surgery. Mapping of the corticospinal tract, at the level of the cerebral peduncle as well as mapping of the VII, IX-X and XII cranial nerve motor nuclei on the floor of the fourth ventricle, is of great value to identify "safe entry-zones" into the brainstem. Mapping techniques allow recognizing anatomical landmarks such as the facial colliculus, the hypoglosseal and glossopharyngeal triangles on the floor of the fourth ventricle, even when normal anatomy is distorted by a tumor. On the basis of neurophysiological mapping, specific patterns of motor cranial nuclei displacement can be recognized. However, brainstem mapping cannot detect injury to the supranuclear tracts originating in the motor cortex and ending on the cranial nerve motor nuclei. Therefore, monitoring techniques should be used. Standard techniques for continuously assessing the functional integrity of motor cranial nerves traditionally rely on the evaluation of spontaneous free-running EMG in muscles innervated by motor cranial nerves. Although several criteria have been proposed to identify those EMG activity patterns that are suspicious for nerve injury, the terminology remains somewhat confusing and convincing data regarding a clinical correlation between EMG activity and clinical outcome are still lacking. Transcranial mMEPs are also currently used during posterior-fossa surgery and principles of MEP monitoring to assess the functional integrity of motor pathways are similar to those used in brain and spinal-cord surgery. Recently, current concepts in muscle MEP monitoring have been extended to the monitoring of motor cranial nerves. So-called "corticobulbar mMEPs" can be used to monitor the functional integrity of corticobulbar tracts from the cortex through the cranial motor nuclei and to the muscle innervated by cranial nerves. Methodology for this purpose has appeared in the literature only recently and mostly with regards to the VII cranial nerve monitoring. Nevertheless, this technique has not yet been standardized and some limitations still exist. In particular, with regards to the preservation of the swallowing and coughing reflexes, available intraoperative techniques are insufficient to provide reliable prognostic data since only the efferent arc of the reflex can be tested.

Twelve pairs of nerves (the cranial nerves) lead directly from the brain to various parts of the head, neck, and trunk. Some of the cranial nerves are involved in the special senses (such as seeing, hearing, and taste), and others control muscles in the face or regulate glands. The nerves are named and numbered (according to their location, from the front of the brain to the back).

Image 1: Twelve pairs of cranial nerves emerge from the underside of the brain, pass through openings in the skull, and lead to parts of the head, neck, and trunk. The nerves are named and numbered, based on their location, from the front of the brain to the back. Thus, the olfactory nerve is the 1st cranial nerve, and the hypoglossal nerve is the 12th cranial nerve

Unlike spinal nerves whose roots are neural fibers from the spinal grey matter, cranial nerves are composed of the neural processes associated with distinct brainstem nuclei and cortical structures.[1]

The names of the cranial nerves (CN) are: CN I - Olfactory, CN II - Optic, CN III - Oculomotor, CN IV - Trochlear, CN V - Trigeminal, CN VI - Abducens, CN VII - Facial, CN VIII - Vestibulocochlear, CN IX - Glossopharyngeal, CN X - Vagus, CN XI - Accessory, and CN XII - Hypoglossal. [2] Link: Introduction to Neuroanatomy

The names of the cranial nerves sometimes correspond with their individual function. Some of the cranial nerves are purely sensory, others are purely motor, and the rest have both sensory and motor components. [4]

Dysfunction of certain cranial nerves may affect the eye, pupil, optic nerve, or extraocular muscles and their nerves; thus, they can be considered cranial nerve disorders, neuro-ophthalmologic disorders, or both.

Cranial nerve disorders can also involve dysfunction of smell, vision, chewing, facial sensation or expression, taste, hearing, balance, swallowing, phonation, head turning and shoulder elevation, or tongue movements (see table below). One or more cranial nerves may be affected.

Review summary: The optic nerve is evaluated by visual evoked potentials. Measurements of latency, amplitude, and waveform morphology are especially useful in detecting demyelinating lesions. Brain stem auditory evoked potentials evaluate the auditory portion of the eighth cranial nerve. Using an auditory stimulus, a number of waveforms are generated, and changes in the normal patterns of response can detect abnormalities. Assessment of the trigeminal and facial nerves is done using a series of electrical stimulation techniques including the blink, masseter, and masseter inhibitory reflexes and facial motor nerve conduction studies. The blink reflex detects lesions of the first division of the trigeminal nerve and the facial nerve. The masseter reflex evaluates the third division of the trigeminal nerve. Changes in responses are measured and, using a combination of these techniques, localization of lesions at specific sites can be made. Accessory motor nerve conduction is useful not only in focal nerve injury, but repetitive stimulation on the accessory and facial nerves is used in diagnosing neuromuscular junction disorders. In addition, many of the voluntary muscles innervated by the cranial nerves are accessible to needle electrode examination, and evaluation can aid in identification of focal nerve lesions, as well as diagnosis in diffuse nerve and muscle disorders.

The Spinal Accessory Nerve (SAN) or Cranial Nerve 11 is termed a cranial nerve as it was originally believed to originate in the brain. It has both a cranial and a spinal part, though debate still rages regarding if the cranial part is really a part of the SAN or part of the vagus nerve. [1] The cranial part , along with the cranial nerves 9 and 10, supplies innervation to the soft palate, larynx and pharynx. The spinal part supplies innervation to the Sternocleidomastoid and Trapezius muscles.[2]

Spinal Portion: Travels upwards as the C1-C5/6 spinal root section join together and exit out the jugular foramen, with the cranial portion. It goes on to supply the Sternocleidomastoid and Trapezius muscles.[2]

The nerve traverses the posterior cranial fossa to reach the jugular foramen. It briefly meets the cranial portion of the accessory nerve, before exiting the skull (along with the glossopharyngeal and vagus nerves).

An isolated third cranial nerve palsy may cause variable ipsilateral involvement of the superior, inferior, and medial recti muscles and/or inferior oblique muscle. Multiple cranial nerve palsies might indicate lesions of the brainstem, cavernous sinus, skull base, or a more generalized peripheral nerve process such as Miller Fisher Syndrome.

Convergence in accommodation: When shifting one's view from a distant object to a nearby object, the eyes converge (are directed nasally) to keep the object's image focused on the foveae of the two eyes. This action involves the contraction of the medial rectus muscles of the two eyes and relaxation of the lateral rectus muscles. The medial rectus attaches to the medial aspect of the eye and its contraction directs the eye nasally (adducts the eye). The medial rectus is innervated by motor neurons in the oculomotor nucleus and nerve. 041b061a72


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