Artbased Question Chapter 15 Question 13 Part a Identify the Structure That Forms Cranial Nerve I

Learning Objectives

By the end of this department, you volition be able to:

• Depict the functional grouping of cranial nerves
• Match the regions of the forebrain and brain stalk that are connected to each cranial nervus
• Suggest diagnoses that would explain certain losses of function in the cranial fretfulness
• Chronicle cranial nerve deficits to impairment of adjacent, unrelated structures

The twelve cranial nerves are typically covered in introductory anatomy courses, and memorizing their names is facilitated past numerous mnemonics developed by students over the years of this do. But knowing the names of the nerves in order often leaves much to exist desired in understanding what the nerves exercise. The nerves tin can be categorized by functions, and subtests of the cranial nerve test can clarify these functional groupings.

Three of the nerves are strictly responsible for special senses whereas four others comprise fibers for special and full general senses. Iii nerves are connected to the extraocular muscles resulting in the control of gaze. Four nerves connect to muscles of the face, oral cavity, and pharynx, controlling facial expressions, mastication, swallowing, and speech. Four fretfulness make up the cranial component of the parasympathetic nervous organisation responsible for pupillary constriction, salivation, and the regulation of the organs of the thoracic and upper abdominal cavities. Finally, one nerve controls the muscles of the cervix, assisting with spinal control of the movement of the head and neck.

The cranial nerve exam allows directed tests of forebrain and brain stem structures. The twelve cranial nerves serve the head and neck. The vagus nerve (cranial nerve X) has autonomic functions in the thoracic and superior abdominal cavities. The special senses are served through the cranial nerves, as well as the full general senses of the head and neck. The movement of the optics, confront, tongue, throat, and neck are all under the control of cranial nerves. Preganglionic parasympathetic nerve fibers that command pupillary size, salivary glands, and the thoracic and upper abdominal viscera are institute in four of the fretfulness. Tests of these functions tin can provide insight into harm to specific regions of the encephalon stem and may uncover deficits in side by side regions.

Sensory Nerves

The olfactory, optic, and vestibulocochlear fretfulness (cranial nerves I, 2, and VIII) are defended to 4 of the special senses: olfactory property, vision, equilibrium, and hearing, respectively. Gustatory modality awareness is relayed to the brain stalk through fibers of the facial and glossopharyngeal nerves. The trigeminal nerve is a mixed nerve that carries the general somatic senses from the caput, like to those coming through spinal fretfulness from the rest of the body.

Testing smell is straightforward, as mutual smells are presented to one nostril at a time. The patient should be able to recognize the smell of coffee or mint, indicating the proper performance of the olfactory organization. Loss of the sense of olfactory property is called anosmia and can exist lost post-obit blunt trauma to the head or through aging. The short axons of the first cranial nervus regenerate on a regular basis. The neurons in the olfactory epithelium have a limited life span, and new cells grow to supercede the ones that die off. The axons from these neurons grow back into the CNS by following the existing axons—representing i of the few examples of such growth in the mature nervous system. If all of the fibers are sheared when the brain moves within the cranium, such every bit in a motor vehicle accident, then no axons can notice their fashion dorsum to the olfactory bulb to re-establish connections. If the nerve is not completely severed, the anosmia may be temporary as new neurons tin can eventually reconnect.

Olfaction is non the pre-eminent sense, only its loss can be quite detrimental. The enjoyment of food is largely based on our sense of smell. Anosmia means that food will not seem to have the same taste, though the gustatory sense is intact, and food will oft be described as beingness banal. However, the taste of food can be improved by adding ingredients (due east.g., common salt) that stimulate the gustatory sense.

Testing vision relies on the tests that are common in an optometry office. The Snellen chart (Figure 16.7) demonstrates visual acuity by presenting standard Roman messages in a variety of sizes. The result of this test is a rough generalization of the acuity of a person based on the normal accepted acuity, such that a alphabetic character that subtends a visual angle of v minutes of an arc at 20 feet can exist seen. To take 20/60 vision, for example, means that the smallest letters that a person can see at a 20-human foot altitude could be seen by a person with normal acuity from 60 feet away. Testing the extent of the visual field means that the examiner tin can establish the boundaries of peripheral vision every bit simply as holding their easily out to either side and asking the patient when the fingers are no longer visible without moving the eyes to rail them. If it is necessary, farther tests can constitute the perceptions in the visual fields. Concrete inspection of the optic disk, or where the optic nerve emerges from the eye, can be achieved by looking through the pupil with an ophthalmoscope.

Figure xvi.vii The Snellen Chart The Snellen chart for visual acuity presents a limited number of Roman letters in lines of decreasing size. The line with letters that subtend 5 minutes of an arc from 20 feet represents the smallest letters that a person with normal acuity should be able to read at that distance. The different sizes of letters in the other lines represent crude approximations of what a person of normal acuity can read at different distances. For case, the line that represents xx/200 vision would have larger messages then that they are legible to the person with normal vigil at 200 feet.

The optic nerves from both sides enter the cranium through the respective optic canals and meet at the optic chiasm at which fibers sort such that the 2 halves of the visual field are candy by the opposite sides of the brain. Deficits in visual field perception often suggest damage along the length of the optic pathway between the orbit and the diencephalon. For example, loss of peripheral vision may be the result of a pituitary tumor pressing on the optic chiasm (Figure sixteen.8). The pituitary, seated in the sella turcica of the sphenoid os, is direct inferior to the optic chiasm. The axons that decussate in the chiasm are from the medial retinae of either eye, and therefore conduct information from the peripheral visual field.

Figure sixteen.8 Pituitary Tumor The pituitary gland is located in the sella turcica of the sphenoid bone within the cranial flooring, placing it immediately junior to the optic chiasm. If the pituitary gland develops a tumor, it tin can press confronting the fibers crossing in the chiasm. Those fibers are conveying peripheral visual information to the opposite side of the encephalon, then the patient will feel "tunnel vision"—meaning that just the central visual field will be perceived.

The vestibulocochlear nerve (CN Viii) carries both equilibrium and auditory sensations from the inner ear to the medulla. Though the ii senses are not direct related, anatomy is mirrored in the two systems. Bug with residue, such as vertigo, and deficits in hearing may both signal to issues with the inner ear. Within the petrous region of the temporal bone is the bony labyrinth of the inner ear. The antechamber is the portion for equilibrium, composed of the utricle, saccule, and the 3 semicircular canals. The cochlea is responsible for transducing sound waves into a neural signal. The sensory nerves from these two structures travel side-past-side as the vestibulocochlear nerve, though they are really divide divisions. They both emerge from the inner ear, laissez passer through the internal auditory meatus, and synapse in nuclei of the superior medulla. Though they are office of singled-out sensory systems, the vestibular nuclei and the cochlear nuclei are close neighbors with adjacent inputs. Deficits in i or both systems could occur from damage that encompasses structures shut to both. Damage to structures near the two nuclei can issue in deficits to one or both systems.

Balance or hearing deficits may exist the upshot of harm to the middle or inner ear structures. Ménière'south disease is a disorder that can impact both equilibrium and audience in a variety of ways. The patient can suffer from vertigo, a low-frequency ringing in the ears, or a loss of hearing. From patient to patient, the exact presentation of the disease can be different. Additionally, within a unmarried patient, the symptoms and signs may change as the disease progresses. Employ of the neurological examination subtests for the vestibulocochlear nerve illuminates the changes a patient may go through. The disease appears to be the result of accumulation, or over-production, of fluid in the inner ear, in either the vestibule or cochlea.

Tests of equilibrium are of import for coordination and gait and are related to other aspects of the neurological test. The vestibulo-ocular reflex involves the cranial nerves for gaze control. Rest and equilibrium, as tested by the Romberg test, are part of spinal and cerebellar processes and involved in those components of the neurological exam, as discussed later.

Hearing is tested by using a tuning fork in a couple of different ways. The Rinne examination involves using a tuning fork to distinguish between conductive hearing and sensorineural hearing. Conductive hearing relies on vibrations being conducted through the ossicles of the heart ear. Sensorineural hearing is the transmission of audio stimuli through the neural components of the inner ear and cranial nervus. A vibrating tuning fork is placed on the mastoid process and the patient indicates when the sound produced from this is no longer present. Then the fork is immediately moved to only next to the ear canal and so the sound travels through the air. If the audio is not heard through the ear, meaning the sound is conducted better through the temporal bone than through the ossicles, a conductive hearing arrears is present. The Weber test also uses a tuning fork to differentiate between conductive versus sensorineural hearing loss. In this test, the tuning fork is placed at the elevation of the skull, and the sound of the tuning fork reaches both inner ears past travelling through bone. In a good for you patient, the sound would appear every bit loud in both ears. With unilateral conductive hearing loss, however, the tuning fork sounds louder in the ear with hearing loss. This is because the sound of the tuning fork has to compete with background noise coming from the outer ear, simply in conductive hearing loss, the background racket is blocked in the damaged ear, allowing the tuning fork to sound relatively louder in that ear. With unilateral sensorineural hearing loss, nevertheless, harm to the cochlea or associated nervous tissue means that the tuning fork sounds quieter in that ear.

The trigeminal arrangement of the head and neck is the equivalent of the ascending spinal cord systems of the dorsal column and the spinothalamic pathways. Somatosensation of the confront is conveyed along the nerve to enter the brain stem at the level of the pons. Synapses of those axons, all the same, are distributed beyond nuclei establish throughout the brain stalk. The mesencephalic nucleus processes proprioceptive information of the face, which is the movement and position of facial muscles. It is the sensory component of the jaw-wiggle reflex, a stretch reflex of the masseter muscle. The chief nucleus, located in the pons, receives information about light affect besides as proprioceptive information virtually the mandible, which are both relayed to the thalamus and, ultimately, to the postcentral gyrus of the parietal lobe. The spinal trigeminal nucleus, located in the medulla, receives data about rough touch, pain, and temperature to be relayed to the thalamus and cortex. Essentially, the projection through the chief nucleus is analogous to the dorsal column pathway for the torso, and the projection through the spinal trigeminal nucleus is analogous to the spinothalamic pathway.

Subtests for the sensory component of the trigeminal system are the aforementioned as those for the sensory exam targeting the spinal nerves. The primary sensory subtest for the trigeminal organization is sensory discrimination. A cotton-tipped applicator, which is cotton attached to the terminate of a sparse wooden stick, tin be used easily for this. The wood of the applicator can be snapped so that a pointed end is opposite the soft cotton-tipped end. The cotton fiber end provides a touch stimulus, while the pointed stop provides a painful, or abrupt, stimulus. While the patient's optics are airtight, the examiner touches the two ends of the applicator to the patient'south face, alternate randomly between them. The patient must place whether the stimulus is sharp or irksome. These stimuli are candy past the trigeminal system separately. Contact with the cotton tip of the applicator is a lite bear on, relayed by the main nucleus, but contact with the pointed end of the applicator is a painful stimulus relayed past the spinal trigeminal nucleus. Failure to discriminate these stimuli tin can localize problems within the brain stem. If a patient cannot recognize a painful stimulus, that might indicate impairment to the spinal trigeminal nucleus in the medulla. The medulla also contains important regions that regulate the cardiovascular, respiratory, and digestive systems, too every bit beingness the pathway for ascending and descending tracts between the brain and spinal string. Damage, such as a stroke, that results in changes in sensory discrimination may indicate these unrelated regions are afflicted too.

Gaze Control

The three nerves that control the extraocular muscles are the oculomotor, trochlear, and abducens nerves, which are the third, fourth, and sixth cranial fretfulness. As the name suggests, the abducens nerve is responsible for abducting the eye, which it controls through wrinkle of the lateral rectus musculus. The trochlear nerve controls the superior oblique muscle to rotate the eye along its axis in the orbit medially, which is called intorsion, and is a component of focusing the optics on an object close to the face. The oculomotor nerve controls all the other extraocular muscles, also as a muscle of the upper eyelid. Movements of the two eyes need to be coordinated to locate and runway visual stimuli accurately. When moving the eyes to locate an object in the horizontal plane, or to rail motility horizontally in the visual field, the lateral rectus muscle of one middle and medial rectus muscle of the other heart are both active. The lateral rectus is controlled past neurons of the abducens nucleus in the superior medulla, whereas the medial rectus is controlled by neurons in the oculomotor nucleus of the midbrain.

Coordinated movement of both optics through different nuclei requires integrated processing through the brain stem. In the midbrain, the superior colliculus integrates visual stimuli with motor responses to initiate heart movements. The paramedian pontine reticular formation (PPRF) will initiate a rapid eye movement, or saccade, to bring the eyes to bear on a visual stimulus quickly. These areas are connected to the oculomotor, trochlear, and abducens nuclei by the medial longitudinal fasciculus (MLF) that runs through the majority of the brain stem. The MLF allows for cohabit gaze, or the movement of the eyes in the same direction, during horizontal movements that require the lateral and medial rectus muscles. Control of cohabit gaze strictly in the vertical management is contained within the oculomotor complex. To elevate the optics, the oculomotor nerve on either side stimulates the contraction of both superior rectus muscles; to depress the eyes, the oculomotor nervus on either side stimulates the wrinkle of both inferior rectus muscles.

Purely vertical movements of the optics are not very common. Movements are often at an angle, so some horizontal components are necessary, adding the medial and lateral rectus muscles to the movement. The rapid movement of the eyes used to locate and straight the fovea onto visual stimuli is called a saccade. Find that the paths that are traced in Figure 16.9 are non strictly vertical. The movements betwixt the nose and the mouth are closest, only still take a camber to them. Also, the superior and inferior rectus muscles are not perfectly oriented with the line of sight. The origin for both muscles is medial to their insertions, and then superlative and depression may require the lateral rectus muscles to recoup for the slight adduction inherent in the contraction of those muscles, requiring MLF activeness every bit well.

Effigy sixteen.9 Saccadic Eye Movements Saccades are rapid, conjugate movements of the eyes to survey a complicated visual stimulus, or to follow a moving visual stimulus. This prototype represents the shifts in gaze typical of a person studying a face. Notice the concentration of gaze on the major features of the face and the large number of paths traced between the eyes or around the mouth.

Testing centre move is simply a matter of having the patient track the tip of a pen as it is passed through the visual field. This may appear similar to testing visual field deficits related to the optic nerve, just the departure is that the patient is asked to non move the eyes while the examiner moves a stimulus into the peripheral visual field. Here, the extent of movement is the indicate of the exam. The examiner is watching for conjugate movements representing proper function of the related nuclei and the MLF. Failure of one eye to abduct while the other adducts in a horizontal movement is referred to as internuclear ophthalmoplegia. When this occurs, the patient volition feel diplopia, or double vision, as the 2 optics are temporarily pointed at different stimuli. Diplopia is not restricted to failure of the lateral rectus, considering any of the extraocular muscles may neglect to move i center in perfect conjugation with the other.

The final aspect of testing eye movements is to move the tip of the pen in toward the patient'due south face. As visual stimuli motion closer to the face, the two medial recti muscles crusade the eyes to motion in the ane nonconjugate movement that is part of gaze control. When the 2 eyes move to expect at something closer to the face, they both adduct, which is referred to as convergence. To go on the stimulus in focus, the eye also needs to modify the shape of the lens, which is controlled through the parasympathetic fibers of the oculomotor nerve. The change in focal power of the center is referred to every bit adaptation. Accommodation ability changes with age; focusing on nearer objects, such equally the written text of a volume or on a computer screen, may require cosmetic lenses after in life. Coordination of the skeletal muscles for convergence and coordination of the smooth muscles of the ciliary body for accommodation are referred to every bit the accommodation–convergence reflex.

A crucial function of the cranial nerves is to keep visual stimuli centered on the fovea of the retina. The vestibulo-ocular reflex (VOR) coordinates all of the components (Figure sixteen.10), both sensory and motor, that make this possible. If the head rotates in one management—for example, to the right—the horizontal pair of semicircular canals in the inner ear signal the movement by increased activity on the right and decreased action on the left. The data is sent to the abducens nuclei and oculomotor nuclei on either side to coordinate the lateral and medial rectus muscles. The left lateral rectus and right medial rectus muscles will contract, rotating the optics in the opposite direction of the head, while nuclei decision-making the right lateral rectus and left medial rectus muscles will be inhibited to reduce antagonism of the contracting muscles. These actions stabilize the visual field past compensating for the head rotation with contrary rotation of the optics in the orbits. Deficits in the VOR may be related to vestibular harm, such equally in Ménière'southward disease, or from dorsal encephalon stalk damage that would affect the eye movement nuclei or their connections through the MLF.

Effigy sixteen.10 Vestibulo-ocular Reflex If the head is turned in ane direction, the coordination of that movement with the fixation of the eyes on a visual stimulus involves a circuit that ties the vestibular sense with the middle motility nuclei through the MLF.

Fretfulness of the Face and Oral Cavity

An iconic part of a doctor'south visit is the inspection of the oral cavity and pharynx, suggested by the directive to "open your mouth and say 'ah.'" This is followed by inspection, with the aid of a tongue depressor, of the back of the mouth, or the opening of the oral fissure into the throat known every bit the fauces. Whereas this portion of a medical exam inspects for signs of infection, such as in tonsillitis, it is also the ways to test the functions of the cranial fretfulness that are associated with the oral cavity.

The facial and glossopharyngeal nerves convey gustatory stimulation to the brain. Testing this is as uncomplicated as introducing salty, sour, biting, or sugariness stimuli to either side of the natural language. The patient should respond to the taste stimulus before retracting the tongue into the mouth. Stimuli practical to specific locations on the tongue volition dissolve into the saliva and may stimulate gustation buds connected to either the left or right of the nerves, masking whatever lateral deficits. Along with taste, the glossopharyngeal nerve relays general sensations from the pharyngeal walls. These sensations, along with sure gustatory modality stimuli, tin can stimulate the gag reflex. If the examiner moves the tongue depressor to contact the lateral wall of the fauces, this should arm-twist the gag reflex. Stimulation of either side of the fauces should elicit an equivalent response. The motor response, through contraction of the muscles of the pharynx, is mediated through the vagus nerve. Normally, the vagus nerve is considered autonomic in nature. The vagus nerve directly stimulates the contraction of skeletal muscles in the pharynx and larynx to contribute to the swallowing and speech functions. Further testing of vagus motor function has the patient repeating consonant sounds that require motility of the muscles around the fauces. The patient is asked to say "lah-kah-pah" or a similar fix of alternating sounds while the examiner observes the movements of the soft palate and arches between the palate and tongue.

The facial and glossopharyngeal fretfulness are also responsible for the initiation of salivation. Neurons in the salivary nuclei of the medulla project through these two nerves as preganglionic fibers, and synapse in ganglia located in the head. The parasympathetic fibers of the facial nerve synapse in the pterygopalatine ganglion, which projects to the submandibular gland and sublingual gland. The parasympathetic fibers of the glossopharyngeal nerve synapse in the otic ganglion, which projects to the parotid gland. Salivation in response to food in the oral cavity is based on a visceral reflex arc within the facial or glossopharyngeal nerves. Other stimuli that stimulate salivation are coordinated through the hypothalamus, such as the aroma and sight of food.

The hypoglossal nerve is the motor nervus that controls the muscles of the tongue, except for the palatoglossus musculus, which is controlled by the vagus nerve. In that location are two sets of muscles of the tongue. The extrinsic muscles of the tongue are connected to other structures, whereas the intrinsic muscles of the tongue are completely contained within the lingual tissues. While examining the oral cavity, movement of the tongue will betoken whether hypoglossal role is impaired. The test for hypoglossal part is the "stick out your tongue" function of the exam. The genioglossus muscle is responsible for protrusion of the natural language. If the hypoglossal fretfulness on both sides are working properly, then the tongue will stick straight out. If the nerve on ane side has a deficit, the tongue volition stick out to that side—pointing to the side with harm. Loss of function of the tongue can interfere with speech and swallowing. Additionally, because the location of the hypoglossal nerve and nucleus is near the cardiovascular center, inspiratory and expiratory areas for respiration, and the vagus nuclei that regulate digestive functions, a tongue that protrudes incorrectly can suggest damage in adjacent structures that have nothing to practice with controlling the tongue.

Interactive Link

Lookout this curt video to encounter an examination of the facial nerve using some elementary tests. The facial nerve controls the muscles of facial expression. Astringent deficits will be obvious in watching someone employ those muscles for normal control. 1 side of the confront might not move similar the other side. But directed tests, especially for wrinkle confronting resistance, require a formal testing of the muscles. The muscles of the upper and lower face need to be tested. The strength test in this video involves the patient squeezing her eyes shut and the examiner trying to pry her eyes open. Why does the examiner ask her to try a second fourth dimension?

Motor Nerves of the Neck

The accessory nervus, also referred to equally the spinal accessory nerve, innervates the sternocleidomastoid and trapezius muscles (Figure xvi.11). When both the sternocleidomastoids contract, the caput flexes forward; individually, they cause rotation to the opposite side. The trapezius can act every bit an antagonist, causing extension and hyperextension of the neck. These 2 superficial muscles are of import for irresolute the position of the caput. Both muscles also receive input from cervical spinal fretfulness. Forth with the spinal accessory nervus, these nerves contribute to elevating the scapula and clavicle through the trapezius, which is tested by asking the patient to shrug both shoulders, and watching for asymmetry. For the sternocleidomastoid, those spinal nerves are primarily sensory projections, whereas the trapezius also has lateral insertions to the clavicle and scapula, and receives motor input from the spinal cord. Calling the nerve the spinal accessory nervus suggests that it is aiding the spinal nerves. Though that is not precisely how the name originated, it does help make the clan between the function of this nerve in controlling these muscles and the part these muscles play in movements of the trunk or shoulders.

Figure sixteen.11 Muscles Controlled by the Accessory Nerve The accessory nerve innervates the sternocleidomastoid and trapezius muscles, both of which attach to the caput and to the trunk and shoulders. They can human action as antagonists in head flexion and extension, and equally synergists in lateral flexion toward the shoulder.

To exam these muscles, the patient is asked to flex and extend the cervix or shrug the shoulders against resistance, testing the strength of the muscles. Lateral flexion of the neck toward the shoulder tests both at the same fourth dimension. Whatsoever difference on i side versus the other would suggest damage on the weaker side. These force tests are common for the skeletal muscles controlled by spinal nerves and are a pregnant component of the motor exam. Deficits associated with the accessory nerve may accept an issue on orienting the head, every bit described with the VOR.

Homeostatic Imbalances

The Pupillary Low-cal Response

The autonomic control of pupillary size in response to a vivid light involves the sensory input of the optic nervus and the parasympathetic motor output of the oculomotor nerve. When lite hits the retina, specialized photosensitive ganglion cells ship a bespeak forth the optic nervus to the pretectal nucleus in the superior midbrain. A neuron from this nucleus projects to the Edinger–Westphal nuclei in the oculomotor circuitous in both sides of the midbrain. Neurons in this nucleus give rise to the preganglionic parasympathetic fibers that project through the oculomotor nerve to the ciliary ganglion in the posterior orbit. The postganglionic parasympathetic fibers from the ganglion projection to the iris, where they release acetylcholine onto circular fibers that constrict the pupil to reduce the amount of lite hitting the retina. The sympathetic nervous system is responsible for dilating the pupil when lite levels are low.

Shining light in one eye volition elicit constriction of both pupils. The efferent limb of the pupillary low-cal reflex is bilateral. Light shined in ane eye causes a constriction of that pupil, as well as constriction of the contralateral pupil. Shining a penlight in the eye of a patient is a very artificial situation, as both optics are normally exposed to the same light sources. Testing this reflex can illustrate whether the optic nerve or the oculomotor nervus is damaged. If shining the light in one eye results in no changes in pupillary size but shining light in the contrary eye elicits a normal, bilateral response, the damage is associated with the optic nerve on the nonresponsive side. If light in either eye elicits a response in only one middle, the problem is with the oculomotor system.

If low-cal in the correct center only causes the left pupil to constrict, the direct reflex is lost and the consensual reflex is intact, which means that the right oculomotor nerve (or Edinger–Westphal nucleus) is damaged. Damage to the correct oculomotor connections volition be axiomatic when calorie-free is shined in the left eye. In that case, the direct reflex is intact but the consensual reflex is lost, meaning that the left student will constrict while the right does not.

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Source: https://openstax.org/books/anatomy-and-physiology-2e/pages/16-3-the-cranial-nerve-exam

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