what cranial nerves are dedicated to smell, vision, equilibrium and hearing?

The Neurological Test

The Cranial Nervus Examination

OpenStaxCollege

Learning Objectives

By the cease of this section, you will be able to:

• Describe the functional grouping of cranial fretfulness
• Match the regions of the forebrain and brain stem that are connected to each cranial nervus
• Propose diagnoses that would explain sure losses of function in the cranial fretfulness
• Chronicle cranial nervus deficits to harm of adjacent, unrelated structures

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

Three of the nerves are strictly responsible for special senses whereas 4 others contain fibers for special and general senses. Iii nerves are continued to the extraocular muscles resulting in the control of gaze. 4 nerves connect to muscles of the face, rima oris, and pharynx, controlling facial expressions, mastication, swallowing, and speech. Four nerves brand up the cranial component of the parasympathetic nervous arrangement 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 neck, assisting with spinal command of the move 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 general senses of the caput and neck. The movement of the eyes, face, natural language, pharynx, and cervix are all under the command of cranial nerves. Preganglionic parasympathetic nerve fibers that control pupillary size, salivary glands, and the thoracic and upper abdominal viscera are found in four of the nerves. Tests of these functions can provide insight into damage to specific regions of the encephalon stem and may uncover deficits in adjacent regions.

Sensory Nerves

The olfactory, optic, and vestibulocochlear nerves (cranial fretfulness I, Two, and VIII) are defended to iv of the special senses: odour, vision, equilibrium, and hearing, respectively. Sense of taste sensation is relayed to the brain stem through fibers of the facial and glossopharyngeal nerves. The trigeminal nervus is a mixed nerve that carries the general somatic senses from the caput, similar to those coming through spinal nerves from the rest of the trunk.

Testing smell is straightforward, as common smells are presented to one nostril at a time. The patient should be able to recognize the smell of java or mint, indicating the proper functioning of the olfactory arrangement. Loss of the sense of smell is called anosmia and can be lost following blunt trauma to the head or through aging. The curt axons of the first cranial nerve regenerate on a regular basis. The neurons in the olfactory epithelium take a limited life span, and new cells grow to replace the ones that dice off. The axons from these neurons grow dorsum into the CNS past following the existing axons—representing one of the few examples of such growth in the mature nervous system. If all of the fibers are sheared when the encephalon moves within the attic, such as in a motor vehicle accident, then no axons tin find their mode back to the olfactory seedling to re-establish connections. If the nervus is not completely severed, the anosmia may exist temporary as new neurons can eventually reconnect.

Olfaction is non the pre-eminent sense, but its loss can be quite detrimental. The enjoyment of food is largely based on our sense of odour. Anosmia means that nutrient will not seem to have the same taste, though the gustatory sense is intact, and food volition often be described as beingness bland. However, the taste of food tin can be improved past calculation ingredients (e.g., salt) that stimulate the gustatory sense.

Testing vision relies on the tests that are common in an optometry part. The Snellen chart ([link]) demonstrates visual acuity by presenting standard Roman letters in a variety of sizes. The result of this examination 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 5 minutes of an arc at 20 feet tin be seen. To accept 20/lx vision, for case, means that the smallest letters that a person can run into at a 20-foot altitude could be seen by a person with normal acuity from sixty feet away. Testing the extent of the visual field means that the examiner can establish the boundaries of peripheral vision every bit but equally holding their easily out to either side and asking the patient when the fingers are no longer visible without moving the optics to track them. If it is necessary, further tests can establish the perceptions in the visual fields. Physical inspection of the optic disk, or where the optic nerve emerges from the eye, can be accomplished by looking through the pupil with an ophthalmoscope.

The Snellen Nautical chart

The Snellen chart for visual acuity presents a express 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 altitude. The different sizes of letters in the other lines stand for rough approximations of what a person of normal acuity tin read at dissimilar distances. For example, the line that represents 20/200 vision would have larger letters so that they are legible to the person with normal acuity 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 ii halves of the visual field are processed by the opposite sides of the brain. Deficits in visual field perception often suggest damage along the length of the optic pathway betwixt the orbit and the diencephalon. For example, loss of peripheral vision may be the upshot of a pituitary tumor pressing on the optic chiasm ([link]). The pituitary, seated in the sella turcica of the sphenoid os, is directly junior to the optic chiasm. The axons that decussate in the chiasm are from the medial retinae of either eye, and therefore carry information from the peripheral visual field.

Pituitary Tumor

The pituitary gland is located in the sella turcica of the sphenoid bone inside the cranial floor, placing it immediately inferior to the optic chiasm. If the pituitary gland develops a tumor, it can press against the fibers crossing in the chiasm. Those fibers are conveying peripheral visual data to the opposite side of the encephalon, so the patient volition experience "tunnel vision"—significant that simply 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 two senses are not directly related, anatomy is mirrored in the ii systems. Problems with balance, such as vertigo, and deficits in hearing may both point to problems 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 three 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 separate divisions. They both emerge from the inner ear, pass 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 close to both. Damage to structures near the 2 nuclei can issue in deficits to i or both systems.

Balance or hearing deficits may exist the event of damage to the middle or inner ear structures. Ménière's disease is a disorder that can affect both equilibrium and audition in a variety of ways. The patient can suffer from vertigo, a depression-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 single patient, the symptoms and signs may change equally the disease progresses. Use of the neurological examination subtests for the vestibulocochlear nerve illuminates the changes a patient may go through. The illness appears to be the result of accumulation, or over-production, of fluid in the inner ear, in either the lobby or cochlea.

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

Hearing is tested by using a tuning fork in a couple of different ways. The Rinne test 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 middle ear. Sensorineural hearing is the transmission of audio stimuli through the neural components of the inner ear and cranial nerve. A vibrating tuning fork is placed on the mastoid procedure and the patient indicates when the sound produced from this is no longer present. And so the fork is immediately moved to just next to the ear culvert so the audio travels through the air. If the sound is not heard through the ear, significant the sound is conducted better through the temporal bone than through the ossicles, a conductive hearing arrears is present. The Weber exam also uses a tuning fork to differentiate betwixt conductive versus sensorineural hearing loss. In this test, the tuning fork is placed at the top of the skull, and the audio of the tuning fork reaches both inner ears past travelling through bone. In a salubrious 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 dissonance coming from the outer ear, simply in conductive hearing loss, the background noise is blocked in the damaged ear, allowing the tuning fork to audio relatively louder in that ear. With unilateral sensorineural hearing loss, still, damage to the cochlea or associated nervous tissue means that the tuning fork sounds quieter in that ear.

The trigeminal system of the head and cervix is the equivalent of the ascending spinal cord systems of the dorsal column and the spinothalamic pathways. Somatosensation of the face 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 across nuclei found throughout the encephalon stem. The mesencephalic nucleus processes proprioceptive information of the face up, which is the motion and position of facial muscles. It is the sensory component of the jaw-jerk reflex, a stretch reflex of the masseter muscle. The master nucleus, located in the pons, receives information about light touch every bit well as proprioceptive information almost 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 information about crude touch, pain, and temperature to be relayed to the thalamus and cortex. Essentially, the project through the chief nucleus is coordinating to the dorsal column pathway for the trunk, and the projection through the spinal trigeminal nucleus is coordinating to the spinothalamic pathway.

Subtests for the sensory component of the trigeminal system are the same as those for the sensory test 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 end of a thin wooden stick, can be used easily for this. The wood of the applicator tin can be snapped and then that a pointed stop is opposite the soft cotton fiber-tipped end. The cotton end provides a touch stimulus, while the pointed end provides a painful, or sharp, stimulus. While the patient's eyes are closed, the examiner touches the two ends of the applicator to the patient'southward face, alternating randomly between them. The patient must identify whether the stimulus is precipitous or dull. These stimuli are processed by the trigeminal system separately. Contact with the cotton tip of the applicator is a light bear on, relayed by the chief nucleus, but contact with the pointed end of the applicator is a painful stimulus relayed by the spinal trigeminal nucleus. Failure to discriminate these stimuli can localize issues within the brain stalk. If a patient cannot recognize a painful stimulus, that might indicate harm to the spinal trigeminal nucleus in the medulla. The medulla also contains of import regions that regulate the cardiovascular, respiratory, and digestive systems, too as being the pathway for ascending and descending tracts between the brain and spinal cord. Harm, such every bit a stroke, that results in changes in sensory discrimination may point these unrelated regions are affected every bit well.

Gaze Control

The three nerves that command the extraocular muscles are the oculomotor, trochlear, and abducens nerves, which are the third, fourth, and 6th cranial fretfulness. Every bit the name suggests, the abducens nervus is responsible for abducting the centre, which it controls through contraction of the lateral rectus muscle. 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 eyes on an object shut to the face. The oculomotor nervus controls all the other extraocular muscles, as well equally a muscle of the upper eyelid. Movements of the two eyes need to be coordinated to locate and track visual stimuli accurately. When moving the optics to locate an object in the horizontal airplane, or to runway movement horizontally in the visual field, the lateral rectus muscle of one eye and medial rectus muscle of the other eye are both active. The lateral rectus is controlled by 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 motion of both optics through dissimilar nuclei requires integrated processing through the brain stalk. In the midbrain, the superior colliculus integrates visual stimuli with motor responses to initiate eye movements. The paramedian pontine reticular formation (PPRF) will initiate a rapid eye movement, or saccade, to bring the eyes to deport on a visual stimulus chop-chop. These areas are continued 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 conjugate gaze, or the motion of the eyes in the aforementioned direction, during horizontal movements that require the lateral and medial rectus muscles. Control of conjugate gaze strictly in the vertical direction is contained inside 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 nerve on either side stimulates the contraction of both junior rectus muscles.

Purely vertical movements of the eyes are not very mutual. Movements are often at an bending, so some horizontal components are necessary, adding the medial and lateral rectus muscles to the motility. The rapid movement of the eyes used to locate and direct the fovea onto visual stimuli is called a saccade. Detect that the paths that are traced in [link] are non strictly vertical. The movements between the olfactory organ and the oral cavity are closest, but still have a slant to them. Too, the superior and inferior rectus muscles are non perfectly oriented with the line of sight. The origin for both muscles is medial to their insertions, then superlative and low may require the lateral rectus muscles to compensate for the slight adduction inherent in the contraction of those muscles, requiring MLF action every bit well.

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 image 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 optics or around the mouth.

Testing eye move is simply a thing of having the patient track the tip of a pen as it is passed through the visual field. This may appear like to testing visual field deficits related to the optic nerve, but the difference is that the patient is asked to not movement the eyes while the examiner moves a stimulus into the peripheral visual field. Here, the extent of movement is the indicate of the examination. The examiner is watching for conjugate movements representing proper function of the related nuclei and the MLF. Failure of one middle to abduct while the other adducts in a horizontal motion is referred to equally internuclear ophthalmoplegia. When this occurs, the patient will feel diplopia, or double vision, as the two optics are temporarily pointed at unlike stimuli. Diplopia is not restricted to failure of the lateral rectus, because any of the extraocular muscles may neglect to movement one eye 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'south face up. As visual stimuli move closer to the face, the two medial recti muscles cause the eyes to move in the one nonconjugate movement that is part of gaze control. When the ii eyes move to look at something closer to the face, they both adduct, which is referred to as convergence. To continue 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 middle is referred to every bit accommodation. Adaptation ability changes with age; focusing on nearer objects, such equally the written text of a book or on a reckoner screen, may crave corrective lenses after in life. Coordination of the skeletal muscles for convergence and coordination of the smooth muscles of the ciliary torso for accommodation are referred to as the adaptation–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 ([link]), both sensory and motor, that make this possible. If the head rotates in ane direction—for example, to the right—the horizontal pair of semicircular canals in the inner ear indicate the movement by increased action on the right and decreased action on the left. The information 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 eyes in the opposite direction of the head, while nuclei controlling the correct lateral rectus and left medial rectus muscles will be inhibited to reduce animosity of the contracting muscles. These actions stabilize the visual field by compensating for the head rotation with opposite rotation of the eyes in the orbits. Deficits in the VOR may be related to vestibular impairment, such as in Ménière'southward disease, or from dorsal brain stem damage that would touch the center movement nuclei or their connections through the MLF.

Vestibulo-ocular Reflex

If the caput is turned in one management, the coordination of that motility with the fixation of the optics on a visual stimulus involves a circuit that ties the vestibular sense with the center motion nuclei through the MLF.

Fretfulness of the Face and Oral Crenel

An iconic part of a doctor's visit is the inspection of the oral crenel 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 crenel into the pharynx known as the fauces. Whereas this portion of a medical exam inspects for signs of infection, such as in tonsillitis, it is also the means to test the functions of the cranial nerves that are associated with the mouth.

The facial and glossopharyngeal nerves convey gustatory stimulation to the brain. Testing this is as simple every bit introducing salty, sour, bitter, or sweet stimuli to either side of the natural language. The patient should reply to the gustatory modality stimulus before retracting the tongue into the mouth. Stimuli applied to specific locations on the tongue will dissolve into the saliva and may stimulate gustatory modality buds connected to either the left or correct of the nerves, masking any lateral deficits. Along with gustation, the glossopharyngeal nerve relays full general sensations from the pharyngeal walls. These sensations, along with certain gustation stimuli, can stimulate the gag reflex. If the examiner moves the natural language 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 nervus 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 crave move of the muscles around the fauces. The patient is asked to say "lah-kah-pah" or a similar ready of alternating sounds while the examiner observes the movements of the soft palate and arches betwixt the palate and tongue.

The facial and glossopharyngeal nerves are besides responsible for the initiation of salivation. Neurons in the salivary nuclei of the medulla project through these 2 nerves equally 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 fretfulness. Other stimuli that stimulate salivation are coordinated through the hypothalamus, such as the smell and sight of food.

The hypoglossal nervus is the motor nerve that controls the muscles of the tongue, except for the palatoglossus muscle, which is controlled by the vagus nerve. At that place 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 natural language are completely contained within the lingual tissues. While examining the oral cavity, movement of the tongue volition indicate whether hypoglossal part is dumb. The exam for hypoglossal function is the "stick out your tongue" role of the examination. The genioglossus muscle is responsible for protrusion of the tongue. If the hypoglossal fretfulness on both sides are working properly, then the tongue will stick straight out. If the nerve on 1 side has a deficit, the tongue will stick out to that side—pointing to the side with harm. Loss of part of the natural language tin interfere with voice communication and swallowing. Additionally, because the location of the hypoglossal nervus and nucleus is nigh the cardiovascular center, inspiratory and expiratory areas for respiration, and the vagus nuclei that regulate digestive functions, a tongue that protrudes incorrectly can advise impairment in adjacent structures that have nothing to exercise with controlling the tongue.

Watch this short video to see an exam of the facial nerve using some simple tests. The facial nerve controls the muscles of facial expression. Severe deficits will be obvious in watching someone use those muscles for normal command. One side of the face might not move like the other side. Only directed tests, especially for wrinkle confronting resistance, require a formal testing of the muscles. The muscles of the upper and lower face demand to exist tested. The strength test in this video involves the patient squeezing her optics shut and the examiner trying to pry her optics open. Why does the examiner ask her to try a 2nd time?

Motor Nerves of the Neck

The accompaniment nerve, also referred to every bit the spinal accessory nerve, innervates the sternocleidomastoid and trapezius muscles ([link]). When both the sternocleidomastoids contract, the head flexes forwards; individually, they cause rotation to the contrary side. The trapezius can deed as an adversary, causing extension and hyperextension of the neck. These 2 superficial muscles are important for changing the position of the head. Both muscles also receive input from cervical spinal nerves. Along 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 disproportion. 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 nerve suggests that it is aiding the spinal nerves. Though that is non precisely how the name originated, it does assist brand the clan between the function of this nerve in controlling these muscles and the office these muscles play in movements of the trunk or shoulders.

Muscles Controlled past the Accessory Nervus

The accessory nervus innervates the sternocleidomastoid and trapezius muscles, both of which adhere to the head and to the trunk and shoulders. They tin can act as antagonists in head flexion and extension, and equally synergists in lateral flexion toward the shoulder.

To test these muscles, the patient is asked to flex and extend the neck or shrug the shoulders confronting resistance, testing the strength of the muscles. Lateral flexion of the neck toward the shoulder tests both at the same time. Any difference on one side versus the other would suggest damage on the weaker side. These strength tests are common for the skeletal muscles controlled by spinal fretfulness and are a significant component of the motor exam. Deficits associated with the accessory nervus may have an upshot on orienting the caput, as described with the VOR.

Homeostatic Imbalances

The Pupillary Light Response
The autonomic control of pupillary size in response to a bright low-cal involves the sensory input of the optic nerve and the parasympathetic motor output of the oculomotor nerve. When light hits the retina, specialized photosensitive ganglion cells send a point forth the optic nerve to the pretectal nucleus in the superior midbrain. A neuron from this nucleus projects to the Eddinger–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 nervus to the ciliary ganglion in the posterior orbit. The postganglionic parasympathetic fibers from the ganglion project to the iris, where they release acetylcholine onto circular fibers that tuck the student to reduce the amount of light hitting the retina. The sympathetic nervous system is responsible for dilating the pupil when light levels are low.

Shining lite in one heart will arm-twist constriction of both pupils. The efferent limb of the pupillary calorie-free reflex is bilateral. Calorie-free shined in one eye causes a constriction of that pupil, equally well equally constriction of the contralateral student. Shining a penlight in the eye of a patient is a very bogus state of affairs, as both optics are normally exposed to the same lite sources. Testing this reflex can illustrate whether the optic nervus or the oculomotor nerve is damaged. If shining the lite in one center results in no changes in pupillary size but shining light in the opposite eye elicits a normal, bilateral response, the damage is associated with the optic nervus on the nonresponsive side. If light in either eye elicits a response in only 1 eye, the problem is with the oculomotor organisation.

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

The Cranial Nerve Examination

The cranial nerves can be separated into four major groups associated with the subtests of the cranial nervus exam. First are the sensory nerves, then the nerves that control heart movement, the nerves of the oral cavity and superior pharynx, and the nerve that controls movements of the neck.

The olfactory, optic, and vestibulocochlear nerves are strictly sensory fretfulness for scent, sight, and residue and hearing, whereas the trigeminal, facial, and glossopharyngeal nerves carry somatosensation of the face, and taste—separated between the anterior two-thirds of the natural language and the posterior one-third. Special senses are tested by presenting the particular stimuli to each receptive organ. General senses can be tested through sensory discrimination of touch versus painful stimuli.

The oculomotor, trochlear, and abducens nerves control the extraocular muscles and are continued by the medial longitudinal fasciculus to coordinate gaze. Testing conjugate gaze is as elementary as having the patient follow a visual target, like a pen tip, through the visual field ending with an arroyo toward the face to test convergence and adaptation. Along with the vestibular functions of the eighth nerve, the vestibulo-ocular reflex stabilizes gaze during head movements by coordinating equilibrium sensations with the eye motion systems.

The trigeminal nerve controls the muscles of chewing, which are tested for stretch reflexes. Motor functions of the facial nerve are ordinarily obvious if facial expressions are compromised, simply can exist tested by having the patient raise their eyebrows, smile, and frown. Movements of the tongue, soft palate, or superior pharynx can exist observed straight while the patient swallows, while the gag reflex is elicited, or while the patient says repetitive consonant sounds. The motor control of the gag reflex is largely controlled by fibers in the vagus nerve and constitutes a test of that nerve because the parasympathetic functions of that nerve are involved in visceral regulation, such as regulating the heartbeat and digestion.

Movement of the head and neck using the sternocleidomastoid and trapezius muscles is controlled by the accessory nerve. Flexing of the neck and strength testing of those muscles reviews the part of that nerve.

Interactive Link Questions

Lookout man this short video to see an examination of the facial nerve using some simple tests. The facial nervus controls the muscles of facial expression. Severe deficits will exist obvious in watching someone use those muscles for normal command. One side of the face might non movement like the other side. Just directed tests, particularly for contraction against resistance, require a formal testing of the muscles. The muscles of the upper and lower face up need to exist tested. The force exam in this video involves the patient squeezing her eyes shut and the examiner trying to pry her optics open. Why does the examiner ask her to try a second time?

She has just demonstrated voluntary control by endmost her eyes, but when he provides the resistance that she needs to hold tight against, she has already relaxed the muscles enough for him to pull them open. She needs to squeeze them tighter to demonstrate the force she has in the orbicular oculi.

Review Questions

Without olfactory sensation to complement gustatory stimuli, food will taste bland unless information technology is seasoned with which substance?

1. salt
2. thyme
3. garlic
4. olive oil

A

Which of the following cranial fretfulness is not part of the VOR?

1. optic
2. oculomotor
3. abducens
4. vestibulocochlear

A

Which nerve is responsible for controlling the muscles that result in the gag reflex?

1. trigeminal
2. facial
3. glossopharyngeal
4. vagus

D

Which nerve is responsible for taste, also as salivation, in the anterior oral cavity?

1. facial
2. glossopharyngeal
3. vagus
4. hypoglossal

A

Which of the following fretfulness controls movements of the neck?

1. oculomotor
2. vestibulocochlear
3. spinal accessory
4. hypoglossal

C

Disquisitional Thinking Questions

Every bit a person ages, their ability to focus on near objects (accommodation) changes. If a person is already myopic (near-sighted), why would corrective lenses not be necessary to read a volume or estimator screen?

If the person already has problems focusing on far objects, and wears corrective lenses to see further objects, then every bit accommodation changes, focusing on a reading surface might yet exist in their naturally nearly-sighted range.

When a patient flexes their neck, the caput tips to the correct side. Also, their tongue sticks out slightly to the left when they try to stick it directly out. Where is the damage to the brain stem almost likely located?

The medulla is where the accessory nervus, which controls the sternocleidomastoid musculus, and the hypoglossal nerve, which controls the genioglossus musculus, are both located. The weakness of the left side of the cervix, and the tendency of the tongue to point to that side, both show that the impairment is on the left side of the brain stem.

Glossary

accommodation

in vision, a change in the ability of the eye to focus on objects at dissimilar distances

accommodation–convergence reflex

coordination of somatic command of the medial rectus muscles of either middle with the parasympathetic control of the ciliary bodies to maintain focus while the eyes converge on visual stimuli well-nigh to the face

conductive hearing

hearing dependent on the conduction of vibrations of the tympanic membrane through the ossicles of the middle ear

conjugate gaze

coordinated motility of the two eyes simultaneously in the same direction

convergence

in vision, the move of the optics so that they are both pointed at the aforementioned point in infinite, which increases for stimuli that are closer to the subject

diplopia

double vision resulting from a failure in conjugate gaze

extrinsic muscles of the tongue

muscles that are continued to other structures, such as the hyoid bone or the mandible, and control the position of the natural language

fauces

opening from the oral cavity into the pharynx

internuclear ophthalmoplegia

arrears of conjugate lateral gaze considering the lateral rectus muscle of one eye does non contract resulting from damage to the abducens nerve or the MLF

intorsion

medial rotation of the heart effectually its axis

intrinsic muscles of the tongue

muscles that originate out of, and insert into, other tissues within the natural language and command the shape of the natural language

jaw-wiggle reflex

stretch reflex of the masseter muscle

medial longitudinal fasciculus (MLF)

fiber pathway that connects structures involved in the control of heart and head position, from the superior colliculus to the vestibular nuclei and cerebellum

paramedian pontine reticular formation (PPRF)

region of the brain stem next to the motor nuclei for gaze control that coordinates rapid, cohabit center movements

Rinne test

use of a tuning fork to test conductive hearing loss versus sensorineural hearing loss

saccade

small, rapid movement of the eyes used to locate and direct the fovea onto visual stimuli

sensorineural hearing

hearing dependent on the transduction and propagation of auditory information through the neural components of the peripheral auditory structures

Snellen nautical chart

standardized organisation of messages in decreasing size presented to a subject at a altitude of xx feet to test visual acuity

vestibulo-ocular reflex (VOR)

reflex based on connections between the vestibular arrangement and the cranial fretfulness of eye movements that ensures that images are stabilized on the retina as the head and body motility

Weber exam

use of a tuning fork to test the laterality of hearing loss by placing it at several locations on the midline of the skull

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Source: http://pressbooks-dev.oer.hawaii.edu/anatomyandphysiology/chapter/the-cranial-nerve-exam/

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