Chapter 11: Special Senses: The Eyes and Ears

Overview of Structures, Combining Forms, and Functions of the Eyes and Ears

• Major Structures:

  • Eyes: receptor organs for the sense of sight

    • Iris: controls the amount of light entering the eye

    • Lens: focuses rays of light on the retina

    • Retina: converts light images into electrical impulses and transmits them to the brain

    • Lacrimal Apparatus: accessory structures that produce, store, and remove tears

  • Ears: receptor organs for the sense of hearing; also helps maintain balance

    • Outer Ear: transmits sound waves to the middle ear

    • Middle Ear: transmits sound waves to the inner ear

    • Inner Ear: receives sound vibrations and transmits them to the brain

Vocabulary Related to the Special Senses

• Word Parts:

  • blephar/o: eyelid

  • -cusis: hearing

  • irid/o: iris, colored part of eye

  • kerat/o: horny, hard, cornea

  • myring/o: tympanic membrane, eardrum

  • ophthalm/o: eye, vision

  • -opia: vision condition

  • opt/o: eye, vision

  • ot/o: ear, hearing

  • phak/o: lens of eye

  • retin/o: retina, net

  • scler/o: sclera, white of eye, hard

  • trop/o: turn, change

  • tympan/o: tympanic membrane, eardrum

• Medical Terms:

  • adnexa: accessory or adjoining anatomical parts of an organ

  • amblyopia: reduced vision in one eye

  • ametropia: refractive error of the eye

  • anisocoria: unequal pupil size

  • astigmatism: irregular curvature of the cornea

  • audiometry: measurement of hearing acuity

  • cataract: clouding of the lens of the eye

  • conjunctivitis: inflammation of the conjunctiva

  • dacryoadenitis: inflammation of the lacrimal gland

  • diplopia: double vision

  • ectropion: outward turning of the eyelid

  • emmetropia: normal vision

  • entropion: inward turning of the eyelid

  • esotropia: inward deviation of the eye

  • exotropia: outward deviation of the eye

  • fluorescein angiography: imaging of blood vessels in the retina

  • glaucoma: increased intraocular pressure

  • hemianopia: loss of vision in half of the visual field

  • hordeolum: stye, infection of the eyelid

  • hyperopia: farsightedness

  • infectious myringitis: infection of the tympanic membrane

  • iridectomy: surgical removal of part of the iris

  • iritis: inflammation of the iris

  • keratitis: inflammation of the cornea

  • labyrinthectomy: surgical removal of the inner ear

  • laser trabeculoplasty: laser treatment for glaucoma

  • mastoidectomy: surgical removal of the mastoid bone

  • mydriasis: dilation of the pupil

  • myopia: nearsightedness

  • myringotomy: surgical incision of the tympanic membrane

  • nyctalopia: night blindness

  • nystagmus: involuntary eye movement

  • ophthalmoscopy: examination of the interior of the eye

  • optometrist: healthcare professional who examines eyes for vision problems

  • otitis media: inflammation of the middle ear

  • otomycosis: fungal infection of the ear

  • otopyorrhea: discharge of pus from the ear

  • otorrhea: discharge from the ear

  • otosclerosis: abnormal growth of bone in the middle ear

  • papilledema: swelling of the optic disc

  • periorbital edema: swelling around the eyes

  • photophobia: sensitivity to light

  • presbycusis: age-related hearing loss

  • presbyopia: age-related loss of near vision

  • ptosis: drooping of the eyelid

  • radial keratotomy: surgical procedure to correct nearsightedness

  • retinopexy: surgical reattachment of the retina

  • scleritis: inflammation of the sclera

  • sensorineural hearing loss: hearing loss due to damage to the inner ear or auditory nerve

  • stapedectomy: surgical removal of the stapes bone

  • strabismus: misalignment of the eyes

  • tarsorrhaphy: surgical closure of the eyelids

  • tinnitus: ringing in the ears

  • tonometry: measurement of intraocular pressure

  • tympanometry: measurement of middle ear function

  • vertigo: dizziness

  • vitrectomy: surgical removal of the vitreous humor

  • xerophthalmia: dryness of the eyes

Learning Goals

• Describe the functions and structures of the eyes and their accessory structures

• Recognize, define, spell, and pronounce the primary terms related to the eyes and vision

• Describe the functions and structures of the ears

• Recognize, define, spell, and pronounce the primary terms related to the ears and hearing

Functions of the Eyes

• The eyes are the receptor organs of sight

• Functions:

  • Receive images and transmit them to the brain

Structures of the Eyes

• Eyeball and adnexa

• Adnexa of the Eyes:

  • Structures outside the eyeball

  • Include the orbit, eye muscles, eyelids, eyelashes, conjunctiva, and lacrimal apparatus

• Adnexa means the accessory or adjoining anatomical parts of an organ

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The Orbit

• The orbit is the bony cavity of the skull that contains and protects the eyeball and its associated muscles, blood vessels, and nerves.

Muscles of the Eye

• There are six major eye muscles attached to each eye, arranged in three pairs:

  • Superior and inferior oblique muscles

  • Superior and inferior rectus muscles

  • Lateral and medial rectus muscles

• These muscles allow for a wide range of precise eye movements.

Binocular Vision

• Binocular vision occurs when the muscles of both eyes work together to provide normal depth perception.

• Depth perception is the ability to see things in three dimensions.

The Eyelids, Eyebrows, and Eyelashes

• The upper and lower eyelids, eyebrows, and eyelashes help protect the eyeball from foreign matter, excessive light, and injuries.

• The canthus is the angle where the upper and lower eyelids meet.

• The edges of the eyelids contain oil-producing sebaceous glands.

• The eyebrows and eyelashes are made up of small hairs called cilia.

• The tarsus, or tarsal plate, provides stiffness and shape to the eyelids.

The Conjunctiva

• The conjunctiva is the transparent mucous membrane that lines the underside of each eyelid and covers the surface of the eyeball.

The Lacrimal Apparatus

• The lacrimal apparatus consists of the structures that produce, store, and remove tears.

• The lacrimal glands secrete tears to maintain moisture on the surface of the eyeball.

• Blinking helps distribute the tears across the eye.

• The lacrimal canal collects tears and empties them into the lacrimal sacs.

• The lacrimal sac is an enlargement of the upper portion of the lacrimal duct.

• The lacrimal duct drains excess tears into the nose.

The Eyeball

• The eyeball is a 1-inch sphere with only about one-sixth of its surface visible.

• Optic and ocular mean pertaining to the eye or sight.

• Extraocular means outside the eyeball, while intraocular means within the eyeball.

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Walls of the Eyeball

• The walls of the eyeball are made up of three layers: the sclera, choroid, and retina.

• The sclera, also known as the white of the eye, maintains the shape of the eye and protects the inner layers of tissue.

• The choroid is the opaque middle layer of the eyeball that contains blood vessels and provides the blood supply for the eye.

• The retina is the sensitive innermost layer that receives nerve impulses and transmits them to the brain via the optic nerve.

Segments of the Eyeball

• The interior of the eyeball is divided into the anterior and posterior segments.

Anterior Segment of the Eye

• The anterior segment makes up the front one-third of the eyeball and is divided into anterior and posterior chambers.

• The anterior chamber is located behind the cornea and in front of the iris.

• The posterior chamber is located behind the iris and in front of the ligaments holding the lens in place.

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• Aqueous humor fills the chambers of the eye

  • Aqueous humor helps maintain eye shape and nourishes intraocular structures

  • Aqueous humor is filtered and drained through trabecular meshwork and canal of Schlemm

• Intraocular pressure (IOP) is a measurement of fluid pressure inside the eye

  • IOP is regulated by the rate at which aqueous humor enters and leaves the eye

• Posterior segment of the eye is lined with the retina and filled with vitreous humor

  • Vitreous humor is a soft, clear, jelly-like mass that helps maintain eye shape

• Retina contains rods and cones that receive images and transmit nerve impulses to the brain

  • Rods are black and white receptors, cones are color receptors

• Macula is the light-sensitive area in the center of the retina responsible for sharp central vision

  • Fovea centralis is a pit in the middle of the macula with a high concentration of cones and no rods

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• Optic disk is the blind spot where the nerve endings of the retina enter the optic nerve

• Optic nerve transmits nerve impulses from the retina to the brain

• Uvea is the pigmented layer of the eye consisting of the choroid, ciliary body, and iris

• Ciliary body adjusts the thickness of the lens to focus light rays on the retina

• Iris controls the amount of light entering the eye through the pupil

• Cornea is the transparent outer surface of the eye that focuses light rays

• Pupil is the circular opening in the center of the iris that permits light to enter the eye

• Lens is the clear, flexible structure that focuses images on the retina

• Accommodation is the process of adjusting the eyes for seeing objects at various distances

• Convergence is the inward movement of the eyes toward each other to maintain binocular vision

• Emmetropia is the normal relationship between the refractive power of the eye and the shape of the eye

• Refraction is the ability of the lens to bend light rays to focus on the retina

• Visual acuity is the ability to distinguish object details and shape at a distance

Medical Specialties Related to the Eyes:

• Ophthalmologist specializes in diagnosing and treating diseases and disorders of the eyes

• Optometrist provides primary eye care and measures vision accuracy to determine if corrective lenses are needed

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Optician

• Designs, fits, and dispenses lenses for vision correction

Pathology of the Eyes and Vision

Eyelids

• Ptosis: drooping of the upper eyelid, usually due to paralysis

• Chalazion: nodule or cyst on the upper eyelid caused by obstruction in a sebaceous gland

• Ectropion: eversion of the edge of an eyelid, usually affects the lower lid

• Entropion: inversion of the edge of an eyelid, usually affects the lower eyelid

• Hordeolum: pus-filled and often painful lesion on the eyelid resulting from an acute infection in a sebaceous gland

• Periorbital edema: swelling of the tissues surrounding the eye or eyes

Additional Adnexa Pathology

• Conjunctivitis: inflammation of the conjunctiva usually caused by an infection or allergy

• Dacryoadenitis: inflammation of the lacrimal gland caused by a bacterial, viral, or fungal infection

• Subconjunctival hemorrhage: bleeding between the conjunctiva and the sclera

• Xerophthalmia: drying of eye surfaces, including the conjunctiva

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Uvea, Cornea, Iris, and Sclera

• Uveitis: inflammation of the uvea causing swelling and irritation

• Iritis: inflammation of the uvea affecting primarily structures in the front of the eye

• Corneal abrasion: injury, such as a scratch or irritation, to the outer layers of the cornea

• Corneal ulcer: pitting of the cornea caused by an infection or injury

• Diabetic retinopathy: damage to the retina as a complication of uncontrolled diabetes

• Keratitis: inflammation of the cornea

• Scleritis: inflammation of the sclera

The Eye

• Anisocoria: condition in which the pupils are unequal in size

• Cataract: loss of transparency of the lens causing a progressive loss of visual clarity

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• Floaters: particles of cellular debris that float in the vitreous humor and cast shadows on the retina

  • Occur normally with aging or in association with retinal detachment, retinal tears, or intraocular inflammation

• Photopsia: presence of flashes of light

  • Often caused by damage to the eye or migraine headaches

• Miosis: contraction of the pupil

  • Normally in response to exposure to light, but also possibly due to the use of prescription or illegal drugs

• Mydriasis: dilation of the pupil

  • Causes include diseases, trauma, or drugs

• Nystagmus: involuntary, constant, rhythmic movement of the eyeball

  • Can be congenital or caused by a neurological injury or drug use

• Papilledema: swelling and inflammation of the optic nerve at the point of entrance into the eye through the optic disk

  • Caused by increased intracranial pressure, can be due to a tumor pressing on the optic nerve

• Retinal detachment and retinal tears: separation of the light-sensitive retina from the choroid

  • Can cause blindness if not treated

  • Can be caused by head trauma, aging, or separation of the vitreous humor from the retina

• Retinitis pigmentosa: progressive degeneration of the retina that affects night and peripheral vision

  • Detected by the presence of dark pigmented spots in the retina

• Glaucoma: group of diseases characterized by increased intraocular pressure

  • Causes damage to the retinal nerve fibers and the optic nerve

  • Open-angle glaucoma: most common form, gradual blockage of the trabecular meshwork

  • Closed-angle glaucoma: sudden narrowing of the opening between the cornea and iris, can cause rapid increase in intraocular pressure

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• Macular degeneration: progressive condition damaging the macula at the center of the retina

  • Results in loss of central vision, but not total blindness

  • Age-related macular degeneration: most common in older people, leading cause of legal blindness in those older than age 60

  • Dry macular degeneration: slow deterioration of the cells of the macula

  • Wet macular degeneration: damage caused by formation of new blood vessels, resulting in rapid and severe vision loss

• Diplopia: perception of two images of a single object

  • Can be a symptom of a serious underlying disorder

• Hemianopia: blindness in one-half of the visual field

• Monochromatism: inability to distinguish certain colors in a normal manner

  • Genetic condition caused by deficiencies in or absence of certain types of cones in the retina

• Nyctalopia: difficulty seeing at night despite normal daytime vision

• Photophobia: excessive sensitivity to light

• Presbyopia: common changes in the eyes that occur with aging

  • Near vision declines as the lens becomes less flexible and the muscles of the ciliary body become weaker

• Strabismus: disorder in which the eyes point in different directions or are not aligned correctly

  • Esotropia: inward deviation of one or both eyes

  • Exotropia: outward deviation of one eye relative to the other

• Refractive Disorders: focusing problem due to the lens and cornea not bending light properly on the retina

  • Myopia (nearsightedness): light rays focus in front of the retina

  • Hyperopia (farsightedness): light rays focus beyond the retina

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• Ametropia is any error of refraction in which images do not focus properly on the retina

  • Astigmatism, hyperopia, and myopia are all forms of ametropia

• Astigmatism is a condition in which the eye does not focus properly because of uneven curvatures of the cornea

• Hyperopia, also known as far-sightedness, is a defect in which light rays focus beyond the retina

  • This condition can occur in childhood, but usually causes difficulty after age 40

• Myopia, also known as nearsightedness, is a defect in which light rays focus in front of the retina

  • This condition occurs most commonly around puberty

• Amblyopia is a dimness of vision or the partial loss of sight, especially in one eye, without detectable disease of the eye

• Scotoma, also known as blind spot, is an abnormal area of diminished vision surrounded by an area of normal vision

DIAGNOSTIC PROCEDURES FOR VISION AND THE EYES

• Snellen chart is used to measure visual acuity

  • Results are recorded as a fraction with 20/20 being considered normal

• Refraction is an examination procedure to determine an eye's refractive error

  • Best corrective lenses can be prescribed based on the results

• Cover test is an examination of how the two eyes work together and is used to assess binocular vision

• Visual field testing, also known as perimetry, is performed to determine losses in peripheral vision

  • Blank sections in the visual field can be symptomatic of glaucoma or an optic nerve disorder

• Ophthalmoscopy, also known as funduscopy, is the use of an ophthalmoscope to visually examine the back part of the eye

  • This examination includes the retina, optic disk, choroid, and blood vessels

• Dilation of the eyes is required in preparation for the ophthalmoscopic examination of the interior of the eye

  • Mydriatic drops are used to achieve artificial enlargement of the pupils

• Slit-lamp ophthalmoscopy is a diagnostic procedure in which a narrow beam of light is focused onto parts of the eye to permit examination of the structures at the front of the eye

  • Fluorescein staining is often used to help detect foreign bodies or an infected or injured area of the eye

• Fluorescein angiography is a radiographic study of the blood vessels in the retina of the eye following the intravenous injection of a fluorescein dye as a contrast medium

  • Angiograms are used to determine whether there is proper circulation in the retinal vessels

• Tonometry is the measurement of intraocular pressure

  • Abnormally high pressure can be an indication of glaucoma

TREATMENT PROCEDURES OF THE EYES AND VISION

• Orbitotomy is a surgical incision into the orbit

  • This procedure is performed for biopsy, abscess drainage, or removal of a tumor or foreign object

• Tarsorrhaphy is the partial or complete suturing together of the upper and lower eyelids to protect the eye when the lids are paralyzed and unable to close normally

• Corneal transplant, also known as keratoplasty, is the surgical replacement of a scarred or diseased cornea with clear corneal tissue from a donor

• Enucleation is the removal of the eyeball, leaving the eye muscles intact

• Ocular prosthesis, also known as an artificial eye, may be fitted to wear over a malformed eye or to replace an eyeball that is either congenitally missing or has been surgically removed

• Iridectomy is the surgical removal of a portion of the tissue of the iris

  • This procedure is most frequently performed to treat closed-angle glaucoma

• Radial keratotomy is a surgical procedure to treat myopia

  • Incisions are made in the cornea to cause it to flatten, improving distance vision

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• Scleral buckle is a silicone band or sponge used to repair a detached retina

  • Detached layers are brought closer together by attaching the band onto the sclera of the eyeball

  • Creates an indentation or buckle effect inside the eye

• Vitrectomy is the removal of the vitreous humor and its replacement with a clear solution

  • Used to treat retinal detachment or when diabetic retinopathy causes blood to leak and cloud the vitreous humor

• Lensectomy is the surgical removal of a cataract-clouded lens

• Phacoemulsification is the use of ultrasonic vibration to shatter and remove the lens clouded by a cataract

  • Intraocular lens is surgically implanted as a replacement for the natural lens

Corrective Lenses

• Refractive errors in the eye can be corrected with lenses

• Concave lenses are used for myopia (nearsightedness)

• Convex lenses are used for hyperopia (farsightedness)

• Corrective lenses can combine different refractive powers for better distance and near vision

• Bifocals have two powers, trifocals have three powers

• Strabismus can be treated with corrective lenses or an eye patch

• Contact lenses float directly on the tear film in front of the eye

  • Rigid gas-permeable lenses cover the central part of the cornea

  • Disposable soft lenses cover the entire cornea

Laser Treatments of the Eyes

• Laser iridotomy uses a focused beam of light to create a hole in the iris of the eye

  • Used to treat closed-angle glaucoma

• Laser trabeculoplasty is used to treat open-angle glaucoma by creating openings in the trabecular meshwork

• LASIK is used to treat vision conditions caused by the shape of the cornea

  • A flap is opened in the surface of the cornea and a laser is used to change the shape of a deep corneal layer

• Photocoagulation is the use of a laser to treat wet macular degeneration and repair small retinal tears

• Retinopexy is used to reattach a detached area in a retinal detachment

• Pneumatic retinopexy involves injecting a gas bubble into the vitreous cavity to put pressure on the area of repair

• Lasers are used to remove clouded tissue in the posterior portion of the lens capsule after cataract extraction

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• The ears are the receptor organs of hearing and help maintain balance

• Auditory means pertaining to the sense of hearing

• Acoustic means pertaining to sound or hearing

Structures of the Ears

• The ear is divided into three regions: outer ear, middle ear, and inner ear

• The pinna, or auricle, captures sound waves and transmits them into the external auditory canal

• Cerumen, or earwax, is secreted by ceruminous glands in the auditory canal

• The tympanic membrane, or eardrum, transmits sound waves by vibrating

• The mastoid process is the temporal bone containing hollow air space that surrounds the middle ear

• The auditory ossicles are three small bones located within the middle ear

  • Malleus, incus, and stapes

• The role of the auditory ossicles is to transmit sound waves from the eardrum to the inner ear by vibration

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• The Eustachian Tubes

  • Narrow tubes that lead from the middle ear to the nasal cavity and throat

  • Purpose is to equalize air pressure within the middle ear with that of the outside atmosphere

• The Inner Ear

  • Contains sensory receptors for hearing and balance

  • Structures include the oval window, cochlea, organ of Corti, semicircular canals, and acoustic nerves

• Normal Action of the Ears

  • Air conduction: sound waves enter the ear through the pinna and travel down the external auditory canal to the tympanic membrane

  • Bone conduction: eardrum vibrates and causes the auditory ossicles to vibrate, transmitting sound waves to the inner ear

  • Sensorineural conduction: sound vibrations reach the inner ear, where they are received and relayed to the auditory nerve for transmission to the brain

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• Pathology of the Ears and Hearing

  • The Outer Ear

    • Impacted cerumen: accumulation of earwax that forms a solid mass in the external auditory canal

    • Otalgia: pain in the ear

    • Otitis: inflammation of the ear

    • Otomycosis: fungal infection of the external auditory canal

    • Otopyorrhea: flow of pus from the ear

    • Otorrhea: any discharge from the ear

    • Otorrhagia: bleeding from the ear

  • The Middle Ear

    • Barotrauma: pressure-related ear condition caused by pressure changes or blocked eustachian tube

    • Cholesteatoma: destructive epidermal cyst in the middle ear and/or mastoid process

    • Mastoiditis: inflammation of the mastoid bone cells

    • Infectious myringitis: contagious inflammation causing painful blisters on the eardrum

    • Otitis media: inflammation of the middle ear

      • Acute otitis media: associated with upper respiratory infection, common in young children, can lead to ruptured eardrum

      • Serous otitis media: fluid buildup in the middle ear without infection symptoms

    • Otosclerosis: ankylosis of the bones of the middle ear, resulting in conductive hearing loss

  • The Inner Ear

    • Labyrinthitis: inflammation of the labyrinth, can cause vertigo and deafness

    • Vertigo: sense of whirling, dizziness, and loss of balance

    • Ménière’s disease: chronic disorder with intermittent increase of fluid in the inner ear, producing attacks of vertigo, fluctuating hearing loss, and tinnitus

    • Tinnitus: condition of ringing, buzzing, or roaring sound in one or both ears, often associated with hearing loss and prolonged exposure to loud noises

Page 18: Hearing Loss

• Acoustic neuroma is a brain tumor that develops near the cranial nerve running from the brain to the inner ear

  • Can cause hearing loss, vertigo, and tinnitus

• Deafness is the complete or partial loss of the ability to hear

• Presbycusis is a gradual loss of sensorineural hearing that occurs as the body ages

• Conductive hearing loss occurs when sound waves are prevented from passing from the air to the fluid-filled inner ear

  • Causes include earwax buildup, infection, fluid in the middle ear, punctured eardrum, otosclerosis, and scarring

• Sensorineural hearing loss, also known as nerve deafness, develops when the auditory nerve or hair cells in the inner ear are damaged

  • Usually due to age, noise exposure, or an acoustic neuroma

• Noise-induced hearing loss is caused by repeated exposure to loud noises

  • Can permanently damage hair cells in the cochlea

  • Any sound above 85 decibels can cause hearing loss if the exposure is prolonged

Page 18: Diagnostic Procedures of the Ears and Hearing

• Audiological evaluation measures the ability to hear and understand speech sounds

  • Best achieved in a sound-treated room with earphones

• Audiometry uses an audiometer to measure hearing acuity

  • Audiometer produces acoustic stimuli of a set frequency and intensity

• Sound is measured in hertz and decibels

• Tympanometry uses air pressure in the ear canal to test for disorders of the middle ear

• Weber and Rinne tests use a tuning fork to distinguish between conductive and sensorineural hearing losses

Page 19: Treatment Procedures of the Ears and Hearing

• Otoplasty is the surgical repair, restoration, or alteration of the pinna of the ear

• Ear tubes (tympanostomy tubes) are placed through the eardrum to provide drainage for fluids and relieve pressure

• Mastoidectomy is the surgical removal of mastoid cells, used to treat mastoiditis or in preparation for a cochlear implant

• Myringotomy is a surgical incision in the eardrum to relieve pressure or create an opening for ear tubes

• Stapedectomy is the surgical removal of the top portion of the stapes bone and the insertion of a prosthetic device

• Tympanoplasty is the surgical correction of a damaged middle ear

• Labyrinthectomy is the surgical removal of all or a portion of the labyrinth, performed to relieve vertigo but causes complete hearing loss

• Vestibular rehabilitation therapy is a form of physical therapy to treat balance disorders caused by problems in the inner ear and vestibular nerve

Page 20: Treatments for Hearing Loss

• Assistive listening devices transmit, process, or amplify sound and can be used with or without a hearing aid

• Cochlear implant bypasses damaged portions of the ear and stimulates the auditory nerve

• Fenestration is a surgical procedure to create a new opening in the labyrinth to restore lost hearing

Page 21: Hearing Aids

• Hearing aids are electronic devices worn to correct a hearing loss.

• Sensorineural hearing loss can sometimes be corrected with a hearing aid.

• Analog hearing aids use a microphone to detect and amplify sounds.

• Digital hearing aids use a computer chip to convert incoming sound into a code that can be filtered before being amplified.

• Digital hearing aids are designed to best compensate for a specific type of hearing loss.

Abbreviations Related to the Special Senses (Table 11.3)

• Table 11.3 presents an overview of abbreviations related to the terms introduced in this chapter.

• It is important to be cautious when using abbreviations to avoid errors or confusion.

Cochlear Implant (Figure 11.17)

• An external speech processor captures sound and converts it to digital signals.

• The processor sends digital signals to an internal implant.

• The internal implant turns signals into electrical energy and sends it to a receptor inside the cochlea.

• Electrodes stimulate the auditory nerve