Auditory System

Physiology of hearing

1 The auricle directs sound waves into the external auditory canal. 2 When sound waves strike the tympanic membrane, the alternating waves of high and low pressure in the air cause the tympanic membrane to vibrate back and forth. The tympanic membrane vibrates slowly in response to low-frequency (low-pitched) sounds and rapidly in response to high frequency (high-pitched) sounds. 3 The central area of the tympanic membrane connects to the malleus, which vibrates along with the tympanic membrane. This vibration is transmitted from the malleus to the incus and then to the stapes. 4 As the stapes moves back and forth, its oval-shaped footplate, which is attached via a ligament to the circumference of the oval window, vibrates in the oval window. The vibrations at the oval window are about 20 times more vigorous than the tympanic membrane because the auditory ossicles efficiently transmit small vibrations spread over a large surface area (the tympanic membrane) into larger vibrations at a smaller surface (the oval window). 5 The movement of the stapes at the oval window sets up fluid pressure waves in the perilymph of the cochlea. As the oval window bulges inward, it pushes on the perilymph of the scala vestibuli. 6 Pressure waves are transmitted from the scala vestibuli to the scala tympani and eventually to the round window, causing it to bulge outward into the middle ear. 7 The pressure waves travel through the perilymph of the scala vestibuli, then the vestibular membrane, and then move into the endolymph inside the cochlear duct. 8 The pressure waves in the endolymph cause the basilar membrane to vibrate, which moves the hair cells of the spiral organ against the tectorial membrane. This leads to bending of the stereocilia and ultimately to the generation of nerve impulses in first-order neurons in cochlear nerve fibers. 9 Sound waves of various frequencies cause certain regions of the basilar membrane to vibrate more intensely than other regions. Each segment of the basilar membrane is "tuned" for a particular pitch.

Hearing threshold

140 dB pain and damage to ear. 120 dB uncomfortable to ear. 80 dB shouting. 60 dB normal conversation. 20 dB whispering. 0 dB hearing threshold.

Eustachian tube dysfunction

Alter pressure on either side of TM→ stretch TM → pain

Impedance matching

Amplification process helps to compensate the loss.Two processes are involved in the impedance matching mechanism of middle ear. They are: 1. The area of the tympanic membrane is larger than that of the stapes foot plate in the cochlea. The forces collected over the ear drum are concentrated over a smaller area, thus increasing the pressure over oval window. The pressure is increased by the ratio of these two areas i.e. 18.75 times. 2. The second process is the lever action of the middle ear bones. The arm of the incus is shorter than that of the malleus, and this produces a lever action that increases the force and decreases the velocity at the stapes. Since the malleus is 2.1 times longer than the incus, the lever action multiplies the force by 2.1 times.

Middle ear

Auditory ossicles:Transmit and amplify vibrations from tympanic membrane to oval window. Auditory (eustachian) tube:Equalizes air pressure on both sides of tympanic membrane.

External Ear

Auricle (pinna):Collects sound waves. External auditory canal (meatus):Directs sound waves to eardrum. Tympanic membrane (eardrum):Sound waves cause it to vibrate, which in turn causes malleus to vibrate.

Conduction deafness

Caused by interruption of the passage of sound waves through the external or middle ear. Includes the following causes: Obstruction by wax (cerumen) or a foreign body in the external auditory meatus Otosclerosis: Produced by neogenesis of the labyrinthine spongy bone around the oval window, resulting in fixation of the stapes. The most frequent cause of progressive conduction deafness. Otitis media: An inflammation of the middle ear. The most common cause of meningitis (excluding meningococcus) and the most common cause of brain abscesses.

Internal Ear

Cochlea: Contains a series of fluids, channels, and membranes that transmit vibrations to spiral organ (organ of Corti), the organ of hearing; hair cells in spiral organ produce receptor potentials, which elicit nerve impulses in cochlear branch of vestibulocochlear (VIII) nerve.

Vestibular nystagmus

Consists of involuntary to-and-fro, up-and-down, or rotary movements of one or both eyes. It consists of a slow component, opposite the direction of rotation, and a fast compensatory component, in the direction of rotation. It is named after the fast component. It results from the stimulation of hair cells within the semicircular ducts on rotation.

Tympanic reflex

Contraction of the tensor tympani and stapedius muscles of the middle ear cause the manubrium of the malleus to be pulled inward and the footplate of the stapes to be pulled outward. This decreases sound transmission. Loud sounds initiate a reflex contraction of these muscles called the Tympanic reflex.

Oculocephalic reflex

Doll's head eye movements: normally suppressed vestibulo-ocular reflex. Test method : consists of rapid movement of the head in horizontal or vertical planes. Test results: With intact proprioception and brainstem (vestibular nuclei), the eyes move conjugately in the opposite direction. Doll's head eye movements are absent or abnormal when lesions of the vestibular nuclei and MLFs are present.

Presbycusis

Hearing loss occurring with aging. It results from degenerative disease of the organ of corti in the first few millimeters of the basal coil of the cochlea (high-frequency loss of 4000-8000 hz). The most common cause of hearing loss.

Vertigo

In this condition, the person has a sensation of turning or rotation in space in the absence of actual rotation. Usually, debris from the otolithic membrane, located in the saccule and utricle, accumulates at the ampulla of the posterior semicircular canal and adheres to the cupula, making it more sensitive to angular movement. Vertigo is often accompanied by nausea, vomiting, and gait ataxia. It can be caused by peripheral vestibular lesions that affect the labyrinth of the inner ear or the vestibular division of CN VIII. It can also be caused by central lesions that affect the brainstem vestibular nuclei or their connections.

Gaze Palsy

Inability to move both eyes in the same direction Involves both eyes. Lesion in cranial nerve six nucleus or P PRF would cause ipsilateral gaze palsy

Cold Water Caloric Nystagmus

Irrigation of the left ear with cold water causes a response by the same mechanism in the opposite direction (Fig. B). Cold water cools the endolymph in the most lateral portion of the horizontal canal; the endolymph falls, deflecting the cupula laterally, bending the hair cells laterally, and decreasing their firing rate. Decreased activity of hair cells in the left horizontal semicircular canal results in decreased activity of neurons in left vestibular nuclei(Fig. B; pathways shown in blue), relative to the right vestibular nuclei (Fig. B; pathways shown in red). The activity of neurons in the left (ipsilateral) oculomotor nucleus and right (contralateral) abducens nucleus is decreased. Relatively, there is greater activity of neurons in the right oculomotor nucleus, which results in contraction of the medial rectus muscle of the right eye. Relative increased activity of the left abducens nucleus results in contraction of the lateral rectus muscle of the left eye, causing slow deviation of both eyes to the left side. Thus, irrigation of the left ear with cold water creates a sensation of rotation of the head to the right, causing a slow deviation of the eyes to the left

Motion Sickness

It consists of nausea, weakness, and other dysfunctions caused by stimulation of the semicircular canals during motion, such as in a boat, automobile, airplane, swing, or amusement park ride. It may progress to vomiting. Antiemetics such as anticholinergic or antihistamine medications can be taken to counter the nausea and vomiting associated with motion sickness. Scopolamine is an anticholinergic drug that reduces the excitability of vestibular receptors. Cyclizine (Marezine), dimenhydrinate (Dramamine), and diphenhydramine (Benadryl) are antihistamines that affect the neural pathways from the vestibule.

Tinnitus

It consists of noises such as ringing, clicking, whistling, or booming in the ears. These noises may occur as a result of disorders in the middle or inner ear or along the central neuronal pathways.

Meniere disease

It is an inner ear disease associated with an increase in endolymphatic fluid pressure. Characterized by episodic attacks of vertigo, tinnitus, hearing loss, nausea, vomiting, and a sensation of fullness and pressure in the ear. Also characterized by the presence of horizontal nystagmus during the attack. The fast phase is to the opposite ear; past-pointing and falling occur to the affected side.

Caloric nystagmus

It is induced with cold- or hot-water irrigation of the external auditory meatus. It is used to stimulate each labyrinth separately. used to evaluate unconscious patients. Used to stimulate individual semicircular canals. Test method (used to stimulate the horizontal semicircular canal) While sitting erect, the subject tilts the head back 60°, or the recumbent subject elevates the head 30° from a horizontal position. Cold or hot water is syringed into the external auditory meatus. Test results (Mnemonic COWS Cold, Opposite; Warm, Same) Cold-water irrigation results in nystagmus to the opposite side and past-pointing and falling to the same side. Hot-water irrigation results in the reverse reactions.

Perilymph

It resembles extracellular fluid, plasma, and cerebrospinal fluid and surrounds the membranous labyrinth (in the perilymphatic space). It communicates with the subarachnoid space via the cochlear aqueduct.

Endolymph

It resembles intracellular fluid and is found within the membranous labyrinth (endolymphatic space). It is secreted by the stria vascularis of the cochlear duct.

Regulation of posture

Neural Pathways involved in maintenance of equilibrium & balance. Stimulation of hair cells in vestibular apparatus activates sensory neurons of VIII. Sensory fibers transmit impulses to cerebellum and vestibular nuclei of medulla. Sends fibers to oculomotor center. Neurons in oculomotor center control eye movements Neurons in spinal cord stimulate movements of head, neck, and limbs

Caloric nystagmus in Comatose subjects

No nystagmus is seen. With the brainstem intact, the eyes deviate to the side of cold irrigation. With bilateral MLF transection, the abducting eye deviates to the side of cold irrigation. With lower brainstem damage to vestibular nuclei, the eyes do not deviate.

Bilateral MLF Lesion

Normally, Irrigation of the right ear with cold water creates a sensation of rotation of the head to the left, causing a slow deviation of the eyes to the right. With bilateral MLF transection, the abducting eye deviates to the side of cold irrigation.

Movement of fluid in cochlea

Over all stiffness of basilar membrane is 100 times less at helicotrema than near oval window.

Vestibulo-ocular reflex

Reflex movement of the eyes to compensate for head movement to keep objects of interest on the center of the retina. The VOR involves a 3-neuron arc, consisting of the oculomotor nuclei, vestibular nuclei, and vestibular ganglion. The 3-neuron arc, as the simplest VOR, consists of the following: Primary Afferent Fibers: from the cristae of the semicircular ducts. Vestibular Nuclei: neurons send their axons to the nuclei of the extraocular muscles (passing in the medial longitudinal fasciculus). Motor Neurons: send their axons to the extraocular muscles. Abducens nuclei which innervates the lateral rectus muscle is responsible for rotating the eyes laterally. Oculomotor nuclei which is primarily responsible for controlling medial rectus (causing eyes to pull in) Can be tested in conscious or unconscious subjects by stimulating the kinetic labyrinth. Afferent limb is CN VIII. Efferent limb is CN III, VI.

Acoustic neuroma

Schwannoma or neurilemoma ● Consists of a peripheral nerve tumor of the vestibulocochlear nerve (CN VIII). ● Located in the internal auditory meatus or in the cerebellopontine angle of the posterior cranial fossa. ● Includes symptoms such as unilateral deafness and tinnitus(ear ringing)

Nerve deafness

Sensorineural or perceptive deafness Disease of the cochlea, cochlear nerve, or central auditory pathway (acoustic neuroma). ● Can result from the action of drugs and toxins(e.g., Quinine, aspirin, streptomycin). ● Can be the result from prolonged exposure to loud noise. ● Can result from rubella infection in utero, cytomegalovirus, or syphilis. ● Include the following:

Human hearing

Sound wave travel in air at speed of 340m/s @200 C at sea level. Increase with increase in temp. and altitude. Audible range is 20 to 20,000 Hz Amplitude of sound wave determines the loudness or intensity. Hearing threshold varies with frequency. Intensity measured in decibel(dB) Pitch- subjective sensation produced by the frequency of sound. M-120 Hz , F- 250 Hz

Postrotational nystagmus

Test method :The subject is rotated several times in the same direction and then is suddenly stopped. Test results : The subject with normal labyrinths will have a horizontal nystagmus opposite the direction of rotation (fast phase). The subject will past-point and tend to fall in the direction of rotation, and experience a sensation of turning (vertigo) to the opposite side.

Auditory Vs Vestibular Pathway

The auditory system is exteroceptive and concerned with perception of sound. The vestibular system, in contrast, is proprioceptive and concerned with the maintenance of equilibrium and orientation of the body in space and, hence, involved in motor activities. The receptors (mechanoreceptors) are hair cells located within specialized neuroepithelial structures. They are responsible for converting mechanical energy in the form of displacement of their surface elements caused by sound waves (for hearing) and head movements (for balance) into electrochemical energy to be transmitted to the auditory (cochlear) or vestibular root of the vestibulocochlear nerve (cranial n.VIII), respectively. The hair cells are located within the membranous labyrinth of the inner ear, which is a closed tubular system filled with endolymph. The auditory hair cells are in the spiral organ of Corti in the cochlea.

Rinne test

Tuning Fork test ● Compares air and bone conduction. ● Performed by placing a vibrating tuning fork on the mastoid process until it is no longer heard; Then it is held in front of the ear. ● Normal subject hears vibration in the air after bone conduction is gone. ● Patient with unilateral conduction deafness fails to hear vibrations in the air after bone conduction is gone. ● Patient with unilateral partial nerve deafness hears vibrations in the air after bone conduction is gone.

Schwabach test

Tuning Fork test ● Compares bone conduction of a patient with that of a person with normal hearing. ● Demonstrates bone conduction to be better than normal in cases of conduction deafness. ● Demonstrates bone conduction to be less than normal in cases of nerve deafness.

Genesis of AP in afferent vestibular nerve

The otolith organs include two chambers within the labyrinth, the utricle and the saccule. Hair cells are located in a sensory epithelium called the macula. The tips of hair cell stereocilia project into a gelatinous cap, which is covered in small calcium carbonate crystals called otoliths. Movement of the head displaces otoliths and bends the stereocilia, resulting in development of a receptor potential in the same way as that described for auditory hair cells. The otolith organs transduce two types of information, the static angle (tilting) of the head and the presence of linear acceleration. 1. Tilting of the head changes the angle between the otolith organs and the direction of the force of gravity. Different degrees of tension are placed on hair cell stereocilia, depending on their orientation. All possible angles are represented because the macula of each utricle is oriented horizontally and the macula of each saccule is oriented vertically. 2. Linear acceleration(e.g., starting and stopping when riding in a vehicle) also displaces the otoliths and excites hair cells in the maculae. (Note: when traveling at a constant velocity, there is no acceleration, resulting in the sensation of being perfectly still.)

Vestibular System

The sensory system that responds to gravity and keeps people informed of their body's location in space.

Mechanism of sound conduction

The tips of the stereocilia on the hair cells are embedded in the tectorial membrane, and the bodies of hair cells rest on the basilar membrane. An upward displacement of the basilar membrane creates a shearing force that results in lateral displacement of the stereocilia. Mechanical displacement of the stereocilia and kinocilium in a lateral direction causes depolarization of the hair cell. A downward displacement of the basilar membrane creates a shearing force that results in hyperpolarization of the hair cell.

Semicircular ducts

These sensory organs respond to angular acceleration. The ampulla is a localized dilatation at one end of the semicircular duct. A patch of innervated hair cells is found at the base of the ampulla in a structure termed a crista (meaning crest). The crista contains hair cells with stereocilia oriented in a consistent direction. The cupula, a thin vane, sits atop this crest, filling the lumen of the semicircular duct. The stereocilia of the hair cells are embedded in the gelatinous cupula.

Otolithic organs, the saccule and utricle

These two similar organs lie against the walls of the inner ear between the semicircular ducts and the cochlea. The receptors, called maculae (meaning "spot"), are patches of hair cells topped by small, calcium carbonate crystals called otoconia. The saccule and utricle lie at 90 degrees to each other. Thus, with any position of the head, gravity will bend the cilia of one patch of hair cells, due to the weight of the otoconia to which they are attached by a gelatinous layer. This bending of the cilia produces afferent activity going through the VIIIth nerve to the brainstem. The utricle is most sensitive to tilt when the head is upright. The saccule is most sensitive to tilt when the head is horizontal. Unlike the semicircular ducts, the kinocilia of hair cells in the maculae are NOT oriented in a consistent direction. The kinocilia point toward (in the utricle) or away from (in the saccule) a middle line called the striola.

Weber test

Tuning Fork test ● Performed by placing a vibrating tuning fork on the vertex of the skull. ● Normal subject hears equally on both sides. ● Patient with unilateral conduction deafness hears the vibration louder in the diseased ear. ● Patient with unilateral partial nerve deafness hears the vibration louder in the normal ear.

Vestibular pathway

Vestibular nerves -> Vestibular nuclei (located at junction of medulla and pons) -> Synapse with 2nd order neurons that send fibers to cerebellum, reticular formation and vestibulospinal tract -> these signals balance facilitation and inhibition of antigravity muscles thus controlling equilibrium.

Warm Water Caloric Nystagmus

When the left external auditory canal is irrigated with warm water (Fig. 16.12A), the heat of the water reaches the lateral part of the horizontal canal and the endolymph in that region is warmed. The endolymph rises and exerts pressure on the lateral side of the cupula, causing it to bend in the medial direction. Medial bending of the cupula causes medial bending of the hair cells, increasing their firing rate. Hair cells are innervated by neurons located in the vestibular (Scarpa's) ganglion. Increased activity of hair cells results in increased activity of central processes of neurons located in the vestibular ganglion. These central processes travel in vestibulocochlear nerve, which projects to the ipsilateral vestibular nuclei. Therefore, increased activity of hair cells in the left horizontal semicircular canal results in stimulation of left vestibular nuclei. Increased activity of the left vestibular nuclei (Fig. 16.12A; activated pathways shown in red), relative to the right vestibular nuclei (Fig. 16.12A;pathways shown in green), causes stimulation of the left (ipsilateral) oculomotor (nucleus of CN III) and right (contralateral) abducens (nucleus of CN VI) nuclei. Activation of left oculomotor nucleus results in contraction of medial rectus muscle of the left eye and stimulation of right abducens nucleus results in contraction of the lateral rectusmuscle of the right eye, causing slow deviation of both eyes to the right side. Thus, irrigation of the left ear with warm water creates a sensation of rotation of the head to the left, causing a slow deviation of the eyes to the right.

Internuclear ophthalmoplegia (INO)

a deficit in the control of conjugate eye movements, which results from damage to the medial longitudinal fasciculus (MLF). The MLF carries internuclear neurons to connect nuclei of the brain stem, including the nucleus of the abducens nerve (CN VI) in the pons to the contralateral subnucleus of the oculomotor nerve in the midbrain (CN III) that supplies the medial rectus. The medial rectus subnucleus of cranial nerve III and the motorneurons of CN VI are responsible for mediating adduction and abduction of the eye, respectively. Thus, the MLF allows for coordination of eye movements between both eyes and allows both eyes to move conjugately in the same direction of gaze. INO is characterized by a deficit in adduction along with contralateral abducting nystagmus Patients most commonly complain of horizontal diplopia due to dysconjugate gaze, or less commonly vertical-oblique diplopia resulting from an associated skew deviation. Due to a mismatch in saccadic movements between the eyes, patients may also report difficulties in tracking fast-moving objects.