Neurology of Hearing and Balance

What is another name for hearing

audition

What happens during hearing

A process where acoustic energy waves are changed into neural impluses that are interpreted by the brain

What are the two main divisions of the ear

• peripheral auditory system

• central auditory system

What is part of the peripheral auditory system?

• outer ear

• middle ear

• inner ear

• cranial nerve 8

What is part of the central auditory system?

• brainstem

• brain

definition of sound

audible variations in air pressure

• dependent on the motion of air molecules associated with sound

• propagation of sound

The Nature of Sound

pitch = frequency

loudness = amplitude = intensity

The Outer Ear

• The canal lined by skin containing hair follicles, sebaceous glands, and ceruminous glands that produce wax.  earwax. The cerumen lubricates the skin and coats hairs near the opening to impede the entry of foreign particles into the ear.  Excessive accumulation of cerumen can plug the meatus, however, and result in conductive hearing loss

Tympanic Membrane - ear drum

• boundary between outer and middle ears

• vibrates when sound waves entering the external auditory meatus hit it

• a framework of connective tissue fibers covered by skin on the external ear side and mucous membrance on the side of the middle ear cavity

• perforation may cause transient or permanent hearing impairment

What are the three ossicles

help with the transduction of vibratory energy into mechanical energy

• malleus

• incus

• stapes

malleus

• hammer

  • attached to the tympanic membrane

incus

• anvil

  • connects the malleus to the stapes

stapes

• stirrup

• fits into the oval window

What is the structure of the attenuation reflex

tensor tympani inserts on the malleus, stapedius inserts on the stapes

What is the function of the attenuation reflex

• protects the innear ear (cochlea) from damaging vibrations

• masks background noise in noisy environments

• decreases sensitivity to one’s own voice

what is the purpose of the eustachian tube?

equalizes pressure in middle ear to atmospheric pressure

Ear Infections

• more common in childrne

• their eustahcian tubes are shorter, narrower, more horizontal than adults, makes movement of air and fluid difficult.

• Bacteria trapped in the eustachian tube may produce an ear infection that pushes on the eardrum cuasing it to become red, swollen, and sore

Inner Ear: Anatomy

• the rocking of the stapes in the oval window creates waves in the cochlear fluid

• this is another energy change: mechanical energy of ossicular movement that has been changed into hydraulic energy

• these waves disrupt the hair cells in the organ of Corti causing a third energy change: hydraulic energy changed to electrochemical energy

Fluid Movement in the Cochlea

• Pressure at oval window, pushes perilymph into scala vestibuli, which then travels around to the scala tympani and causes the round window membrane to bulge out

The Basilar Membrane

• At the apex of the cochlea, the scala media is closed off, and the scala tympani becomes continuous with the scala vestibuli at a hole in the membranes called the Helicotrema

• The Response of Basilar Membrane to Sound

  Structural properties: Wider at apex, stiffness decreases from base to apex. Perilymph movement bends basilar membrane near base, wave moves towards apex

Why the cochlea is spiral-shaped

It increases the strength of vibrations produced by sound waves, especially at low pitch.

Bending of Stereocilia

• At rest, the hair cells are held between the reticular lamina and the basilar membrane, and the tips of the outer hair cell stereocilia are attached to the tectorial membrane.

• When sound causes the basilar membrane to deflect upward, the reticular lamina moves up and inward toward the modiolus, causing the stereocilia to bend outward. 

Depolarization of a Hair Cell

• Ion channels on stereocilia tips are opened when the tip links joining the stereocilia are stretched.

 

• The entry of K⁺ (potassium) depolarizes the hair cell, which opens voltage-gated calcium channels. Incoming Ca²⁺ leads to the release of neurotransmitter from synaptic vesicles, which then diffuses to the postsynaptic neurite from the spiral ganglion. 

Hair Cells: Scanning Electron

(a)Hair cells and their stereocilia.

A higher-resolution view of the stereocilia on an outer hair cell. The stereocilia are approximately 5 µm in length.

Innervation of Hair Cells

One spiral ganglion fiber receives input from only 1 inner hair cell; each inner hair cell feeds about 10 spiral ganglion neurites.

One spiral ganglion fiber synapses with many outer hair cells.

Therefore, vast majority of information leaving cochlea comes from INNER hair cells.

Cochlear Amplifier: Outer Hair Cells Amplify Movement of Basilar Membrane

Outer hair cells amplify the response of the basilar membrane à stereocilia on inner hair cell bend more à increased transduction in inner hair cells à greater response in auditory nerve.

Without cochlear amplifier:

  100 fold smaller peak movement of basilar membrane

  Excessive exposure to antibiotics (kanamycin) damages outer hair cells cochlear amplifier à deafness

Cochlear Efferent Nerves

About 1000 efferent fibers project from brain stem to cochlea

Synapse on inner and outer hair cells à changes the shape of the outer hair cell à affects response of inner hair cell.

Thus descending input from brain to cochlea can regulate auditory sensitivity

Otoacoustic Emissions

Ears emit sounds present click à echo that can be recorded w/ mic in auditory canal. Normally too faint for us to hear.

Occur because the sensitivity of the cochlear amplifier very high

If spontaneous otoacoustic emissions loud enough:

  can cause a form of tinnitus

     usually the result of cochlear damage from exposure to extreme loud sound

Can be used to test function of ears: play series of sounds, record evoked echos.

    especially useful for those that cannot voluntarily respond – e.g., newborn babies 

Overview of Auditory Pathway

Pathway is complex: many synapses from medullary cochlear nuclei to other brainstem nuclei

Important Pathway

1 a. Ipsilateral cochlea to ventral and dorsal cochlear nuclei in medulla

1 b. Bilateral to Superior Olivary Nuclei Medulla

2. Bilateral thru lateral lemniscus (bundle of fibers) to Inferior Colliculus in Midbrain

3. Bilateral to Medial Geniculate Nucleus Thalamus

4. Bilateral to Primary Auditory Cortex

Majority of ascending fibers cross midline

All ascending pathways converge onto Inferior Colliculus

Important Points

1. Other projections and brainstem nuclei contribute.

    e.g., Inferior Colliculus to MGN & also to Superior Colliculus (integration of auditory & visual info)

2. Auditory Pathways have extensive feedback.

   e.g., brain stem neurons send axons to outer hair cells

   e.g., auditory cortex sends axons to MGN & Inferior Colliculus

3. Each Cochlear Nucleus receives input from only Ipsilateral Ear;

    All other auditory nuclei in brainstem receive input from Both ears

      à only way brainstem damage can cause deafness in one ear is if cochlear nucleus or auditory nerve damaged on that side

Tonotopic Maps

On basilar membrane and cochlear nucleus. From base to apex, the basilar membrane resonates with increasingly lower frequencies. This tonotopy preserved in auditory nerve and cochlear nucleus. In the cochlear nucleus, bands of cells with similar characteristic frequencies which increase progressively from anterior to posterior.

Sound Localization

• Sound waves coming from the right side will reach the right ear first, and there will be a large interaural delay before the sound propagates to the left ear.

• If the sound comes from straight ahead, there is no interaural delay. Delays for three different sound directions are shown. 

Sound Localization: Ineraural Intensity

• With high-frequency sound, the head will cast a sound shadow to the left when sound waves come from the right. Lower-intensity sound in the left ear is a cue that the sound came from the right.

• If the sound comes from straight ahead, a sound shadow is cast behind the head but the sound reaches the two ears with the same intensity.

• Sound coming from an oblique angle will partially shadow the left ear.

Vertical Sound Localization

based on reflections from the pinna

Tonotopic Maps Pt. 2

• Primary auditory cortex (purple) and secondary auditory areas (yellow) on the superior temporal lobe.

• Tonotopic organization within primary auditory cortex. The numbers are characteristic frequencies. 

Primary Auditory Cortex

Heschl’s gyri

Maintenance of tonotopic representation

Discrimination of timing patterns of auditory stimuli

Essential for human speech perception

Secondary Auditory Area

Superior temporal gyrus

Processing of timing patterns

Recognition of spatial attributes typical of human speech

Acoustic aphasia- impaired perception & discrimination of human speech sounds

Association Cortex of Wernicke

Comprehension of spoken language and generalization of meaning

Hearing Disorders: Conductive

•Interrupted sound transmission to cochlea

•Middle ear pathologies

• Otitis media (inflammation of middle ear fluid) and ottosclerosis (impeded stapes movement)

•Symptoms

• Fluctuating hearing loss, good word-speech recognition, mostly softly spoken speech, impaired auditory reflex, and air-bone gap

Hearing Disorders: Sensorineural

Dysfunction of hair cells and/or auditory nerve fibers

•Commonly implicated with

• Ménière's disease- progressive & fluctuating hearing loss with vertigo & tinnitus

• Presbycusis- old age induced hearing loss of high frequencies

• Acoustic neuroma/vestibular schwannoma- hearing impairment and disequilibrium, and ataxic symptoms in later stages

•Symptoms

• Difficulty in understanding speech, reduced self-monitoring in speech recruitment

Hearing Disorders: Mixed

•A combination of conductive and sensorineural hearing loss.

 

•There may be a problem in the outer or middle ear and in the inner ear or auditory nerve. 

•

•It can happen after a head injury, long-term infection, or because of a disorder that runs in your family.

Hearing Disorders: Central

•Can result from damage to lower brainstem upper brainstem or primary auditory cortex

•

•Clinical Characteristics

•Near-normal sensitivity to stimuli

•Impaired processing of linguistic signals

Cortical Involvement: Unilateral Cortical Lesion

normal hearing thresholds with imparied perception and discrimination of speech

Cortical Involvement: Bilateral Cortical Lesion

profound hearing impairment

Cortical Involvement: Primary and Secondary Auditory Lesion

Acoustic aphasia

•Impaired discrimination of speech sounds & phonemes- skills necessary for learning phonemes and understanding language

Cortical Invovlement: Language association cortex lesion

Wernicke aphasia

•Impaired  comprehension of spoken and written language

•Word finding deficit & asemantic verbal output

Cortical Invovlement: Right Temporal lesion

Impaired processing of environmental sounds, non-verbal memory, & musical properties

Balance

Also known as vestibular system

Our spatial orientation system that helps us sit upright, walk, and perform other functions in our spatial environment

The Vestibular Labyrinth

Locations of the otolith organs (utricle and saccule) and semicircular canals.

A vestibular labyrinth resides on each side of the head, with the semicircular canals arranged in parallel planes

Macular Hair Cells Responding To Tilt

• When the utricular macula is level (the head is straight), the cilia from the hair cells also stand straight.

• When the head and macula are tilted, gravity pulls the otoliths, which deform the gelatinous cap, and the cilia bend. 

Macular Orientation

The macula in the utricle is horizontal.

The macula in the saccule is vertical. The arrows on each macula show how the hair cells are polarized. Bending the hairs in the direction of the arrow depolarizes them. 

Essentially, these otolithic organs sense how quickly you are accelerating forward or backward, left or right, or up or down

What do semicircular canals do?

Sense rotation of the head

Ampula of Semicircular Canals

• The cilia of hair cells penetrate into the gelatinous cupula, which is bathed in the endolymph that fills the canals.

• When the canal rotates leftward, the endolymphlags behind and applies force to the cupula, bending the cilia within it.

Activation of Semicircular Canals

Head rotation causes the excitation of hair cells in one horizontal semicircular canal and the inhibition of hair cells in the other. (Graphs) Long-lasting head rotation leads to adaptation of the firing in vestibular axons. When rotation is stopped, the vestibular axons from each side begin firing again, but with opposite patterns of excitation and inhibition

Vestibular Pathlogies

•Vestibular schwannoma – slow-growing, unilateral, benign tumor on vestibular branch of cranial nerve VIII

• main symptoms are hearing loss and tinnitus as well as vertigo and balance issues

•Labyrinthitis – infection of the inner ear

• leading to vertigo, nausea, and vomiting

•Ménière's disease – unknown cause likely involves both genetic and environmental factors

• episodes of feeling like the world is spinning (vertigo), ringing in the ears (tinnitus), hearing loss, and a fullness in the ear

•Benign paroxysmal positional vertigo (BPPV) - probably caused when pieces that have broken off otoliths have slipped into one of the semicircular canals

• dizziness, vertigo and nystagmus

•Motion Sickness - the vestibular system reports no movement but the visual system reports movement

• nausea