CSD 208 Quiz 6

Process of energy transformation

Acoustic energy (sound waves) → mechanical → hydraulic → electrochemical aka neural

Outer ear (function + parts)

• Function: To gather, channel, and funnel acoustic energy

• Parts: Pinna and external auditory meatus (EAM)

Pinna (function)

• Aka auricle, the external extension of the ear (what we see as an ear)

• Elastic cartilage folds/ridges provide structure and act as a filter for sound, changing their resonance

• Paired, which aids in localization

Resonant frequency of pinna

5,000 Hz (very high)

Parts of pinna (6)

• Helix (top)

• Antihelix (ridge opposite to helix)

• Triangular fossa (hollow part near antihelix)

• Concha (big curve)

• Tragus (little flap near inner, connected side)

• Antitragus (bony flap above lobe, opposite to tragus)

Localization (function and process)

Detecting where sound came from, important for safety

• We have two ears/pathways. The head acts as a barrier

• Sound waves hit either side, are partially funneled through that side’s ear, are partially bounced off of the face, eventually hitting the other ear

• By this time it’s slightly dampened and delayed in the second ear

• Where the two signals meet up in the brain (on the side of the second ear, because the first signal will keep running until the second signal comes in and they can meet) will give our brain a sense of the degree (0-360) at which it came from

External auditory meatus (EAM)

External ear canal that conveys acoustic energy to TM

• Slightly angled forward toward nose

• S-shaped to help with resonance

• Has two portions

  • Lateral 1/3: Outer, cartilagenous

  • Medial 2/3: Osseous (made of temporal bone)

Resonant frequency of EAM

2,500-4,000 Hz

Lateral 1/3 of EAM protective measures

Lined with epithelium that secretes cerumen (earwax)

• Purpose of earwax is to protect from things getting too far into our ear, to maintain ear canal health, to help from drying out, and to repel insects

• Hair also helps to resist things from going into ear

Microtia

Small pinna

• Can hear with a device that bypasses the outer and middle ears and stimulates the mastoid bone

Types of microtia (4)

• Microtia I: Small ear, narrow ear canal, other structures are normal

• Microtia II: Small ear, very narrow canal, some missing components

• Microtia III: Ear is vertical mass of soft tissue, external canal is absent

• Anotia: Absent ear structures, no hearing ability

Otitis externa

Swimmer’s ear

Outer ear as an amplifier

• Pinna contributes gain of 20 dB at 2,000 Hz

• EAM amplifies sound between 1,500-8,000 Hz

• Transduction results in a partial lss of amplitude, so we need speech sounds (most important to hear) to be boosted in this part

  • Speech sounds occur around 500-6,000 Hz

Middle ear space

Air-filled cavity within the temporal bone, bony walls create enclosed space

• Lateral wall: Separates outer ear from middle ear

• Medial wall: Houses oval and round window

• Posterior wall: Communicates with mastoid bone

• Anterior wall: Eustachian tube, connects to nasopharynx, helps with drainage and pressure

Structures of middle ear

• Tympanic membrane

• Auditory ossicles

Tympanic membrane form

• Cone-shaped

• Fibrous membrane (thin)

• End of external auditory meatus (EAM)

• Attached to bony wall of EAM by the annulus

Why is the tympanic membrane a fibrous membrane?

• It’s really thin which allows us to see behind it

• Important when looking for fluid behind ear drum

Annulus

Ring of cartilage, seals middle ear cavity off from external canal, attaches EAM wall to TM

Tympanic membrane vibration

Vibrates in response to sound waves traveling down EAM

• Occurs in buckle motion

• Low frequencies: Vibrates as a whole

• High frequencies: Vibrates selectively

• Intensity is measured by amount of displacement

Layers of tympanic membrane

• Lateral: Epithelium

• Intermediate: Fibrous lamina

• Medial: Continuation of mucous membrane lining of middle ear

Fibrous lamina

• Has circular and radial fibers

• Gives stiffness and strength to TM

Otoscopic exam

Looking into ear with an otoscope (pointy lit device) to note any abnormalities

• Normal TM appears light gray or pink

Tympanic muscles

Smallest muscles in body, respond reflexively

• Tensor tympani muscles

• Stapedius muscle

Tensor tympani muscles

• Decreases range of motion of TM

• Innervated by V trigeminal

Stapedius muscle

• Acoustic reflect threshold (ART) tests it

• Driving force of rotating stapes and stiffening TM

Auditory ossicles parts (3)

• Malleus

• Incus

• Stapes

Auditory ossicles functions (3)

• Transmits vibrations to fluid-filled inner ear

• Increases pressure on that fluid

• Protects inner ear from being overdriven by excessively strong vibrations

Malleus (4 parts + relation to TM)

• Parts include manubrium, neck, head, lateral process

• Eardrum is attached to malleus at the manubrium

  • It moves the malleus → incus → rotates stapes → hits oval window

Incus parts (4)

• Short process

• Long process

• Lenticular process

• Incudostapedial joint

Stapes parts (5)

• Head

• Neck

• Anterior crus

• Posterior crus

• Footplate

  • Rests in oval window of inner ear, sealed there by annular ligament

Smallest bone in human body

Stapes

Annular ligament

Seals footplate of stapes to oval window

Middle ear functions (4)

• Transduction

• Equalization of air pressure

• Protection from loud sounds

• Overcome impedance mismatch

Transduction in middle ear

• Energy transformation from acoustic → mechanical

• Occurs at TM → malleus → incus → stapes → inner ear

Equalization of pressure in middle ear

• For TM vibration to occur

  • Air pressure in ME = air pressure in EAM

  • Middle ear cavity must be sealed to the acoustic energy signal

  Eustachian tube helps equalize pressure

Eustachian tube

• Runs from middle ear to nasopharynx

• Typically closed off at nasopharyngeal entrance

• Can be opened by “swallowing” with the tensor veli palatini

Protection from loud sounds in middle ear

Acoustic reflex

• Tensor tympani stiffens the TM

• Stapedius pulls stapes away from oval window

Overcoming impedance mismatch in middle ear

In order to transform acoustic waves → hydraulic waves without loss of energy, impedance mismatch must be overcome!

Sound amplification in middle ear

• TM buckles add 4-6 dB

• Ossicular chain multiplies vibration with 2 dB gain

• TM to oval window multiplies force with 25 dB gain

Lever effect in middle ear

Malleus and incus act as a lever, force is multiplied due to the size offset between these two parts, has 2 dB gain

Area effect

The surface area of the TM is much larger than the oval window. When vibration is transferred, it’s concentrated, resulting in a 14x increase of mechanical force and a 25 dB gain

Tympanometry

Procedure looking at the integrity of the middle ear system

• Assesses compliance, pressure, volume (size of canal) by comparing results to normative ranges

Results of tympanometry

• Type A: Normal

• A (shallow): Thick tympanic membrane

• A (deep): Flaccid tympanic membrane

• B: Otitis media

• C: Negative pressure, otitis media with no fluid

Movement of sound through ear

1. Sound is collected by pinna

2. It travels through external auditory meatus, which sends the acoustic energy along to the TM

3. Sound waves hit eardrum, which vibrate

4. This hits the malleus, converting the force into mechanical energy

5. This hits the incus → stapes → stapes’ footplate hits oval window

6. This displaces fluid in inner ear, converting the sound into hydraulic energy

7. Eventually reaches auditory nerve

How is pressure boosted in the ear?

The surface area needs to decrease, concentrate the force

• Buckle action of TM

• Lever action of ossicles

• Area effect between TM and footplate of stapes at oval window

Boundaries of inner ear

• Lateral boundary: Middle ear cavity (ossicles)

• Vestibule: Shared space for structures of inner ear, has two windows (oval, round)

Sensory structures in vestibular system (5)

3 paired semicular canals

• Horizontal

• Posterior

• Anterior

2 paired otolith organs

• Utricle

• Saccule

Labyrinths

House the 5 sensory structures within them, aka canals, seen by slicing into vestibule

• Osseous labyrinth: Outer canal, bony, filled with perilymph fluid (sea water)

• Membraneous labyrinth: Inner canal, filled with endolymph

Semicircular canals

• Each has an ampulla (receptor bulb) at the base

• Filled with fluid to manage low-frequency, angular (based on spatial awareness) movement

Horizontal semicircular canals

Shaking head left to right

• Right HC is paired with left HC

Posterior semicircular canals

Nodding head up and down

• Right PC pairs with left AC, vice versa (one of each on opposite sides to manage 360 motion)

Anterior semicircular canals

Moving head/ears toward shoulder

• Left AC pairs with right PC, vice versa

Otoliths

Manage gravity in linear motions

• Utricle: Gives horizontal movement

• Saccule: Gives vertical movement (up/down)

• Contain otochonia to create static equilibrium

Otochonia

Otolithic carving crystals that create static equilibrium

• Have mass to them, so when we move in a way that adds weight to them, the mass shifts and bends stereocilia, leading to polarization

What happens when otochonia become loose?

They float into the vestibular system and move in the semicircular canals, freely moving in our head which makes our brain think that our body is spinning after quick movements → vertigo

• Found through a test that identifies the location of the impairment

Stereocilia

Shorter hairs in bunches that project from the organs of balance into endolymph fluid

• Disturbed by head movements, which sends a signal to the brain with info

• The CNS interprets the info and uses it to make postural adjustments

Depolarization process

1. Head moves

2. Endolymph fluid shifts due to inertia

3. This moves the basilar membrane, which moves stereocilia toward kinocilia

4. This creates a shearing action along the TM

5. This creates pressure on tip links, which open up ion channels

6. This causes ions (potassium and calcium) to flow in and create action potential

7. After the flow, glutamate (a neurotransmitter) is released at synaptic cleft

8. Energy changes from hydraulic → electrical/neurochemical

Cochlea

A snail shell-like coiled structure with 2.5 turns, about 1.5 inches long from base to apex

• Has both a bony and membraneous labyrinth

• Houses organ of corti and scala

• Transduces mechanical → hydraulic energy

Organ of corti

Our hearing organ within the cochlea

Modiolus

Fibers of CN VIII auditory nerve, becomes the internal auditory meatus (internal tunnel for CN)

Scala

Fluid-filled, cornucopia-shaped canals

Scalas (3)

• Scala vestibuli (top)

• Scala media (middle)

• Scala tympani (bottom)

Osseous labyrinth

Houses scala vestibuli and scala tympani

• Both filled with perilymph fluid

• Outer, bony parts

Helicotrema

Apex of canals, where the scala tympani and scala vestibuli meet and communicate

Membraneous labyrinth

Houses scala media aka cochlear duct

• Filled with endolymph fluid

• Inner membrane surrounding organ of corti

Membranes of cochlear duct (aka scala media)

• Reisner’s membrane

• Basilar membrane

• Tectorial membrane

Reisner’s membrane

Superior cochlear duct membrane, acts as the border between scala media and scala vestibuli

Basilar membrane

Inferior cochlear duct membrane, acts as the border between scala media and scala tympani

• Supported by attachments to the bony walls of the osseous labyrinth

• Organ of corti is embedded here

Frequency-related function of the basilar membrane

Tonotopic organization (specific locations have different resonant frequencies) happens here because it’s narrow at the base and wider at the apex

Tectorial membrane hairs

Outer hair cells make contact with the TM and are embedded, while the inner hair cells of the organ of corti don’t make contact with the TM

Afferent nerves

Input toward the brain collected by hair cells

• Inner hair cells: One row, many fibers to one nerve, 3000 of them

• Outer hair cells: Three rows, one fiber to many nerves, 12000 of them

Efferent nerves

Input away from the brain, inhibitory (reduce afferent activity)

• Requires a feedback loop to know if we need to enhance or change auditory signal

Polarization

Moving head → endolymph fluid moves kinocilia (tall hairs) in relation to stereocilia with fast, angular motions, causing them to bend

• If stereocilia bends toward kinocilia → depolarization (activated, action potential is fired)

• If stereocilia bends away from kilocilia → hyperpolarization

Hyperpolarization process

Occurs when we need sound signals to stop being transmitted and action potentials to conclude

• The basilar membrane stops vibrating, causing the stereocilia to bend away from kinocilia

• The tip links contract and close ion channels, meaning that no NTs are released and no action potentials are created

Presbycusis

An age-related hearing loss caused by hair cells dying, leading to a lack of frequency distinction, requiring a louder input