CSD 301 Midterm

First use of the word “audiology”

Raymond Carhart and Norton Canfield (independently) in 1945

• Field started during WWII for soldiers with HL

Notable figures in audiology (2)

• Raymond Carhart

  • “Father of audiology”

  • Background in speech science and psych

  • Became director of rehab center for war heroes with HL

• James Jerger

  • Founded American Academy of Audiology

  • Worked at audiology journal

Amount of US children born with HL in at least one ear

2-3 out of every 1000

How many children experience ear infections by 3 y/o?

5 out of 6

Percentage of deaf children born to hearing parents

Over 90%

Percentage of US adults who report some trouble hearing

15%

Percentage of US adults with tinnitus lasting >5 mins in the last year

10%

How many US adults could benefit from using hearing aids?

28.8 million

How many CIs have been implanted worldwide?

More than 1 million

Audiologist

A professional who is uniquely qualified to provide a range of services related to…

• Prevention of HL

• Identification, assessment, diagnosis, treatment of auditory or vestibular impairment

Audiology scope of practice (7)

Hearing, balance, related disorders

• Identification of HL

• Assessment and diagnosis

• Non-medical treatment of HL

• Hearing conservation

• Intraoperative neurophysiologic monitoring

• Basic and applied research

• Sales and marketing

Non-medical treatment of HL (4)

• Hearing aids

• Cochlear implants

• Assistive listening devices (not prescribed)

• Audiologic rehab

Audiologic rehab techniques (5)

• Auditory training

• Communication strategies

• Frequent communication partner

• Speechreading training

• Vestibular rehab training

Intraoperative neurophysiologic monitoring

• Monitoring the vitality of auditory nerves in head and neck surgery

• Confirming correct insertion of implant in ear prior to wound closure

Types of audiologic research (3)

• Basic: Using animal models, figuring out how processes work

• Translational: Taking findings from non-human subjects and seeing if it also occurs in humans, using this for treatment

• Clinical: Seeing if protocols are effective

Where audiology applies to SLP scope of practice (3)

• Prevention and wellness (education about hearing)

• Screening (for communication disorders)

• Auditory rehabilitation (speech and language are impacted by HL and APD)

Audiology work settings (7)

• Veteran medical centers

• Specialty hospitals (childrens’)

• ENT or private practices

• Hearing instrument manufacturers

• Research

• Colleges/unis

• K-12 schools

Education and training requirements for audiology

• AuD (accredited program required for CCCs)

  • Most programs are 4 years incl. clin externship

• Obtain state license

• Continuing education to maintain license

Trend in audiology private practices

More audiologists are starting them

Percentage of audiologists who held AuD as their only higher-ed degree

78%

• 11% had masters, 6% had PhD

Percentages of audiologists’ perceptions of job openings vs. job seekers

A third for each

• More job openings than job seekers (nonres healthcare)

• Balanced (industry)

• Fewer job openings than job seekers (schools)

Two overarching brain systems

• Central NS: Brain, brainstem, nerves

• Peripheral NS: Primarily composed of sensory organs

Function of each ear segment

• Outer: Collects sound

• Middle: Overcomes impedance mismatch and conducts mechanical energy

• Inner: Converts hydraulic energy to electric signals sent to brain

How many dB are lost in impedance mismatch? What effect does this have?

30 dB

• Turns a loud noise perceived by outer ear into a quiet noise perceived by inner ear

Parts of outer ear (2)

• Pinna

• External auditory meatus

Parts of middle ear (6)

• Tympanic membrane

• Malleus

• Incus

• Stapes

• Oval window

• Round window (covered by membrane)

Oval window

An opening that connects middle ear to inner ear

• Stapes is inserted here

Parts of inner ear (3)

• Cochlea

• Nerves

• Semicircular canals

Chambers within the semicircular canals

• Scala vestibuli: Top, perilymph

• Scala media: Middle, endolymph

• Scala tympani: Bottom, perilymph

Helicotrema

A gap in the semicircular canals that connects the three chambers (SV, SM, ST)

Perilymph and endolymph compositions

• Perilymph: High sodium (saltwater), low potassium

  • Similar to CSF

• Endolymph: High potassium, low sodium

Prevention of mixing between perilymph and endolymph

There’s two membranes that separate the chambers (SV, SM, ST) so the fluids don’t mix

• This would be toxic to your hair cells and cause HL

Importance of impedance matching

Necessary for smooth transmission of energy from low-pressure air-filled cavity (outer ear) → high-pressure fluid-filled cavity (inner ear)

Transmission of energy from outer → inner ear

1. Sound waves enter OE as acoustic energy and goes through ear canal

2. Waves hit ear drum in ME and become mechanical energy that sets the ossicles into motion AND amplifies sound to mediate pressure

3. Vibrations from stapes footplate going in and out of oval window displaces IE fluid, creating hydraulic waves

4. Waves travel along basilar membrane, hitting cilia which sends neuro-electric energy to the brain

5. Energy travels along brainstem through cerebrum until it reaches the auditory center of the brain

Bone conduction

Sound vibrates the bones of the skull, stimulating the cochlea directly

Air conduction

Sound passes through the air and into the ear canal → middle ear → inner ear

Auricle (parts and function)

Aka outer ear, composed of cartilage, soft tissue, and skin

• Pinna and lobule

• Function: Collect and localize sound

Auricle function in humans vs. animals

Limited function (collecting and localizing sound) in humans because musculature is not as developed as animals, so it can’t move as much

Pinna

Oval-shaped appendage on the lateral surface of the head, part of the auricle that collects sound

Lobule

Aka lobe, composed of fat and soft tissue

External auditory meatus (shape, function)

Aka ear canal

• S-shaped, 2.5 cm long

• Functions: Transmits and amplifies sound from auricle to TM, protects TM from damage, maintains clear passageway to TM through self-cleaning

Outer vs. inner parts of EAM

• Outer 1/3: Cartilage, hair, sebaceous and ceruminous glands

• Inner 2/3: Bone (tympanic and squamous portion of temporal bone), skin (thin, sensitive)

Epithelial migration

Self-cleaning process of EAM where skin cells move laterally from TM to extenral opening

Resonance (and relevance to EAM)

A phenomenon where a vibrating system or external force drives another system to oscillate with greater amplitude at a specific preferential frequency

• Ear canal is an open-closed tube that contains a series of natural resonant freqs

Acoustic pressure transfer function of EAM

The resonant frequency of a typical adult ear can range from 2700-3000 Hz with a gain of up to 17-22 dB SPL

Tympanic membrane (description and parts)

Aka ear drum, a thin, elastic, cone-shaped membrane that separates outer ear from middle ear

• Pars tensa

• Pars flaccida

• Umbo

Pars tensa

Tense portion of TM, has three layers

• Outer: Cutaneous (smooth muscle, reflects light)

• Middle: Fibrous (2 layers)

• Inner: Mucosal (single layer)

Pars flaccida

Flexible portion of TM

Umbo

Most central portion of TM, most sunken part due to concave shape

Tympanic membrane functions

• Barrier protecting middle and inner ears from objects

• Vibrate in response to sound pressure waves converting acoustic energy to mechanical energy, which is transmitted to ME bones

Eustachian tube (description and functions)

Canal that connects the middle ear to the nasopharynx

• Functions: ME pressure adjustment, draining fluids, and aeration

  • Without this, prolonged unequal pressure between atmosphere and ME can cause pain

Middle ear muscles

Protect the ear from damage by damping sounds, this happens when bones are pulled during contractions which makes the ossicular chain stiffer and harder to vibrate)

• Tensor tympani

• Stapedius

Tensor tympani

Middle ear muscle that connects to manubrium (long bone attached to TM) of malleus

• Pulls malleus forward and inwards when contracted

Stapedius

Middle ear muscle that connects to neck of stapes

• Pulls stapes back when contracted due to acoustic reflex

Acoustic reflex (what it is and its functions)

Contracts stapedius to pull back stapes and dampen sounds

• Protects inner ear from loudd sounds, improves hearing among low-freq sounds (prioritizes speech sounds in presence of background noise)

• Limitation: Response may be too slow to respond to sudden sounds, contraction can’t be maintained

Acoustic reflex’s effect on low/high frequency sounds

Contraction of muscles stiffens ossicular chain, making it harder to vibrate

• Dampens flow of low-frequency sounds

• Enhances high frequencies, these flow through stiff areas easier

When not contracted, it enhances LFs and attenuates HFs

• Think of guitar string, loose → low

2 pathways of acoustic reflex

• Ipsilateral (lower threshold)

  • ME→ cochlea → CN 8 → SOC → CN 8 → ME

• Contralateral (higher threshold)

  • ME → same cochlea → same CN 8 → other SOC → other CN 8 → other ME

Neither goes to the brain, reflexes are shortcuts between sensory cells and muscles

Conductive mechanism (and HL)

Consists of outer and middle ear, where acoustic signals are converted into mechanical energy

• Conductive HL occurs when there is a problem conducting sound waves anywhere along this route

2 types of nerve fibers in inner ear

• Afferent: Ascending (ear → brain), 95% synapse with inner hair cells

  • Primary nerves for auditory info

• Efferent: Descending (brain → ear), majority synapse with outer hair cells

Organ of corti

Responsible for sensing sound

Organ of corti parts (and their functions)

• Spiral ligament (wall of scala media)

• Stria vascularis (produces potassium for endolymph)

• Supporting cells (Phalangeal and Deiter’s, fill in gaps between hair cells)

Frequency allocation in cochlea

Due to tonotopic organization of basilar membrane

• Base: Stiff, narrow, processes high frequencies

• Apex: Floppy, wide, processes low frequencies

Mass-stiffness gradient

The length of the basilar membrane varies in stiffness and width, which leads to frequency allocation

Hydro-mechanical function of inner ear

As the stapes footplate moves in and out of the oval window, there are corresponding increases/decreases in cochlear fluid pressure

Outer hair cells

On the outer side of the Organ of Corti

• 3 rows

• Longest stereocilia are imbedded in tectorical membrane

• Amplify and sharpen signal

• Have electromotile response

Electromotile response (and relation to polarization)

Causes OHC to compress and shorten repetitively, making displacement of the basilar membrane larger

• Depolarization: ER activates compression of prestin (protein) on side of cell → shortens OHC

• Hyperpolarization: ER causes prestin to stretch → lengthens OHC

Inner hair cells

On the inner side of the Organ of Corti

• 1 row

• Stereocilia float freely in endolymph

• Transduce sound waves into neural impulses

Tip links

Extracellular filaments that connect stereocilia to one another

• When stereocilia tilt, TLs stretch under tensile force → opens ion channel which signals nerves

• This bending can de- or hyper-polarize hair cells depending on the direction the stereocilia are pulled

Polarization produced by tip links

• Depolarization: Opens ion gate → makes hair cells less negatively charged → increase in neural firing from cochlear nerve

• Hyperpolarization: Closes ion gate → positively charged ions can’t get in, increases negative charge → decrease in neural firing

Glutamate

Only neurotransmitter found in the IE, transmits through synapses during depolarization

• Excitatory, meaning it increases activity of connected neurons (encouraging to fire APs)

• Released by inner hair cells

Spiral ganglion

Group of cell bodies that form a bulb-like structure which gather information in a nerve and send them to the axons of connected neurons

• Mostly made up of Type I neurons

Resting vs. action potential

• Resting: Electrical charge when neuron isn’t sending a signal

• Action: Neuron sends info down an axon, results in an increase of neural, electrical firing activity

CN VIII

Auditory Nerve or Vestibulocochlear Nerve

• Bridge between sensory organs (hair cells in cochlea) and central auditory system (cochlear nucleus in brain)

Is CN VIII a bundle nerve?

Yes, it branches out into two parts (cochlear and vestibular)

Type I neurons

Large in diameter, myelinated (quick transmission)

• Encodes input strength

• Fires in response to depolarizing stimuli

• Makes up 95% of ganglions

Relationship between type I neurons and inner hair cells

Each fiber innervates 1 IHC, each IHC connects to 10 T1 nerve fibers

• If something severs relationship between neuron and IHC → nerve fiber dies (can’t connect to new)

• If something damages all 10 fibers → IHC could also die due to overload of NTs

• If something damages some fibers → IHC will slow down, can’t transport info quickly

Two types of coding for type I neurons

• Frequency coding

• Intensity coding

Frequency coding (and 2 types)

Hair cells sitting along basilar membrane send info to CN VIII about how often each resonance is struck

• Time coding

• Place coding

Time coding

How the brain tells, from the pattern of firing in CN VIII, what frequencies are present

• Brain only knows neural impulses, not vibrations

• Happens through perception of phase locking

Phase locking

Auditory nerve will tend to fire (spike of action potential) at the same phase of the sound wave cycle for a low-frequency tone

• This gets weaker as the frequency increases, neural firing becomes random with high-freqs

Place coding

Different nerve fibers have different characteristic frequencies, and these are derived from the parts of HCs and BM that they communicate with

• Measures the place of maximal excitation during an auditory signal

• More reliable than time coding

Intensity coding

How the brain tells, from the pattern of firing in CN VIII, the intensities of different frequencies present

Different types of fibers involved in intensity coding

• High-spontaneous rate fibers

• Medium-spontaneous rate fibers

• Low-spontaneous rate fibers

High-spontaneous rate fibers

• Produce over 18 spikes/sec for intensity coding

• Responsible for hearing sensitivity to quiet sounds

• Can be saturated by loud background noise

Medium-spontaneous rate fibers

• Between 0.5 to 18 spikes/sec for intensity coding

Low-spontaneous rate fibers

• Less than 0.5 spikes/sec for intensity coding

• Responds to loud sounds

• Important for hearing speech in presence of background noise

Type II neurons

Small in diameter, unmyelinated, might have role in signaling pain and damage

• Comprises 5% of ganglions

• Synapse with OHC

Sensorineural hearing loss

Collective term that refers to HL due to damage in inner ear, irreversible

• Sensory HL: Damage to cochlea

• Neural HL: Damage along CN VIII

Possible bone conduction mechanisms

• Inertial

• Compressional

• Osseotympanic

Inertial mechanism for bone conduction

• Vibration to skull → moves cochlea → oval window moves around stapes footplate → fluids are displaced → rest of normal pathway

• Occurs at all frequencies but mostly low

Compressional mechanism for bone conduction

• Segmental (each bone) vibration results in compression of cochlea during high frequency sounds → sets fluid in motion → rest of normal pathway

  • Above 1500 Hz, skull bones vibrate at different phases

  • Below 1500 Hz, skull vibrates as a whole

Osseotympanic mechanism for bone conduction

• Vibration to skull bones → bony part of ear canal vibrates and produces air-conducted sound in ME → stimulates TM → continues normal pathway

  • Enhanced if canals are plugged from occlusion effect

Occlusion effect

When you create a blocking for vibration (such as plugging your ears), it gets louder

• Present in air-conduction process of osseotympanic mechanism of bone conduction

• Maximal below 1000 Hz, amplifies low freqs

Principal nuclei in central afferent auditory pathways

ECOLIMA

• Eighth cranial nerve

• Cochlear nucleus

• Superior olivary complex

• Lateral lemniscus

• Inferior colliculus

• Medial geniculate body

• Auditory cortex

Ipsilateral pathway (afferent)

Comes up the same side, COLIMA

• Cochlear nucleus

• Superior olivary complex

• Lateral lemniscus

• Inferior colliculus

• Medial geniculate body

• Auditory cortex

Contralateral pathway (afferent)

Crosses over to the other side at every OLIMA

• (I) Ventral cochlear nucleus

• (C) SOC

• (C) LL

• (C) IC

• (C) MGB

• (C) AC

Cochlear nucleus

First major nucleus of central auditory system, located in medulla, tonotopically organized

Two regions of cochlear nucleus

• Ventral: Timing and pattern of firing from CN VIII

  • Only causes excitation

• Dorsal: Spectral analysis (organizes freqs)

  • Causes excitation and inhibition

Tonotopic organization of cochlear nucleus

• Higher frequencies → midline

• Lower frequencies → outside

Superior olivary complex (SOC)

Second major nucleus of the central auditory system, located in lower pons

• Main processing center of binaural info (localization)

• Has both ipsilateral and contralateral pathways

• Has three parts, all tonotopically organized