10. Audiology and Hearing Disorders Flashcards
1
Q
Outer Ear
A
Auricle or pinna: funnels sound to the ear canal and helps localize sound; mostly cartilage
External auditory canal/meatus: goes from pinna to the TM or eardrum; resonates sound; cells within canal excrete cerumen
2
Q
Middle Ear
A
- Air-filled cavity; separated from outer ear by the TM
- 3 small bones in the ME form the ossicular chain
- Eustachian tube connects ME to nasopharynx
3
Q
ME: Tympanic Membrane (aka eardrum)
A
- Elastic, thin, cone-shaped
- Flexible and tough and vibrates in response to sound pressure
- Entire TM responds to low-freq sounds, but only certain portions respond to high-freq sounds
4
Q
ME: Ossicular Chain
A
- Suspended in the ME by ligaments
- Composed of 3 tiny bones: malleus, incus, stapes
- Malleus (“hammer”): one end embedded in TM; b/c of this, vibations of TM are transmitted to the malleus
- Incus (“anvil”): attached to malleus; malleus + incus are attached to a tight joint that permits very little movemnt
- Stapes (“stirrup”): attached to incus; footplate, or other end of stapes, is inserted into oval window, a small opening that leads to the inner ear
- Ossicular chain transmits sound efficiently and w/ no distortion; also amplifies incoming sound by approx. 30 dB before transmitting it into the fluids of the inner ear
5
Q
ME: Muscles and Reflexes
A
- Two small muscles in ME dampen vibrations of the TM and ossicular chain: tensor tympani and stapedius muscles
- Tensor tympani is innervated by CN V; stapedius by VII
- Stapedius stiffens ossicular chain so its vibrations are reduced; tensor tympani tenses the TM so its vibrations are reduced
- When someone hears a very loud sound that could cause damage, the ME muscles contract in a reflexive action called the “acoustic reflex.” The acoustic reflex stiffens the ME system, especially the TM
6
Q
ME: Eustachian (or auditory) Tube
A
- Connects ME with nasopharynx
- ET helps maintain equal air pressure w/in and outside ME; yawning and swallowing opens nasopharyngeal end
- Opening of ET is assisted by the contraction of the tensor veli palatini and the levator veli palatini
- ET can also allow germs and infections to spread into the ME, causing hearing probs, esp. in infants whose ETs are more horizontal than those of adults
- Infants w/ cleft palate freq have ET dysfunction, making them vulnerable to conductive HL
7
Q
Inner Ear
A
- Begins w/ oval window (opening in temporal bone that houses IE); Through stapes footplate movement in oval window, IE receives mechanical vibrations of sound
- IE is a sys of interconnecting tunnels aka “labyrinths” w/in temporal bone; tunnels filled w/ perilymph
- 2 major structures: VESTIBULAR SYSTEM (contains 3 semicircular canals, which are responsible for equilibrium SO vestibular sys is related to movement, balance and posture) amp COCHLEA (filled w/ endolymph)
- Floor of cochlear duct is the “basilar membrane,” which contains organ of corti (endolymph, contains cilia which respond to sound vibrations)
- Vibrations created by footplate of stapes into oval window create wavelike movements in the perilymph and through Reissner’s membrane, those movements are transmitted to the endolymph, where the endolymph then transmits movements to the basilar membrane
- Re: basilar- low freq sounds stimulate the tip, high-freq sounds stimulate the base; stimulating sound signals set off waves of fluid, creating movements of the membrane
- The hair cells in the organ of Corti respond to the vibrations of the basilar membrane; vibrations create a shearing force on those cells; at this point, mechanical forces of vibration are transformed into electrical energy, which can stimulate nerve endings
- This energy transformation w/in organ of Corti is critical, b/c nerve fibers that carry sound to brain don’t respond to mech. vibration, only electrical impulses
8
Q
Auditory Nervous System
A
*CN VIII picks up neural impulses created by movement of hair cells in cochlea
*2 branches of CN VIII: vestibular (body equilibrium/balance) and auditory/acoustic branch (supplies many hair cells of cochlea and conducts electrical sound impulses from cochlea to brain)
*CN VIII exits the IE through the internal auditory meatus
and enters BS; at BS level, most CN VIII fibers from one ear decussate (cross over) to oppo. side, forming contralateral pathways; some continue on same side, forming ipsilateral pathways; this crossover of signals allows brain to compare sounds received from each ear and helps brain localize and interpret sounds
*From BS, CN VIII fibers project sound to temporal lobe of brain; temporal lobe contains primary auditory area, which is responsible for receiving and interpreting sound stimuli
9
Q
SUMMARY thus far
A
- Outer ear, composed of auricle and pinna, funnels sound to middle ear. The TM, which separates the outer ear from the middle ear, vibrates in response to sound
- Sound is then responded to by the ossicles, which conduct the sound to the inner ear. The vestibular system and the cochlea are the major structures of the inner ear. Here, energy is converted to electrical impulses, which stimulate the acoustic nerve
- The auditory branch of CN VIII carries electrical sound impulses from the cochlea to the brain, where the sound is interpreted in the primary auditory area of of the temporal lobe
10
Q
Source of Sound
A
- Source: mechanical vibrations of an elastic object; waves travel through mediums that must be elastic to carry sound; disturbance of molecules must be audible
- Sources: VFs, strings of an instrument, tuning fork, etc
- Vibrations occur in cycles; frequency refers to number of times a cycle of vibration repeats itself within a sec; a Pure tone: tone of single freq; Simple harmonic motion or sinusoidal motion: tone of single freq that repeats itself; Complex tone: 2+ sounds of differing freq
- Complex tone vibrations may be periodic or aperiodic (i.e., repeat itself at regular or irregular intervals)
11
Q
Sound Waves
A
- When an object such as guitar strings moves back and forth, it displaces air molecules, causing them to move,
- This causes movement in molecules lying farther and farther away from vibrating object, called sound waves
- Molecules near vibrating object swing back and forth while remaining where they are
- These swings disturb adjacent molecules, which then swing back and forth, thus disturbing the molecules next to them, and process continues
- Back-and-forth movements of molecules change air pressure b/c the movements consist of an instance in which the molecules are compressed together (compression) and an instance in which they are farther apart (rarefaction or expansion)
- A single cycle consists of one instance of compression and one instance of rarefaction within a second; Hz refers to cycles per second
12
Q
Frequency and Intensity
A
- Human ear responds to freqs in 20-20,000 Hz range
- Variations in freq of vibratory cycle cause sensation of different pitches; pitch is perceptual, freq is physical
- Loudness is perceptual, intensity is physical
- Intensity is related to amplitude, which is the extent of displacement of molecules in their to-and-fro motion
- Logarithmic scale used to measure audible range of intensity; one number is multiplied by itself a specific number of times; on logarithmic scale, ear is sensitive to 130 units called dB (1/10 of a bel, basic measure of sound pressure measurement)
13
Q
Sound Pressure Level and Hearing Level
both are ways to measure human responses to sound
A
- dB: measure of sound pressure; also measures intensity of one sound against another
- Intensity of sound is expressed in terms of dBs at a certain sound pressure level, or SPL
- Normal speech varies bet. 50 and 70 dB SPL; People feel pain when sound level reaches 140 dB SPL
- Hearing level: lowest intensity of a sound necessary to stimulate the auditory system; hearing level is the dB level used on audiometers; it is the dB level of sound referenced to audiometric zero
- Human ear is most sensitive to sounds ranging bet. 1,000 and 4,000 Hz (and easier to hear even if less loud); this can create complications in measuring hearing
- To deal with this prob, scientists first determine the SPLs necessary to stimulate auditory system at different frequencies, then those SPLs were considered the 0 db hearing level
14
Q
Normal Hearing: Air Conduction
Sound travels through medium of air
A
- Sound waves strike TM
- The movements of TM causes ossicles to move, creating movement of IE fluids
- These movements cause vibrations in the basilar membrane of cochlea
- Hair cells supplied by CN VIII respond to these vibrations, and sound is carried to brain by CN VIII
15
Q
Normal Hearing: Bone Conduction
bones conduct the sound to the inner ear
A
- Fluids of IE are housed in the skill
- The larger bones of the skill conduct sound, as does the ossicular chain in the middle ear
- The skull bones vibrate in response to airbourne sound waves, causing movements in the IE fluids (also displacing hair cells)
16
Q
Hearing Impairments: Hard of Hearing vs Deaf
A
HoH:
- Child: loss between 16 and 75 dB
- Adult: loss between 25 and 75 dB
Deaf:
- Children and adults: loss exceeds 75 dB and often greater than 90 dB
- Deaf with a capital D refers to deafness as cultural ID
17
Q
Hearing Loss: Severity Categories
A
NORMAL: Up to 15 dB (Normal hearing in children. In adults the upper limit may extend to 25 dB)
MILD: 16 to 40 dB (Mild hearing loss in children. In adults the range is 25 to 40 dB)
MODERATE: 41 to 55 dB
MODERATELY SEVERE: 56-70 dB (or 56-65)
SEVERE: 71 to 90 dB (or 66-89)
PROFOUND: 91 + dB (or 90+)
18
Q
CONDUCTIVE HEARING LOSS
A
- Efficiency with which sound is conducted to the ME or IE is diminished
- In pure conductive HL, the IE, CN VIII, and the auditory centers of brain are all working normally
- Person’s bone conduction (skull bones, not ossicular chain) are also fairly normal
- Even when ossicular chain is not conducting sound, bones of skull do. Thus, conductive hearing loss is never profound
- People w/ conductive loss tend to hear own speech well, thus tend to speak too softly, esp. when b/g noise
19
Q
Conductive Hearing Loss: Causes
A
- Abnormalities of EAM, TM, or ossicular chain
- Birth defects, diseases; foreign bodies can block EAM
- Some kids born w/ cleft palate or other craniofacial abnormalities may have aural atresia (closed EAM), which is often associated with microtia (small and deformed pinna)
- Stenosis (birth defect; narrow EAM; sound waves usu. don’t strike TM)
- External otitis (infection/swelling of skin on EAM)
- Foreign objects, growths, and tumors can block canal
- Otosclerosis (growth on stapes footplate)
- Carhart’s notch (pattern of bone-conduction thresholds characterized by reduced B-C sensitivity predominantly at 2,000 Hz; often found in pts with otosclerosis)
- Otospongiosis (stapes becomes too soft to vibrate)
- Collapsed ear canals, impacted cerumen, ossicular fixation, and disarticulation of ossicular chain aka ossicular discontinuity
- Otitis media aka middle ear effusion
20
Q
Conductive Hearing Loss Causes: Otitis Media aka Middle Ear Effusion
A
*ME infection often associated with upper-respiratory and ET dysfunction
*Frequent in infants, rare in adults
*Usu creates a conductive hearing loss of 20-35 dB HL
(Often undetected by reg. pure tone screenings, which are carried out at 25 dB HL)
*3 types: Serous, Acute, Chronic
21
Q
Otitis Media Types: Serous
A
- ME inflamed and filled with watery or thick fluid
- ET is blocked, preventing fresh air to ventilate the ME
- ME gets airtight; then air inside thins/pressure reduces
- Increased air pressure outside ear pushes TM inward, reducing its mobility
- The retracted membrane vibrates inefficiently, resulting in conductive hearing loss
- Tx: antibiotics and PE tubes
22
Q
Otitis Media Types: Acute
A
- Sudden onset due to infection
- Quick buildup of fluid and pus causes mod-severe pain
- Pressure buildup in ME may rupture TM, giving instant relief as pus is discharged from ruptured membrane
- Tx: medical and surgical procedures, e.g., myringotomy
