special senses I Flashcards
1
Q
normal range of human hearing
A
- 20-20,000Hz
- -> people under 25 cannot typically hear above 16,000Hz
2
Q
What range of frequencies are humans most sensitive to
A
- 1-4kHz
3
Q
describe normal range of hearing changes with age
A
- Presbycusis = normal hearing loss in older individuals, first appears in high frequency range (16kHz)
- Possible notch in audiogram due to exposure to explosions, gunshots, etc
4
Q
describe dB scale
A
- loudness is a psychophysical measurement of the PERCEPTION of intensity of sound
- depends on both frequency and sound pressure
- SPL = 20log(sound pressure/reference pressure)
- 0 dB = pressure of sound equals reference pressure
- if sound pressure is weaker than reference pressure, sound will have NEGATIVE dB value
5
Q
function of outer ear
A
- pinna = importnat in localizing sound in the vertical plane
- External auditory meatus = has resonant frequency of 3,500Hz; increases sound pressure at the tympanic membrane for certain frequencies (esp around 2.5-4kHz)
6
Q
- Function of middle ear
A
- Auditory tube/Eustachian tube = air pressure in middle ear is equalized with that in the external enviornment
- Ossicular system (maleus, incus, stapes) = provides impedance match between outer and inner ear
- Middle ear muscles:
- -> stapedius = innervated by CN VII
- -> tensor tympani = innervated by CN V
7
Q
Describe how sound energy is transformed into neural signals
A
- Sound enters external auditory canal
- pressure of sound waves moves tympanic membrane
- ossicles transmit pressure from sound waves into cochlea
- cochlea contains organ of corti, which contains auditory hair cells
- pressure from sound waves move endolymph, which in turn moves hair cells and depolarizes them
- depolarization results in action potential sent to brain via auditory nerve and sound is “heard”
8
Q
describe middle ear attenuation reflex
A
- won’t have the reflex until already heard a loud noise, damage has already occured
- Both muscles contract in response to loud noise, but most attenuation (muscle contraction) is from stapedius (can also contract immediately before vocalization and in response to swallowing, closing the eyes and other non-acoustic stimuli
- MOST attenuation is for sounds below 1kHz (thought to aid in hearing in noisy environments
- BELLS PALSEY CAN CAUSE loss of this reflex in one ear due to functional denervation of stapedius
9
Q
describe sensory input and motor output of middle ear attenuation reflex
A
- Sensory input = stapedius muscle (CN VIII)
- Motor output:
- -> CN VII to stapedius muscles
- -> CN V to tensor tympani
10
Q
describe the place principle for determination of sound frequency
A
- HIGHER FREQUENCIES recognized near BASE
- Lower frequencies recognized near APEX
11
Q
describe basilar membrane movement
A
- traveling waves always start at stiffer part of basilar membrane
- basilar membrane varies in width and stiffness
- -> near stapes or base of cochlea = membrane NARROW AND STIFF
- -> near helicotrema or apex = membrane is WIDER and FLOPPY
12
Q
describe causes of conductive deafness
A
- impaired sound transmission in external or middle ear
- plugging of external ear canal w/ wax/foreign body
- otosclerosis
- fibrosis due to repeated middle ear infection
- otitis media w/ effusion
- damage to tympanic membrane
13
Q
causes of sensorineural deafness
A
- NERVE DEAFNESS
- loss of cochlear hair cells
- hair cells damaged by ototoxic drugs (aminoglycoside antibiotics)
- noise exposure (temporary or permanent damage)
- vestibular sxhwannoma (unilateral hearing loss
- If CN VIII intact and enough hair cells remain (can Tx w/ hearing aid)
- If not enough hair cells (can tx with cochlear implant)
14
Q
causes of central hearing loss
A
- caused by lesion to central auditory nuclei and pathways
- sometimes called retrocochlear lesions
- due to much midline crossing of auditory information, unilateral damage to ascending pathways has ONLY subtle effects on hearing
- MS can cause defective sound localization
15
Q
describe audiometry
A
- measure hearing thresholds at specific frequencies
- patient fitted with calibrated earphones
- pure tones are presented at various frequencies and intensities
- patient indicates whether or not tone is heard
- each ear tested separately
