Lecture 22: Auditory System

What are the 2 functions of the inner ear?

hearing and equilibrium

What structure marks the end of the middle ear and beginning of the inner ear?

oval window

What structure brings air from the nasopharynx to the middle ear?

auditory tube

Number of complete sound wave cycles per unit of time

frequency

Maximum displacement of a sound wave from its resting position

amplitude

What does the amplitude of a sound wave determine?

loudness (intensity)

What does the frequency of a sound wave determine?

pitch

How do quiet and loud sounds differ in amplitude?

- quiet sounds have low amplitude

- loud sounds have high amplitude

How do low-pitched and high-pitched sounds differ in frequency?

- low pitch has low frequency

- high pitch has high frequency

What is the normal frequency range of human hearing?

20-20,000 Hz

Sound frequencies below 20 Hz

infrasound

Sound frequencies above 20,000 Hz

ultrasound

What unit measures sound intensity?

decibels (dB)

What is the approximate intensity of a whisper?

30 dB

What is the approximate intensity of soft music?

40 dB

At approximately what sound intensity indicates risk of hearing loss and the need for hearing protection?

110 dB

What is the approximate pain threshold for sound?

130 dB

What sound intensity is considered harmful?

140 dB

What is the function of the ear in sound processing?

breaks down complex sounds into simple, discrete vibrations for subsequent analysis by the brain

Picture of sound showing frequencies from low to high and how they change over time

spectrogram

What is the pathway of sound through the external and middle ear?

external auditory meatus → eardrum → ear ossicles → oval window

How much does the middle ear amplify sound waves?

nearly 15-20 times

What is the central bony structure around which the cochlea spirals?

modiolus

(spirals 2¾ turns)

Where is the cochlear duct attached?

sides of the bone

What fluid fills the cochlear duct (membranous labyrinth)?

endolymph high in K+

What structure holds the cochlear duct laterally?

spiral ligament

what bony projection holds the cochlear duct medially?

osseous spiral lamina

What are the 3 chambers of the bony cochlea?

- scala vestibuli

- scala media

- scala tympani

What fluid fills the scala vestibuli and tympani?

perilymph

What fluid fills the scala media?

endolymph

Which cochlear chamber is the membranous cochlear duct?

scala media

What structure connects the scala vestibuli and scala tympani at the apex of the cochlea?

helicotrema

What structure receives vibrations from the stapes?

oval window

What structure is associated with the scala tympani and allows pressure release?

round window

Where is the organ of hearing located?

floor of the scala media

Sensory organ responsible for hearing

organ of Corti

What membrane forms the roof of the cochlear duct?

vestibular (Reissner's) membrane

What membrane forms the floor of the cochlear duct?

basilar membrane

What structure forms the lateral wall of the cochlear duct?

stria vascularis

What is the function of the stria vascularis?

source of K+ for endolymph

What bony projection is associated with the medial attachment of the tectorial membrane?

spiral limbus

Gelatinous membrane located over the organ of Corti

tectorial membrane

How many rows of hair cells are present in the organ of Corti?

4 rows total

How are the hair cells arranged in the organ of Corti?

1 row of inner hair cells and 3 rows of outer hair cells

Conversion of mechanical energy into electrochemical activity by hair cells

mechanotransduction

How does basilar membrane movement activate cochlear hair cells?

basilar membrane moves → relative movement between tectorial membrane and hair cells → hair cell deflection → ion channels open

What happens when cochlear hair cell stereocilia deflect toward the kinocilium in the mechanotransduction diagram?

mechanically gated channels open → K+ enters → depolarization → Ca2+ enters → neurotransmitter release

What is the relationship between cochlear location and sound frequency?

cochlear tonotopy

Which region of the cochlea detects high-frequency sounds?

base

Which region of the cochlea detects middle-frequency sounds?

middle

Which region of the cochlea detects low-frequency sounds?

apex

What is the key difference between auditory and most other sensory pathways?

auditory pathway projects bilaterally

What are the 4 main relay stations of the auditory pathway?

- cochlear nuclei

- superior olivary complex

- inferior colliculus

- medial geniculate nucleus

Where are the cochlear nuclei located?

medulla

Where is the superior olivary complex located?

pons

Where is the inferior colliculus located?

midbrain

Where is the medial geniculate nucleus located?

thalamus

Where are the 1st-order neurons of the auditory pathway located?

spiral ganglion

What type of neurons are found in the spiral ganglion?

bipolar neurons

Where are the 2nd-order neurons of the auditory pathway located?

cochlear nuclei

What are the subdivisions of the cochlear nuclei?

- dorsal cochlear nucleus (DCN)

- posterior ventral cochlear nucleus (VCNp)

- anterior ventral cochlear nucleus (VCNa)

What structures carry auditory fibers across the midline?

acoustic striae

What are the 3 acoustic striae?

- dorsal acoustic stria (d.a.s.)

- intermediate acoustic stria (i.a.s.)

- ventral acoustic stria (v.a.s.)

Which acoustic stria has the largest output?

ventral acoustic stria

What is the pathway for auditory fibers that bypass the superior olive?

cochlear nuclei → acoustic striae → contralateral lateral lemniscus

What is the pathway for auditory fibers that relay through the superior olivary complex?

cochlear nuclei → bilateral superior olivary complex → lateral lemniscus

What is the function of the superior olivary complex?

compares auditory inputs from both ears to localize sounds in space

What timing information is detected by cells of the superior olivary complex?

differences in arrival time of auditory stimuli between the 2 ears

What happens when a sound source is directly in front of the head?

sound waves arrive at both ears at the same time and intensity

What happens when a sound source is closer to one ear?

sound arrives earlier and louder at the closer ear and later and softer at the farther ear

What pathway carries auditory information from the lower brainstem toward the inferior colliculus?

lateral lemniscus

Where do some lateral lemniscus fibers terminate before reaching the inferior colliculus?

nuclei of the lateral lemniscus

What do the nuclei of the lateral lemniscus do?

communicate with each other and send ipsilateral fibers to the inferior colliculus

Where do some lateral lemniscus fibers project to participate in alertness?

reticular formation

What system is associated with auditory input to the reticular formation?

reticular activating system (RAS)

Where do some lateral lemniscus fibers project directly?

inferior colliculus

What are the projections of the inferior colliculus?

opposite inferior colliculus and ipsilateral medial geniculate nucleus

What connects the left and right inferior colliculi?

commissure of the inferior colliculus (c.i.c.)

What pathway connects the inferior colliculus to the medial geniculate nucleus?

brachium of the inferior colliculus (b.i.c.)

What structure receives auditory input from the inferior colliculus and projects to the cortex?

medial geniculate nucleus

What fibers carry information from the medial geniculate nucleus to the primary auditory cortex?

acoustic (auditory) radiation

Where is the primary auditory cortex located?

floor of the lateral fissure

What gyri contain the primary auditory cortex?

transverse temporal gyri of Heschl (areas 41 and 42)

What Brodmann areas are associated with the auditory association cortex?

areas 42 and 22

What is the function of the auditory association cortex?

puts meaning to a sound

Complete absence of hearing

anacusis

Reduction in hearing

hypacusis

Hearing loss associated with aging

presbycusis

Hearing loss caused by problems in the external or middle ear that prevent sound vibrations from reaching the oval window

conductive hearing loss

What are the causes of conductive hearing loss listed in the lecture?

- otosclerosis

- otitis media

- excess cerumen

- foreign material in the external auditory canal

Fixation of the footplate of the stapes

otosclerosis

Hearing loss caused by disease of the cochlea, cochlear division of CN VIII, or cochlear nuclei in the medulla

sensorineural hearing loss

(permanent - cannot regenerate hair cells)

What structures can be affected in sensorineural hearing loss?

- cochlea

- cochlear division of CN VIII

- cochlear nuclei in the medulla

What are the causes of sensorineural hearing loss listed in the lecture?

- prolonged loud noise exposure

- toxic pharmaceutical effects

- acoustic neuroma

- Meniere's disease

Hearing test comparing air conduction with bone conduction

Rinne test

What is the normal result of the Rinne test?

air conduction greater than bone conduction

What Rinne test result indicates conductive hearing loss?

sound heard equally or less near the ear canal compared with mastoid (bone conduction ≥ air conduction)

Hearing test detecting relative hearing loss between the 2 ears

Weber test

What is the normal result of the Weber test?

sound heard equally in both ears

What is the Weber test result in unilateral conductive hearing loss?

sound heard louder in the abnormal ear