3. Auditory System

Question 1

external ear:

function and components

Answer 1

• fxn: collect and transmit sounds
• components
  
  • auricle: outer ear; shape to collect sound
  • external auditory meatus: transmitting sound
  • tympanic membrane: transform sound wave to mechanical vibration

Question 2

middle ear:

fxn and components

Answer 2

• fxn: transmit and amplify sound signal
• components:
  
  • malleus, incus, stapes
  • middle ear cavity
  • oval window
  • round window

Question 3

inner ear:

function and components

Answer 3

• fxn: transforms sound to electric signals
• components:
  
  • scala vestibuli
  • scala tympani
  • cochlea duct or scala media

Question 4

which 2 small mucles are important in protecting the ear?

Answer 4

• Tensor tympani muscle: innervated by trigeminal nerve (CN V)
• Stapedius muscle: connected to stapes; inn. by facial nerve

Question 5

malleus, incus, and stapes correspond with which shapes?

Answer 5

• malleus = hammer
• incus = anvil
• stapes = stirrups

Question 6

which inner ear structure contains perilymph?

contents of perilymph?

Answer 6

• scala vestibuli
• low potassium, high sodium (similar to ECF)

Question 7

which inner ear structure contains endolymph?

contents of endolymph?

Answer 7

• cochlear duct/ or scala media
• high potassium, low sodium

Question 8

where is the helicotrema found?

Answer 8

at the apex of the cochlea (curling structure)

Question 9

which type of hair cells receive the majority of spiral ganglion innervation?

Answer 9

• 90% of spiral ganglion innervate Inner hair cell; as many as 20 spiral ganglion per 1 inner hair cell
• whereas only 10% of spiral ganglion innervate the 3 outer hair cells; 1 spiral ganglion can cover 12 outer hair cells

Question 10

what supports hair cells? what is the function?

Answer 10

hair cells are supported by phalangeal (supportive) cells –>

which act to “stiffen” the superior surface of the organ of corti

Question 11

where are the 1st order neurons in audition?

Answer 11

cell bodies in the modiolus (spiral ganglion neurons);

cell body is in the cochlea –> sends the process to the hair cells

Question 12

process of inhibition of hair cells

Answer 12

1. stereocilia is deflected AWAY from longer stereocilia
2. hair membrane depolarizes
3. afferent nerve fibers are not stimulated

Question 13

mechanism for excitation of hair cells?

Answer 13

• stereocilia deflected TOWARDS the longer stereocilia
• results in influx of K+ ions
• hair cell membrane depolarizes
• influx of calcium ions
• then NTs are released –>
• afferent nerve fibers are stimulated (excitation)

Question 14

where is there tonotopic localization?

Answer 14

in organ of corti

Question 15

which type of frequency sounds are found at the helicotrema?

Answer 15

lower frequency tones

Question 16

which type of frequency sounds are found at the base/ closer to middle ear?

Answer 16

high frequency sounds

Question 17

what movement causes the difference in sound frequencies?

how?

Answer 17

the basilar membrane oscillation allows for diff’t frequencies;

• the base (closer to middle ear) is more stiff/narrow, so higher frequency oscillations will occur here
• whereas the apex/helicotrema is floppy/wide/elastic –> lower frequency stimulation/tone

Question 18

path of axons from hair cells?

Answer 18

• hair cell axons that are excited –>
• travel from spiral ganglion –>
• via cranial nerve 8 (vestibulocochlear) –>
• to cochlear nucleu –>across midline to
• superior olivary nucleus –> ascends to inferior colliculus –>
• via brachium of inf. colliculus –> medial geniculate nucleus —>
• via auditory radiation –>
• to transverse temporal gyrus of heschi
• into the medulla

Question 19

where are the cochlear nuclei found?

Answer 19

cochlear division (CN VIII)

the dorsal & ventral cochlear nuclei are found in the medulla

Question 20

where do cochlear axons synapse?

Answer 20

on the ventral and dorsal cochlear nuclei

w/in the medulla

Question 21

CC: sxs of lesion of nerve or cochlear nuclei?

Answer 21

deafness

Question 22

in coronal section, which cochlear nuclei is more inferolateral?

also identify where 4th ventricle would be

Answer 22

ventral nuclei

4th ventricle is medial, empty white space

Question 23

why would there not be deafness with a unilateral lesion above the cochlear nuclei?

Answer 23

there are a lot of bilateral projections above the cochlear nuclei –>

so it would be able to compensate

Question 24

where do the cochlear nuclei travel to as they ascend?

Answer 24

to the caudal pons;

specifically the:

• Superior olivary nucleus FIRST
• Lateral lemniscus –> fibers to LL ascend from SON
• Trapezoid body –> fibers from SON decussate/cross over midline through trapezoid body

Question 25

would a unilateral lesion in the Lateral Lemniscus, Trapezoid body, or Superior olivary nuceli cause deafness?

Answer 25

no, bc there are **bilateral projections at multiple sites in the brainstem**

Question 26

How do **auditory axons** travel from the **superior olivary nucleus?**

Answer 26

can travel via lateral lemniscus --\> * either continue ascending the brainstem thru cerebral crus to the **inferior collilulus**, OR * synapse at nucleus of lateral lemniscus, and travel across the commissure of lateral lemniscus --\> caudal mibdbrain

Question 27

describe pathway from **midbrain to thalamus?**

Answer 27

the **interior colliculus** of the caudal midbrain --\> to the **medial geniculate nucleus in the thalamus**

Question 28

where does the **medial geniculate nucleus** of the thalamus project to?

Answer 28

the primary auditory cortex | (aka transverse temporal gyri)

Question 29

describe the tonotopic organization of the auditory cortex?

Answer 29

* **medial** side of auditory cortex = high frequency sounds (base) * **lateral** side of auditory cortex = low frequency sounds (apex/helicotrema)

Question 30

where are low frequency sounds found | (w/ regards to tonotopic organization)

Answer 30

**lateral side of auditory cortex** = low frequency sounds from the **apex/helicotrema**

Question 31

which gyri contain the **primary auditory cortex?**

Answer 31

transverse temporal gyri; information from here goes to unimodal --\> then heteromodal association cortex

Question 32

**tone**: define

Answer 32

frequency; can be high or low

Question 33

what affects how loud or quiet a sound is?

Answer 33

the **amplitude of sound;** i.e. differential firing of afferent fibers

Question 34

where does **sound localization** occur? what does it require?

Answer 34

occurs in **superior olive nucleus** requires information from both ears to be able to localize the sound

Question 35

where do unilateral lesions occur to produce **hearing loss?**

Answer 35

\*but not higher than the cochlear nuclei * cochlear nuclei * CN 8 (vestibulocochlear nerve) * cochlea

Question 36

a **unilateral lesion** in the pons, midbrain, or cortex would not produce deafness --- why?

Answer 36

due to bilateral projections at multiple sites in the brainstem

Question 37

**superior olivary nuclei**: pathway and function

Answer 37

* project back to **cochlea** to inhibit auditory nerve terminals on hair cells * **regulates hair cell sensitivity --\>** regulating selective attention to certain sounds by affecting **olivococlear efferent fibers**

Question 38

**conductive hearing loss:** define, and possible causes

Answer 38

* (sound transmission is the problem; issue w/ outer or middle ear, or TM) * causes incl: * foreign bodies in external acoustic meatus * perforated tympanic membrane * otitis media * otosclerosis * cholesteatoma

Question 39

otosclerosis: define

Answer 39

overgrowth of bones around stapes; if bones are stuck, we can't hear

Question 40

cholesteatoma: define

Answer 40

overgrowth of **desquamated keratin debris** w/in middle ear cavity; preventing sound transmission

Question 41

sensorineual hearing loss: define and causes

Answer 41

* in which the root cause lies in the inner ear or sensory organ (cochlea and associated structures) or the vestibulocochlear nerve (cranial nerve VIII) * causes incl: * congenital hearing loss -genetic mutations, developmental insults * acquired hearing loss * noise-induced hearing loss * aging-related hearing loss * lesions in auditory pathway

Question 42

noise-induced hearing loss: define

Answer 42

types of hearing loss **damage to sterociliated cells in organ of corti**, loss of high frequency hearing first

Question 43

Aging related progressive bilateral/symmetric sensorineural hearing loss: define

Answer 43

often of higher frequencies due to the destruction of hair cells at the cochlear base.

Question 44

which 2 tests can be used to distinguish **conductive vs. sensory neural hearing loss**

Answer 44

* **Rinne:** air conduction --\>and then placing on temporal bone to see if you hear the sound through the bone conduction * **Weber**: place tuning fork on midline of forehead; sound is conducted through temporal bone; ask where they hear the sound,

Question 45

**bone** conduction: define

Answer 45

Sound vibration can be transmitted **through temporal bone vibration to inner ear, less effective than air conduction**

Question 46

**air** conduction: define

Answer 46

Sound transmission through tympanic membrane vibration and ossicle movement, more effective than bone conduction

Question 47

which is more effective? air or bone conduction?

Answer 47

air conduction

Question 48

results of weber and rinne tests for: ## Footnote **conductive hearing loss**

Answer 48

* Weber: localizes to **AFFECTED** ear * Rinne: **abnormal (BC \> AC)**

Question 49

results of Weber and Rinne tests for: ## Footnote **sensory neural hearing loss**

Answer 49

* Weber: localizes to **UNAFFECTED** ear * Rinne: **normal** (BC \< AC), w/ AC being more effective