29 – Auditory System

Question 1

What are the anatomical components of the auditory system?

Answer 1

-external ear (pinna)
-external ear canal
-middle ear
-inner ear
*CN VII is also there

Question 2

External ear canal:

Answer 2

-up to tympanic membrane

Question 3

Middle ear:

Answer 3

-air filled
-auditory ossicles (x3)
>malleus
>stapes
>incus

Question 4

Inner ear:

Answer 4

-vestibular apparatus (filled with endolymph)
-cochlea (ventral)
>spiral surrounded by perilymph
>filled with endolymph

Question 5

Perilymph:

Answer 5

-continuation of the CSF

Question 6

Endolymph in the cochlea:

Answer 6

-controls buoyancy of structures
*important for conduction of sound waves!

Question 7

How does sound travel?

Answer 7

-through pinna
-waves hit tympanic membrane=vibrate=vibrate 3 ossicles
-bones vibrate and amplify the sound waves
-perilymph around it will change (in bony labyrinth) (fluid wave)
-causes movement of endolymph and hair cells detect that

Question 8

Cochlea and organ of Corti:

Answer 8

-on the basilar membrane
- ‘lever’ on it and hair cells
-acts like a pump and it moves up and down=vibrates on hair cells
>hair cells secrete NTs and directly synapse onto CN VIII then to the medulla

Question 9

Auditory system is working on both sides:

Answer 9

-able to localize where the sound is coming from
>time delay and decreased intensity

Question 10

Detection of sound (2):

Answer 10

1. Frequency of sound
2. Timing of sound

Question 11

Cochlea hair cells and frequency:

Answer 11

-different areas for high and low frequencies
>low: close to apex of cochlea
>a nerve innervates each hair cell at EACH frequency
-when moved=NTs released into medulla then to auditory cortex
*all hair cells are similar, LOCATION is what determines their ‘sound code’

Question 12

High vs. low frequency and vibrations:

Answer 12

-high: faster vibrations
-low: slower vibrations

Question 13

Frequency of sound:

Answer 13

-determines which axons will be activated
-combination of frequencies produce complex sounds (ie. Talking)
*info provides animal with an indication of WHAT the sound is
Ex. bark, growl, owner calling them, explosion, predator

Question 14

Timing of sound:

Answer 14

-timing of when sound reaches each ear allows the animal to localize WHERE the sounds was located

Question 15

What are the auditory pathways?

Answer 15

1. CN VIII
2. Medulla
3. Thalamus
4. Auditory Cortex
   *lots of crossing over

Question 16

Medulla:

Answer 16

-responsible for integrating all the information and amplifying it
*maintains the signals and send the info to the thalamus

Question 17

What does it mean if there is UNILATERAL DEAFNESS:

Answer 17

-peripheral lesion on ipsilateral side
>cochlea or the inner ear bones
>endolymph
>CNVIII
*anything before the medulla

Question 18

What is the most common cause of deafness?

Answer 18

-infections
>especially in older animals
*not examined in a regular neuro exam (not easy to do)

Question 19

What are 2 types of deafness?

Answer 19

1. Conduction deafness
2. Sensorineural deafness

Question 20

Conduction deafness:

Answer 20

-something preventing sound waves from coming into cochlea
>external ear/middle ear

Question 21

Sensorineural deafness:

Answer 21

-damage to structures of inner ear and rest of auditory system
>cochlea/CN VIII/damage in brainstem/auditory cortex

Question 22

Deafness:

Answer 22

-progressive degeneration (loss) of hair cells occurs over lifespan of an animal
*excessive or chronic stimulation of hair cells results in deafness (loud environments: hunting dogs, boarding kennels)

Question 23

Which frequencies are lost first and why?

Answer 23

-high frequencies
-these hair cells are most prone to degeneration/cell death

Question 24

What drugs may cause death of the hair cells which can cause deafness?

Answer 24

-aminoglycosides (systemic antibiotics)
*don’t use them anymore

Question 25

Congenital deafness:

Answer 25

-typically associated with pigmentation and coat colour
-unilateral and bilateral deafness
*no genetic markers yet
Ex. white coats, merle (Australian shepherds, etc.), dalmatians (30%)

Question 26

White coloured dogs:

Answer 26

-don’t have much pigment=less melanocytes=less nutrients into the endolymph=hair cells die
>will hear for the first 4-5 weeks of life, but it can’t be maintained
>test hearing after 6 weeks
-50% with white coat and blue eyes=deaf

Question 27

BAER test:

Answer 27

-brainstem auditory evoked response
-electrodes placed on head around the ear
>recording brain activity
-earphones placed inside to deliver sounds (‘clicks’ of various frequencies)

Question 28

Normal animal BAER test:

Answer 28

-normal waves at:
>CN VIII
>brainstem
>midbrain
>auditory cortex

Question 29

Periphery damage BAER test:

Answer 29

-no waves at all=flat line