Physiology of hearing and balance

Question 1

What is the ion balance of perilymph?

Answer 1

Similar to ECF with a high sodium level and low potassium level

Question 2

What is the ion balance of endolymph?

Answer 2

Similar to ICF - high potassium level and a low sodium level

Question 3

What is the central pathway of hearing?

Answer 3

E.coli Ma:
Eight nerve 
Cochlear nucleus
superior Olivary complex
Lateral lemniscus
Inferior colliculus
Medial longitudinal fasciculus 
Auditory complex (brodmanns area 41 and 42 of the left superior temporal gyrus)

Question 4

What age can a foetus hear?

Answer 4

18 weeks

Question 5

What age can a foetus respond to sound/ voice?

Answer 5

26 weeks

Question 6

How is neonatal hearing tested?

Answer 6

Outer hair cells produce sound and therefore if they are functioning, it can be assumed there is a normal functioning cochlea

Question 7

What is normal development in hearing in language in children?

Answer 7

3 months - cooing, recognize mother voice
6 months - babbling, turns to sound or eyes towards sound, makes happy and sad sounds
12 months - may make simple noises, mama/dada, follows instructions, turns to sound, follow simple instructions
18 months - few words, recognize body parts, animals, simple commands
24 months - two word phrases, 50+ words, understands questions and follows demands
36 months - 3 word sentences, understand order, positioning

Question 8

When will a child be referred to a speech therapist if they are not speaking?

Answer 8

27 months

Question 9

What are the 5 vestibular end organs?

Answer 9

Ampullae of 3 SCC (rotational movement)

Organs of otolith: saccule and utricle (linear movement)

Question 10

What does the superior vestibular nerve supply?

Answer 10

Lateral and anterior SCC

Utricle

Question 11

What does the inferior vestibular nerve supply?

Answer 11

Posterior SCC

Saccule

Question 12

What does the stapedius muscle do and what supplies it?

Answer 12

Pulls stapes away from the oval window

Supplied by facial nerve

Question 13

What does the tensor tympani muscle do and what supplies it?

Answer 13

Pulls the malleus, tensing the tympanic membrane

Supplies by V3

Question 14

What is contained within the cochlear duct?

Answer 14

Endolymph (sacula media)

Question 15

What is contained within the cochlea?

Answer 15

Perilymph (scala vestibuli and scala tympani)

Question 16

What window is at the scala vestibuli?

Answer 16

Oval window

Question 17

What window is at the scala tympani?

Answer 17

Round window

Question 18

What does the organ of corti sit on?

Answer 18

The basilar membrane of the scala medius (endolymph)

Question 19

Where is low pitch and high pitch sound heard?

Answer 19

Low pitch at the apex (longest radial fibres)

High pitch at the base (short radial fibres)

Question 20

Where are the afferent neurones of the cochlear nerve found?

Answer 20

Bipolar cell bodies in the spiral ganglion which is located within the modiculus

Question 21

How is sound transmitted from the outer ear to the inner ear?

Answer 21

Pressure waves impact the tympani membrane
Vibration transmitted from handle of malleus to incus to the stapes where the base rocks against the oval window to produce waves in the perilymph of the scala vestibuli

Question 22

How will the pressure wave move around the cochlea duct?

Answer 22

Perilymph of scala vestibuli
Endolymph of scala media - APs produced which travel to brain
Perilymph of scala tympani to leave cochlea via round window

Question 23

What effect will the pressure wave have on the basilar membrane?

Answer 23

It will impart a rocking action, proportional to the degree of displacement of the organ or corti. The stereocilia will be displaced, opening K+ channels leading to depolarisation of hair cells, releasing glutamate and firing an AP to the cochlear nerve

Question 24

What characteristics of the wave form determine pitch and volume?

Answer 24

Pitch = frequency 
Volume = amplitude

Question 25

Where are the vestibular hair cells located?

Answer 25

The otolith organs (utricle and saccule) where they will detect linear movements of the head (saccule vertical and utricle horizontal) 3 SCC

Question 26

What movements do the otolith organs sense?

Answer 26

Linear - saccule vertical and utricle horizontal

Question 27

What movements do the ampullae of the 3 SCC (supeiror, lateral and posterior) detect?

Answer 27

Rotational, angular movements

Question 28

Where are the cell bodies of the vestibular nerve located?

Answer 28

Vestibular ganglion in the outer part of the IAM

Question 29

What occurs when the head rotates?

Answer 29

The endolymph will lag behind, exerting a pressure in the cupula in the opposite direction

Question 30

What hair cells are present in the vestibular system?

Answer 30

Kinocilium (tallest) Bending of the stereocilia towards to kinocilium will depolarize the cell and result in an increased afferent activity Bending away will hyperpolarize the cell

Question 31

Where can the macula of the otolith organs be found?

Answer 31

Patches of hair cells topped by small calcium carbonate crystals called otoconia The saccule and utricle lie at 90 degrees so with any position of the head, gravity will bend the cilia of one macule due to the weight of the otoconia to which they are attached via a gelatinous layer

Question 32

Are the kinocilium in the macula oriented in the same way?

Answer 32

NO In the utricle the kinocilia point towards to striola In the saccule they point away from the striola

Question 33

Where do the axons of the 1st order vestibular afferents travel to after they synapse in the vestibular ganglion at the IMA?

Answer 33

Vestibular portion of the 8th CN through the IMa to enter the brainstem at the cerebellopontine angle

Question 34

What are the 4 2nd order vestibular nuclei?

Answer 34

Inferior, medial, lateral and superior | All found beneath the floor of the 4th ventricle

Question 35

What function does the lateral vestibulospinal tract have?

Answer 35

Walk upright - descends ipsilateral from where it originates. Originates from the lateral vestibular nuclei

Question 36

What function does the medial vestibulospinal tract have?

Answer 36

Tracts bilaterally along the medial longitudinal fasciculus to track head movements and help to coordinate head movements with eyes (VOR)

Question 37

Describe inter-nucleus ophthalmoplegia?

Answer 37

Patients gaze directed AWAY from lesion, the ipsilateral (adducting) eye will not adduct and the contralateral (abducting) eye will demonstrate horizontal nystagmus

Question 38

What causes INO?

Answer 38

Lesion in the medial longitudinal fasciculus which provides a connection between CN3 in the midbrain and CN6 in the pons and facilitates conjugate eye movements on lateral gaze

Question 39

What can cause INO?

Answer 39

``` MS - bilateral Vascular brainstem lesion Pontine glioma Inflammatory encephalitis Myasthenia gravis ```

Question 40

What is nystagmus?

Answer 40

An imbalance of the CNS perceiving a rotating head due to vestibular loss on one side and a firing VOR, you come to the end of the orbit and saccade back But then the brain still interprets rotating to the eye will flick back

Question 41

What are the fast and slow movements of a nystagmus?

Answer 41

VOR is slow moving outwards | It will quickly saccade (flick) back to the midline