W11 Hearing and Equilibrium 1

Question 1

What type of receptor cell is used in both the auditory and vestibular systems?

Answer 1

Hair cells

Question 2

What are 3 modalities detected by the vestibular system?

Answer 2

1. Sense of position of head (sensing gravity)
2. Linear acceleration of the head
3. Rotation of the head

Question 3

What modality is detected by the auditory system?

Answer 3

Rapid vibration of the air surrounding the ear (sound) in the 20 to 20,000 Hz range

Question 4

What type of reflexes involve the vestibular system?

Answer 4

Vestibulo-spinal reflexes and vestibulo-ocular reflexes

Generates skeletal muscle contraction to maintain balance and orientation

Question 5

Dyfunction of the vestibular system leads to what condition?

Answer 5

Vertigo

Question 6

Hair cells are a modified form of what type of cell?

Answer 6

Epithelial cells

Question 7

What neurotransmitter(s) are secreted by hair cells when they are stimulated?

Answer 7

Glutamate primarily, but also Aspartate

These NT’s go to the innervating neurons and stimulate action potentials

Question 8

What two lymphatic compartments are separated by the epithelial cell layers (some of which are hair cells) ?

Answer 8

Endolymph and Perilymph

Question 9

What are the potassium and sodium concentrations of Perilymph? What bodily fluid compartment is this composition similar to?

Answer 9

[K+] ~ 2mM
[Na+] ~ 140 mM

Similar to CSF

Question 10

What are the potassium and sodium concentrations of Endolymph? What bodily fluid compartment is this composition similar to?

Answer 10

[K+] ~ 150mM
[Na+] ~ 1 mM

Similar to Cytosol

Question 11

What is the potential (voltage) of endolymph in the cochlea?

And the vestibular system?

Answer 11

Cochlea endolymph = +80mV

Vestibular endolymph = 0 mV

Question 12

Hair cells have 1 long mobile cilium, what is its name?

What are the many shorter cilia called? What connects them?

Answer 12

1 long mobile cilium = Kinocilium

other cilia = Stereocilia. There are 50-150 of them
Connected by Tip Links

Question 13

What are the 4 types of hair cells?

Answer 13

2 Vestibular hair cells: Type I and II (the difference is not hugely important in this class, slight difference in the placement of efferent axon)

2 Cochlear hair cells: Inner and Outer (difference is more important, but so far the difference hasn’t been explained. outer hair cells have stronger connections to efferent nerves that allow for fine tuning)

Question 14

Where is endolymph produced in the cochlea? How does it work?

(see lecture slide/drawing)

Answer 14

Produced in the Stria Vascularis

Layer of epithelial cells separates perilymph from endolymph. On the first layer of the perilymph side, the Na+,K+,2 Cl- Transporter brings those 3 in, Na+ and Cl- are brought back to perilymph, whereas K+ ions are transported towards endolymph over several epithelial layers

Question 15

What type of mechanosensitive ion channel at the top of the stereocilium is responsible for hair cell depolarization?

Answer 15

TRPA1 (transient receptor potential A1), a non-selective cation channel. Allows K+, Na+ and Ca2+ influx from the endolymph side, but mostly K+ is important because endolymph is full of potassium

Question 16

What is positive mechanical deformation of hair cells?

Answer 16

Mechanical stimulation occurs on the side of the stereocilia, pushing them towards the kinocilium. This causes the TRPA1 channels to open, potassium enters the hair cell, the cell depolarizes, NT’s are released, and increased frequency of action potentials results.

Question 17

What is negative mechanical deformation of hair cells?

Answer 17

Mechanical stimulation pushes the kinociulm over the stereocilium, causing their TRPA1 channels to close. This causes hyperpolarization, less NT’s are released, and so action potential frequency is lower

Question 18

What is the electrochemical gradient of potassium in the vestibular hair cells versus the endolymph space?

And in the cochlear hair cells?

Answer 18

Vestibular hair cells is only +40mV (because endolymph has 0 potential, hair cell has -40mV)

Cochlear hair cell is +120mV (80mV endolymph + 40mV of hair cell)

There is no chemical gradient because the concentration of K+ is ~ the same in hair cell cytoplasm and endolymph. Only an electrical gradient.

Question 19

What are the two “otolith organs?”

Answer 19

The Saccule and Utricle
(two of the three vestibular locations for hair cells)

These are sensitive to linear acceleration

Question 20

What is the otolithic membrane?

Answer 20

Membrane on top of the macula (where saccule and utricle hair cells are located)
Otolithic membrane is a gel composed of mucopolysaccharides and otoliths (small CaCO3 stones) that give the membrane a high density which can be affected by inertia/gravity

Movement of the membrane moves the stereocilia, influencing AP frequency

Question 21

When in anatomical position, what is the direction of the stereocilia in the utricle?
And in the saccule?

Answer 21

Utricle stereocilia = vertical orientation

Saccule = horizontal

Question 22

What is the function of the striola in the otolithic organs?

Answer 22

The striola is a line through the middle of the otolithic organs. In the saccule, all hair cells point away from the striola (opposite directions on either side of the line). In the utricle, all hair cells face towards the striola.

In any position, hair cells on one side of the striola will be depolarized, and on the other side will be hyperpolarized.

Question 23

How are semicircular canals different than the otolithic organs? (both are in the vestibular system)

Answer 23

Semicircular canals have no otolithic membrane and cannot detect linear acceleration. Instead they have a cupula gel with no otoliths. With this gel and the orientation of the semicircular canals, they can detect rotation of the head which causes the gel to flow and stimulate hair cells.

Question 24

What is the name of the region where hair cells are located in the semicircular canals?

Answer 24

Crista Ampullaris

again, to differentiate, the saccule and utricle have hair cells in the macula

Question 25

How long is it possible for stimulation of rotation of the head to occur?

Answer 25

About 15 seconds, at which point the endolymph rotates at the same speed as the head and there is no difference to cause hair cell depolarization (delay of rotation)

Question 26

In the vestibular system, what are the afferent fibers? What are the afferent fiber bodies? Where does their axon go?

Answer 26

Fibers = dendrites
Bodies = Scarpa’s Ganglion
Axons go to vestibular nuclei on ipsilateral side

Question 27

When rotating the head, what is the effect of the vestibular systems existing on both sides of the body?

Answer 27

Action potential frequency slows down on one side of the body, while speeding up on the other. This creates a contrast that helps distinguish the motion more clearly.

When the head rotation stops, the cupula is briefly moved in the opposite direction.