Sensory Receptors and the Vestibular System (Dr. Gale)

Question 1

What are the vestibular system’s main functions ?

Answer 1

To detect head mvnts and counter them w/ eye mvnts + postural adjustments to maintain stable vision.

Question 2

What are the two set of organs in the inner ear ?

What are their roles ?

Answer 2

The semicircular canals, which respond to rotational movements (angular acceleration); and the utricle and saccule (that make the otolithic organs) within the vestibule, which respond to changes in the position of the head with respect to gravity (linear acceleration).

Question 3

What is the otolithic membrane ?

Why is it important ?

Answer 3

The otolithic membrane is a fibrous structure covering the surface of the hair cells, and is itself covered with otoconia (CaCO3 crystals).
It plays a critical role in the brain’s interpretation of equilibrium. The membrane serves to determine if the body or the head is tilted, in addition to the linear acceleration of the body.

Question 4

What are the semi-circular canals (SSCs) ?

What is their collective role ?

Answer 4

The SScs (or ampullae) are canals or ducts lined with cilia and filled with endolymph. Every time the head moves, the endolymph moves the cilia, which project into the cupula. This works as a type of motion (angular acceleration) sensor, as the movements of the cilia are communicated to the brain.
The 3 SSCs are aprxn perpendicular to each other: horizontal, vertical and anterior vertical. This helps sense all possible head-rotational angles in 3D.

Question 5

What is push-pull activation of the SSCs and why is it useful ?

Answer 5

Push-pull activation refers to the fact that, when hair cells in the capulla of a SSC on one side of the ear are “pushed one way”, they are “pulled” the opposite way on the other side. This is possible because each SSC is paired w/ another on the opposite side of the head.
Opposite activation gives response to push-pull response in vestibular nerve fibers and is v imp for VOR.

Question 6

How does the VOR work and what is its fct ?

Answer 6

When the head rotates, the SSCs detect that rotation and the vestibular nucleus signals to the accumulator system (III) to excite extra ocular muscles. By this mechanism, the eye mvnt is able to match the head mvnt.
This allows the line of sight to be fixed on a specific visual target.

Question 7

Which nerves emerge form the inner ear and where do they converge ?

Answer 7

The 3 ampullary nerves, the utricular nerve and the saccular nerves all first cluster in the vestibular ganglia and then converge to make the vestibulocochlear nerve (VIII) (or vestibular branch).

Question 8

What are hair cells ?

Answer 8

They are the sensory receptors for hearing and balance. They are connected an afferent bipolar neurons that carry this info to the CNS.

Question 9

What are stereocilia ?

What are their dimensions and how sensitive are they ?

Answer 9

Stereocilia are non-motile apical modifications of the hair cells. They are composed of tightly-packed actin filaments (also some espin and fibrin). These cells are about 4μm long and ca detect changes in lateral movements as small as 0.3nm.

Question 10

How do stereocilia detected mvnt ?

Answer 10

All stereo cilia are cross-linked, so all move when only some are pushed. The deflection of stereocilia physically opens ion channels via tip links (or transduction links) that can pull on them.
When the stereocilia bundle is pushed towards the largest stereocilia, this opens transduction channels and depolarises the cell.
Conversely, pushing the stereocilia bundle away from the largest stereocilia closes transduction channels and hyperpolarises the cell.

Question 11

When the channels are pulled open by the tip links and current can flow inside the call, what kind of potential is generated ?
What effect does this have ?

Answer 11

A graded receptor potential (RP). This activates Ca2+ entry and neurotransmitter release.

Question 12

What is the advantage of the mechanical gating to stereocilia ?

Answer 12

It is v fast and involves no 2d messenger (like in vision).

Question 13

What type of cations enter the ions channels of the stereocilia ?

Answer 13

K+ ions enter from the endolymph that bathes the apical surface of hair cells.

Question 14

Of what proteins are tip links made of ?

Answer 14

The tip link is made of two different cadherin molecules, protocadherin 15 and cadherin 23.
Extra: It has been found that the tip links are relatively stiff, so it is thought that there has to be something else in the hair cells that is stretchy which allows the stereocilia to move back and forth.
It is hypothesized that the tip link is attached to the myosin motor which moves along actin filaments.

Question 15

What is the aprox conduction of a streocilia ion channel ?

Answer 15

100pS

Question 16

How many channels are their per hair cell ?

Answer 16

Around 100

Question 17

What does neutrotransmitter release depending on in hair cells ?

Answer 17

CA2+/calmodulin(Calcium Modulated-Protein)

Question 18

What is the organ of Corti ?

Answer 18

The organ of Corti is the sensitive element in the inner ear and can be thought of as the body’s microphone. It is situated on the basilar membrane in one of the three compartments of the cochlea. It contains four rows of hair cells which protrude from its surface.

Question 19

What are the different types of hair cells and what are their roles ?

Answer 19

There are two types of hair cells in the organ of Corti, the inner and outer hair cells. The outer hair cells (OHC) perform an amplifying role and it is the inner hair cells (IHC) that detect the sound and transmit it to the brain via the auditory nerve.