Sensory Transduction & Ion Channels: Mechanoreceptors

Question 1

What are mechanoreceptors?

Answer 1

This is a type of receptor responds to mechanical forces such as pressure, touch, vibrations, or stretch.

Question 2

What are hair cells?

Answer 2

mechanoreceptors (sensory receptors) of both the auditory and vestibular systems in the inner ears of all vertebrates

They have stacks of stereocilia on their tip/upper surface

Question 3

What are the Different types of mechanoreceptors?

Answer 3

• Touch receptors
• Baroreceptors
• Proprioceptors
• Hair cells

Question 4

What is the inner ear formed of?

Answer 4

Semicircular canals: form the vestibular system

Cochlea: forms the auditory system (fluid filled spiral canal divided by a flexible membrane)

Question 5

What is the Structure of the cochlea?

Answer 5

3 fluid-filled chambers:

• scala vestibuli
• cochlear duct (scala media)
• scala tympani

Reissner’s membrane separates scala vestibuli and cochlear duct (scala media)

Basilar membrane separates cochlear duct (scala media) and scala tympani

Question 6

What permits us to hear sound?

Answer 6

Spiral organ of cochlea, also known as organ of Corti, is the receptor organ for hearing

Question 7

Where does the organ of corti sit?

Answer 7

It sits on top of the basilar membrane within the cochlear duct

Question 8

What does the organ of corti do?

Answer 8

It consists of inner and outer hair cells that translate fluid vibration of sounds into electrical impulses that are carried to the brain by sensory nerves

Question 9

What sits on top of hair cells?

Answer 9

tectorial membrane (can move up and down)

Question 10

What do hair cells contain on their tips?

Answer 10

Stereocilia which are arranged in ‘bundles’ (e.g. 30-300 stereocilia in each bundle in the ear)

Question 11

How are sterocilia connected?

Answer 11

Within the bundle, stereocilia can be connected via a number of links:

· Lateral-link connectors: top connectors, shaft connectors and ankle links

· Tip links: found at the top of the cilia and are critical to the transduction process

Question 12

What is the function of the lateral-link connectors and the tip links on stereocilia?

Answer 12

The lateral-link connectors between the shafts of stereocilia hold the bundle together to allow it to move as a unit.

The tip links are the important links which attach the shorter stereocilia to its taller neighbour, and there are non-specific cation channels at the base of the tip links which allow Ca2+ influx

Question 13

What is the result of tip-link destruction?

Answer 13

hearing loss

Question 14

What is the difference between outer hair cells and inner hair cells?

Answer 14

• Outer hair cells are directly attached to the tectorial membrane and therefore it is the physical movement of the tectorial membrane which moves the outer hair cells
• Inner hair cells are not directly attached to the tectorial membrane, and can move just due to fluid vibration of sounds

Question 15

Describe the Transduction mechanism of sound.

Answer 15

1) Basilar membrane vibrates due to fluid vibration from sound
2) Organ of Corti sitting on the basilar membrane moves, creating a shearing force in the tectorial membrane and causing it to move
3) This will push hair cell bundles sideways towards the tallest stereocilia
4) Tip-links on the tip of the sterocilia open ion channels
5) Outside of the stereocilia is a potassium (K+) rich fluid called Endolymph. When the tip-links stretch the tip of the stereocilia and distort it, they allow the influx of K+ influx, depolarising the cell
6) Non-specific cation channels at the base of the tip links then open, and Ca2+influx triggers neurotransmitter release at the synapse
7) Neurotransmitter binds to afferent nerve triggering an action potential

Question 16

What happens when the hair bundle is pushed towards the shortest stereocilia?

Answer 16

When the hair bundle is pushed in the other direction towards the shortest stereocilia, the ion channels close and there is decreased in firing of the afferent nerve.

Question 17

The endolymph however, has roughly the same composition as intracellular fluid (high in K+, low in Na+). So, what is making K+ move into the sterocilia and cause depolarisation of the hair cell?

Answer 17

There is a force which pushes K+ into the cell considering the extracellular and intracellular have the same concentration (there is no concentration gradient).

Endocochlear Potentials (charge)

• endolymph potential +80mV compared to intracellular potential of hair cell (-40 to -60mV)
• big electrical gradient which pushes positive charge into those stereocilia

Question 18

What is the advantage of using K+ to depolarise the hair cell?

Answer 18

Extremely rapid response (endolymph potential co-varies with high frequency hearing limit)

Outsourcing metabolically demanding activity (K+ moves in via electrical gradient and depolarises cell, then passively leaks back out again to repolarise the cell; no need for opening and closing of channels). Therefore, hair cells don’t have to maintain the metabolic rate necessary to deal with the rapid ion fluxes occurring as they are transducing sound.

Question 19

What do hair cells need to balance?

Answer 19

Sensitivity and saturation

Question 20

How do hair cells balance sensitivity and saturation?

Answer 20

They do this mostly via external mechanisms that either damp down or boost the vibrations of the membrane. However, hair cells can also participate:

Slow Adaptation
-top end of tip link is anchored to myosin which crawls along the actin cytoskeleton of sterocilia, maintaining optimal tip-link tension and therefore sensitivity

Fast Adaptation
-Ca2+ entering binds to transducer channels and reduces their opening, and also ‘slam’ close the channels, possibly resulting in a tug on the top-link. This could be a mechanism for speeding up repolarisation