Sensing mechanical stimuli, temperature and pain LEcture 4

Detecting mechanical stimuli is fundamental for sensing…

1. Hearing

2. touch

3. balance

4. proprioception

5. Sensing stomack or bladder distenction

6. osmotic pressure

7. arterial pressure

How are mechanical stimuli sensed?

• directly by ion channels

BUT

• little is knwon about the molecular machinery

How do ainimals sense physical contact

• Mechanoreceptors of their body surface

How do aquatic creates detect movement and pressure changes in the water

The lateral line system

Lateral line system strucuture

• Mechanoreceptors→ Neuromasts

• Arranged in an interconnected netword

• along the head and body

Constrasting to marine creates, terrestrial vertebrates must…

• sense vibrations in the air

How did the lateral line evolve in to the inner ear

• Most rostral part of the lateral line evolved into

  1. the inner ear

  2. cochlea (meachnosensory organ responsible for hearing

  3. balance (semicircular canalas and vestibular organ)

Why is this evolution hypothesessied: evidence

1. Lateral line and inner ear share the same meachnorecpetors: Hair cells

2. Mechanotransduction in the inner eat occurs in liquid-filled cavities

   • endolymph

Alternate explanation for this?

• convergent evolution

Hair cells→ where found

• vestibular apparatus → balance

• cochlea→ hearing

• lateral lines of fish→ waterbourne vibrations

Strucutre of the hair cells

Apex

• hair bundle→ set of sterocilia (like microvissli)

• gradually increase in height

• Next to the largest of these is→ Kinocilium

What happens when thebundle of sterocilia is bent in the direction of the largest one

Depolarised

When bent in direction of the smallest one?

hyperpolarised

Therefore, how dot ehy detect any stimulus

• from the deflection of the sterocilia in the right direction

Transduction: iono or metabotropic?

Ionotropic

• Not G-proteins!!

• much faster than that!

Transduction: How is depolarisation caused?

1. Thin tip links of cadherin connect the tip of each sterocilium to the shaft of the next longer neighbour

2. hair bundle deflected in direction of longer

3. tip links stretched

4. tension physically opens ion channels at the tips of sterocilia

5. Allow influx of cations

6. → depolarisation → K+ enters the cell (the endolymph has very high K+

Wht is this channel?

• Not really sure yest

• maybe a complex of several proteins

• non-selective ion channels

• biggest net effect of its opeing is that K+ enters the cell

What happens to this depolarisation? encoding

Does not generate an action potential!

1. Leads to increaed released of transmitter vesciles

2. at presynaptic terminals at the base of the cell

3. onto primary afferent neurons

How much of a deflection is needed to do this?

• 1/15 degree in the angle of the % um sterocila

• cause a noticeable change in membrane potential and transmitter release

Contrast to olfactory/photo receptors

• Hair cells are all pretty much the same!!!

unlike

• The 350-1000 different protein receptor types in olfactory receptors

• 3 different types of cones we have

If they are all the same, how do they respond to different things in different organs

e.g gravity, sound tones, vibrations in water

Use: different accessory strucutures

• determine which mechanical stimuli actually result in deflection of the hair cell bundles

E.g of parts of the inner ear and what they detect

1. Vestibular→ linear acceleration (gravity)

2. Semicircular canals→ rotational/angular acceleration and movements

3. Cochlea→ Sound vibrations

Vestibular organs (gravity)→ strucutre

• Saccule and utricle arranged at right angles

what consist of:

1. Array of hair cells BUT oritenated in different directions

2. embedded in gel→ DENSER than the endolymph

   • contains calcium carbonate crystals (otoliths)

What is the role of otoliths and detecting gravity!

Head is tilted

1. otolith sink undergravity

2. bends all the hair bundles in the direction of tilt

3. depolaristes that subset of hair who sterocilia are in the approraite direciton

4. BUT

5. those not in the position→ will be hyperpolarised

6. OVERALL different head tilts→ different unique codes for the direction of gravirt!

Similarities with plants?

• Use statoliths→ to detect gravity

→ BOTH: depend on dense strucutres sinking in fluid

Once detected, how is reponse coordinated

1. send neural messages to the brain

2. evalulates them against other information about head posture (visual cues)

3. almost like a calibration

4. then

5. sends reflec efferent signals to the appropriate neck or limn muscles

6. corect posutre and stop you from falling over!

Compare this reponse to plants?

• statoliths accelerate flux of growth-inhibiting auxin

Semicircular canals: sensing angular motion→ strucutre

• 3 semicircular canals

• orientated at right angles to each other (x,y,z axes)

• detect angular motion of the head

How do they do this?

1. 1. clusters of hair cells found at the base of each canal in an ampulla

2. sense the fluid movement around the canal

3. Compare the outsputs of the three canals→ accutate estimate of angular movements in 3 DIMENSIONS

Inner ear and Cochlea→ detecting sound vibrations, but with liquid?

Sound vibrations of the ear drum→ virbtrations BUT

• inner ear is filled with endolymph

BUT PROBLEM
- airborne sounds are all reflected when they strike a liquid surface

How is this solved

Middle Ear

• Ossicles (malleus, incus and stapes) in the air-filled tympanic middle ear

  → Allow air-borne sound energy to get to the cochlea!

Where is this middle ear found in organims

• frogs, reptiles, birds and mmamls

BUT needed of aquatic species

Now air born sounds can get through, how does the cochlea sense sound (strucutre)

• Has 3500 inner hair cells

• laid out in a single row along the length of a flexible 30mm membrane

  • basilar membrane

• Sterocilla loosley embedded in the overlying tectorial membrane

Strucutre of basilar membrane and how it can sense different frequencies

• Width gradually increases from base to apex

• Thin peice vibrates/ resolates at a higher frequency than a thicker peice

• When a higher tone present→ naow basal part will responate

  → these specific inner hair cells get activated

What happens when sound is detected

1. Basilar membrane vibrates in time with the sound waves

2. hair bundles on the hair cells deflect to and fro (in time with sound waves)

3. Whichever freuency it was, corresponding width of the basilar membrane will vibrate

4. So even though teh hair cells are the same, they response to different tones due to their different locations

How does the basilar mmebrane filter sound (and act like auditory prism)

• different tone freunqcies of different groups of hair cells

• acitivate different groups of inner hair cells

→ can filter out from 20 to 20kHz

How is this sound freunciy detection passed on

1. Each inner hair cell communicates synaptically with

2. several afferent nerve fibres (which do not contact any other hair cells)

3. Has own set of personalised afferent nerve fibres!

Overall: detet specific frenecy→ encode for specific frequency

• Activity in ‘x’ particular nerve fibres→ ‘x’ particular frequency is being playwed

What is the mechanism called

Labelled lines coding meachnism

Can plants hear sounds?

• Seem to be able to sense specific vibrations

• e.g caterpillars

Can plants make sounds

• Apprently they get stressed with chewing sounds!

Different things we must detect in out skin (somatosensation)

1. light touch

2. vibration

3. pressure

4. nociception

All use different sensitive receptors

Types of mechnosensitive response in skin

1. Meissner corpuscles

2. Merkel cells

3. Ruffini endings

4. Pacinian corpuscles

5. free nerve enedings

How do these all differ?

• morphology

• innervation pattern

• location in the skin

• receptive field sie

• phyiological response

But they all transduce the signal the same

1. ion channels in the nerve terminals are activated

• by mechnical stimuli

DUE TO

• Stretch or Deform the cell membrane

  • Directly

• or

  • via linker proteins

2. THEN→ mechanical displacement leads to ion channel opeing and depolsiation!

Overall how tranduce signal

1. Stretch or deform ion channels (mechnosensitive ion channels)

   • directly

   • or via linker proteins (spring open thingy)

2. Opens channels

3. Depolarisation

What are these meachnosensitive ion channel?

Piezo channels

What are Piezo channels used by?

Merkel cells→ the most sensitive mechanoreceptor cell

Strucutre of Piezo channels

• largest proteins knwon

• three subunits

• but with 30 transmembrane helices

• In plants and protists!

If all these touch receptors use ion channels, they most encode and adapt differently for specific sensations

Major differences in the speed of their adaptationn

• FAST→ Meissner and Pacinian

• SLOW→ Merkel and Ruffini

Touch in plants?

• Mimosa pudica

• Venus fly trap

Mixing up sensory modaliteies

• Some taste sensing is done my touchin octupuses

How is temperature sensed?

• Useing Transient Receptor Potential (TRP) ion channels

• 6 transmembrane domains proteins

• gated by different temperates and/or chemical ligands

Where are they also used?

• invertebrates photoreceptions

Features of TRPs

• non-selective tetrameric cation channels

• most with infinite Ca2+ permeability

• Polymodal activation and regulation

How are they polymodal actiavted/regulated?

• once channel is opened by several things

→ So have many phyiological roles nad found in many sensory receptors

E.g TRPV1 in pain nerve enedings opened by

1. Chilli type chemicals

2. acid (H+)

3. nasty electrophilic chemicals (mustard)

4. mechanical stimuli

5. painfully hot/cold temperatures

6. indireclty by tissue damage