topic #9: sensory transduction l

what are all animals capable of

- capable of sensing & responding to signals in external environment (light, sound, touch, vibration,heat.taste, oder,etc)

- as well as internal signals (muscle or tendon stretch, joint movement, body temp, pH, glucose [ ])

what do nervous systems have

have evolved specialized sensory cells that receive sensory info & transduce (transform) it into neural activity

what is adequate stimulus

specific sensory modality that most commonly activates a sense organ

what is sense

ability to detect a modality

what are the major classes of sensory cells ( based on "adequate stimulus)

- mechanoreceptors

- photoreceptors

-chemoreceptors

- thermal ( warm & cold) receptors

- nociceptors

-electroreceptors

-magnetoreceptors

what are mechanoreceptors

w/ stretch gated cation channels

what are photoreceptors

G- protein coupled receptors ; rhodopsin

what are chemoreceptors

Ex:taste & olfactory receptors

- all olfactory receptors are G-protein-coupled

- some taste receptors are G- protein coupled (sweet & most bitter)

- other taste receptors involve direct interaction btwn tastant molecules & ion channels (sour,salt, some bitter)

what are thermal (warm & cold) receptors

in skin, warm & cold receptors

- cold receptors are activated by menthol

- in pre-optic/anterior hypothalamus, mammals have warm & cold receptor neurons

- infrared receptors in "pit-organs" of some snakes

what are nociceptors

pain receptors

- found in skin, cornea, muscle, joints, tooth pulp & viscera

- some are mechanically sensitive, responding to intense tissue-damaging stimuli, others are "polymodal" (responding to mechanical chemical & temp stimuli )

what are electroreceptors

ampullae of lorenzini & tuberous organs

mainly in fish

- also found in bills of platypus & spiny anteater

what are magnetoreceptors

- respond to weak magnetic field

- found in migratory birds, bees & some bacteria

- presumed receptors contain "magnetite" - ferrous ferrite & very little known about those receptors

* recent evidence also suggest that a light-sensitive protein in eyes of animals (from flies to birds to humans), called "cryptochrome" can also sense earth's magnetism

what are EOD

electric organ discharge

*wave type EOD (hummers)

what are the 2 types of electroreceptors

- tuberous

- ampullary

what are receptors pot's

graded change in memb pot of a sensory receptor cell in response to sensory stimulation

- amplitude of pot is proportional to intensity of stimulus

what is transduction

transformation of energy first into graded receptor pot - then into a spike freq code

how do receptor pot & freq coding work

- size of a sensory stimulus determines size of receptor pot

- this, in turn, is encoded in freq of firing of sensory cell &/or its postsynaptic neuron

what are "short receptors"

- some sensory receptors are specialized non-neuronal cells that transduce sensory info, produce local graded sensory pot's (no APs) & release neurotransmitter onto 1 primary sensory neurons ; e.g. Merkels disks in skin, vestibular & cochlear hair cells, photoreceptors , taste bud receptors

what are "long receptors"

- other receptors are neurons, have axons capable of conducting APs & transmitting sensory info over long distances in spike-freq code " e.g, most skin mechanoreceptors, muscle & tendon stretch receptors, olfactory receptors, temp (warm & cold) receptors, pain receptors

what are mechanoreceptors

- respond to mechanical displacement/stretch

ex: skin mechanoreceptors, hair cells of inner ear (vestibular & cochlear)

what is adaptation

decreasing sensory response to maintained stimulus

what are the diff types of cutaneous mechanoreceptors

Merkels disks = steady skin indentation [SA= slowly adapting]

reffinis endings = steady skin indentation [SA= slowly adapting]

hair follicle receptors = flutter (velocity detectors ) [RA= rapidly adapting]

meissners corpuscles = flutter (velocity detectors) [RA= rapidly adapting]

pacinian corpuscles = vibration (acceleration detectors) [ v.RA= very rapidly adapting]

free nerve endings = damage/pain [v.SA= very slowly adapting]

what are piezo ion channels

trimers & are shaped like propeller (or triskelion) w/ around a central pore

what is piezo 1

mechanosensitive channels expressed in non-sensory tissues like lung, bladders, skin red blood cells

What is Piezo 2?

mechanosensitive channels expressed in somatosensory neurons, responsible for tactile sensation & proprioception

what is a "cap"

C- terminal extracell domain , CED

what is pacinian corpuscles

- subcutaneous, around claws & footpads, interosseous membs of leg, forearm

- vibration-sensitive

- very rapidly adapting

removal of layered capsule changes what

changes receptor pot from rapidly to slowly adapting

- local anesthetic (procaine) was used in this experiment to block APs

what did most recent work by pawson show

- pacinian corpuscles are actually "mechano-chemical" receptors

- modified schwann cells making up layered corpuscle release both glutamate & GABA during mechanical stimulation

- GABAergic inhibition appears to contribute to very rapid adaptation of nerve ending

- gabazine or picrotoxin (GABA antagonists) enable slowly adapting activity

what do hair cells of inner ear contain

vestibular organs & cochlea

what do vestibular organs do

- balance organs

- provide info about static position (tilt) & acceleration of head in space

what is function of cochlea

- hearing organ

- provides info about intensity & freq of sound

what is the location of stretch activated ion channels

near tip links connecting tips of stereocilia

what was A.J hudspeth experiment when you move hair bundle toward kinocilium & away from it

extracel recording

movie hair bundle toward kinocilium --> increase K+/Ca2+ influx through stretch gated cation channels at tips of stereocilia --> depolarize hair cell --> open voltage gated Ca channels --> increase transmitter release

move hair bundle away from kinocilium --> decrease K+/Ca2+ influx through stretch-gated cation channels --> hyperpolarize hair cell --> close voltage gated Ca channels --> decrease transmitter release

what did sakaguchi schematic representation of tip link complex illustrating show

- CDH23 & PCDH15 comprise tip link, which inserts into stereocilia memb at sites of upper & lower tip densities

- tip densities are presumed to contain scaffolding proteins

what is MET

mechanoelectrical transduction channel

- MEt is a heteromeric dimer of TMC1 & TMC 2 proteins

what are TMC

transmemb channel like protein

what is Ushers syndrome

causes deafness via mutation that disrupts tip link protein "protocadherin 15"

what did the elusive hair cell transduction channel revel

- family of transmemb proteins has been shown to modulate both Ca permeability & single channel conductance of vert hair cell mechanosensor, implicating them directly in inner ear mechanosensation

- Beethoven (Bth) mouse mutant = exhibits progressive hearing loss & eventual deafness

- pan et al showed that Bth mice, which carry a point of mutation in transmemb channel-like 1 gene (TMC1) , exhibit reduced single channel conductance & Ca++ permeability in hair cells compared to wild type mice during mechanical stimulation

how can K+ flow in to hair cells through stretch gated cation channels ? it normally flows out of cells, doesn't it ?

FIRST:

- endolymph bathing hair bundles has high K+ & low Na+ [ ] 's compared to perilymph bathing hair cell bodies (low K+ & hgh Na+)

- high K+ [ ] of endolymph makes K+ equilibrium pot less (-) than in other cells & produces a memb pot of only about -45 mv

SECOND:

- endolymph as positive exrtracell pot of +80mV

- total resting memb pot of hair cells, relative to endolymph is about -125mV!! (-45mV resting pot relative to an outside +80mV in endolymph

cochlear hair cells encode 2 types of info about sound?

- intensity

- freq

how is sound intensity encoded

- relatively straight forwrd translation of increasing sound intensity (larger ampli sound waves) into larger movements of hair bundles --> larger memb pot oscillations in hair cells --> larger changes in transmitter release --> increased # freq of action pots in postsynaptic auditory neurons

what are the 2 mechs for encoding sound freq

1. mechanical properties of cochlea & amplification by outer hair cells (mammals)

2. electrical resonance properties of hair cells (non-mammalian verts)

how does freq work in mammals

- cochlear hair cells are tuned to diff sound freqs by their positions in cochlea

- sound freq is encoded by mechanical properties of cochlea

base of cochlea --> hair cells sensitive to high freq sound (nearest eardrum)

apex of cochlea --> hair cells sensitive to low freq sound

HOW??

- low freq sound causes greated vibration at apex; high freq sound causes greatest vibration at base [discovered by georg von bekesy]

in mammals, how is the cochlea constructed

- contains 3 rows of outer hair cells & 1 row of inner hair cells

Inner hair cells : transduce sound & pass afferent sound info on to brain

Outer hair cells : act as cochlear amplifiers that boost amplitude & freq selectivity of basilar memb vibrations

- outer hair cells do not participate in sound transduction

- exhibit "electromotility" - they change their length in response to a change in Em

how is the cochlear vibrations produced

produced by outer hair cells are strong enough to generate sound in form of spontaneous or evoked otoacoustic emissions (OAE), 1st described by david kemp

where do outer hair cells amplify motion

- amplify motion of cochlear partition (basilar memb & organ o corti) via active contraction during depolarization (corresponding to rarefaction phase of sound wave) & elongation during hyperpolarization (corresponding to compressive phase of sound wave )

in birds, reptiles & amphibians where does freq tuning occur

- occurs via electrical resonance properties of hair cells

- mo outer hair cells in these species

cochlear hair cells in birds, reptiles & amphibians exhibit what

- exhibit spontaneous (endogenous) memb pot oscillations

- these oscillations occurs at diff freqs in diff hair cells

- cells located at base of cochlear exhibit high-freq oscillations ; cells at apex exhibit ;ow-freq oscillations

- characteristic oscillation freq of these cells decrease continuously along length of cochlea from base to apex

when a hair cell is stimulated by sound at its characteristic "resonance freq" what does it exhibit & what does a depolarizing & repolarizing phase cause

exhibits a maximal electrical response (memb pot oscillation)

*depolarizing phase of these oscillations --> produced by Ca++ influx

* repolarizing phase produced by --> calcium-activated K+ efflux (Ik(ca))

- diff btwn high & low freq cells is the fast vs slow activation of Ik(ca)