Senses Flashcards
fovea
where photoreceptors are most dense
structure of retina
highly organised layered structure
light from bottom/back where ILM/muller cell endfeet
photoreceptors at opposite end
info from photoreceptors to ganglion cells, axon from ganglia cells form optic nerve
why does light come in at opposite end to where photoreceptors are?
don’t want light to bounce around eye ball because won’t be able to see if light is scattered
retinal pigment epithelial is black so light can’t get reflected off so can be detected here
photoreceptors
in rod cell
outer segement where photopigment is located on discs
cone cell also has outer segment
rods
high sensitivity low temporal resolution more sensitive to scattered light low acuity achromatic
496nm
cones
lower sensitivity high temporal resolution most sensitive to direct axial rays high acuity trichromatic
419, 531, 559nm
rhodopsin
photopigment
light hits rhodopsin
light turns 11-cis retinal to all-trans retinal
changes conformation of opsin (ligand binding to GPCR) - activates transducin (specialised G protein)
trans-retinal dissociates from protein and recycled back to cis via retinal pigment epithelium (which is why outer segments of photoreceptors close association with pigment epithelium)
ganglion cells have receptive fields
light into central field: ganglia excited and AP
light in outer circle: inhibit AP
(off centre field has opposite effect)
receptive fields
are concentric
photoreceptors and receptive fields converge..
onto bipolar cell and then ganglion cell
light not on ganglion cell itself but on receptors wired to ganglion cell
convergent signalling in retina
2 ganglion cells with receptive fields adjacent to each other
they share photoreceptors so receptive field contribute to both ganglion cells
photoreceptor to bipolar cells, on-centre
voltage response in wrong direction
retina turns upside down
glutamate as NT but can be inhibitory in retina
more depolarisation means more glutamate (off-centre)
on centre: glutamate inhibits so light causes less glutamate so less inhibition and depolarisation in bipolar cells.
on-centre ganglion cells
off-centre
signal rapid increases in light intensity
signal rapid decreases in light intensity
lateral geniculate neurones have…
concentric visual fields
M channel
P channel
analysis of movement
analysis of fine detail and colour
receptive fields of simple cells in visual cortex
rectangle
specific retinal position
discrete excitatory and inhibitory regions
specific axis of orientation
all axes of orientation are represented for each part of the retina
complex cells
no on off surround
sensitive to bar of light along long axis but has to be moving
for firing if bar not orientated properly
general principles on eye lecture
convergent processing
receptive fields
hierarchial processing
ordered maps in the brain
rods and cones are not the only photoreceptors in the retina
light sensitive ganglion cells (ipRGC)
melanopsin
ancient opsin in ipRGC
control how pupil dilates and light entrained circadian clock
P
sound pressure
loudest tolerable sound
120dB SPL
basilar membrane
mechanical analyser of sound
3 compartments of cochlea are filled with fluid
IHC inner hair cell
OHC - outer
width varies along its length
membrane thin and floppy at apex and thicker and taught at base
thick stiff - high frequency sounds
vibrations of the basilar membrane
round window pushed out while oval window pushed in because fluid goes round
creates standing wave, created at place depending on frequency
hair cells
sound transducing components of cochlea
hair cells have sterocilia - organise and in contact with tectorial membrane
hair cells vibrate and cause potential in hair cells
vibrations of basilar membrane
presses against tectorial membrane and sterocilia of hair cells are deflected in a direction
why are different hair cells in diff locations
look at sound frequency most efficient at activating each hair cell, so need to be in diff places to be efficient
why must there be a way to amplify sound especially at low sound intensities?
sensitivity of cochlea too great and frequency selectivity too sharp to result solely from passive mechanical properties
active mechanisms of the cochlea
outer hair cells change their length in response to sound
pulls on membrane
triangle vibrates
and amplifies souns because making vibrations bigger
prestin
motor protein in plasma membrane
mechanosensitive ion channels in sterocilia open when sterocilia pressed
current changes membrane potential of outer hair cells and change in motor protein in membrane (conformational change)
OHC doesn’t develop without prestin
499 prestin mutation
no voltage sensitive conformational change (change length)
removes electromotility from hair cells
increases threshold for hearing across the frequency range
mechanosensitive ion channels
hardly any effect on hearing when knock it out
maybe few channels only in stereocilia
potassium and calcium entry
how are potential generated in hair cells?
movements of cilia generate membrane potential - graded with changes in potential in stereocilia
scala vestibuli and scala tympani have…
normal extracellular fluid
high sodium, low potassium
scala media
high potassium in fluid
because of stria vascularis (spiral ligament)
stria vascularis
contains marginal cells - with tight junctions between them
secretes potassium rich scala media
endocochlear potential
80mV - 120mV
provides driving force on potassium to give inward currents into hair cells during mechanosensory transduction
potassium through gap junctions through basal cells to intermediate cells where pumped out/ion channels into space
marginal cells pumping potassium out cells so potassium can diffuse in
then through potassium ion channels to endolymph - creates endocochlear potential
like opposite resting potential
hearing loss
1:800 children born with hearing impairment
> 60% of people older than 70
> 50 chromosomal loci associated with non-syndromic hearing loss
> 14 genes identified
GJB2 (Cx26) mutation
most common hearing loss K recycling reduce endocochlear potential interfere with cochlear development if deletion early (postnatal days) later on deletion - doesn't affect
hair cell degeneration
less electromotility
each spiroganglia cell inovates only 1 hair cell, why?
separate sound frequencies
first place to get input from both ears
medial superior olive - spatial localisation of sound
Wernicke’s area
Broca’s area
language comprehension
language production
