Sight/sound (33-25) Flashcards
Is vision static?
No → the eye scans the subject to bring the image into the fovea
human retina ~ 576 megapixels
How does the image captured by eyes differ from reality?
Image is inverted and smaller than reality
→ the brain is important in interpreting the retinal image
How is the retina structured?
Layered structure of neurones interconnected with synapses
→ pigment epithelium
→ photoreceptor outer segments: rods/cones
→ outer nuclear layer
→ outer plexiform layer: pedicles spherules
→ inner nuclear layer: horizontal bipolar cells
→ inner plexiform layer: amacrine cells
→ ganglion cell layer: glaglions, axons
→ nerver fibre layer: muller cell end feet
What are photoreceptors?
Convert light stimuli into electrical nerve impulses (transmitted to via bipolar cells to ganglion cells)
→ rod/cone structure
→ made up of photoreceptor sensory cilium and cell body
Whats the difference between rod and cone photoreceptors?
Rods (outnumber cones 20-fold)
→ high sensitivity: more photopigment, high amplification, saturate in daylight
→ low temporal resolution - slow response - longer integration time
→ more sensitive to scattered light
→ low acuity - high convergence in retinal pathways, achromatic
Cones:
→ lower sensitivity: less photopigment, less amplification, saturate only in intense light
→ high temporal resolution: fast responding - short integration times
→ most sensitive to direct axial rays
→ high acuity - low convergence, concentrated in flea
→ trichromatic: 3 types of pigment, each sensitive to different parts of visible spectrum
What is rhodopsin?
Photopigment
→ 10^8 pigment molecules/rod
→ 7 transmembrane segments
→ 348 aa
→ homologous to GPCRs
isomerisation of retina changes the conformation of the opsin (c.f. ligan binding to GPCR) which leads to activation of transducin - a specialised G-protein
→ all trans retinal dissociates from the protein and is recycled via the retinal pigment epithelium
→ outer segments of photorecprots are in close association with the pigment epithelium
What is the retina?
Complex bit of neural machinery
→ detects light and involved in first processing of visual information
→ light focused onto the retina at the back of the eye
What is the fovea?
Where photoreceptors are most dense
What is the point of the retina pigment epithelium?
Black pigment → absorbs scattered light
What occurs during the light sensitive step?
Rhodopsin converted from cis to trans form
→ isomerisation occurs due to absorption of photon
What is the order of the phototransduction cascade?
light → rhodopsin cis-trans → tranducin activated → cGMP phosphodiesterase activated → cGMP levels fall → cGMP-gated channels close → photoreceptor depolarises (more -ve)
What happens when cGMP-gated channels open?
Ca2+ also enters inhibits granulate cyclase → inhibits cGMP production
What occurs during constant light?
cGMP used up → channels close → no Ca2+ entry → intracellular [Ca2+] falls → activity of granulate cyclase increases → increased [cGMP]
Why don’t photoreceptors fire action potentials?
Close to next processing structure → no need to fire AP if you don’t need to
ganglia cells do fire APs → AP good over long distances
What occurs at the ganglion receptive fields?
On centre field (off surround)
light on centre → rapid APs
light on peripheral → inhibits APs
Off centre field (on surround)
light on centre → inhibits APs
light on peripheral → rapid APs
What is used for the transmitter in bipolar cells for signal processing?
Glutamate
→ excitatory - constant
on-centre → in dark photoreceptors continual inhibition by glutamate - depolarisation
How are the central visual pathways crossed?
Hemi-retina - one half of retina of one eye
→ left of each eye goes to the left side of the brain
→ visual information from the retina projected to the brain in an ordered fashion (visuotopic)
What pathways do the lateral geniculate neurons take?
Concentric visual fields (circular)
On-centre field → M pathway P pathway
Off-centre field → M pathway P pathway
M channel - analysis of movement
P channel - analysis of fine detail and colour
What is the shape of the visual cortex simple cell receptive fields?
Visual cortex → receives, integrates, and processes visual information relayed from the retinas
Receptive field → rectangle - you can work out orientation, they have:
1. specific retinal position
2. discrete excitatory and inhibitory regions
3. specific axis of orientation
4. all axis of orientation are represented for each part of the retina - many synapse onto the same simple cell
What happens when you stimulate a simple cell?
Correct orientation + centre → rapid APs
Correct orientation + peripheral → some APs post stimulus
Incorrect orientation → no APs
Entire cell → no APs
What do complex cells do?
Detect moving bars in correct orientation of light → generate action potentials
What are light sensitive ganglion cells?
A subset of ganglion cells directly respond to light
→ rods and cones aren’t the only photoreceptors
Melanopsin → only found intrinsically in ipRGC - an ancient opsin, encoded by OPN4M, related to invertebrate opsins
What are the roles of light sensitive ganglion cells?
Detect light and send it to optic nerve - fix circadian clock (circadian clock out of time with environment clock - jet lag)
Pupillary dilation
Low acuity images
How can degenerating retinas be fixed?
Photoreceptor transplantation
→ implant stem cells into retina - differentiate into photoreceptors
How is sound transduced?
Sound waves captured → transmitted → transduced (into electrical impulses)
ear canal → ear drums (vibrates)
What is the cochlea?
Cochlea → hollow, spiral-shaped bone found in the inner ear that plays a key role in the sense of hearing and participates in the process of auditory transduction
→ fluid filled - not compressible
round window → bulges out when fluid moves
How do we measure sound?
Decibel sound pressure level
→ logarithmic measure of sound relative to a threshold
dB SPL = 20 x log10(P/Pref)
P = sound pressure
Pref = threshold for human hearing 4kHz
What is the basilar membrane?
A stiff structural element within the cochlea of the inner ear
→ mechanical analyser of sound
→ vibrates up and down
How does the basilar membrane vary?
33mm long
→ apex is ~ 10 times wider than at the base
→ membrane is thin and floppy at apex, thicker and taught at base
What do the vibrations of the basilar membrane do?
Compression causes it to flex down
Refraction causes it to flex up
Separates different frequencies of sound into different physical characteristic regions - tonotopic map
What are the sound transducing components of the cochlea?
Outer hair cells, inner hair cells
~ 30,000 hair cells in the 2 cochlea
→ like cilia but organised
→ turn waves into electric signal
→ can be damages - loud rock concerts
vibrations of basilar membrane → inner/outer hair cells move forwards/backwards
What are otoacoustical emissions?
Sounds produced by the ear
→ given off by one small part of the cochlea when it is stimulated by soft clicking sounds
sounds given off by one small part of the cochlea when it is stimulated by soft clicking sounds.
→ when the sound stimulates the cochlea, the outer hair cells vibrate
→ the vibration produces a nearly inaudible sound that echoes back into the middle ear
What happens to outer hair cells in the cochlea when you hear sound?
Contracting and lengthening
What is prestin?
Motor protein in the plasma membrane - found in OHC of cochlea
→ to drive the cochlear amplifier, increasing auditory sensitivity and sharpening frequency tuning
→ removing prestin changes morphology of OHC
What is required for hearing signal transduction?
Links between tips of stereocilia - the sites of mechanotranduction
Mechanosensitive ion channels
→ movements of cilia generate potentials in hair cells
How is influx of K+ into the membrane of apical hair cells induced?
Mechanosensitive transducer channels at the tip of the hair cell stereocilia allow K+ to flow into cells
→ endocochlear potential provides the driving force on K+ to give the inward current into hair cells during mechanical-sensory transduction
→ K+ leaving cells - creates -ve resting potential
→ K+ entering cells = encocholear potential +80 charge
occurs at stria vascularis
What is end-cochlear potential?
K+ diffusion potential generated across the KCNJ10 K+ channel by the very low [K+] in intrastrial fluid spaces and the high [K+] in the cytosol of intermediate cells
What is the scala media?
Flui filled compartment - one of the 3 canals that run the length of the cochlea in the inner ear
→ has stria vascularis - endolymph: generated fluid from blood with high [K+]
What are the advantaged of K+ in the inner ear?
Influx of K+ ions into the sensory cells causes the least change in cystolic conc compared to any other ion → K+ is the most abundant ion in the cytosol
Don’t need to spend energy pumping K+ out → influx and extrusion of K+ are energetically inexpensive for the sensory cell since both occur down an electrochemical gradient
Why do hair cells have a negative resting membrane potential?
Their basolateral membrane is not in the high [K+] endolymph
→ drives K+ in
How is hearing loss generated?
1:800 children born with serious hearing impairment
>60% of older people suffer sufficient haring loss to benefit from a hearing aid
>50 chromosomal loci associated with non-syndromic haring loss
>14 genes indetified
What molecules are associated with deafness?
Some of the molecules in hair cells
→ mutations in K+ channel - don’t have proper resting potential
mutations affect potassium recycling in cochlea → effect development of endococholear potential
What does the GJB2 (Cx26) mutation affect?
Development of cochlear if deleted early - but not if deleted later (can’t explain why 90day postnatal babies suffer hearing loss)
What can the Cx26 deletion cause?
Hair cell degeneration → (hair cells don’t express connexions) - degeneration can take time to occur
Affects electromobily of the outer hair cells → OHCs don’t express connexions
The OHCs still shows electromotility but the active cochlear amplification is reduced
What are spiral ganglion neurons?
The afferent neurons contacting hair cells
→ several spiral ganglion cells innovate one hair cell
→ but don’t make multiple connections to different hair cells - as they have gone to the effort to separate waves out by frequency
→ also code the intensity of sound - bigger response as intensity of sound increases
What is a cochlear implant?
A small, complex electronic device that can help to provide a sense of sound to a person who is profoundly deaf
→ thread an electrode around cochlear to stimulate spiral ganglion neurons
How is the tonotopic map preserved?
Axons in the cochlear nucleus terminate at different locations
What do the different neurons of the auditory nerve fibre do?
Different neurons extract different features of the firing pattern - sound
PST = peristimulus histogram
Pyrimidal → pauser PST
Octopus cell → onset PST
Globular blush cell → primary-like with notch PST
Multipolar cell → chopper PST
Spherical blush cell → primary-like PST
→ relays response from autarky nerve
What is the medical superior olive (MSO)?
An important brain center that computes sound location by comparing small differences in arrival time at the two ears
→ sound from a source nearest to one ear reaches that ear quicker than the other ear giving rise to inter-aural delays
→ MSO gets input from both ears
→ projection of inputs from the cochlear nucleus to the MSO and wiring within the MSO gives rise to a place code
→ condition pathway length changes depending on location - encodes position of sound
How is the brain and language linked?
Wernicke’s area → language comprehension
Broca’s area → language production
Overview of the cochlear:
Transduction of sound to electrical activity occurs in cochlear
→ the cochlear is an active sound transducer - mechanically amplifies the discord od the basilar membrane caused by sound pressure
→ mechxnosensitive ion channels transduce distortion of sterocilia into graded changes of membrane potential
Over view of tonotopy:
Different frequency of sounds separated on basilar membrane → preserved all the way through:
→ basilar membrane
→ hair cells
→ spiral ganglion cells
→ cochlear nucleus
→ auditory cortex
