Sight/sound (33-25)

Question 1

Is vision static?

Answer 1

No → the eye scans the subject to bring the image into the fovea

human retina ~ 576 megapixels

Question 2

How does the image captured by eyes differ from reality?

Answer 2

Image is inverted and smaller than reality
→ the brain is important in interpreting the retinal image

Question 3

How is the retina structured?

Answer 3

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

Question 4

What are photoreceptors?

Answer 4

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

Question 5

Whats the difference between rod and cone photoreceptors?

Answer 5

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

Question 6

What is rhodopsin?

Answer 6

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

Question 7

What is the retina?

Answer 7

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

Question 8

What is the fovea?

Answer 8

Where photoreceptors are most dense

Question 9

What is the point of the retina pigment epithelium?

Answer 9

Black pigment → absorbs scattered light

Question 10

What occurs during the light sensitive step?

Answer 10

Rhodopsin converted from cis to trans form
→ isomerisation occurs due to absorption of photon

Question 11

What is the order of the phototransduction cascade?

Answer 11

light → rhodopsin cis-trans → tranducin activated → cGMP phosphodiesterase activated → cGMP levels fall → cGMP-gated channels close → photoreceptor depolarises (more -ve)

Question 12

What happens when cGMP-gated channels open?

Answer 12

Ca2+ also enters inhibits granulate cyclase → inhibits cGMP production

Question 13

What occurs during constant light?

Answer 13

cGMP used up → channels close → no Ca2+ entry → intracellular [Ca2+] falls → activity of granulate cyclase increases → increased [cGMP]

Question 14

Why don’t photoreceptors fire action potentials?

Answer 14

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

Question 15

What occurs at the ganglion receptive fields?

Answer 15

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

Question 16

What is used for the transmitter in bipolar cells for signal processing?

Answer 16

Glutamate
→ excitatory - constant

on-centre → in dark photoreceptors continual inhibition by glutamate - depolarisation

Question 17

How are the central visual pathways crossed?

Answer 17

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)

Question 18

What pathways do the lateral geniculate neurons take?

Answer 18

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

Question 19

What is the shape of the visual cortex simple cell receptive fields?

Answer 19

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

Question 20

What happens when you stimulate a simple cell?

Answer 20

Correct orientation + centre → rapid APs
Correct orientation + peripheral → some APs post stimulus
Incorrect orientation → no APs
Entire cell → no APs

Question 21

What do complex cells do?

Answer 21

Detect moving bars in correct orientation of light → generate action potentials

Question 22

What are light sensitive ganglion cells?

Answer 22

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

Question 23

What are the roles of light sensitive ganglion cells?

Answer 23

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

Question 24

How can degenerating retinas be fixed?

Answer 24

Photoreceptor transplantation
→ implant stem cells into retina - differentiate into photoreceptors

Question 25

How is sound transduced?

Answer 25

Sound waves captured → transmitted → transduced (into electrical impulses)

ear canal → ear drums (vibrates)

Question 26

What is the cochlea?

Answer 26

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

Question 27

How do we measure sound?

Answer 27

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

Question 28

What is the basilar membrane?

Answer 28

A stiff structural element within the cochlea of the inner ear
→ mechanical analyser of sound
→ vibrates up and down

Question 29

How does the basilar membrane vary?

Answer 29

33mm long
→ apex is ~ 10 times wider than at the base
→ membrane is thin and floppy at apex, thicker and taught at base

Question 30

What do the vibrations of the basilar membrane do?

Answer 30

Compression causes it to flex down
Refraction causes it to flex up

Separates different frequencies of sound into different physical characteristic regions - tonotopic map

Question 31

What are the sound transducing components of the cochlea?

Answer 31

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

Question 32

What are otoacoustical emissions?

Answer 32

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

Question 33

What happens to outer hair cells in the cochlea when you hear sound?

Answer 33

Contracting and lengthening

Question 34

What is prestin?

Answer 34

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

Question 35

What is required for hearing signal transduction?

Answer 35

Links between tips of stereocilia - the sites of mechanotranduction
Mechanosensitive ion channels

→ movements of cilia generate potentials in hair cells

Question 36

How is influx of K+ into the membrane of apical hair cells induced?

Answer 36

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

Question 37

What is end-cochlear potential?

Answer 37

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

Question 38

What is the scala media?

Answer 38

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+]

Question 39

What are the advantaged of K+ in the inner ear?

Answer 39

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

Question 40

Why do hair cells have a negative resting membrane potential?

Answer 40

Their basolateral membrane is not in the high [K+] endolymph
→ drives K+ in

Question 41

How is hearing loss generated?

Answer 41

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

Question 42

What molecules are associated with deafness?

Answer 42

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

Question 43

What does the GJB2 (Cx26) mutation affect?

Answer 43

Development of cochlear if deleted early - but not if deleted later (can’t explain why 90day postnatal babies suffer hearing loss)

Question 44

What can the Cx26 deletion cause?

Answer 44

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

Question 45

What are spiral ganglion neurons?

Answer 45

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

Question 46

What is a cochlear implant?

Answer 46

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

Question 47

How is the tonotopic map preserved?

Answer 47

Axons in the cochlear nucleus terminate at different locations

Question 48

What do the different neurons of the auditory nerve fibre do?

Answer 48

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

Question 49

What is the medical superior olive (MSO)?

Answer 49

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

Question 50

How is the brain and language linked?

Answer 50

Wernicke’s area → language comprehension
Broca’s area → language production

Question 51

Overview of the cochlear:

Answer 51

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

Question 52

Over view of tonotopy:

Answer 52

Different frequency of sounds separated on basilar membrane → preserved all the way through:
→ basilar membrane
→ hair cells
→ spiral ganglion cells
→ cochlear nucleus
→ auditory cortex