sight, taste and smell Flashcards
what is the spectrum of visible light?
400 nm - 700 nm, blue at lower end, red at higher end
describe the gross anatomy of the eye
pupil - lets light in
iris - muscles controlling amount of light entering the eye
Cornea - transparent covering of pupil and iris, refracts light onto retina
Sclera - outer layer (minus cornea), tough and protective, maintains shape
Extraocular muscles, controlled by oculomotor nerve (CN III)
Optic nerve -carries axons from retina to brain (CN II)
what is the retina?
where light is refracted on to, has cones and rods, fovea has highest visual acuity and is surrounded by the macula (good but visual acuity due to fewer blood vessels in this area)
where are the blood vessels of the eye?
at the optic disk, the blind spot
what is the aqueous humor and the vitreous humor?
aqueous humor is behind the cornea and supports cells without the need for blood vessels which would interfere with refraction
vitreous humor is fluid filled main body of the eye and maintains the shape
how does the cornea refract light?
due to it having a significant difference in refractive index to air
refracts 80% of light in the eye
define focal length
why does the lens become thinner for distant objects?
focal length = distance from refractive surface to point of convergence of parallel light rays
light rays from far apart are closer to being parallel, which requires less refraction, which the accommodates for
to flatten and weaken the lens (far objects) what happens?
suspensory ligaments tighten and ciliary muscles relax
what is:
1) emmetropia
2) hyperopia
3) myopia and how are they fixed?
1) things are all working correctly
2) far-sight, close objects are harder to see because light converges behind the retina as eye is too short - use a convex lens to increase refraction
3) short sight, far objects are harder to see as light converges too early, before the retina, so use a concave lens to make light rays less parallel
the retina has laminar organisation
how does this not effect the photoreceptors of the fovea?
they bend out of the way
there are 7 layers in the organisation of the retina, what are they?
the ganglion cell layer, closest to the vitreous humor, outputs info from retina to brain
inner plexiform layer - synapses, between ganglion, amacrine and bipolar cells
inner nuclear layer - amacrine cells, horizontal cells involved in modulating info
and bipolar cell bodies
outer plexiform layer - closer to back of the eye - synapses, between bipolar and photoreceptor cells
outer nuclear layer - cell bodies of the photoreceptors
photoreceptor outer segments
pigmented epithelium - absorbs light the photoreceptors don’t to prevent reflection back to the retina, increasing visual acuity
*signal transduced at photoreceptors (GP), bipolar cells (GP) send signal onto ganglion cells (AP)
where are the light-sensitive pigments in photoreceptors?
membranous disks
compare rods and cones
Rods -
High sensitivity
More disks, with higher concentration of photopigments in each disks, so 1000x more sensitive than cones (so at night - only rods used, so no colour seen)
Low resolution - many receptors to one bipolar neuron
Cones -
High resolution/visual acuity (and colour)
Fewer disks and lower photopigment concentrations
Not as easily saturated (less sensitive) so better at continuous response to stimuli
where are rods and cones found?
fovea = only cones
blind spot = neither
either side of the fovea = lots of rods maybe a few cones
peripheral retina = mostly rods
central retina = more cones
why in the peripheral retina are there many bipolar neurons synapsing eith one ganglion cell?
allows for high sensitivity (light levels are lower0 but low acuity as it is unclear exactly where the light came from, seeing as there is multiple inputs
what photopigments are used and why? why are some relied on more at different times?
rhodopsin in rods, only ones used at night when we are most sensitive to 500 nm
S, M and L opsins in cones for short medium and long wavelengths, cones relied on more in the day when we are most sensitive to 560 nm
retinal ganglion cells have melanopsin which allows them to respond to huge changes in light, important in circadian rhythm, distinguishing between night and day
what makes photoreceptors so unusual (and how does it work)?
they are depolarised at rest and hyperpolarised when activated
ligand gated ion channel on the inside, ligand being cGMP
when it’s dark cGMP keeps these channels open allowing influx of cations (mostly Na+), keeping the cell depolarised (open K+ channels allow K+ to leave so as not to depolarise too far)
when its light cGMP levels drop, channels close, hyperpolarise the cell
explain the signalling cascade in photoreceptors
the opsin is a GPCR in the membranous discs
this GPCR has a small molecule - retinal - within
photons change the retinal from cis to trans, exchanging the GPCR’s GDP for GTP
alpha subunit activates phosphodiesterase, an enzyme that converts cGMP to GMP, meaning light causes low cGMP levels and the LGICs close
what occurs in saturation (rods vs cones)?
it’s when cGMP levels are so low you have closed all the ion channels you can and cannot hyperpolarise any further
rods are easily saturated, cones are able to maintain a longer response to high light intensities
rhodopsin gets bleached and time is required to reset retinal back to it’s cis state
how does light adaption work, specifically the role of calcium?
initial light = high hyperpolarisation
if it continues, photoreceptors gradually depolarise in order to make room for response if light intensity increases
the LGICs allow in calcium as it is a cation
calcium inhibits guanylyl cyclase, the enzyme making cGMP, so in the dark calcium comes in and prevents too much cGMP form overly depolarising the cell
in the light, when LGICs are closing, no calcium allows guanylyl cyclase to go crazy, opening those channels and allowing depolarisation even in the presence of light which initially causes hyperpolarisation
