lecture 15 Flashcards
Name structures that light passes through on the way to retina
- Cornea: translucent connective tissue light first passes through
- Aqueous humour: fluid behind cornea
- Lens: rounded structure or can be flattened to change directionality of light (malleable)
- vitreous humour: jelly like behind lens
- Retina: layer that contains photoreceptors (where light will fall behind eye)
when light is sensed in the retina
photoreceptors transduce light energy into an electrical signal (AP) to the brain via the optic nerve
what are the two regions that do the most bending
- Cornea (2/3, right away)
- Lens (1/3 malleable)
what is the key point to see an image clearly
focal point needs to fall on the retina for clear vision
mechanism of lens accommodation
- when ciliary is relaxed ligaments are pulling causing lens to flatten (helps focus retina on far objects)
- when ciliary muscle contracts ligaments are let go and lens assumes it’s normal rounded shape (helps focus retina on close objects)
- loose accommodation with age
the fovea and the macula
region of greatest focus and center of visual field
fovea within the maculla
opsin
- light activated GCPRs
- start photo transduction cascade
- are proteins
outer and inner segments of photoreceptors
outer segment: light sensing part of photoreceptors
inner segment: cell business part where you hav nucleus, mitochondria, ER, etc.
preventing scatter means
preventing light from reflecting back
Rods
- responsible for low light or night vision
- opsin GCPR expressed in rods is rhodopsin which is a photo pigment that is a signal transducer
cones
- are responsible for sharp vision and color vision
- have 3 different types of cones each have a different photo pigment (related to rhodopsin) and responds to a specific color (red, blue, green)
what can we do with just 3 cone types
- see 200 different hues (color)
- see 20 levels of saturation
- see 500 levels of brightness
retinal is a derivative of
vitamin A
what is required for opsin to become photosensitive
11-cis retinal
what activates opsin
the conformational change from 11-cis retinal to all-trans retinal
rhodopsin=
opsin + retinal
when opsin and retinal are tightly bound
rhodopsin is inactive
in outer segment
- there is intracellular flattened disks where we find the opsin
signal transduction in rods (darkness)
- 11-cis retinal is bound to opsin which mean rhodopsin is inactive and transducin is off
- cyclic GMP levels are high because transducin is inactive so phosphodiesterase is inactive
- high levels of cGMP lead to opening of CNG Na+ channels which cause an influx of Na+ and rod cell is depolarized
- leading voltage gated Ca2+ channels to open
- Ca2+ enters cell and causes neurotransmitter glutamate release onto bipolar cells so they are inhibited
photo transduction in rods (in light)
- a photon of light is absorbed by 11-cis retinal and it photoisomerizes to all trans retinal (kink—> unkink)
- this causes a conformational change that activates rhodopsin and opens up a binding site for transducin to be activated
- transducin activates phosphodiesterase that breaks down cGMP
- cGMP levels decrease which causes CNG channels to close
- which causes cell to hyperpolarize
- voltage gates Ca2+ channels close which results in less Ca2+ to enter
- this causes a decrease in neurotransmitter glutamate release so we remove inhibition on bipolar cells
- this causes bipolar cells to excite which causes a receptor potential which then cause release of neurotransmitter glutamate
- retinal ganglion cells will excite and cause and action potential
ganglion cells axons form _________ and project to _______
- optic nerve
- CNS
Pathways for vision
- optic nerve
- then we cross at optic chiasm
- optic tract
- thalamus
- visual cortex (occipital lobe)
Loss of accommodation is called
Presbyopia
