Retina Phototransduction Flashcards
What is Cataracts and what causes it? What are the symptoms?
Clouding of the lens that affects vision; it is due to disruption of the order of the organization of the lens cell fibers or aggregation of proteins. Symptoms are hazy vision, poor night vision, glare and faded colors
What is Glaucoma and how is it caused? What are the symptoms?
Group of diseases that damage the eye’s optic nerve and can result in vision loss. It is due to high intraocular pressure. The vision loss is very slow and happens at the periphery, patients often do not know that they are losing their vision until they are tested
Cells along the vertical path (photoreceptors, bipolar and ganglion cell) in the eye release what neurotransmitter? What do cells mediate later information transmission (horizontal and amacrince) release?
Glutamate - stimulatory
GABA or glycinergic (with some exception) - inhibitory
In darkness what do rods and cones do?
They are depolarized at -40mV and constantly secrete neurotransmitter.
When stimulated by light how do photoreceptors react? How is the signal passed on?
They do not fire action potentials but instead respond with graded hyperpolarizations. The hyper polarization then spreads passively to the synapse where it reduces the release of the neurotransmitter glutamate.
In the dark explain what the ions are doing in photoreceptors:
There is circulating current in the dark. Na and Ca cations flow inward through cGMP-gated channels at the outer segment. K ions flow outward through K-selective channels at the inner segment. The combined action of the currents results in -40mV resting potential.
In the light explain what the ions are doing in photoreceptors:
The inward cationic current is suppressed. cGMP levels are reduced and the channels are closed reducing Ca and Na flow. K ions continue to still flow. Hyperpolarization and neurotransmitter release is reduced.
What is important about Ca levels in the photoreceptors?
The reduction of inward current when stimulated by light results in a reduction in intracellular Ca levels/ Light-induced reductions in Ca levels are important for light adaptation.
What is phototransduction? What occurs at the cellular level?
It is the conversion of light into electrical signals. The visual pigment protein rhodopsin (cone opsin in cones) with a light-absorbing chromophore. Opsins are in the GPCR superfamily. Light absorption results in a conformational change in retinal which leads in turn to activation of rhodopsin which activates the G-protein. This leads to a reduction in cGMP levels
What is special about the phototranduction pathway?
The absorption of single photon closes more than 200 channels. This leads to a membrane hyper polarization of ~1mV careful observers can see light flashes so dim that only 1 in 100 rods absorbs a single photon.
What is the visual cycle? Where does it occur?
It is the conversion of all trans to 11-cis isomere. It is the restoration of retinal capable of signaling photon capture. It occurs largely in the retinal pigment epithelium in a process known as the visual cycle. It is essential for maintaining the light sensitivity in photoreceptors.
Where are cones mostly located?
The fovea. It is used for high spatial vision. the ganglion cells, Inner Plexiform Layer, and Inner Nuclear Layer are pushed away for higher acuity.
Describe the pathology in open angle glaucoma:
Slow development caused by the obstruction of drainage canals
Describe the pathology in closed angle glaucoma:
Sudden increase in intraocular pressure caused by closed or narrow angle between the iris and cornea.
Where are rods mostly located?
In the periphery
There is a high convergence of rods onto rod bipolar cells, what are the consequences of this?
High sensitivity and low resolution. Tradeoff high sensitivity for low spatial resolution.
