Physiology Of Vision II Flashcards
What are Visual pigments ?
Disc’s of outer segment in both rods and cones contain this pigment and this is where light absorption occurs and is a derivative of rhodopsin
What are the 3 types of cones?
Red
Green
Blue
What is colour blindness?
Inability to distinguish colours
When one or more classes of cones are non-functional and unable to manufacture necessary visual pigments
What is Photoreception?
It begins when photon hits the retinal portion of a rhodopsin molecule that’s in membrane of disc the opinion is activated
What are the 2 possible configurations of Bound retinal molecule?
11 cis firm and 11 trans form
What happens to the 11 cis form?
When it absorbs light it changes to a more linear 11 trans form this then activates opsin molecule
What is Transducin?
It is a G proteins a membrane bound enzyme complex
How is transducin activated?
It is activated by opsin and transducin will activate PDE
What is PDE?
It is an enzulyme that breaks down cGMP and the levels decline and gated Na close
How are the Na channels inactivated?
When cGMP is removed and this will the decrease Na entry into cytoplasm
What happens at -40Mv during photoreception?
Photoreceptors are continously releasing NT (glutamate) across synapses at the inner segment and Na is constantly being pumped out
What is dark current?
The constant movement of Na from outer segment to inner segment and out of cell
What happens when Na channels close?
The transmembrane potential drops towards-70MV and reduction of Na entry will reduce dark current because the active transport continues to move Na from cytoplasm
What happens when membrane hyperpolarizes?
The rate of glutamate release will decrease and this will cause Na channels on bipolar cells to open indicating to the adjacent bipolar cell that the photoreceptir has absorbed the photon
What is bleaching?
After absorbing the photon the retinal does not revert the 11 cis form the entire rhodopsin breaks down into retinal and opsin and reassemble before it revombine with opsin retinal enzymatically converts 11 cis form
What us the cause of night blindness?
Lack of vit A because Vit A is a source of retinin
What us the Dark adapted state?
After 30 min or more being in the dark the visual pigments will have recovered from photo bleaching and fully receptive to stimulation
What are ganglion cells?
They monitor a specific portion of the visual field and are made of two types M and P cells
What are M cells?
They monitor the rod and are really big they provide information on motion,depth perception,location of objects and general form of object
What are P cells?
They monitor the cones and are much smaller provide information about edges,colour and fine detail
What are on center neurons?
Excited by light arriving in center and inhibited when light strikes edges of receptive field
What are off center neurons?
Inhibited by light in central zone and are stimulated by illumination at edges
What are visual pathways?
They begin at photoreceptors
And end at visual cortex of cerebral hemispheres
Message crosses two synapses before it heads towards brain
What are the two synapses of visual pathways?
Photoreceptor to bipolar cell
Bipolar cell to ganglion cell
What are the central pathways of visual information?
Axons from ganglion cells converge on optic disc and penerate wall of eye and more to the diencephalon as optic nerve 2 the two optic nerves reach duencephalon at optic chiasm
What us visual data?
It is the combined field of vision that arrives at visual cortex of opposite occipital lobe
Left half at right occipital lobe
Right half at left
Want is optic radiation?
A bundle of projection fibers that link lateral geniculate with visual cortex
