PHOTOCHEMISTRY OF VISION

BY

DR. SUMAIRA IQBAL

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RHODOPSIN VISUAL CYCLE

• Outer segment contains 40% photopigment– rhodopsin/visual purple
• Combination of retinal and scotopsin
• Retinal– 11-cis type
• Readily binds to scotopsin

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RHODOPSIN VISUAL CYCLE

• Rhodopsin decompose when light falls on retina
• Rhodopsin decompose in fraction of a second
• Photoactivation of electrons in retinal portion of rhodopsin
• cis form of retinal is converted into all-trans form
• All trans form has no binding sites for scotopsin
• Decomposed into retinal and scotopsin

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RHODOPSIN VISUAL CYCLE

• First product to be formed is BATHORHOPSIN in psec after light falls
• Followed by formation of LUMIRHODOPSIN in nsec
• Leads to formation of METARHODOPSIN-I in μsec
• Followed by formation of METARHODOPSIN-II in msec
• Then in seconds is converted into SCOTOPSIN and 11 cis retinal
• It takes minutes to recompose Rhodopsin
• For synthesis retinal must be converted to trans form(occurs in DARK)

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EXCITATION OF RODS

• Rods receptor potential is hyperpolarizing
• Inner side of the membrane becomes more negative
• Excitation of the rod causes increased negativity of intrarod membrane potential
• Opposite to other sensory receptors
• Reason of hyperpolarization
• When rhodopsin decompose reduce the Na conductance in the outer segment

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EXCITATION OF RODS

• Sodium ions moves in a circuit through the inner and outer segments of rods
• Inner segment pumps sodium from inside to the outside
• Creating a negative potential inside the entire cell
• Phototransduction converts light stimuli in to electrical signals

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IN DARK

• Outer segment of the rod is very leaky to sodium ions
• Sodium ions leak back to the inside of the rod
• Neutralize negativity on the inside of the entire cell
• Dark conditions—rod is not excited
• Electronegativity can be -70 to -80mV, when receptor potential is generated
• Reduces to -40mV

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IN LIGHT

• Rhodopsin decompose thus decreases membrane conductance of sodium to the interior of the rod
• Sodium ions are pumped outward from the inner segment
• Loss of positive ions from the inside of the cell leads to increased negativity inside the membrane
• The greater the negativity is more the degree of hyperpolarization

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IN LIGHT

• At maximum light intensity, membrane potential is –70 to –80mv
• Equivalent to equilibrium potential for potassium ions across the membrane
• There is increased negativity of intra rod membrane potential.
• When rhodopsin decomposes it decreases the rod membrane conductance for Na+ ions in the outer segment
• There is rather hyperpolarization than depolarization

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SUMMARY

• Inner segment continually pumps na+ from inside to the outside of rod
• So  –ve potential.
• Outer segment is very leaky to Na+ in dark state.
• So much of +ve Na leaks back into inside of rod to neutralize most of the negativity.
• So in darkness there is decreased negativity of -40 mv rather than -70-80 mv in sensory receptors.
• When rhodopsin in outer segment is exposed to light & decomposes, it decreases the outer segment membrane conduction for Na+

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DURATION OF THE RECEPTOR POTENTIAL

• Light strikes the retina-- Hyperpolarization leads to receptor potential
• Reaches peak in 0.3 second, stays for a second
• In cones, change occurs four times faster than rods
• Image may last for second on retina
• Receptor potential is proportional to the logarithm of light intensity
• Helps to discriminate light intensities

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PHOTOTRANSDUCTION

• Photon of light activates electron in 11-cis portion of rhodopsin
• Structural change in retinene photo pigment– metarhodopsin-II forms
• Conformational change in photo pigment
• Activates an enzyme—Transducin
• Activated transducin activates Phosphodiestrase
• Decreased intracellular cGMP

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PHOTOTRANSDUCTION

• cGMP bound with Na channel portion of outer segment to splint it in open state (in dark/inactivated state)
• cGMP is destroyed by hydrolysis
• Closure of sodium channel(Na+ stays out of cell)—Hyperpolarization
• Rhodopsin kinase (in a sec) inactivates activated rhodopsin and entire cascade reverses back to normal

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PRINCIPAL STEPS IN PHOTOTRANSDUCTION

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THANK YOU

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