PHOTOCHEMISTRY OF VISION Physiology Lecture SlideshowPHOTOCHEMISTRY OF VISION Physiology Lecture Slideshow


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


  • 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
  • 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


  • 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


  • 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
  • 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


  • 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+


  • 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


  • 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


  • 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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