RHODOPSIN VISUAL CYCLE
- 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)
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
- 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
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
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
- 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
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+
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
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
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
PRINCIPAL STEPS IN PHOTOTRANSDUCTION
