Light stimulation of sensory cells
Figure 1: Cleavage of photopigments and transduction in rods and cones due to light stimulation. This causes a chemical bond change, resulting in a straightened molecule. Retinal detaches from opsin, causing conformational change in its shape. G-protein transducin binds to opsin and is activated. Hyperpolarization of the cell and reduced release of neurotransmitter ends the chain reaction.
Source: Sjaastad O.V., Sand O. and Hove K. (2010) Physiology of domestic animals, 2nd edn., Oslo: Scandinavian Veterinary Press.
Darkness
- When light strikes the retina, photons are trapped by the receptor molecules (rods and cones) located in the membrane discs in the outer segments
- Membrane discs in rod = "light trap"
- Receptor molecules in retina = photopigments
- Rods contain photopigment, rhodopsin
- Different types of cones have its own type of photopigments
- Photopigments compose of opsin (protein) and retinal (produced in cell from vitamin A, retinol)
- Photopigments are G-protein-coupled receptors
Figure 1: Cleavage of photopigments and transduction in rods and cones due to light stimulation. This causes a chemical bond change, resulting in a straightened molecule. Retinal detaches from opsin, causing conformational change in its shape. G-protein transducin binds to opsin and is activated. Hyperpolarization of the cell and reduced release of neurotransmitter ends the chain reaction.
Source: Sjaastad O.V., Sand O. and Hove K. (2010) Physiology of domestic animals, 2nd edn., Oslo: Scandinavian Veterinary Press.
Darkness
- Outer segment of rods and cones:
- Concentration of cGMP in outer segment is high
- Surface membrane:
- High density of Na+ channels so Na+ can diffuse into cells.
- Channels open when bound to cGMP
- Ion channels open in dark à Na+ enters & depolarize sensory cells (light)
- Inner segment of rods and cones:
- Na+ pumped out via Na+-K+ pump
- Depolarization of inner segment keeps voltage-gated Ca2+ channels open à continuous neurotransmitter (glutamate) release from synaptic terminal of sensory cell
Light
- When light (photons) is absorbed, 11-cis-retinal of the photopigment, rhodopsin, changes its conformational shape to all-trans-retinal
- All-trans-retinal leaves photopigment, causing a conformational change of the photopigment
- Now, there is a exposed binding receptor site on photopigment
- G-protein transducin can now attach to the photopigment and is activated
- This protein structure activates the enzyme, phosphodiesterase
- Phosphodiesterase hydrolyses cGMP resulting in
- closure of Na+ channels
- Na+ concentration in cell decreases and membrane potential = -ve
- Rods & cones hyperpolarize
- Release of neurotransmitter, Glutamate, is reduced
- Results in inhibition & stimulation of ganglion cells
Figure 2: Ion currents through rods and cones in darkness. Na+ diffuses into the outer segment through open ion channels while Na+ is pumped out via Na+K+ pump in the inner segment. Light closes the ion channels.
Source: Sjaastad O.V., Sand O. and Hove K. (2010) Physiology of domestic animals, 2nd edn., Oslo: Scandinavian Veterinary Press.
Notes
- Single photon can close many hundreds of ion channels
- Prevents more than a million Na+ from entering cell
- Single photon evokes detectable receptor potential in cell
Reference
- Sjaastad O.V., Sand O. and Hove K. (2010) Physiology of domestic animals, 2nd edn., Oslo: Scandinavian Veterinary Press.
