Physiology - Adaptation to light and dark

Adaptation to light and dark

Light adaptation

• Adaptation to light > adaptation to dark
• Recovers normal vision in less than a minute
• Pupil constriction à restrict amount of light entering eye
• Bleaching of photopigments
• In bright light:
• Sensory cells contain little uncleaved photopigment
• Low light sensitivity
• sensory cells are light adapted

Dark adaptation

Figure 1: Dark adaptation curves. Purple: rod adaptation curve. Green: cone adaptation curve. Red: two-stage dark adaptation curve of both rods and cones. The initial time delay between "light-adapted sensibility" and beginning of the curves is due to delay between time the lights are switched off and when measurement of curves start.

Source: Goldstein E.B. (2014) Sensation and Perception, 9th edn., Wadsworth: Cengage Learning.

• Photopic (light-adapted) state to scotopic (dark-adapted) state

• Maximal light sensitivity
• Cones adapt more quickly to darkness than rods
• Cones: 5 minutes
• Rods: 20-30 minutes
• Species dependent
• Dependent on pre-existing light level
• Brighter pre-existing light, lower rhodopsin stores à longer to reach maximal light sensitivity
• Increase light sensitivity by
• Dilation of pupil
• Synaptic adaptation of retinal neurons
• Increase rhodopsin available in rod outer segments à regeneration of rod photopigments

References

1. Akers R.M. and Denbow D.M. (2013) Anatomy and physiology of domestic animals, 2nd edn., Iowa: John Wiley & Sons, Inc.
2. Goldstein E.B. (2014) Sensation and Perception, 9th edn., Wadsworth: Cengage Learning.
3. Maggs D.J., Miller P.E. and Ofri R. (2013) Slatter's fundamentals of veterinary ophthalmology, 5th edn., Missouri: Elsevier.
4. Sjaastad O.V., Sand O. and Hove K. (2010) Physiology of domestic animals, 2nd edn., Oslo: Scandinavian Veterinary Press.

Physiology - Light stimulation of sensory cells

Light stimulation of sensory cells

• 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

1. Sjaastad O.V., Sand O. and Hove K. (2010) Physiology of domestic animals, 2nd edn., Oslo: Scandinavian Veterinary Press.