Eye Physiology: Photosignalling

Phototransduction

The process in which the body converts light energy into electrical energy for signalling to the brain.

This is a 3-stage biochemical cascade redulting in ion current flow and hence changes in membrane potential.

1. Light is absorbed by, and activates pigment molocules in, the photoreceptors.
   1. Rhodopsinis the primary photopigment in rods.  It contains a large structural protein opsin and a small light absorbing compound called retinal (a derivative of vitamin A).  In the dark, retinal is in the form- 11-cis-retinal.  In the light, retinal changes its conformation to all-trans-retinal.
   2. 
2. Activated pigment stimulates a G-protein, leading to an activation of cyclic GMP phosphodiesterase which converts cGMP into 5′-GMP.
   1. The G-protein in rods is called transducin.
   2. 
3. Reduction in the concentration of cGMP in the cytoplasm closes ion channels, resulting in a hyperpolarising shift of membrane potential.
   1. In the dark, 2 currents flow
      1. the inward current of Na(+) through cGMP gated channels in the outer segment of photoreceptor cells (see retina)
      2. the outward current of K(+) through non-gated channels in the inner segment
   2. In the light
      1. [cGMP] is reduced, closing cGMP-gated channels
      2. Na(+) inward flow is reduced but K(+) flow continues
      3. Cell hyperpolarises.
   3. NB despite the changes in membrane potential, the concentrations of various ions are maintained at normal still by the Na/K pump.
   4. 

Cell pathways

1. Light enters from the ganglion cell layer, and must      penetrate all cell types before reaching the rods/cones.
2. The outer segments of the rods/cones transduce the light and send signals through the cell bodies of the ONL and out to their axons.
3. In the OPL, photoreceptor axons contact the dendrites of bipolar cells and horizontal cells.
   1. Horizontal cells are interneurons that aid with signal       processin
4. The bipolar cells in the INL process input from the      photoreceptors and horizontal cells, and transmit the signal down thier axons.
   1. Horizontal cells’ communicate between photoreceptors and bipolar cells and are thought to be mainly send inhibitory signals to help regulate signalling
5. In the IPL, bipolar axons contact dendrites of the ganglion cells and amacrine cells
   1. Amacrine cells are another class of interneuron that are thought to be important to modify and send combined signals originating from both rods and cones to the correct ganglionic cells.
6. The ganglion cells of the GCL send their axons through      the OFL to the optic disk to make up the optic nerve.  They travel all the way to the lateral geniculate nucleus.

Other cells involved are the interplexiform cells, which are post-synaptic to amacrine cells and presynaptic to bipolar/horizontal cells to form a feedback loop to further regulate the input to the ganglionic cells.

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