Changes in the light flux on the retina produce electrical changes in all retinal cells, including the RPE and Müller cells, as well as neurons. These electrical changes result from ionic currents that flow when ion-specific channels are opened or closed. These currents reach the vitreous and the cornea, where they can be detected noninvasively and form the basis of the electroretinogram (ERG) (Figs 12-7, 12-8). The currents are initiated by the ionic response started in the rods and cones. This response influences the ionic current directly by changes in Na+, K+, and Ca2+ fluxes and indirectly by synaptic modification of second-order retinal neurons. The ionic changes are due to shifts in the photoreceptors’ conductivity of Na+, K+, and Ca2+; this conductivity is facilitated by the CNG channels (see Fig 12-4).
As discussed previously, light hyperpolarizes cones and rods. The cone response is rapid; it turns off while the light is still on and overshoots the dark potential (see Fig 12-7). The rod response is more prolonged and turns off very slowly. Darkness depolarizes the cone and has little influence on the rod, which is saturated at high light levels and too slow to respond to the shadow.
Figure 12-7 Electroretinogram (ERG) showing the responses of a rod and a cone to a pulse of light and a pulse of darkness. The light pulse hyperpolarizes both photoreceptors. The rod responses are prolonged. The cone responses turn off quickly, even while the pulse of light is on. Darkness rapidly depolarizes the cone but has only a small effect on the slower rod response.
Figure 12-8 Origins of measurable electrical signals from the retina. EOG = electro-oculogram; ERG = electroretinogram.
(Modified with permission from Forrester JV, Dick AD, McMenamin PG, Roberts F, Pearlman E.
The Eye: Basic Sciences in Practice.
4th ed. St Louis: Saunders; 2016:297.)
See BCSC Section 12, Retina and Vitreous, for detailed discussion of ERGs and retinal responses.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.