11. What is an electroretinogram? What are the cellular origins 
       of its components?
The electro-oculogram measures the difference in electrical charge between the front and back of the eye. The difference between the negative charge at the back of the eye and the positive charge at the front is about 6 mV. 

The EOG actually measures the electrical potential of retinal pigment epithelium. In light adapted eye there is a steady rise in this potential, recordable as a reversible potential on horizontal eye movement and known as the light rise. This is caused by extracellular flow of current caused by changes in the potassium concentration in the interphotoreceptor matrix.

Electrodes are placed on the skin near the inner and outer canthus
a. The subject looks right and left while the change in electrical potential is recorded.
b. The EOG value is the difference in the electrical charge between the front (+) and back (-)of the eye.
c. Begin by letting the patient dark adapt for about 10 minutes while recording. The room lights are turned on the recording continues as the patient light adapts. The voltage difference between the front and back of the eye varies depending on light or dark adaptation and the health of the eye.

The RPE is critical to the normal function of the retina but can fail in a number of disease states. Measurements of its ability to adapt to changing light intensity is often performed in conjunction with an ERG. Best's disease, a form of inherited macular disorder can often be diagnosed with EOG (but not on ERG) alone.

More model answers