'A' wave in Electro Retinogram corresponds to activity in
High-Yield Explanation
The electroretinogram (ERG) is a diagnostic test that measures the electrical activity generated by neural and non-neuronal cells in the retina in response to a light stimulus. The electrical response is a result of a retinal potential generated by light-induced changes in the flux of trans retinal ions, primarily sodium and potassium. A-wave: initial corneal-negative deflection, derived from the cones and rods of the outer photoreceptor layers. This wave reflects the hyperpolarization of the photoreceptors due to closure of sodium ion channels in the outer segment membrane. Absorption of light triggers the rhodopsin to activate transducin, a G-protein. This leads to the activation of cyclic guanosine monophosphate phosphodiesterase (cGMP PDE) eventually leading to a reduction in the level of cGMP within the photoreceptor. This leads to closure of the sodium ion channels resulting in a decrease of inwardly directed sodium ions, or a hyperpolarization of the cell. The a-wave amplitude is measured from baseline to the trough of the A-wave. Ref.textbook of physiology by A K Jain 7th edition page no.1110