Vision begins with the absorption of light by special cells in the retina. Ultimately, the energy is used to close ligand-gated ion channels, causing sizable changes in the transmembrane potential. The pulse of electric potential travels through the optical nerve and into the optical cortex, where it is interpreted as a signal and incorporated into the web of events we call visual perception. Taking the resting potential as -30 mV, the temperature as 325 K, permeabilities of the K+ and Cl– ions as PK+ = 1.0 and Po– = 0.45, respectively, and the concentrations as (K11.= 100 mM, Nalin= 10 mM, (Cl–bn = 10 mM [‘Clout = 5 mM, [Nalout = 140 mM, and [Cl]out = 100 mM, calculate the relative permeability of the Na+ ion.
