You apply ACh and activate nicotinic receptors on a muscle cell. Which way will current flow through the receptor channels when Vm = –60 mV? When Vm = 0 mV? When Vm = 60 mV? Why?
Nicotonic ACh receptors are permeable to both sodium and potassium. When Vm = –60 mV, net current flow through ACh-gated ion channels is inward, toward the equilibrium potential of sodium, causing depolarization. At Vm = 60 mV, the direction of net current flow through the ACh-gated ion channels is outward, toward the equilibrium potential of potassium, causing the membrane potential to become less positive. The critical value of membrane potential at which the direction of current flow reverses is called the reversal potential. In this case, the reversal potential is 0 mV because this is the value between the equilibrium potentials of sodium and potassium. At 0 mV, no current flows.