Write 8 pages with APA style on Computer Simulation of Action Potentials in Squid Axon. Action potentials are rapid changes in the membrane potential. In turn, this potential is based upon the differences in concentrations of ions, each of which is charged either negative (anion) or positive (cation), across the membrane. The concentration difference is due to a selectively permeable membrane, which prevents the ions from transferring sides to equalize the number of ions between inside and outside a cell.But why is there a concentration gradient in the first place? The Na+-K+ pumps along with the cell membrane force three Na+ outside and two K+ inside the cell. As a result, there is a net deficit of positive ions and a resulting negative potential inside the cell. In a resting state, the membrane potential is -90 millivolts (90 mV). Upon depolarization, the membrane rapidly becomes very permeable to Na+, through its voltage-gated channels, allowing the excess of Na+ to pass through into the cell. As a result, the resting potential is changed to as much as +35 mV. Through repolarization, the resting potential is gained back not long after depolarization, when Na+ voltage-gated channels close and K+ passively diffuse down its concentration gradient through its own voltage-gated channels (Guyton and Hall, 2006).However, the entry of Na+ does not immediately cause depolarization. The number of Na+ that enter the cell must be more than the amount of K+ that gets out of the cell since the membrane is more permeable to K+ than Na+. Thus, the sudden change of membrane potential to -65 mV is the said threshold for stimulating the action potential (Guyton and Hall, 2006). Any electrical stimuli above this threshold produce an action potential with the same amount of strength, as stated by the all-or-none concept (Purves et al., 2004). In addition, a new action potential cannot occur unless the membrane is still depolarized. This is because the Na+ voltage-gated channels necessary for depolarization is still deactivated during repolarization. At this point, called the refractory period, no amount of stimulus can initiate an action potential.