Computer Simulation of Action Potentials in Squid Axon Assignment

Computer Simulation of Action Potentials in Squid Axon - Assignment Example During the experiment, it was seen that the latency of the response was dependent on the strength of the stimulus, and not on the duration by which the membrane is exposed to the stimulus. However, in contrast to latency, the strength of the action potential was not modified by either strength or duration of stimulus. As well, it was observed that another stimulus cannot produce an action potential if given immediately after a previous stimulus. This refractory period is caused by the deactivation of voltage-gated Na+ channels and opening of K+ channels, resulting to the return of the membrane potential to its negative state. In conclusion, the action potential, and subsequently the signal transmission based on it, is dependent on the opening and closing of ion channels present along the membrane. INTRODUCTION 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 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 gradien t through its own voltage-gated channels (Guyton and Hall, 2006). However, 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 action potential (Finkler). Hodgkin and Huxley characterized the voltage-gated channels involved in the generation of action potential. According to these scientists, the Na channels have two gates (the activation and inactivation gates), while K channels only have one. At resting state, the Na+ channels have the activation gate (facing extracellularly) closed while the other gate is opened. At this time the K+ channels are closed as well. When the channels are activated, both the activation and inactivation gates of Na+ channels are opened. Finally, upon repolarization, the inactivation gate is closed, while the other is opened. K+ channels are opened as well. To mathematically describe the effects of such changes on membrane potential, they also provided equations to describe the relationship among Na+