Here we come to the crux of the matter. We want to understand what happens to the membrane potential when gated ion channels open. This occurs not only as neurons code and transmit information, but also in many other integrative activities in the body.
The opening of an ion channel and the subsequent movement of ions through the channel necessarily changes the membrane potential. This, of course, is because the moving ions are moving charges. But in what direction does the membrane potential change? You are now equipped to answer this in every case.
But typically, more than one specific type of ion can cross the membrane. In this case, the membrane potential moves towards the "average" of the equilibrium potentials for the specific ions that can cross the membrane. The "average" is not a true average, but an average weighted by the relative permeabilities for the different ions -- the more permeable the membrane is for an ion, the more the equilibrium potential of that ion will influence the membrane potential.
Consider a "resting" neuron. In our example, we have been using a resting membrane potential of -70 mV. Refer back to the table of equilibrium potentials. We know that the resting membrane is mainly permeable to K+, and thus the resting membrane potential should be close to the K+ equilibrium potential. However, the resting membrane potential is not exactly at the equilibrium potential for K+, but a bit higher at -70 mV. This is because the resting membrane is slightly permeable to Na+.
What about Cl-? Since Cl- is not actively transported, its intracellular concentration typically must adjust to put the ion near equilibrium. For this reason, the equilibrium potential for Cl- is equal here to the resting membrane potential at -70 mV.
Now let's now follow what happens to the membrane potential of a cell as specific gated ion
channels open. This is illustrated in the animation referenced just below. After you open
the animation, observe how the membrane potential
tends to move towards the equilibrium potential of the ion that moves through the opening
channel. Don't worry if you don't follow everything the first time. The animation will
continue to repeat until you close the window.