Written by Travis M. Moore
Last edited 6-Jun-2020
Ok, brief summary: we know the inside of a neuron is more negative relative to the fluid surrounding it. We also know that Na+ rushes into the neuron and causes it to depolarize (i.e., "fire"). This briefly makes the inside of the neuron more positive relative to the fluid outside. Finally, we saw the Na+ ions are pumped back outside the neuron, restoring the outside positivity so the neuron can fire again. Let's look at the whole process again from the beginning, this time adding in more details. We'll follow the events shown in Figure 1 below.
Remember that before a neuron fires, it's just sitting around with more Na+ ions outside the cell and more K+ ions inside the cell. By the way, the term for the environment outside the neuron is the interstitial fluid. This gradient of ions results in a negative charge inside the neuron compared to the interstitial fluid. Specifically, the difference across the cell membrane is around -70 mV in most neurons. Because the neuron isn't doing anything at this point, the -70 mV is referred to as the resting potential. Box 1 in Figure 1 shows the resting potential. The resting potential channel activity is shown in Figure 2.
Life would be pretty boring if neurons stayed at their resting potential. But what makes them fire? Neurons are connected to our sensory systems. When we hear a sound, see an image, feel pressure, etc. those sensations are relayed to neurons. In the case of the auditory system, when the basilar membrane moves and causes the inner hair cells to "fire" (yes, they also depolarize!), that signal tells the neurons in the auditory nerve that something happened. In turn, the neurons fire in order to relay the "something happened at this frequency" signal to the brain for more advanced processing.
The presence of sound tells the auditory neurons to begin to depolarize. This causes some of the Na+ channels to open, and Na+ starts to pour into the neuron (Figure 3). When that happens, the resting potential changes to become more positive. Remember the Na+ moves into the neuron because there is an imbalance between the outside and inside of the neuron both in charge and in the number of Na+ ions. So when given the chance, the Na+ moves through the open channels to try to balance things out.
If enough sodium enters the neuron to raise the voltage to -55 mV (Figure 1, box 2), the neuron officially "fires" and all of its sodium channels open (Figure 1, box 3, Figure 4). This is when the cell becomes fully depolarized. The official term for this rapid depolarization is the action potential, which makes sense because the neuron is no longer resting, but active. This is the heart of neural activity.
Eventually the Na+ ions make the inside of the neuron more positive than the interstitial fluid, raising the voltage to around +30 mV. At this point, the Na+ channels close and the K+ channels open. Because there are more K+ ions inside the neuron than outside, when those channels open, K+ rushes outside the cell to try and balance out the gradient. What we're left with is the exact opposite of what we started with: there is now more K+ in the interstitial fluid than inside the neuron.
The new imbalance of more K+ outside the neuron causes the voltage inside the neuron to plummet (i.e., repolarize). However, the K+ is a little too good at doing this, and the voltage actually drops below -70 mV (Figure 1, box 4). When this happens, both Na+ and K+ gates close (Figure 6). During this time, the neuron cannot fire again until the voltage returns to -70 mV. The resting potential is reestablished through the use of sodium potassium pumps that use energy to force the Na+ back outside the neuron and the K+ back inside (Figure 1, boxes 5 and 1). Now the process can begin all over again.
There is so much more to say about neural transduction, but it is not the focus of this course. To solidify the concepts learned above, the videos below are fantastic overviews by excellent teachers.