1.3 Action Potential Fundamentals
At a potential of -70mV, the membrane is said to be polarized. Polarized is a fancy word for different, and simply refers to the fact that there is a charge differential across the membrane. A resting state neuron is thus said to be polarized; a state that is maintained by the aforementioned pumps, channels, and non-permeable factors. Any event that brings membrane potential closer to 0 (zero charge difference across the membrane) is referred to as depolarization (making less polar).
While leak channels are constitutively open, and the Na/K pump is always active, most channels are gated. One subtype of gated channels are opened by the binding of a ligand to the extracellular portion of the channel, and promptly close after the separation of the ligand from its binding site. On the other hand, voltage-gated channels are opened at a voltage threshold; they remain open when voltage is at threshold or greater, and are closed at voltage’s less than threshold. An image of a voltage gated channels is shown below. One of the peptides that composes the polypeptide, termed the S4 region of the protein, is made of many positively charged amino acids and is flexibly attached on a hinge structure near the base of the channel’s intracellular portion. When the inside is negatively charged (-70mV relative to the outside), the S4 region is attracted to the intracellular portion and keeps the channel closed. However, when membrane potential is at -50mV or greater, this attraction becomes too weak to keep the channel closed. The S4 region changes it’s position and with it, the conformation of the protein, resulting in an open channel.
Gated channels mediate activity-dependent changes in membrane potential, which cause a neuron to “fire”, effectively sending out an electrical signal onto other neurons. “Firing” or sending out an electrical signal refers to the process of coordinated membrane depolarization, originating at the cell body and propagating along the axon until it reaches the axon terminals. While a polarized membrane has a voltage of -70mV, the depolarization process increases the voltage such that it becomes closer to 0 (hence less polar).
How does depolarization occur? Hypothetically, if gated sodium, calcium, and potassium channels open, then the membrane would suddenly become much more permeable to positively charged ions. These ions would chaotically enter the cell, rapidly altering the membrane potential in the process and bringing it to a more de-polar state. In this hypothetical scenario, Calcium and Sodium will rush in as they push the membrane potential closer to each ion’s respective equilibrium potential (+135 and +50mV respectively), while potassium exits the cell in a similar attempt to push membrane potential closer to its equilibrium of -90mV. Depolarization can thus be thought of as the process of converting the potential energy (ie voltage) of the ions into kinetic energy (i.e current / velocity of ions