The K+ Channel, A New Hope For a Better Understanding
The axons of our neurons are the pathway for the communication that exists in our nervous system. This communication takes the form of an electric signal, also called an action potential. The action potential occurs due to a change in voltage across the membrane of the axon. The change in voltage is achieved by a change in the concentrations of the ions, Na+, Ca+, and K+(1) . The cell starts with a large concentration of potassium ions, K+, inside the cell, and a large concentration of sodium ions, Na+, outside the cell. The action potential propagates down the axon due to openings and closing of different channels allowing changing of the concentrations of the ions(10).
Channels are proteins that span the membrane of the axon. These proteins have a structure so that they can be allow ions to flow through pores that are only open at the appropriate times. Some of the channels are opened and closed by other chemicals, while some are initiated by a change in the membrane potential.
This particular K+ channel, which is greatly studied, is a voltage-gated channel. This means that the channel opens in response to a certain voltage difference that occurs across the membrane. The channel is closed when the cell is at rest. Following inactivation the channel opens via a complicated mechanism, which scientists are still trying to decipher(7)(8). The specific voltage is that which occurs after the Na+ channel has opened and allowed a significant amount of Na+ to be released from the cell. So, the K+ channel is induced to an open state by a depolarization of the membrane potential. The K+ channel opens at the beginning of the repolarization, or after the depolarization has almost reached its peak. The opening of this channel allows K+ ions to flow outside of the membrane of the cell, bringing the voltage of the cell back down to its normal level. The K+ returns to the inside of the cell through a pump that exchanges it for Na+ so that there is little voltage change.
Scientists have struggled for a long time to understand how voltage-gated channels work. "How and where changes in the structure of the voltage-sensing domains [work] to gate ion conduction is not understood," said Li-Smerin et al. in a paper published in February 2000(2). Since then research in the field of voltage-gated channels has reached great heights. Now, scientists view K+ channels as those that are best understood(3). There have been multiple experiments done to determine the structure of the channel through x-ray crystallography, flourometry, and mutagenesis(2)(4). That picture has been almost perfected. Mutations have been done to determine structures that have a big influence on the voltage sensing and gating properties of the channel. For example, Li-Smerin et al. did a series of 37 mutations that led to two groups that they call the major impact and the minor impact residues(2). They were defined as being major...