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| Voltage-Gated Channels |
The neuron, like all cells, possesses a cell membrane that is mostly lipid. Ions like sodium and potassium cannot cross the lipid membrane on their own. In all cells transport of ions, as well as some small molecules, is carried out by channels, which are very tiny openings in the membrane formed by protein pores. These channels are often gated - that is, opened or closed - depending on the conditions of the cell. When open, the ions can enter and pass through channels by diffusion. Ions will always travel down their electrochemical gradient. For example, sodium is much more plentiful outside the cell than inside. It is also positively charged, while the inside of the cell is typically negatively charged relative to outside. Thus, both the chemical and electrical components of the gradient will drive sodium ions into the cell when sodium channels open. Voltage-gated channels are those in which the membrane potential of the cell determines whether they are opened or closed. Other channels can be opened or closed by various chemicals, such as neurotransmitters.
Channel proteins that span the cell membrane form the ion channels. To determine the structure of proteins, scientists have often used X-ray crystallography. (See the Proteins and Proteomics unit.) In 2003 Roderick MacKinnon and his colleagues used this technique to examine the structure of a voltage-gated potassium channel from a unicellular archaea. Previous studies have shown that ion channels have a central ion-conduction pore. Like all proteins, ion channel proteins are made up of amino acids, some of which are charged. When voltage changes occur, these charged components of the protein make very small movements. This can result in more dramatic conformational changes, causing the channels to open and close. MacKinnon's group found that "voltage-sensor paddles" surround this pore. It appears that with voltage changes in the membrane, these paddles will move and thus permit potassium ions across the membrane. 2 Further study of the structure of the different classes of ion channels from other species will help elucidate the mechanisms by which they allow ion transport.