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Dominic John Repici
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Stated simply, ion channels in neurons, are trans-membrane protein molecules that selectively permit or block the flow of specific ions in to, or out of the cell. They can be free-running, voltage sensitive, or controlled by a staggering array of chemical factors, such as neurotransmitters.
There are many configurations of ion channel proteins, each with a particular function and a particular ion type (or set of ion types) that it selects for.
Ion channels act as pores for a particular ion. Many open and close to the particular ion they operate on, and the quantity of ions they permit to pass can be modulated by chemical signals, or voltage levels. At the lowest level, they are binary, that is, they are in either an opened or a closed state. At this level, they are often said to gate ion-flow. They are able to effectively open to varying degrees, though, by adjusting the ratio of opened time, to closed time. They can open and close very quickly; hundreds or thousands of times per second.
As stated, a given ion channel is usually selective for a particular ion. Many modulate the passage of ions based on mechanical, three-dimensional, conformal changes to the molecule.
Ion channels can be controlled by a variety of signaling factors, such as the presence of various neurotransmitters at a synaptic cleft, second- and third-messenger chemical factors within the cell itself, or voltage differences across the cell-membrane.
Some ion channels can move ions through at a very fast pace, on the order of 120 million ions per second.
One of a growing number of examples of a well understood ion channel protein is the KcsA potassium channel, which was solved by Rod MacKinnon and colleagues.