As the name suggests, Ion Channels (IC's) allow specific ions into and out of the cell.
Ion channels are selective and allow ions with certain characteristics to move through them. This selectivity is based on size and polarity. Channels with a negative polarity allow positive ions but not negative ions. Similarly, channels with a positive polarity allow negative ions but not positive ions. Channels discriminate based on size also. A cation-selective channel, or negative channel, will permit Na+ but not K+ because Potassium is bigger in size than Sodium. Another cation-selective channel (the nicotinic receptor on the motor endplate) might have less selectivity and permit the passage of several different small cations.
A cation is a positive ion and an anion is a negative ion.
Ion channels are controlled by gates, and, depending on the position of the gates, the channels may be open or closed.
When a channel is open, the ions for which it is selective can flow through it by passive diffusion (high to low concentration), down the existing electrochemical gradient. In the open state, there is a continuous path between ECF (extracellular fluid) and ICF (intracellular fluid), through which ions can flow.
When the channel is closed, the ions cannot flow through it, no matter what the size of the electrochemical gradient. The conductance of a channel depends on the probability that it is open. The higher the probability that the channel is open, the higher is its conductance or permeability.
The gates on ion channels are controlled by three types of sensors.
Voltage-gated ion channels respond to changed in membrane potential.
Second messenger-gated ion channels respond to changes in signaling molecules.
Ligand-gated ion channels respond to changes in ligands (a molecule that binds to the channel) such as hormones or neurotransmitters.
Voltage-gated ion channels are controlled by changes in membrane potential. This type of ion channel is most prevalent in neurons where the difference in activation times caused by different voltage changes can change the shape and time of the action potential.
<aside> ⚠️ The activation gate on the nerve Na+ channel is opened by depolarization of the nerve cell membrane and the opening is responsible for the upstroke of the action potential. Another gate on the Na+ channel is closed by depolarization (an inactivation gate). Because the activation gate responds more rapidly to depolarization than the inactivation gate, the Na+ channel first opens then closes. This difference in response times of the two gates accounts for the shape and time course of the action potential.
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Second messenger–gated ion channels have gates that are controlled by changes in levels of intracellular signaling molecules (second messengers).
As learned in most AP Biology classes, examples of second messengers include cyclic adenosine monophosphate (cAMP) and inositol 1,4,5-triphosphate (IP3).
Because these are internal (intracellular) the sensors are located on the inside of the cell.
<aside> ⚠️ The gates on Na+ channels in cardiac sinoatrial node are opened by increased intracellular cAMP.
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