Ion channels

- are integral proteins that span the membrane and, when open, permit the passage of certain ions.

1. Ion channels are selective; they permit the passage of some ions, but not others. Selectivity is based on the size of the channel and the distribution of charges that line it.

-For example, a small channel lined with negatively charged groups will be selective for small cations and exclude large solutes and anions. Conversely, a small channel lined with positively charged groups will be selective for small anions and exclude large solutes and cations.

2. Ion channels may be open or closed. When the channel is open, the ion(s) for which it is selective can flow through. When the channel is closed, ions cannot flow through.

3. The conductance of a channel depends on the probability that the channel is open. The higher the probability that a channel is open, the higher the conductance, or permeability. Opening and closing of channels are controlled by gates.

a. Voltage-gated channels are opened or closed by changes in membrane potential.

- For example, the activation gate of the Na+ channel in nerve is opened by depolarization; when open, the nerve membrane is permeable to Na+ (e.g., during the upstroke of the nerve action potential). The inactivation gate of the Na+ channel in nerve is closed by depolarization; when closed, the nerve membrane is impermeable to Na+ (e.g., during the repolarization phase of the nerve action potential).

b. Ligand-gated channels are opened or closed by hormones, second messengers, or neurotransmitters.

- For example, the nicotinic receptor for acetylcholine (ACh) at the motor end plate is an ion channel that opens when ACh binds to it. When open, it is permeable to Na+ and K+, causing the motor end plate to depolarize.

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