Ion channels can alter membrane potential

Many ion channels in the plasma membranes of neurons behave as if they contain a "gate" that opens under some conditions, but closes under other conditions. Voltage-gated channels open or close in response to a change in the voltage across the plasma membrane. Chemically gated channels open or close depending on the presence or absence of a specific molecule that binds to the channel protein, or to a separate receptor that in turn alters the channel protein. Both voltage-gated and chemically gated channels play important roles in neuronal function.

Changes in gated channels may perturb the resting potential. Imagine what happens, for example, if sodium channels in the plasma membrane open. Na+ ions diffuse into the neuron because of their higher concentration on the outside, plus they are attracted to the inside of the cell by its negative charge. As a result of the entry of Na+ ions, the inside of the cell becomes less negative. When the inside of a neuron becomes less negative (or more positive) in comparison to its resting condition, its plasma membrane is said to be depolarized (Figure 44.8a).

An opposite change in the resting potential occurs if gated Cl- channels open. The concentration of Cl- ions is normally greater in the extracellular fluid than inside the neuron. This difference is large enough so that the opening

(a) Na+channel

K+ channel Na+channel open voltage gate

Voltage gate closed o

K+ channel Na+channel open voltage gate

Voltage gate closed

Gated Na+_

channel open

KK+ +chcahnanenlel poepnen

Cl- channel voltage gate open

KK+ +chcahnanenlel poepnen

Cl- channel voltage gate open

Voltage gate closed

channel open

(mV)

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