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As an action potential (nerve impulse) travels down an axon there is a change in electric polarity across the membrane of the axon. In response to a signal from another neuron, sodium- (Na +) and potassium- (K +)–gated ion channels open and close as the membrane reaches its threshold potential.
In excitable cells, such as neurons, the delayed counterflow of potassium ions shapes the action potential. By contributing to the regulation of the cardiac action potential duration in cardiac muscle, malfunction of potassium channels may cause life-threatening arrhythmias. Potassium channels may also be involved in maintaining vascular tone.
This combination of closed sodium channels and open potassium channels leads to the neuron re-polarizing and becoming negative again. The neuron continues to re-polarize until the cell reaches ~ –75 mV, [2] which is the equilibrium potential of potassium ions. This is the point at which the neuron is hyperpolarized, between –70 mV and –75 mV.
The cell membrane potential created by potassium and sodium ions allows the cell to generate an action potential—a "spike" of electrical discharge. The ability of cells to produce electrical discharge is critical for body functions such as neurotransmission, muscle contraction, and heart function. [22]
Inward-rectifier potassium channels: These channels allow potassium ions to flow into the cell in an "inwardly rectifying" manner: potassium flows more efficiently into than out of the cell. This family is composed of 15 official and 1 unofficial member and is further subdivided into 7 subfamilies based on homology.
The channels then close, de-inactivate, and regain their ability to open in response to stimulus. The relative refractory period immediately follows the absolute. As voltage-gated potassium channels open to terminate the action potential by repolarizing the membrane, the potassium conductance of the membrane increases dramatically.
This inactivation shuts off the sodium current and plays a critical role in the action potential. Ion channels can be classified by how they respond to their environment. [21] For example, the ion channels involved in the action potential are voltage-sensitive channels; they open and close in response to the voltage across the membrane.
The potassium channels exhibit a delayed reaction to the membrane repolarisation, and, even after the resting potential is achieved, some potassium continues to flow out, resulting in an intracellular fluid that is more negative than the resting potential, and during which no action potential can begin (undershoot phase/refractory period). This ...