E1: Lab 3 membrane graded, action, potential

Explain how an increase in extracellular K⁺ concentration can have a dramatic effect on the equilibrium potential for K, the resting membrane potential, and the excitability of cells.

Increasing extracellular K⁺ reduces the K⁺ concentration gradient, making the equilibrium potential for K⁺ less negative. Because the resting membrane potential is largely determined by K⁺, Vm also becomes less negative (depolarizes). This initially increases excitability by bringing Vm closer to threshold, but very high extracellular K⁺ can inactivate Na⁺ channels and reduce excitability.

Define threshold, and relative and absolute refractory period. Explain the state of ion channels and the stimulus amplitude required to generate an action potential during each.

Threshold is the membrane potential at which enough voltage-gated Na⁺ channels open to trigger an action potential. During the absolute refractory period, Na⁺ channels are inactivated and no stimulus can produce an action potential. During the relative refractory period, some Na⁺ channels have recovered but K⁺ channels remain open, making the membrane hyperpolarized; a stronger-than-normal stimulus is required to generate an action potential.

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