Which step of nerve or muscle firing would be directly affected by a change in extracellular K\(^+\) concentration?

• A. Opening of voltage-gated Na\(^+\) channels
• B. Release of neurotransmitter at the synapse
• C. Reuptake of Ca\(^{2+}\) into the sarcoplasmic reticulum
• D. Resting membrane potential

Answer: (D)

Verified step by step guidance

1. Understand that the resting membrane potential is primarily established by the distribution of ions, particularly K extsuperscript{+} (potassium), across the cell membrane. The movement of K extsuperscript{+} is governed by its concentration gradient and the membrane's permeability to K extsuperscript{+}.
2. Recall that the resting membrane potential is maintained by the sodium-potassium pump (Na extsuperscript{+}/K extsuperscript{+} ATPase), which actively transports 3 Na extsuperscript{+} ions out of the cell and 2 K extsuperscript{+} ions into the cell, creating a high intracellular K extsuperscript{+} concentration and a low extracellular K extsuperscript{+} concentration.
3. Recognize that a change in extracellular K extsuperscript{+} concentration would alter the K extsuperscript{+} concentration gradient. This would directly affect the equilibrium potential for K extsuperscript{+}, as described by the Nernst equation: $E=\frac{\mathrm{RT}\ln }{\left(\mathrm{[K}\right]}$_outside/[K]_inside)
4. Understand that the resting membrane potential is closely tied to the equilibrium potential of K extsuperscript{+}, as the membrane is most permeable to K extsuperscript{+} at rest. A change in extracellular K extsuperscript{+} concentration would therefore directly affect the resting membrane potential, potentially making it less negative (depolarized) or more negative (hyperpolarized).
5. Conclude that the other steps listed (opening of voltage-gated Na extsuperscript{+} channels, release of neurotransmitter at the synapse, and reuptake of Ca extsuperscript{2+} into the sarcoplasmic reticulum) are not directly influenced by extracellular K extsuperscript{+} concentration, as they involve different ions or processes. The resting membrane potential is the correct answer.