Across the cell membrane of a skeletal muscle cell in the resting state with a non-electrogenic pump
**Core Concept**
The resting skeletal muscle cell has a stable resting membrane potential due to the balance between the permeability of the cell membrane to ions and the activity of ion pumps. In this context, a non-electrogenic pump is one that maintains the ion gradient without generating a net movement of charge across the membrane.
**Why the Correct Answer is Right**
The Na+/K+-ATPase pump, also known as the sodium-potassium pump, is a non-electrogenic pump that maintains the resting membrane potential in skeletal muscle cells. It uses ATP to actively transport three sodium ions out of the cell and two potassium ions into the cell, creating an electrochemical gradient that favors the influx of sodium ions and the efflux of potassium ions. This gradient is essential for the maintenance of the resting membrane potential, which is around -90 millivolts in skeletal muscle cells.
**Why Each Wrong Option is Incorrect**
* **Option A:** The Na+/Ca2+ exchanger is an electroneutral exchanger that helps regulate calcium levels in the cell, but it is not responsible for maintaining the resting membrane potential.
* **Option B:** The Cl-/HCO3- exchanger is involved in the regulation of chloride and bicarbonate levels in the cell, but it is not a non-electrogenic pump and does not play a primary role in maintaining the resting membrane potential.
* **Option C:** The Na+/K+/2Cl- cotransporter is an electrogenic cotransporter that helps regulate sodium, potassium, and chloride levels in the cell, but it is not a non-electrogenic pump and can generate a net movement of charge across the membrane.
**Clinical Pearl / High-Yield Fact**
The Na+/K+-ATPase pump is essential for maintaining the resting membrane potential in skeletal muscle cells, and its activity is critical for muscle contraction and relaxation.
**Correct Answer:** C.