At rest, muscle cells exhibit a distinct difference in charge between the inside and outside of the cell membrane, known as the membrane potential. The inside of the muscle cell is negatively charged relative to the outside. This charge difference arises primarily from the uneven distribution of ions, particularly sodium (Na+) and potassium (K+) ions.
Inside the cell, there is a higher concentration of potassium ions compared to the outside, while the outside has a higher concentration of sodium ions. The cell membrane is more permeable to potassium ions than sodium ions, leading to the movement of potassium ions out of the cell. This efflux of positively charged ions contributes to making the interior of the cell more negative, creating a resting membrane potential of approximately -70 mV.
This charge difference is crucial for various cellular functions, including the generation of action potentials that are essential for muscle contraction and communication between nerve cells. The maintenance of this resting charge is primarily facilitated by the sodium-potassium pump, which actively transports sodium out of the cell and potassium into the cell, further establishing the resting membrane potential.