Which of the following is an intracellular constituents of neuromuscular system?

The nerve and muscle fibers, like other bodily cells, maintain a fluid internal environment that is distinctly different from the external or interstitial medium. The main intracellular constituents are potassium (K), magnesium (Mg), and phosphorus (P), whereas those outside the cell are sodium (Na), calcium (Ca), and chloride (Cl). Ref: Ropper A.H., Samuels M.A. (2009). Chapter 45. Electrophysiologic and Laboratory Aids in the Diagnosis of Neuromuscular Disease. In A.H. Ropper, M.A. Samuels (Eds), Adams and Victor's Principles of Neurology, 9e.