The squid giant axon occupies a unique position in the history of our understanding of cell membrane potentials and nerve action. When an electrode is stuck into an intact giant axon, the membrane potential registers a negative number. Resting membrane potential at room temperature is solely determined by the sodium and potassium channels. See below for the ion gradients.
Ion Cytoplasm Seawater
Na+ 60 mM 420 mM
K+ 400 mM 20 mM