Most of the electrophysiological studies involve bioelectrical potentials generated by sources inside the body such as nerve, brain, pacemakers, muscles etc. The body as a whole acts as volume conductor and it helps in the extracellular spread of current (i.e. volume conduction). The source of current lies in the cell membrane where millions of voltage gated and ligand gated ion channels are placed which is being operated under dynamic equilibrium with various neuro-hormonal regulatory systems of body. The intricately tuned regulatory control system changes the behavior of the membrane property by changing the permeability of the membrane to a particular ion which in turn is being controlled by the variable expression of the channels for those particular substance or ions over the cell surface. The change in the channel behavior alters the potential difference between the two surfaces of the cells, alters the Calcium or other ionic level inside the cell and may also create a train of propagating impulse without any decrement in the voltage called action potential.
One of the important determinants of the current strength and its initiation is the intracellular versus extracellular ionic distribution and its balance. For example a small change in potassium concentration extracellularly may cause cardiac emergency or flaccid paralysis because of the change in the cardiac or skeletal muscle ionotropic properties. As many of the ion channels are voltage gated channels, a slight change in the ionic concentration gradient across the cell membrane may cause a significant change in the resting membrane potential or the threshold local potential to generate action potential. In other words, it may cause a change in the membrane excitability. Tetany may be cited as a good example of similar change in the membrane property. Tetany is a condition in which the skeletal muscle becomes hyper excitable resulting in the spasmodic contraction of a group of skeletal muscles upon very trivial stimulation. The condition is caused by slight decrease in the extracellular ionic Calcium concentration. The ionic Calcium in its normal extracellular concentration is responsible for the stability of the skeletal muscle membrane Sodium channels. When it decreases due to any reason, the membrane Sodium channel becomes unstable and even a trivial stimuli is capable of opening sufficient number of Sodium channels to create a threshold voltage for generation of action potential resulting in the hyperexcitability of the skeletal muscles.