Electrophysiology and Excitability

The transmembrane potential is sometimes called resting membrane potential, although a viable cell constitutes an ‘arena’ of many complex biochemical transformations, and, thereby, is never at rest. The membrane itself is continually traversed (both passively and actively) by electrical ionic currents. However, the resting membrane potential has a special significance in the case of excitable cells, and it is to be distinguished from local and action potentials, discussed later on in this chapter.

The most common method of determining the transmembrane potential (Fig. 6.1) relies on measuring the potential difference between a large reference electrode (RE) immersed in the suspending medium of the cell and a microelectrode (μE) inserted through the membrane into the interior of the cell (Koester, 1991; Guyton, 1992; Bear et al., 2001). It can also be measured by quantifying the fluorescence intensity and/or spectral shift of voltage-sensitive fluorescent dyes as they approach the membrane (Emaus et al., 1986; Hibino et al., 1993). More recently, an interesting variant of the dye-based method has been developed, which relies on dyes that generate second harmonics when hit by intense ultrashort (i.e., femtosecond to nanosecond) laser pulses (Sacconi et al., 2005).