In the original Hodgkin & Huxley analysis of the membrane currents in squid giant axon, the time dependence of the currents was attributed to voltage-dependent changes in the permeability of the membrane to the ions involved. According to this theoretical framework, the time constants for the rate of change of current should be unique functions of the membrane potential, and should be independent of the previous history of the voltage and current flowing. In recent years, however, there has been increasing awareness of the fact that, for many physiological preparations, the space just outside the cell membrane is not in good diffusive contact with the bulk extra-cellular fluid perfusing the preparation. Consequently, when current flows across the cell membrane, there are changes in the ion concentrations in this so-called ‘restricted extra-cellular space’ (RECS).

At the end of the last century it was established that the different nerve cells along a neuronal path do not come into direct physical contact with one another, but that there are narrow gaps between them, called synapses (Sherrington, 1897; Ramón y Cajal, 1906). Elliot (1905) made the basic experimental observation that the propagation of nerve impulses across a synapse might be mediated by specific chemical agents (see Fig. i). Such substances are now called neurotransmitters, and some 20 different compounds putatively responsible for synaptic transmission in different parts of the nervous system are known at present, e.g. a few recently isolated polypeptides. The most extensively studied transmitters are acetylcholine and the catecholamine group, consisting of dopamine (a), noradrenaline (b), and adrenaline (c).

DNA restriction and modification enzymes are responsible for the hostspecific barriers to interstrain and interspecies transfer of genetic information that have been observed in a variety of bacterial cell types. Although the phenomenon of host specificity was initially observed in the early 1950s (Luria & Human, 1952; Bertani & Weigle, 1953), it was nearly a decade later that Arber and his colleagues accurately predicted the molecular basis of the phenomenon. Their experiments with bacteriophage λ demonstrated that a given host-specificity system imparts a specific modification to the viral DNA, and further, that restriction of DNA lacking the appropriate modification is s consquence of nucleolytic hydrolysis upon entry into the host cell (Arber & Dussoix, 1962; Dussoix & Arber, 1962; Arber, Hattman & Dussoix, 1963).

Given the existence of a difference in electrical potential between the interior of a nerve cell and the media surrounding it, where the cytoplasm is some 70 mV negative (Hodgkin, 1958), it must be expected that any positively charged ion to which the cell membrane is permeable is more concentrated in the cell interior. For monovalent cations such as Na and divalent cations such as Ca and Mg this is not the case in the majority of the cells such as the squid giant axon. In other words, nerve cells maintain a lower intracellular concentration of these ions, as compared with their concentration in the extracellular fluid. For Mg, Ca and Na ions, this lower internal concentration must, in the steady state, be effected by some membrane based mechanism which consumes energy.

(Quart. Rev. Biophys. 12, 1, February 1979, page 51) contained several drafting errors. It should be replaced by the corrected version below.