Nerve Excitation

This well-written monograph is designed for students and investigators. Full appreciation requires a high level of sophistication in physical chemistry, as well as biophysics and mathematics. However, the historical introduction, the textual development of ideas, and the summary (which may best be read first) should make the broad outline comprehensible to any reader who is reasonably familiar with a modern textbook understanding of the mechanism of membrane excitability in nerve and muscle. Tasaki’s new concepts are based largely upon recent experiments from his laboratory involving elaborate manipulations of both the internal and external fluid media bathing the excitable membrane of the squid axon. Using models of inanimate physical chemistry, especially ion exchanger membranes, Tasaki develops a hypothesis which not only accounts for the now classical Hodgkin-Huxley thesis but also for the complex newer data which are otherwise difficult to interpret. He suggests that the excitable membrane exists in two stable conformational statesthe resting and the excited. The macromolecular membrane complex is thought to have a relative excess of fixed negative charge at the external layer which provides cation exchanger properties. “In the resting stable state, the anionic sites in the membrane are occupied primarily by divalent cations (largely calcium) derived from the external medium; in the excited stable state these sites are occupied predominantly by univalent cations (largely potassium). The conformation of the membrane macromolecules and the properties of the cation exchanger are determined primarily by the univalent/divalent cation ratio within the membrane. . . . The excitation process is triggered by the transport of internal univalent cations (potassium) into the membrane. . . . When a limited area of the membrane is transformed into its excited conformation, a local circuit is effected.” The propagated impulse carries on as understood in the classical theory. The generally increased membrane conductance of’ excitation is associated with increased cation interdiffusion across the membrane and a consequent change in the local ionic environment of the excited membrane. Excitation is terminated (inactivation process) when divalent cations from the external medium diffuse back into the membrane in sufficient concentration again to form stable complexes with fixed anionic sites. This effort to bring together concepts of changing ionic conductance and the structure of the protein-phospholipid complex of the membrane is obviously significant. Although thermodynamic aspects are given important emphasis, metabolic features that distinguish the living from the inanimate membrane are not considered in these relatively short-term experimental studies. Although the material is complicated and controversial, the interrelations of electrophysiological function with membrane structure and electrolyte transport are clearly important to clinical neurology. This book provides a useful and necessary introduction to a field that requires our attention.