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Neural pacemakers and rhythmicity.

This review will draw on material gathered both from nerve cells and from other types of secretory cells that display excitability characteristics similar to those of neurons. Indeed, in view of the similarities, it may be profitable to regard many types of secretory cells as neurons with vanishingly short axons and dendrites. I will emphasize the possible mechanisms linking the internal state of the cell to rhythmic electrical activity, predominantly a membrane phenomena. This seems a sensible relationship to bring out because, in the broad sense, rhythmic neurons are a component of control loops in which the output of an effector organ is modulated by neural firing. With the obvious exception of stretch receptors and the like, where it is probable that the primary stimulus input directly alters membrane properties (cL 99), many environmen­ tal factors (for neurons) have their strongest effects on cytoplasmic conditions or levels of cytoplasmic second messengers, raising the question of how these changes are coupled to the cell membrane. At the present there is no particular difficulty in generating periodic electrical discharge in computer simulations based upon experimentally defined mem­ brane conductances. The characteristic frequencies of these discharges lie in the millisecond-to-minute range, depending upon which parameters are used in the simulations and from what preparation they were derived. Data for these simulations have come exclusively from invertebrate neurons, which are much more amenable to the voltage-clamp analysis necessary to describe membrane conductances than are neurons from vertebrates. The data base of longest standing is the one derived from the squid giant axon by Hodgkin & Huxley (51). A number of investigators demonstrated that this set of conductances (or in more modem terminology, the Na and delayed K-ion