Factors Influencing Facilitation in Actinozoa. The Action of Certain Ions

In at least some of the muscles in Coelenterates the size of response is controlled by the neuromuscular junction. The sphincter in Calliactis and the longitudinal mesenteric retractor muscles of Metridium senile are served by part of the nerve network which acts as a single excitable system. The response is not graded through a large number of separate motor units. A single stimulus applied to this nerve net is ineffective, or only causes a trivial response ; but it leaves behind it an effect which facilitates the transmission of a second impulse to the muscle. Each impulse in a series facilitates the transmission of excitation to more and more muscle fibres. After each stimulus the facilitating effect gradually dies away (Pantin, 1935 b). In a short discussion, the relation of this “facilitation” process to other cases of junctional transmission was considered (Pantin, 1937). The process by which excitation is only effective if one stimulus follows its predecessor before the period of facilitation has departed, is analogous to the condition of vertebrate skeletal muscle during partial curarization. But at present the nature of the facilitation process is still unknown. A study of the action of ions and other substances throws considerable light upon its nature, and it is the object of this paper to describe and discuss these effects.