Factors Affecting Muscle Activation in the Hydroid Tubularia

Activating the distal opener system (DOS), one of the conducting systems of the hydroid Tubularia, causes synchronous opening of the distal tentacles. Simultaneously recorded potentials (DOS pulses = DOSP's) from the vicinity of the responding muscles have two components: (a) an initial, short, all-or-nothing pulse reflecting activity in the conducting system, and, after a brief delay; (b) a second, slower potential thought to be a muscle action potential because its amplitude varies monotonically with that of tentacle movement. The amplitude of the second component is here used as a measure of the intensity of muscle activation.2. The second DOSP component is depressed by preceding DOS activity (defacilitation); by spontaneous tentacle movement including but not restricted to spontaneous DOS firing; by firing of the HP sytsem, one of the pacemaker systems in the polyp; and during the mouth opening associated with defecation. In all instances the first DOSP component is unchanged, indicating that the depression is of transmission processes between the conducting system generating the first component and the muscle membrane producing the second component. The recovery from depression is slow, taking tens of seconds following HP activity and minutes after DOS firing.3. The HP system and the DOS are mutually inhibitory. Previous work has shown that activity in the DOS inhibits HP firing; here it was found that HP firing inhibits the tentacle response which is a consequence of DOS activity. The depression following spontaneous tentacle movements suggests that spontaneous activity and DOS firing may excite the muscles through a common final pathway.