The effectiveness with which different contractions in a number of muscles can be inhibited was investigated. As a measure of this effectiveness the frequency of inhibition which can just inhibit a contraction with a given frequency of excitation was determined. It was found that in all systems the ratio (Rc) of such inhibitory frequencies to that of the excitatory frequencies they can suppress was constant for a wide range of frequencies.
At high frequencies either the inhibition or the excitation may become less effective. This is explained by failure of the respective system to function normally at such a frequency.
The effectiveness of inhibition of different systems was determined. Some systems show a very constant Rc value; in a second group Rc varies within wider limits; and a third group shows two distinct Rc's sometimes in the same preparation at different times.
Rc values have been found to vary widely. For instance, in the bender inhibitor-slow bender system of Pachygrapsus three excitatory impulses are suppressed by one inhibitory impulse; in the closer inhibitor-slow closer system of Cambarus one excitatory impulse needs five inhibitory impulses to counteract its effect. The fast closer contraction of Cambarus and the fast closer and fast bender contraction of Pachygrapsus were found to be uninhibitable, i.e. no effect of inhibition whatsoever was noticed on any of these contractions. All three systems are distinguished by giving a mechanical response to a single stimulus in contrast with all the inhibitable systems which do not respond to single impulses.
Reduction of the action potentials during inhibition is obtainable in only a few systems, namely, the opener inhibitor-opener and the stretcher inhibitor-stretcher systems of Cambarus and the crabs. (In the crabs this applies only to the ‘true’ inhibitors.) In all other systems, including every system of Panulirus, no reduction of the muscle action potential is obtained.