ABSTRACT
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 Re’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 what-soever 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 ‘true1 inhibitors.) In all other systems, including every system of Panultrus, no reduction of the muscle action potential is obtained.
This has been established with certainty for Panulirus only, but is also very likely for Camfrorm and the crabs.
This innervation of the acceaaory flexor was first demonstrated during the present investigation. In the crabs and the crayfish the innervation of this muscle has not yet been found.
Facilitation of excitation has been shown to work at two places, for there is a facilitation of the action potential and of the contractile mechanism (Wiersma & van Harreveld, 1939). In the present discussion we have taken these two together, thus deliberately simplifying the picture. It seems possible that in the inhibitory process two similar facilitations are active, but at present it is not fruitful to discuss the theoretical consequences of such an arrangement.
