Extracts of whole nerve (chelipeds of Cancer magister) cause inhibition of impulse generation of the crayfish stretch receptor preparation, similar to that produced by gamma-aminobutyric acid (GABA). This is not found with extracts containing only sensory or sensory and motor fibers. Extracts of inhibitory fibers inhibit the stretch receptor discharge-indicating an inhibitory action equivalent to that of up to 30,000 micrograms of GABA per gm. wet weight of inhibitor fiber. This high value is taken as an indication that the inhibitory substance in crab inhibitory fibers is not identical with gamma-aminobutyric acid. Whole nerves were found to contain 1.7 to 6.7 microg. acetylcholine per gm. nerve tissue (clam ventricle and frog rectus abdominis muscle). No acetylcholine could be detected in extracts of motor and inhibitory fibers. The acetylcholine content of sensory fibers can account for the acetylcholine activity of whole nerve extract. It is concluded that the factor I of crustacean nerve is an exclusive property of the inhibitory fibers. The results support the assumption that factor I is the transmitter substance of inhibitory neurons in these animals. The absence of acetylcholine in motor fibers indicates that this substance does not function as a transmitter of motor impulses in Crustacea, and explains the previously observed failure of the substance to elicit motor responses in these animals. The function of acetylcholine in sensory fibers is not yet clarified.

Acetylcholine↗

PubMed1960

Studies on the nervous regulation of the heart beat in decapod crustacea.

The effect of electrical stimulation of cardioaccelerator and cardioinhibitor nerves on the mechanically recorded heart beat of crayfish was studied. Similar experiments were performed with the lobster, Homarus americanus. Quantitative relationships between uni- and bilateral accelerator and/or inhibitor nerve stimulation and the resulting change in frequency and amplitude of the heart beat were established. With increasing frequency of stimulation the accelerator nerves show a relative decrease in their action, while that of the inhibitor nerves increases. It appears that left and right regulator nerves have synaptic contacts at the same areas of the postsynaptic cells within the heart ganglion. Similar results are obtained whether all impulses arrive over one, or over the other, or over both accelerator (or inhibitor) nerves; the resulting acceleration or inhibition depends strictly on the number of accelerator, or inhibitor impulses arriving at the ganglion. The ganglion cells can adapt to the inhibitor action. This is shown to be a postsynaptic phenomenon. Adaptation to accelerator stimulation is virtually absent. Characteristic after-effects of the accelerator and inhibitor action were observed and quantitatively evaluated. The interpretation of the results is based on the assumption of chemical transmitter substances. It is concluded that the accelerating transmitter decays slowly while the inhibitory transmitter is inactivated relatively rapidly.

Animals↗

PubMed1959

The effects of factor I and of gamma-aminobutyric acid on smooth muscle preparations.

Explore the source record for details and available documents.

Amino Acids↗

PubMed1958

Studies on the distribution of Factor I in mammalian brain.

Explore the source record for details and available documents.

Animals↗

PubMed1957

Chemical transmission and adaptation.

Acetylcholine and factor I appear to be transmitter substances of excitatory and inhibitory regulatory nerve fibers supplying the sensory neurons of stretch receptor organs of the crayfish. Sudden application of a low concentration of acetylcholine causes the impulse frequency to jump to a peak value. But immediately the frequency falls again and gradually reaches a steady state which is not far above the previous frequency level. If the acetylcholine is now withdrawn there follows a silent period after which the frequency returns to its original level. The time course of these events is identical with that of adaptations to sudden increase or decrease of stretch. Factor I in sufficiently low concentrations causes an immediate fall in impulse frequency (silent period) which is followed by a return to a value near the previous frequency level. Withdrawal of factor I is followed by excitation and again return of the frequency to the rate measured before the application of factor I. The time course of these phenomena is identical with that of adaptations to sudden decrease and increase of stretch. It is suggested that adaptation may be a property not only of sensory neurons but of neurons in general and that even central neurons may be considered as receptor neurons inasmuch as they respond to chemically transmitted excitatory and inhibitory stimuli.

Acetylcholine↗

PubMed1957