Slow Axons Research Articles

Functioning of the Inhibitory Conditioning Axon Innervating Insect Muscles*

ABSTRACT The jumping muscle of locusts is supplied by three motor axons (Hoyle, 1955 a), one of which gives rise to large twitch contractions when stimulated with a single shock and is termed the fast (F) axon, whilst another gives rise to slow contractions only, when repetitively stimulated, and is termed the slow (S) axon (Hoyle, 1955 b). The third axon was found not to give a contraction when stimulated alone but it did cause a hyperpolarization in some of the muscle fibres Thus, by comparison with peripheral inhibitory action at crustacean neuromusclar junctions (Fatt & Katz, 1953), amphibian heart (Hutter & Trautwein, 1956) and cat spinal cord (Eccles, 1953), its function could be expected to be inhibitory; but Hoyle (1955b) found that stimulation of this axon at the same time as stimulation of either of the two excitatory ones did not lead to mechanical inhibition. Instead, the mechanical responses were enhanced, though the effect was slight so functional significance could not be ascribed to it with confidence. Since this phenomenon could not be fully understood it has remained in the background awaiting further investigation.

The mechanism of dual responsiveness in muscle fibers of the grasshopper Romalea microptera.

Dually innervated Romalea muscle fibers which respond differently to stimulation of their fast and slow axons are excited by intracellularly applied depolarizing stimuli. The responses, though spike-like in appearance, are graded in amplitude depending upon the strength of the stimuli and do not exceed about 30 mv. in height. In other respects, however, these graded responses possess properties that are characteristic of electrically excitable activity: vanishingly brief latency; refractoriness; a post-spike undershoot. They are blocked by hyperpolarizing the fiber membrane; respond repetitively to prolonged depolarization, and are subject to depolarizing inactivation. As graded activity, these responses propagate decrementally. The fast and slow axons of the dually responsive muscle fibers initiate respectively large and small postsynaptic potentials (p.s.p.'s) in the muscle fiber. These responses possess properties that characterize electrically inexcitable depolarizing activity. They are augmented by hyperpolarization and diminished by depolarization. Their latency is independent of the membrane potential. They have no refractory period, thus being capable of summation. The fast p.s.p. evokes a considerable or maximal electrically excitable response. The combination, which resembles a spike, leads to a twitch-like contraction of the muscle fiber. The individual slow p.s.p.'s elicit no or only little electrically excitable responses, and they evoke slower smaller contractile responses. The functional aspects of dual responsiveness and the several aspects of the theoretical importance of the gradedly responsive, electrically excitable component are discussed.

STUDIES ON NEUROMUSCULAR TRANSMISSION IN LIMULUS

1. The electrical responses occurring in single muscle fibers of the closer muscles of the chelae of the walking legs of Limulus have been studied with the aid of intracellular electrodes and electrical stimulation of the motor axons. At the same time the total tension of the muscle was recorded at the tarsal tip.2. The muscle is supplied by only two motor nerve fibers, one of which (the "fast" axon) evokes larger mechanical and electrical responses than does the other (the "slow" axon).3. No inhibitory nerve fiber was found.4. The electrical responses consist typically of junctional potentials resembling small end-plate potentials. The fast junctional potentials may give rise to small spike potentials.5. On repetitive stimulation both axons give rise to plateaux of depolarization, from which small spikes may arise.6. The mechanical responses consist of very small twitches to single shocks and tetani to repetitive excitation. The tetanus/twitch ratio is more than 30:1 for the fast axon, more than 100:1 for the slow axon.7. There is post-tetanic potentiation of the twitch response of up to 5 times in the mechanical response to a single shock applied to the fast axon. This decays slowly over a period of about a minute.

DOUBLE DISCHARGES IN HUMAN MOTOR UNITS

DOUBLE DISCHARGES IN HUMAN MOTOR UNITS