Abstract
PREVIOUS work on the histology of the peripheral nerves of annelids has failed to reveal the nature of the termination of the motor axons on the muscle fibres. Retzius1 and Smallwood2 both described the motor axons of Lumbricus as ending simply with small swellings on the surface of the muscle fibres after producing a few lateral branches. Smallwood described a second type of ending which may be similar to those presented here, but his figure is inadequate. Fig. 1 is a photograph of a methylene blue preparation of Nereis diversicolor showing the termination of three motor axons on the parapodial levator muscle. They are located in the dorsal region of the parapodium where the fibres of this muscle originate on the connective tissue underlying the epidermis. I have also found similar endings in this position in Nereis virens. Each axon divides into a cluster of 15–20 end plates and transverse sections of the preparation showed that these were on the surface of a small group of muscle fibres. The endings (Fig. 2) are similar to the grape type shown by Gray3 to occur on the tonic muscle fibres in the frog. If the simple type of termination described by Retzius represents a true functional ending, the occurrence of a second type is of great interest in view of the evidence put forward by Horridge4 and Wilson5 for fast and slow systems of muscular contraction. It suggests the possibility that each type of muscular response may be mediated through morphologically distinct endings.
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References
Retzius, G., Biol. Unters. Neue Folge (Leipzig), 3, 1 (1892).
Smallwood, W. M., J. Comp. Neurol., 42, 35 (1926).
Gray, E. G., Proc. Roy. Soc., B, 146, 416 (1957).
Horridge, G. A., Proc. Roy. Soc., B, 150, 245 (1959).
Wilson, D. M., J. Exp. Biol., 37, 46 (1960).
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DORSETT, D. The Motor Axon Terminations of Annelids. Nature 198, 406 (1963). https://doi.org/10.1038/198406a0
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DOI: https://doi.org/10.1038/198406a0
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