Giant Axons and Synergic Contractions in Branchiomma Vesiculosum

Abstract

Conduction in the giant axons of annelids has been clarified by the work of Stough, Eccles and colleagues, Bullock, Rushton and others (reviewed by Nicol, 1948a). They are now known to be motor axons, or at least nerve fibres on the efferent side of reflex arcs, and they intermediate quick withdrawal movements effected by synergic contractions of widespread longitudinal muscles. Probably the most highly developed axons of this kind occur in the Serpulimorpha (Nicol, 1948 a, b, c, d). Our knowledge of the physiological activity of these nerve fibres, as components of a neuro-muscular unit, is still incomplete, however, and the sabellid worm Branchiomma vesiculosum (Montagu) has been investigated, as a suitable experimental animal. The present paper gives an analysis of giant axon functioning in the intact animal as revealed through muscular activity. A few observations on the functioning of the giant fibres in Myxicola infundibulum (a related species) have already been published (Nicol, 1948 c), and will be referred to in the present account. Budington (1902) described the results of stimulating pieces of the earthworm which contained nerve cord. His records show contractions of the longitudinal muscles, and it is probable that he was stimulating the giant axons of the animal.