Nervously mediated change in the mechanical properties of the cirral ligaments of a crinoid

Abstract

Abstract The cirri of crinoid echinoderms are jointed appendages which show variable rigidity but lack muscles. The cirral joints are subtended by collagenous ligaments which determine the passive mechanical behaviour of cirri under tensile load. Creep and stress relaxation tests were performed on the cirri of Antedon bifida. In rigid, initially flexed cirri, the ligaments behave like stiff visco‐elastic solids. When the external K‐ion concentration is elevated by as little as 5 mM, the ligaments become pliant, allowing such cirri to straighten under low loads. Responsiveness to K is depressed or abolished by anaesthetic agents. It requires external Ca, but Sr or Ba can be substituted for Ca. It is enhanced by excess Ca or Mg‐lack, and inhibited by Ca‐lack, excess Mg, or Mn, Co, and La ions. The cirral ligaments are known to be permeated by the processes of neurosecretory‐like cells which are innervated by a branch of the cirral nerve. It is argued that the response to K is nervously mediated and that by depolarizing neuronal elements K may activate a local nervous reflex which brings about destabilizing changes within the ligaments.