Slow Contraction and Its Relation to Spontaneous Activity in the Sea-AnemoneMetridium Senile(L.)

Abstract

1. The very slow responses of the body wall of the actinian Metridium senile have been studied, both in intact animals and in partially isolated tissue preparations. 2. The slow responses to electrical stimulation differ from the rapid facilitated responses of the retractor muscle. There is an enormous latent period of peripheral origin. The contraction is sigmoid, and does not show the step-like character of the retractor response. The relation to frequency and number of electric stimuli also differs. The responses vary with the state of the animal. 3. Some tissues, such as the marginal sphincter region, give two distinct kinds of contraction: a quick facilitated response and a slow delayed response. There is no striking histological differentiation into two kinds of muscle fibres, though there is some anatomical differentiation into two regions. 4. In contrast with the total through-conduction to the sphincter and the retractor, the radial muscle of the disk shows radial ‘linear through-conduction’, the response remaining localized circumferentially. Nevertheless, in the disk also there are both quick and slow responses. 5. The slow responses in the intact animal are not simple contractions. They consist of a co-ordinated sequence in several muscles which may continue for many minutes. There is evidence of reciprocal inhibition between the circular and parietal muscles. The responses to the same stimulus vary greatly at different times. 6. The slow responses to electrical excitation show a detailed resemblance to the spontaneous contractions of the same muscle systems. The muscle systems excited are not passive, but spontaneously active systems. 7. The parietal muscle system can be excited to contract locally by specific stimuli. Electrical excitation excites always the whole parietal system. The co-ordinating system in the latter case is considered to be the through-conduction system. The complete identity of the spontaneous parietal-circular sequence with that resulting from electrical excitation indicates that the through-conduction system co-ordinates the spontaneous sequence as well as the response to the stimulus. There is evidence of occasional impulses in the through-conduction system during spontaneous activity. 8. Partly isolated rings of the circular muscle in such a preparation of the body wall, connected by a strip of tissue, respond to electrical excitation. In contrast, in the intact animal the contractions of the parts of the circular muscle system are co-ordinated to give an exceedingly slow peristaltic or antiperistaltic wave. 9. The excitation system of the slow muscle resembles the visceral neuromuscular system of vertebrates rather than that of skeletal muscles such as the limb muscles.