Feedback cue and switching characteristics

Abstract

Asexual reproduction (fissioning) in planarians is controlled by group size; increased group size suppresses fissioning, isolation releases it. This effect is mediated through the brain; social stimulation from cohorts causes the brain to inhibit the segmental plexus fissioning system from initiating the events of fissioning. Experiments were conducted to elucidate 1) the stimulus cue conveying information regarding cohort number and 2) the switching characteristics of the control system. The results of these show that the social stimulus responsible for the effect involves neither vision nor discharge of pheromonal substances into the habitat; the sensory event apparently involves direct contact and chemoreceptive, as well as tactile, modalities. The switching characteristics (graduated response, long release time, short reinstatement time) found for the brain inhibitory system agree with those expected from the mathematical model derived for a simple neurohormonal control system in which it is postulated that social stimulation causes neurosecretory cells of the brain to release a fissioning inhibitory hormone (FIH) that inhibits the segmental plexus fissioning (SPF) system from initiating fissioning.

Localization and identification of the receptors by electron micrographic and ablation studies

“…Or why, in isolated worms, should ablation of the cephalic margins yield any further increase in fissioning? A previous study (Best et al, 1974) indicated some suppression of fissioning by nonspecific background stimulation of intact isolates which, although not large in comparison to social stimulation effects, was significant. This finding, taken altogether with the fact that our marginal ablation removes most, but not all, neural cilia of the cephalic surface, permits one to answer the two questions posed a t the beginning of this paragraph.…”

“…The inhibitory effect of the brain becomes attenuated with distance along the axis of the worm; under comparable conditions, long worms are more likely to fission than short ones. The "switching" characteristics are also suggestive of those to be anticipated from a rieurohumoral meclhanism; release of fissioning, following isolation, is sluggish but the inhibition of fissioning, following regrouping, is rapid (Best et al, 1974).…”

“…These were from the same cultures, maintained in the same way, as described previously Best et al, 1974). The glassware used in these experiments was cleaned in the same manner described before (Best et al, 1974).…”

“…These were from the same cultures, maintained in the same way, as described previously Best et al, 1974). The glassware used in these experiments was cleaned in the same manner described before (Best et al, 1974). This clean glassware was then preslimed, prior to being used in an experiment, by placing a number of nonexperimental planarians, in aged tap water, in each vessel for 24 hr.…”