Wednesday, 3 June 2009

9. Formal inversion in a Radiata model.

We have been edging our way from molecular stimulus-response systems and brain-mind homologues towards the emergence of their neural counterparts, which first appeared in the Radiata. A standard laboratory model for these typically jar-shaped, blind-gutted, radially symmetric animals is the freshwater Hydra. The hydra polyp is attached by its base (B) and has an upward pointing mouth/anus (MA) surrounded by a zone of tentacles (T).

The nervous system of a hydra is a body-wide nerve net (NN) via which any stimulus anywhere provokes a full body response. The key structure for our purpose is the nerve ring, which is a thick neuron bundle running round the circumference of the animal between the mouth/anus and the tentacles. Its key developmental feature is its stability. Whereas the rest of the hydra nervous system undergoes constant cell replenishment, the nerve ring does not. The nerve ring is thus a highly credible candidate for a simple brain.

The Radiata represent the most likely evolutionary stage at which the circularity factor became an integral part of our own brain heritage (some 1300 million years ago according to modern 'molecular clocks'). There are at least six arguable theories as to how Radiata nervous systems became Bilateria nervous systems (like those of humans) and one of these is simplistically shown. The main point for the present argument is that at this stage, before the advent of bilaterality and forward foraging, all brains were unicyclic.

When the nerve ring is examined closely, as schematised in the lower diagrams, it is seen to be made of chains of individual neurons in which signals from any stimulus travel in both (mutually inverted) directions round the ring.

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