12. Formal inversion in C. elegans - C.

Diagram developed from data published by White et al. (1986).

The four neurons have been artificially separated. To envisage the natural state, imagine the nearly circular parts superimposed on one another. These nearly circular parts contribute to the nerve ring -- the 'highest' structure in the worm brain and the homologue of our own cerebral cortex. Expressed simplistically, the homologous structures are the sites where the respective animals (representing minimal and maximal Bilateria) do the most sophisticated 'thinking' of which they are capable.

The black dots connected by coloured lines are sample connections between neurons.

• Green(ish) lines represent connections on the same side of the animal,
• Red(ish) lines between the two sides and different neuron pairs, and
• Blue lines between two sides and within a neuron pair.

[the 'ish' of the green and red lines refers to the fact that the scanning method has lost some of the colour of the original artwork. I shall hope to improve these colours in due course.]

In evolutionary terms, the green connections will eventually become the critically important association fibres and the red and blue connections the critically important commissural fibres in our own brains -- so that the scientific value of the minimal model brain in beginning to understand our own thinking processes can hardly be overstated.

Two other important points about this stage of Minimal Bilateria neural evolution should be appreciated. First, neurons were already manifest as three broad types: cells to receive stimuli (sensory neurons), cells to trigger responses (motor neurons) and cells to connect the two (interneurons) -- the basic arrangement that continues in all subsequent Bilateria including humans. In later animals, the three neuron types were to become increasingly structurally complex but whereas the sensory and motor neurons also increased their number in each circuit to a relatively small degree, the interneuron parts of circuits were to expand hugely to produce more and more powerful brains.

Second, connections between neurons were already of the two main types that occur in all later animals (chemical and electrical) but such connections were made via relatively simple 'touching points' between adjacent neuron processes -- a situation best imagined by the superimposition mentioned above. Such en passant connections were the earliest kind of neuron linkage in bilateral animals. In later animals such direct links were generally replaced by much more complex devices.

Because the worm brain has only about 180 neurons and only around 4000 such connections, it is possible to work out various basic circuits, as in the example in the next post.