Extending the cellbrain analogy

We have covered the main point of this chapter: the possibility that mind "emerges from the brain" in much the same way as life "emerges from the cell". There is a striking formal analogy between our characterisation of the living state and our provisional characterisation of mind. Like all analogies, this one has limits, but it is interesting to see how much further it can be extended.

1. "Luxury" and "housekeeping" functions. These are rather old-fashioned terms in molecular biology but they have not lost their meaning. Some proteins are necessary for all cells, so they are made in all cells. We describe these as "housekeeping" proteins. Examples include the enzymes of central metabolism and the main components of the cytoskeleton. Genes for these ubiquitous proteins are called "housekeeping" genes. Other proteins are only necessary, at least in significant quantities, in particular differentiated cells, i.e. cells that have become dedicated to specialist functions at the expense of their capacity to divide. Genes for these specialist proteins are called "luxury" genes.

Some parts of the brain are necessary for overseeing basic physiological functions, e.g. respiratory muscle contractions and the beating of the heart. No vertebrate could survive without these functions. Other parts of the human brain are devoted to specifically human functions such as facial recognition, communication of emotions, language and abstract thought. The basic-physiology parts of the brain can be regarded as analogous to "housekeeping" genes, and the higher-function parts to "luxury" genes.

2. Redundancy. There is a good deal of redundancy in both cell and brain. Many signalling pathway components in cells are the products of genes that have been duplicated and then modified during the course of evolution53. In the brain, "back-up" systems can be brought into play if an area is damaged; and sensory inputs are parcelled into blocks of information that are sent to disparate areas. These are examples of redundancy. The human visual system has at least six distinct and apparently largely independent processing pathways. Apparently this is the result of duplication during embryonic brain development.

Redundancy in both cell and brain provides emergency back-up mechanisms in the event of system failure. It also affords more possibilities for cross-talk among different signalling pathways. And as Kauffman and others have observed, redundancy seems to be a prerequisite for robustness in complex systems.

3. Stress responses. When external conditions exceed the limits of normal cell functioning, "heat-shock" genes are expressed. The resultant proteins shut off almost all luxury functions. Just enough housekeeping functions are sustained to keep the cell alive. The heat-shock proteins bind to regulators of luxury function. They release them again when conditions become less hostile and normal cell activity can be resumed.

When conditions exceed the capacity of the brain to tolerate them, individuals suffer clinical depression, or in some cases catalepsy. Many higher brain functions are down-regulated, though housekeeping activities remain intact. Stress hormones are released, ensuring a continuing glucose supply. This is necessary (though it is unlikely to be sufficient) for restoring normal brain function.

53 There are numerous examples. A prominent one is protein kinase C, which is essential for transducing many extracellular signals. Protein kinase C exists in many different isoforms, the products of a much-duplicated and modified gene.

There is therefore some parallelism between the effects of "stress" on the cell and the effects of "stress" on the brain.

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