The brain, our most magnificent organ, is among other things a generator of decisions. It routinely receives input, matches it against an internal representation about the exterior world, and adjusts our behavior in appropriate ways. (More often appropriate than not, that is.)
In our staggeringly complex human brains, it is impossible to pinpoint any decision to a single cell. Most researchers subscribe to a model where decisions in the brain are determined by the architecture and dynamics of the neural network, i.e. the synaptic connections, transmissions, and responses.
But what happens in a nerve system with less than our approx. 1014 connections? Brain sizes come in a huge continuum, ranging from 4-5 kg in the elephant, over approximately 1.3 kg in the human, down to invertebrates with only a few nerve cells in their entire bodies. Yet all these creatures share the ability to make decisions.
With diminishing complexity in the nervous system, the actual decision should become easier and easier to pinpoint, and eventually it might converge on a single cell. This is in fact what has been found in the Aplysia, a sea slug with a fairly simple and very well-characterised nervous system, and a common model organism in neuroscience. A neuron called B51 has been shown to make the decision to carry out feeding behaviour. Successful feeding is rewarded by dopaminergic signaling from the esophagus back to neuron B51. This is known in psychology as operant conditioning, meaning roughly that the organism learns from the consequences of its behavior.
A recent paper by Fred Lorenzetti and co-workers in the journal Neuron begins to shed light on how decision-making is carried out by neuron B51. Lorenzetti and his colleagues found that reward of the feeding behavior led to changes in the membrane structure of the neuron, reducing the threshold for firing and therefore making it more probable that feeding behavior will be initiated. They were also able to block the activities of a few intracellular proteins, and found two protein kinases that were crucial for operant conditioning to take place.
We can attribute meaning to these biochemical changes. In its context, a reduced firing threshold of neuron B51 probably means that there is a greater abundance of food in the environment. This piece of knowledge is a part of the internal representation of the outside world. And this particular cell possesses enough complexity to both carry this part of the internal representation and function as a decision generator on its own!
Of course, since the cell is part of the neural network this doesn’t mean that a network-centered view is any less correct or useful. The information processing of neuron B51 can only be made meaningful in the context of its neuronal connections. And it is not necessary to know, from a network-perspective, which specific changes in the cell that are underpinning the cell’s altered electrical activity. And finally, as a caveat, I should add that much is still unknown about the regulation of neuron B51. This model may well have to be questioned in the light of future evidence.
But this story illustrates three important things:
That a single cell can be capable of making decisions
That the internal workings underlying decision-making in the cell are attracting attention, although more from neurobiologists than from cell biologists.
That the decisions of single cells in multicellular organisms may require other cells to decode the decision and translate it into behavior, which means that the decision is only meaningful in that highly specific context.