Wednesday, June 22, 2011

roundworm currently unexplained

The roundworm has only 302 neurons in its brain, yet complete understanding of its behavior is not available at present - see here:

Why is the wiring diagram produced by Dr. White so hard to interpret? She pulls down from her shelves a dog-eared copy of the journal in which the wiring was first described. The diagram shows the electrical connections that each of the 302 neurons makes to others in the system. These are the same kind of connections as those made by human neurons. But worms have another kind of connection.

Besides the synapses that mediate electrical signals, there are also so-called gap junctions that allow direct chemical communication between neurons. The wiring diagram for the gap junctions is quite different from that of the synapses.

Not only does the worm’s connectome, as Dr. Bargmann calls it, have two separate wiring diagrams superimposed on each other, but there is a third system that keeps rewiring the wiring diagrams. This is based on neuropeptides, hormonelike chemicals that are released by neurons to affect other neurons.

The neuropeptides probably help control the brain’s general status, or mood. A strong hint of how they work comes from the npr-1 gene, which makes a protein that responds to neuropeptides. When the npr-1 gene is active, its neuron becomes unavailable to its local circuit.

That may be a reason why the worm’s behavior cannot be computed from the wiring diagram: the pattern of connections is changing all the time under the influence of the worm’s 250 neuropeptides.

The connectome shows the electrical connections, and hence the quickest paths for information to move through the worm’s brain. “But if only a subset of neurons are available at any time, the connectome is ambiguous,” she says.

The human brain, too, has neuropeptides that set mood and modify behavior. Neuropeptides are probably at work when the pain pathways are cut off in acute crises, allowing people to function despite serious wounds.

The human brain, though vastly more complex than the worm’s, uses many of the same components, from neuropeptides to transmitters. So everything that can be learned about the worm’s nervous system is likely to help with the human system.

Though the worm’s nervous system is routinely described as simple, that is true only in comparison with the human brain. The worm has 22,000 genes, almost as many as a person, and its brain is a highly complex piece of biological machinery. The work of Dr. Bargmann’s and other labs has deconstructed many of its operational mechanisms.

What would be required to say that the worm’s nervous system was fully understood? “You would want to understand a behavior all the way through, and then how the behavior can change,” Dr. Bargmann says.

That goal is not unattainable,” she adds.

[D.G. Of course, one must be optimistic - how else will one justify new grant applications? But this should give pause to those who claim that the workings of the human brain are well understood. For example, here is a summary of all the brain's major functions:]