Neurobiology

The human brain is highly complicated and unique. It gives us the power to think, imagine, and decide. Moreover, it contains billions of neurons to perform such specific tasks like regulating body temperature, perceiving and interpreting sensations, and causing secretions of hormones. All of these were coordinated and controlled just by an organ weighing approximately 1.3 kilograms.

In our third year Advanced Biology Class, one of the topics we’d discussed was the human nervous system, the main component of which is the brain. Because of this, we have become interested in the human brain, its functions, and how it is able to control virtually every other part of the human body. Fortunately, Mr. Greenside also specializes in neurobiology - the study of the nervous system - and he was able to explain a few things about the human brain that were certainly new to us.

"All details about how brains work are poorly understood at this point, not just by physicists but by everyone, including neurobiologists. The parts of brains that are well understood tend to be the parts of brain that are involved with input (the neurons that convert sound, light, chemicals, touch, etc into signals that the brain can understand) and with output (so-called motor neurons which directly connect to muscles and that cause the muscles to contract). But once you are more than about two neurons deep into the brain, when each neuron is in contact with 5,000 or more other neurons and all are actively transmitting information simultaneously, almost nothing is understood about how brains represent information or how they process the information. The situation is rather like chemistry before the discovery of the periodic table. People know a huge number of facts about brains: what happens if you destroy certain parts of brain tissue, what happens to behavior if you apply drugs or electrical stimulations to various parts of brains, how electrical activity of particular neurons correlates with stimuli. But scientists still lack a conceptual and theoretical framework to know which of these many details are important or central.

As one example of the difficulties involved, scientists still don't understand how memory works in any brain. They know from experiments that certain anatomical parts of brains called synapses, little micron-size nanomachines that connect one neuron to another, change their internal properties when an animal learns something new or when it forgets something, and all scientists believe that learning and memory is related to changes in synapses. (Human brains have a large number of these synapses, about 10^14 = 100,000,000,000,000, and that partly explains your ability to remember so much information.) But we don't know how brains read out the changes in synapses to actually extract a memory; we don't know the algorithm brains use to change the synaptic properties to encode memory.

So from one point of view, scientists studying the brain are educated guesses as to what might be happening, and we need to wait for (or help to develop) future inventions of new technology, and perhaps the invention of new kinds of mathematics, that will be able to confirm our guesses.

Still, there are experiments that strongly suggest what might be happening in some parts of the brain and there is enough experience in physics and mathematical modeling for people like myself to suggest plausible mechanisms. For example, ingenious experiments have shown that a part of a songbird's brain called "HVC" (high vocal center) has neurons that behave in an unusual way as a bird sings: each neuron is completely quiet until the song reaches a certain point, and then the neuron fires a brief 6 millisecond pulse of action potentials and then becomes quiet again. Different neurons fire their pulses at different times during the song and people believe that each part of the song is represented by at least one neuron. We don't know how these neurons are connected to one another or the purpose of the bursts they fire, but a simple guess that explains this sparse firing is that the neurons form a long chain, like a line of dominoes. When one neuron fires, it activates the next neuron in the chain and so on like dominoes falling over one after the other. My group has done mathematical calculations to explore the hypothesis that the neurons form a chain and our calculations show that a chain is compatible with the known experimental data, although only if extra assumptions are made which then suggest new experiments to try. For example, one can show theoretically that the chain, if it exists, can't be a single row of neurons, there has to be redundancy with multiple chains that are cross connected. This redundancy then leads to further calculations and predictions, e.g., what happens if you kill a neuron in the chain or what happens if you add noise via an electrical stimulus to a neuron in the chain, or what happens to the bird's song if you modify the chain in various ways.

It is not known but an extremely interesting question is whether human brains have something like HVC, which seems to be a digital clock that provides a high resolution time sequence with which one can organize various time dependent behaviors. One thing humans can do but songbirds cannot is speed up and slow down their speech at will (songbirds always sing their songs at the same speed). It is an interesting puzzle about how to set up a circuit of chains that would allow you to vary the duration of speech at will, or whether some completely different arrangement of neurons is needed to provide this capability."

~ Henry Greenside