Monday, April 02, 2007

Some Thoughts on Information – V

(Click on a graphic to enlarge)

In this installment, I hope to cover how the information transferred from a photon to rhodopsin makes it to the brain. Visual information arrives at the retina of the eye as a spatial and temporal array of photons. In one sense, information is already digitized.

In Thoughts on Information III, we introduced the transduction of information from photons to G proteins. Let me just review that. Recall that the rod cell in the retina has the following structure:

If a photon of light activates the rhodopsin molecule, a G protein is generated. While it is possible that a single photon will activate a single molecule that will then generate a nerve impulse and pass the information up the chain to the brain (consider yourself in a pitch black cave; you might possibly be able to “see” one photon as a flash), in the more mundane case of day to day visual accrual of information, it probably takes many, many photons to cause a rod cell to fire. (if you are interested in the actual molecules involved, i.e. rhodopsin tranducin, please go here.

Recall that each of these reactions is catalyzed by an enzyme which is governed by the restrictions we discussed in Some Thoughts on Information IV.

That is, each chemical reaction is governed by the activation energy and statistics.

In summary, visual information arrives in the eye as a choreography of photons, a pattern if you will. This digital information (a photon is the ultimate in digital information) is transduced into G proteins. The G proteins cause a change in the sodium/potassium distribution across the rod cell membrane which creates an action potential in the axon of the rod cell.

Please go here for a dramatization of an action potential:

Please note that the action potential, just like the activation energy of a chemical reaction, has a threshold and that it is either generated or not. The action potential of any nerve is an either/or process. That is, again, it is digital information. This either/or process (no apologies to Kierkegaard) exists throughout the pathways and in the brain. Most importantly, because of this threshold, there is only one molecule difference between the non firing state and the firing state (action potential). Let me emphasize this:

In the generation of an action potential in the rod cells of the eye, it is the difference of a single molecule (e.g. G protein) which makes the difference between not firing and firing.

This action potential travels up the rod cell axon to the synapse with other cells in the outer layer of the retina as you will remember from the diagram presented before:

Please note that there are synapses at two layer in the retina before it leaves for the optic nerve.

Even in the retina of the eye there is manipulation of information. This manipulation of information is entirely understandable as basic electro chemical processes (action potentials, synapses, further action potentials) as we will discuss in a minute. It is highly likely that the basic scheme here is replicated in the brain in a much more complex way. But, the fact that information has pre analysis has got to be interesting.

One can assume that this processing of information in the retina served a purpose in evolution, perhaps streamlining the transmission to the brain. In addition, this just reinforces the observation that what the brain “sees” and what the eye “sees” are much different patterns. That is, the eye does not map the image onto the brain.

The transmission of the nerve impulse down the nerve is an electrochemical phenomena as previously described:

As each action potential reaches a synapse, the connection with the dendrite of the next nerve cell, there is a transfer of information across the space involving neurotransmitters.

Once again let me emphasize that in order for the action potential to be produced in the second neuron, there has to be threshold reached, i.e. a certain concentration of sodium ions diffusing into the cell through the gated channels. Since there is a condition when there is no action potential and one where there is such a potential, there is the situation where one further ion of sodium is enough to reach the threshold.

When the nerve impulse leaves the retina, it travels via the optic tracts to the visual cortex in the occipital lobes of the brain.

There are many aspects of this information transfer that of interest both mechanically and from an evolutionary standpoint:

1. Because of the existence of neurotransmitters, one can assume that they were the way in which information was transferred in a primitive organism (this method is still used in humans in the endocrine system). It was only as the organism grew larger and became multicellular (there is a maximum size beyond which a cell will not economically function) that the evolution of nerves occurred. Still, instead of evolving a direct connection, organisms still retain the intermediary step of neurotransmitters. The system has to always be amenable to evolution even if it would be more efficient not to be. I suppose that survivability is superior to utility.

2. Previously, in Thoughts on Information IV, we suggested that one of the functions of enzymes was to bring the rates of biochemical reactions within the same time frame and that the overall rate was dependent on the slowest rate in the chain. In a similar way, the rate of information processing is dependent on the speed with which the organism can access information. This is, in part, dependent on the transfer rate of information. One can see that the simple diffusion of an informational molecule from a receptor to the brain would be far too slow to allow the organism to benefit from the information without the electrochemical conduction of the nerve. It is probably either the synthesis of neurotransmitters, or their release (a physical process that involves multiple steps) that is rate limiting in neurotransmission. All in turn are limited by the Arrhenius equation with the requirement that there be sufficient activation energy. (As an aside, in “The Stars My Destination” the hero gets a rewired nervous system. I assume that this involves getting rid of all those rate determining slow steps.)

3. Information transfer is quantized in that there is a threshold that has to be reached in order for the action potential to be generated either at the primary site (e.g. the eye) or in the transmission to the brain (at the multiple synapses). This threshold is dependent upon a single molecule. If the threshold is not reached there is no action potential. I feel strongly that this enormous implications for what transpires in the brain.

In summary, we have followed the quantized information in photons from the eye to the brain. We have observed that the electrochemical processes are governed by activation energy of enzymes but that the generation of an action potential is an either/or process. Finally, we have observed that there is preprocessing of information at the retinal level.

In the final installment, we hope to address information processing in the brain using the observations that we have made on the acquisition of information above.

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