A Brain in a Single Cell

From Lettvin

"...when you have eliminated the impossible, whatever remains, however improbable, must be the truth."
-- Doyle, Sir Arthur Conan: The Sign of the Four

"Great fleas have little fleas upon their backs to bite ’em,
And little fleas have lesser fleas, and so ad infinitum.
And the great fleas themselves, in turn, have greater fleas to go on;
While these again have greater still, and greater still, and so on."
-- De Morgan, Augustus: A Budget of Paradoxes

One of the areas that remains out of focus in current understanding of cells is the ability of the cell to seemingly process the information surrounding it, its environment and react appropriately. Recent research (citations) shows that individual unicellular organisms, and colonies of them, can learn.

JDL: decades old research, and I do not have the references.

Vertebrates are eukaryotes, as are plants, jellyfish, worms, and much of life as we know it. Prokaryote include bacteria and archaea, but, for the sake of simplicity, we will just look at eukaryotic cells. We will also restrict the discussion to certain parts of the cell. For a full explanation of the structure and physiology of a cell please see (citation).

JDL: Archaea is the first of the three eukaryote, prokaryote, archaea; not sub to prokaryote.

The eukaryote cell is surrounded by a membrane made up of protein. These membrane proteins are the cellular gateways that admit and expel substances. They choose whether to allow or refuse passage in both directions.

JDL: membrane is made up of phospholipid, where living cells stud the membrane with proteins.

Within the cell are long chain proteins made of actin, or neurofibrils called polymers. (There are other polymer chains comprising many monomers chained together, usually unbranching.) These polymers often, as one chain, cross from one membrane protein to another, through the cytoskeleton, passing near each other and, should the nucleus allow it, near the nucleus as well.

JDL: actin filaments are polymers made from chains of actin monomers glued end to end by an ATP hidden within a cup-like depression.

JDL: neurofibrils are polymers made from chains of tau protein monomers.

We will concentrate on tubulin chains. Tubulin is capable of supporting a propagating bolus of ions that travels along its length in one direction. The bolus traverses the cell from one membrane protein to another at the other end, unless it is intercepted.

JDL: I tried to correct a mistake I made. Tubulin is not the channel for the propagating bolus, neurofibrils are.

JDL: The bolus can travel in either direction, but it propagates away from where it was formed.

The bolus, as it travels down the polymer, may encounter another polymer close enough that the energetics of jumping from one polymer to another does not favor either strand, giving a 50% chance of the bolus leaving its original polymer and propagating, instead, down the other polymer.

The nucleus moves around within the cell, much like a spider in its web. As it moves, it may choose to move two strands closer together or further apart. By changing this distance, the nucleus can cause signals to be rerouted from one place to another.

JDL: it can change the probability that the signal will jump and be rerouted.

You probably noticed that we used the word "choose" which may seem to be an odd choice of terms for a process contained within a single cell, but there are operations being performed for which there is no better term. The routing of the signal has convergences and divergences that operate in the same way that we believe the nervous system does. This concept is normally reserved for more complex organisms with dedicated nerve cells, but this processing occurs in a non-random manner, and choose is the most appropriate term.

JDL: and may be the mechanism underlying unicellular learning.

You can teach unicellular critters to tolerate and swim into bad tasting water to get to sugar, when untrained critters will not. This "behavior" is not possible without an information processing system. A "learning" mechanism is required. There is no option but to assume that the mechanisms available are doing the work.

It is also not possible to make an information processing system that operates on the surface of the cell (membrane) that would process information in the required way. I do not deny that some of the processing occurs there. The membrane proteins are interlinked and sometimes are long and loop through the membrane many times in many shapes. But the operations I expect to see require divergence/convergence operations in 3D organizations.

JDL: I mis-spoke. I would say information processing on a 2D surface, like membrane, lacks the facility to perform convergences and divergences that we believe underly living system information processing systems.

There are proteins on the surface of the nucleus. These proteins move about on the surface of the nucleus in a manner similar to spiders moving their legs to walk about their webs. The proteins can grab a polymer, pull the polymer from here to there, glue the polymer in place, or unglue it, and then release the polymer, to walk to some other site. The operation is very skilled appearing and involves an extraordinary amount of active processing.

If this seems like a sophisticated system to reside in a tiny cell, remember that, the DNA strands, an even smaller component of the cell, coil and uncoil, hiding and exposing lengths. Proteins are constructed and fragmented as the engine for doing so is instructed by the DNA. This coiling and uncoiling is done within the context of proximity to the nuclear membrane. The spider walk is achieved by DNA instruction based on signals arriving at the nucleus from the cell membrane. Signal can take the form of ion bolus (fast) or resource for the engine to use (energy and raw materials) or outgoing signals and excreta.

JDL: I will review the links later.

External Links

• Microbe Super-Communicator
• Plant intelligence
• Bacterial Intelligence, Lynn Margulis, Astrobiology Magazine
• Slimemold Intelligence

Cajal's excellent two volume "Histology of the Vertebrates".

I have pieced this together from a variety of sources. Some are decades old Russian monographs, others by folks like Lynn Margulies, others by the recent work on water around Gerald Pollack, some around the century old drawings of Cajal, century-and-a-half old work by duBois-Reymond, and much much more.