Copyright © 2007-2017 Russ Dewey
So far our description of the nerve cell resembles the classic neuron doctrine. The classic doctrine can be traced to the work of Santiago Ramon y Cajal and Charles Scott Sherrington in the first decades of the 1900's. Their ideas lasted half a century almost unchanged.
In the 1970s there was a "quiet revolution" in neuroscience (Schmitt, Dev, and Smith, 1976). This was due to the emergence of the electron microscope, a tool that allowed researchers to see neural structures much tinier than could be seen before.
In 1900, Cajal had to strain to see a whole neuron in an optical microscope. By 1965, researchers could look directly at synapses in an electron microscope.
New findings accumulated quickly without much of a splash. The cumulative effect was to introduce a universe of complexity. Most of the ideas from the classic doctrine, taught to introductory students as typical of neurons, proved to be oversimplifications.
What caused the "quiet revolution" in neuroscience?
This change occurred so quickly that many people outside the field of neuroscience missed it. Pick up any science textbook for children, even half a century after the quiet revolution, and chances are you will see the classic doctrine of the 1930s and 1940s.
If you see references to dendrites as "input structures" and axons as "output structures," that is a clue that you are reading a 1940s-era description of neurons. The findings that dendrites and axons could act as both input and output structures came early in the quiet revolution.
Synapses were found to occur between any two parts of neurons. There were dendrodendritic synapses (from dendrite to dendrite), somatodendritic synapses (from soma to dendrite), dendrosomatic synapses (from dendrite to soma) and even axoaxonic and somatosomatic synapses.
The statement that "dendrites are input structures" is obsolete in another way. It fails to acknowledge the detailed information processing inside dendrites.
In a Science article titled, "Dendrites Shed Their Dull Image," Barinaga (1995) summarized the newer perspective:
"...Dendrites, once thought to be mere adding machines, seem to be more actively involved in shaping the responses of neurons..."
"...their finely branched network acts as a two-way highway, not only conveying incoming messages to the cell body, but also relaying information from the cell body back to their own outer reaches..."
In perhaps the biggest surprise about dendrites, researchers found that dendrites–as they stretch away from the cell body–sometimes turn into axons (Dacey, 1988).
What were some surprises discovered during the quiet revolution?
In other words, a structure which looks and acts like a dendrite, complete with dendritic spines receiving inputs from axons, may travel onward, branch, and become an axon. The basic parts of a neuron are not as easy to identify and distinguish as once thought.
In the classic neuron doctrine, neurons communicated by chemical means. During the quiet revolution, it was discovered that some neurons were directly connected.
These connections are called gap junctions or electrotonic (not "electronic" but "electrotonic") synapses. These are synapses without transmitter substances. At these locations, a nerve impulse is passed immediately to the next neuron through gaps in the membranes.
These synapses are extremely fast. There is no synaptic delay caused by chemical transmitters going across a synaptic cleft. Thus they are often found in escape systems where they help trigger quick getaway movements.
What are gap junctions? Where are they commonly found?
Gap junctions are especially common in fish and invertebrates (animals without backbones). They also exist in the retina of the human eye, where fast information processing is at a premium.
The classical neuron doctrine said a neuron communicated with other neurons by firing nerve impulses when pushed over a threshold by many excitatory inputs. Each neuron was said to be a little on/off switch, activated only when it received enough nerve impulses from other neurons.
However, the quiet revolution of the mid-1960s revealed that neurons were not on-off switches. Some neurons conduct all their business with weak electrical interactions.
These electrical potentials are different from nerve impulses in two ways. They are hundreds of times weaker than the electrical disturbances of a nerve impulse.
Unlike nerve impulses which are "all or none" (sometimes compared to a toilet flushing) weak interactions are graded and variable in their effect on neighboring synapses. Some neurons using these weak electrical interactions never fire nerve impulses.
How do weak electrical interactions differ from nerve impulses?
Weak electrical influences are important in dendrodendritic synapses: synapses between two dendrites. Distances between two parts of the same dendritic tree are often so small there is no room for nerve impulses to travel anyway.
Dendrodendritic synapses form local circuits: tiny areas, only a few microns (millionths of a meter) across. In local circuits, several twigs from dendritic trees may form many synapses with each other, producing feedback loops of various kinds.
Local circuits are so small that a thousand would fit inside the dot of an exclamation mark. These circuits are especially common in humans. During the quiet revolution, researchers discovered the human brain had more local circuits per cubic centimeter than any other brain studied (Schmidt, Dev, and Smith, 1976).
All the simple assumptions embodied in the textbook neuron of the 1930s and 1940s have been replaced by complexities and variations. Your brain, and its neurons, are much more complicated that scientists first suspected.
One important modern finding is that neurons can exchange large biomolecules such as proteins and RNA. This is important because proteins and RNA are capable of carrying enormous amounts of information.
Sending nerve impulses to a neuron is like tapping out a message in Morse code (a combination of dots and dashes). Nerve impulses are all-or-none signals, like the dots and dashes of Morse Code.
Transmitting a biomolecule such as a protein or RNA to another neuron, by contrast, is more like passing a book through a window. The speed may be slower, but the amount of information passed in a single act is much greater.
What is biomolecular transfer, and why is it important?
Axons and dendrites have narrow transportation channels within them called microtubules, 200-300 angstroms wide, and neurofilaments, 100 angstroms wide. An angstrom is one ten-millionth of a meter.
Microtubules are capable of "rapid and precise" delivery of transmitters, proteins, and other large molecules. Biomolecules can be transported to specific areas within the neuron or to synapses with other neurons (Aletta and Goldberg, 1982).
Within a single axon of a neuron, different rate components can be detected, each for a distinct macromolecular complex. In other words, axons are great, multi-laned highways. Each type of substance has its own lane, and each lane moves at its own speed.
What are microtubules and neurofilaments? What happens inside them?
One highly speculative theory (championed by Roger Penrose in Britain and Stewart Hameroff in the United States) suggested consciousness was generated by quantum effects in microtubules.
Why microtubules? To Penrose, they were just small enough to be subject to quantum effects, just large enough to avoid getting lost in the thermal noise of the brain.
Penrose wanted to use quantum effects to explain consciousness because quantum effects are something special, outside the normal bounds of the physical sciences. Penrose does not believe consciousness arises "merely" from computational processes in the brain.
That is not a very scientific form of reasoning. The brain is plenty complex enough to generate surprising emergent phenomena found nowhere else.
Why do Penrose and Hameroff suggest that quantum effects in microtubules might be involved in consciousness?
Physicist Max Tegmark published an article in 2000 showing quantum changes operate on a time scale far too fast for neurons. The effects proposed by Penrose and Hameroff could not work, he said, unless the brain was cooled to near Absolute Zero to avoid background noise and heat. "It's reasonably unlikely that the brain evolved quantum behavior," concluded one physicist (Seife, 2000).
Even if they do not carry out quantum computing, nerve cells obviously participate in very complex and subtle forms of information processing. If you add up the findings of the quiet revolution–gap junctions, local circuits, biomolecular transfer–there is complexity for scientists to investigate for centuries to come.
Half a century ago, psychologists modeling nerve cell interactions on computers tended to assume information was passed between neurons in a binary (on/off) code. This provided a handy analogy to the binary code of newly-emerging computer technology.
How is a previous assumption of psychologists now known to be dramatically wrong?
Now it is clear that analogy is dramatically wrong, or at the very least, incomplete. Much information processing of neurons involves tiny weak electrical potentials that are graded (not all-or-none, therefore not binary) and precise exchange of complex molecules.
If you add up all the findings of the quiet revolution, it becomes clear that neurons are highly complex systems in their own right. They are not just tiny beads in a chain, the way they looked in textbook diagrams of the 1950s.
The more a scientist works with individual, identifiable neurons in a creature like Aplysia, the sea slug (which has large, easy to see neurons), the more each neuron seems like an individual. The eminent neuroscientist Theodore H. Bullock is said to have remarked, "Neurons are people."
What did Bullock mean by saying "Neurons are people"?
What did Bullock mean? Probably he was referring to what might be called personality in neurons. Each has predictable ways of responding to certain situations.
We refer to pets and even inanimate systems such as cars as having personalities, when we get to know their quirks and charms. Apparently neurons can evoke the same response.
Aletta, J. M. & Goldberg, D. J. (1982). Rapid and precise down regulation of fast axonal transport of transmitter in an identified neuron. Science, 218, 913-916
Barinaga, M. (1995) Dendrites shed their dull image. Science, 268, 200.
Dacey, D. M. (1988). Dopamine-accumulating retinal neurons revealed by vitro fluorescence display a unique morphology. Science, 240, 1196-1198.
Schmitt, F. O., Dev, P. & Smith, B .H. (1976). Electronic processing of information by brain cells. Science, 193, 114-120.
Seife, C. (2000). "Cold Numbers Unmake the Quantum Mind". Science, 287, 791.
Tegmark, M. (2000). "Importance of quantum decoherence in brain processes". Physical Review E., 61, 4194-4206.
Write to Dr. Dewey at email@example.com.
Copyright © 2007-2017 Russ Dewey