Book T of C
Chap T of C
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.
What caused the "quiet revolution" in neuroscience?
In the 1970s there was a "quiet revolution " in neuroscience (Schmitt, Dev, & Smith, 1976). This was due to the emergence of the electron microscope, a tool which allowed researchers to see neural structures much, much tinier than was previously possible. 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 whole new universe of complexity to scientists' view of the neuron. Most of the assumptions of the classic doctrine were found to be oversimplifications. For example, we now know that dendrites are not exclusively input structures, axons are not exclusively output structures, and that neurons communicate in ways other than by sending nerve impulses. A scientist from the 1940s surveying the neuroscience literature today might well conclude, "Everything I thought I knew about neurons was wrong."
This sharp turn occurred so quickly that many people outside the field of neuroscience simply missed it. Pick up any science textbook for school children, even in the 2000s, and chances are you will see a description of neurons that bears a closer resemblance to the classic doctrine of the 1940s than the quiet revolution of the 1970s. 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 both act as 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.
What are some complications discovered during the quiet revolution? What was perhaps the biggest surprise?
The statement that "dendrites are input structures" is obsolete in another way. It fails to acknowledge the detailed information processing which occurs in dendrites. In a Science article titled, "Dendrites Shed Their Dull Image," Barinaga (1995) summarized research showing "dendrites, once thought to be mere adding machines, seem to be more actively involved in shaping the responses of neurons" and "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 of all, researchers found that dendrites—as they stretch away from the cell body—sometimes turn into axons (Dacey, 1988). 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.
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Copyright © 2007 Russ Dewey