The neuron doctrine is the now fundamental idea that neurons are the basic structural and functional units of the nervous system. The theory was first proposed by Santiago Ramón y Cajal and completed by the eminent Heinrich Wilhelm Gottfried von Waldeyer-Hartz in the late 19th century. It holds that neurons are discrete cells (not connected in a meshwork), which are metabolically distinct units with cell bodies (somata), axons, and dendrites.
The Law of Dynamic Polarization further states that neural transmission goes only in one direction, from dendrites toward axons. There are notable exceptions to this "law"; for example, electrical synapses can allow transmission in both directions (i.e. bidirectional transmission).
The initial failure to accept the doctrine was due in part to inadequate ability to visualize cells using microscopes, which were not developed enough to provide clear pictures of nerves. With the cell staining techniques of the day, a slice of neural tissue appeared under a microscope as a complex web and individual cells were difficult to make out. Since neurons have a large number of neural processes an individual cell can be quite long and complex, and it can be difficult to find an individual cell when it is closely associated with many other cells. Thus, a major breakthrough for the neuron doctrine occurred in the late 1800s when Ramón y Cajal used a technique developed by Camillo Golgi to visualize neurons. The staining technique, which uses a silver solution, only stains one in about a hundred cells, effectively isolating the cell visually and showing that cells are separate and do not form a continuous web. Further, the cells that are stained are not stained partially, but rather all their processes are stained as well. Ramón y Cajal altered the staining technique and used it on samples from younger, less myelinated brains, because the technique did not work on myelinated cells. He was able to see neurons clearly and produce drawings like the one at right.
For their technique and discovery respectively, Golgi and Ramón y Cajal shared the 1906 Nobel Prize in Physiology or Medicine. Golgi could not tell for certain that neurons were not connected, and in his acceptance speech he defended the reticular theory. Ramón y Cajal, in his speech, contradicted that of Golgi and defended the now accepted neuron doctrine.
While the neuron doctrine is a central tenet of modern neuroscience, recent studies suggest that there are notable exceptions and important additions to our knowledge about how neurons function.
First, electrical synapses are more common in the central nervous system than previously thought. Thus, rather than functioning as individual units, in some parts of the brain large ensembles of neurons may be active simultaneously to process neural information. Electrical synapses are formed by gap junctions that allow molecules to directly pass between neurons, creating a cytoplasm-to-cytoplasm connection.
Second, dendrites, like axons, also have voltage-gated ion channels and can generate electrical potentials that carry information to and from the soma. This challenges the view that dendrites are simply passive recipients of information and axons the sole transmitters. It also suggests that the neuron is not simply active as a single element, but that complex computations can occur within a single neuron.
Third, the role of glia in processing neural information has begun to be appreciated. Neurons and glia make up the two chief cell types of the central nervous system. There are far more glial cells than neurons: glia outnumber neurons by as many as 10:1. Recent experimental results have suggested that glia play a vital role in information processing.
Finally, recent research has challenged the historical view that neurogenesis, or the generation of new neurons, does not occur in adult mammalian brains. It is now known that the adult brain continuously creates new neurons in the hippocampus and in an area contributing to the olfactory bulb. This research has shown that neurogenesis is environment-dependent (eg. exercise, diet, interactive surroundings), age-related, upregulated by a number of growth factors, and halted by survival-type stress factors. Of particularly compelling interest, Charles Gross and Elizabeth Gould provided evidence suggesting that neurogenesis occurred in neocortex after birth, in areas of the brain known to be important for cognitive function. Strong challenges to this work have come from more well-controlled studies by Pasko Rakic and others which support Rakic's original hypothesis that neurogenesis after birth is restricted to the olfactory bulb and hippocampus. Rakic argues that the Princeton group's work has not been substantiated by multiple other groups.