Neuron Theory | Research & Encyclopedia Articles

Neuron Theory

The existence of nerves was recognized very early in the study of the human body. Ancient Greek and Roman scholars conducted primitive studies of the structure of nerves and hypothesized their functions. This research moved forward very slowly, however, because of the special nature of nerve cells. Early scientists recognized the existence of nerve cells and nerve fibers, but were unable to determine how these two were associated with each other. Especially in the brain, the complex network of dendrites and axons makes it difficult to trace the connection between fibers (dendrites and axons) and cell bodies. Two developments were critically important in the advance of knowledge about nerve cells: microscopy and cell staining.

Some of the earliest and most elegant microscopic observations of nerve cells were made by Rudolf Albert von Kölliker in the 1840s. Kölliker was able to see that nerve fibers are actually attached to the nerve cell itself. This finding laid the basis for the theory that cell and fiber might constitute a single functional unit in the nervous system.

Improvements in the staining of nerve cells by Camillo Golgi provided a second line of study. In 1873, Golgi developed a new technique for staining cells, one that used silver salts instead of organic compounds. With this technique, Golgi was able to identify fine details of nerve cell structures and to discover that cells are separated by tiny gaps, or synapses.

This finding confirmed a hypothesis made somewhat earlier by Wilhelm von Waldeyer-Hartz. Waldeyer-Hartz might, in fact, be called the father of modern neuron theory. Although he did relatively little original research himself, Waldeyer coined the term neuron and suggested that the nerve cell is the basic unit of the nervous system anatomically and physiologically. He also proposed the term neuron theory for his hypothesis.

The neuron theory was examined from a variety of angles during the mid-1850s. In addition to microscopic and cell-staining techniques, researchers studied the growth and degeneration of nerve cells. For example, Wilhelm His (1831-1904) showed that nerve cells taken from embryos act as centers out of which nerve fibers grow. This finding suggested a connection between cell bodies and fibers. The major problem that remained, however, was the relative importance of nerve fibers and nerve cell bodies in the transmission of nerve messages. The popular view through much of the nineteenth century was that fibers are critically important in this process, while cell bodies play a largely structural, supportive role for the fibers.

This issue was finally cleared up through the research of the great Spanish histologist Santiago Ramón y Cajal, who improved both microscopic and staining techniques used in studying nerve tissue. He was eventually able to see very clearly the structure and association of nerve cells. He showed that the axon of one cell lies very close to the dendrite of a second cell. In his Law of Transmission by Contact, Ramón y Cajal argued that complete neurons--not just their fibers--were the functioning unit of the nervous system. With this research, the neuron theory was largely regarded as having been proved.

An important line of supplementary research on the neuron theory was conducted by the American zoologist Ross Granville Harrison (1870-1959) in the early 1900s. Biologists had been uncertain as to how nerve fibers develop. Various theories had been suggested. One theory, for example, held that fibers develop out of pre-existing protoplasmic bridges between neurons. In 1907, however, Harrison was able to demonstrate that nerve fibers grow as projections that develop from the neuron body itself.

Scientists have long observed that nerve cells in the central nervous system (brain and spinal cord) of mammals don't regenerate new connections after an injury the way those in the peripheral nervous system can. In the 1990s, provocative research by Michal Schwartz of the Weizmann Institute of Science in Rehovot, Israel, shed light on this observation. Schwartz believes that the main barrier to nerve cell regeneration in mammals may be the need to protect complex brain circuits from accidental remodeling by immune cells, especially macrophages. In other words, it is the suppression of macrophage activity that may stop nerve cell regeneration. In one study, Schwartz and her team were able to induce nerve cell regeneration in rats by transplanting macrophages into damaged areas of the central nervous system. Such approaches could one day lead to new treatments for spinal cord injuries.