Neuron Doctrine

the doctrine that the nervous system is composed of individual cells—neurons—that are contiguous but maintain their genetic, morphological, and functional individuality.

The neuron doctrine regards nervous activity as the aggregate result of individual neuronal interactions in the body. In the late 19th and early 20th centuries, the neuron doctrine was opposed by the theory of continuity, which assumed that cellular matter can pass directly from the cytoplasm of one neuron to the cytoplasm of another; thus, according to the theory of continuity, the outgrowths of neurons form a single cytoplasmic network. Advocates of the theory of continuity, for example, the Hungarian scientist S. Apathy and the German scientist A. Bethe, believed that neurofibrils maintain the cytoplasmic continuity of nerve tissue. Convincing evidence in support of the neuron doctrine was amassed by several scientists, including S. Ramon y Cajal, A. A. Zavarzin, and B. I. Lavrent’ev, in studies of the micro-structure and embryology of the nervous system and in studies on the degeneration and regeneration of neurons. In light of recent electrophysiological and electron-microscopic data, the feasibility of the neuron doctrine is no longer open to challenge.

In all organisms, including primitive ones, the nervous system is composed of separate neurons that interact at sites of contact—synapses—that contain complex structures. Deviations from this general pattern are rare. The functional isolation of a neuron may be lost during the synchronous excitation of a group of neurons; an example of this can be found in the nerve center that innervates the electric organ in fish. The cytoplasmic merging of the outgrowths of several neurons accounts for the occurrence of giant axons in squid; in the process of so merging, the neurons lose their morphological separateness.