Elements of The Neuron Doctrine
Neuron Theory is an example of consilience where low level theories are absorbed into higher level theories that explain the base data as part of higher order structure. As a result the neuron doctrine has multiple elements, each of which were the subject of low level theories, debate, and primary data collection. Some of these elements are imposed by the necessity of cell theory that Waldeyer was trying to use to explain the direct observations, and other elements try to explain observations so that they are compatible with cell theory.
Neural units The brain is made up of individual units that contain specialized features such as dendrites, a cell body, and an axon.
Neurons are cells These individual units are cells as understood from other tissues in the body.
Specialization These units may differ in size, shape, and structure according to their location or functional specialization.
Nucleus is Key The nucleus is the trophic center for the cell. If the cell is divided only the portion containing the nucleus will survive.
Nerve Fibers are Cell Processes Nerve fibers are outgrowths of nerve cells.
Cell Division Nerve cells are generated by cell division.
Contact Nerve cells are connected by sites of contact and not cytoplasmic continuity. Waldeyer himself was neutral on this point, and strictly speaking the neuron doctrine does not depend upon this element. The heart is an example of excitable tissue where the cells connect via cytoplasmic continuity and yet is perfectly consistent with cell theory. This is true of other examples such as connections between horizontal cells of the retina, or the Mauthner cell synapse in goldfish.
Law of Dynamic Polarization Although the axon can conduct in both directions, in tissue there is a preferred direction for transmission from cell to cell.
Later elements that were not included by Waldeyer, but were added in the following decades.
Synapse A barrier to transmission exists at the site of contact between two neurons that may permit transmission.
Unity of Transmission If a contact is made between two cells, then that contact can be either excitatory or inhibitory, but will always be of the same type.
Dale's Law Each nerve terminal releases a single type of transmitter.
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