Neuron doctrine
The neuron doctrine is the concept that the nervous system is made up of discrete individual cells, a discovery due to decisive neuro-anatomical work of Santiago Ramón y Cajal and later presented by, among others, H. Waldeyer-Hartz.[1] The term neuron (spelled neurone in British English) was itself coined by Waldeyer as a way of identifying the cells in question. The neuron doctrine, as it became known, served to position neurons as special cases under the broader cell theory evolved some decades earlier. He appropriated the concept not from his own research but from the disparate observation of the histological work of Albert von Kölliker, Camillo Golgi, Franz Nissl, Santiago Ramón y Cajal, Auguste Forel and others.[2][3]
Historical context
A landmark development came from Camillo Golgi who invented a
Elements
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 .
- 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 cellsynapse 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.
Update
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.
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.[9] Electrical synapses are formed by gap junctions that allow molecules to directly pass between neurons, creating a cytoplasm-to-cytoplasm connection, known as a syncytium.[10]
Furthermore, the phenomenon of
References
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- ^ Renato M.E. Sabbatini (2003). "Neurons and Synapses: The History of Its Discovery". Brain & Mind Magazine. Retrieved 23 August 2013.
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- ISBN 9780123865250.
- Bullock, T.H.; Bennett, M.V.L.; Johnston, D.; Josephson, R.; Marder, E.; Fields, R.D. (2005). "The Neuron Doctrine, Redux". Science. 310 (5749): 791–793. S2CID 170670241.