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The neuron doctrine is the concept that the
nervous system
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...
is made up of discrete individual cells, a discovery due to decisive neuro-anatomical work of
Santiago Ramón y Cajal
Santiago Ramón y Cajal (; 1 May 1852 – 17 October 1934) was a Spanish neuroscientist, pathologist, and histologist specializing in neuroanatomy and the central nervous system. He and Camillo Golgi received the Nobel Prize in Physiology or Med ...
and later presented by, among others,
H. Waldeyer-Hartz. The term ''
neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
'' (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
In biology, cell theory is a scientific theory first formulated in the mid-nineteenth century, that living organisms are made up of Cell (biology), cells, that they are the basic structural/organizational unit of all organisms, and that all cell ...
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
Camillo Golgi (; 7 July 184321 January 1926) was an Italian biologist and pathologist known for his works on the central nervous system. He studied medicine at the University of Pavia (where he later spent most of his professional career) betwee ...
,
Franz Nissl
Franz Alexander Nissl (9 September 1860, in Frankenthal – 11 August 1919, in Munich) was a German psychiatrist and medical researcher. He was a noted neuropathologist.
Early life
Nissl was born in Frankenthal to Theodor Nissl and Maria Haas ...
,
Santiago Ramón y Cajal
Santiago Ramón y Cajal (; 1 May 1852 – 17 October 1934) was a Spanish neuroscientist, pathologist, and histologist specializing in neuroanatomy and the central nervous system. He and Camillo Golgi received the Nobel Prize in Physiology or Med ...
,
Auguste Forel
Auguste-Henri Forel (1 September 1848 – 27 July 1931) was a Swiss myrmecologist, neuroanatomist, psychiatrist and eugenicist, notable for his investigations into the structure of the human brain and that of ants. For example, he is considered a ...
and others.
Historical context
Theodor Schwann
Theodor Schwann (; 7 December 181011 January 1882) was a German physician and physiologist. His most significant contribution to biology is considered to be the extension of cell theory to animals. Other contributions include the discovery of ...
proposed in 1839 that the tissues of all organisms are composed of cells. Schwann was expanding on the proposal of his good friend
Matthias Jakob Schleiden
Matthias Jakob Schleiden (; 5 April 1804 – 23 June 1881) was a German botanist and co-founder of cell theory, along with Theodor Schwann and Rudolf Virchow.
Career
Matthias Jakob Schleiden was born in Hamburg. on 5 April 1804. His father was ...
the previous year that all plant tissues were composed of cells. The nervous system stood as an exception. Although nerve cells had been described in tissue by numerous investigators including
Jan Purkinje,
Gabriel Valentin Gabriel Gustav Valentin (July 1810 - 24 May 1883), also Gabriel Valentin, was a German physiologist and professor of physiology at the University of Bern.
Gabriel Gustav Valentin was born at Breslau in July 1810. He was Jewish, the son of a Jewis ...
, and
Robert Remak
Robert Remak (26 July 1815 – 29 August 1865) was a Jewish Polish-German embryologist, physiologist, and neurologist, born in Poznań, Posen, Prussia, who discovered that the origin of cells was by the Cell division, division of pre-existing cel ...
, the relationship between the nerve cells and other features such as dendrites and axons was not clear. The connections between the large cell bodies and smaller features could not be observed, and it was possible that
neurofibrils would stand as an exception to cell theory as non-cellular components of living tissue. Technical limitations of microscopy and tissue preparation were largely responsible.
Chromatic aberration
In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wave ...
,
spherical aberration
In optics, spherical aberration (SA) is a type of optical aberration, aberration found in optical systems that have elements with spherical surfaces. Lens (optics), Lenses and curved mirrors are prime examples, because this shape is easier to man ...
and the dependence on natural light all played a role in limiting microscope performance in the early 19th century. Tissue was typically lightly mashed in water and pressed between a glass slide and cover slip. There was also a limited number of dyes and fixatives available prior to the middle of the 19th century.
A landmark development came from Camillo Golgi who invented a
silver staining In pathology, silver staining is the use of silver to selectively alter the appearance of a target in microscopy of histological sections; in temperature gradient gel electrophoresis; and in polyacrylamide gels.
In traditional stained glass, silv ...
technique in 1873 which he called ''la reazione nera'' (
black reaction), but more popularly known as Golgi stain or Golgi method, in his honour. Using this technique nerve cells with their highly branched dendrites and axon could be clearly visualised against a yellow background. Unfortunately Golgi described the nervous system as a continuous single network, in support of a notion called
reticular theory Reticular theory is an obsolete scientific theory in neurobiology that stated that everything in the nervous system, such as brain, is a single continuous network. The concept was postulated by a German anatomist Joseph von Gerlach in 1871, and was ...
. It was reasonable at the time because under light microscope the nerve cells are merely a mesh of single thread. Santiago Ramón y Cajal started investigating nervous system in 1887 using Golgi stain. In the first issue of the ''Revista Trimestral de Histología Normal y Patológica'' (May, 1888) Ramón y Cajal reported that the nerve cells were not continuous in the brain of birds. Ramón y Cajal's discovery was the decisive evidence for the discontinuity of nervous system and the presence of large number of individual nerve cells. Golgi refused to accept the neuron theory and hung on to the reticular theory. Golgi and Ramón y Cajal were jointly awarded the 1906
Nobel Prize for Physiology or Medicine
The Nobel Prize in Physiology or Medicine is awarded yearly by the Nobel Assembly at the Karolinska Institute for outstanding discoveries in physiology or medicine. The Nobel Prize is not a single prize, but five separate prizes that, according ...
, but the
controversy
Controversy is a state of prolonged public dispute or debate, usually concerning a matter of conflicting opinion or point of view. The word was coined from the Latin ''controversia'', as a composite of ''controversus'' – "turned in an opposite d ...
between the two scientists continued. The matter was finally resolved in the 1950s with the development of electron microscopy by which it was unambiguously demonstrated that nerve cells were individual cells interconnected through
synapses
In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell.
Synapses are essential to the transmission of nervous impulses from ...
to form a nervous system, thereby validating the neuron theory.
Elements
Neuron theory is an example of
consilience
In science and history, consilience (also convergence of evidence or concordance of evidence) is the principle that evidence from independent, unrelated sources can "converge" on strong conclusions. That is, when multiple sources of evidence are ...
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
Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...
, a
cell body, and an
axon
An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...
.
;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
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucle ...
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
Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
: Nerve cells are generated by cell division.
;Contact: Nerve cells are connected by sites of contact and not
cytoplasmic
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. Th ...
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 The Mauthner cells are a pair of big and easily identifiable neurons (one for each half of the body) located in the rhombomere 4 of the hindbrain in fish and amphibians that are responsible for a very fast escape reflex (in the majority of animals ...
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
In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell.
Synapses are essential to the transmission of nervous impulses from ...
: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
Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions and disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, development ...
, 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. 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
A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus) ...
.
Furthermore, the phenomenon of
cotransmission
Neurotransmission (Latin: ''transmissio'' "passage, crossing" from ''transmittere'' "send, let through") is the process by which signaling molecules called neurotransmitters are released by the axon terminal of a neuron (the presynaptic neuron), ...
, in which more than one neurotransmitter is released from a single presynaptic terminal (contrary to Dale's law), contributes to the complexity of information transmission within the nervous system.
References
*
External links
The discovery of the neuron
{{Neuroscience
Neurohistology
History of neuroscience