View source for Santiago Ramón y Cajal

=== Neuron doctrine ===

[[Image:dynamic polarization.png|thumb|300px|right|F7| Schematic section of the retina of birds. Arrows indicate the direction of electric impulses, thus illustrating the ''law of dynamic polarization''. From Cajal's Nobel lecture "The structure and connexions of neurons", 1906.]]
The theory of the individuality of the nerve cell was first formulated in 1891 by Cajal, according to his recent discoveries. Shortly after, Waldeyer coined the word "neuron" to designate the individual nerve cells described by Cajal. Then, the term "neuron doctrine" was born. This theory constitutes the fundamental principle of neuroscience ([[#Bullock05|Bullock et al., 2005]]). In current terminology, it can be formulated as follows: 

'''1'''. The neuron is the structural and functional unit of the nervous system.

'''2'''. Neurons are individual cells, which are not anatomically continuous to other neurons.

'''3'''. The neuron has three parts: dendrites, soma (cell body) and axon. The axon has several terminal arborizations, which make close contact to dendrites or the soma of other neurons.

With its physiological corollary:

'''4'''. Conduction of nerve impulses is directional and follows the theory of axipetal polarization. According to Cajal, the polarization of the nerve impulse is due to the pre-established relations between the neurons and the initial position of the excitation; if the point of entry of the current varies, the excitation wave can go from the axon to the cell body or from an axonal branch to its main trunk; and the same can apply to dendrites.    
 
In proving that the ''cell theory'' applied to the nervous system, Cajal made the definitive argument that the ''cell theory'' provided an accurate description of all tissues in every living thing, and thus helped alongside Schwann, Schleiden and Virchow to establish this pillar of modern biology. The ''neuron doctrine'' also had its own merits, especially regarding the physiological characterization of neurons as information processing units. It became complete after the introduction of the concept of synapse by Sherrington in 1897. Here one of the weaknesses of Cajal’s exclusive grounding in anatomical studies becomes apparent: despite his talent for inferring function from morphological data, Cajal could not envision the one-way property of synaptic transmission ([[#Berlucchi99|Berlucchi, 1999]]).  

'''The ''neuron doctrine'' today''' 

The description in 1952 of the mechanisms underlying the generation and propagation of action potentials opened up new directions for research in functional neurobiology ([[#HH52|Hodgkin & Huxley, 1952]]). In light of the considerable progress that has been made since then, some aspects of the ''neuron doctrine'' must be reevaluated ([[#Bullock05|Bullock et al., 2005]]). For instance, although neurons are indeed anatomically discrete units, they are not always single functional units in the sense envisioned by Cajal. Some neurons are connected by gap junctions, a type of channel that provides the neurons with cytoplasmic continuity. These electrical synapses cause groups of neurons to be functionally coupled in a way reminiscent of the ''reticular theory''.  

The ''theory of axipetal polarization'' has exceptions as well. In many neurons, action potentials can travel backward from the axon and soma regions into the dendrites ([[#Stuart93|Stuart et al., 1993]]), a possibility already considered by Cajal. Moreover, under certain conditions action potentials can be initiated in dendrites, remaining local or sometimes propagating into the soma to initiate single or multiple spikes of activity in the axon ([[#Golding98|Golding & Spruston, 1998]]).

Finally, current research points to a role for glia (non-neuronal cells) in information processing by the central nervous system, a feature that Cajal and his contemporaries  began already to envision ([[#Garcia08|García-Marín et al., 2007]]). Two-way communication between neurons and glial cells is actually essential for axonal conduction, synaptic transmission, and information processing ([[#Fields02|Fields & Stevens-Graham, 2002]]).