The Neuron Doctrine Today

As the genetic unit, the neuronal nucleus contains the chromosomal DNA, the human genome of 30-40,000 genes. A high percentage probably concern the nervous system, whereas the degree of alternate processing and the number of proteins expressed, though unknown, may be far greater.

As the anatomical unit, the picture proof by Sanford Palay and George Palade in 1954 that at the synapse the apposed neuronal membranes are separated by a 10 to 20-nm cleft was a triumph for electron microscopy. But for gap junctions (see previous discussion) and fine filaments seen in the EM crossing the gap between pre- and postsynaptic cytoplasm, doubts about the separateness of nerve cells were laid to rest.

As the functional unit, modification of the doctrine is mandatory: groups of cells are the working units of the nervous system, not single cells themselves.

As the trophic unit, evidence has accumulated over the last century (in neurology, neuropathology, and experimental biology) to show that the cell body of a neuron is vital to its own life. If its processes are severed or injured but the soma is left intact or little harmed, new ones may be formed, or at least the damage controlled. But if the soma is severely damaged, the nerve cell dies.

Still (to paraphrase neuroanatomist Alf Brodal), ''The concept of a neuron as a trophic unit is scarcely acceptable as originally formulated. A single neuron is dependent on other neurons for its viability.'' In this regard, neurons and their processes exert nurturing and supporting effects on other tissues, but such trophic interactions have not been examined as fully in the nervous system itself.

Studies of neurotrophisms are numerous, and they continue. But trophic functions of neurons have not been investigated as intensively as their communicative and endocrine abilities. Less is known about them. Their clinical significance is profound, as in managing patients with spinal cord injury in which muscular atrophy and decubitus ulcers are secondary to even more debilitating problems.

Finally as to trophisms, the multitudinous, ubiquitous neuroglial cells deserve attention. The neuron doctrine was formulated a century ago for neurons and for good reasons. But, as it was for the laws of dynamic polarization and specified connections, a corollary for the neuroglia could be added. Its production, by astrocytes, of numerous growth factors supporting vital attributes of certain neurons merits mention in relation to the trophic tenet of the revised neuron doctrine.

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