Challenges

Nerve cell bodies stained with basophilic dyes show numerous microscopic clumps of Nissl substance (named after German psychiatrist and neuropathologist Franz Nissl, 1860–1919), which consists of rough endoplasmic reticulum and associated ribosomes. The prominence of the Nissl substance can be explained by the fact that nerve cells are metabolically very active, and hence are involved in large amounts of protein synthesis.

The cell body of a neuron is supported by a complex meshwork of structural proteins called neurofilaments, which are assembled into larger neurofibrils. Some neurons also contain pigment granules, such as neuromelanin (a brownish-black pigment, byproduct of synthesis of catecholamines) and lipofuscin (yellowish-brown pigment that accumulates with age).

There are different internal structural characteristics between axons and dendrites. Axons typically almost never contain ribosomes, except some in the initial segment. Dendrites contain granular endoplasmic reticulum or ribosomes, with diminishing amounts with distance from the cell body.

Challenges to the neuron doctrine The neuron doctrine is a central tenet of modern neuroscience, but recent studies suggest that this doctrine needs to be revised.

First, 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. [6]

Second, dendrites, like axons, also have voltage-gated ion channels and can generate electrical potentials that carry information to and from the soma. This challenges the view that dendrites are simply passive recipients of information and axons the sole transmitters. It also suggests that the neuron is not simply active as a single element, but that complex computations can occur within a single neuron. [7].

Third, the role of glia in processing neural information has begun to be appreciated. Neurons and glia make up the two chief cell types of the central nervous system. There are far more glial cells than neurons: glia outnumber neurons by as many as 10:1. Recent experimental results have suggested that glia play a vital role in information processing. 

Finally, recent research has challenged the historical view that neurogenesis, or the generation of new neurons, does not occur in adult mammalian brains. It is now known that the adult brain continuously creates new neurons in the hippocampus and in an area contributing to the olfactory bulb. This research has shown that neurogenesis is environment-dependent (eg. exercise, diet, interactive surroundings), age-related, upregulated by a number of growth factors, and halted by survival-type stress factors. . Of particularly compelling interest, Charles Gross and Elizabeth Gould provided evidence suggestive that neurogenesis occurred in neocortex after birth, in areas of the brain known to be important for cognitive function.Strong challenges to this work have come from more well-controlled studies by Pasko Rakic and others which support Rakic's original hypothesis that neurogenesis after birth is restricted to the olfactory bulb and hippocampus. Rakic argues that the Princeton group's work has not been substantiated by multiple other groups

Copyright © 2006 vlgn.com                                 Powered by Engineer Partner one stop outsouce