Chemistry Reference
In-Depth Information
Difficulties in the chemical studies of brain tissue plagued researchers
for many years. By examining the brains of recently deceased humans,
the French researcher Jean-Pierre Courbe reported in 1833 that normal
brains consisted of 2-2.5 percent of the element phosphorous, whereas
the brains of people with low intelligence had a lower percentage, and the
brains of the “insane” had a higher one. If true, this finding suggested that
phosphorous had an excitatory effect on the nervous system—too little
of the element dampened activity, while too much caused an unnatural
outburst. But the finding was not true, evidently because of mistakes in
the difficult procedures of isolating and chemically analyzing tissue. Al-
though mistaken, Courbe's research was an interesting prelude to a simi-
lar set of findings in the 20th century, described below.
A great advance in brain science occurred in 1873, when the Italian
physician Camillo Golgi (1843-1926) developed a method of staining tissue
with silver nitrate. Neurons, as shown in the figure on page 74, have long,
thin projections called axons, which often course through and compose the
brain's white matter. A typical neuron has a diameter of about 0.002 inches
(0.005 cm), though it generally covers a larger area with its branches called
dendrites, which receive input from other neurons. The axon projects to
other neurons or muscles that may be nearby or, in extreme cases, as far
away as 40 inches (100 cm) in humans (and longer in bigger animals). Prior
to Golgi's staining technique, biologists could not make a lot of progress
studying brain tissue with microscopes because the transparent cells and
projections were impossible to track. But the stain filled the whole cell, in-
cluding the axon, allowing scientists to pick it out of the drab background.
(For some as yet unknown reason, applying Golgi's technique to brain tis-
sue stains only a fraction of neurons. This selectivity is essential, otherwise
the stain would cover everything and biologists would be unable to pick out
a cell in the uniformly dark background.)
With tools such as Golgi staining, researchers studied neurons and
their projections. But it was not obvious if the projections made contact
with the target neuron or stopped just before reaching the neuron, leav-
ing a small gap. Some researchers, including Golgi, believed that the brain
was a giant net consisting of cells in physical contact with one another;
other researchers, such as the Spanish anatomist Santiago Ramón y Cajal
(1852-1934), were convinced that a gap existed between axons and their
target neurons. Cajal realized the brain might be simpler to understand
if the cells were joined as one, as Golgi believed, but Cajal was certain
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