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Cajal's Drawings

Dendritic trees and axonal arborizations can be spectacular and complex. One of the biggest and most complex dendritic trees is that of the Purkinje cell, a type found in the cerebellum. Purkinje cells have a huge, flat, spreading dendritic tree. The cells are arranged in layers, with axons from other cells crossing through them, forming a huge grid which may have something to do with coordinating movements in space: one of the functions of the cerebellum.

Where is the Purkinje cell found?


The Purkinje cell with its huge dendritic tree

The drawing here is a classic by Santiago Ramon-y-Cajal (ray-MON ee ka-YAL), a neuroanatomist who lived from 1852 to 1934. The name Ramon-y-Cajal is composed of the father's and mother's surnames and is often shortened to "Cajal." Cajal won the Nobel Prize in 1906 for his pioneering work on neurons. The drawing of the Purkinje cell dendritic tree was one of his favorites. Cajal said it reminded him of a grape arbor.

How did Cajal make his drawings?

Ramon-y-Cajal made drawings by injecting neurons with dye. The dye spread within the cell but could not cross the cell membrane. Neurons are naturally transparent and tightly packed together. Without a stain of some kind, one cannot see much in nervous tissue, using a light microscope. With his dye injection technique, Cajal was able to see complex neural processes (dendritic trees and axonal arborizations) using only a light microscope. It is packed full of structures too small for scientists of Cajal's era to see, including the cell's nucleus, which regulates many aspects of the cell's activity. The little black blob is the cell body or soma of the neuron. It is packed full of structures too small for scientists of Cajal's era to see, including the cell's nucleus, which regulates many aspects of the cell's activity.

The gaps between neurons are so tiny that in the 1890s there was a tremendous controversy over whether the nervous system formed a continuous network (the reticular theory) or whether it consisted of separate cells (what was then called the neurone theory). Cajal finally resolved the issue. He showed that neurons were distinct cells, separated by gaps that—in those days—were too tiny for microscopes to see.

What was Cajal's proof that neurons were separate?

Santiago Ramon-y-Cajal spent much of his professional life from about 1889 onward trying to persuade other scientists to drop the reticular theory in favor of the neuron theory. Two years before his death, when he was in his 80s, Cajal marshaled all the evidence in favor of the neurone theory one last time...although by then (1932) most people realized Cajal was right all along. Neurons were individual cells, separate and distinct. Cajal knew this because of his dye-injection technique. The dye did not spread between cells, so he knew there was some barrier or gap between them.


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