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Unformatted text preview: 233 CHAPTER 22 STRUCTURE OF THE CEREBRAL CORTEX In order to understand what the brain does and how it works, investigators have relied on three principal methods. One method is to evaluate changes in behavior or perception when a region is lesioned or removed. In the last chapter we saw some of the remarkable changes in complex functions that resulted from damage to one or another portion of association cortex. These insights came from clinical studies and such insights are also obtained from more controlled studies where restricted regions of the brain are lesioned in animals and their behavior before and after lesions are assessed. The second method is to record the action potential discharges from a particular region of the brain evoked when stimuli are presented to an animal by an investigator. In this way, investigators can determine exactly what feature of a stimulus the neuron is responding to and how it codes for different types of stimuli. The results of such studies on the visual system will be discussed in detail in subsequent chapters. The third method is to evaluate the types of cells, how they are arranged and how they are connected in a region of the brain. The arrangement of various types of cells in a region of the brain is called its neuronal architecture. The evaluation of neuronal architecture requires that the investigator be able to see the cells in the brain, and it is for this reason that the development of various stains is so important. Indeed, one eminent neuroscientist, Floyd Bloom, once wrote a review article that he charmingly titled, The Gain in Brain is Mainly in the Stain. Franz Nissl, the German physician who introduced a stain for RNA that is one of the most important stains for studying the nervous system Neuronal architecture of the cerebral cortex Around the turn of the last century, neuroanatomists began looking at the cortex with a variety of compounds that stained different portions of the neuron. Two stains were particularly important: the Nissl stain and the Golgi stain. The Nissl method was introduced by the German physican, Franz Nissl, and uses basic aniline to stain RNA blue. The rough endoplasmic reticulum in nerve cells, which is called Nissl substance, appears dark blue due to the staining of ribosomal RNA, giving the cytoplasm a mottled appearance. Individual granules of ribosomes (extranuclear RNA) are named Nissl granules. The Nissl substance in a neuron is present only in the cell body and not in the axons or dendrites. Nissl stains, then, are selective for the nucleic acids and stain the cell body, but not the dendrites or axons, of every neuron in the brain, and thus can reveal important structural features of neurons and differences among neurons in various parts of the cortex. 234 Golgi stains, on the other hand, use heavy metals, like silver nitrate (the same compound used in black-while photographic paper). The Italian anatomist, Camillo Golgi, in about 1875, came about this method by which, seemingly at random, only a very small proportion of neurons in a region are stained...
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- Spring '08