HA "Far Side" cartoon by Gary Larson shows a classroom with pupils at their desks. One pupil with a noticeably small head and his hand raised says, "Teacher, may I be excused? My head is full." This amusing cartoon suggests a deep question: What is the capacity of the human brain? Is it limited by size, number of neurons, number of synapses?
Since the earliest studies of the cellular structure of the brain, the dogma has been that we are born with a certain number of neurons, we steadily lose neurons throughout life, and we do not get any new neurons. The evidence: In adult brains, no neurons with mitotic structures, or neurons at different stages of maturation, are visible. The explanation for our lifelong ability to learn has been that we have enormous potential to form and re-form synapses. Synaptic plasticity is certainly an important property of the nervous system; however, one of the most exciting and revolutionary recent discoveries in neurobiology is that new neurons are formed in adult avian and mammalian brains, and that the formation of these new neurons seems to be stimulated by experience and learning.
The birth of new cells can be revealed by giving an animal an injection of ra-dioactively labeled thymidine, which becomes incorporated into new DNA. The surprise discovery was the appearance of some labeled neurons when adult rats were injected. Lots of reasons were generated as to why they could not really be new neurons, and few scientists gave up the old dogma. This debate received much more attention when Fernando Not-tebohm and his colleagues at Rockefeller University showed that new neurons are formed in those parts of the bird brain responsible for song at the time of the year when birds come into reproductive condition. Furthermore, they showed that sex hormones and hearing song stimulated the birth of new neurons. (We will return to this story in Chapter 52.)
The exciting results from bird brains gave new impetus to mammalian studies. Two parts of the adult mammalian brain have now been shown to acquire new neurons. One is the olfactory bulb, which is not surprising, since the olfactory bulb neurons extend into the nasal epithelium, which is regularly shed. The other area is the hippocampus, which, as we will learn in this chapter, is a region involved in the formation of long-term memories. As in birds, experience and learning stimulate neurogenesis in the mammalian hippocampus. For those who still doubt, Fred Gage and his colleagues at the Salk Institute have recorded action potentials from new hippocampal neurons and have
An adult mouse was injected with viral particles that carried a gene for green fluorescent protein (GFP).The genetic material from the virus was taken up and subsequently expressed by dividing cells. When the mouse brain was examined two weeks later, it contained neurons labeled with GFP—neurons that must have arisen after the viral injection. Mice that were allowed to run in wheels or mazes produced even greater numbers of labeled (new) neurons.
shown that they mature into functional neurons with properties identical to those of older neighboring cells.
Many new questions arise from these studies. Could neu-rogenesis be stimulated in other parts of the nervous system to repair damage and to counter the effects of aging? Since brains don't continue to get bigger, is there regular loss and replacement of neurons? Does loss of neurons contribute to forgetting? Do new neurons facilitate learning? Will you generate new neurons by studying this chapter?
The unit of function of the brain is the neuron. The human brain consists of about 100 billion neurons, which account for its ability to handle vast amounts of information. In the previous two chapters we learned about the cellular properties of neurons. In this chapter we take on the challenge of understanding some functions of the human nervous system in terms of these cellular mechanisms.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.