Nervous System | Research & Encyclopedia Articles

Nervous System

How are human functions such as movement, learning, thought, and reflexive action regulated by the body? For a very long time, scholars divided these processes into two large categories: those controlled by the brain and those controlled by the mind. According to this duality, intellectual processes such as critical thinking and memory were a function of the mind, an immaterial entity that was not subject to scientific study, but was to be understood in terms of philosophical analysis. The brain, on the other hand, was thought to be a concrete, physical entity responsible for phenomena such as muscular activity that could be investigated by scientific means. During the Middle Ages, for example, many scholars assigned the control of higher intellectual processes to an immortal and invisible soul and the control of muscular behavior to a tangible and mortal soul that could be studied scientifically.

Studies of the physical aspect of this duality have a very long history. For example, the Greek physicians Erasistratus and Herophilus discovered the existence of nerves and traced their connection through the central nervous system into the brain, thereby refuting the then widely held belief that the heart was the center of sensation. Both carried out detailed investigations of the brain and identified many of its regions. The great Roman physician Galen carried out a number of experiments on the nervous system and identified seven of the twelve cranial nerves. He also expanded on an earlier theory that nerves are hollow tubes through which "animal spirits" flow. Galen's work was among the last research on the nervous system for nearly 1,500 years.

When such research began again, it involved a new assumption; namely, that all forms of animal behavior--both intellectual and physical--are functions of the physical brain and, thus, subject to concrete, scientific analysis. Over time, it has become more and more clear that this assumption is valid and that all animal behaviors can be understood in terms of the nervous system and, on a more basic level, in terms of biochemical and biophysical changes that take place within this system. Today, there are probably no human or animal behaviors ascribed by scientists to the control of an intangible "mind" or "soul."

One landmark figure in early research on the nervous system was Thomas Willis (1621-1675). In 1664, Willis wrote the Cerebri anatome, a long article that summarized all that was then known about the brain and nervous system. Some historians claim that this article provided the first complete description of the nervous system. In that article, Willis also reported on his own research, describing the role of the cerebellum and the cerebrum, respectively, in the control of involuntary and voluntary activity.

At about the same time, the brilliant French scientist René Descartes applied his mechanistic philosophy to the analysis of animal behavior. He taught that muscular actions are controlled by a nervous system that consists of a complex arrangement of pumps and tubes. Messages are carried from the brain to the muscles by means of "vital spirits" that flow through these tubes, Descartes wrote. Although his theory was proved incorrect, Descartes did introduce an important new concept to the study of the nervous system: the reflex. The term reflex refers to any involuntary response that a body makes when exposed to a stimulus. The knee-jerk reaction that occurs when a physician tests your reflexes is perhaps the most familiar of these reflex actions.

Understanding reflex action became one of the major goals among researchers of the nervous system over the next century and a half. In 1811, the English surgeon Sir Charles Bell (1774-1842) made an important discovery. He found that nerves leading into the spinal column consist of two different kinds. Some (the sensory nerves) carry messages from receptors in the eyes, ears, tongue, skin, and other places on the body, while others (motor nerves) transmit messages from the central nervous system to the muscles. Bell's discovery was confirmed a decade later by François Magendie, working with no knowledge of Bell's own research. The term reflex action was not actually introduced until 1833. It was first used by the English physiologist Marshall Hall (1790-1857), who did extensive studies on the phenomenon.

A detailed explanation of the reflex process was eventually provided by Charles Scott Sherrington, working with the "scratching" reflex in dogs. Sherrington was able to trace the path of a nerve message from regions on a dog's back, across sensory nerves, to the spinal cord, across synapses, to motor neurons, and then to muscle cells in the dog's leg. Because of his extensive research on nerve processes such as this one, Sherrington has become known as the father of modern neurophysiology. Studies of reflex action have become an important part of psychological research also. The Russian physiologist Ivan Petrovich Pavlov carried out his now-famous research on conditioned reflexes in the 1880s and 1890s. He showed that, by exposing an animal to two stimuli at the same time (for example, food and a bell), he could train an animal to respond (by salivating) to the stimulus (the bell) that was otherwise unconnected with this response. Throughout much of the twentieth century, the American psychologist B. F. Skinner (1904-1990) carried this type of research on conditioned responses to even higher levels of sophistication.

Nineteenth-century neurophysiologists also explored a number of other features of the nervous system. For example, the German physiologist Johannes Müller (1801-1858) focused on the sensory organs, especially the response of the eye to color and of the ear to sound. Some of his most important discoveries are summarized in his "doctrine of specific nerve energies." According to this doctrine, a sensory nerve can respond in only one way. For example, no matter how one stimulates the optic nerve, that stimulation will always be interpreted in the brain as light.

The exact mechanism by which messages travel along nerve fibers was also a topic of great interest. As early as 1771, the Italian anatomist Luigi Galvani had found that nerve messages somehow have an electrical component. This line of research was further developed by Emil du Bois-Reymond (1818-1896), sometimes called the father of modern electrophysiology. The electrical nature of nerve transmission was put to use in the 1920s by Joseph Erlanger (1874-1965) and Herbert Gasser (1888-1963) in their adaptation of the cathode ray tube for the measurement of nerve messages. With their cathode ray tube, Erlanger and Gasser were able to greatly amplify the naturally small electrical currents passing along nerve cells and to study them in much better detail. This research allowed them to calculate, for example, the speed at which messages travel along a nerve.

A related problem concerning nerve transmission involved the mechanism by which a message travels from one cell to the next. In the 1890s, Wilhelm von Waldeyer-Hartz had suggested that adjacent nerve cells do not actually touch each other, but are separated by a small gap. This view was confirmed experimentally by Camillo Golgi shortly thereafter. How nerve messages are transmitted across these tiny gaps--synapses--soon became a topic of intense research. In 1903, the English physiologist Thomas R. Elliott (1877-1961) suggested that such messages are carried across the gap by chemicals, now known as neurotransmitters. Over the next two decades, Elliott's colleague Henry Dale (1875-1968) and the German-American physiologist Otto Loewi (1873-1961) were able to confirm this view and to identify the first neurotransmitter, acetylcholine.

Yet another line of research on the nervous system involved the process of "mapping" the brain. This term refers to the localization of certain specific bodily functions in particular parts of the brain. Some of the most important research in this field was carried out by Gustav Fritsch (1838-1927) and Julius Hitzig (1838-1907). By operating on the brains of living dogs, Fritsch and Hitzig were able to show that specific regions of the brain are responsible for specific kinds of muscular action. They were thus able to draw a map of the brain showing the specific location for each of these functions. The work of Fritsch and Hitzig was advanced and extended by Sir David Ferrier (1843-1928), who not only improved the earlier maps, but also modified them to include regions responsible for sensory organs and other functions.

At the cellular level, scientists know that brain cells fall into two broad groups: neurons and glial cells. According to scientific tradition, neurons do most of the information processing and communicating, while glial cells are little more than space-holders. However, new research is revealing that glial cells not only communicate with each other but also with neurons. Glial cells also seem to play a vital role in brain development, partly because they produce substances that help neurons grow. In addition, drug and biotechnology companies are now exploring ways to use glial cells to develop treatments for conditions such as Alzheimer's disease.