Synapse
Nerve cells differ from other types of cells in an important way: They contain long projections, nerve fibers that consist of axons and dendrites. These nerve fibers are very long and complex. In an animal, they overlap each other in a network whose basic structure is difficult to unravel. For this reason, early biologists had a great deal of trouble determining exactly what it is that constitutes a single nerve cell and how two nerve cells are related to each other.
According to one popular early theory, nerve fibers are actually in contact with each other, and nerve messages are transmitted directly from one fiber to the next, or from one cell to the next. An alternative to this view was suggested in the late 1800s by the German anatomist Wilhelm von Waldeyer-Hartz. Waldeyer-Hartz hypothesized that two nerve cells or nerve fibers are not actually in contact with each other, but are separated by a small gap. Waldeyer-Hartz's hypothesis was based on modest experimental evidence, and it was not until the work of Camillo Golgi some time later that this idea could be confirmed in the laboratory. Using a new type of cell stain made with silver salts, Golgi prepared some excellent slides that clearly showed the structure of nerve cells. In those preparations, the gap hypothesized by Waldeyer-Hartz was easily observed.
In spite of this discovery, Golgi could not explain how this gap was involved, if at all, in the transmission of nerve impulses from one cell to another. Probably the clearest description of nerve transmission first appeared in Charles Scott Sherrington's monumental work, The Integrative Action of the Nervous System (1906). In this book, Sherrington argued that nerve messages travel along the axon on one cell, across the gap between two cells, and into the dendrite of a second cell. He was unable to say, however, how the message passes across the gap, which he called the synapse. Sherrington's description inspired efforts by other researchers to find the mechanism by which nerve messages pass across the synapse. As early as 1903, the British physiologist Thomas R. Elliott (1877-1961) suggested that these messages might be carried across the synapse by chemicals. Over the next two decades, research by a number of investigators, particularly Henry Dale (1875-1968), Otto Loewi (1873-1961), and Sir John Carew Eccles (1903-), confirmed this hypothesis and uncovered the first of these chemicals, acetylcholine.
Today most drugs that affect the nervous system do so through their effects on the synapses. Such drugs may alter the release of a neurotransmitter or the effects of a neurotransmitter on postsynaptic membranes--the neuron membranes at which messages are received.
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