The nervous system has intrigued scientists and philosophers since ancient times. The Roman physician Galen thought that the brain pumped a vapor called psychic pneuma through hollow nerves and squirted it into the muscles to make them contract. The French philosopher René Descartes still argued for this theory in the seventeenth century. It finally fell out of favor in the eighteenth century, when Luigi Galvani discovered the role of electricity in muscle contraction. When microscopes and histological staining methods were better developed, the Spanish histologist Santiago Ramón y Cajal (1852-1934) was able, with great effort, to trace the course of nerve fibers through tissue sections. He demonstrated that the nervous pathway was not a continuous "wire" or tube, but a series of separate cells separated by synapses. His theory, now called the neuron doctrine, suggested another direction for research—how do neurons communicate? The two key issues in neurophysiology are (1) How does a neuron generate an electrical signal and (2) How does it transmit a meaningful message to the next cell? These are the questions to which this section and the next are addressed.
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