As the neuron doctrine became generally accepted, it raised the question of how neurons communicate with each other. In the early twentieth century, biologists assumed that synaptic communication was electrical—a logical hypothesis given that neurons seemed to touch
Figure 12.16 Synaptic Knobs on the Soma of a Neuron in a Marine Slug, Aplysia (SEM).
Are these synapses axodendritic, axosomatic, or axoaxonic?
each other and signals were transmitted so quickly from one to the next. Closer histological examination, however, revealed a 20- to 40-nm gap between neurons—the synap-tic cleft—casting doubt on the possibility of electrical conduction.
In 1921, the German pharmacologist Otto Loewi (1873-1961) conclusively demonstrated that neurons communicate by releasing chemicals. The vagus nerves supply the heart, among other organs, and slow it down. Loewi opened two frogs and flooded the hearts with saline to keep them moist. He stimulated the vagus nerve of one frog, and its heart rate dropped as expected. He then removed some of the saline from that heart and squirted it onto the heart of the second frog. The solution alone reduced that frog's heart rate. Evidently it contained something released by the vagus nerve of the first frog. Loewi called it Vagusstoffe ("vagus substance") and it was later renamed acetylcholine—the first known neurotransmitter.
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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.