Although the theory that neurons are true secreting cells which act upon one another by the passage of chemical substances was enunciated at the beginning of the 1900s (Scott 1905), it was only half a century later that the significance of humoral transmission for the processing of information in the central nervous system became fully appreciated. It was then established that the transfer of neural impulses occurs principally at morphologically differentiated contact sites, the synapses, and that the chemical intermezzo in the otherwise electrical flow of signals, though causing some delay, still falls within the millisecond range. It appeared, moreover, that the influence exerted by a presynaptic element via a chemical mediator, or neurotransmitter, could be either excitatory or inhibitory (e.g. Anderson et al. 1964; Eccles 1957, 1969).
KeywordsTransmitter Substance Neural Impulse Presynaptic Element Tamic Acid Chemical Neuroanatomy
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