Nutrition and Neurotransmitters

Abstract

In 1921, Otto Loewi was the first to show that transmission of nerve impulses was mediated by chemicals. In his classic experiments, he demonstrated that when ventricular fluid of a stimulated heart was transferred to a nonstimulated frog heart, the effects of stimulation were observed in this heart. This meant that the nerve stimulus of the first heart was reproduced by the chemical activity of the solution transferred to the second heart (Loewi, 1960). Subsequent analysis in his laboratory demonstrated that this chemical substance was acetylcholine, a rather simple and relatively small molecule. Ever since this discovery, acetylcholine has been identified as a signal transmitter that has many functions in the human body. For example, it can slow down the heart rate, constrict involuntary muscle, and participate in complex integrating functions of the brain and spinal cord. A neurotransmitter is now defined as a chemical substance discharged from a nerve fiber ending, which reaches and is immediately recognized by a receptor on the surface of a postsynaptic nerve cell, and the net effect is either a stimulation or inhibition of a receptor cell. A search for other neurotransmitter substances, for example, a substance that would increase heart rate, led to the isolation of noradrenaline or norepinephrine (NA) by von Euler (1956). The established criteria used to define a substance as a neuro-transmitter include (1) secretion following stimulation of the nerve; (2) binding with a specific receptor on the postjunctional cell; (3) production of a biological response, stimulation, or inhibition; and finally (4) a mechanism to rapidly terminate the biological effect. A number of neurotransmitters that would satisfy most of these requirements have been identified (Krnjevic, 1974; Bachelard, 1974) (Fig. 29).