FoundationalConceptsNeuroscience

him during sleep, he resolved to attempt to recover them. His inten-
tion was successful: the next night he awoke around 3 a.m. with an
idea for an experiment. This time, he got up and went to his labora-
tory to perform the following experiment.
Loewi was studying neural regulation of the heart using a prepa-
ration in which the heart from a frog was kept functioning in a jar
containing a solution of ions in water. Animal hearts contain their
own rhythmic oscillators, and a heart in ajar can maintain a regular
rhythm even when not connected to the animal’s brain or body. Ina
living animal, the heart rate is modulated by signals from the brain
via the autonomic nervous system. Loewi left some of these auto-
nomic fibers attached to the heart and by stimulating one of them, the
vagus nerve, the beating of the heart slowed. This much Loewi had
already investigated. The new experimental idea, emerging from his
dream, was this...
The experiment was set up as described above. After the vagus
nerve was electrically stimulated and the beating frog heart exhibited
the expected slowing, Lowei collected fluid from the jar containing
that frog heart and poured it into another jar containing a second
beating frog heart. Lowei then observed that this second frog heart
slowed its beating, too, but without any electrical stimulation of its
vagus nerve. Loewi’s conclusion was that some soluble chemical
substance must be released when the vagus nerve is stimulated, and
it is this chemical substance that mediates the signal from the vagus
nerve to the heart to slow its beating. Chemical neurotransmission!
Loewi called the mystery substance Vagusstoff, German for “sub-
stance from the vagus” nerve. Vagusstoff was later identified to be
acetylcholine, the first molecule recognized as a neurotransmitter.