CHEMISTRY OF THE BRAIN: MOLECULES, MOOD, AND MENTAL ILLNESS - Chemistry: Notable Research and Discoveries - page 76

sending messages composed of action potentials. As scientists began
to study this process, they realized that the transmission of messages
across the gap was extremely important. The British physiologist Sir
Charles Sherrington (1857-1952) coined the term synapse in 1897 to
describe this gap. In most cases, the gap is less than 0.000001 inches
(0.0000025 cm).
Early researchers thought of two means by which synaptic transmis-
sion could occur. Action potentials could jump the gap by some electri-
cal mechanism such as induction—this is the same process by which an
electrical current in one coil of a transformer induces a current in the
other coil, even though the coils are not in contact. Another possible
mechanism of synaptic transmission involves the use of a chemical in-
termediate. Suppose an action potential caused molecules of a certain
chemical to be released at the tip of the axon. These molecules would
diffuse across the gap, reaching the other side in a short period of time.
But researchers were unsure what effect these molecules could have on
the postsynaptic neuron that receives the message. (The transmitting
neuron is known as the presynaptic neuron.) In 1906, the British re-
searchers Thomas Elliot (1877-1961) and John Langley (1852-1925)
considered the idea of a receptor on the postsynaptic neuron, on which
the chemical might dock and exert its effects.
This idea of chemical transmission in the nervous system was not
widely accepted until researchers began conducting ingenious experi-
ments. In 1921, Otto Loewi (1873-1961), a German-born physician
who later immigrated to Austria and then to the United States, exper-
imented with a nerve known as the vagus nerve. A nerve consists of
a bundle of axons—these axons constitute the projections of a group
of neurons to another group of neurons or to muscles (such as heart
muscle or skeletal muscle) or glands (such as the adrenal gland, which
sits above the kidney and releases hormones into the bloodstream). The
vagus nerve contains projections from neurons in the brain to the heart
and is the means by which the brain can regulate heart function (for
instance, speeding the rate in times of excitement). As in the case of
neuron-to-neuron transmission, messages conveyed to the muscles and
glands must cross synapses.
Loewi used frogs in his experiments. He exposed the vagus nerve
and heart of one frog, then stimulated the nerve artificially by shock-
ing it with an electrical current. The heart began to beat faster. Loewi