CHEMISTRY OF THE BRAIN: MOLECULES, MOOD, AND MENTAL ILLNESS - Chemistry: Notable Research and Discoveries - page 75

sity.” Improvements in microscopes and staining techniques eventually
resolved the debate in favor of Ramón y Cajal by the early 20th century.
Meanwhile, scientists studied the electrical activity of neurons. They
discovered that the messages neurons pass to one another consist of brief
electrical impulses known as action potentials. Neurons (and most oth-
er cells) have a small electrical potential of about 60 millivolts between
their interior and exterior—this voltage, which is only about 4 percent
as strong as a typical flashlight battery, exists across the cell's membrane.
The watery solution inside a cell is different from the solution outside the
cell, but both solutions contain various nutrients as well as charged par-
ticles called ions. Ions come from compounds that dissolve in water, such
as sodium chloride (table salt) that separates into a positively charged so-
dium ion and a negatively charged chloride ion. The reason salt dissolves
is because the chemical bond between sodium and chlorine is ionic—the
bond forms when sodium donates an electron to chlorine. Water mol-
ecules pull apart the bond, and the electron stays with the chloride ion,
giving it an excess negative charge while leaving sodium with a deficit.
What makes a neuron special is the presence of protein molecules
that are sensitive to the voltage across its membrane. Membranes do
not allow ions to pass—one of the main jobs of a membrane is to regu-
late the flow of substances into and out of the cell—but certain pro-
teins embedded in the membrane contain channels to allow ions to pass
through. The flow of ions constitutes an electrical current. In neurons,
these proteins, known as ion channels, can open and close quickly, pro-
ducing currents that briefly change the electrical potential of the cell.
This process gives rise to an action potential, which lasts only a few milli-
seconds and travels down the neuron's axon.
But how does the action potential bridge the gap between the axon
and the target neuron? The answer to this question lies at the founda-
tion of many important topics in brain chemistry.
SynAPTICTrAnSMISSIon
Action potentials generated by neurons convey information in their
rate—how many occur in a given period of time, which can be up to
a few hundred per second—and sometimes in their timing relative to
one another. Networks of neurons process sensory information, control
movement, and create the still mysterious nature of consciousness by