air about electricity.Our description of the microscopic nature of neural signaling begins
with a discussion of the electrical properties of neurons, leading up
to how a signal is propagated along an axon. These electrical proper-
ties begin with ions. Electricity is the movement of charge; ions are
charged particles, and ions abound in living organisms. Calcium, chlo-
rine, potassium, and sodium are among the top ten most abundant
chemical elements in the human body. And these elements occur as
ions dissolved in the fluids of the body: Ca*, Cl, K, and Na’*.
Particles in solution are constantly moving as a result of the energy
of thermal agitation. The movement is random and tends to cause
particles to move apart and distribute uniformly over whatever vol-
ume of fluid is available, a process called diffusion. Diffusion can be
readily observed by putting a spoonful of milk in a cup of tea. The milk
spreads out and is soon uniformly distributed through the entire cup
of tea. If the tea is hot, the energy of thermal agitation is relatively
large and the diffusion process takes place rapidly. If the tea is cold,
the lower energy of thermal agitation makes for a noticeably slower
diffusion.
The neuronal boundary, like all cell boundaries, is composed of a
phospholipid bilayer membrane (described in Chapter 3). An impor-
tant property of such membranes is that they are impermeable to
ions, meaning that ions cannot pass through them. This is because
most of the thickness of the bilayer is composed of the phospholipid
hydrocarbon chains and is a highly hydrophobic environment. Ions,
however, are very hydrophilic and will collect polar water molecules
around them. This shell of HyO molecules prevents ions from pen-
etrating into and through the hydrophobic core of a phospholipid
bilayer.