FoundationalConceptsNeuroscience

potential changes between cells. The signaling mediated by electrical
synapses is relatively simple: communication of ion concentration or
membrane potential changes from one cell to another, and perhaps
the transfer of small molecules having regulatory functions within
cells.
The chemical synapse is more complex and provides opportunities
for additional kinds of regulation, such as changes in strength,
feedback, and varied effects on different target cells. When the term
synapse is used without further qualification, it generally refers toa
chemical synapse. Figure 6.2 shows a diagram of a chemical synapse.
A narrow gap, the synaptic cleft, separates the presynaptic axon
terminal and the postsynaptic cell. The synaptic cleft is very small,
around 20 nanometers (nm) in width, and is filled with water and
ions. Although this gap is too small to see with a light microscope—
an electron microscope is required to visualize it—the space is still
considerably larger than the tiny gap separating cells connected by
electrical synapses. Cells connected by electrical synapses can be seen
as essentially touching or in direct physical contact, whereas cells
connected by chemical synapses are separated by a space that a signal
passing from one cell to the next must traverse.