CHAPTER 6Synapses, Neurotransmitters, and Receptors
Synapses (Greek syn = together, haptein = to fasten, join, clasp) are the
points of communicative contact between neurons. While most stud-
ies of synapses have involved neuron-to-neuron communication, it is
now appreciated that synaptic connections also involve glial cells—
between neurons and glia and between glia and glia. The word synapse
was introduced into the lexicon of science in the 1890s. Synapses
come in two varieties: electrical and chemical.
An electrical synapse (also called a gap junction) is built from clus-
ters of proteins that form channels in the membranes of two adjacent
cells. A single channel is called a connexon, and each connexon is made
up of several component proteins called connexins (Fig. 6.1). An elec-
trical synapse forms when one or more connexon pairs join together,
allowing ions to pass directly from one cell to the next. The size of
the channel pore is larger than that of the ion channels discussed in
Chapter 5, such as those involved in action potentials, and ions of var-
ious sizes and charges may all pass through the connexon pore. The
pore is even large enough for small molecules like ATP and glucose
to pass through. In electrical synapses, ions can flow rapidly from
one cell to another, resulting in very fast propagation of membrane