36
౪
1 Introduction
gether are called the “synapse”. The membrane of the axon before the synapse has an
electrochemical potential, as we have been discussing. That potential is temporarily re-
versed by an action potential. When that change in potential reaches the tip of the axon,
another voltage-gated channel opens and in this case it allows calcium ions to enter. The
increase in calcium concentration inside triggers the release of the neurotransmitter
acetylcholine into the synaptic cleft via exocytosis (more about exocytosis and neuro-
transmitter a little later). The acetylcholine is now in the little space between the two
neurons, the synaptic cleft. It travels over to the other side of the cleft and binds to a
transmembrane receptor – which also happens to be an ion channel. In this case, it is a
ligand-gated ion channel and the ligand is acetylcholine. The channel opens and potas-
sium and sodium ions enter the dendrite of that neuron. Since the dendrite also has a
potential across its membrane, the influx of all of these positive charges changes its po-
tential and this initiates an action potential that carries the information to the soma of
the neuron, where all incoming action potentials are processed. If the processing results
in an action potential at the axon hillock, that action potential moves along the axon to
the next synapse and the process repeats.
What is exocytosis? Basically, at the axon side of the synapse there are a lot of small
vesicles that are held tight by the cytoskeleton (Figure 1.39). They are filled with com-
pounds called neurotransmitters. Neurotransmitters, as their name suggests, transmit
information between neurons. There are different neurotransmitters for different types
of neurons; also, some of the neurotransmitters will make it more likely and some less
likely that the following neuron will end up with an action potential. In the example
above the neurotransmitter was acetylcholine, a common neurotransmitter that results
Figure 1.39: Exocytosis into the synaptic cleft.