Chapter 5: Synaptic transmission
Two classes of synapses
Synapses in the brain fall into two classes: electrical synapses and chemical synapses. These
two classes van be distinguished based on their structures and the mechanism they use to
transmit signals from the presynaptic to the postsynaptic neuron.
Electrical synapses permit direct, passive flow of electrical current from one neuron to
another due to the potential difference generated locally by the presynaptic action
potential. Current flow at electrical synapses arises at the gap junction, where membranes of
the two communicating neurons are linked. Gap junctions contain a connexon, which
provides the path for electrical current to flow from one neuron to to another.
With chemical synapses, the space between the presynaptic and postsynaptic neuron is
called the synaptic cleft. Synaptic vesicles in the synaptic cleft are filled with
neurotransmitters. These are secreted from the presynaptic neuron and detected by
receptors on the postsynaptic membrane.
Signaling transmission at electrical synapses
Connexons are composed of a unique family of ion channel proteins, the connexins, which
serve as subunits to form connexon channels. All connexons have four transmembrane
domains, and all connexons consist of six connexins that come together to form a hemi-
channel in both the pre- and postsynaptic neurons. These hemi-channels are precisely
aligned to form a pore that connects the two cells and permits electrical current to flow. A
variety of substances can simply diffuse between the cytoplasm of the pre- and postsynaptic
neurons. This permits important intracellular metabolites, such as ATP and second
messengers, to be transferred between neurons.
One advantage of electrical synapses is that transmission is extraordinarily fast. Because
passive current flow across connexons is virtually instantaneous, communication can occur
without the delay that is characteristic of chemical synapses. Another advantage of electrical
synapses is that transmission can be bidirectional. This allows electrical synapses to
synchronize electrical activity among populations of neurons.
Signaling transmission at chemical synapses
In the presynaptic terminal, the active zone is the place where synaptic vesicles discharge
their neurotransmitters in the synaptic cleft.
, Neuroscience Purves et al. (2018)
Sequence of events involved in transmission at a typical chemical synapse:
1) Transmitter is synthesized and then stored in vesicles.
2) An action potential invades the presynaptic terminal.
3) Depolarization of the presynaptic terminal causes opening of voltage-gated Ca2+
channels.
4) Ca2+ flows into the presynaptic cell through channels because of the concentration
gradient.
5) Ca2+ causes vesicles to fuse with the presynaptic membrane.
6) Neurotransmitters are released into the synaptic cleft via exocytosis.
7) Neurotransmitter binds to receptor molecules in the postsynaptic membrane.
8) Postsynaptic channels open or close.
9) Postsynaptic current causes excitatory or inhibitory postsynaptic potential
(EPSP/IPSP) that changes the excitability of the postsynaptic cell.
10) Neurotransmitters are removed by glial uptake or enzymatic degradation.
11) The vesicular membrane is retrieved from the plasma membrane.
Properties of neurotransmitters
Neurotransmitters are classified in two broad categories: small-molecule neurotransmitters,
such as acetylcholine, and neuropeptides. Multiple neurotransmitters can produce different
types of responses on individual postsynaptic cells. The speed of postsynaptic responses
produced by different transmitters also differs, allowing control of electrical signaling over
different timescales. In some cases, neurons synthesize and release two or more different
neurotransmitters, in which case the molecules are called co-transmitters.
In general, small-molecule neurotransmitters mediate rapid synaptic actions, whereas
neuropeptides tend to modulate slower, ongoing neuronal functions. The synthesis of small-
molecule neurotransmitters occurs locally within presynaptic terminals. Small-molecule
neurotransmitters are packed in small clear-core vesicles. Neuropeptides are synthesized in
the cell body of a neuron, and peptide-filled vesicles are transported down to the synaptic
terminal via axonal transport in large dense-core vesicles.
Quantal release of neurotransmitters
End plates are synapses between spinal motor neurons and skeletal muscle cells. They are
simple, large, and peripherally located. The postsynaptic action potential that is triggered by
an end plate potential (EPP) causes the muscle fiber to contract. Spontaneous changes in
miniature end plate potential (MEPP) occur even in the absence of stimulation of the
presynaptic motor neuron. A presynaptic action potential cause a postsynaptic EPP because
it synchronizes the release of many transmitter quanta (MEPPs).
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