Video Transcript
The diagram provided shows a simple
outline of a cholinergic synapse. What part of the synapse is
indicated by the question mark?
This question asks us to identify a
part of the synapse in a cholinergic neuron. Let’s review the structures
involved in a synapse in order to answer this question correctly.
Neurons make up the nervous system,
which is one of the most complex biological systems known to man. Neurons link to each other and
their effectors, muscles and glands, through junctions called synapses. Information is passed on through
the nervous system via nerve impulses. These are first received by the
dendrites of the neuron. The impulse then moves past the
soma, or cell body, along the axon and to the axon terminals. It is here at the end of the axon
that the synapses are found.
Let’s now look at a synapse in more
detail. There are three main parts to a
synapse, as we can see in this diagram. The neuron coming into the synapse
is the presynaptic neuron, pre- meaning before, so it is the neuron before the
synapse. At the end of the presynaptic
neuron are the axon terminals, which are shaped like a doorknob, so are called
presynaptic knobs. These knobs contain vesicles of
neurotransmitter. After the presynaptic knob comes
the synaptic cleft or gap.
When an electrical signal arrives
at the presynaptic knob, it causes a rapid influx of calcium ions. This influx causes the fusion of
the vesicles with the membrane of the presynaptic knob and the release of the
neurotransmitter into the synaptic cleft. This is the step that is
illustrated in the diagram provided with the question.
In this question, we are looking at
a cholinergic synapse, so the type of neurotransmitter released is
acetylcholine. This then diffuses across the cleft
to the third part of the synapse, the postsynaptic neuron. On the membrane of the postsynaptic
neuron are receptors for the neurotransmitter. In the case of this cholinergic
synapse, the neurotransmitter acetylcholine binds to its receptors, causing sodium
ion channels to open and sodium ions to enter, as illustrated on our diagram. This depolarizes the postsynaptic
neuron, and so the process of neural transmission begins all over again.
Now that we have reviewed the
process of signal transduction and how it involves the synapse between each neuron,
we are able to take another look at our question. This question asks us to identify
the term for space between two neurons where the neurotransmitter can be released
and received, as indicated by the question mark. We now know the correct answer is
the synaptic cleft.
