Video Transcript
Which of the following is a
defining feature of synapses? (A) Synapses can only pass
information in one direction. (B) Synapses transmit information
as electrical signals only. (C) Synapses only form between two
neurons. Or (D) acetylcholine receptors are
only located on the presynaptic neuron.
This question is asking us about
synapses, so let’s review the key points about their features to help us answer
it. The nervous system is one of the
body’s internal communication networks that helps us to respond to changes in the
environment around us. Neurons are the nerve cells that
make up this network. And they are interconnected to
carry nerve impulses all around the body. The junctions between neurons or
between neurons and effectors, such as muscles, are called synapses. Synapses transmit the impulse from
cell to cell.
Let’s now take a closer look at
their structure and how this allows them to carry out their role. Here you can see there are three
main parts to a synapse. Coming into the synapse is the
presynaptic neuron, pre- meaning before, so it is the neuron before the synapse. Then there is the synaptic cleft or
gap and, finally, the postsynaptic neuron, post- meaning after. The presynaptic neuron carries the
electrical impulse towards the synapse.
At its ends are synaptic bulbs, or
synaptic knobs. When the impulse arrives here, it
causes a special type of chemical called a neurotransmitter to be released into the
synaptic cleft. An example of a neurotransmitter is
acetylcholine. The neurotransmitter will then
diffuse across the synaptic cleft down its concentration gradient to the other
side. The synapse shown here is a
cholinergic synapse. So in this example, acetylcholine
binds to receptors on the membrane of the postsynaptic neuron, causing an impulse to
be triggered. The neurotransmitter can only be
released from the presynaptic neuron. And there are only receptors for it
on the postsynaptic neuron, so the impulse can only pass in one direction. This is important to prevent
confusion with the messages.
Now that we have revised over the
structure and function of the synapse, let’s return to the question, which asks,
which of the following is a defining feature of synapses? We can rule out option (B) as we
now know that chemicals carry the information across the synaptic cleft. We can also rule out option (C) as
we know that synapses can form between one or more neurons and also with
effectors. Option (D) is also incorrect as the
receptors are found on the postsynaptic neuron membrane not the presynaptic. This leaves us with the correct
answer (A). A defining feature of a synapse is
that they can only pass information in one direction.
