Video Transcript
The diagram provided shows a simplified unlabeled outline of a cholinergic
synapse. The influx of which ion generates an action potential in the postsynaptic neuron? (A) Calcium, (B) oxygen, (C) sodium, (D) potassium, (E) hydrogen.
This question is asking us to recall the ions at the synapse that generate an action
potential in the postsynaptic neuron. To answer this question, let’s review the ions involved in the transmission of an
action potential at a cholinergic synapse. First, let’s review what the synapse is and its function in propagating the action
potential from one neuron to the next. The synapse is the connection between two neurons or sometimes a neuron and an
effector organ or gland. The structure of a synapse between two neurons includes a presynaptic neuron, which
precedes the synaptic junction, the synaptic cleft, which is the gap between two
neurons, and the postsynaptic neuron, which is the neuron following the synaptic
cleft.
For an action potential to be propagated from the presynaptic neuron to the
postsynaptic neuron, it must first be converted from an electrical signal to a
chemical signal. This happens when the action potential arrives at the end of the presynaptic neuron,
which is called the synaptic knob. The arrival of the action potential causes the synaptic knob to depolarize. This triggers the voltage-gated calcium-ion channels to open and allows calcium ions
to diffuse into the synaptic knob. The influx of calcium ions triggers the synaptic vesicles found in the synaptic knob
that contain neurotransmitters to fuse with the presynaptic membrane.
Since the question tells us that the diagram is of a cholinergic synapse, we know
that the neurotransmitter in the vesicles are acetylcholine. When the synaptic vesicles fuse with the presynaptic membrane, the acetylcholine is
released into the synaptic cleft via exocytosis. Acetylcholine will then travel across the synaptic cleft via passive diffusion, the
acetylcholine neurotransmitters bind to the receptor sites on the sodium-ion
channels located on the postsynaptic membrane.
Binding of acetylcholine to sodium-ion channels causes them to open, allowing sodium
ions to diffuse into the postsynaptic neuron. The influx of sodium ions into the postsynaptic neuron triggers the generation of a
new action potential in the postsynaptic neuron. So, without the influx of sodium ions into the postsynaptic neuron, the generation of
a new action potential would not be possible.
With this information, we are now ready to answer our question. The influx of sodium ions generates an action potential in the postsynaptic
neuron.
