Video Transcript
The nerve agent sarin prevents
acetylcholine in the synaptic cleft from being broken down after an action potential
has been generated. What effect could this have?
In order to answer this question,
we first need to understand some more information about how neurons communicate with
each other in the human nervous system. Dendrites receive signals from
other neurons. The signal is turned into an
electrical impulse called an action potential, which travels down the axon. There is a junction between each
axon terminal at the end of the neuron and the dendrites of the adjacent neuron
called a synapse. The neuron before the synapse is
called the presynaptic neuron, and the neuron that follows the synapse is called the
postsynaptic neuron. This is easy to remember as the
prefix pre- means before, while the prefix post- means after.
Let’s take a closer look at where
the presynaptic and postsynaptic neurons meet at the synapse. There is a space between the two
neurons in the synapse called the synaptic cleft. When an action potential arrives at
the end of the presynaptic neuron to a region called the synaptic knob, it triggers
calcium-ion channels to open and calcium ions diffuse into the synaptic knob. This triggers vesicles containing
chemical messengers called neurotransmitters to move toward the presynaptic membrane
and release the neurotransmitter into the synaptic cleft.
The neurotransmitter diffuses
across the synaptic cleft and binds to specific receptor sites present on sodium-ion
channels embedded in the postsynaptic membrane. This causes the sodium-ion channels
to open, leading to an influx of sodium ions into the postsynaptic neuron. This influx of sodium ions causes a
new action potential to be generated in the postsynaptic neuron. Cholinergic synapses, which are
common in the parasympathetic nervous system, use acetylcholine as the
neurotransmitter. Once acetylcholine triggers the
generation of a new action potential in the postsynaptic neuron, it is usually
broken down in the synaptic cleft by an enzyme called acetylcholinesterase so that
it does not overstimulate the postsynaptic neuron and generate too many action
potentials.
The question tells us that the
nerve agent sarin prevents acetylcholine from being broken down. If sarin prevents
acetylcholinesterase from degrading acetylcholine, then the neurotransmitter will
remain in the synaptic cleft and continue stimulating the postsynaptic neuron and
generating action potentials. Now we can answer the question as
to the effect of the nerve agent sarin. Action potentials continue to be
generated in the postsynaptic neuron.
