In a neurophysiology study, researchers stimulate the axon of a post-synaptic motor neurone, generating an action potential that travels backwards (antidromically) toward the dendrites and the post-synaptic membrane. However, this electrical excitation fails to cross the synaptic cleft to initiate an action potential in the pre-synaptic sensory neurone.
Which of the following biological principles explains why synaptic transmission remains strictly unidirectional in this system?
Voltage-gated Ca2+Ca^{2+}Ca2+ channels are only present on the post-synaptic membrane, which prevents the entry of calcium ions required to trigger exocytosis in the pre-synaptic neurone.
Synaptic vesicles containing neurotransmitters are localized exclusively within the pre-synaptic knob, and complementary neurotransmitter receptors are found only on the post-synaptic membrane.
Acetylcholinesterase is present only in the synaptic cleft, meaning any backward-travelling chemical signal is degraded before reaching the pre-synaptic membrane.
The threshold potential of the pre-synaptic membrane is significantly higher than that of the post-synaptic membrane, preventing depolarization by retrograde electrical currents.