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.
135 exam-style questions on OCR GCSE Biology 3.1 Coordination and control – the nervous system, covering 3.1.1 Structure of the nervous system, 3.1.2 Coordinated response, 3.1.3 The reflex arc, 3.1.4 Structure of the eye, 3.1.5 Defects of the eye, 3.1.6 Structure and function of the brain, 3.1.7 Difficulties of investigating brain function, and 3.1.8 Limitations in treating nervous system damage. Each one has a worked solution and a mark scheme showing where the marks go.