Mini MCAT passage: Manipulating synaptic transmission between neurons

A researcher is interested in uncovering mechanisms of synaptic transmission between neurons cultured from embryonic mouse neural cells. The researcher first stimulates a presynaptic neuron to fire an action potential and measures the response of the postsynaptic neuron. Consistently, the postsynaptic neuron depolarizes and propagates an excitatory potential. She then performs a series of manipulations:

In a first set of experiments, she alters the extracellular concentration of various ions and observes how this affects neurotransmitter release and target cell behavior.

In a second set of experiments, she examines the effects of a known neurotoxin, Toxin A, which prevents synaptic vesicles from fusing with the presynaptic membrane of axon terminals.

In a third set of experiments, she studies Toxin B, a neurotoxin with an unknown mechanism. When she administers Toxin B, she finds that it decreases the neurotransmitter concentration in the synapse and causes an absence of synaptic vesicles in the axon terminal of the presynaptic cell.