What happens to the endplate potential when botulinum toxin is present?

• A. It increases significantly
• B. It decreases
• C. It remains the same
• D. It becomes erratic

Answer: (B)

Botulinum toxin functions by blocking the release of acetylcholine at the neuromuscular junction. When this toxin binds to the presynaptic terminals, it prevents the fusion of neurotransmitter-filled vesicles with the presynaptic membrane, effectively inhibiting the exocytosis of acetylcholine. Consequently, with less acetylcholine being released into the synaptic cleft, there are fewer molecules available to bind to the nicotinic receptors on the postsynaptic membrane.

This reduction in acetylcholine leads to a decrease in the endplate potential, which is the change in membrane potential due to the influx of sodium ions through the acetylcholine receptors. If the endplate potential is insufficient to reach the threshold for action potentials in the muscle fiber, it results in muscle weakness or paralysis. Thus, the presence of botulinum toxin directly correlates with a decrease in the endplate potential.