Final answer:
Synaptic strength at the neuromuscular junction is enhanced by repeated activation of the synapse, leading to increased neurotransmitter release and receptor sensitivity.
Step-by-step explanation:
Synaptic strength at the neuromuscular junction is enhanced through D) Repeated activation of the synapse. When a synapse is repeatedly activated, synaptic efficacy is increased, a phenomenon known as synaptic plasticity. This involves a greater release of neurotransmitters from the presynaptic neuron, increased sensitivity and density of postsynaptic receptors, and structural changes that increase the synapse's effectiveness over time. For instance, acetylcholine (ACh) is the neurotransmitter that is released from nerve endings at neuromuscular junctions to stimulate muscle contraction. Drugs that inhibit acetylcholinesterase, the enzyme which degrades acetylcholine, effectively increase synaptic transmission at cholinergic synapses because they allow acetylcholine to remain in the synaptic cleft longer, continually activating the postsynaptic membrane.