What is contained within synaptic vesicles at a neuromuscular junction?

At a neuromuscular junction, synaptic vesicles serve as storage containers for neurotransmitter molecules. Specifically, when an action potential reaches the synaptic terminal of a motor neuron, these vesicles fuse with the presynaptic membrane and release their contents into the synaptic cleft. The primary neurotransmitter at the neuromuscular junction is acetylcholine, which binds to receptors on the muscle cell membrane (sarcolemma) to initiate muscle contraction.

The other options do not pertain to the contents of synaptic vesicles. Calcium ions, while indeed important for neurotransmitter release, are not stored in synaptic vesicles. Rather, they flow into the synaptic terminal from extracellular space when the action potential arrives, triggering the release of the vesicles’ contents. Myosin proteins and actin filaments are key components of muscle structure and function but are not found in synaptic vesicles. Myosin is part of the thick myofilaments, while actin forms the thin myofilaments in the contractile apparatus of muscle fibers. Therefore, the presence of neurotransmitter molecules within synaptic vesicles is what is critical for communication between nerves and muscles at the neuromuscular junction.