Which component of the neuromuscular junction contains vesicles filled with neurotransmitters?

The presynaptic terminal is the portion of the neuromuscular junction that is responsible for the release of neurotransmitters, which facilitate communication between the nerve and muscle cells. This terminal contains synaptic vesicles, which are small membrane-bound sacs filled with neurotransmitters such as acetylcholine. When an action potential travels down the motor neuron, these vesicles fuse with the presynaptic membrane, leading to the release of neurotransmitters into the synaptic cleft.

By contrast, the postsynaptic terminal is located on the muscle fiber side of the junction and is lined with receptors that bind to the neurotransmitters released from the presynaptic terminal. The presynaptic membrane is the specific part of the neuron involved in neurotransmitter release, but it is the terminal itself where the vesicles are stored. Neurotransmitter receptors are proteins that respond to neurotransmitters but do not contain vesicles. Therefore, the presynaptic terminal is identified as the correct answer because it is the component fundamentally responsible for containing and releasing the neurotransmitters needed for muscle contraction.