It's the synapse between our neuron, the motor neuron, and our cell membrane of that muscle. So if we zoom in, we basically get the following picture. So, we have the axon terminal of our somatic motor neuron, and we have the cell membrane of the skeletal muscle inside our biceps muscle. So notice that the cell membrane contains these receptor proteins, and the axon terminal contains these synaptic vesicles that carry special neurotransmitters. Now, in the case of the motor division of the somatic nervous system, these neurotransmitters are always acetylcholine. So as the action potential travels and eventually ends up on the exxon terminal, it basically causes the release of these synaptic vesicles, and that releases our acetylcholine.