Here's why:

* Motor neurons are the nerve cells responsible for transmitting signals from the central nervous system to muscles, causing them to contract.

* Synapses are the specialized junctions where nerve cells communicate.

* At the neuromuscular junction (the synapse between a motor neuron and a muscle fiber), the motor neuron is the presynaptic cell. It releases the neurotransmitter acetylcholine, which binds to receptors on the muscle fiber (the postsynaptic cell), triggering muscle contraction.

Other examples of presynaptic cells include:

* Sensory neurons: These neurons transmit signals from sensory receptors (like those in your skin, eyes, or ears) to the central nervous system.

* Interneurons: These neurons connect other neurons within the central nervous system and play a crucial role in processing information.

In all these cases, the presynaptic cell is the one sending the signal across the synapse, while the postsynaptic cell receives the signal.