Answer:
Motor neuron.
Step-by-step explanation:
Muscle contraction begins with signal. The contraction occur when nervous system send signals. The impulse which is called signal is travel through a motor neuron.
The whole process include following three steps:
1) Message is traveled from nervous to muscular system and trigger the chemical reaction.
2) This chemical reaction causes the muscle fibers to reorganize in such a way that shorter the length of them which is called contraction.
3) when this signal is no longer remain , reverse chemical process occur and muscles relax.