Question

when you swat a fly, which type of neuron stimulates your arm to move ?

Answer 1

Motor neurons arising from the cerebullum(which controls most voluntary muscle movements)

Answer 2

DrAmaQuenn is correct in stating that motor neurons stimulate muscle fibres. I think that they have their cell bodies in the spinal cord. There is a link below to a page which will tell you all about motorneurons. The cerebellum is important in controlling movements so I've included a link to a page about this part of the brain.

http://en.wikipedia.org/wiki/Motor_neurons
http://en.wikipedia.org/wiki/Cerebellum