What is the neurological end in of a full body spasm?

It happens sometimes when your almost sleeping but not comparatively.. Your whole body or a portion of you body just jump. What causes this? Is your body dumping/releasing a definite neurotransmitter ie: seretonin?What is the neurological end in of a full body spasm?
in the intermediary stage between one awake and being asleep, it's thought that inhibitory impulse from the brain are decreased- so in essence you hold dis-inhibition. that explains all sorts of phenomemon such as those vivid dreams inhabitants sometimes get as they're any waking up or going to sleep. that just now publicized phenomenon os "sleep sex" could also be explained this way. spasms or any tonic-clonic involuntary movements while sleeping could also be explained by this dis-inhibition.
and it can also due to several neurological and biological transmitter.