The peripheral nervous system is then composed of three sub-systems: the somatic nervous system, the autonomic nervous system, and the visceral nervous system.
The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system; some textbooks also include sensory systems.
Salivary alpha amylase levels have been found to correlate with heightened autonomic nervous system activity levels, reacting in similar ways to the hormone norepinephrine.