The peripheral nervous system is classically divided into two parts, themselves bifurcated once more. The first half is the somatic nervous system, consisting of the sensory nerves (e.g. touch, proprioception, and temperature), as well as voluntary-motor branches and reflex arcs (e.g. knee-jerk reflex). The cell bodies for sensory nerves are primarily located dorsal to the spinal cord – they make up the dorsal root ganglia. The motor neurons (more precisely, lower motor neurons) are located in the ventral horn of the spinal cord or in the ventral root. These neurons are responsible for both sending out efferent signals to the muscles and bringing back afferent signals to the brain. These afferent signals include proprioceptive information, from e.g. the Golgi tendon organ.
The second half is the autonomic nervous system which implements the unconscious, automatic regulation of the function of internal organs such as digestion, breathing, heat rate, urination, and sexual arousal. It also implements reflexes such as coughing, swallowing, sneezing, vomiting, and orgasm. The autonomous nervous system thereby supports the "Four F" behavioral states: fighting, fleeing, feeding, and mating. The former two are the province of the sympathetic branch and its hormonal henchmen, cortisol and the epinephrines (née adrenalines). The latter two are handled by the parasympathetic branch and mediated primarily by oxytocin and vasopressin. Further detail about the operations of these systems quickly exits neuroscience and enters physiology.
- Kandel's Principles of Neural Science provides little information on this topic.