One point that is worth mentioning when discussing the autonomic nervous system is the fact that visceral efferent fibers are accompanied by visceral afferent fibers. These afferent fibers provide information to the central nervous system about the internal environment including blood pressure and blood oxygen and carbon dioxide levels. This information gets integrated in the CNS, which generates visceral reflexes such as changing heart rate to maintain homeostasis.
In general, we are unaware of visceral sensations. When we are aware, the sensations are usually poorly localized and vague, such as the sensation of nausea. There are, however, certain stimuli, especially pathological conditions, that evoke true pain. For the most part, unconscious sensation is carried on afferent fibers accompanying parasympathetic fibers, and visceral pain is carried on afferent fibers accompanying sympathetic fibers. These visceral pain fibers travel through dorsal roots, as do other sensory fibers, and this is the cause of referred pain, such as the pain of a heart attack felt down the left upper limb.
