The Autonomic Nervous System manages unconscious systems in the body, such as heart-rate and digestion. It is largely autonomous, with only a few actions available for conscious alteration.
Because the actions of the ANS are mostly automatic, it does not react as much to external stimulus as other systems. However, if mismatched sensory input is detected, the ANS can take some steps to respond and keep its automatic actions consistent or healthy.
For example, if the ANS detects toxins from bad food, it can trigger vomiting before the body is aware of any ill-health effects. It can also speed up the heart-rate if blood-pressure drops, even if there is no initial effect on the body; this is a common experience for people who sit for long periods and suddenly stand up.
The ANS also affects the body when motion-sickness is felt; movement in the inner-ear that is not perceived by the eyes is interpreted as hallucination from poison, which results in nausea. Sensory mismatch most often results in nausea, since most ANS issues are interpreted as poison ingestion.