Final answer:
The autonomic nervous system, including the sympathetic and parasympathetic divisions, usually has antagonistic control over target organs but sometimes achieves a balance that results in no net change.
Step-by-step explanation:
The autonomic nervous system is a component of the nervous system responsible for controlling involuntary functions such as heart rate, blood flow, and digestion. It consists of the sympathetic and parasympathetic nervous systems, which typically have opposing effects on target organs. However, there are cases where an effector does not have antagonistic control from these systems. This could occur if the signaling pathways for both systems result in a balanced state, where the actions of one system are effectively neutralized by the other, leading to no net change in the function of the target organ.
Normally, the sympathetic nervous system is known for preparing the body for stress-related activities ('fight or flight'), whereas the parasympathetic nervous system is associated with routine operations ('rest and digest'). In some instances, as with certain drugs acting on these systems, the balance of signals from the systems results in no significant shift in organ activity. An example of a lack of antagonistic control is in the gastrointestinal system, where sympathetic activation causes sphincters to contract but parasympathetic activation contracts other muscles to propel contents along, leaving the movement of food unaltered.