Answer: The enteric nervous system can operate independently of sympathetic or parasympathetic innervation.
The nervous system is often divided into three functional parts. The central nervous system (CNS) consists of the brain and spinal cord. The autonomic nervous system (ANS) consists of the organs that are innervated by the sympathetic and parasympathetic nerves. The enteric nervous system (ENS) is a separate entity which controls the activity of the gastrointestinal tract, including the stomach and intestines.
The gastrointestinal tract is under control of sympathetic and parasympathetic control. For example, during sympathetic nervous system activity, digestion is slowed, while vagal nerve stimulation (parasympathetic nervous system activation) increases digestion. However, it was found that stimulation of the enteric nervous system can change gastrointestinal activity independent of autonomic nervous system activity.
It is estimated that the enteric nervous system consists of about 500 million neurons. While this is a small fraction of the 80 billion neurons in the brain, it is more than the 100 million in the spinal cord. ("Gut Instincts: The Secrets of your Second Brain")
The ENS in sometimes regarded as the “second brain.” It uses many of the same neurotransmitters that our brain uses. In fact, 90% of the serotonin in the whole body is in the gut. The ENS has sensory neurons that respond to physical distention and chemical conditions. It also has motor neurons which can encourage peristalsis, the movement of food through the digestive system.