The<unk>  is capable of autonomous functions like the coordination of reflexes; although it receives considerable innervation from the autonomic<unk>  system, it can and does operate independently of the brain and the spinal cord. Its study is the focus of neurogastroenterology. Complexity The<unk>  has been described as a "second brain" for several reasons. The<unk>  can operate autonomously. It normally communicates with the central<unk>  system (CNS) through the parasympathetic (e.g., via the vagus nerve) and sympathetic (e.g., via the prevertebral ganglia)<unk>  systems. However, vertebrate studies show that when the vagus nerve is severed, the<unk>  continues to function. In vertebrates, the<unk>  includes efferent neurons, afferent neurons, and interneurons, all of which make the<unk>  capable of carrying reflexes and acting as an integrating center in the absence of CNS input. The sensory neurons report on mechanical and chemical conditions. Through intestinal muscles, the motor neurons control peristalsis and churning of intestinal contents. Other neurons control the secretion of enzymes. The<unk>  also makes use of more than 30 neurotransmitters, most of which are identical to the ones found in CNS, such as acetylcholine, dopamine, and serotonin. More than 90% of the body's serotonin lies in the gut, as well as about 50% of the body's dopamine, which is currently being studied to further our understanding of its utility in the brain. The<unk>  has the capacity to alter its response depending on such factors as bulk and nutrient composition. In addition,<unk>  contains support cells which are similar to astroglia of the brain and a diffusion barrier around the capillaries surrounding ganglia which is similar to the blood–brain barrier of cerebral blood vessels. Peristalsis A simplified image showing peristalsis Peristalsis is a series of radially symmetrical contractions and relaxations of muscles which propagate down a muscular tube. In humans and other mammals, peristalsis is found in the smooth muscles of the digestive tract to propel contents through the digestive system. The word is derived from New Latin and comes from the Greek peristallein, "to wrap around," from peri-, "around" + stallein, "to place". Peristalsis was discovered in 1899 by the work of physiologists William Bayliss and Ernest Starling. Working on the small intestines of dogs, they found that the response of increasing the pressure in the intestine caused the contraction of the muscle wall above the point of stimulation and the relaxation of the muscle wall below the point of stimulation. Segmentation Segmentation contractions are the contractions in intestines carried out by the smooth muscle walls. Unlike peristalsis, which involves the contraction and relaxation of muscles in one direction, segmentation occurs simultaneously in both directions as the circular muscles alternatively contract. This allows for thorough mixing of intestinal contents, known as chyme, to allow greater absorption. Secretion The secretion of gastrointestinal hormones, such as gastrin and secretin, is regulated through cholinergic neurons residing in the walls of the digestive tract. Hormone secretion is controlled by the vagovagal reflex, where the neurons in the digestive tract communicate through both afferent and efferent pathways with the vagus nerve.
Enteric nervous system
Peripheral nervous system
Central nervous system
Nervous system
Cranial nerves
Reproductive system
Urinary system
Gastrointestinal tract