These sensory neurons, in turn, activate neurons of the myenteric plexus, which then proceed to split into two cholinergic pathways: a retrograde and an anterograde. Activated neurons of the retrograde pathway release substance P and acetylcholine to contract the smooth muscle behind the bolus. The activated neurons of the anterograde pathway instead release nitric oxide and vasoactive intestinal polypeptide to relax the smooth muscle caudal to the bolus. This allows the food bolus to effectively be pushed forward along the digestive tract. Esophagus
After food is chewed into a bolus, it is swallowed and moved through the esophagus. Smooth muscles contract behind the bolus to prevent it from being squeezed back into the mouth. Then rhythmic, unidirectional waves of contractions work to rapidly force the food into the stomach. The migrating motor complex (MMC) helps trigger peristaltic waves. This process works in one direction only, and its sole esophageal function is to move food from the mouth into the stomach (the MMC also functions to clear out remaining food in the stomach to the small bowel and remaining particles in the small bowel into the colon). In the esophagus, two types of peristalsis occur:

First, there is a primary peristaltic wave, which occurs when the bolus enters the esophagus during swallowing. The primary peristaltic wave forces the bolus down the esophagus and into the stomach in a wave lasting about 8–9 seconds.
In Egypt, 60% of the population in the Northern and Eastern parts of the Nile Delta and only 6% in the Southern part are infected.S. mansoni is commonly found in places with poor sanitation. Because of the parasites fecal-oral transmission, bodies of water that contain human waste can be infectious. Water that contains large populations of the intermediate host snail species is more likely to cause infection. Young children living in these areas are at greatest risk because of their tendency to swim and bathe in cercaria-infected waters longer than adults
. Anyone travelling to the areas described above, and who is exposed to contaminated water, is at risk of schistosomiasis. History
The intermediate hosts Biomphalaria snails are estimated to originate in South America 95–110 million years ago. But the parasites Schistosoma originated in Asia. In Africa, the progenitor species evolved into modern S. mansoni and S. haematobium around 2–5 million years ago.A German physician Theodor Maximillian Bilharz was the first to discover the parasite in 1851, while working at Kasr el-Aini Hospital, a medical school in Cairo. Bilharz recovered them from autopsies of dead soldiers, and noticed two distinct parasites. He described one of them as Distomum haematobium (now S. haematobium) in 1852, but failed to identify the other. In one of his letters to his mentor Karl Theordor von Siebold, he mentioned some of the eggs were different in having terminal spines while some had lateral spines. Terminal-spined eggs are unique to S. haematobium, while lateral spines are found only in S. mansoni.