Gastrointestinal physiology


Gastrointestinal physiology is a branch of human physiology addressing the physical function of the gastrointestinal (GI) system. The major processes occurring in the GI system are that of motility, secretion, regulation, digestion and circulation. The function and coordination of each of these actions is vital in maintaining GI health, and thus the digestion of nutrients for the entire body. The GI tract generates motility using smooth muscle subunits linked by gap junctions. These subunits fire spontaneously in either a tonic or a phasic fashion. Tonic contractions are those contractions that are maintained from several minutes up to hours at a time. These occur in the sphincters of the tract, as well as in the anterior stomach. The other type of contractions, called phasic contractions, consist of brief periods of both relaxation and contraction, occurring in the posterior stomach and the small intestine, and are carried out by the muscularis externa. [edit]Stimulation The stimulation for these contractions likely originates in modified smooth muscle cells called interstitial cells of Cajal. These cells cause spontaneous cycles of slow wave potentials that can cause action potentials in smooth muscle cells. They are associated with the contractile smooth muscle via gap junctions. These slow wave potentials must reach a threshold level for the action potential to occur, whereupon Ca2+ channels on the smooth muscle open and an action potential occurs. As the contraction is graded based upon how much Ca2+ enters the cell, the longer the duration of slow wave, the more action potentials oc

ur. This in turn results in greater contraction force from the smooth muscle. Both amplitude and duration of the slow waves can be modified based upon the presence of neurotransmitters, hormones or other paracrine signaling. The number of slow wave potentials per minute varies based upon the location in the digestive tract. This number ranges from 3 waves/min in the stomach to 12 waves/min in the intestines.[1] [edit]Contraction Patterns The patterns of GI contraction as a whole can be divided into two distinct patterns, peristalsis and segmentation. Occurring between meals, the migrating motor complex is a series of peristaltic waves cycles in distinct phases starting with relaxation followed by an increasing level of activity to a peak level of peristaltic activity lasting for 515 minutes.[2] This cycle repeats every 1.52 hours but is interrupted by food ingestion. The role of this process is likely to clean excess bacteria and food from the digestive system.[3] [edit]Peristalsis Peristalsis Animation Peristalsis is one of the patterns that occur during and shortly after a meal. The contractions occur in wave patterns traveling down short lengths of the GI tract from one section to the next. The contractions occur directly behind the bolus of food that is in the system, forcing it toward the anus into the next relaxed section of smooth muscle. This relaxed section then contracts, generating smooth forward movement of the bolus at between 225 cm per second. This contraction pattern depends upon hormones, paracrine signals, and the autonomic nervous system for proper regulation.