GIT system
GENERAL PRINCIPLES OF GASTROINTESTINAL
The digestive system is a tube running from mouth to anus.It provides the body with a continual supply of water, electrolytes, vitamins, and nutrients, which requires:
(1) movement of food through the alimentary tract;
(2) secretion of digestive juices and digestion of the food;
(3) absorption of water, various electrolytes, vitamins, and digestive products;
(4) circulation of blood through the gastrointestinal organs to carry away the absorbed substances; and(5) control of all these functions by local, nervous, and hormonal systems.
PHYSIOLOGICAL ANATOMY OF THE GASTROINTESTINAL WALL
A typical cross section of the intestinal wall show the following layers from the outer surface inward: (1) the serosa, (2) a longitudinal smooth muscle layer, (3) a circular smooth muscle layer, (4) the submucosa, and (5) the mucosa. In addition, sparse bundles of smooth muscle fibers, the mucosal muscle, lie in the deeper layers of the mucosa..
NEURAL CONTROL OF GASTROINTESTINAL FUNCTION—ENTERIC NERVOUS SYSTEM
The gastrointestinal tract has a nervous system all its own called the enteric nervous system. It lies entirely in the wall of the gut, beginning in the esophagus and extending all the way to the anus. The number of neurons in this enteric system is about 100 million, nearly equal to the number in the entire spinal cord. This highly developed enteric nervous system is especially important in controlling gastrointestinal movements and secretion.The enteric nervous system is composed mainly of two plexuses, shown in Figure below: (1) an outer plexus lying between the longitudinal and circular muscle layers, called the myenteric plexus or Auerbach’s plexus, and (2) an inner plexus, called the submucosal plexus or Meissner’s plexus, which lies in the submucosa.
The myenteric plexus controls mainly the gastrointestinal movements, and the submucosal plexus controls mainly gastrointestinal secretion and local blood flow.. Although the enteric nervous system can function independently of these extrinsic nerves, stimulation by the parasympathetic and sympathetic systems can greatly enhance or inhibit gastrointestinal functions.
DIFFERENCES BETWEEN THE MYENTERIC AND SUBMUCOSAL PLEXUSES
The myenteric plexus consists mostly of a linear chain of many interconnecting neurons that extends the entire length of the gastrointestinal tract. Because the myenteric plexus extends all the way along the intestinal wall and lies between the longitudinal and circular layers of intestinal smooth muscle, it is concerned mainly with controlling muscle activity along the length of the gut.When this plexus is stimulated, its principal effects are (1) increased tonic contraction, or “tone,” of the gut wall; (2) increased intensity of the rhythmical contractions; (3) slightly increased rate of the rhythm of contraction; and (4) increased velocity of conduction of excitatory waves along the gut wall, causing more rapid movement of the gut peristaltic waves.
The myenteric plexus should not be considered excitatory because some of its neurons are inhibitory; their fiber endings secrete an inhibitory transmitter, possibly vasoactive intestinal polypeptide or some other inhibitory peptide. The resulting inhibitory signals are especially useful for inhibiting some of the intestinal sphincter muscles that impede movement of food along successive segments of the gastrointestinal tract, such as the pyloric sphincter, which controls emptying of the stomach into the duodenum, and the sphincter of the ileocecal valve, which controls emptying from the small intestine into the cecum.
The submucosal plexus, in contrast to the myenteric plexus, is mainly concerned with controlling function within the inner wall of each minute segment of the intestine. For instance, many sensory signals originate from the gastrointestinal epithelium and are then integrated in the submucosal plexus to help control local intestinal secretion, local absorption, and local contraction of the submucosal muscle that causes various degrees of infolding of the gastrointestinal mucosa.
TYPES OF NEUROTRANSMITTERS SECRETED BY ENTERIC NEURONS
different neurotransmitter substances that are released by the nerve endings of different types of enteric neurons, including: (1) acetylcholine, (2) norepinephrine, (3) adenosine triphosphate, (4) serotonin, (5) dopamine, (6) cholecystokinin, (7) substance P, (8) vasoactive intestinal polypeptide, (9) somatostatin, (10) leu-enkephalin, (11) met-enkephalin, and (12) bombesin.Acetylcholine most often excites gastrointestinal activity. Norepinephrine almost always inhibits gastrointestinal activity, as does epinephrine, which reaches the gastrointestinal tract mainly by way of the blood after it is secreted by the adrenal medullae into the circulation.
FUNCTIONAL TYPES OF MOVEMENTS IN THE GASTROINTESTINAL TRACT
Two types of movements occur in the gastrointestinal tract: (1) propulsive movements, which cause food to move forward along the tract at an appropriate rate to accommodate digestion and absorption, and (2) mixing movements, which keep the intestinal contents thoroughly mixed at all times.PROPULSIVE MOVEMENTS—PERISTALSIS
A contractile ring appears around the gut and then moves forward; this mechanism is analogous to putting one’s fingers around a thin distended tube, then constricting the fingers and sliding them forward along the tube. Any material in front of the contractile ring is moved forward.Peristalsis is an inherent property of many syncytial smooth muscle tubes; stimulation at any point in the gut can cause a contractile ring to appear in the circular muscle, and this ring then spreads along the gut tube. (Peristalsis also occurs in the bile ducts, glandular ducts, ureters, and many other smooth muscle tubes of the body.)
The usual stimulus for intestinal peristalsis is distention of the gut. That is, if a large amount of food collects at any point in the gut, the stretching of the gut wall stimulates the enteric nervous system to contract the gut wall 2 to 3 centimeters behind this point, and a contractile ring appears that initiates a peristaltic movement. Other stimuli that can initiate peristalsis include chemical or physical irritation of the epithelial lining in the gut. Also, strong parasympathetic nervous signals to the gut will elicit strong peristalsis.
MIXING MOVEMENTS
Mixing movements differ in different parts of the alimentary tract. In some areas, the peristaltic contractions cause most of the mixing. This is especially true when forward progression of the intestinal contents is blocked by a sphincter so that a peristaltic wave can then only churn the intestinal contents, rather than propelling them forward.At other times, local intermittent constrictive contractions occur every few centimeters in the gut wall. These constrictions usually last only 5 to 30 seconds; new constrictions then occur at other points in the gut, thus “chopping” and “shearing” the contents first here and then there.
