BIOL 1051H Chapter 25: _The Digestive System part 2
The Digestive System Part 2
The pancreas is located behind the stomach. It has both endocrine and exocrine
functions. The endocrine functions are performed by the islets of Langerhans which secrete
insulin and glucagon. The exocrine pancreas consists of secretory units called acini. The
exocrine secretions include bicarbonate (which neutralizes stomach acid) and digestive
enzymes that are stored in the acini as zymogen granules. These pass in the pancreatic duct to
the small intestine. The islets of Langerhans and acini of the pancreas are intermingled
throughout the organ giving it a homogenous appearance.
Pancreatic juice contains water, bicarbonate, and digestive enzymes. These enzymes
include amylase which digests starch (carbohydrates), trypsin which digests protein, and lipase
which digests fats. These enzymes are free floating in the chyme as opposed to those that are
associated with the brush border of microvilli.
Complete digestion requires both pancreatic and brush border enzymes. Most
pancreatic enzymes are produced in an inactive form called zymogens. This prevents the
enzymes from digesting the pancreas itself. Trypsin is activated by brush border enzyme called
enterokinase. Trypsin in turn activates other zymogens so they become active enzymes as well.
The digestive system is controlled by the endocrine system and the nervous system. The
system most heavily involved with regulating and coordinating digestive activities is the
parasympathetic system, via the vagus nerve. However, the GI tract is also highly regulated by
the action of hormones. In fact, the GI tract is itself both an endocrine gland and a target for the
action of hormones
The enteric nervous system is a specific branch of the autonomic nervous system
contained entirely within the gastrointestinal tract itself. It is organized into ganglia which are
interconnected by the 2 nerve plexuses of the gut. The outer myenteric (Auerbach’s) plexus is
found along the entire length of the GI tract. The inner submucosal (meissner’s) plexus is
located only in the small and large intestine.
The enteric nervous system is sometimes called the brain gut. It is capable of
selfregulating the actions of the gut. However, input from the parasympathetic, sympathetic, and
endocrine systems modify the behavior. Intestinal reflexes can be mediated by the enteric
nervous system and paracrines produced intrinsically, but are regulated by the extrinsic control
of the autonomic nervous system (sympathetic and parasympathetic) and hormones.
Some intestinal reflexes are as follows. The gastroileal reflex involves the motility of the
ileum and the movement of chyme through the ileocecal sphincter in response to increased
gastric activity. In other words, when the stomach is moving material through, the ileocecal
sphincter also allows material through. The ileogastric reflex involves the gastric motility in
Document Summary
The endocrine functions are performed by the islets of langerhans which secrete insulin and glucagon. The exocrine pancreas consists of secretory units called acini. The exocrine secretions include bicarbonate (which neutralizes stomach acid) and digestive enzymes that are stored in the acini as zymogen granules. These pass in the pancreatic duct to the small intestine. The islets of langerhans and acini of the pancreas are intermingled throughout the organ giving it a homogenous appearance. Pancreatic juice contains water, bicarbonate, and digestive enzymes. These enzymes include amylase which digests starch (carbohydrates), trypsin which digests protein, and lipase which digests fats. These enzymes are free floating in the chyme as opposed to those that are associated with the brush border of microvilli. Complete digestion requires both pancreatic and brush border enzymes. Most pancreatic enzymes are produced in an inactive form called zymogens. This prevents the enzymes from digesting the pancreas itself. Trypsin is activated by brush border enzyme called enterokinase.
