Introduction to the Gastrointestinal System

Hello :) This post will introduce the gastrointestinal (GI) system. We'll discuss at the main compartments of the GI system as well as their functions. We'll also go over the basic digestive processes as well as the anatomy of the enteric nervous system. We'll also take a look at the phases of digestion, some of the main hormones secreted in this system as well as some of the hormones that regulate appetite.

The Gastrointestinal Tract (GIT)

In monogastric animals, the GIT is composed of:

• the oral cavity: this is where food enters the GIT. Some digestion of starches by amylase begins here however mechanical digestion via mastication (chewing) mainly occurs here. Saliva is also secreted and this lubricates the food (which is now called a bolus) as it moves down the GIT.
• pharynx: this allows the passage of the food into the oesophagus and not into the trachea!
• oesophagus: this is a muscular tube which directs food into the stomach.
• stomach: here hydrochloric acid as well as some enzymes such as pepsinogen are secreted. The low pH allows for protein denaturation and creates a larger surface are for the action of pepsinogen. 
• small intestine: this is the major site for nutrient absorption and digestion.
• large intestine: this is the major site for water absorption
• rectum: undigested food exits the body from here as feces.   

Basic Digestive Processes

Mechanical Digestion:

This involves the mechanical breakdown of food to increase the surface area on which the enzymes will act. This form of digestion occurs in the mouth through mastication and rumination. Mixing also occurs and this facilitates digestion and absorption. Mixing mainly occurs in the stomach of monogastrics (some grinding may happen here too) and the forestomach of ruminants. In the small intestine, mixing occurs and this allows the ordered passage of food through the intestine aborally (away from the mouth). In the large intestine mixing and retention occurs and this allows fermentation and the absorption of nutrients, water and electrolytes.

Chemical Digestion:

Glandular cells in the GIT secrete enzymes into the lumen and this enables intraluminal nutrient breakdown. The salivary glands, pancreas and liver also secrete substances and these reach the GIT via ducts. The enzymes which are secreted operate at an optimum pH. This pH will match the pH of the area of the GIT in which they work. Mucous is secreted and this lubricates the contents and protects the mucosal epithelium of the GIT.  

The nutrients which the animal ingests contains complex carbohydrates, proteins and fats which need to be broken down in order for them to be absorbed. The initial breakdown by enzymes occurs in the lumen of the intestine and further breakdown occurs on the surface of the epithelia. This is enhanced by the brush border which increases the surface area of the epithelium and hence allows more nutrients to be broken down.

 Absorption: 

Water, ions, vitamins and small molecules which are a result of the breakdown of the nutrients move from the GIT lumen to the blood and lymph. Absorption is optimised by the large surface area created by villi and microvilli and may occur via active or passive transport. Any material that is not absorbed is eliminated from the body as feces.

The Enteric Nervous System

The enteric nervous system is an intrinsic system that is integrated within the autonomic nervous system. It forms two plexuses:

• Myenteric Plexus: this exists between the circular and longitudinal muscle layers of the GIT and primarily regulates motility.
• Submucosal Plexus: this exists between the submucosal circular muscle layers and regulates secretion and reabsorption. 

The parasympathetic nervous system includes preganglionic (cholinergic) fibres which arise from the medulla via the vagus nerve and from the sacral spinal cord via the pelvic nerve. These fibres form synapses with the ganglion cells in the enteric nervous system and stimulate intestinal motility, secretion and release of hormones. The mediator at the target cells is usually acetylcholine. 

The sympathetic nervous system involves preganglionic cholinergic nerve fibres which synapse in sympathetic ganglia outside the GIT. From these ganglia, post ganglionic adrenergic fibres innervate cells in the myenteric and submucosal plexuses. Some of the post ganglionic nerves also innervate some blood vessels and the muscularis mucosae. The sympathetic nervous system constricts blood vessels and inhibits intestinal motility, secretion and hormone release.

The vagus nerve in the GIT is a mixed nerve, and is composed of approximately 75% sensory (afferent) fibres. The receptors in the mucosa and smooth muscle relay information back to the CNS via vagal afferent fibres. The signals communicated by the afferent fibres trigger the long vagovagal reflex, in which the efferent signal is also in the vagus nerve. Reflex signals from the vagus nerve are not consciously perceived. 

Approximately 50% of the GIT sympathetic nerves are afferent nerves. Information from the GIT is sent to the spinal cord, there is some reflex activity however most signals travel to the cerebral cortex where they are consciously perceived. These sympathetic nerves are involves in the transmission of signals from nociceptors to the brain where they are perceived as pain. 

The enteric system has both short and long reflexes. Long reflexes to the GIT involve a sensory neuron that sends an impulse to the brain where it is integrated and another impulse is sent to the digestive system. The stimulus may come from the GIT or another area such as salivary glands or the pancreas and liver and may travel along the vagus, pelvic or sympathetic nerves. Short reflexes occur when only the enteric nervous system receives, integrates and acts upon the stimulus. An example can be seen when sensory cells are stimulated in the wall of the GIT. These cells then send signals to the nerve plexuses in the GIT. 

Gastrointestinal Hormones

There are three main peptide hormones secreted in the GIT that we need to be familiar with:

• Gastrin: this is released from G cells (G for Gastrin) in the antral and duodenal mucosa. Proteins and protein digestion products from a meal contact the antral mucosa and cause the release of gastrin. It is also released by vagal stimulation through gastrin-releasing peptide as well as through distention of the gastric wall. Its actions are to stimulate gastric acid secretion and growth of gastric oxyntic glands and colonic mucosa.
• Cholecystokinin (CCK): this is released from I-cells in the duodenal and jejunal mucosa. It is released in response to fatty acids, peptides and amino acids. Its actions are to stimulate: gall bladder contractions, pancreatic enzyme secretion, growth of exocrine pancreas and gall bladder as well as pancreatic water and bicarbonate secretion. It also inhibits gastric emptying.
• Secretin: This is released from S cells (S for Secretin) in the duodenal mucosa. It is released in response to acid to a lesser extent by fatty acids. Its actions are to stimulate: pancreatic and liver water and bicarbonate secretion, growth of the exocrine pancreas, and gastric pepsin secretion. It also inhibits gastric acid secretion and the gastric trophic (stimulating the activity of another endocrine gland) effects of gastrin.

 Phases of Contraction:

 There are three phases of digestion:

•  Cephalic: this includes changes in secretion and motility that occur in response to sight, smell, taste and mastication of food. An example of this is when a dog drools saliva when it watches a person eat. This phase is affected by input from the CNS and is regulated via long reflexes. 
• Gastric: this phase includes changes in GIT secretion and motility initiated in the stomach. Its stimuli include gastric distension and the release of peptides from protein digestion. It involves the release of gastrin and activation of both the long and short reflexes.
• Intestinal: this includes changes in GI secretion and motility including pancreatic secretion. It is initiated by alterations in the volume and composition of digesta, particularly in the duodenum. It involves the release of secretin, CCK and the activation of both long and short reflexes.

The Hormones Which Regulate Appetite

Leptin is a hormone which is secreted by adipose cells. When the intake of energy from food exceeds the body's need for energy fat is deposited in adipose tissue. This tissue responds by secreting leptin. Leptin acts on the hypothalamus to reduce the sensation of hunger. It also increases the metabolic rate which reduces the amount of fat stored in adipose tissue. Leptin is important in the long-term maintenance of body weight.

CCK is also secreted by the intestinal mucosa in response to the ingestion of proteins and fat. This activates the receptors on the vagus nerve which sends a satiety signal to the hypothalamus in order to limit the size of the meal. Leptin and insulin may also increase the hypothalamus' sensitivity to CCK.

That's it for this post, if you have any questions please fell free to ask :)