9.5 Regulation of gastrointestinal function

The digestive system is a complex network of organs and processes regulated by neural and hormonal signals. The enteric nervous system, working with the central nervous system, orchestrates digestion through neurotransmitters and hormones like gastrin and secretin.

Digestive function can be disrupted by various factors, including stress, infections, and medications. Understanding these influences is crucial for maintaining gastrointestinal health and addressing common digestive disorders that impact overall well-being.

Neural and Hormonal Regulation of Digestion

Enteric Nervous System (ENS) and Central Nervous System Communication

• The enteric nervous system (ENS) is a complex network of neurons and ganglia embedded within the walls of the gastrointestinal tract regulates digestive functions independently of the central nervous system
• The ENS communicates with the central nervous system through the parasympathetic and sympathetic branches of the autonomic nervous system coordinates digestive activities with the body's overall state
  • Parasympathetic stimulation, primarily via the vagus nerve, increases gastrointestinal motility, secretion, and blood flow
  • Sympathetic stimulation generally has inhibitory effects on digestive functions

Gastrointestinal Hormones and Local Factors

• Gastrointestinal hormones, such as gastrin, secretin, cholecystokinin (CCK), and motilin, are released by enteroendocrine cells in response to various stimuli act on target cells to regulate digestive processes
  • Gastrin stimulates gastric acid secretion promotes growth of the gastric mucosa
  • Secretin stimulates pancreatic bicarbonate secretion inhibits gastric acid secretion
  • CCK stimulates pancreatic enzyme secretion, gallbladder contraction, and satiety (feeling of fullness)
  • Motilin stimulates gastrointestinal motility is involved in the migrating motor complex
• Local paracrine factors, such as histamine and somatostatin, also play a role in modulating gastrointestinal function at a tissue level
  • Histamine, released by enterochromaffin-like cells in the stomach, stimulates gastric acid secretion
  • Somatostatin, released by D cells in the stomach and pancreas, inhibits the secretion of other gastrointestinal hormones (gastrin, CCK)

Cephalic Phase of Digestion

Sensory Stimuli and Digestive Responses

• The cephalic phase is a set of digestive responses triggered by sensory stimuli, such as the sight, smell, taste, or thought of food, before food enters the stomach
• Cephalic phase responses are mediated by the parasympathetic nervous system, primarily through the vagus nerve, prepare the gastrointestinal tract for the arrival of food
  • Salivary secretion is increased during the cephalic phase aids in the lubrication and initial digestion of food through the action of salivary amylase on starches
  • Gastric acid secretion is stimulated during the cephalic phase creates an acidic environment in the stomach that activates pepsinogen facilitates protein digestion
  • Pancreatic and intestinal secretions are also enhanced during the cephalic phase ensures the presence of digestive enzymes and bicarbonate in the small intestine

Importance of the Cephalic Phase

• The cephalic phase accounts for up to 30-50% of the total digestive response to a meal highlights its importance in the overall digestive process
• The cephalic phase optimizes digestion by preparing the gastrointestinal tract for the arrival of food reducing the workload on the stomach and small intestine
• Impairment of the cephalic phase, such as in patients with vagotomy (surgical removal of the vagus nerve), can lead to reduced digestive efficiency and potential nutritional deficiencies

Migrating Motor Complex in Gastrointestinal Health

Characteristics and Functions of the Migrating Motor Complex (MMC)

• The migrating motor complex (MMC) is a cyclical pattern of electrical and contractile activity occurs in the gastrointestinal tract during fasting periods between meals
• The MMC consists of four phases: Phase I (quiescent), Phase II (irregular contractions), Phase III (regular, high-amplitude contractions), and Phase IV (transition back to Phase I)
  • Phase I is characterized by minimal contractile activity lasts approximately 30-60 minutes
  • Phase II involves intermittent, irregular contractions gradually increase in frequency and amplitude
  • Phase III, also known as the "housekeeper wave," consists of intense, regular contractions that propagate from the stomach to the ileum
  • Phase IV is a brief transitional period that marks the return to Phase I
• The primary function of the MMC is to sweep residual undigested material, mucus, and bacteria from the stomach and small intestine prevents bacterial overgrowth maintains gastrointestinal health

Control and Importance of the MMC

• The MMC is controlled by the enteric nervous system is modulated by hormones, such as motilin, which is released during Phase III stimulates smooth muscle contraction
• Disruption of the MMC, such as that caused by stress, certain medications, or disease states, can lead to impaired gastrointestinal motility an increased risk of bacterial overgrowth and associated digestive disorders
  • Stress can inhibit the MMC through the action of corticotropin-releasing factor (CRF) on the enteric nervous system
  • Medications, such as opioids and proton pump inhibitors, can disrupt the MMC by altering gastrointestinal motility and secretion
• The presence of a functioning MMC is considered a marker of gastrointestinal health its absence or impairment is often observed in various digestive disorders, such as small intestinal bacterial overgrowth (SIBO) and irritable bowel syndrome (IBS)

Factors Disrupting Gastrointestinal Function

Psychological and Infectious Factors

• Psychological stress can alter gastrointestinal motility, secretion, and permeability through the gut-brain axis contributes to the development or exacerbation of digestive disorders such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD)
  • Stress activates the hypothalamic-pituitary-adrenal (HPA) axis releases cortisol, which can disrupt the balance of the gut microbiome and increase intestinal permeability
  • Stress also modulates the activity of the enteric nervous system through the release of neurotransmitters like serotonin and norepinephrine
• Infections by pathogenic bacteria, viruses, or parasites can cause acute or chronic gastrointestinal inflammation leads to conditions such as gastroenteritis, Helicobacter pylori-associated peptic ulcers, and post-infectious IBS
  • Bacterial infections, such as Salmonella or Campylobacter, can cause acute gastroenteritis and damage the intestinal mucosa
  • Viral infections, like rotavirus or norovirus, are common causes of acute gastroenteritis in children and adults
  • Parasitic infections, such as Giardia or Cryptosporidium, can cause chronic diarrhea and malabsorption

Dietary, Medication, and Lifestyle Factors

• Food intolerances and allergies, such as lactose intolerance and celiac disease, can cause digestive symptoms damage to the gastrointestinal mucosa when trigger foods are consumed
  • Lactose intolerance results from a deficiency in the enzyme lactase leads to the fermentation of undigested lactose in the colon, causing bloating, diarrhea, and abdominal pain
  • Celiac disease is an autoimmune disorder triggered by the consumption of gluten damages the small intestinal mucosa, leading to malabsorption and various digestive symptoms
• Medications, such as nonsteroidal anti-inflammatory drugs (NSAIDs), antibiotics, and opioids, can disrupt the balance of the gut microbiome, alter gastrointestinal motility, cause mucosal damage, leading to various digestive side effects and disorders
  • NSAIDs, like aspirin and ibuprofen, can cause gastric mucosal damage increase the risk of peptic ulcers and gastrointestinal bleeding
  • Antibiotics can disrupt the balance of the gut microbiome lead to the overgrowth of opportunistic pathogens, such as Clostridium difficile
  • Opioids can cause constipation by reducing gastrointestinal motility increasing water absorption in the colon
• Lifestyle factors, such as a diet high in processed foods, alcohol consumption, smoking, and irregular meal patterns, can contribute to the development of digestive disorders by altering the gut microbiome, increasing inflammation, disrupting normal gastrointestinal function
  • A diet high in processed foods and low in fiber can alter the composition of the gut microbiome promote the growth of pro-inflammatory bacteria
  • Alcohol consumption can cause gastric mucosal damage increase intestinal permeability, leading to conditions like alcoholic gastritis and leaky gut syndrome
  • Smoking can increase the risk of peptic ulcers, Crohn's disease, and gastrointestinal cancers by promoting inflammation and impairing mucosal healing
  • Irregular meal patterns, such as skipping meals or eating late at night, can disrupt the circadian rhythm of the gastrointestinal tract lead to impaired motility and secretion

Neurological and Endocrine Disorders

• Neurological disorders, such as Parkinson's disease and multiple sclerosis, can affect the autonomic nervous system lead to gastrointestinal dysmotility and associated digestive symptoms
  • Parkinson's disease can cause delayed gastric emptying (gastroparesis) and constipation due to impaired vagal nerve function and reduced dopamine signaling in the enteric nervous system
  • Multiple sclerosis can lead to various gastrointestinal symptoms, such as dysphagia, gastroparesis, and constipation, due to demyelination of nerve fibers controlling digestive functions
• Endocrine disorders, such as diabetes and thyroid dysfunction, can cause gastrointestinal complications by altering motility, secretion, and absorption within the digestive tract
  • Diabetes can cause gastroparesis, diarrhea, and constipation due to autonomic neuropathy and impaired glucose control
  • Hyperthyroidism can cause diarrhea and increased gastrointestinal motility due to the stimulatory effects of thyroid hormones on the enteric nervous system
  • Hypothyroidism can lead to constipation and reduced gastrointestinal motility due to the inhibitory effects of low thyroid hormone levels on digestive function