Digestive Physiology Lecture 1 - Digestive Regulation

Vocabulary flashcards covering the intrinsic and extrinsic regulation of the gastrointestinal tract as described in the lecture notes.

Intrinsic system

Located in the GI wall; includes the enteric nervous system and endocrine secretions; regulates digestive processes via local reflexes and intrinsic signaling.

Extrinsic system

Located outside the GI tract wall; influences the intrinsic system via autonomic nerves and extrinsic hormones.

Enteric Nervous System (ENS)

The intrinsic nervous system of the GI tract; can function independently from the CNS and communicates with the SNS and PNS.

Myenteric (Auerbach’s) plexus

Neural network between the longitudinal and circular smooth muscle layers; coordinates gut motility and peristalsis.

Submucosal (Meissner’s) plexus

Neural network in the submucosa; regulates fluid and electrolyte movement across the mucosa.

Afferent (sensory) neurons

Nerve cells that detect chemical and mechanical conditions in the gut (mechanoreceptors and chemoreceptors).

Interneurons (ENS)

Neurons that connect the myenteric and submucosal plexuses to enable reflexes and integration within the ENS.

Efferent (motor) neurons

Innervate smooth muscle and glands; signals can be excitatory or inhibitory to regulate contraction and secretions.

Neurocrines

Neurotransmitters released by enteric neurons; include acetylcholine, substance P, somatostatin, NO, VIP, etc.

Excitatory neurocrines (ACh)

Acetylcholine; promotes GI motility and secretion.

Inhibitory neurocrines

Peptides like somatostatin and others (e.g., NO, VIP) that reduce motility or secretion.

Vasoactive intestinal peptide (VIP)

Neurotransmitter with dual roles: inhibitory in gut muscle, stimulatory on mucosal glands.

Autocrine

Regulatory molecules that act back on the same cell that secreted them.

Paracrine

Regulatory molecules that diffuse locally to nearby cells via extracellular fluid.

Endocrine

Hormones released into the bloodstream to act at distant sites; e.g., gastrin, secretin.

Gastrin

Produced in the distal stomach; stimulates parietal cells to secrete HCl and increases stomach motility; release triggered by stomach distension, vagal input, and protein; inhibited by low pH.

Gastrin feedback loop

Negative feedback where acidified stomach reduces gastrin release; sequence: meal → acid increase → gastrin release → more acid → pH decrease inhibits gastrin.

Secretin

Produced in the duodenum; stimulates pancreatic and biliary bicarbonate secretion; inhibits gastric acid; released in response to low duodenal pH.

Cholecystokinin (CCK)

Produced in the small intestine; stimulates pancreatic enzyme secretion and inhibits gastric emptying; released in response to proteins and fats in chyme.

Gastric Inhibitory Polypeptide (GIP) / Glucose-dependent Insulinotropic Polypeptide

Produced in the duodenum/upper jejunum; inhibits gastric motility and secretions; stimulates insulin release when glucose is present; oral glucose increases insulin via GIP.

Motilin

Produced in the duodenum and jejunum; increases gut motility between meals and helps regulate tone of the lower esophageal sphincter; release stimulated by acetylcholine.

Aldosterone

Extrinsic endocrine hormone from the adrenal cortex; promotes Na+ and water reabsorption in gut and salivary glands in exchange for K+; mainly acts in kidneys but also affects GI tract.

Gut-Associated Lymphoid Tissue (GALT)

Mucosal immune tissue with phagocytes and lymphocytes; cytokines from these cells influence ENS and endocrine activity, affecting motility and secretions.

Cytokines

Immune signaling molecules released by sensitised lymphocytes; can increase GI motility and secretions and influence neural and endocrine regulation.

Regulatory peptides (trophic effects)

Peptides that increase GI regulatory proteins with higher food intake, leading to mucosal hypertrophy and greater digestive capacity.

Visceral reflex

Reflexes triggered by stimuli like stretch, temperature, chemical change; can be long (CNS-involved) or short (within ENS).

Short visceral reflex

Local reflex within the ENS with no CNS involvement.

Long visceral reflex

Reflex arc involving CNS input and return to the GI tract; integrates broader regulation.

Mechanoreceptors

Receptors in muscle layers that detect stretch or distension of the gut.

Chemoreceptors

Receptors in the mucosa that detect chemical conditions (pH, peptides) in the gut lumen.

Parasympathetic vs Sympathetic influence on ENS

Parasympathetic input typically promotes digestion; sympathetic input inhibits digestion and slows GI activity.

Sphincter control by ANS

Sympathetic stimulation constricts sphincters; parasympathetic stimulation relaxes sphincters to allow passage of contents.

Varicosities

Bead-like strings of axon terminals in autonomic fibers; release neurotransmitters at multiple end organs for broad action.

Neurocrines in ENS

Transmitters released by enteric neurons; can be excitatory or inhibitory (e.g., ACh, NO, serotonin).

Five major intrinsic GI hormones

Gastrin, Secretin, CCK, GIP, and Motilin; produced by GI epithelium and regulate digestion through various actions.