Gastrointestinal Physiology Quiz

Physiology Study Notes

• Gastrointestinal Physiology: Four major processes occur along the GI tract:
  • Motility
  • Secretion
  • Digestion
  • Absorption
• Functional Structures in the Gastrointestinal Tract:
  • Smooth muscle cells
  • Interstitial cells of Cajal
  • Secretory cells
• Other Related Structures:
  • Control systems of GI functions (Neural control: Enteric nervous system & Autonomic nervous system; Hormonal control: GI endocrine)
  • Blood flow to the GI
• Smooth Muscle Cells in the GI Tract:
  • Two main layers (Longitudinal & Circular)
  • Form bundle-like structure connected by gap junctions
  • This organization ensures effective contractions along the GI tract
  • Third layer exists between mucosa and submucosa called Muscularis mucosa
• Characteristics of Smooth Muscle Cells:
  • Electrical activity characterized by Slow Waves (Basic Electrical Rhythm-BER).
  • These are undulating changes in membrane potential.
  • Spike Potentials—true action potentials appearing at the peak of slow waves.
• Calcium (Ca²⁺) in Smooth Muscle Contraction:
  • Ca²⁺ enters the cell from interstitial fluid through activated Ca²⁺ channels triggered by spike potentials
  • Ca²⁺ is released from the Sarcoplasmic Reticulum (SR) via the formation of inositol triphosphate (IP₃).
• Mechanism of Contraction:
  • Ca²⁺ binds to calmodulin, forming a complex that activates myosin filaments, resulting in contraction.
• Chemical Control of Smooth Muscle Activity:
  • Smooth muscle cells respond to a wide range of stimuli (Neurotransmitters or hormones)
  • Activity appears through receptor activation and subsequent cellular response
• Integration of Smooth Muscle Responses:
  • Smooth muscle cells respond to ligands binding to their receptors
  • Tonic contractions: variations in intensity
  • Rhythmic contractions (phasic): rhythmic cycles of contraction and relaxation, observed in small intestine motility
• Summary of Control for GI Smooth Muscle Cells Activity:
  • Electrical control (slow waves, spike potentials)
  • Neurochemical control (responses of smooth muscles to numerous neurotransmitters released by neurons in the ENS)
• Effective Activity of Smooth Muscle Cells in the GI Tract:
  • Smooth muscle cells operate in a syncytium, synchronized contractions
  • Coordinated by the enteric nervous system (ENS)
  • Interstitial cells of Cajal (ICCs) play crucial role in coordinating and synchronizing this activity
• Interstitial Cells of Cajal (ICCs):
  • Widely spread throughout the GI tract
  • Possess many processes enabling communication through gap junctions with other ICCs & smooth muscle
  • Generate electrical activity potentially as pacemaker cells of the GI tract
• ICCs Communication:
  • Communication occurs via gap junctions between ICCs and smooth muscle cells, synchronizing electrical activity
  • Resulting in smooth muscle cells function as a syncytium
• ICCs Generate Slow Waves:
  • ICCs are excitable cells capable of generating slow waves
  • Characterized by upstrokes from a constant resting potential (-70mV)
  • Believed to be metabolism-dependent
• ICCs Receive Inputs from the ENS:
  • ICCs receive inputs from the enteric nervous system, further regulating smooth muscle activity
• Secretory Cells:
  • Exist as solitary cells or in glands; specialized for synthesizing & secreting substances (enzymes, hormones, factors, or mucus).
    • Can secrete water and electrolytes (serous secretions)
• Enteric Nervous System (ENS):
  • A network of neurons in the GI tract
  • Consists of Myenteric plexus (Auerbach's plexus) & Submucosal plexus (Meissner's plexus).
  • Regulates gut movements and secretion.
• Neural Connectivity and Sensory Input:
  • Neurons within each plexus are interconnected
  • Sensory neurons transmit signals from the GI epithelium to enteric plexuses, prevertebral ganglia, spinal cord and the brainstem (via the vagus nerve)
• Function of Enteric Neurons:
  • Regulate GI functions by releasing neurotransmitters that can be inhibitory or excitatory (motility, secretion, vascular blood flow)
• Key Neurotransmitters in the ENS:
  • Acetylcholine (Ach)
  • Substance P (SP)
    • Vasoactive Intestinal Peptide (VIP)
    • Calcitonin Gene-Related Peptide (CGRP)
    • Gastrin-Releasing Peptide (GRP)
• Autonomic Nervous System:
  • Parasympathetic Nervous System:
    • Cranial division (vagus nerve) and Sacral division (pelvic nerves) innervate various parts of the GI tract.
    • Stimulates GI activity, increasing motility, secretion, and blood flow.
  • Sympathetic Nervous System:
    • Originates from spinal segments T5-L2.
    • Decreases GI activity, reduces motility, and reduces smooth muscle activity
• Endocrine Cells & Hormones:
  • Many hormones identified in the GI tract, some with functions yet to be determined
    • Include Gastrin, Cholecystokinin (CCK), Secretin, Gastric Inhibitory Peptide (GIP), Glucagon-like Peptide-1 (GLP-1), Motilin, Ghrelin, Amylin, Enterostatin, Neuropeptide Y (NPY), and Pancreatic Polypeptide
  • Scattered endocrine cells release other hormones such as Somatostatin, Neurotensin, Thyrotropin-Releasing Hormone (TRH), and Adrenocorticotropic Hormone (ACTH)
    • These diverse hormones regulate various GI functions and associated systems.
• Blood Flow & Local Activities in the GI:
  • Blood flow to the GI is related to activities (absorption, secretion, motility)
    • Factors like the release of vasodilators, kinins, or reduced oxygen can increase blood flow.
  • Enteric and autonomic nervous systems regulate blood flow
• Summary of Pathways Controlling Digestive System Activities: Diagram demonstrates factors, including external influences, intrinsic nerve plexuses, extrinsic autonomic nerves, and gastrointestinal hormones, coordinating digestive activity

Test your knowledge on the key aspects of gastrointestinal physiology, including motility, secretion, digestion, and absorption. This quiz will cover the functional structures within the GI tract and their roles, as well as the regulation by neural and hormonal control systems.