Human Anatomy and Physiology The Digestive System

Organization Gastrointestinal tract (alimentary canal) Tube open at both ends for the transit of food during processing. Functional segments include the mouth, esophagus, stomach, small intestine, and large intestine. Accessory structures Contribute to the food processing. Teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Figure 24.1 Organs of the Digestive System Accessory organs are highlighted in pink

Six Basic Digestive Processes Ingestion Secretion Mixing and propulsion Digestion Mechanical digestion Chemical digestion Absorption Defecation

Catabolism Breaking down complex substances into simpler ones with the release of energy (ATP).

LAYERS OF THE GI TRACT

Four Layers of the GI tract Mucosa - innermost Submucosa Muscularis Serosa - outermost

Mucosa Composed of three layers: Epithelium Lamina Propria (contains MALT) Muscularis mucosa Plays an important role in digestion and absorption Muscularis mucosae Mucosa-associated lymphatic tissue (MALT) MUCOSA: Epithelium Lamina propria

Mucosa Epithelium Mucus and enteroendocrine cells Nonkeratinized stratified squamous epithelium Simple columnar epithelium Mouth Pharynx Esophagus Anal canal FNC: Protection Stomach Intestines FNC: Secretion and Absorption Mucosa Epithelium Mucus and enteroendocrine cells

Mucosa 2. Lamina Propria Areolar connective tissue Blood and lymphatic vessels Nerves and sensors Mucosa-associated lymphatic tissues (MALT) immune system cells All throughout G.I tract Prominently tonsils, S.I., appendix and L.I. Supports epithelium, binds to muscularis mucosa

Mucosa 3. Muscularis mucosa Smooth muscle Creates folds in stomach and S.I Increases surface area

Submucosa Composed of: Areolar connective tissue (highly vascular) Submucosal plexus (part of the ENS) Binds mucosa to muscularis Regulates chemical digestion Glands in submucosa Submucosal plexus (plexus of Meissner) SUBMUCOSA

Muscularis Skeletal muscle in mouth, pharynx, superior esophagus, and external anal sphincter Rest of GI tract is composed of smooth muscle: Inner circular muscle Outer longitudinal muscle Contains myenteric plexus between (part of ENS) Plays an important role in mixing and propulsion MUSCULARIS: Circular muscle Longitudinal muscle Myenteric plexus (plexus of Auerbach)

Serosa Serous membrane of areolar connective tissue and simple squamous epithelium Secretes serous fluid Also called the visceral peritoneum Esophagus lacks a serosa – replaced with adventitia (single layer of loose areolar CT)

Figure 24.2 Layers of the GI tract

NEURAL INNERVATION OF THE GI TRACT

Enteric Nervous System Second brain  “Brain of the gut.” Consists of 100 million motor neurons, interneurons, and sensory neurons that extend from the esophagus to the anus. Located in sheaths of tissue lining the esophagus, stomach, S.I and colon Neurons arranged into the myenteric plexus and the submucosal plexus.

Enteric Nervous System Myenteric neurons control gastric motility. Submucosal neurons control the secretory cells. Can function independently of the CNS.

Figure 24.3 Organization of the Enteric Nervous System

Autonomic Nervous System Vagus nerve (X) supplies parasympathetic fibers that stimulate ENS neurons. Stimulate GI secretion and motility Sympathetic nerves (from thoracic and upper lumber regions of the spinal cord) inhibit synapse of ENS neurons. Anger, fear and anxiety slow digestion

Gastrointestinal Reflex Pathways Regulate secretions and motility in response to stimuli present in the lumen. Reflexes begin with receptors associated with sensory neurons of the ENS. ENS, CNS, and ANS can modulate reflexes.

Sensory Receptors in G.I Tract Chemoreceptors – respond to chemicals in food in the lumen. Mechanoreceptors – stretch receptors that are activated when food distends the wall of a GI organ. Controls of the digestive activity are both extrinsic and intrinsic (nervous and hormonal)

PERITONEUM

Key Terms Serous Membrane - Smooth, transparent, two-layered membrane that lines internal cavities of the body, lubricated by a fluid derived from serum. Mesothelium. Peritoneum – body’s largest serous membrane Parietal peritoneum – lines abdominal wall Visceral peritoneum - covers most abdominal organs (serosa); some organs (kidneys, pancreas) are retroperitoneal Peritoneal cavity - potential space between parietal and visceral peritoneum; contains serous fluid.

Figure 24.4 Relationship of the peritoneal folds to one another and to organs of the digestive system

Main extensions of the peritoneum Greater omentum Falciform ligament Lesser omentum Mesentery Mesocolon

Greater Omentum Arises from stomach and duodenum, covers small intestine, and attaches to transverse colon. Adipose tissue

Falciform Ligament Attaches liver to abdominal wall and diaphragm.

Lesser Omentum Suspends stomach and small intestine from liver.

Mesentery Largest fold. Binds small intestine to posterior abdominal wall

Mesocolon Binds large intestine to posterior abdominal wall. Works with mesentery to keep the intestines in place.

MOUTH

Mouth (oral or buccal cavity) Formed by cheeks, hard and soft palate, lips, and tongue. Covered externally by skin. Covered internally by mucous membrane.

Mouth (oral or buccal cavity) Divided into: Vestibule - bounded externally by cheeks and lips and internally by gums and teeth. Oral cavity proper - extends from gums and teeth to fauces (the opening between oral cavity and pharynx)

Cheeks – contain buccinator muscles Lips (labia) – contain orbicularis oris muscles; attached to gum by labial frenulum Lips and cheeks keep food between teeth and assist in speech. Uvula – the hangy-thing in the back of the throat; blocks nasopharyngeal opening during swallowing