BIO 226 Chap 22: Digestive System

Chapter 22: The Digestive System

Organization of the Digestive System

• Nutrients are provided for cell maintenance and growth.
• The digestive system includes a muscular tube called the digestive tract, also known as the gastrointestinal (GI) tract or alimentary canal.
• The cardiovascular system, respiratory system, and urinary system support tissues with no direct contact with the outside environment.
• The respiratory system works with the cardiovascular system to supply oxygen and remove carbon dioxide from cells.
• The urinary system removes organic wastes generated by cell activity.
• The digestive tract is where food passes along its length from the mouth to the anus.
• Accessory organs secrete products into the digestive tract.
• Organs include the oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and large intestine, accessory organs are the tongue, salivary glands, liver, gallbladder, and pancreas.

Digestive Tract as a Muscular Tube

• The digestive tract is a long muscular tube.
• The tube is lined with permanent ridges and temporary folds.
• Both of these features increase surface area for absorbing nutrients.

Mesentery

• Mesentery consists of double sheets of peritoneal membrane.
• Areolar tissue lies between mesothelial layers.
• This tissue provides access route for blood vessels, nerves, and lymphatics.
• In addition, it stabilizes attached organs and prevents entanglement of intestines.

Major Layers of the Digestive Tract

• The four major layers of the digestive tract are the mucosa, submucosa, muscular layer, and serosa.

Mucosa (Inner Lining)

• Mucosa consists of a mucous membrane of epithelium, moistened by glandular secretions, and lamina propria of areolar tissue.

Submucosa

• Submucosa is a layer of dense irregular connective tissue.
• It contains blood vessels and lymphatic vessels.
• In some regions, it contains exocrine glands.
• These secrete buffers and enzymes into the digestive tract.

Muscular Layer

• The Muscular Layer is made of smooth muscle in two layers: inner circular layer and outer longitudinal layer.
• It is involved in mechanical processing and movement along the tract.

Serosa

• Serosa is a layer of visceral peritoneum along the digestive tract in the abdominal cavity.
• The oral cavity, pharynx, esophagus, and rectum have no serosa.
• These are covered instead by adventitia, a sheath formed from a dense network of collagen fibers.
• Adventitia firmly attaches the tract to adjacent structures.

Components of the Mucosa

• The tract begins and ends with stratified squamous epithelium, while the stomach, small and large intestines have simple columnar with goblet cells.
• Villi (singular, villus) refer to small mucosal projections that increase surface area for absorption.
• Lamina propria is areolar tissue containing blood vessels, sensory nerve endings, lymphatic vessels, smooth muscle cells, lymphoid tissue, and some mucous glands.
• The muscularis mucosae refers to two concentric layers of smooth muscle: the inner circular layer and outer longitudinal layer, which alter the shape of the lumen and move the circular folds and villi
• Circular folds (plicae circulares) refer to permanent transverse folds in the intestinal lining.

Nerve Plexuses

• These are involved in local control of digestive activities.
• Parasympathetic stimulation increases digestive muscle tone and activity.
• Sympathetic stimulation decreases muscle tone and activity.
• Submucosal neural plexus is located in the submucosal layer and innervates the mucosa and submucosa, contains sensory neurons, autonomic nerve fibers.
• Myenteric plexus refers to network of sensory neurons and autonomic nerve fibers (mys, muscle + enteron, intestine).
• It is in the muscularis externa between the circular and longitudinal layers.
• This works with the submucosal plexus to coordinate local control of digestive activity.

Smooth Muscle Characteristics and Role

• Smooth muscle is found throughout the body, and forms sheets, bundles, or sheaths around tissues.
• It is around blood vessels, regulates blood flow, and ring-shaped sphincters regulate movement along passageways in the digestive and urinary systems.
• It is in the digestive tract, is organized into inner circular layer and outer longitudinal layer in which cells are aligned parallel to each other.
• Relatively long and slender, diameter is 5–10 μm; length 30–200 μm, contain actin and myosin filaments.
• Organization of actin and myosin filaments differs from skeletal and cardiac muscle, contains no T tubules.
• The sarcoplasmic reticulum forms a loose network throughout the sarcoplasm, without myofibrils or sarcomeres.
• The cell has no striations, giving the tissue a "smooth" microscopic appearance.
• Thin filaments are attached to dense bodies, distributed throughout sarcoplasm, similar to Z lines of skeletal muscle tissue, that are attachment points for adjacent smooth muscle cells
• Thick filaments scattered throughout the sarcoplasm, with more myosin heads per thick filament than in skeletal or cardiac muscle.
• Cell shortens, but not in a straight line, the cell twists like a corkscrew as it contracts.

Types of Smooth Muscle

• Multi-unit smooth muscle cells are innervated in motor units like skeletal muscle, each cell may be connected to more than one motor neuron.
• Multi-unit Locations: Iris of eye (regulates diameter of the pupil), portions of male reproductive system, walls of large arteries, arrector pili muscles of skin.
• Visceral smooth muscle cells Lack any direct connection with motor neuron, arranged in sheets or layers.
• Electrically connected by gap junctions and mechanically connected by dense bodies, cells contract in a wave as a single unit.
• Stimulation can be neural, hormonal, or chemical plus rhythmically stimulated by pacesetter cells.
• These located in digestive tract walls, the gallbladder, urinary bladder, and many other internal organs.
• Plasticity relates to the ability to function over a wide range of lengths due to the scattered arrangement of thick and thin filaments.
• With Plasticity, tension development and resting length are not directly related, important for digestive tract and other organs that change size and shape

Smooth Muscle Tone

• Normal background activity and tension due to various stimulations.

Smooth Muscle Contractions and Motility

• Peristalsis describes wave of muscle contraction.
• Food enters the digestive tract as a bolus, a moist, compact mass of material.
• Bolus is propelled along the tract by contractions of the muscularis externa (peristalsis).
• Circular muscles contract behind bolus, and longitudinal muscles ahead of bolus contract, and then this process repeats.
• Segmentation involves cycles of contraction that churn and fragment the bolus.
• Segmentation mixes contents with intestinal secretions.
• These cycles have no set pattern of contractions, so there is no particular direction of movement, and occurs in most areas of the small intestine and some portions of the large intestine

Digestive Regulation Mechanisms: Local Factors

• Local factors are the primary stimulus for digestive activities.
• Examples: Changes in pH of contents in lumen, physical distortion of digestive tract wall, presence of chemicals (specific nutrients or chemical messengers released by the mucosa).

Neural Control Mechanisms

• Short reflexes (myenteric reflexes) are triggered by chemoreceptors or stretch receptors in digestive tract walls.
• Controlling neurons are in the myenteric plexus.
• Long reflexes have a higher level of control involving interneurons and motor neurons of the CNS.
• These generally control large-scale peristalsis, moving material from one region of the tract to another; which may involve parasympathetic motor fibers that synapse in the myenteric plexus.

Hormonal Control Mechanisms

• Involve at least 18 hormones that affect digestive function, and some affect other systems as well.
• Hormones are peptides produced by enteroendocrine cells, are endocrine cells in the epithelium of the digestive tract

Congenital Megacolon

• Hirschsprung disease is characterized by absences or marked reduction in number of ganglion cells in the myenteric plexus of rectum.
• It causes paralysis of smooth muscle (absence of peristaltic movement), and abnormal dilation and hypertrophy of colon.
• Results in chronic constipation, bloating, abdominal pain.

The Digestive Tract Description

• The Digestive Tract: A muscular tube about 10 m (33 ft) long with the major organs : oral cavity (mouth), pharynx, esophagus, stomach, small intestine and large intestine.
• The oral cavity (mouth) has mechanical processing (with the teeth and tongue), moistening, mixing with salivary secretions
• The Pharynx has muscular propulsion of food into the esophagus
• The Esophagus: transport of materials to the stomach
• The Stomach involves chemical breakdown, mechanical processing, and enzymatic digestion and absorption.
• The Small intestine includes enzymatic digestion and absorption.
• The Large intestine involves dehydration and compaction of indigestible materials.

Digestive Functions

• Ingestion: Occurs when solid food and liquid enter the oral cavity
• The oral cavity Mechanical digestion and propulsion, Involves crushing and shredding of food and mixing and churning in the stomach
• Chemical digestion, Chemical and enzymatic breakdown of food into small organic molecules that can be absorbed by the digestive epithelium
• Secretion The release of water, acids, enzymes, buffers, and salts by the digestive tract epithelium and by accessory digestive organs
• Absorption is the movement of nutrients across the digestive epithelium and into the bloodstream
• Defecation Indigestible food is compacted into material waste called feces, which are eliminated by defecation.

Oral Cavity (Mouth) Boundaries

• Lined by oral mucosa (stratified squamous epithelium) that is keratinized in areas that are exposed to severe abrasion (superior tongue surface, hard palate) and thin, nonkeratinized lining on cheeks, lips, and inferior tongue surface
• Thin mucosa inferior to the tongue allows for rapid absorption of lipid-soluble drugs (example: nitroglycerin)
• Nutrients are not absorbed here, but digestion of carbohydrates and lipids begins here.
• The superior boundary is the Hard palate that is Formed by the palatine processes of the maxillary bones and horizontal plates of the palatine bones and the Soft Palate - Muscular region posterior to the hard palate
• The anterior and lateral boundary: Cheeks - Form the lateral walls of the oral cavity that is Supported by pads of fat and the buccinator muscles that Are anteriorly, cheek mucosa is continuous with the labia
• The anterior and literal boundary: Labia (lips) - Form anterior boundary
• The oral cavity and boundaries Posterior boundary: Uvula Dangling process extending from the soft palate Helps prevent food from entering pharynx prematurely: Swings upward during swallowing to prevent food from entering the nasopharynx
• Palatine tonsils, located on either side of the oropharynx
• The Posterior boundary; Root of tongue — Fixed portion projecting into the oropharynx, marked by a V-shaped line of vallate papillae, and Lingual tonsils, located in the root of the tongue.
• Inferior boundary includes the Body of the tongue that is the Anterior, mobile portion; Geniohyoid and mylohyoid muscles that provide extra support to the inferior boundary.
• Oral vestibule - The space between the cheeks (or lips) and teeth
• Frenulum of the upper lip (frenulum, a small bridle) that Attaches gums to upper lip and Thick mucosa, ridges cover hard palate that Provides traction for compression of food by the tongue
• Frenulum of the lower lip Attaches gums to lower lip
• Gingivae (gums) - Ridges of oral mucosa surrounding the base of each tooth that Is Firmly attached to the periostea of the underlying bone.
• Palatal arches Located on either side of the uvula. that Palatoglossal arch - Extends between soft palate and base of tongue and Palatopharyngeal arch - Extends from soft palate to pharyngeal wall
• Space between oral cavity and oropharynx.
• Tongue: the floor is attached by the frenulum of the tounge also known as the lingual frenulum, it is the organ which manipulates materials inside the mouth Surface flushed by secretions of small glands Secretions contain water, mucins, and lingual lipase (an enzyme that starts the digestion of lipids)

Ankyloglossia (Tongue-Tie)

• Present at birth.
• Frenulum of tongue is too short.
• Interferes with breastfeeding in newborns.
• Interferes with learning to speak in toddlers.

Components of a tooth

• The bulk of each tooth is composed of dentin, a mineralized matrix similar to bone but contains no cells
• The pulp cavity - The interior chamber of the tooth that the Components of the tooth Occlusal surface
• Enamel - Covers the dentin of the crown - Hardest biologically manufactured substance - Composed of calcium phosphate that Requires calcium, phosphate, and vitamin D for formation and resistance to decay.
• Gingival sulcus and the Shallow groove surrounding the base of the neck that Epithelial attachment blocks bacteria from accessing deeper tissues around the root
• Cement - Covers the dentin in the root that Is Less resistant to erosion than dentin and the Periodontal ligament - Creates gomphosis articulation between root dentin and alveolar bone
• Narrow tunnel within the root of the tooth and Root canal Passageway for blood vessels and nerves to the pulp cavity and also the Opening into the root canal is the apical foramen
• The Crown: Portion projecting into the oral cavity from the surface of the gums.
• Neck: The boundary between the crown and root.
• Root: The Portion is below the gum line and Sits in a bony tooth socket called an alveolus.
• Blade shaped teeth with single root - Located at the front of the mouth - Useful for clipping and Cutting of teeth
• Conical: Conical with a sharp ridgeline and pointed tip Used for tearing or slashing that Have a single root Four types of teeth
• Flattened crowns with prominent ridges Used for crushing, mashing, or grinding the Have one or two roots

Molars

• Very large flattened crowns with prominent ridges.
• Adapted for crushing and grinding.
• Typically have three roots (upper jaw) or two roots (lower jaw).

Deciduous Teeth

• Formed during embryonic development.
• Also called primary teeth, milk teeth, or baby teeth.
• At 2 years of age, there are 20 deciduous teeth. 5 on each side of upper and lower jaws with 2 incisors, 1 canine, 2 deciduous molars.

Permanent Teeth

• Gradually replace deciduous teeth Periodontal ligaments and roots of primary teeth erode Primary teeth fall out or are pushed aside by secondary teeth and Three additional molars appear on each side of the upper and lower jawsThird molars are called wisdom teeth
• Three additional molars appear on each side of the upper and lower jaws, and the third molars are called wisdom teeth
• There are 32 total permanent teeth
• Fails to erupt because of overcrowding from adjacent teeth or from twisting and tilting within jaw bone Most commonly happens with wisdom teeth Treatment ranges from nothing to tooth extraction

Pharynx (Throat) and Esophagus

• The pharynx is Membrane-lined cavity posterior to the nose and mouth, also Continuous with the esophagus Includes the Common passageway for solid food, liquids, and air Three regions Nasopharynx. Larynx Laryngopharynx
• Transports - Hollow, muscular tube: ~25 cm (10 in) long and 2 cm (0.8 in.) wide Narrowest point at the beginning (posterior to cricoid cartilage) Descends posterior to the trachea Enters the abdominopelvic cavity through the esophageal hiatus (opening in the diaphragm)
• Actively moves food and liquids to the stomach
• By the parasympathetic and sympathetic fibers from the esophageal plexus Maintain resting muscle tone in circular muscle LayerKeeps the lumen closed, except when you swallow
• Band of smooth muscle that functions as sphincter Prevents air from entering the esophagus At the inferior end of the esophagus that is Normally contracted, this prevents backflow of stomach contents, plus layers of the esophageal wall features The layers in the of the wall are Mucosa (nonkeratinized stratified squamous epithelium) and submucosa form large folds extending the length of the esophagus Allow for expansion with passage of a bolus and Muscularis externa Superior third is composed of skeletal muscle Middle third is a mix of skeletal and smooth muscle Inferior third is composed of smooth muscle only plus there is No serosa Adventitia of connective tissue anchors esophagus to posterior body wall