BIOM 3200 Chapter Notes - Chapter 18: Muscularis Mucosae, Submucous Plexus, Simple Columnar Epithelium

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Unit 12: Digestive System
Within the lumen of the gastrointestinal tract food molecules are hydrolyzed into their monomers.
The monomers then pass through the inner layer (mucosa) of the small intestine to enter the blood
lymph via absorption.
Digestion and Absorption are the primary functions of the digestive system
Enzymes that digest food are also capable of digesting a person’s own tissue, this doesn’t normally
happen because protective devices inactivate digestive enzymes in the body and keep them away
from form the cell’s cytoplasm
Lumen and gastrointestinal tract is continuous with the environment
Indigestible materials (cellulose) pass through the digestive tract without crossing its epithelial lining
One-way transport enables different regions of the GI tract to be specialized for different functions:
1. Motility: the movement of food through the digestive tract through the processes of:
a. Ingestion: taking food into the mouth
b. Mastication: chewing the food and mixing it with saliva
c. Deglutition: swallowing food
d. Peristalsis and segmentation: rhythmic, wavelike contractions (peristalsis), and mixing
contractions in different segments (segmentation), move food through the gastrointestinal tract
2. Secretion: Includes endocrine and exocrine secretions
a. Exocrine Secretions: Water, HCL, bicarbonate and many digestive enzymes are secreted into the
lumen. The stomach alone secretes 2-3 L of gastric juices a day
b. Endocrine Secretions: the stomach and small intestine secrete regulatory hormones
3. Digestion: the Breakdown of food molecules into their smaller subunits which can be absorbed
4. Absorption: this refers to the passage of digested end products into the blood or lymph
5. Storage and Elimination: temporary storage and subsequent elimination of indigestible molecules
6. Immune Barrier: the simple columnar epithelium that lines the intestine, with tight junctions between
cells provides a physical barrier to the penetration of pathological organisms and toxins
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The GI tract can be divided into the alimentary canal and the accessory digestive organs.
Organs of the GI tract: oral cavity, pharynx, esophagus, stomach, small intestine and large intestine
Accessory Organs: teeth, tongue, salivary glands, liver, gallbladder and pancreas
Viscera: the abdominal organs used in digestion, or just any of the organs in the thoracic and
abdominal cavities
Layers of the Gastrointestinal Tract
The GI tract is composed of 4 layers (tunics), each tunic contains a dominant tissue that performs a
specific function in the digestive process.
The four tunics (form the inside out) are the: mucosa, submucosa, muscularis and serosa
Mucosa: lines the lumen and is the absorptive and major secretory layer of the GI tract. It is a simple
columnar epithelium supported by a thin layer of areolar connective tissue (lamina propria) that
contains numerous lymph nodules that help protect against disease. The Muscularis mucosae is
external to the lamina propria, and is a thin layer of muscle forming folds in the GI tract which greatly
increase its absorptive surface area. Goblet cells in the mucosa secrete mucus through the GI tract.
Submucosa: A thick, highly vascular layer of connective tissue. Absorbed molecules pass through the
mucosa and enter the blood and lymphatic vessels of the submucosa. Contains glands and nerve
plexuses as well. The submucosal plexus (Meissner’s Plexus) provides a nerve supply to the
muscularis mucosae of the small and large intestine.
Muscularis (Muscularis externa): Enables segmental contractions and peristaltic movement
throughout the GI tract. Consists of an inner circular and outer longitudinal layer of smooth muscle,
the contraction of which moves food through the tract and mixes it with digestive enzymes. The
myenteric/ Auerbach’s plexus is located between the 2 muscle layers and provides nerve supply to
the entire GI tract, it consists of ganglia and fibers from the sympathetic and parasympathetic
divisions of the autonomic nervous system.
Serosa: The binding and protective layer consisting of areolar connective tissue covered with a layer
of squamous epithelium
Regulation of the Gastrointestinal Tract
The GI tract is innervated by the sympathetic and parasympathetic divisions of the ANS
Vagus nerve is the source of parasympathetic activity in the esophagus, stomach, pancreases,
gallbladder, small intestine and the upper portion of the large intestine
The lower portion of the large intestine receives parasympathetic innervation from spinal nerves in
the sacral region
Submucosal and myenteric plexuses are where parasympathetic preganglion fibers synapse with
postganglionic neurons to innervate the smooth muscle of the GI tract
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Sympathetic nerves reduce peristalsis and secretory activity, but stimulate the contraction of
sphincters – making them antagonistic to the effects of parasympathetic nerve stimulation
Intrinsic Sensory Neurons: cell bodies are within the gut and are not part of the autonomic system.
These help the local regulation of the digestive tracts by a complex neural network- Enteric Nervous
System/ Enteric Brain. ENC works alongside paracrine regulation, as molecules act locally, within the
tissue of the GI tract, and hormonal regulation of the hormones secreted by the mucosa
Extrinsic regulation via ANS and Endocrine system.
Intrinsic regulation via the Enteric nervous system and paracrine regulators.
From the Mouth to Stomach
Peristaltic contraction in the esophagus bring food to the stomach, which secretes acidic gastric juices
that mix with food.
The proteins that result are called chyme, and they are partially digested by the enzyme pepsin
Mastication mixed food with saliva secreted by the salivary glands. Saliva contains mucus,
antimicrobial agents, and salivary amylase which catalyses the partial digestion of starch
Deglutition is divided into the: oral, pharyngeal and esophageal phases. Requires 25 muscles.
Muscles of the mouth, pharynx and upper esophagus are striated and innervated by somatic motor
neurons- muscles in the middle and lower section are smooth and innervated by autonomic neurons
Oral phase is voluntary, others are automatic and controlled by the swallowing center in the brain
In the oral phase, food mixes with saliva to create a bolus, that is moved towards the oropharynx
Receptors in the posterior portion of the oral cavity and oropharynx stimulate the pharyngeal phase
The palate lifts to close off the nasopharynx form the oropharynx; vocal chords close off the opening
to the larynx and the epiglottis covers the vocal chords; larynx moves out of the way of the bolus and
towards the esophagus, and then the upper esophageal sphincter relaxes
These activities in the pharyngeal phase take less then 1 second
Esophageal Phase takes about 5-6 seconds, as the bolus is moved by peristaltic contractions towards
the stomach
Esophagus
Connects the pharynx to the stomach- a muscular tube approximately 25 cm long
Posterior to the trachea within the mediastinum of the thorax
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