Lecture 39 - Digestive System Overview - From Oral Secretions to the Stomach

Question 1

What is the function of the digestive system?

Answer 1

To processes what we ingest and absorb the nutrients from food to fuel the body

Question 2

What is ingestion and how are nutrients from the food absorbed?

Answer 2

Ingestion is the act of taking food (or water) into the body. Most food must be digested in order to be absorbed

Question 3

Describe the path food takes in the digestive system

Answer 3

1. Food is ingested, then digested through physical and chemical means
2. It’s propelled along the digestive tract
3. Nutrients are absorbed into the bloodstream and lymphatic system
4. Non-nutrients (and some wastes) are excreted by defecation

Question 4

What is the digestive system made up by?

Answer 4

Made up by gastrointestinal tract and accessory organs

The gastrointestinal tract (alimentary canal) is a muscular tube that runs from the mouth to the anus. Movement is (generally) in one direction

Question 5

What is the alimentary canal and describe its organization

Answer 5

A hollow muscular tube lined by a mucus-secreting epithelium

• Mucosa - epithelium + areolar connective tissue
• Submucosa - dense connective tissue, vessels (and sometimes glands)
• Muscle layer - usually 2 layers of smooth muscle
• Serosa (or adventitia) - epithelium + areolar tissue (or dense connective tissue)

Question 6

What is the function of mesenteries and peritoneum

Answer 6

They are associated with the alimentary canal

Mesenteries - membranes that anchor abdominal organs to the peritoneum

Peritoneum - a double-layered membrane encasing most of the abdominal cavity

Question 7

What are the locations of neural plexuses?

Answer 7

Neurons of the ENS and ANS are found throughout the digestive tract and organized into 2 plexuses (networks)

Submucosal plexus - contains visceral sensory fibres and parasympathetic and sympathetic postganglionic neurons (ANS)

Myenteric plexus - contains ENS neurons (interneurons and motor neurons)

Question 8

In most of the digestive tract, what is the type of muscle found and how are they organized?

Answer 8

You mostly see smooth muscle into 2 layers with different orientations:

1. Circular layer - arranged around the circumference of the tube
2. Longitudinal layer - arranged along the length of the tube

Question 9

What are the exceptions to the typical organization of muscle layer in the digestive tract?

Answer 9

1. Oral cavity, pharynx, upper esophagus have skeletal muscle in the muscularis
2. The stomach has 3 layers of smooth muscle
3. The colon has an incomplete outer layer

Question 10

What is the organization of visceral smooth muscle cells?

Answer 10

They are connected by gap junctions and influenced by rhythmically active pacesetter cells. They are a single-unit organization

The pacesetter cells create regular rhythms of depolarization and repolarization (slow wave potentials) which can spread throughout the muscle layer

NTs released from ANS neurons can enhance or diminish slow wave potentials

Question 11

Name the 5 different processes that occur in the digestive tract

Answer 11

1. Propulsion - forward movement (swallowing, salivation)
2. Digestion - food breakdown (chewing, salivation)
3. Absorption - nutrients into bloodstream
4. Coordination - feedback and feedforward loops (tasting)
5. Protection - keeping pathogens out of the rest of the body

Question 12

What is propulsion? What helps it occur?

Answer 12

Refers to the movement of ingested material through the digestive tract (ie. pushing the bolus from the mouth to the anus)

Peristalsis helps this to occur - it involves spreading waves of contraction in both muscle layers in a proximal-to-distal direction. These waves propel food through the tract

Question 13

What is digestion and what are the 2 parts? Describe the role segmentation plays

Answer 13

Refers to the breakdown of ingested material into smaller components. It involves segmentation, which are rhythmic cycles of circular muscle contraction that fragment the food bolus, but don’t produce net forward movement

1. Physical digestion - the fragmentation of food by purely physical forces (ex. mastication, segmentation)
2. Chemical digestion - the breakdown of macromolecules into smaller components by chemicals or enzymes

Question 14

What enhances digestion and propulsion?

Answer 14

Primary and some accessory digestive organs secrete fluids into the lumen of the digestive tract. These secretions typically contain digestive enzymes or chemicals, but as fluids, they also help to reduce friction of food moving through the tract

Almost all of these secretions are reabsorbed before the wastes are excreted

Question 15

What is absorption?

Answer 15

The uptake of nutrients, water, and vitamins from the digestive tract lumen into the bloodstream. It refers to the movement of nutrients across the mucosa (epithelial layer) into the body tissue

Only certain parts of the digestive tract are actually capable of absorption

Question 16

What is mastication (chewing)?

Answer 16

It contributes to physical digestion by bringing 2 rows of teeth together. Chewing is a coordinated rhythmic motor behaviour involving movements of the mandible, and puts food in contact with moving, grinding teeth

Question 17

What is the function of the tongue?

Answer 17

It is a muscular organ which manipulates food and is also involved in gustation (taste). It contains skeletal muscle tissue. Its mobility is increased by extrinsic muscles

The tongue:
- Contributes to physical digestion and propulsion
- Secretes lingual lipase which contributes to chemical digestion of fats
- Is the principal organ for our sense of “taste”

Question 18

How is the sense of taste formed?

Answer 18

It is detected by taste cells found within taste buds and is important for several digestive reflexes. The superior surface of the tongue contains several types of papillae, which house taste buds

Question 19

What is the function of salivary glands?

Answer 19

Their secretions contributes to chemical digestion, propulsion, and protection. Saliva is an aqueous solution, but also contains a digestive enzyme (salivary amylase), antibacterial enzymes, antibodies, and electrolytes that buffer the osmolarity and pH of the oral cavity

Question 19

What 3 reflexes are triggered during the activation of taste afferent neurons?

Answer 19

1. Salivation reflex
2. Gag reflex
3. Gastric reflexes

Question 19

What division of the ANS innervate salivary glands?

Answer 19

Both divisions of the ANS innervate the salivary glands:

• Parasympathetic activity - enhances the rate of secretion
• Sympathetic activity - stimulates secretion, but reduces the volume of saliva produced (by causing local vasoconstriction)

Question 19

How is the secretion of saliva enhanced? What is salivatory reflex?

Answer 19

It can be enhanced by increasing parasympathetic activity

Salivatory reflex - activation of chemoreceptors (taste cells) and mechanoreceptors in the oral cavity trigger salivation via long reflexes which are integrated in the medulla

Question 20

Describe the general process of swallowing

Answer 20

Moves food from the oral cavity through the pharynx and esophagus to the stomach. Must stay out of the trachea

The pharynx (and upper esophagus) contains 2 layers of skeletal muscle fibres, which enables some voluntary control over swallowing movements

Question 21

What are the 3 phases of swallowing?

Answer 21

1. Buccal phase
2. Pharyngeal phase
3. Esophageal phase

Question 22

Describe the buccal phase of swallowing

Answer 22

Mostly involves movements of the tongue. The tongue elevates, pushing the bolus against the hard palate. The tongue retracts, forcing the bolus into the oropharynx

The buccal phase is initiated voluntarily

Question 23

Describe the pharyngeal phase of swallowing

Answer 23

Its coordinated reflex responses prevent propelled food from entering the nasopharynx and larynx

The presence of a bolus in the oropharynx triggers:
- coordinated peristaltic-like contraction of pharyngeal muscles
- laryngeal and hyoid muscles contract and elevate the larynx, causing the epiglottis to close over the trachea
- muscles of the soft palate elevate the uvula, blocking the nasopharynx

This phase is reflexive (triggered in response to mechanoreceptor activation)

Question 24

Describe the esophageal phase of swallowing

Answer 24

As the pharynx pushes the bolus into the esophagus, a wave of peristaltic contractions begin, caused by the neural activity in the myenteric plexus.

Near the distal end of the esophagus, stretch receptors trigger the relaxation of the smooth muscle in the lower esophageal sphincter, allowing movement into the stomach

This phase is reflexive (triggered in response to mechanoreceptor activation)