Chapter 21 (1-2)

What is another name for the digestive system?

The gastrointestinal (GI) system

Where does the GI system begin?

Mouth and pharynx

Describe the path of food through the GI system

Mouth and pharynx --> Food receptors

Esophagus

Stomach

Small intestine

Large intestine

What does the GI tract refer to?

The stomach to the anus

In the oral cavity, how does digestion begin?

With chewing and the secretion of saliva

Where does saliva come from?

3 pairs of salivary glands

Parotid, Sublingual, submandibular

Describe the muscular structure of the walls of the esophagus.

Top 1/3 --> Skeletal muscle walls

Bottom 2/3 --> Smooth muscle walls

What are the 3 sections of the stomach?

Fundus

Central body

Antrum

How does the stomach continue digestion?

By mixing food with acid and enzymes to create chyme

What does the stomach create by mixing food with acids and enzymes?

Chyme

Where does chyme, produced by the stomach, pass through to reach the small intestine?

The pylorus

What is the pyloric valve?

Smooth muscle that gates the pylorus

only allows small amounts of chyme to enter the small intestine at a time

Where does most digestion occur?

Small intestine

Describe the 3 sections of the small intestine

Duodenum

Jejunum,

Ileum

What performs digestion?

What aids digestion?

Performs --> intestinal enzymes

Aids --> exocrine secretions from pancreas and liver to gallbladder

Where are almost all digestive nutrients and secreted fluids absorbed?

Small intestine

What is the proximal portion of the large intestine?

The colon

Where are water and electrolytes absorbed from chyme?

Large intestine (colon)

proximal portion(duodenum) of small intestine

What does the colon produce?

Feces

Where does feces go after leaving the colon?

The rectum

What leads to a defecation reflex?

Stretching of the rectal wall as feces fills the rectum

This causes feces to leave the body through the anus due to elaxation of the anal sphincters.

What is the function of the digestive processes?

Transfer nutrients, water, and electrolytes to the internal environment of the body from the external environment

What is the digested substance of the digestive tract called?

Chyme

What are the 3 challenges the GI tract faces while carrying out digestive processes?

-Avoiding autodigestion

-Mass balance

-Defense

Describe why autodigestion may occur

Digestion is the process of breaking down food into constituent parts.

Autodigestion would occur if digestion were to break down the cells of the digestive system

What is mass balance of the digestive system?

Matching fluid input with fluid output

How is defense an obstacle of the digestive system?

Lumen of digestive tract has the greatest surface area in contact with the internal and external environment

What are defensive mechanisms of the digestive system?

Mucus

Digestive enzymes

Acid

lymphoid tissue

Where is the largest collection of lymphoid tissue located?

Digestive system

What are different secretions of the digestive system?

Saliva --> salivary glands

Bile --> liver

Gastric secretions --> stomach

Pancreatic secretions --> pancreas

Intestinal secretions --> intestinal epithelial cells

How are substances removed from the digestive system?

Absorption --> small/large intestines (8.9L)

Excretion --> feces (0.1L)

What do exocrine glands secrete?

Digestive enzymes

Where are digestive enzymes secreted from?

Exocrine glands

Epithelial cells of stomach and small intestine

What are digestive enzymes?

Proteins released on demand

Many digestive enzymes are secreted in an inactive form.

What is the inactive form called?

Zymogens

Where are zymogens activated?

Only in the GI lumen

Why are zymogens only activated on the GI lumen?

So they can be safely stockpiled in the cells without destroying the cells that harbor them

What challenge that the gut faces is addressed by zymogens?

Avoiding autodigestion

What suffix correlates to zymogens?

-ojen

ex --> pepsinogen activates to pepsin

What is the function of Mucus in the gut?

Creates a protective coating to help lubricate the gut

Protection from acid

What does chewing and churning do?

Breaks down food to create a greater surface area that will interact with digestive enzymes

Define motility

The movement of substances through the GI tract

What are the 2 functions of motility?

-Move food from the moth to the anus

-Mechanical break down of food (contributes directly to digestion)

What process in the GI system does the mechanical breakdown of food most directly contribute to?

Digestion

What gives rise to motility in the digestive system?

Smooth muscle modified by different inputs

What are the inputs that modify smooth muscle in the digestive system resulting in motility?

Nerves from the enteric nervous system

Hormonal inputs

Paracrine inputs

Most smooth muscle of the digestive system is single-unit.

What is a key unique characteristic of single-unit smooth muscle (compared to multi-unit smooth muscle)?

Muscle cells are interconnected by gap junctions

Describe the different contractions of the GI tract

Tonic contraction --> Contraction sustained for minutes or hours. It occurs in sphincters and interior stomach

Phasic contractions --> Alternating contraction and relaxation. Associated with de/repolarization in autorhythmic cells

How is muscle contraction force related to action potential firing?

The more action potentials that fire, the stronger the force will be

What are interstitial cells of Cajal (ICCs)?

Autorhythmic cells of the digestive system

Where are Interstitial cells of Cajal (ICCs) present?

Between layers of smooth muscle throughout the GI tract

What are the 3 patterns of contraction in the GI tract?

Migrating motor complex --> housekeeping

Peristalsis --> pushes food down

Segmented contraction --> churns food

Describe the migrating motor complex pattern of movement

Happens between meals when GI tract is mostly empty and passes slowly from section to section over 1.5 hours

Moves food ruminates and bacteria out of upper GI tract to large intestine

Housekeeping function

Describe peristalsis

Progressive waves of contraction that pushes a bolus of food from one section to the next using circular muscles

Describe segmented contraction

Contraction and relaxation of short segments of intestine using circular muscles with occasional longitudinal muscles that help churn food

What process does the churning of food in the GI tract contribute to?

Digestion and absorption

Which 2 digestive processes are primarily subject to regulation by the enteric nervous system?

Motility

Secretion

Why is motility regulated?

If food moves through the gut too quickly, there wont be enough time for reabsorption

Why is secretion regulated?

So that digestive enzymes that break down food are released at the correct time

The enteric nervous system (ENS) can carry out reflexes independent of the CNS.

True or False

True

What are short reflexes of the ENS?

Reflexes that happen entirely within the GI tract

What are long reflexes of the ENS?

Reflexes integrated with the CNS

Activation of which branch of the autonomic nervous system would you expect to increase secretion and motility in the gut?

Parasympathetic branch

"Rest and digest"

What is the cephalic phase?

The earliest phase of digestion when the brain prepares the body in anticipation of food

What region of the brain sends signals to autonomic neurons during the cephalic phase?

Medulla oblongata

What kind of reflex is the anticipatory response to the sight/smell of food?

Positive feedforward

How is the cephalic phase a positive feed-forward system?

Before a stimulus arrives, the brain feeds-forward information to the gut/stomach to prepare it for digestion

Describe are the 4 functions of saliva?

-Soften and moisten food --> make it easier to swallow (works with chewing)

-Starts digestion --> breaking down starch into glucose using salivary amylase

-Taste --> Dissolved foods are easier to taste

-Defense --> Antibacterial enzymes and immunoglobins present to help fight infection

What is mastication?

Chewing

What is the function of salivary amylase?

Breaking starch into glucose

Which autonomic system primarily controls saliva?

Parasympathetic

What is a swallowing (deglutition) reflex?

A reflex that pushes a bullous into the esophagus

What is the stimulus for a swallowing reflex?

Pressure from the tongue on the soft palate in the back of the mouth

What nerve produces the swallowing reflex?

Where does it project?

Glossopharyngeal nerve projects to the medulla

Where does the medulla send output information for the swallowing reflex?

Somatic motor neurons projecting to the pharynx and upper esophagus

Autonomic neurons in the lower esophagus

Describe the swallowing reflex

-Tongue creates pressure on soft palate in the back of the mouth

-Sensory neurons on glossopharyngeal nerve activate and project to swallowing centers in the medulla

-Medulla output projects to somatic motor neurons that control muscles of pharynx and upper esophagus

**Also to autonomic neurons of lower esophagus

-Nasopharynx closes with a contraction that moves larynx up and forward to close trachea

**Esophageal sphincter opens

-Epiglottis folds down as the bolus of food moves down to close off upper airway

-Bolus moving down triggers wave of peristalsis to push food into the stomach

What is the first movement of the swallowing reflex?

Closing of the nasopharynx by contracting the larynx to close the trachea and open the esophageal sphincter

Describe the gastic phase of digestion

Stretching (distension) of the stomach and the presence of food activates endocrine cells and enteric neurons to release hormones, neurotransmitters, and other paracrine molecules to influence motility and secretion

What components of food are specifically used to activate endocrine cells and enteric neurons during the gastric phase of digestion?

Peptides and amino acids

What digestive processes does the gastric phase influence?

Motility and secretion

The gastric phase uses _______ reflexes when food is present.

both reflexes

Before food is present, _______ reflexes are used to prepare the stomach for digestion

Before food is present, long reflexes are used to prepare the stomach for digestion

What are the 3 functions of the stomach?

-Storage

-Digestion

-Defense

What happens when food enters the stomach from the esophagus?

The stomach relaxes and expands to hold the increased volume

What is it called when the stomach relaxes due to increased food volume?

Receptive relaxation

Receptive relaxation is a neurally mediated reflex.

True or False

True

What is the name of the upper portion of the stomach?

Fundus

How is food processed in the upper/lower portion of the stomach?

Upper (fundus) --> holds food

Lower --> digests food

What waves pushes food into the bottom of the stomach (pylorus) for digestion?

Peristaltic waves

What does food mix with in the pylorus to produce chyme?

Acid and digestive enzymes

As food becomes chyme in the pylorus, how does is move into the duodenum of the small intestine?

Peristaltic waves push chime from the pylorus into the small intestine

What are the 4 layers of the stomach wall?

Which one is connective tissue?

Inner mucosa

Submucosa

Muscularis externa

Serosa --> connective tissue surrounding gut

Describe the 3 layers of mucosa

• Epithelium – protection, secretion, and absorption

• Lamina propria – support, nourishment, and immune defense

• Muscularis mucosae – local mucosal movement to aid digestion and absorption

What are the folds in the stomach wall called?

Rugae

What is the function of rugae?

increase surface area in the stomach

The mucosa epithelium contains transporting epithelial cells.

What are transporting epithelial cells important for?

Moving nutrients and solutes between lumen of gut and interstitial space

Describe the apical and basolateral portion of mucosa epithelium

Apical (facing lumen) --> secretion of ions, enzymes, mucus, paracrine molecules

Basolateral (facing interstitial space) --> molecules reabsorbed are secreted into interstitial space for capillaries or lymphatic system

In the stomach and colon, connections between epithelial cells are ______ _______, but are ______ in the small intestine

In the stomach and colon, connections between cells are tight junction but are leaky in the small intestine

Why are epithelial cells 'leaky' in the small intestine?

It is the site for most nutrient reabsorption

Why are stem cells present in the mucosa?

Epithelial cells only live a few days