Anatomy GI System

Functions of the digestive system

• Ingestion and motility

• Secretion

• Mixing and propulsion

• Digestion

• Absorption

• Defecation

What 2 groups of organs make up the digestive system?

1. GI tube - a continous tube that extends from the mouth to the anus through the throacic and abdominopelvic cavities

2. Accessory digestive organs

Longitidunal muscle

Regulates length

Circular muscle

regulates diameter

Enteric Nervous System

• brain of the gut

• 100 million neurons

• controls involuntary GI propulsion

• controls acid and hormonal secretions

Myentric Plexus

Controls GI intrinsic movements

• frequency and strength of muscularis contraction

Submucosal Plexus

Controls the secretions of the organs of the GI tract and local blood flow

Vagus nerves

Supply parasympathetic fibers to most of the GI tract

Slow waves

Depolarized rhythmic waves in the membrane

• determined by the frequency of the slow waves of the smooth muscle membrane potential

Where do smooth waves originate?

In the interstitial cells of Cajal

Slow waves are ______ depolarizations in GI smooth muscle

Spontaneous

Peristaltic Contactions

Responsible for forward movement

Segmental contractions

Responsible for mixing

Mechanical digestions

Consists of all movement in the GI tract that causes:

• Change in the physical state of ingested food

• Churning of the contents in the GI lumen

• Propelling of food forward along the GI tract

Chewing is a ______ movement

Voluntary

Functions of Chewing

• Masticate the bolus

• Mix the bolus with saliva

• Bring the bolus into contact with the digestive organs

Microscopic Layers Outer to Inner

Serosa → Longitudinal Muscle → Circular muscle → submucosa → muscularis mucosae → mucosa

Smooth muscle has ______ contractability

High

Parasympathetic effects on GI

• increase motility

• relax sphincters

• increase secretion

Sympathetic effects on GI

• decrease motility

• contract sphincters

• decrease secretion

• activated by stress/emotions

Types of Salivary Glands

Parotid: serous, amylase

Submandibular: mixed but mostly serous

Sublingual: mostly mucous

How are carbohydrates digested?

Through salivary amylase

Sympathetic regulation of saliva

• decreases volume of saliva → dry mouth

• thick mucus

Parasympathetic regulation of saliva

• watery

• enzyme rich saliva

• increases secretion

Functions of saliva

• lubrication

• dissolving food

• defense

• digestion

• oral hygiene

• water regulation

Three phases of swallowing

1. Oral → voluntary

2. Pharyngeal → involuntary

3. Esophageal → involuntary

Motors functions of the stomach

• fundus relaxes to receive food

• content seperates by density

Stimulators of motor function in the stomach

PNS, gastin, motilin

Inhibitors of GI motor functions

SNS, secretion, CCK, GIP

Pepsin

• secreted as pepsinogen

• activated by HCl

• inactivated in duodenum, pH > 5

Parietal cells

• HCL → activates pepsinogen, denatures protein, bactericidal

• Intristic factor → B12 absorption

Brunners Glands

• Duodenum

• Alkaline mucus

  • protects from acidic chyme

Crypts of Lieberkuhn

• Intestinal juice

• Enterokinase activates trypsinogen → trypsin

Does the large intestine have digestive enzymes?

No

• Lots of mucus

The myenteric plexus (Auerbach) primarily controls:

A. Secretion
B. Blood flow
C. GI motility
D. Nutrient absorption

C. GI motility

Parasympathetic stimulation generally causes:

A. Increased motility and secretion
B. Decreased motility and secretion
C. Sphincter contraction
D. Dry mouth

A. Increased motility and secretion

Slow waves by themselves DO NOT cause contractions because:

A. They are hyperpolarizing events
B. They do not reach threshold for action potentials
C. They occur too slowly
D. They occur only in the stomach

B. They do not reach threshold

Slow waves originate from which specialized cells?

A. Chief cells
B. Parietal cells
C. Interstitial Cells of Cajal
D. Kupffer cells

C. Interstitial Cells of Cajal

Which gland produces mostly mucous secretions?

A. Parotid
B. Submandibular
C. Sublingual
D. Brunner’s glands

C. Sublingual

Parasympathetic stimulation of salivary glands produces:

A. Thick, mucous-rich saliva
B. Very little saliva
C. Watery, enzyme-rich saliva
D. No change

C. Watery, enzyme-rich saliva

Which stomach region is responsible for storage and receptive relaxation?

A. Antrum
B. Fundus
C. Pylorus
D. Body

B. Fundus

Pepsinogen is activated in the stomach by:

A. CCK
B. Bicarbonate
C. HCl
D. Gastrin

C. HCl

Which hormone slows gastric emptying, especially in response to fats?

A. Gastrin
B. Secretin
C. CCK
D. Motilin

C. CCK

Which enzyme is primarily responsible for lipid digestion?

A. Salivary amylase
B. Pepsin
C. Pancreatic lipase
D. Trypsin

C. Pancreatic lipase

Bile salts are mainly reabsorbed in the:

A. Jejunum
B. Duodenum
C. Ileum
D. Colon

C. Ileum

Which hormone stimulates gallbladder contraction?

A. Secretin
B. VIP
C. CCK
D. Ghrelin

C. CCK

Motilin is released during:

A. Feeding
B. Fasting
C. Sleep
D. Defecation

B. Fasting

A major function of the proximal colon is:

A. Storage of feces
B. Absorption of water and electrolytes
C. Protein digestion
D. Enzyme secretion

B. Absorption of water and electrolytes

Describe the roles of the myenteric and submucosal plexuses

Myenteric plexus (Auerbach): controls GI motility (frequency and strength of contractions)

Submucosal plexus (Meissner): regulates secretion and local blood flow

What are slow waves, and do they cause contraction?

Slow waves are rhythmic oscillations in GI smooth muscle membrane potential produced by Interstitial Cells of Cajal.

• They do not cause contraction unless they reach threshold

What is retropulsion in the stomach?

The forceful backward movement of chyme from the pylorus into the stomach, aiding mixing and particle size reduction.

What is the main function of Brunner’s glands?

Secretion of alkaline mucus rich in bicarbonate to neutralize gastric acid entering the duodenum.

What stimulates gallbladder contraction?

Cholecystokinin (CCK) released in response to fats and partially digested proteins in the duodenum

What hormones decrease gastric emptying?

• CCK

• Secretin

• GIP

What are mass movements in the colon and how often do they occur?

Powerful peristaltic contractions that move fecal contents long distances.

• Occur 1–3 times per day.

What is the function of the longitudinal muscle layer in the GI tract?

It regulates the length of the GI tract segment

What is the function of the circular muscle layer in the GI tract?

It regulates the diameter of the GI tract lumen

What is the primary function of the myenteric plexus?

Controls GI intrinstic movment, particularly the frequency and strength of contraction of the muscularis

What is the primary function of the submucosal plexus?

It controls the secretions of the organs of the GI tract and local blood flow

What is the general effect of parasympathetic stimulation on the GI tract?

It causes an increase in GI secretion and motility by increasing the activity of ENS neurons

What is the general effect of sympathetic stimulation on the GI tract?

It causes a decrease in GI secretion and motility by inhibiting the neurons of the ENS

The ____ nerves supply parasympathetic fibers to most of the GI tract, except for the last half of the large intestine

Vagus (X)

How does parasympathetic stimulation affect sphincters in the GI tract?

It relaxes sphincters

What is another name for the slow waves in the GI smooth muscle membrane potential?

Basal electric rhythm

What specialized cells are considered the electrical pacemakers for GI smooth muscle cells and originate slow waves?

Interstitial cells of Cajal

What determines the rhythm of most GI contractions?

The frequency of the slow waves of the smooth muscle membrane potential

How do smooth waves propagate through smooth muscle cells?

Through gap junctions

In the small intestine, the frequency of slow waves decreases in the ____ direction (away from the mouth)

Aboral

Which ions in saliva activate salivary amylase?

Chloride ions

What is the approximate pH of saliva?

Slightly acidic

• 6.35 to 6.85

Which salivary gland produces mainly serous secretion containing salivary amylase?

The parotid gland

What is the effect of parasympathetic stimulation on saliva secretion?

Promotes continous secretion of a moderate amount of watery, enzyme rich saliva

Weak peristaltic contractions in the middle part of the stoamch are known as _____ waves

Mixing

The process where gastric contents are pushed back from the pylorus for further mixing is called ____

Retropulsion

What is the migrating motor complex?

A series of contractions initiated by motilin during fasting to cleanse the stomach and small intestine of any residue

What is the function of intrinsic factor?

It is required for the absorption of vitamin B12 in the ileum

What two enzymes are secreted by chief cells in the stomach?

Pepsinogen and gastric lipase

How is the inactive enzyme pepsinogen converted into the active enzyme pepsin?

It is activated by hydrochloric acid in the stomach

Where are G cells located?

Pyloric antrum of the stomach

Pancreatic enzymes are secreted in an inactive form called a ____

Zymogen

How is trypsinogen activated to trypsin in the small intestine?

It is activated by the intestinal enzyme enterokinase

What is the function of bicarbonate in pancreatic juice?

Neutralizes the acidic chyme coming from the stomach, creating an optimal pH for pancreatic enzymes

What are the two major functions of the liver related to digestion?

• Synthesis of bile salts

• Excretion of bilirubin

What is the main digestive function of bile?

To emulsify fats

Does bile contain any digestive enzymes?

No

What is the function of the gallbladder?

Stores and concentrates bile

The hormone _____ stimulates the gallbladder to contract and release bile

Cholecystokinin (CCK)

The hormone ____ stimulates the pancrease to release a juice rich in bicarbonate

Secretin

Where does most nutrient absorption occur in the GI tract?

The small intestine

What are the saclike segments of the large intestine called?

Haustra

What are the two main functions of the large intestine?

• Absorption of water and electrolytes from chyme

• Storage of feces

Which component of the Enteric Nervous System (ENS) is primarily responsible for regulating secretions from the gastrointestinal organs and local blood flow?

A. Submucosal plexus (plexus of Meissner)

B. Vagus nerve fibers

C. Myenteric plexus (plexus of Auerbach)

D. Sensory neurons in the mucosal epithelium

A. Submucosal plexus

Gastric emptying is a regulated process. Which of the following conditions would tend to inhibit or slow down gastric emptying?

A. Activation of the parasympathetic nervous system.

B. The release of gastrin and motilin.

C. A large volume of food stretching the stomach wall.

D. The presence of fatty acids and acidic chyme in the duodenum.

D. The presence of fatty acids and acidic chyme in the duodenum

What is a major functions of the bacterial flora residing in the large intestine?

To break down undigested carbohydrates and synthesize certain vitamins like vitamin K