Control of the GI Tract

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Questions and Answers

What is the primary role of sensory neurons in the enteric nervous system?

Control motility of the GI tract

Receive information from the mucosa (correct)

Regulate secretion of digestive enzymes

Relay information from motor to sensory neurons

Which plexus is responsible for controlling the motor function of the gastrointestinal tract?

Plexus of Auerbach (correct)

Submucosal Plexus

Meissner’s Plexus

Vagus Plexus

Which neurotransmitter is considered excitatory in the enteric nervous system?

GABA

Serotonin

Norepinephrine (NE)

Acetylcholine (ACh) (correct)

What type of neurons in the enteric nervous system act on smooth muscle and endocrine/exocrine cells?

Motor neurons Signup and view all the answers

What is the main function of Meissner’s Plexus in the gastrointestinal tract?

Control secretion of the mucosa Signup and view all the answers

Which neurotransmitter is released by inhibitory fibers in the enteric nervous system?

Nitric Oxide (NO) Signup and view all the answers

What type of control does the autonomic nervous system (ANS) primarily exert on the gastrointestinal tract?

Extrinsic control Signup and view all the answers

How many types of neurons are contained within the enteric nervous system?

Three Signup and view all the answers

What is the primary role of gastric juice in the stomach?

To mediate digestion in the stomach Signup and view all the answers

Which enzyme initiates lipid digestion in the stomach?

Gastric lipase Signup and view all the answers

What do parietal cells in the oxyntic glands secrete?

Intrinsic Factor and HCl Signup and view all the answers

In the stomach, salivary amylase is primarily inactivated by which condition?

Low pH levels Signup and view all the answers

What is the primary secretory function of G cells in the pyloric glands?

Producing gastrin Signup and view all the answers

What stimulates the waves of smooth muscle contraction in the digestive tract?

Interstitial Cells of Cajal Signup and view all the answers

At what interval does the duodenum have its slow waves compared to the ileum?

Every 4-5 seconds Signup and view all the answers

What is the approximate membrane potential (mV) required for an action potential in intestinal smooth muscle?

-40 mV Signup and view all the answers

Which of the following mechanical functions does the stomach perform?

Short-term storage of ingested food Signup and view all the answers

Which part of the digestive tract has the slowest frequency of slow waves?

Colon Signup and view all the answers

What role do gap junctions play in smooth muscle contraction?

They allow for the transmission of depolarization cycles between cells Signup and view all the answers

What happens to membrane potential during stimulation by the sympathetic nervous system in smooth muscle?

It results in hyperpolarization Signup and view all the answers

Which of the following is NOT a function of the stomach?

Absorption of small molecules Signup and view all the answers

What is the primary function of receptive relaxation in the stomach?

To maintain stable intraluminal pressure despite increased volume Signup and view all the answers

Which nervous system primarily controls gastric accommodation and dilation of the proximal stomach?

Enteric nervous system Signup and view all the answers

What happens when the vagus nerve is transected?

Receptive relaxation is impaired Signup and view all the answers

Where do contractions in the stomach primarily begin?

At the mid-stomach Signup and view all the answers

What role does the pyloric sphincter play in gastric emptying?

It regulates the passage of small amounts of food into the intestines. Signup and view all the answers

What is the relationship between volume and pressure in the stomach during receptive relaxation?

Pressure remains stable despite increased volume. Signup and view all the answers

What does the spontaneous electrical rhythm in the distal stomach facilitate?

A coordinated contraction pattern for mixing and emptying Signup and view all the answers

What occurs during the contractions as they approach the pyloric sphincter?

The contractions both increase in force and speed. Signup and view all the answers

What primarily causes the emptying of the stomach?

Coordinated peristaltic contractions in the stomach Signup and view all the answers

How does the fed state affect stomach contractions?

Contractions are continuous Signup and view all the answers

Which hormone is associated with increasing gastric contractions?

Gastrin Signup and view all the answers

What factor most influences the time food spends in the stomach?

Size and composition of food Signup and view all the answers

Which of the following mechanical stimuli can increase contractions in the stomach?

Stretching of the stomach wall Signup and view all the answers

Which physiological state does the stomach exhibit prominent contractions?

In the fed state Signup and view all the answers

Which of the following is a primary factor that triggers hormone release affecting gastric motility?

Composition of food entering the duodenum Signup and view all the answers

In which phase does the stomach generally remain quiescent?

Fasted state Signup and view all the answers

What effect does the sympathetic nervous system primarily have on gastrointestinal motility?

It decreases motility and secretions. Signup and view all the answers

Which neurotransmitter is primarily responsible for the excitatory control of gastrointestinal motility?

Acetylcholine (ACh) Signup and view all the answers

Which of the following statements accurately describes the physiological antagonism between the sympathetic and parasympathetic nervous systems in the GI tract?

The sympathetic system decreases motility while the parasympathetic system increases it. Signup and view all the answers

What is the main source of innervation for the lower gastrointestinal tract?

Pelvic nerve Signup and view all the answers

How does the sympathetic nervous system primarily influence the secretion of gastrointestinal glands?

Inhibits secretion by decreasing glandular activity. Signup and view all the answers

Which component primarily activates gastric lipase in the stomach?

Acidic pH Signup and view all the answers

What primary role does gastrin serve in the gastrointestinal system?

Stimulates secretion of hydrochloric acid Signup and view all the answers

Which cells in the oxyntic glands secrete intrinsic factor critical for vitamin B12 absorption?

Parietal cells Signup and view all the answers

What is the primary composition of gastric juice secreted by the stomach?

Water and hydrochloric acid Signup and view all the answers

Which of the following best describes the process of digestion initiated in the stomach?

Hydrolysis of proteins into small peptides Signup and view all the answers

What is the role of inter-neurons in the enteric nervous system (ENS)?

To relay information between sensory and motor neurons Signup and view all the answers

Which of the following correctly describes the location of the Plexus of Auerbach?

Between the longitudinal and circular smooth muscle Signup and view all the answers

Which type of neuron primarily controls the secretion of the mucosa in the enteric nervous system?

Motor neurons Signup and view all the answers

What kind of control does the autonomic nervous system (ANS) exert over the enteric nervous system?

Integration of both sympathetic and parasympathetic influences Signup and view all the answers

Which neurotransmitter is released by excitatory fibers in the enteric nervous system?

Acetylcholine (ACh) Signup and view all the answers

What is the primary function of Meissner’s Plexus?

Controlling secretion of mucosal glands Signup and view all the answers

Which statement accurately depicts the organization of the enteric nervous system (ENS)?

ENS is composed of two main plexuses along the digestive tract Signup and view all the answers

Which of the following best describes the relationship between the enteric nervous system and the autonomic nervous system?

The enteric nervous system is a complex that relies on both intrinsic and extrinsic control Signup and view all the answers

What primarily influences the time food spends in the stomach?

The composition of the food Signup and view all the answers

Which hormone is responsible for decreasing contractions in the stomach?

Secretin Signup and view all the answers

What mechanism is responsible for the coordinated emptying of the stomach?

Peristaltic contractions in the stomach Signup and view all the answers

Which of the following factors does NOT directly affect gastric motility?

Psychological stress Signup and view all the answers

What physiological process primarily involves the relaxation of the stomach to maintain stable intraluminal pressure despite increased volume?

Receptive relaxation Signup and view all the answers

What occurs in the stomach during the cephalic phase?

Secretion of gastric juices begins Signup and view all the answers

How do intestinal receptors influence gastric emptying?

By altering hormonal release affecting motility Signup and view all the answers

What happens to the ability of the stomach to distend when the vagus nerve is transected?

The stomach's distensibility is impaired. Signup and view all the answers

During mixing and emptying in the stomach, where do contractions primarily begin?

Mid-stomach region Signup and view all the answers

Which condition best describes the state of the stomach in the fasted state?

Quiescence Signup and view all the answers

What role does the pyloric sphincter play in gastric physiology?

It regulates the passage of chyme into the duodenum Signup and view all the answers

What effect does the narrow pyloric sphincter have on gastric emptying?

It restricts the amount of food emptied from the stomach. Signup and view all the answers

What neurotransmitter is suggested to play a role in the process of receptive relaxation in the stomach?

Nitric oxide Signup and view all the answers

Which system is primarily responsible for the regulation of gastric accommodation and proximal stomach dilation?

Enteric nervous system Signup and view all the answers

What is the primary outcome of the contractions that occur as they approach the pyloric sphincter?

Contents are primarily pushed back into the stomach. Signup and view all the answers

What is the relationship between volume and pressure in the stomach during receptive relaxation?

Pressure remains stable despite increasing volume. Signup and view all the answers

Study Notes

Control of the GI Tract

GI tract control is a combination of neural and hormonal factors.
The enteric nervous system (ENS) provides intrinsic control.
The autonomic nervous system (ANS) provides extrinsic control.
Intrinsic factors include the ENS, and extrinsic factors include the ANS and GI hormones.

Enteric Nervous System (ENS)

The ENS is contained entirely within the GI tract.
It has three types of neurons: sensory, motor, and interneurons.
Sensory neurons receive information from the mucosa.
Motor neurons control motility and secretions.
Interneurons relay information from sensory to motor neurons.
Two plexuses, the myenteric (Auerbach's) and submucosal (Meissner's) plexuses, are responsible for controlling specific GI actions.
The myenteric plexus controls motor function.
The submucosal plexus controls the secretion of the mucosa.

Extrinsic Control by the ANS

The ANS plays a role in controlling GI function.
The parasympathetic nervous system (PNS) stimulates GI activity.
The sympathetic nervous system (SNS) inhibits GI activity.

Smooth Muscle Slow Waves

Slow waves of smooth muscle contraction are triggered by the Interstitial Cells of Cajal (ICCs).
ICCs are between the circular and longitudinal smooth muscle layers.
Slow waves cycle through depolarization that doesn't reach threshold.
These slow waves, initiated by ICCs, are passed to smooth muscle through gap junctions.
The frequencies of slow waves in different GI sections vary.

The Stomach - Regions

The stomach has three regions: the oesophagus, proximal stomach (orad), and distal stomach (caudad).
The pyloric sphincter regulates flow between the stomach and the duodenum.

Gastric Motor Function

The stomach's function involves storage of ingested food, mixing contents, and emptying into the intestines.
Receptive relaxation, a vagovagal reflex, relaxes the lower esophagus and proximal stomach, allowing for increased volume without pressure increase.

Mixing & Emptying

The distal stomach shows spontaneous electrical rhythm.
Contractions begin in the mid-stomach and spread distally, with the force and speed increasing as they reach the pyloric sphincter.
This creates mixing, resulting in small amounts of food being emptied with each contraction, while most is pushed back for the next wave.

Factors Affecting Contractions

Gastric contractions change with the presence of food.
Neurohumoral and mechanical stimuli, such as distension, ACh, gastrin, and secretin, impact contractions.

Emptying

Emptying is due to coordinated contractions and relaxation.
The time spent in the stomach depends on the composition of food. Liquids empty quicker than solids.
Composition of food entering the duodenum activates intestinal receptors, triggering hormone release, which in turn affects gastric motility and emptying.

Gastric Juice

The stomach secretes gastric juice, composed mainly of water with HCl, HCO3-, Na+, K+, mucous, and pepsin.
Gastric juice is vital for digestion in the stomach and includes swallowed saliva and refluxed duodenal fluids.

Gastric Digestion

Digestion breaks down macromolecules into smaller, more absorbable components.
Carbohydrate digestion starts in the mouth and continues in the stomach with gastric lipase, which is activated by low pH.
Protein/peptide digestion begins with pepsin, activated by low pH.

Gastric Glands

The stomach contains pyloric and oxyntic glands.
Surface epithelial cells secrete mucous and HCO3- for lubrication and protection.
Pyloric glands are located in the antrum and contain mucous cells, gastrin (G cells), and somatostatin (D cells).
Oxyntic glands are in the fundus and body, containing oxyntic or parietal cells, which secrete HCl and intrinsic factor.

Enteric Nervous System (ENS)

Located entirely within the GI tract
Contains sensory, motor, and inter-neurons
Sensory neurons detect information from the mucosa (muscle stretch, chemical nature)
Motor neurons control motility and secretions (smooth muscle, endocrine/exocrine cells)
Inter-neurons relay information between sensory and motor neurons
Organized into two plexuses:
- Plexus of Auerbach (Myenteric Plexus): Located between the longitudinal and circular smooth muscle, controlling motor function
- Meissner's Plexus (Submucosal Plexus): Located in the submucosa, controlling mucosal secretions
Contains excitatory fibers (releasing acetylcholine) and inhibitory fibers (releasing nitric oxide or vasoactive intestinal peptide)

Extrinsic Control by the ANS

Sympathetic Nervous System:
- Originates from the prevertebral ganglion
- Post-ganglionic fibers synapse with both plexuses, also directly innervate GI smooth muscle/glands
- Primarily releases norepinephrine, decreasing motility and secretions
Parasympathetic Nervous System:
- Vagus nerve (upper GI) and pelvic nerve (lower GI)
- Contains sensory afferents and pre-ganglionic efferents innervating the plexuses
- Releases acetylcholine, increasing contractions and secretions
- Represents physiological antagonism of norepinephrine - "rest and digest"

Receptive Relaxation

Occurs during swallowing, relaxing the lower esophagus and proximal stomach.
Facilitated by the vagovagal reflex, releasing nitric oxide to stabilize intraluminal pressure despite increased volume.
Gastric accommodation and dilation of the proximal stomach also accommodate increased volume without pressure increase.
Controlled by the enteric nervous system, with a marked increase in pressure beyond certain volumes.

Mixing & Emptying

Distal stomach: Has spontaneous or basal electrical rhythm, similar to the small intestine.
Contractions: Begin mid-stomach and spread distally, increasing in force and speed towards the pyloric sphincter/gastroduodenal junction.
Effect: Contractions overtake stomach contents, emptying a small amount due to the narrow pyloric sphincter, while most content is pushed back for the next wave (mixing).

Factors Affecting Contractions

Fasted state: Stomach is quiescent, depending on the phase (e.g., cephalic phase).
Fed state: Contractions are fairly continuous.
Control: Controlled by various neurohumoral and mechanical stimuli:
- Mechanical stimuli: Distension
- Acetylcholine (vagus): Increase number and force of contractions
- Gastrin: Increases contractions
- Secretin: Decreases contractions

Emptying

Mechanism: Coordinated contraction: peristaltic contractions in the stomach, relaxed pylorus, and lack of contraction in the duodenum.
Time in stomach: Depends on composition:
- Liquid empties quickly
- Solids require size reduction, taking longer in the stomach
Duodenal regulation: Composition of food entering the duodenum activates intestinal receptors, releasing hormones that alter gastric motility and emptying (acidic, lipid, osmotic pressure).

Gastric Juice

Secretion: ~1,500 ml/day
Composition: Predominantly water, also includes HCl, HCO3-, other ions, mucous, and pepsin.
Includes swallowed saliva and refluxed duodenal fluids (bile, pancreatic juice).
Mediates digestion in the stomach.

Gastric Digestion

Carbohydrates: Digestion begins in the mouth with salivary amylase, largely inactivated by the acidic stomach.
Lipids: Digestion begins in the stomach with gastric lipase, activated by the low stomach pH.
Proteins/peptides: Digestion starts in the stomach with pepsin, activated by low pH from inactive precursor pepsinogen.

Gastric Glands (Pyloric & Oxyntic)

Surface epithelial cells (throughout stomach): Secrete thick mucous with mucin and HCO3-, lubricates and protects against low pH and enzymes.
Pyloric glands (antrum of stomach):
- Predominantly mucous cells
- Gastrin (G cells): Secretes gastrin, a peptide hormone released into the blood (endocrine function)
- Somatostatin (D cells): Secretes somatostatin, a peptide hormone with endocrine and paracrine actions.
Oxyntic glands (fundus and body):
- Oxyntic or Parietal cells: Secrete HCl (kills bacteria, lowers pH for digestion) and intrinsic factor (vitamin B12 absorption).

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Description

This quiz focuses on the control mechanisms of the gastrointestinal (GI) tract, highlighting the roles of the enteric nervous system (ENS) and the autonomic nervous system (ANS). It delves into intrinsic and extrinsic factors that affect GI function, including specific neuron types and plexuses involved in motility and secretion. Test your knowledge on these vital processes governing digestion.