GI Reflexes Overview L8

GI Reflexes

• Three main types of GI reflexes control gastrointestinal function.
• Intrinsic reflexes: These are integrated within the gut wall and control local functions such as secretion, peristalsis, mixing contractions, and inhibition.
• Extrinsic reflexes: These involve the prevertebral sympathetic ganglia and transmit signals long distances within the GI tract. Examples include:
  • Gastrocolic reflex: Signals from the stomach trigger colonic evacuation.
  • Enterogastric reflex: Signals from the colon and small intestine inhibit stomach motility and secretion.
  • Colonoileal reflex: Signals from the colon inhibit ileal emptying into the colon.
• Central reflexes: These involve the spinal cord or brainstem and regulate GI activity.
  • Reflexes from the stomach and duodenum to the brainstem control gastric motor and secretory activity.
  • Pain reflexes can cause general inhibition of the entire GI tract.
  • The defecation reflex originates in the colon and rectum, travels to the spinal cord, and returns to stimulate contractions for defecation.

Peristaltic Reflex Overview

• The passage of a bolus through the intestinal wall triggers a reflex that constricts the lumen behind the bolus and dilates the lumen ahead.
• This coordinated movement of the longitudinal and circular musculature propels the chyme along the digestive tract.
• Peristalsis is a characteristic of many syncytial smooth muscle cells.
• Stimulation of a point in the gut can cause a contractile ring to form in the circular muscle, which then spreads along the gut tube.
• This phenomenon also occurs in other tissues like the bile duct and ureters.

Peristalsis Mechanism

• Stretching of the intestinal wall stimulates afferent sensory neurons. These mechanosensitive neurons communicate with interneurons, triggering two effects:

  • Proximal to the site of distention: Excitatory motor neurons are stimulated, releasing acetylcholine (Ach) and substance P (SP). This leads to smooth muscle contraction, specifically in the circular muscle layer.
  • Distal to the site of distention: Inhibitory motor neurons are stimulated, releasing nitric oxide (NO), adenosine triphosphate (ATP), and vasoactive intestinal peptide (VIP). This results in smooth muscle relaxation, particularly in the circular muscle layer.

• The direction of chyme propulsion is from the oral (mouth) end towards the aboral (anus) end.

• For the coordinated movement, circular and longitudinal musculature exhibit opposite actions: one contracts while the other relaxes.

Vago-vagal Reflex

• The vagovagal reflex is a gastrointestinal reflex circuit.
• The reflex is coordinated by afferent and efferent fibers of the vagus nerve.
• The dorsal vagal complex in the brain is involved in the reflex.
• Stimulation of mechanical receptors in the gastric mucosa stimulates the vagus afferent fibers.
• The vagovagal reflex is active during receptive relaxation of the stomach in response to food mastication and deglutition.
• Once food is in the stomach, the vagovagal reflex causes active relaxation of smooth muscle in the stomach wall.
• The reflex involves the stomach, brain, and then back to the stomach.

Functional and Motor Subdivision of the Stomach

• The stomach can be divided into two main regions: the distal region and the proximal region.
• The distal region, also known as the antrum or lower part of the corpus, is responsible for grinding and sieving food.
• This function is often referred to as the "gastric pump."
• The distal region breaks down solid food particles into smaller pieces suitable for digestion in the small intestine.
• The proximal region, also known as the fundus or upper part of the corpus, acts as a storage compartment for food.
• It retains food until it is ready to be passed into the small intestine.

Gastric Pump Dynamics

• Inflow into the "gastric pump" is orchestrated by a series of sequential events:

  • Tonic contraction of the fundus (gastric store)
  • Strong peristaltic wave in the corpus
  • Passage into the grinder and then into the pylorus
  • Emptying of fluid and pre-digested particles into the duodenum, with fluids emptying faster than solid particles.

• Interstitial cells of Cajal (ICC) act as pacemakers, generating slow waves in smooth muscle cells.

• Contraction of smooth muscle cells occurs when spike potentials occur. This is crucial for the peristaltic wave and the movement of food through the stomach.

Intestinal Motility Functions

• Mixing and Contact: Intestinal motility mixes food with digestive juices and enhances contact between the intestinal wall and food.
• Propulsion: Peristalsis propels chyme (partially digested food) distally (toward the anus).

Small Intestine Motility Phases

• Digestive Period:
  • Propulsive pattern: peristaltic waves move chyme distally.
  • Peristaltic wave speed varies: fastest in the duodenum, medium in the jejunum, and slowest in the ileum.
• Interdigestive Period:
  • Non-propulsive pattern: segmentation contractions occur.
  • Segmentation contractions: localized contractions of circular muscle, dividing the gut into constricted and dilated segments.
  • Segmentation contractions are important for nutrient absorption when concentration is high, but do not contribute significantly to propulsion.

Intestinal Motility Functions

• Facilitates mixing of food with digestive juices
• Enhances contact between food and the intestinal wall
• Propels chyme (partially digested food) towards the anus through peristalsis

Small Intestine Motility Phases

• Digestive Phase:
  • Characterized by both propulsive and non-propulsive contractions
  • Propulsive Pattern: Peristaltic waves propel chyme aborally (towards the anus)
    • Fastest in the duodenum
    • Moderate speed in the jejunum
    • Slowest in the ileum
  • Non-Propulsive Pattern: Segmentation contractions
    • Localized contractions of circular muscle divide the intestine into constricted and dilated segments
    • Promotes mixing and nutrient absorption
    • Does not contribute to significant aboral propulsion
• Interdigestive Phase:
  • Occurs when the stomach and small intestine are empty (between meals)
  • Characterized by the "Migrating Motoric Complex" (MMC)
    • Sweeps undigested material towards the colon
    • Regulates bacterial population and growth

Migrating Motor Complex (MMC)

• Comprises three phases characterized by distinct contraction patterns
• Phase 1: Resting phase - no contractions.
• Phase 2: Intermittent and irregular contractions, characterized by varying strengths.
• Phase 3: Strong peristaltic contractions originating in the stomach and moving distally.

Large Intestine Motility

• Peristaltic waves move ingesta forward.
• Antiperistaltic waves move ingesta backward, leading to more intense mixing.

Pathologic Contractions

• Oral migrating contractions cause vomiting.
  • A defense mechanism that eliminates gastrointestinal content.
  • Uses striated muscle and non-digestive structures coordinated by the brainstem.
• Aboral migrating contractions cause diarrhea.
  • Increases the frequency or volume of defecation, often due to increased water content.
  • Occurs when there is a mismatch between water secretion and absorption.

Causes and Sequence of Vomiting

• Causes include:
  • Stimuli before food intake (smell, emotions, appearance).
  • Stimuli after food intake (visceral afferents to the vomiting center).
  • Stimuli after absorption (drugs and toxins stimulate the chemoreceptor trigger zone).
• Sequence of events:
  • Antiperistaltic waves from the duodenum propel ingesta toward the stomach.
  • Contraction of the abdominal musculature increases intra-abdominal pressure.
  • Expansion of the chest cavity with a closed glottis lowers intrathoracic pressure.
  • The lower esophageal sphincter relaxes.
  • The upper esophageal sphincter opens.

Types of Diarrhea

• Malabsorptive Diarrhea:
  • Inadequate absorption of secreted water.
  • Can be caused by viral, bacterial, or protozoan infections.
  • Destruction of villi and reduced length lead to decreased surface area and less absorption.
• Secretory Diarrhea:
  • Increased intestinal secretion overwhelms absorptive capacity.
  • Caused by enterotoxins.
  • No destruction of villi, but interference with absorption.

Defecation Reflex

• Involves the internal (smooth muscle) and external (striated muscle) sphincters.
• Internal Sphincter:
  • Para- and sympathetic innervation.
  • Normally tonically contracted, providing continence.
• External Sphincter:
  • Voluntarily constricted, allowing for potty training.
• Rectosphincteric Reflex:
  • Feces accumulate in the rectum, a reservoir.
  • Peristaltic movement of feces into the rectum leads to relaxation of the internal sphincter.
  • Triggers the urge to defecate.