Control of the GI Tract

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).

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.