Gastrointestinal Motility

The primary functions of the GI tract are digestion Digestion Digestion refers to the process of the mechanical and chemical breakdown of food into smaller particles, which can then be absorbed and utilized by the body. Digestion and Absorption and absorption Absorption Absorption involves the uptake of nutrient molecules and their transfer from the lumen of the GI tract across the enterocytes and into the interstitial space, where they can be taken up in the venous or lymphatic circulation. Digestion and Absorption, which require coordinated contractions of the smooth muscles present in the GI tract. Peristaltic waves, segmentation contractions, and the migrating motor complex are all important contraction patterns that help to mix contents, get them in contact with the intestinal walls (where they are further digested by brush-border enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body's constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes and absorbed into the enterocytes), and propel material down the tract at appropriate times and in appropriate amounts.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Table of Contents

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Overview of GI Motility

Definition

GI motility refers to the contraction of smooth muscle within the walls of the GI tract, which:

  • Move foodstuff along the tract
  • Mix and grind
  • Store foodstuff (via contracted sphincters, keeping material in its current location)

Important types of movements/contractions

  1. Peristalsis
  2. Segmentation
  3. Sphincter contraction and relaxation

Peristalsis

  • Coordinated, wave-like contractions that move material forward through the tract, ultimately toward the rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal
  • Acetylcholine (ACh) contracts one small area of the tract.
  • NO and vasoactive intestinal peptide (VIP) cause relaxation of the area immediately ahead of the contracting area.
  • This relaxation pushes foodstuff from areas of high pressure (area that is contracting) to areas of low pressure (area that is relaxing).
Peristalsis

Coordination of smooth muscle in peristalsis:
Acetylcholine (ACh) causes constriction of the muscle just behind the foodstuff, while NO and vasoactive intestinal peptide (VIP) cause relaxation ahead of it. This creates a pressure gradient, forcing the food downward. Nerves from the enteric nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System (with input from the autonomic nervous system Autonomic nervous system The ANS is a component of the peripheral nervous system that uses both afferent (sensory) and efferent (effector) neurons, which control the functioning of the internal organs and involuntary processes via connections with the CNS. The ANS consists of the sympathetic and parasympathetic nervous systems. Autonomic Nervous System), coordinate the wave of constriction and relaxation.

Image by Lecturio.

Segmentation

  • Circumferential muscle contractions (i.e., squeezing)
  • Forces some of the contents forward and some backward
  • Segmentation contractions occur in multiple nearby locations all at once.
  • Allows for mixing, churning, and kneading of food material 
  • Occurs in the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach and intestines
Segmentation contractions in the stomach

Segmentation contractions in the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach:
Note how the subsequent contraction achieves mixing of contents and enough increase in pressure to overcome the relaxed pyloric sphincter.

Image by Lecturio.

Sphincter contraction and relaxation

  • Sphincters are circular bands of smooth muscle that are normally contracted at baseline → keep foodstuff from passing into the next segment of the tract 
  • Regulating sphincter relaxation allows for control of movement through the tract.
  • Baseline contraction is maintained by:
    • Enkephalins (neurotransmitters in the enteric nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System (ENS))
    • The ability of smooth muscle to maintain a latch-bridge state: 
      • A state in which myosin is dephosphorylated (no longer cross-bridge cycle), however, remains attached to actin = maintains some tension 
      • Allows the muscle to maintain tone without expending much energy
  • Relaxation is triggered by VIP.

Review of Nervous System Control over GI Smooth Muscle

GI motility is controlled primarily via the parasympathetic nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System (PNS) and the ENS.

Parasympathetic nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System

  • Stimulation of the PNS:
    • ↑ GI motility 
    • ↑ GI secretions
    • ↑ Blood flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure
    • ↓ Constriction of GI sphincters (allows food to move through the tract)
  • Primary neural signaling molecules:
    • ACh
    • VIP

Enteric nervous system Nervous system The nervous system is a small and complex system that consists of an intricate network of neural cells (or neurons) and even more glial cells (for support and insulation). It is divided according to its anatomical components as well as its functional characteristics. The brain and spinal cord are referred to as the central nervous system, and the branches of nerves from these structures are referred to as the peripheral nervous system. General Structure of the Nervous System

A specialized portion of the ANS located within the walls of the GI tract

Consists of:

  • Meissner plexus (also called the submucosal plexus): 
    • Located in the submucosa
    • Controls the muscularis mucosa (independently of the muscular layer of the intestines)
  • Auerbach plexus (also called the myenteric plexus) 
    • Located in the muscularis, between the circular and longitudinal layers 
    • Includes both motor and sensory neurons Sensory neurons Neurons which conduct nerve impulses to the central nervous system. Autonomic Nervous System
    • Provides motor input to the smooth muscle → generates contractions
    • Receives sensory input from chemoreceptors and mechanoreceptors and coordinates appropriate responses (often as a reflex) 
  • Interstitial cells of Cajal (ICCs): 
    • Specialized pacemaker cells located in the muscularis
    • Generate the slow-wave electrical activity responsible for triggering action potentials that result in baseline GI motility
Layers and folds in intestinal walls

Structure of the intestinal walls:
The Meissner plexus is located within the submucosa, and the Auerbach plexus (also known as the myenteric plexus) is located between the circular and longitudinal muscular layers.

Image by Lecturio.

Function: local control of GI function

  • Controls peristalsis and segmentation
  • Stimulates the secretion of regulatory hormones Hormones Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body. Hormones play critical roles in coordinating cellular activities throughout the body in response to the constant changes in both the internal and external environments. Hormones: Overview and neurotransmitters (see sections below)

Control and regulation of motility

  • The ICCs control the rate of smooth muscle contraction Smooth muscle contraction Smooth muscle is primarily found in the walls of hollow structures and some visceral organs, including the walls of the vasculature, GI, respiratory, and genitourinary tracts. Smooth muscle contracts more slowly and is regulated differently than skeletal muscle. Smooth muscle can be stimulated by nerve impulses, hormones, metabolic factors (like pH, CO2 or O2 levels), its own intrinsic pacemaker ability, or even mechanical stretch. Smooth Muscle Contraction via slow (electrical) waves: 
    • Refers to the slowly changing membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential of pacemaker cells (e.g., ICCs)
    • Pacemaker cells do not have a stable resting membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential.
    • Instead, specific ion channels allow for a slow flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure of ions (i.e., current) across the membrane → slowly brings the membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential closer to threshold for depolarization
    • If threshold is reached, an action potential (AP) occurs → muscle cells contract
    • These contractions are known as slow-wave contractions.
  • Frequencies of contractions:
    • Stomach: 3–5 contractions per minute
    • Small intestine Small intestine The small intestine is the longest part of the GI tract, extending from the pyloric orifice of the stomach to the ileocecal junction. The small intestine is the major organ responsible for chemical digestion and absorption of nutrients. It is divided into 3 segments: the duodenum, the jejunum, and the ileum. Small Intestine: 12–20 contractions per minute
    • Large intestine Large intestine The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix: 6–8 contractions per minute
  • Frequency of contractions does not change significantly with regulation, but strength of contraction does.
  • Smooth muscle contractions are graded: 
    • ↑ Release of Ca2+ from the sarcoplasmic reticulum (SR) in response to an AP → stronger contraction
    • Amount of Ca2+ released from the SR can be regulated by the ANS and hormones Hormones Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body. Hormones play critical roles in coordinating cellular activities throughout the body in response to the constant changes in both the internal and external environments. Hormones: Overview
  • The ENS receives input from:
    • Vagus nerve (parasympathetic, stimulatory → ↑ motility)
    • Prevertebral ganglia (sympathetic, inhibitory → ↓ motility)
Slow electrical waves

Schematic depiction of slow electrical waves and how they affect smooth muscle tone Muscle tone The state of activity or tension of a muscle beyond that related to its physical properties, that is, its active resistance to stretch. In skeletal muscle, tonus is dependent upon efferent innervation. Skeletal Muscle Contraction:
If the peak of the wave reaches the threshold required for an action potential (seen as a vertical spike in the membrane potential Membrane potential The membrane potential is the difference in electric charge between the interior and the exterior of a cell. All living cells maintain a potential difference across the membrane thanks to the insulating properties of their plasma membranes (PMs) and the selective transport of ions across this membrane by transporters. Membrane Potential), muscle contraction occurs. When action potentials occur back to back in smooth muscle (Stimulated panel), contractions are stronger. If threshold is not reached (Inhibited panel), contractions do not occur.

Image by Lecturio.

Swallowing and Esophageal Motility

Swallowing

Swallowing (also called deglutition) is a complex process involving > 22 muscles in the mouth, pharynx Pharynx The pharynx is a component of the digestive system that lies posterior to the nasal cavity, oral cavity, and larynx. The pharynx can be divided into the oropharynx, nasopharynx, and laryngopharynx. Pharyngeal muscles play an integral role in vital processes such as breathing, swallowing, and speaking. Pharynx, and esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus, which moves food in the mouth through the upper esophageal sphincter and into the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus.

  • The tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Oral Cavity: Lips and Tongue shapes and lubricates the bolus.
  • Initiation of swallowing is a voluntary process → tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Oral Cavity: Lips and Tongue voluntarily moves the bolus toward the back of the mouth → initiates involuntary reflexive process
  • Mechanoreceptors in the oropharynx are activated.
  • Afferents travel through the glossopharyngeal nerve (IX) to the swallowing centers in the medulla oblongata and pons.
  • Motor efferents travel through the vagus nerves (X) → stimulate the pharyngeal muscles:
    • Base of the tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Oral Cavity: Lips and Tongue compresses against the hard palate Hard palate The anteriorly located rigid section of the palate. Oral Cavity: Palate → closes off the oropharynx from the rest of the mouth
    • Soft palate Palate The palate is the structure that forms the roof of the mouth and floor of the nasal cavity. This structure is divided into soft and hard palates. Oral Cavity: Palate moves upward → closes off the nasopharynx
    • Laryngeal muscles move forward and upward → close the airways
  • Upper esophageal sphincter muscles first relax and then contract → force foods down into the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus and then close to prevent immediate reflux
  • Stretch in the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus triggers esophageal peristaltic waves
Swallowing

Swallowing:
First, the tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Oral Cavity: Lips and Tongue compresses the food against the hard palate Hard palate The anteriorly located rigid section of the palate. Oral Cavity: Palate (note the glottis is open). Next, the tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Oral Cavity: Lips and Tongue forces the bolus to the back of the mouth; the soft palate Palate The palate is the structure that forms the roof of the mouth and floor of the nasal cavity. This structure is divided into soft and hard palates. Oral Cavity: Palate closes off the nasopharynx. Finally, as the foodstuff is forced downwards, the epiglottis is pulled forwards, closing the glottis (airway), and the upper esophageal sphincter relaxes, letting food into the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus. Peristaltic waves move the food down towards the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach.

Image by Lecturio.

Normal esophageal motility

  • The 1st ⅓ of the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus has a striated muscle layer.
  • The remaining ⅔ has a smooth muscle layer.
  • Primary peristalsis: 
    • 1st wave of esophageal peristalsis that begins as food enters the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus
    • Coordinated by the PNS and ENS
  • Secondary peristalsis: a 2nd wave of peristalsis responsible for:
    • Clearing the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus of remnants of the foodstuff
    • Removing acidic content that may have refluxed 
  • Relaxation of the lower esophageal sphincter and upper stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach allowing the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach to accept food (mediated by VIP)
Esophagus innervation

Innervation of the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus:
Note how the swallowing center integrates striated and smooth muscle activity.

Image by Lecturio.

Vomiting

  • Induced by:
    • Excessive stretching of the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach
    • Psychological stimuli
    • Chemical irritants (e.g., bacterial toxins, alcohol)
  • Controlled by the emetic center in the medulla, which stimulates:
    • Relaxation of the lower esophageal sphincter
    • Contraction of the diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm and abdominal muscles
  • These muscles compress the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach → force food back up the esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus

Gastric Motility

  • Regions of the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach and their functions:
    • Lower esophageal sphincter: controls movement between the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach and esophagus Esophagus The esophagus is a muscular tube-shaped organ of around 25 centimeters in length that connects the pharynx to the stomach. The organ extends from approximately the 6th cervical vertebra to the 11th thoracic vertebra and can be divided grossly into 3 parts: the cervical part, the thoracic part, and the abdominal part. Esophagus 
    • Cardia: relaxes to allow food entry into the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach 
    • Fundus and body: 
      • Act as a reservoir
      • Contain ICCs: pacemaker cells triggering regular contractions in the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach
    • Antrum: mixing and grinding 
    • Pylorus: controls release into the duodenum
  • Reception–relaxation response: 
    • A property of GI smooth muscle cells
    • The stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach briefly resists stretching → reception–relaxation reflex then triggers it to relax
  • Segmentation: back and forth movement of contents, causing mixing 
  • Peristaltic movement:
    • Triggered by ICCs 3‒5 times per minute (approximately every 15 sec) 
    • Causes a pressure wave starting at the fundus → pylorus
    • Pressure increases enough to overcome the pyloric valve and move a small amount (approximately 3 mL) of chyme into the duodenum with each wave
  • Gastric emptying:
    • Contents leave the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach after differing lengths of time (in order from fastest to slowest):
      • Carbohydrates Carbohydrates Carbohydrates are one of the 3 macronutrients, along with fats and proteins, serving as a source of energy to the body. These biomolecules store energy in the form of glycogen and starch, and play a role in defining the cellular structure (e.g., cellulose). Basics of Carbohydrates > proteins > lipids Lipids Lipids are a diverse group of hydrophobic organic molecules, which include fats, oils, sterols, and waxes. Fatty Acids and Lipids 
      • Liquids > solids
      • High-osmolality contents (i.e., ↑ Na+ content) > lower-osmolality content
    • Typical gastric emptying time: 
      • 4 hours for a typical meal
      • Up to 6 hours for salty, high-fat meals
Regions of the stomach and their functions

Regions of the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach and their functions:
In the body of the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach, the interstitial cells of Cajal are the pacemaker cells triggering regular peristaltic movements.

Image by Lecturio.

Intestinal Motility and Defecation

Segmentation

Mixes chyme:

  • With bile and intestinal and pancreatic juices
  • To ensure adequate contact with the intestinal wall and its membrane-bound proteins:
    • Brush-border digestive enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body's constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes 
    • Absorptive transport proteins

Migrating motor complex (MMC)

  • Describes a pattern of internal electrical stimulation through the intestines that triggers large peristaltic waves
  • These waves propel residual material down the tract:
    • The 1st wave begins in the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach or duodenum → travels approximately 10–70 cm before dying out
    • The 2nd wave begins slightly farther down the tract from where the 1st one started.
    • The 3rd wave begins even farther down than the 2nd one, and so on.
  • “Milks” remaining content toward the colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix
  • Takes about 2 hours to move through the entire intestinal tract
  • Occurs during fasting (known as the interdigestive phase)
  • A 2nd MMC may occur:
    • Moves bacteria Bacteria Bacteria are prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission. Some of these organisms play a significant role in the pathogenesis of diseases. Bacteriology: Overview from the small intestine → large intestine
    • MMC dysfunction can lead to small intestinal bacterial overgrowth
  • Phases of the MMC:
    • Phase 1: quiescence
    • Phase 2: increasing number of intermittent, nonpropulsive contractions
    • Phase 3: 5–10 minutes of strong, propagating contractions
    • Phase 4: declining activity before quiescence of phase 1
  • Motilin: hormone thought to initiate an MMC
Waves of the migrating motor complex

A graph showing the waves of the migrating motor complex during fasting:
The red dotted line shows the phase 3 waves.

Image by Lecturio.

Defecation

The rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal contains stretch receptors that stimulate the defecation reflex when the rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal begins to fill with feces.

  • Stretch receptors activated in rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal → 
  • Sensory nerve fibers carry signal to the sacral spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord → 
  • Synapse Synapse The junction between 2 neurons is called a synapse. The synapse allows a neuron to pass an electrical or chemical signal to another neuron or target effector cell. Synapses and Neurotransmission with parasympathetic motor fibers → 
  • Send signals for peristaltic waves to the myenteric nerve plexus in the muscular layers of:
    • Descending colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix
    • Sigmoid colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix
    • Rectum
    • Internal anal sphincter
  • Peristaltic contractions in the colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix and rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal move feces downward.
  • Internal anal sphincter relaxes.
  • Defecation occurs only if the external anal sphincter is voluntarily relaxed at the same time via motor impulses from the cerebral cortex Cerebral cortex The cerebral cortex is the largest and most developed part of the human brain and CNS. Occupying the upper part of the cranial cavity, the cerebral cortex has 4 lobes and is divided into 2 hemispheres that are joined centrally by the corpus callosum. Cerebral Cortex
  • If defecation is consciously suppressed, peristaltic contractions cease within a few minutes.
The defecation reflex

The defecation reflex:
1. Feces stretch the rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal and stimulate the stretch receptors, transmitting the signal to the spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord.
2. A spinal reflex sends parasympathetic motor signals to the myenteric nerve plexus, resulting in contraction of the smooth muscles within the rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal and pushing feces downward.
3. The same spinal reflex also sends parasympathetic motor signals to relax the internal anal sphincter.
4. Voluntary impulses from the brain prevent defecation by keeping the external anal sphincter contracted. Defecation will occur if voluntary signals allow the external anal sphincter to relax.

Image by Lecturio.

Clinical Relevance

  • Achalasia Achalasia Achalasia is a primary esophageal motility disorder that develops from the degeneration of the myenteric plexus. This condition results in impaired lower esophageal sphincter relaxation and absence of normal esophageal peristalsis. Patients typically present with dysphagia to solids and liquids along with regurgitation. Achalasia: primary esophageal motility disorder that develops from degeneration of the myenteric plexus. This degeneration results in impaired lower esophageal sphincter relaxation and absence of normal esophageal peristalsis. Presentation is typically with dysphagia Dysphagia Dysphagia is the subjective sensation of difficulty swallowing. Symptoms can range from a complete inability to swallow, to the sensation of solids or liquids becoming "stuck." Dysphagia is classified as either oropharyngeal or esophageal, with esophageal dysphagia having 2 sub-types: functional and mechanical. Dysphagia to solids and liquids along with regurgitation. 
  • Gastroparesis: impaired or delayed gastric emptying without evidence of obstruction. Gastroparesis can be idiopathic or due to systemic causes, such as diabetes mellitus Diabetes mellitus Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus or scleroderma Scleroderma Scleroderma (systemic sclerosis) is an autoimmune condition characterized by diffuse collagen deposition and fibrosis. The clinical presentation varies from limited skin involvement to diffuse involvement of internal organs. Scleroderma
  • Irritable bowel syndrome Irritable bowel syndrome Irritable bowel syndrome (IBS) is a functional bowel disease characterized by chronic abdominal pain and altered bowel habits without an identifiable organic cause. The etiology and pathophysiology of this disease are not well understood, and there are many factors that may contribute. Irritable Bowel Syndrome: group of diseases consisting of various GI symptoms (e.g., bloating, diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea, abdominal pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain) without an identifiable organic cause. Some individuals have GI motility abnormalities.
  • Constipation Constipation Constipation is common and may be due to a variety of causes. Constipation is generally defined as bowel movement frequency < 3 times per week. Patients who are constipated often strain to pass hard stools. The condition is classified as primary (also known as idiopathic or functional constipation) or secondary, and as acute or chronic. Constipation: symptom that is generally defined as bowel movement frequency < 3 times per week. Constipated individuals often pass hard stools with straining.
  • Hirschsprung disease Hirschsprung Disease Hirschsprung disease (HD), also known as congenital aganglionosis or congenital megacolon, is a congenital anomaly of the colon caused by the failure of neural crest-derived ganglion cells to migrate into the distal colon. The lack of innervation always involves the rectum and extends proximally and contiguously over variable distances. M Hirschsprung Disease: also called congenital megacolon Megacolon Megacolon is a severe, abnormal dilatation of the colon, and is classified as acute or chronic. There are many etiologies of megacolon, including neuropathic and dysmotility conditions, severe infections, ischemia, and inflammatory bowel disease. Megacolon. Hirschsprung disease Hirschsprung Disease Hirschsprung disease (HD), also known as congenital aganglionosis or congenital megacolon, is a congenital anomaly of the colon caused by the failure of neural crest-derived ganglion cells to migrate into the distal colon. The lack of innervation always involves the rectum and extends proximally and contiguously over variable distances. M Hirschsprung Disease is a condition that occurs as a consequence of the lack of ganglionic cells and ICCs in the last segment of the large intestine, producing permanent contraction of the internal anal sphincter and dilation of the segment that precedes it.
  • Fecal incontinence: unintentional release of fecal mater due to an inability of the anal sphincter complex to maintain adequate tone.

References

  1. Boland, M. (2016). Human digestion—a processing perspective. J Sci Food Agric 96:2275–2283. https://pubmed.ncbi.nlm.nih.gov/26711173/
  2. Cheng, L.K., et al. (2010). Gastrointestinal system. Wiley Interdiscip Rev Syst Biol Med 2:65–79. https://wires.onlinelibrary.wiley.com/doi/abs/10.1002/wsbm.19
  3. Konturek, P.C., Brzozowski, T., Konturek, S.J. (2011). Stress and the gut: pathophysiology, clinical consequences, diagnostic approach and treatment options. J Physiol Pharmacol 62:591–599. https://pubmed.ncbi.nlm.nih.gov/22314561/
  4. Kusano, M., et al. (2014). Gastrointestinal motility and functional gastrointestinal diseases. Curr Pharm Des 20:2775–2282. https://pubmed.ncbi.nlm.nih.gov/23886379/
  5. Sanders, K.M., Koh, S.D., Ro, S., Ward, S.M. (2012). Regulation of gastrointestinal motility—insights from smooth muscle biology. Nat Rev Gastroenterol Hepatol 9:633–645. https://www.nature.com/articles/nrgastro.2012.168

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