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Questions and Answers
What is the primary function of the gastro-colic reflex?
What is the primary function of the gastro-colic reflex?
Which hormone primarily inhibits gastric motility when food enters the small intestine?
Which hormone primarily inhibits gastric motility when food enters the small intestine?
What type of movements in the GI tract are referred to as peristalsis?
What type of movements in the GI tract are referred to as peristalsis?
During which reflex does the brain send signals back to the stomach via the vagus nerve?
During which reflex does the brain send signals back to the stomach via the vagus nerve?
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What characterizes the 'Law of the Gut'?
What characterizes the 'Law of the Gut'?
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What initiates the increased action of the pyloric pump during gastric wall distension?
What initiates the increased action of the pyloric pump during gastric wall distension?
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Which factor is NOT associated with the inhibition of the pyloric pump?
Which factor is NOT associated with the inhibition of the pyloric pump?
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What characteristic of mixing contractions in the small intestine is true?
What characteristic of mixing contractions in the small intestine is true?
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Which hormone is known to inhibit intestinal motility?
Which hormone is known to inhibit intestinal motility?
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What triggers peristaltic rush in the intestines?
What triggers peristaltic rush in the intestines?
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What is the primary function of the ileocecal valve?
What is the primary function of the ileocecal valve?
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What initiates the intrinsic defecation reflex?
What initiates the intrinsic defecation reflex?
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Which of the following statements about mass movements is true?
Which of the following statements about mass movements is true?
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Which mechanism assists in the relaxing of the internal anal sphincter during defecation?
Which mechanism assists in the relaxing of the internal anal sphincter during defecation?
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What is primarily absorbed in the colon?
What is primarily absorbed in the colon?
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What role does the gastro-ileal reflex play in intestinal movement?
What role does the gastro-ileal reflex play in intestinal movement?
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What distinguishes haustrations in the colon's movements?
What distinguishes haustrations in the colon's movements?
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When do mass movements typically occur after a meal?
When do mass movements typically occur after a meal?
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What primarily initiates muscle contractions in the gastrointestinal tract?
What primarily initiates muscle contractions in the gastrointestinal tract?
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What is the frequency of slow waves in the duodenum?
What is the frequency of slow waves in the duodenum?
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What causes muscle fibers to become more excitable?
What causes muscle fibers to become more excitable?
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Which type of gastrointestinal electrical wave is responsible for true action potentials?
Which type of gastrointestinal electrical wave is responsible for true action potentials?
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What is the primary function of the Myenteric plexus?
What is the primary function of the Myenteric plexus?
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How do calcium ions influence muscle contractions in the gastrointestinal tract?
How do calcium ions influence muscle contractions in the gastrointestinal tract?
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Which neurotransmitter is primarily associated with sympathetic inhibition in the gastrointestinal tract?
Which neurotransmitter is primarily associated with sympathetic inhibition in the gastrointestinal tract?
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What does the term 'tonic contraction' refer to in gastrointestinal smooth muscles?
What does the term 'tonic contraction' refer to in gastrointestinal smooth muscles?
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What effect does sympathetic stimulation have on the enteric nervous system (ENS)?
What effect does sympathetic stimulation have on the enteric nervous system (ENS)?
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Which structure lies within the submucosa to integrate sensory signals from the gastrointestinal epithelium?
Which structure lies within the submucosa to integrate sensory signals from the gastrointestinal epithelium?
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Which of the following factors decreases the excitability of muscle fibers in the gastrointestinal tract?
Which of the following factors decreases the excitability of muscle fibers in the gastrointestinal tract?
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What is the primary result of slow wave activity in the gastrointestinal muscles?
What is the primary result of slow wave activity in the gastrointestinal muscles?
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Which statement about the enteric nervous system is correct?
Which statement about the enteric nervous system is correct?
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What is the main function of peristaltic contractions in the intestines?
What is the main function of peristaltic contractions in the intestines?
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Which stage of swallowing is completely involuntary?
Which stage of swallowing is completely involuntary?
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What role does the lower esophageal sphincter serve?
What role does the lower esophageal sphincter serve?
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During the esophageal stage of swallowing, food is moved to the stomach through:
During the esophageal stage of swallowing, food is moved to the stomach through:
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What is most responsible for mixing food in the stomach?
What is most responsible for mixing food in the stomach?
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What condition occurs if the lower esophageal sphincter does not relax satisfactorily?
What condition occurs if the lower esophageal sphincter does not relax satisfactorily?
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What physiological change occurs when food stretches the stomach?
What physiological change occurs when food stretches the stomach?
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Which type of peristalsis continues until all food is in the stomach?
Which type of peristalsis continues until all food is in the stomach?
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What is chyme?
What is chyme?
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How frequently do mixing waves occur in the stomach?
How frequently do mixing waves occur in the stomach?
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What happens to the food in the stomach over time?
What happens to the food in the stomach over time?
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What type of muscle contractions occur every few centimeters in the intestines?
What type of muscle contractions occur every few centimeters in the intestines?
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What triggers the swallowing reflex during the pharyngeal stage?
What triggers the swallowing reflex during the pharyngeal stage?
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What is the role of the vocal cords during the pharyngeal stage?
What is the role of the vocal cords during the pharyngeal stage?
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Study Notes
Overview of GI motility
- GI smooth muscle functions as a syncytium
- Longitudinal muscle layer extends longitudinally down the intestinal tract
- Circular muscle layer extends around the gut
- Fibers within each bundle are connected through gap junctions
- Electrical signals initiating muscle contractions travel readily among fibers
Electrical Activity of GI Smooth Muscle
- GI smooth muscle exhibits continual, intrinsic electrical activity along muscle fiber membranes
- Two basic types of electrical waves:
- Slow waves
- Rhythmical gastrointestinal contractions
- Low changes in the resting membrane potential
- Intensity: 5-15 millivolts
- Frequency: 3/min in the stomach, 8-9/min in the terminal ileum, 12/min in the duodenum
- Caused by interactions between smooth muscle cells and interstitial cells of Cajal (electrical pacemakers)
- Do not cause muscle contraction except in the stomach
- Excite the appearance of intermittent spike potentials that excite muscle contraction
- Spike potentials
- True action potentials
- Occur automatically when the resting membrane potential is more positive than −40 millivolts
- Frequency: 1-10/sec; higher slow wave potential correlates with greater spike frequency
- Duration: 10-20 milliseconds.
- Generated by calcium-sodium channels
- Large numbers of calcium ions enter along with smaller numbers of sodium ions
- Channels are slower to open and close, resulting in a longer action potential duration
- Slow waves
- Resting membrane potential (−56 millivolts)
- Depolarization (less negative) -> muscle fibers become more excitable
- Stretching of the muscle or stimulation by acetylcholine and specific GI hormones
- Hyperpolarization (more negative) -> muscle fibers become less excitable
- Effect of norepinephrine or epinephrine and stimulation of the sympathetic nerves
- Depolarization (less negative) -> muscle fibers become more excitable
- Calcium Ions and Muscle Contraction
- Significant quantities of calcium ions enter the fibers during spike potentials, causing muscle contraction
- Slow waves do not cause calcium ion entry into smooth muscle fibers, therefore no contractions
- Tonic Contraction of Some Gastrointestinal Smooth Muscle
- Continuous contraction that is not associated with the basic electrical rhythm
- Intensity varies and can last several minutes to hours
- Caused by continuous repetitive spikes, hormones, and continuous calcium entry
The Enteric Nervous System
- Entire nervous system of the GI tract
- 100 million neurons
- Located in the gut wall
- Controls GI movements and secretion
- Consists of two plexuses:
- Myenteric/Auerbach’s plexus
- Outer layer, located between longitudinal and circular muscle layers
- Submucosal/Meissner’s plexus
- Inner layer, located in the submucosa
- Myenteric/Auerbach’s plexus
- Sympathetic and parasympathetic fibers connect to both plexuses, enhancing or inhibiting ENS functions.
- Sensory nerve endings from the GI epithelium send signals through afferent fibers in both plexuses
- SNS prevertebral ganglia located in the spinal cord
- Vagus nerves elicit local gut reflexes or reflexes relayed to the gut from prevertebral ganglia or the basal regions of the brain
Differences between Plexuses
- Myenteric Plexus
- Linear chain of neurons extending through the entire GI tract
- Controls muscle activity
- Increased gut wall tonic contraction
- More intense rhythmical contractions
- Increased rate of rhythm of contraction
- Increased velocity of excitatory waves
- More rapid movement of the gut
- More rapid peristaltic waves
- Partial inhibitory effect (VIP) on pyloric sphincter and ileocecal valve sphincter
- Submucosal Plexus
- Controls functions of the inner wall of each segment of the intestine
- Integrates sensory signals from the epithelium to control:
- Local intestinal secretion
- Local absorption
- Local contraction of the submucosal muscle
- Infolding of the GI mucosa
Autonomic Control of the GI Tract
- Parasympathetic Nervous System:
- Cranial fibers (Vagus nerves): Esophagus, stomach, pancreas, small intestine, first half of colon
- Sacral fibers (Pelvic nerves): Distal half of colon
- Parasympathetic stimulation increases ENS activity (promoting defecation)
- Sympathetic Nervous System:
- Pre-ganglionic fibers: T5-L2 sympathetic chains, Celiac ganglion, Mesenteric ganglia
- Post-ganglionic fibers innervate the entire gut
- Sympathetic stimulation inhibits ENS activity
- Norepinephrine directly affects smooth muscle and indirectly affects ENS neurons by inhibiting them
Gastrointestinal Reflexes
- Reflexes entirely integrated in the ENS:
- Secretion, peristalsis, mixing contractions
- Reflexes transmitting signals long distances to other GI areas:
- Gastro-colic reflex: Colon evacuation induced by food presence in the stomach
- Entero-gastric reflex: Inhibition of gastric motility when food passes into the small intestine
- Colono-ileal reflex: Inhibition of ileal emptying into the colon
- Reflexes involving the spinal cord or brain stem:
- Stomach/duodenum -> brain stem -> stomach via vagus nerve: Controls gastric motility and secretion
- Pain reflexes: Inhibition of the GI tract
- Colon/rectum -> spinal cord -> colon/rectum: Defecation reflex (powerful colonic, rectal, and abdominal contractions)
Hormonal Control of GI Motility
- Gastrin
- Cholecystokinin
- Secretin
Functional Types of Movements in the GI Tract
- Propulsive movements (Peristalsis)
- Stimulated by gut distension
- Contractile ring appears 2-3 cm behind and moves forward (5-10 cm)
- Mediated by the myenteric plexus
- "Law of the Gut": Receptive relaxation (gut relaxes several cm downstream allowing easier food propulsion)
- Mixing movements
- Peristaltic contractions cause mixing when forward progression is blocked by a sphincter
- Local intermittent constrictive contractions occur every few cm (lasting 5-30 sec)
- New constrictions at other points result in chopping and shearing movements
Propulsion and Mixing of Food
- Ingestion of Food:
- Mastication (chewing)
- Breaks down indigestible cellulose membranes
- Increases surface area for digestive enzyme action
- Prevents excoriation of the oral mucosa
- Facilitates food transition through the GI tract
- Swallowing:
- Voluntary stage initiates swallowing
- Pharyngeal stage (involuntary) moves food into the esophagus
- Esophageal stage (involuntary) moves food into the stomach
- Mastication (chewing)
Pharyngeal Stage of Swallowing
- Palatopharyngeal folds approximate to form a sagittal slit for passage of food
- Muscular wall of the pharynx contracts, propelling food by peristalsis into the esophagus
- Upper esophageal sphincter relaxes
- Epiglottis swings backward to prevent food entry into the trachea
- Larynx pulls upward and anteriorly
Nervous Initiation of the Pharyngeal Stage of Swallowing
- Stimulation of epithelial swallowing areas (mainly on the tonsillar pillars)
- Triggers the trigeminal and glossopharyngeal nerves to send signals to the medulla oblongata
- Signals travel through the tractus solitarius to the reticular substance of the medulla and lower portion of the pons (deglutition or swallowing center)
- Deglutition center activates the 5th, 9th, 10th, and 12th cranial nerves, triggering pharyngeal and upper esophageal muscle movements
- Pharyngeal stage of swallowing is a reflex act initiated by voluntary food movement to the back of the mouth
- Sensory stimulation of the pharynx elicits the swallowing reflex.
Esophageal Stage of Swallowing
- Conducts food quickly from the pharynx to the stomach
- Primary peristalsis: Continuation of the peristaltic wave from the pharynx to the stomach (lasting 8-10 sec)
- Secondary peristalsis: Initiated by the myenteric NS and reflexes that begin in the pharynx and are relayed via vagal afferent fibers to the medulla. These signals are then transmitted back to the esophagus through glossopharyngeal and vagal efferent nerve fibers, continuing until all food is in the stomach.
- This is a response to esophageal distension by retained food.
Lower Esophageal Sphincter (LES)
- Circular muscle at the lower end of the esophagus extending upward about 3 cm from the stomach
- Maintains tonic constriction to prevent reflux of stomach contents into the esophagus
- Receptive relaxation occurs when a peristaltic wave arrives at its level, allowing easy food propulsion into the stomach.
- Achalasia: LES fails to relax sufficiently.
Motor Functions of Stomach
- Storage of large food quantities until it can be processed by the GI tract
- Mixing of food with gastric secretions to create a suitable semifluid chyme
- Slow emptying of chyme from the stomach into the small intestine at a rate suitable for digestion and absorption
Basic Gastric Anatomy
- The stomach is divided into the cardia, fundus, body, and antrum.
- The pyloric sphincter controls the flow of chyme from the stomach into the duodenum.
Storage Function of Stomach
- Food forms concentric circles in the stomach, with the newest food closest to the esophagus.
- Stretching of the stomach triggers the vagovagal reflex, reducing muscular wall tone and accommodating increasing food quantities (up to 0.8 - 1.5 liters)
Mixing and Propulsion of Food in the Stomach
- Basic electrical rhythm (BER):
- Digestive juices come into immediate contact with stored food.
- Mixing waves move from the mid to upper stomach to the antrum every 15-20 seconds.
- More intense in the antrum, forcing contents under high pressure towards the pylorus.
- Retropulsion: Antral contents squeezed upstream towards the stomach body due to pyloric contraction when the wave arrives (contents are not small enough to pass through the pylorus).
- Chyme: Mixture of food and secretions that passes into the small intestine, forming a murky semifluid or paste.
- Hunger contractions: Intense contractions when the stomach is empty for several hours.
- Rhythmical peristaltic contractions in the stomach body, potentially causing mild pain.
- Begin 12-24 hours after the last food ingestion.
- Peak intensity 3-4 days into starvation.
Stomach Emptying
- Intense antral peristaltic contractions: Pyloric pump
- Decreased constriction of the pyloric sphincter: Allows passage of food particles mixed in chyme to fluid consistency.
Regulation of Stomach Emptying
- Gastric factors:
- Gastric wall distention: Increases pyloric pump action
- Gastrin secreted by the antral mucosa: Increases pyloric pump action
- Duodenal factors:
- Entero-gastric Nervous Reflexes: Inhibit the pyloric pump, increase pyloric sphincter tone. These reflexes respond to:
- Duodenal distention
- Irritated duodenal mucosa
- Acidity and osmolality of the chyme
- Breakdown products in the chyme
- Cholecystokinin (CCK): secreted by the duodenum, inhibits gastric emptying.
- Entero-gastric Nervous Reflexes: Inhibit the pyloric pump, increase pyloric sphincter tone. These reflexes respond to:
Movements of the Small Intestine
- Mixing contractions:
- Induced by wall distension
- Segmentation of the small intestine: Creates a chain of sausages
- Chopping the chyme 2-3 times per minute, with a maximum frequency of 12/min
- Propulsive movements:
- Peristaltic waves stimulated by:
- Gastro-enteric reflex
- Gastrin, CCK, insulin, motilin, serotonin
- Gastro-ileal reflex
- Secretin and glucagon inhibit intestinal motility
- Peristaltic rush: Powerful and rapid peristalsis in response to intense mucosal irritation (e.g., severe infection)
- Peristaltic waves stimulated by:
Function of the Ileocecal Valve
- Prevents backflow of fecal matter from the colon
- Protrudes into the lumen and closes forcefully when excess pressure exists in the cecum
- Ileocecal sphincter: Thickened circular muscle located several cm upstream from the ileocecal valve
- Mildly constricted, but the gastro-ileal reflex intensifies ileal peristalsis, facilitating emptying of ileal contents into the cecum, promoting absorption (1.5-2 liters/day).
Principals Functions of the Colon
- Absorption of water and electrolytes from chyme to form solid feces
- Storage of fecal matter until expulsion.
Movements of the Colon
- Mixing movements (Haustrations):
- Baglike sacs due to combined contractions of circular and longitudinal muscle strips (teniae coli)
- Peak intensity lasts 30 sec, disappears during the next 60 sec
- Minor amount of forward propulsion, but allows for greater fluid absorption
- Expels 80-200 ml of feces daily
- Propulsive movements (Mass Movements):
- Last 8-15 hours from the ileocecal valve to the rectum
- Responsible for further fecal propulsion
- Occur 1-3 times daily, usually 15 minutes after breakfast
- Driven by a constrictive ring in response to distension or irritation in the colon
- 20 cm or more of colon distal to the ring contracts as a unit, propelling "en masse"
- Persist for 10-30 minutes
- Stimulated by: Gastrocolic reflex, duodenocloic reflexes, irritation
Defecation Reflexes
- Intrinsic reflex:
- Initiated when feces enter the rectum
- Spread through the myenteric plexus
- Initiates peristaltic waves in the descending colon, sigmoid, and rectum
- Results in anal sphincter relaxation (both internal and external) promoting defecation
- Parasympathetic defecation reflex:
- Assists the intrinsic reflex
- Originates in the sacral segments of the spinal cord
- Intensifies peristaltic waves
- Relaxes the intentional anal sphincter
- Effectively empties the colon from the splenic flexure to the anus
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