GIT Motility Notes
Swallowing (Deglutition)
Definition:
Passage of food from the oral cavity, through the pharynx, and esophagus to the stomach. A complex process involving coordinated muscle actions and neural control.
Occurs after mastication, which converts food into a bolus, a soft, rounded mass suitable for swallowing. Mastication increases the surface area for enzymatic action.
Stages:
Oral Stage: Voluntary passage of food from the mouth to the pharynx. This stage is under conscious control and prepares the bolus for swallowing.
Pharyngeal Stage: Involuntary (reflex) passage of food through the pharynx to the esophagus. This stage involves multiple protective reflexes to prevent aspiration.
Esophageal Stage: Involuntary (reflex) passage of food through the esophagus to the stomach. Peristalsis, a wave-like muscle contraction, propels the food bolus.
Oral (Buccal) Stage
Voluntary; the individual consciously initiates this stage.
A food bolus is voluntarily forced by the tongue into the pharynx. This is achieved through complex tongue movements.
The tongue collects the food, forming a ball (bolus) by mixing with saliva, which aids in lubrication and enzymatic digestion (e.g., amylase).
It presses the bolus upward and backward against the hard palate to push the food into the pharynx. The pressure and movement trigger the subsequent involuntary stages.
Once the bolus reaches the pharynx, swallowing becomes involuntary (automatic) and cannot be stopped (point of no return). This transition marks the beginning of the pharyngeal phase.
Pharyngeal Stage
The movement of the bolus through the pharynx to the esophagus. This stage is rapid and coordinated to ensure food enters the esophagus and not the airway.
Involuntary, occurs reflexively without voluntary control. The swallowing center in the medulla oblongata controls this phase.
Lasts >2 seconds; a very brief yet critical phase.
Food bolus in the pharynx triggers reflex contractions of pharyngeal muscles to:
Direct the bolus into the esophagus. Multiple muscles contract in a coordinated manner to ensure the bolus moves towards the esophagus.
Prevent the bolus from re-entering the mouth, entering the nasal passages, or entering the trachea. This is achieved through elevation of the soft palate and closure of the larynx.
Swallowing Reflex
Stimulus: Food bolus in the pharynx, particularly at the oropharyngeal junction.
Receptors: Mechanoreceptors on tonsillar pillars at the oropharyngeal junction are sensitive to touch and pressure.
Afferent: 5th (trigeminal), and 9th (glossopharyngeal) cranial nerves transmit sensory information to the swallowing center.
Center: Swallowing center in the reticular formation of the medulla coordinates the swallowing reflex.
Efferent: 5th, 9th, 10th (vagus), and 12th (hypoglossal) cranial nerves control the muscles involved in swallowing.
Effector: Muscles of cheeks, tongue, pharynx, and jaw. These muscles contract in a precise sequence.
Response: Contraction of superior, middle, and inferior constrictor muscles of the pharynx and relaxation of the pharyngoesophageal sphincter, allowing food passage to the esophagus via peristalsis. This coordinated action propels the bolus into the esophagus.
Protective Reflexes During the Pharyngeal Phase
Food is prevented from passing to:
Mouth:- Elevation of the tongue blocks the oral cavity.
Larynx & trachea:- Elevation of the larynx to be covered by the epiglottis. The epiglottis acts as a physical barrier.
Approximation of vocal cords and inhibition of breathing (apnea). This prevents aspiration.
Posterior nasal opening:- Elevation of the soft palate seals off the nasopharynx.
At the same time, relaxation of the upper esophageal sphincter occurs to allow the passage of food into the esophagus. This sphincter must relax to allow the bolus to enter the esophagus.
Once the bolus has entered the esophagus:
The upper esophageal sphincter closes, preventing reflux into the pharynx.
Respiratory airways are opened, allowing normal breathing to resume.
Breathing resumes, indicating the end of the pharyngeal phase.
NB: Swallowing is difficult when the mouth is open because it compromises the initial formation and propulsion of the bolus.
Esophageal Stage
The movement of the food bolus through the esophagus to the stomach.
Involuntary, occurs through a reflex without voluntary control.
Mechanism: Peristaltic wave moves food down the esophagus to the stomach. This is a coordinated contraction of the esophageal muscles.
Types of esophageal peristalsis:
Primary esophageal peristalsis.
Secondary esophageal peristalsis.
Esophageal Peristalsis
Primary Peristalsis
Continuation of pharyngeal peristalsis, initiated by the swallowing center.
Starts in the upper esophagus and travels the whole length of the esophagus in 5-9 seconds. This propels the bolus towards the stomach.
Triggered by the swallowing center via vagus nerves. The vagus nerve (cranial nerve X) plays a key role.
Accompanied by relaxation of the lower esophageal sphincter (release of NO and VIP by its neurons). Nitric oxide (NO) and vasoactive intestinal peptide (VIP) are neurotransmitters that cause smooth muscle relaxation.
Responsible for delivering liquid and semifluid foods down to the stomach effectively.
Secondary Peristalsis
Occurs if the primary peristalsis fails to move all the food entered the esophagus into the stomach, as in the case of a large, sticky bolus. It serves as a backup mechanism.
Distention of the esophagus triggers an ENS reflex to move any remaining food from the esophagus to the stomach. The enteric nervous system (ENS) is a local neural network in the esophageal wall.
Peristalsis
A ring-like contraction of the circular smooth muscle that moves forward, pushing the bolus ahead of the contraction. Longitudinal muscles also contract to shorten the esophagus.
Note that:
Secondary peristaltic waves are stronger than the primary ones. This ensures efficient clearance of the esophagus.
They continue until all the food has emptied into the stomach. They are more forceful in clearing the esophagus.
They do not involve the swallowing center, indicating they are primarily controlled by the ENS.
They are mediated by intrinsic plexuses (myenteric and submucosal plexuses) within the esophageal wall.
The upper 1/3 of the esophagus:- Like the pharynx, is striated muscle, allowing for more forceful contractions.
Controlled by the 9th & 10th cranial nerves (glossopharyngeal and vagus).
The lower part of the esophagus:- Is smooth muscle, enabling sustained contractions.
Innervated by the vagus nerves through their connection with the myenteric nerve plexus, facilitating coordinated peristalsis.
Esophagus
Wave-like Muscular Contractions
Circular smooth muscle contracts behind and relaxes in front of the bolus, propelling it forward.
Followed by longitudinal contraction (shortening) of smooth muscle, which aids in widening the esophageal lumen.
After food passes into the stomach, the LES constricts, preventing reflux.
In case of cutting of the vagus to esophagus:
After several days, the myenteric plexus (without support from the vagus) causes strong secondary peristaltic waves in the lower part of the esophagus, compensating for the loss of vagal control.
After cutting of the vagus and paralysis of the swallowing reflex, feeding can be done by introducing food by tube into the lower esophagus to initiate secondary peristalsis, allowing food to pass into the stomach. This demonstrates the ENS's capability to maintain esophageal motility.
Esophageal Secretion
Entirely mucus, secreted by esophageal glands.
Mucus prevents esophageal wall damage by:-
Lubricating the passage of food, reducing friction during swallowing.
Protecting against acid reflux from the stomach into the lower part of the esophagus by neutralizing acidity and forming a protective layer.
No food or absorption, as the esophagus is primarily a conduit.
No digestion in the esophagus, as no significant digestive enzymes are secreted.
Esophageal Sphincters
Upper Esophageal Sphincter (UES)
Between the pharynx & esophagus.
Closed all the time (tonically contracted), preventing air entry into the esophagus.
Relaxes during swallowing to allow the passage of the bolus.
Prevents the passage of inspired air to the stomach, minimizing gastric distension.
Lower Esophageal Sphincter (LES) (Cardiac Sphincter)
Between the esophagus & stomach.
Tonically contracted, maintaining a barrier between the esophagus and stomach.
Relaxes during swallowing, allowing food to enter the stomach.
Prevents the passage (regurgitation) of acid from the stomach to the esophagus, protecting the esophageal mucosa.
May open when the intragastric pressure is increased (after a heavy meal or carbonated drink), leading to transient reflux.
Gastrin hormone increases its tone, reducing the likelihood of reflux.
Abnormalities of LES pressure may cause abnormalities of swallowing or gastric reflux, leading to conditions like GERD or achalasia.
Abnormalities of the Lower Esophageal Sphincter
LES Incompetence
Is due to reduced tone of LOS, leading to increased susceptibility to reflux.
Permits reflux of gastric acid contents into the esophagus, irritating the esophagus, leading to esophageal discomfort and heartburn. This can result in esophagitis.
May occur during pregnancy (due to hormonal effects and increased abdominal pressure) or due to the presence of hiatus hernia (diaphragmatic hernia), where part of the stomach protrudes into the thorax.
Achalasia
Definition: Failure of relaxation of LOS during swallowing, leading to esophageal dilation.
Cause: Damage of neurons of myenteric plexus in LES leading to decreased NO & VIP, impairing smooth muscle relaxation.
Effects: Collection of food in the esophagus leading to slow passage of food to the stomach, dilatation of the esophagus, Dysphagia (= difficulty in swallowing) & aspiration pneumonia may occur due to regurgitation and inhalation of esophageal contents.
Treated by:
Antispasmodic drugs to relax the smooth muscle at LES, providing temporary relief.
Dilation of the sphincter using balloon dilation.
Incision of the esophageal muscle (myotomy) to reduce LES pressure.
Difficulty swallowing (“Food sticks on way down”).
Solids worse than liquids due to the greater resistance they encounter at the LES.
Summary of Swallowing Reflex
Phases:
Oral (voluntary): The initial phase of bolus formation and propulsion.
Pharyngeal (involuntary): A rapid, reflex-mediated phase ensuring safe passage of the bolus past the airway.
Esophageal (involuntary): Peristaltic movement of the bolus through the esophagus into the stomach.
Movement along the esophagus (by peristalsis) and entry of bolus into the stomach are controlled by local and autonomic reflexes, ensuring a coordinated process.
Mucus is secreted onto the epithelial surface of the esophagus to protect and lubricate the esophageal lining.
Stomach
Motor Functions of the Stomach
Receives food from the esophagus.
Stores food (volume when empty ilda 50 mL; when full < 2 L), allowing for intermittent eating patterns.
Breaks up food (grinding) and mixes it with gastric juice to form chyme, a semi-liquid mixture suitable for intestinal digestion.
Delivers chyme at a controlled rate to the duodenum, optimizing digestion and absorption.
NB:
Fundus: Contains mainly gas, with minimal food content.
Body: Thin muscle layers leading to weak peristalsis, primarily involved in food storage without significant mixing.
Antrum: Thicker muscle layers leading to strong peristalsis and food mixing, essential for gastric emptying.
Gastric Motility
Two distinct motility areas:
Proximal Motor Unit (Fundus + Body)
Functions:
Reservoir to store food, accommodating large volumes without significant pressure increases.
Through receptive relaxation:-
Accommodation of a large volume of food, allowing for efficient storage.
Minimal increase in intragastric pressure via a vago-vagal inhibitory reflex, preventing discomfort and reflux.
Distal Motor Unit (Antrum + Pylorus)
Functions:
Mixing of chyme with gastric secretions, enhancing digestion.
Propulsion of chyme towards the pylorus.
Regulation of gastric emptying into the duodenum, controlling the rate of nutrient delivery.
Controlled by complex interactions between vagal reflexes and gastric and intestinal hormones (gastrin, CCK), coordinating gastric motility with digestive processes.
Gastric Motility:
Gastric Filling and Gastric Storage:
Stomach volume: 50 ml when empty, increasing to 1000 ml during a meal. The stomach accommodates the extra volume of food (20-fold change in volume) with little ↑intragastric pressure (<5 mmHg) due to receptive relaxation of the stomach.
Receptive Relaxation:
Food enters the stomach, the fundus and upper part of the stomach relax, the stomach accommodates larger quantities of food up to 1 liter, with no or little ↑in intragastric pressure, preventing discomfort.
Receptive Relaxation:
Partly mediated by:-
Vagus nerve and triggered by the movement of the pharynx & esophagus, coordinating upper GI activity.
ENS, relaxation of the stomach wall when stretched by food, a local adaptive response.
= a vago-vagal inhibitory reflex involving inhibitory transmitters (e.g., NO, VIP), reducing gastric tone.
If > 1 liter of food is consumed, leading to over-distended stomach that causes ↑intragastric pressure, which results in discomfort and potential vomiting.
Gastric Mixing:
Pacemaker cells in the body of the stomach generate slow wave potentials (SWPs) that move down the length of the stomach toward the pyloric sphincter at a rate of 3-4/min. These SWPs determine the frequency of gastric contractions.
Presence of food increases excitability of smooth muscle, SWPs reach the threshold, leading to muscle contraction (peristaltic waves) at a rate of 3-4/min.
Food in the stomach causes a weak peristaltic wave begins in the lower portion of the body that spreads to the antrum & becomes stronger as smooth muscle of the antrum is thicker than that of the body. This wave mixes the food with gastric secretions.
Forces Chyme
Strong Antral peristaltic contractions have 2 functions:
Force chyme under high pressure toward the pylorus, pushing a small amount of chyme through the pyloric sphincter into the duodenum, initiating gastric emptying.
Mix food with gastric secretion to produce chyme, enhancing digestion.
80% of the contractions in the stomach are segmentation contractions: relatively weak contractions that thoroughly mix ingested food with stomach secretions to form chyme, ensuring efficient digestion.
20% of the stomach contractions are of the peristaltic type, involved in propulsion and emptying.
How Peristaltic Contractions Mix Food?
When the peristaltic wave reaches the pylorus:
Forceful pyloric muscle contraction causes the pylorus to close completely, which blocks further passage into the duodenum, preventing excessive emptying.
The forward propelled food is abruptly stopped at and hits the closed sphincter and is returned back into the antrum, the food is propelled forward by a new peristaltic wave and returned back and son. This Retropulsion (forward and backward movement of food) mixes the food thoroughly with gastric secretion to produce chyme, optimizing the mixing process.
Pyloric Sphincter
Tonic contraction of the pyloric sphincter normally keeps it partially opened (not completely closed) to:
Allow and fluid passage from the stomach easily, facilitating rapid emptying of liquids.
Prevent passage of thick chyme, except with strong antral peristaltic contractions that force a few ml of chyme into the duodenum, controlling the rate of emptying.
Gastric Emptying:
Depends on the intensity of the antral peristaltic contractions:
↑intensity of peristalsis causes ↑ emptying rate, increasing chyme delivery to the duodenum.
↓ intensity of peristalsis causes ↓ emptying rate, reducing chyme delivery.
Gastric Emptying of Solids and Liquids after a Mixed Solid-Liquid Meal
Liquids have an average of 20 min, while solids average 2 hr, reflecting different emptying mechanisms.
Factors Regulating Gastric Emptying:
I- Gastric Factors ↑ Emptying:
Volume of Chyme:
↑food volume in the stomach leads to stretch of the stomach, which then leads to ↑gastric motility by:-
Direct effect on gastric smooth muscle excitability, stimulating contraction.
Indirect effect through: local myenteric reflexes, vagus nerve, and gastrin hormone, coordinating gastric activity.
Degree of Fluidity of Chyme: ↑fluidity causes ↑ gastric emptying, as liquids empty more readily.
Liquid emptying is dependent more on pressure across the pyloric sphincter than on the patency of the pyloric sphincter.
II- Duodenal Factors Affecting Gastric Emptying
Enterogastric reflexes:
Presence of chyme in the duodenum results in + multiple inhibitory nervous reflexes that pass from the duodenal wall back to the stomach:
Directly via ENS, providing local control.
Through the inhibitory nerve fibers to the stomach, coordinating distant control.
Results in reduced gastric emptying by:
↓antral peristaltic contractions, reducing propulsion.
↑the tone of the pyloric sphincter, increasing resistance to flow.
Hormonal responses:
Excess acidic or fatty chyme that enters the duodenum causes the release of CCK, secretin, and GIP, resulting in inhibiting gastric motility [CCK is the most potent one]. These hormones coordinate gastric emptying with intestinal processing.
These hormones are transported by the blood to the stomach to ↓antral contraction and gastric emptying.
CCK and secretin increase the tone of the pyloric sphincter.
The function of these reflexes is to maintain an optimal luminal environment for digestion/absorption and to ensure that the rate of emptying of gastric contents is coordinated with digestive activity, preventing overload.
III- Factors Outside GIT:
Emotions: variable effects:-
Sadness & fear: ↓ gastric motility and emptying, reducing digestive activity.
Anger & aggression: ↑ gastric motility and emptying, increasing digestive activity.
Severe pain: ↑sympathetic activity, leading to ↓ gastric motility and emptying, conserving energy.
Hypoglycemia: ↓ glucose utilization in the hypothalamus that results in ↑vagal activity causes ↑ gastric motility, accompanied by hunger pain, stimulating food intake.
Hunger “Pangs”
Occur when the stomach has been empty for > 8 hrs.
Rhythmic peristaltic contractions of the body.
Successive contractions may merge, which causes the contractions to last for 2-3 minutes.
Maximum intensity after fasting for 3-4 days.
Gradually weaken as satiety increases.
Summary of Regulation of Gastric Emptying
Rate of emptying determined by:
Pressure across the pyloric sphincter (largely determined by intragastric pressure), dictating flow rate.
Patency of the pyloric sphincter relative to particle size, controlling particle passage.
Gastric phase stimuli: promote emptying➔
Stretch, which is mediated by gastric volume through local myenteric reflexes, vagus nerve, and gastrin hormone, stimulating motility.
Protein digestion products that cause Gastrin secretion, promoting acid secretion and motility.
Duodenal phase stimuli: inhibit emptying➔
Neural*:- Intrinsic
Extrinsic. distension, irritation - acidity/osmolar change – from upper intestinal epithelium in response to chemical composition of chyme
Endocrine*:- Secretin
GIP
CCK
Vomiting (Emesis)
Definition: Sudden and Forced retrograde expulsion of upper GI contents through the mouth, Controlled by neurons in the medulla (vomiting center). A protective reflex to remove noxious substances.
Conditions Causing Vomiting:
I) Conditions Inside Digestive Tract:
Touching the back of the throat with a finger, dental instrument, or tongue depressor (gag reflex).
Irritation or distension of the stomach or duodenum, triggering local reflexes.
II) Conditions Outside Digestive Tract:
↑intracranial tension (tumor or cerebral hemorrhage), and vomiting following a head injury is a bad sign suggesting intracranial hemorrhage. Elevated pressure stimulates the vomiting center.
Severe pain (Ex: passage of kidney stone), activating autonomic pathways.
Psychogenic vomiting: induced by emotional factors (unpleasant sights, nauseating odors, or stressful situations) which causes vomiting can be conditioned. Psychological stress can trigger vomiting.
+ of Chemoreceptor cells (in CTZ in the medulla) by certain circulating chemical agents (EX: chemotherapy, emetic drugs (apomorphine), and uremia). The CTZ detects toxins in the blood.
Motion sickness: Rapid changes in the direction of the head stimulate receptors in the inner ear, stimulating the vomiting center. Vestibular input from the inner ear triggers vomiting.
Vomiting – Mechanism: A reflex
Receptors: Inside GIT or outside GIT, detecting various stimuli.
Afferent: Autonomic nerves, transmitting signals to the medulla.
The center: Vomiting center in the medulla, coordinating the vomiting reflex.
Efferent: Motor impulses to the upper GIT, diaphragm, and respiratory muscles, executing the vomiting response.
Vomiting: Sequence of Events
Before Vomiting:
Nausea, sweating, salivation, and tachycardia due to generalized + of ANS. These are premonitory signs of vomiting.
During Vomiting:
Deep inspiration causing descent of the diaphragm downwards and stoppage of breathing in that position, increasing abdominal pressure.
Elevation of the soft palate to close the posterior nasal opening, preventing nasal reflux.
Closure of the glottis to close the larynx and trachea, protecting the airway from aspiration.
Relaxation of the stomach & cardiac sphincter, facilitating expulsion.
Contraction of the pyloric region & Simultaneous contraction of abdominal muscles that causes ↑intra-abdominal pressure, squeezing the flaccid stomach. This causes ↑intragastric pressure and expulsion of gastric contents upward.
The duodenum contracts strongly, forcing some of the intestinal contents back into the stomach and out with the vomitus that leads to bile-staining of the vomited material (the yellowish bile entered from the liver and gall bladder into the small intestine).
The vomiting cycle may be repeated several times until the stomach is emptied, ensuring complete removal of noxious substances.
Note:
The stomach itself does not actively participate in the act of vomiting; it is primarily a passive recipient of pressure.
The stomach, esophagus, and LOS are all relaxed during vomiting, facilitating expulsion of contents.
The major force for expulsion of vomitus comes from the contraction of respiratory muscles (diaphragm and abdominal muscles), generating the necessary pressure.
Excessive vomiting results to the loss of gastric acid causing ↓ concentration in the blood that then causes metabolic alkalosis, disrupting acid-base balance.
Vomiting is useful in removing noxious material from the stomach, serving a protective function.
Intestinal Motility
BER generated in duodenal muscle: ilda12/min, setting the pace for intestinal contractions.
Normal motility patterns:
Segmentation: mixing, non-propulsive, enhancing digestion and absorption.
Peristalsis: propulsion (forward movement), moving contents along the intestine.
Contraction of muscularis mucosa: serves to ‘agitate’ fluid around villi, increasing contact with absorptive surfaces.
Both movements:
Initiated by distension, a local mechanical stimulus.
Require enteric nervous pathways, coordinating local control.
Influenced by autonomic NS:-
↑by parasympathetic activity, enhancing motility.
↓ by sympathetic activity, reducing motility.
Influenced by some GI and other hormones, coordinating with digestive processes.
Small Intestinal Motility
1. Peristalsis
Push the contents forward through the digestive tract at varying speeds. The rate of propulsion depends on the function of the different parts.
Esophagus: very rapid food transit, quickly delivering food to the stomach.
Small intestine (major site for digestion and absorption): the contents are moved slowly, allowing for optimal digestion and absorption.
Peristalsis is a reflex response initiated when the gut wall is stretched by the contents of the lumen that travels via the ENS. This ensures coordinated propulsion.
The stretch initiates a circular contraction (contractile ring) (Ach-mediated) behind the stimulus and an area of relaxation (nitric oxide-mediated) in front of it. Oral to caudal direction, propelling the contents of the lumen forward. This is a coordinated contraction and relaxation pattern.
2. Segmentation:
mix contents to promote digestion & absorption. mixing Movements:
Mix food with digestive enzymes promote digestion➔, enhancing enzymatic breakdown.
Make food come in contact with the absorbing surfaces of the digestive tract facilitating absorption➔, increasing nutrient uptake.
Move the chyme slowly through the small intestine ➔, optimizing digestion and absorption.
They are modified in the different parts of the GIT for proper mixing, adapting to local conditions.
In the small intestine, segmentation contractions (ring-like contractions along the length of the small intestine: Within a seconds, the contracted segments relax and the previously relaxed areas contract which thoroughly mix the food). This is a dynamic process.
Initiation of Segmentation Contraction:
Small intestine's pacemaker cells produce SWPs; if the SWPs reach the threshold, action potentials are triggered, followed by segmentation contraction. These SWPs determine the frequency of contractions.
Rate of segmentation/min = rate of SWP/min, which is inherent (duodenum: 12 waves/min, ileum: 9 waves/min). The frequency decreases distally.
Intensity of segmentation contraction is influenced by:
Mechanical factors: distension of the intestine.
Hormonal factors: gastrin.
Extrinsic nerves: parasympathetic causes ↑excitability of the pacemaker cells
leads to ↑ frequency of the action potentials and ↑ intensity of segmentation
Depressed by sympathetic stimulation, reducing motility. Slight or absent between meals and becomes very vigorous immediately after a meal, adapting to digestive needs.
The duodenum starts to segment primarily in response to local distension caused by the presence of chyme in the duodenum. This is a localized response.
Segmentation of the empty ileum is done by gastrin secreted in response to the presence of chyme in the stomach, which is called the Gastroileal reflex. This coordinates gastric and ileal activity.
NB: chyme is moved very slowly from the upper to the lower part of the small intestine (takes 3-5 hours) to allow sufficient time for the digestive and absorptive processes to take place. This slow transit ensures optimal nutrient uptake.
Intestinal Motility (Other Movements)
Migrating Motility Complex (MMC):
Occurs > 8 hr fasting.
Weak, repetitive peristaltic waves that move a short distance down the intestine.
It takes 80-120 min for one activity front (from antrum to ileum).
MMC regulators:
CCK & gastrin: ↓MMC, inhibiting the housekeeper function during digestion.
Motilin (secreted by the upper small intestine): ↑MMC, stimulating the housekeeper function during fasting.
Functions of MMC:
Clear the stomach and small intestine of luminal contents (remnants of food, mucosal debris, and bacteria) to the colon in preparation for the next meal, which performs the intestinal housekeeper role. This prevents bacterial overgrowth.
Plays a housekeeper role in preventing the overgrowth of microorganisms that might otherwise occur in the small intestinal lumen, maintaining intestinal hygiene.
Note:
Between meals, segmentation contractions are replaced by migrating motility complex. This ensures continuous clearing.
When the next meal arrives: migrating motility complex ceases, and segmentation contraction is triggered again. This adapts to the digestive cycle.
Intestinal Motility (Other Movements)
The Ileal Brake
Unabsorbed nutrients in the lower ileum delays:-
Gastric emptying, reducing gastric activity.
Jejunal transit, slowing small intestinal transit.
Mechanism involves hormones:-
Glucagon-like polypeptide 1 (GLP1)
Neurotensin
Function of the reflex:
To ensure that the rate of transit of food in the gut is optimal for the digestion and absorption of the nutrients and to prevent unabsorbed nutrients entering the colon. This minimizes problems caused by metabolic reactions involving nutrients and colonic microflora. A reflex that reduces proximal GI motility when abnormal levels of nutrients (particularly fat) reach the lower ileum. This optimizes nutrient absorption and prevents colonic overload.
Movement of Chyme into the Cecum
Ileocecal Sphincter: Feedback control
Control:
Via myenteric plexus, providing local control.
Prevertebral sympathetic ganglia, coordinating distant control.
Pressure and chemical irritation in the ileum relax the sphincter and excite peristalsis, promoting emptying.
Fluidity of ileal contents promotes emptying through the sphincter, facilitating flow.
Pressure (distension) and chemical irritation in the cecum (inflamed appendix) contract the sphincter and inhibit peristalsis in the ileum, preventing reflux and stasis.
Mechanism:
When a new meal enters the stomach, segmentation contraction of the ileum and relaxation of the ileocecal sphincter occurs, the gastrin released causes the clean contents of ileum when a new meal enters the stomach, ileal contents pushed to the colon because of the Gastroileal reflex. This coordinates gastric and ileal activity.
Movements of the Colon
Normally sluggish that are non-propulsive. They serve to absorb water and electrolytes and store fecal matter until it can be expelled. The colon primarily functions in absorption and storage.
Long transit time (<24 h), allowing for efficient water absorption.
Two Types of Movement:
1- Mixing (Haustrations)
Large circular constrictions (similar to segmentation in the SI but more marked).
Haustrations = bulging of the large intestine into bag-like sacs because of circular and longitudinal muscle contraction. These increase surface area for absorption.
Depend on the interaction between the ENS and smooth muscle, coordinating colonic motility.
The involvement of the ENS in colonic motility is exemplified by the abnormal motor activity and megacolon found in patients with Hirschsprung’s disease, where intramural nerves are absent or defective. This highlights the importance of the ENS.
2- Propulsive (Mass Movements): Mass movements happen 3-4 times per day.
The colon distension elicits a ring of contraction at the point of distension. This initiates propulsion.
Suddenly, about 18 cm of the colon contract as a single unit, propelling fecal matter in the affected area of the colon down the colon. This moves large quantities of feces towards the rectum.
Usually initiated by food intake (gastrocolic reflex).
Gastrin is involved (➔ gastrocolic reflex probably mediated by gastrin), coordinating gastric and colonic activity.
Move contents from the sigmoid colon to the rectum, preparing for defecation.
The Defecation Reflex
Reflex response to the sudden distension of the rectal wall caused by propulsive movements in the sigmoid colon. This triggers the urge to defecate.
Controlled by neurons in the sacral spinal cord & cerebral cortex = Voluntary + autonomic (parasympathetic) elements, allowing for both reflex and voluntary control.
The rectum is normally empty, maintaining continence.
Sequence of events making up the defecation reflex:
Stimulation of stretch receptors in the rectal wall, sensing distension.
Signals transmitted into the spinal cord, initiating the reflex.
Reflex back to the descending colon, sigmoid, rectum, and anus (pelvic nerves).
Sigmoid colon and rectum contract (controlled by enteric + autonomic nerves), increasing pressure.
Feces move towards the anus.
Involuntary relaxation of the internal anal sphincter + voluntary relaxation of the external anal sphincter, allowing for defecation.
Defecation Reflex also involves the following effects:
Valsalva Maneuver:-
Taking a deep breath
Closure of glottis
Contraction of diaphragm and abdominal wall muscles
Relaxation and movement of pelvis floor downward, facilitating fecal expulsion.
Question 1
Case: A 30‐year‐old man complains of coughing and a choking sensation during meals. He reports that his symptoms occur immediately after he swallows.
Which mechanism normally protects the airway during swallowing?
A. Increased contraction of the tongue muscles B. Voluntary closure of the upper esophageal sphincter C. Elevation of the larynx with epiglottic closure and vocal cord approximation D. Prolonged contraction of pharyngeal constrictor muscles E. Relaxation of the lower esophageal sphincter
Answer: C. Elevation of the larynx with epiglottic closure and vocal cord approximation
Explanation: During the pharyngeal phase of swallowing, the larynx is elevated, and the epiglottis folds over the glottis (with accompanying vocal cord approximation) to prevent food from entering the trachea. This protective reflex is essential in preventing aspiration.
Question 2
Case: A 50‐year‐old patient undergoes surgical nerve-sparing resection for a neck mass. Postoperatively, he has impaired swallowing and difficulty clearing food from his esophagus. Which process likely compensates for the loss of primary peristalsis when the swallowing center is not adequately triggered?
A. Tertiary non-peristaltic contractions B. Primary peristalsis C. Secondary peristalsis D. Esophageal segmentation contractions E. Abnormal retrograde peristalsis
Answer: C. Secondary peristalsis
Explanation: Secondary peristalsis is initiated locally by esophageal distension (via the enteric nervous system) when a residual bolus remains; it does not depend on a swallow trigger and helps clear the esophagus when the primary, centrally mediated peristaltic wave is inadequate.
Question 3
Case: A 28‐year‐old man presents with progressive dysphagia for both solids and liquids, regurgitation of undigested food, and weight loss. Manometry and imaging studies suggest failure of the lower esophageal sphincter (LES) to relax during swallowing.
What is the most likely underlying mechanism?
A. LES incompetence leading to acid reflux B. Obstruction by a distal esophageal mass C. Failure of LES relaxation due to degeneration of inhibitory neurons in the myenteric plexus D. Increased vagal stimulation causing excessive relaxation throughout the esophagus E. Excessive contraction of the upper esophageal sphincter
Answer: C. Failure of LES relaxation due to degeneration of inhibitory neurons in the myenteric plexus
Explanation: Achalasia is characterized by the inability of the LES to relax during swallowing. This is usually due to the loss or dysfunction of inhibitory neurons (which normally release nitric oxide and VIP) in the myenteric plexus, resulting in impaired relaxation and subsequent food stasis.
Question 4
Case: A 40‐year‐old woman reports that she feels uncomfortably full and bloated shortly after beginning a meal, even though she consumes only a small amount of food.
Which normal gastric mechanism is responsible for accommodating an increased volume without a large rise in intragastric pressure?
A. Active antral peristalsis B. Receptive relaxation of the fundus C. Pyloric sphincter dilation D. Rapid segmentation contractions E. Enhanced gastric acid secretion
Answer: B. Receptive relaxation of the fundus
Explanation: Receptive relaxation (a vago-vagal and enteric reflex) enables the proximal stomach—particularly the fundus—to relax and accommodate a large volume of food (up to 1 liter) with little increase in intragastric pressure, thus preventing early satiety.
Question 5
Case: A 30‐year‐old active man is evaluated for nonspecific dyspeptic symptoms. Studies of his gastric function reveal that his stomach is able to effectively mix ingested food with gastric secretions.
Which process best accounts for the mixing of food to form chyme?
A. Vigorous fundal peristalsis B. Strong primary peristaltic waves of the entire stomach C. Segmentation contractions predominantly in the antrum D. Continuous relaxation of the entire gastric wall E. Rapid emptying by the pyloric sphincter
Answer: C. Segmentation contractions predominantly in the antrum
Explanation: Segmentation contractions in the stomach, especially in the antrum, are relatively weak, non-propulsive contractions that serve to mix food thoroughly with gastric secretions, thereby facilitating digestion and the formation of chyme.
Question 6
Case: A 35‐year‐old man notices that when he drinks water with his meals, the liquids seem to pass through his stomach very rapidly, whereas solid foods remain for a prolonged period before passing into the duodenum.
What is the primary reason for the observed difference in gastric emptying between solids and liquids?
A. Liquids require less pressure to move through the pyloric sphincter B. Liquids are inhibited by high CCK levels C. Solids are immediately converted into chyme by gastric acid D. Solids move through the stomach via retrograde peristalsis E. Solids induce a stronger gastrocolic reflex
Answer: A. Liquids require less pressure to move through the pyloric sphincter
Explanation: Gastric emptying of liquids is more rapid because they are already in a fluid state and require little to no grinding. In contrast, solids must be broken down into sufficiently small particles (typically using antral peristaltic action) before they can pass through the pyloric sphincter, resulting in slower emptying.
Question 7
Case: A 42‐year‐old woman with a fatty meal complains of prolonged fullness and occasional nausea. Gastric emptying studies reveal a significant delay in emptying.
Which duodenal hormone is most potent in inhibiting gastric emptying in response to fatty chyme?
A. Gastrin B. Secretin C. Cholecystokinin (CCK) D. Motilin E. Glucose-dependent insulinotropic peptide (GIP)
Answer: C. Cholecystokinin (CCK)
Explanation: CCK is secreted by the duodenal mucosa in response to fat and protein and is the most potent inhibitor of gastric emptying, reducing antral peristalsis and increasing the tone of the pyloric sphincter.
Question 8
Case: A 25‐year‐old man with head trauma develops repetitive vomiting episodes. His clinical picture is concerning for increased intracranial pressure.
Which structure in the medulla plays a key role in integrating signals from both intracranial sources (via the chemoreceptor trigger zone) and the GI tract to initiate the vomiting reflex?
A. Hypothalamus B. Vomiting center in the medulla C. Nucleus ambiguus D. Dorsal motor nucleus of the vagus E. Red nucleus
Answer: B. Vomiting center in the medulla
Explanation: The vomiting (emesis) center in the medulla integrates afferent inputs from the gastrointestinal tract, inner ear, and chemoreceptor trigger zone (area postrema) to orchestrate the coordinated motor actions involved in vomiting.
Question 9
Case: A 45‐year‐old man experiences an urgent need to defaecate soon after eating, often with a sense of abdominal cramping. This “colonic response” is believed to be part of a reflex that involves increased colonic motility following a meal.
What is the name of this reflex, which also serves to propel colonic contents leading to mass movements?
A. Gastroileal reflex B. Colono–rectal reflex C. Gastrocolic reflex D. Enterogastric reflex E. Ileocecal reflex
Answer: C. Gastrocolic reflex
Explanation: The gastrocolic reflex is activated by food intake, especially in the stomach, and triggers colonic mass movements. This reflex helps move colonic contents toward the rectum, often resulting in the urge to defaecate shortly after meals.
Question 10
Case: A 35‐year‐old man, while fasting overnight, experiences a series of low-amplitude, repetitive contractions in his small intestine that slowly propagate over a long distance.
This phenomenon is best described as which of the following?
A. Segmentation contractions B. Primary peristalsis C. Migrating motor complex (MMC) D. Tonic myogenic contraction E. Spontaneous retrograde peristalsis
Answer: C. Migrating motor complex (MMC)
Explanation: The Migrating Motor Complex is a cyclic, recurring motility pattern occurring during fasting (greater than 8 hours) that functions as a "housekeeper" to sweep residual undigested material through the GI tract and prevent bacterial overgrowth.
Question 11
Case: A 40‐year‐old woman eating a high‐fat meal experiences prolonged satiety and delayed gastric emptying. She also reports that she feels full for a long time afterward.
Which mechanism, triggered by unabsorbed nutrients in the distal ileum, best explains the delayed gastric emptying in this patient?
A. Enhanced gastric segmentation B. Activation of the ileal brake C. Stimulation of pancreatic secretions D. Increased release of gastrin E. Inhibition of duodenal peristalsis
Answer: B. Activation of the ileal brake
Explanation: The ileal brake is a feedback mechanism activated by the presence of unabsorbed nutrients (particularly fats) in the distal ileum. It slows gastric emptying and intestinal transit to optimize digestion and absorption.
Question 12
Case: A 50‐year‐old man reports an urgent need to defaecate accompanied by cramping pain after a large meal. On examination, there is increased rectal pressure due to accumulation of feces.
Which reflex is primarily responsible for initiating defaecation in response to rectal distension?
A. Gastroileal reflex B. Defaecation reflex C. Gastrocolic reflex D. Vago-vagal reflex E. Enterogastric reflex
Answer: B. Defaecation reflex
Explanation: The defaecation reflex is triggered by stretch receptors in the rectal wall when fecal matter distends the rectum. This reflex involves both involuntary (spinal) and voluntary (cortical) components to coordinate the contraction of the rectum and relaxation of the internal anal sphincter.
Question 13
Case: A 32‐year‐old woman suffers from frequent heartburn and regurgitation, especially at night, and is diagnosed with gastroesophageal reflux disease (GERD).
Which abnormality in esophageal sphincter function is most likely contributing to her symptoms?
A. Hypertonicity of the upper esophageal sphincter B. Hypotonicity or incompetence of the lower esophageal sphincter C. Excessive primary peristalsis causing rapid transit D. Persistent contraction of the pharyngeal constrictors E. Spasmodic contraction of the esophageal longitudinal muscle
Answer: B. Hypotonicity or incompetence of the lower esophageal sphincter
Explanation: GERD is commonly associated with decreased tone (hypotonicity) or incompetence of the lower esophageal sphincter (LES), allowing gastric acid to reflux into the esophagus and causing symptoms such as heartburn.
Question 14
Case: A 45‐year‐old man complains of early satiety and bloating, and gastric emptying studies reveal delayed emptying of gastric contents. Duodenal hormone measurements show elevated levels of an inhibitory peptide after fatty meals.
Which duodenal hormone is most likely responsible for this inhibition of gastric emptying?
A. Gastrin B. Cholecystokinin (CCK) C. Secretin D. Motilin E. Glucose-dependent insulinotropic peptide (GIP)
Answer: B. Cholecystokinin (CCK)
Explanation: CCK is released in response to fat and protein in the duodenum and is a potent inhibitor of gastric emptying by increasing the tone of the pyloric sphincter and reducing antral peristaltic contractions.
Question 15
Case: A patient who sustained accidental vagotomy (severing of the vagus nerve) during neck surgery shows impaired primary esophageal peristalsis. However, food still eventually passes into the stomach.
What distinguishes the mechanism responsible for this residual esophageal clearance?
A. It is a voluntary swallowing action stimulated by the patient. B. It is secondary peristalsis, initiated by esophageal distension and mediated by the intrinsic enteric nervous system. C. It is due to continuous, tonic contraction of the esophageal muscles. D. It is mediated by an increase in upper esophageal sphincter tone. E. It is the result of direct stimulation by circulating hormones.
Answer: B. It is secondary peristalsis, initiated by esophageal distension and mediated by the intrinsic enteric nervous system.
Explanation: Secondary peristalsis is an automatic, reflexive clearance mechanism triggered by esophageal distension. It is mediated by the intrinsic enteric nervous system and does not rely on the central swallowing mechanism (or the vagus nerve), allowing residual food to be propelled into the stomach even after vagotomy.