Gastrointestinal Physiology: Motility and Digestion

• These are study notes for gastrointestinal physiology, propulsion, digestion and the liver

General Principles of Gastrointestinal Motility

• The GI tube system's functions include absorption of water, macronutrients, vitamins, minerals, and drugs, as well as excretion of waste substances
• The alimentary tract provides the body with a continual supply of water, electrolytes, and nutrients
• This achieved through movement of food, secretion of digestive juices, absorption, circulation of blood, and control by local, nervous, and hormonal systems

Alimentary Tract Structure

• Parts of the alimentary tract are adapted to specific functions, including passage, storage, digestion, and absorption of food
• General shape from esophagus to anus is consistent

Physiologic Anatomy of the Gastrointestinal Wall

• Layers of the intestinal wall, from outer to inner: serosa, longitudinal muscle, circular muscle, submucosa, and mucosa
• Smooth muscle fibers in deeper layers of mucosa, called the mucosal muscle, perform motor functions

Structure of Muscular Layers

• Smooth muscle fibers in the GI tract are 200-500 micrometers long and 2-10 micrometers in diameter, arranged in bundles of up to 1000 parallel fibers
• GI smooth muscle functions as a syncytium because muscle fibers are arranged in bundles and connected electrically through gap junctions
• Electrical signals initiating muscle contractions travel readily within bundles, more rapidly along the bundle length
• Each muscle layer functions as a syncytium, allowing action potentials to travel in all directions
• Action potential travel distance depends on muscle excitability

Electrical Activity of Gastrointestinal Smooth Muscle

• GI smooth muscle is excited by continual slow, intrinsic electrical activity along cell membranes
• Two basic types of electrical waves: slow waves and spikes
• Slow waves are undulating changes in resting membrane potential and control the rhythm of muscle contractions, exciting intermittent spike potentials
• Intensity of slow waves varies between 5 and 15 millivolts, frequency ranges from 3 to 12 per minute in the human GI tract
• Rhythm of contraction varies in different parts of the tract, e.g., about 3/minute in stomach body, 12/minute in duodenum
• Spikes are true action potentials occurring when resting membrane potential becomes more positive than -40 mV, caused by calcium-sodium channels
• Spike potential frequency ranges from 1 to 10 spikes/second, lasting 10-40 times longer than in large nerve fibers

Calcium Ions and Muscle Contraction

• GI smooth muscle contraction results from calcium ions entering the muscle fiber and activating myosin filaments via a calmodulin mechanism

Changes in Resting Membrane Potential

• Baseline voltage level of smooth muscle resting membrane potential can change
• Depolarization (potential becomes less negative) increases excitability due to stretch, acetylcholine, parasympathetic nerves, and GI hormones
• Hyperpolarization (potential becomes more negative) decreases excitability due to norepinephrine/epinephrine and sympathetic nerve stimulation

Types of Muscle Contractions

• Rhythmical contractions: Mediated by calmodulin, caused by spike potentials introducing Ca2+ into cells; calcium-sodium channels open slowly
• Tonic contractions: Continuous, long-lasting contractions independent of slow waves caused by repetitive spikes, partial depolarization by hormones, or continuous Ca2+ entrance

Neural Control of Gastrointestinal Function - Enteric Nervous System

• The enteric nervous system (ENS) lies entirely in the gut wall
• It is composed of myenteric/Auerbach's (outer) and submucosal/Meissner's (inner) plexuses
• Number of neurons in ENS: about 100 million, equivalent to the spinal cord
• ENS controls GI movements and secretion
• Extrinsic parasympathetic and sympathetic systems enhance or inhibit GI functions

Myenteric vs Submucosal Plexuses

• Myenteric plexus: Between longitudinal and circular muscle layers, controls GI movements
• It is a linear chain of interconnecting neurons extending the entire length of GI tract
• Stimulation can have excitatory (increased tonic contraction, rhythm intensity, contraction rhythm ratio, conduction velocity) or inhibitory effects (vasoactive intestinal peptide to open sphincters)
• Submucosal plexus: Lies in submucosa, controls local GI secretion and absorption, blood flow, contraction of mucosal muscle, and integrates sensory receptors

Neurotransmitters Secreted by Enteric Neurons

• Acetylcholine excites
• Norepinephrine inhibits
• Epinephrine acts through blood after adrenal release
• Other neurotransmitters: ATP, 5-HT, dopamine, cholecystokinin, substance P, VIP, somatostatin, leu-enkephaline, met-enkephaline, bombesin

Types of Autonomic Control

• Parasympathetic innervation: Cranial and sacral divisions
• Cranial parasympathetic fibers: Vagus nerves innervate esophagus, stomach, pancreas, and intestines
• Sacral parasympathetics: Pelvic nerves innervate distal large intestine and anus
• Postganglionic neurons in myenteric and submucosal plexuses causing general increase in activity of the entire ENS
• Sympathetic innervation: Fibers originate in spinal cord segments T-5 to L-2, enter sympathetic chains, and pass through ganglia to outlying ganglia
• Innervates entire GI tract, sympathetic nerve endings secrete norepinephrine and epinephrine
• Sympathetic stimulation inhibits activity by direct norepinephrine effect on smooth muscle (except mucosal) and by inhibitory norepinephrine effect on ENS neurons
• Strong sympathetic stimulation blocks food movement

Afferent Sensory Nerve Fibers

• Sensory nerve fibers innervate the gut
• Sensory nerves can be stimulated by irritation of the gut mucosa, excessive distention of the gut or presence of specific chemical substances in the gut
• Signals cause excitation or inhibition of intestinal movements/secretion
• Sensory signals go to spinal cord and brain stem for functions like vagal reflex signals

Reflexes Affecting the Bowel

• Local reflexes: integrated entirely within the gut wall, regulate secretion, movements (peristalsis), inhibitory effects
• Prevertebral sympathetic ganglia reflexes: transmit signals to other intestinal areas (gastrocolic, enterogastric, and colonoileal reflexes)
• Spinal cord and brain stem reflexes: gastric reflexes and defecation reflexes

Hormonal Control of Gastrointestinal Motility

• Hormones also affect GI motility

Functional Types of Movements

• Propulsive movements: Cause food to move forward
• Mixing movements: Keep intestinal contents thoroughly mixed

Propulsive Movements - Peristalsis

• Contractile ring appears around the gut and moves forward, stimulated by distension of the gut, irritation of mucosa, and parasympathetic nervous signals
• Strictly depends on myenteric plexus
• Theoretically, can occur in either direction, usually dies out rapidly in oral direction
• When a segment of the intestinal tract is excited by distension and thereby initiates peristalsis, the contractile ring causing the peristalsis normally begins on the oral side of the distended segment and moves toward the distended segment

Segmentation or Mixing Movements

• Local intermittent constrictive contractions regulated by the neural system
• In some areas, peristaltic contractions themselves cause most of the mixing
• Blocked sphincters make peristaltic waves churn
• Local constrictive contractions occur every few cm, lasting 5-30 seconds

Gastrointestinal Blood Flow – “Splanchnic Circulation”

• Splanchnic circulation: vessels of the GI system
• Includes blood flow through the gut, spleen, pancreas, and liver
• Blood from the gut, spleen, and pancreas flows into the liver via the portal vein, then through liver sinusoids, and out via hepatic veins to the vena cava

Liver Cleansing

• Blood’s flow through the liver allows reticuloendothelial cells lining liver sinusoids to remove bacteria and harmful matter

Nutrient Absorption

• Nonfat, water-soluble nutrients absorbed into the portal venous blood are transported to liver sinusoids, where reticuloendothelial and hepatic cells absorb and store 1/2 to 3/4 of nutrients
• Fat absorbed into intestinal lymphatics are conducted to systemic circulating blood via the thoracic duct, bypassing the liver

Anatomy of Gastrointestinal Blood Supply

• The celiac artery supplies the stomach
• Arteries branch and send smaller arteries circling around the gut, with tips meeting opposite the mesenteric attachment
• Still smaller arteries penetrate intestinal wall and spread along muscle bundles, into villi, and into submucosal vessels to serve secretory and absorptive functions

Effect of Gut Activity and Metabolic Factors on Gastrointestinal Blood Flow

• Blood flow in each area of the GI tract is directly related to local activity level
• Blood flow can increase eightfold or reduce by a third to a fourth
• Blood flow increases after a meal, then decreases back to normal over 2-4 hours
• Vasodilator substances are released from intestinal mucosa during digestion, including cholecystokinin, vasoactive intestinal peptide, gastrin, and secretin
• Glands release kinins into gut wall, causing vasodilation
• Decreased oxygen in the gut wall increases blood flow and adenosine

Countercurrent Flow in the Villi

• Arterial flow into and venous flow out of villi are opposite, in close proximity
• Much of the blood oxygen diffuses from arterioles directly into venules without reaching villus tips
• Up to 80% of oxygen may take short-circuit route
• Disease conditions can cause oxygen deficit in villus tips, leading to ischemia

Nervous Control of Gastrointestinal Blood Flow

• Controlled by adrenergic sympathetic (inhibits movements and reduces flow) and parasympathetic (opposite)
• Parasympathetic stimulation increases blood flow and secretion
• Sympathetic stimulation causes vasoconstriction, decreased flow
• After a few minutes flow returns via autoregulatory escape from local metabolic vasodilator mechanisms
• Sympathetic vasoconstriction allows GI blood flow shut-off during heavy exercise
• Sympathetic stimulation constricts intestinal and mesenteric veins, displacing blood into other body parts

Propulsion and Mixing of Food in Alimentary Tract

• Critical parameters for optimal food processing: Time food remains in each tract part and appropriate mixing
• Multiple nervous and hormonal feedback mechanisms control timing to achieve optimal processing

Ingestion of Food

• Food amount is determined by intrinsic desire (hunger) and type is determined by appetite
• Important regulatory systems maintain nutritional supply
• Discussion confined to mechanics of ingestion, especially mastication and swallowing

Mastication (Chewing)

• Anterior teeth provide cutting action, posterior teeth provide grinding action
• Muscles close teeth with great force: 55 pounds on incisors, 200 pounds on molars
• Chewing is controlled by nuclei in the brain stem, rhythmical chewing movements are regulated by reticular areas and hypothalamus, amygdala, and cerebral cortex
• Chewing reflex: food bolus initiates reflex inhibition of mastication muscles, jaw drops, stretch reflex leads to rebound contraction and repeated cycles

Physiological Importance and Why

• Chewing is important for passing food to the esophagus
• Chewing is important for digestion of all foods, but especially important for most fruits and raw vegetables
• Digestive enzymes act only on food particle surfaces, therefore chewing increases surface contact
• Chewing prevents excoriation of the GI tract and increases ease of emptying

Swallowing

• Complicated mechanism because pharynx subserves respiration and swallowing, converts it for a few seconds at a time into a tract for propulsion of food
• Three stages: voluntary, pharyngeal, and esophageal
• Voluntary
  • Bolus is squeezed/rolled by tongue pressure, automatic

Pharyngeal Stage of Swallowing

• Food bolus stimulates epithelial swallowing receptor areas around pharynx opening, impulses initiate automatic muscle contractions:
• Soft palate pulled upward to close posterior nares
• Palatopharyngeal folds pulled medially to form sagittal slit
• Larynx is pulled upward and anteriorly with vocal cords approximated
• Upper esophageal sphincter relaxes and opens esophagus
• Fast Peristaltic wave of pharynx

Initiation of Pharyngeal Stage

• Most sensitive tactile areas in posterior mouth and pharynx (esp. tonsillar pillars), impulses transmitted via trigeminal and glossopharyngeal nerves to medulla oblongata
• Swallowing reflex is same from swallow to swallow
• Areas in medulla and lower pons that control swallowing are collectively called swallowing center (aka deglutition)
• Motor impulses transmitted successively by cranial nerves 5, 9, 10, 12 and some superior cervical nerves

Effect of Pharyngeal Stage on Respiration

• Entire pharyngeal stage occurs in less than 6 seconds, interrupting respiration briefly
• Swallowing center inhibits respiratory center to halt respiration

Esophageal Stage of Swallowing

• Esophagus conducts food rapidly to the stomach using peristaltic movements
• Primary: Continuation of pharyngeal peristaltic wave, takes 8-10 seconds
• Secondary: Initiated by distention of the esophagus by retained food, continues until food empties

Musculature During Swallowing

• Musculature of pharyngeal wall and upper third of esophagus is striated, controlled by skeletal nerve impulses from glossopharyngeal and vagus nerves
• Lower two thirds of esophagus is smooth muscle, controlled by vagus nerves through myenteric nervous system
• After damage to vagus, myenteric nervous function may enable strong secondary peristalsis

Receptive Relaxation of the Stomach

• Esophageal peristaltic wave approaches: relaxation transmitted through myenteric inhibitory neurons precedes it
• Entire stomach and duodenum become relaxed to receive food

Lower Esophageal Sphincter (LES)

• Broadens circular muscle at lower esophagus, remains tonically constricted
• Intraluminal pressure of about 30 mm Hg
• "Receptive relaxation" during swallowing
• Prevents significant stomach content reflux
• Valve-like mechanism in bottom portion of esophagus also helps prevent reflux

What would happen if there are no Myenteric Plexus

• There would be no inhibitory wave on LES, food would accumulate and vomiting would happen, can cure with cutting the LES

Motor Functions of the Stomach

• Storage of large food quantities
• Mixing with gastric secretions to form chyme
• Slow chyme emptying into intestine at a proper rate

Anatomic vs Physiologic Divisions of the Stomach

• Anatomically: Body and antrum
• Physiologically: Orad (first two thirds of the body) which receives, "caudad" (remainder of the body plus the antrum)

Storage Function of the Stomach

• Concentric circles of food, newest food is closest to the esophageal opening, oldest food is lying near the outer wall of the stomach.
• Food stretches stomach and vagovagal reflex reduces tone in the muscular wall of the body, wall bulges progressively outward to accommodate more food
• Completely relaxed stomach is 0.8 to 1.5 liters
• Pressure in stomach remains low
• Afferent and efferent fibers all belong to the vagus
• Brainstem and medulla.

Mixing and Propulsion of Food - Electrical Rhythm

• Contractions to start in the stomach
• GI juices are secreted
• Weak are known as mixing waves
• Occur almost every 15 or 20 seconds
• Increase during food presence

These waves are initiated by the gut wall

• Consisting of electrical slow waves

Powerful peristalsis also happens - Action Potential

• Powerful rings that force the antral contents to higher pressure toward the pylorus.
• Important in mixing the stomach contents
• Waves make their way thru antral wall to pylorus
• Opening only expels milliliters of contents
• Those contents are squueezed with the proximal ring to be made with the proximal walls near the stomach
• Called propulsion
• Helps stomach mixing
• There is Chyme

Hunger Contractations

• Intestinal secretions, food, water, and digestings mix
• Consistency depends on contents
• Usually like milky semi-fluid or thick paste
• After is passes through the Gut the Food is now called a Chyme and is ready to go

Hunger Contractations

• Rhythmical with empty stomach
• When Contractions become strong there is a tetanic level
• Cause the stomach to be Empty
• 2 - 3 minutes
• Happen more in young people
• They happen due to low level of normal sugar in young people

Stomach Export

• Done by peristaltic contractions at the same time there is some pressure against chyme in the pylorus

Stomach Empty and Pylori Pump

• Weak but mixing, become strong during the peristaltic activity while causing stomach emptying.
• At the same time becomes more gradual until the end.

Pyloric Action

• Narrows export to chyme
• Distal opening of the stomach
• 50 or 100 percent bigger depending on which stomach antrum it is, and it remains in somewhat contraction.
• This the pylori muscle
• Allows certain contents to pass like from fluids with more ease.
• But at the same time, until things can completely combine with the chyme in a consistent way it doesn't allow.
• Nervous and or humoral in nature in both contents coming and going

Control of Stomach Export

• Done by Signals from the contents inside But more so by the duodenum than what is in the stomach though:
• rate of export can't be exceeded with digestive or absorbent abilities of the intestine
• there must be more in stomach for proper digestion of its contents.
• When too basic the export of hcl is triggered from the stomach
• Stretching helps the pylori to close

Promotory Gastric

• stretching of wall
• Presence of certain foods like digested meats

Inhibiting Enterogastric

• When there is food in the duodenum and in sufficient volume.
• Direct communication through stomach intestines
• Excretory nerves travel and work this out as well
• Degree of distention
• Presence of irritation
• Degree of chyme
• Osmolality
• Break dwon product with respect too fats

The intestinal hormone regulation - Small Intestine

• These exist when the small intestin is is emptying
• Fats will trigger the pylori action hormone
• When Duodenum is Empty these hormones have reduced activity
• Most importaint is food triggers

Main Points

• More with feeling then when the pylori opens or closes
• Intestional will not fill in it't empty though and it controls These both provide the signals for the empty Stomach

Movement

• The small intestines movement can be divided into the following.
• Segmentation
• Propulsion There can't be a separation this all occurs together!

Contractions:

• The point when some parts of the small intestines have chyme
• These produce local ringlets that are spaced Some are called squeezing because that what the act as that certain distances

Important

• Electrical slow waves that determine the segmentation contraction, and if at high state 12 per second.
• These weak in activity if block to those parts.

Propulsion Waves

• Contents must be sent throughtout the small intestines to get to the anus very slow at all

Motilitity

Small intestine is also helped through some signals by what occurs after with a meal though, caused partly by duodenal

• distention and also by a thing called the gastroenteric reflex, what the contents and intestines are is helped by signals of certain factors These signals include the following.
• Gastrickin
• CCK
• Intestetinal
• Motiolin All these enhance as there being a food source However other reverse that these hormones are present, however it's been questioned. The large intestines job is also to transfer to be of the chime within the area.

Effect

• Segmentation is different but similar. There is a seperation for such, mixing at and such however they cause the intestine to function better.

Intense Irritation of Small Intestine

• Can create very fast contractions in the intestine with much irritation which helps then move certain factors there such actions is important this helps by creating both the brain system being on in all regions of the intestinal and nerve plexus.

Movements from The Villir Mucosus

Help shorten length of the lumen Increases the rate though of absoprtion However to allow the villi to shorten the space the most there is also help to increase the lymph there though

Colon Functon

• The large intestines function. 1 - To try take as much of the moisture as it can for the chime. 2 - Store it It doesn't have such activity.

Haustartions

• Just how contractions work
• Contracts
• Causes sections to pull outwards and outwards to form those sacks

Colon to Rectum

• Movement helps the material inside go though well though.
• It has not been know if that are something that is bad or has. but to get through needs to be good.

Massive Movements

After they have hit forces the mass to turn force to rectum and to want to go The mixing is again with force though though such, or mixing or helping that process work 1 - Can tell the contents, or the irritation. Also through certain reactions, or the colon system of the brain

• Normal food needs things to move

There is a defecation mechanism

• There is the the parts that are the most complex are through are brains telling it must come out.

The body knows

• Some nerves go to conous medullaris - cord is very involved, there is that part

A great loop happens

Signals help

Sings to colon make bowel work

This all for though

What Helps Stop

• The Sphinter helps the walls well.
• All has their jobs that make defication happen It not though they can work themselves.
• Signals, from the cord
• Then the spinal
• If have an illness has had happened. has happens

3.03 Initiotion or Movements-Colon and the others

• The colon, duolinal can help This helps though this the brain not telling as much and to as long if to be there.
• Can create movement. Can always though create certain types also. Irritative can help as well though. There though what helps with this as often though well.
• Deification

The act

Also that the muscle though also helps Bowel tells. It only after these that more of the process though. It also through colon not just has to be though that they have what it was.

• They help well as though

A summary

• There's the large, intestines help take out.