case 1 Flashcards
layers of the GI tract
Serosa, longditudinal muscle, circular muscle (myenteric plexus), submucosa (meissners plexus), mucosa, epithelial lining.
• Electrical signals that initiate muscle contractions can travel readily from one fibre to the next within each bundle using gap junctions.
enteric nervous system
- The enteric nervous system is part of the autonomic nervous system that is found in the lining of the GI Tract, beginning at the oesophagus and extending down to the anus.
- It is involved in the coordination of reflexes (although it receives innervation by the autonomic nervous system, it can work independently of the brain and the spinal cord).
two main plexuses of the enteric nervous system
- A submucosal plexus/ Meissner’s plexus that lies in the submucosa - controls mainly gastrointestinal secretion and local blood flow.
- A myenteric plexus lying between the longitudinal and circular muscle layers - controls motility.
This also secretes vasoactive intestinal polypeptide. The resulting inhibitory signals are especially useful for inhibiting some of the intestinal sphincter muscles that impede movement of food.
neurons in the enteric nervous system
afferent neurons, interneurons and efferent neurons.
Sensory neurons report on mechanical and chemical conditions.
Through intestinal muscles, the motor neurones control peristalsis and churning of intestinal contents.
two types of movement in the GI tract
propulsive and mixing movements
Propulsive movements-peristalsis
stimulation cause contractile ring in circular muscle which spreads along gut. Stimulus-distention of gut. stretch stim ENS to contract behind it. also stim by irritation of epithelial lining. Parasymp NS to gut elicit peristalsis. requires an active myenteric plexus. starts orad side, moves toward segment, pushing in anal direction 5cm before dying out. there is receptive relaxation down stream allowing food propelled through.
4 stages of swallowing
- Cephalic Stage
- Oral Stage (Voluntary stage)
- Pharyngeal stage – involuntary and constitutes passage of food through the pharynx into the oesophagus
- Oesophageal stage – involuntary phase that transports food from pharynx to the stomach
cephalic stage
• This is the point where one is thinking about having a meal:
All of this is part of the process of which would induce the activity of swallowing
oral stage
- Chewing (mastication)
- Salivation – lubricate the bolus and begin the process of digestion (discussed later)
- Movement of bolus
The bolus is pushed against the hard palate.
The rugae on the hard palate help move the bolus posteriorly into the back of the mouth into the pharynx.
mastication
first step digestion, food ground by teeth, inc surface area food more efficient breakdown by enzymes, food positioned by cheek and tongue between teeth for grinding, 4 muscles-masseter temporalis, lat and medial pterygoid. innervated mandibular branch trigeminal. Stimulation of specific reticular areas in the brain stem taste centres will cause rhythmical chewing movements.
Also, stimulation of areas in the hypothalamus, amygdala, and even the cerebral cortex near the sensory areas for taste and smell can often cause chewing food made softer and warmer and process salivation begins.
teeth and mastication
- The anterior teeth (incisors) provide a strong cutting action.
- The posterior teeth (molars) provide a strong grinding action.
process of mastication - chewing reflex
presence of bolus in mouth initiates reflex inhibition of muscles of mastication so jaw drops, the drop makes stretch reflex rebound contraction. this compresses the bolus against lining of mouth making inhibition again to jaw drop, this causes break down of food, important for digestion carbs as have indigestible cellulose membranes.
pharyngeal stage
- After the voluntary stage, the bolus of food enters the posterior mouth and pharynx.
- Here, it stimulates the epithelial swallowing receptor areas all around the opening of the pharynx, especially on the tonsillar pillars.
- Impulses from these pass to the brain stem to initiate a series of automatic pharyngeal muscle contractions:
contractions of the soft patate and palatopharyngeal folds in the pharyngeal stage
1)The soft palate is pulled upward to close the posterior nares, to prevent the reflux of food into the nasal cavities (nasopharynx) 2)The palatopharyngeal folds on each side of the pharynx are pulled medially to approximate each other. In this way, the folds form a sagittal slit through which the food must pass into the posterior pharynx. This slit performs a selective action, allowing food that has been masticated sufficiently to pass with ease. Because this stage lasts less than 1 second, any large object is usually impeded too much to pass into the oesophagus.
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contractions of the vocal cords in the pharyngeal phase
3) The vocal cords of the larynx are closed, and the larynx is pulled upward and anteriorly by the neck muscles. These actions, combined with the presence of ligaments that prevent upward movement of the epiglottis, cause the epiglottis to swing backward over the opening of the larynx.
All these effects acting together prevent passage of food into the nose and trachea. Most essential is the tight approximation of the vocal cords, but the epiglottis helps to prevent food from ever getting as far as the vocal cords.
changes to the larynx in the pharyngeal phase
4) The upward movement of the larynx also pulls up and enlarges the opening to the oesophagus. At the same time, the upper 3-4cm of the oesophageal muscular wall, called the upper oesophageal sphincter (also called the pharyngoesophageal sphincter) relaxes, thus allowing food to move easily and freely from the posterior pharynx into the upper oesophagus. Between swallows, this sphincter remains strongly contracted, thereby preventing air from going into the oesophagus during respiration.
The upward movement of the larynx also lifts the glottis out of the main stream of food flow, so that the food mainly passes on each side of the epiglottis rather than over its surface; this adds still another protection against entry of food into the trachea.
changes to the pharynx in the pharyngeal phase
5) Once the larynx is raised and the pharyngoesophageal sphincter becomes relaxed, the entire muscular wall of the pharynx contracts, beginning in the superior part of the pharynx, then spreading downward over the middle and inferior pharyngeal areas, which propels the food by peristalsis into the oesophagus.
summary of changes in the pharyngeal phase
Trachea is closed.
Oesophagus is opened.
Fast peristaltic wave initiated by the nervous system of the pharynx forces the bolus of food into the upper oesophagus.
The entire process is less than 2 seconds
upper Esophageal sphincter
consists of the cricopharyngeus muscle, the adjacent inferior pharyngeal constrictor, and the proximal portion of the cervical oesophagus. innervated by vagus, innervation of musculature on UES to facilitate opening is 5,7,12 nerve. • The UES remains closed at rest owing to both its inherent elastic properties and neurogenically mediated contraction of the cricopharyngeus muscle. For the UES to open, it is important that the cricopharyngeus muscle has to relax. This occurs due to cessation of vagal excitation. UES opening is also aided by simultaneous contraction of the suprahyoid and geniohyoid muscles that pull open the UES + upward and forward displacement of the larynx + pulling forward of the hyoid bone
neurophysiology of swallowing
At the centre of the swallowing system is the Brainstem Central Programme Generator (CPG). This is mainly in the medulla and extends into the pons and houses a vast array of neurones and interneurons which link together to produce the activity of swallowing. The interneurons consist of excitatory and inhibitory ones.
There are also nuclei which include the dorsal vagal motor nucleus and the nucleus ambiguus alongside the CN nuclei of CN 5, 7, 9, 10 and 12. All of these CN combine together with the interneurons to allow the sequence to take place. The cortex communicates with the brainstem salivatory nuclei. These send signals via motor neurons to the muscles of swallowing. 26 pairs of muscles are required in the entire swallowing process
different types of receptors in swallowing
Sensory receptors in the oropharynx, larynx and oesophagus detect changes and send signals back to the brainstem and the cortex via CN 5, 7, 9, 10 and 12. Chemical receptors – stimulus (acid); response (feedback control)
Thermal receptors – stimulus (hot/cold); response (non-painful sensation) Mechanical receptors – stimulus (distention); response (burning/pain) – MOST POWERFUL INDUCERS OF A SWALLOW. These help mediate the swallowing response.
Swallowing is not a reflex, but a patterned response.
vagal and spinal afferents
send different types of sensations back into the system via the:
Nadose ganglion of the vagus afferents
Dorsal root ganglion of the spinal afferents
The vagus afferents then go via the thalamus and into the cortex via the medulla.
For spinal afferents, the system involved is the anterolateral system (spinithalamic tract) for nociception and mechanoreception.
cortical input in swallowing
- It has been shown through studies that there are many areas of the cortex that help regulate the swallowing response.
- Cortical dysfunction (i.e. in stroke for example) results in dysphagia.
brainstem and swallowing
• During the process of swallowing, the epiglottis closes off the larynx to prevent aspiration.
For this period swallowing period, respiration is stopped.
The salivatory nuclei are situated close to, if not exactly in the same location, as those nuclei controlling breathing.
They work together to carry out this intricate procedure during swallowing.
Oesophageal stage
• The oesophagus exhibits two types of peristaltic movements:
- Primary peristalsis
- Secondary peristalsis
primary peristalsis
- This is a continuation of the peristaltic wave that begins in the pharynx and spreads into the oesophagus during the pharyngeal stage of swallowing.
- This is quicker in someone sitting up/standing up, due to the influence of gravity
secondary peristalsis
- If the primary peristaltic wave fails to move all the food into the stomach, secondary peristaltic waves result from distention of the oesophagus itself by the retained food.
- These waves continue until all the food has emptied into the stomach.
- The secondary peristaltic waves are initiated partly by intrinsic neural circuits in the myenteric nervous system and partly by reflexes that begin in the pharynx and are then transmitted upward through vagal afferent fibres to the medulla and back again to the oesophagus through glossopharyngeal and vagal efferent nerve fibres.
musculature of oesophagus
•Pharyngeal wall and the upper 1/3 of oesophagus = striated muscle.
Peristaltic waves in these regions are controlled by skeletal nerve impulses from the glossopharyngeal and vagus nerves from the nucleus ambiguus.
•Lower 2/3 of oesophagus = smooth muscle.
This is also strongly controlled by the vagus nerves acting through connections with the oesophageal myenteric nervous system
pressure zones of the Oesophagus
• There are 2 high pressure zones:
The UES - pressure can reach up to 100mmHg
The LES - pressure is around about 20mmHg (can be higher in pathological conditions)
•The inside of the oesophagus had a negative pressure of about -5mmHg.
This acts to help the bolus to be pulled through from the pharynx which is at atmospheric pressure (0mmHg) through the sphincter and into the oesophagus.
The reason why it is negative is because of the lungs and the pleura and there is the mediastinal pleura which pulls against the oesophagus creating this negative pressure.
•In the stomach, there is a slightly higher pressure of +5mmHg than the oesophagus
This doesn’t overcome the LES pressure so reflux is prevented.
•The reason why continuous reflux is prevented is because the pressure in the LES is higher than in the stomach.
receptive relaxation of the stomach
- When the oesophageal peristaltic wave approaches toward the stomach, a wave of relaxation, transmitted through myenteric inhibitory neurons, precedes the peristalsis.
- Furthermore, the entire stomach and, to a lesser extent, even the duodenum become relaxed as this wave reaches the lower end of the esophagus and thus are prepared ahead of time to receive the food propelled into the esophagus during the swallowing act
lower eosophageal sphincter
- This sphincter is found at the lower end of the oesophagus.
- It normally remains tonically constricted with an intraluminal pressure in the oesophagus of 30mmHg.
- When a peristaltic swallowing wave passes down the oesophagus, there is “receptive relaxation” of the lower oesophageal sphincter ahead of the peristaltic wave, which allows easy propulsion of the swallowed food into the stomach.
- Dysfunction of the LSE is called achalasia•Fortunately, the tonic constriction of the lower esophageal sphincter helps to prevent significant reflux of stomach contents into the esophagus except under very abnormal conditions.
Neurotransmitters on sphincters
- Acetylcholine causes constriction of muscles that will close sphincters and also those muscles that aid peristalsis.
- Nitric oxide causes relaxation of these muscles.
protective mechanisms to prevent oesophageal injury from reflux of gastric acid
anti reflux barrier: LES, diaphram, limit frequency of reflux.
Oesophageal clearance: gravity, peristalsis, limit duration of acid contact
Acid neutralization: saliva (HCO3) HCO3 (secreted and blood) limit duration of acid contact
Tissue resistance: cell junctions + membranes, Na/H exchange, Epithelial resitution, blood flow. Protect epithelium during acid contact.
secretory functions of the alimentary tract
•Throughout the alimentary tract, the secretory glands have two functions:
1.Secretion of Digestive enzymes
2.Secretion of Mucus - provides lubrication and protection to alimentary tract
•The digestive secretions are dependent on the presence of food in the alimentary tract.
•In parts of the GI tract, the digestive enzymes secreted are specific to certain foods
goblet cells/mucous cells
These are single-cell mucous glands.
These function mainly in response to local irritation of the epithelium.
They secrete mucous directly onto the epithelial surface to act as a lubrication that also protects the surfaces from excoriation and digestion.
pits
These represent invaginations of the epithelium into the submucosa.
In the small intestine, these pits are called crypts of Lieberkuhn. These are deep and contain specialized secretory cells
TUBULAR GLANDS
These are found in the stomach and the upper duodenum.
These secrete substances such as acid and pepsinogen in the stomach
salivary glands, liver, pancreas
These provide secretions for digestion or emulsion of food.
effect of contact of food with the epithelium
•The presence of food in a particular segment of the GI tract causes the glands to secrete large quantities of juices.
•Direct contact of food with the glandular cells causes this local secretion in the GI tract.
•This local epithelial stimulation also activates the enteric nervous system of the gut wall:
The types of stimuli that do this are:
Tactile stimulation/ Chemical irritation/ Distention of the gut wall
•The resulting nervous reflexes stimulate both the mucous cells on the gut epithelial surface and the deep glands in the gut wall increase their secretion.
autonomic stimulation-parasympatheric
- Stimulation of parasympathetic nerves innervating the glands in the GI tract strongly increases the rate of alimentary glandular secretion.
- This is particularly true in the upper GI tract (innervated by the glossopharyngeal and vagus parasympathetic nerves), e.g. Salivary glands and oesophageal glands.
- Some glands in the distal large intestine (innervated by the pelvic parasympathetic nervous system) secrete secretion as a response to the mechanical presence of food.
- The remainder of the GI tract result in secretion due to local neural and hormonal stimuli in those particular segments of the tract.
autonomic stimulation - sympathetic
•Stimulation of sympathetic nerves innervating the glands in the GI tract has a dual effect:
1.Increase in the amount of secretion.
2.Constriction of the blood vessels that supply the glands.
•Sympathetic stimulation alone usually slightly increases secretion.
•But, if parasympathetic or hormonal stimulation is already causing abundant secretion by the glands, superimposed sympathetic stimulation usually reduces the secretion, sometimes significantly so, mainly because of vasoconstrictive reduction of the blood supply
hormonal regulation
- The hormones in the help regulate the volume and character of the secretions.
- They are particularly important in the stomach and the intestine.
- They are liberated from the GI mucosa in response to the presence of food in the lumen of the gut.
- The hormones are then absorbed into the blood and carried to the glands, where they stimulate secretion.
- This type of stimulation is particularly valuable to increase the output of gastric juice and pancreatic juice when food enters the stomach or duodenum.
basic mechanism of secretion by glandular cells
• The secretory glands secrete two things mainly:
- Organic substances (enzymes etc).
- Water and electrolytes
organic substance secretion
1.The nutrient material needed for the formation of the secretion must first diffuse or be actively transported by the blood in the capillaries into the base of the glandular cell.
2. Mitochondria located inside the glandular cell near its base use oxidative energy to produce ATP.
3.Energy from ATP, along with the appropriate substrates provided by the nutrients, is then used to synthesise the organic secretory substances.
The synthesis occurs in the ER and golgi complex of the glandular cell.
Ribosomes adherent to the ER are responsible for the synthesis of the proteins that are secreted.
4.The secretory materials are transported through the tubules of the ER to the golgi complex.
5.Golgi complex – materials are modified, added to, concentrated, and discharged into the cytoplasm in secretory vesicle, which are stored in the apical end of the glandular cells.
6.Nervous or hormonal signalling causes exocytosis of these vesicles. It happens in the following way:
The control signals increase the cell membrane permeability to calcium ions, and calcium enters the cell.
The calcium causes the vesicles to fuse with the apical cell membrane.
The apical cell membrane breaks open, thus emptying the vesicles via exocytosis.
water and electrolyte secretion
• Water and electrolytes are secreted along with the organic substances.
1.Nerve stimulation of the basal portion of the cell membrane causes an influx of chloride ions.
2.The resulting increase in electronegativity induced inside the cell by excess negatively charged chloride ions then causes an influx of positive ions (e.g. sodium ions).
3.Due to the influx of ions (both positive and negative) an osmotic gradient is created, therefore water enters the glandular cells.
This increases the cell volume and hydrostatic pressure inside the cell, causing the cell itself to swell.
4.The pressure in the cell then initiates minute openings of the secretory border of the cell, causing flushing of water, electrolytes and organic materials out of the secretory end of the glandular cell.
mucus
thick secretion composed mainly of: Water Electrolytes Mixture of several glycoproteins (which themselves are composed of large polysaccharides bound with much smaller quantities of protein) •Functions: Lubrication of the GI tract Protection of the GI tract
mucus functions
- Mucus adheres tightly to the food or other particles and spreads as a thin film over the surfaces.
- It has sufficient body that it coats the wall of the gut and prevents actual contact of most food particles with the mucosa.
- Mucus has a low resistance for slippage, so the particles can slide along the epithelium with great ease, thus preventing excoriative or chemical damage to the epithelium.
- Mucus causes faecal particles to adhere to one another to form the faeces that are expelled during a bowel movement.
- Mucus is strongly resistant to digestion by the GI enzymes.
- The glycoproteins of mucus have amphoteric properties (able to react both as an acid and an alkali). This allows them to buffer small amounts of either acids or alkalis; also, mucus often contains moderate quantities of bicarbonate ions, which specifically neutralize acids.
saliva is secreted by
Parotid glands – serous secretion only
Submandibular glands – serous and mucus secretion
Sublingual glands – serous and mucus secretion (mainly mucus)
Buccal glands – mucus secretion only
• Daily secretion of saliva = 800-1500ml (average = 1000ml).
two major types of protein secretion in saliva
1.Serous secretion
Contains ptyalin (α-amylase), which is an enzyme for digesting starches.
2.Mucus secretion
Contains mucin for lubricating and for surface protective purposes.
•Saliva has a pH of between 6.0-7.0 (a favourable range for the digestive action of ptyalin).
secretion of ions in saliva
- Saliva contains large amounts of potassium and bicarbonate ions.
- Saliva also contains small amounts of sodium and chloride ions. There is a higher concentration of these ions in the plasma as opposed to the saliva.
- The salivary glands contain acini and salivary ducts.
- Salivary secretion is a two stage process
primary secretion of saliva-acini
This contains ptyalin and/or mucin in a solution of ions in concentrations similar to that of the extracellular fluid.
The ions secreted by the acini into the salivary duct are sodium ions (Na+), chloride ions (Cl-) and small amounts of bicarbonate ions (HCO3-).
Sodium ions enter the lumen via tight junctions too.
Water is also added to the salivary duct as a result of osmosis.
This leads to an isotonic, plasma-like primary secretion.
salivary ducts, reabsorption of Nacl
Sodium ions are actively reabsorbed from all the salivary duct lumen and small amounts of potassium ions are actively secreted into the lumen in exchange for the sodium.
The sodium ion concentration of the saliva becomes greatly reduced, whereas potassium ion concentration becomes increased.
There is excess reabsorption of sodium ions over potassium ions.
This creates electrical negativity (around -70mV) in the salivary ducts.
As a result, chloride ions are passively reabsorbed from the lumen (to make the lumen more positive).
Chloride ion concentration in the salivary fluids is greatly reduced, matching the ductal decrease in sodium ion concentration.
Small amounts of bicarbonate ions are secreted by the ductal epithelium into the lumen of the duct. This is caused by the passive exchange of bicarbonate for chloride ions.
summary of salivary duct stage of secretion
There is reabsorption of sodium ions and chloride ions (NaCl) from the duct.
There is some secretion of potassium ions and bicarbonate ions into the duct.
The cell membranes of the epithelial lining of the duct have low water permeability (less aquaporins) and so hardly any water enters the duct via osmosis. This leads to the final saliva being hypotonic.
resting salivation
- Sodium chloride concentration of the saliva (15 mEq/L) is about 1/7 to 1/10 of their concentration in the plasma.
- The concentration of potassium ions (30 mEq/L) is 7 times as great as the plasma.
- The concentration of bicarbonate ions (50-70 mEq/L) is about 2-3 times as great as the plasma
maximal salivation
- During maximal salivation, the salivary ionic concentrations change considerably because the rate of formation of primary secretion by the acini can increase as much as 20x.
- This acinar secretion then flows through the ducts so rapidly that the ductal reabsorption of NaCl is considerably reduced.
- Therefore, when copious quantities of saliva are being secreted, the sodium chloride concentration in the saliva rises only to 1/2 or 2/3 that of plasma (normally 1/7 to 1/10), and the potassium concentration rises to only 4 times (normally 7 times) that of plasma.
what 3 muscles make the UES
cricopharyngeus muscle, the adjacent inferior pharyngeal constrictor, and the proximal portion of the cervical oesophagus
membrane transport
phospholipid bilayer impermeable to ions and polar molecules. channels and carrier transporters are passive down conc gradient, pumps are active against conc gradient.
functions of transport proteins
uptake nutrients substrates and cofactors like glucose, Na amino acid transporter, and export waste like urea and lactate- H+.
Regulate ions PH and vol. Gradient maintanence by K+ Na+. volume inc by Na, K, Cl in. Vol dec Cl and K out. Acid extrusion by Na in H+ out. base extrusion Cl in HCO3 out.
aquaporins
Pore is highly selective to water
•Aquaporin 5 is important in salivary secretion
•Water flow is driven by osmosis
k+ channels
Four subunits arranged around a central pore
•Important in maintaining membrane potential
voltage gated Na channel
Single polypeptide chain
•24 transmembrane domains
•Important in nerve conduction
carrier proteins - classification
facilitator-uniport. cotransporter-symport. exchanger-antiport.