GI motility Flashcards
Where is smooth muscle found in the GI tract?
Everywhere except oesophagus
List the factors that regulate appetite in the common domesticated species
- Nervous system
- Pregastric factors
- Gastrointestinal and post-absorptive factos
- Hormonal
Explain how the nervous system regulates appetite in the common domesticated species
- Arcuate nucleus in hypothalamus, many NTs and hormones
- Lateral hypothalamus = hunger centre
- Ventromedial hypothalamus = satiety centre
What happens if there is a lesion in the lateral hypothalamus?
- Is the hunger centre
- Animal will become anorexic and lose weight
What happens if there is a lesion in the ventromedial hypothalamus?
- Satiety centre
- Will overeat (and likely become obese)
List some of the pregastric factors that regulate appetite in the domestic animal
- Appearance of food
- Taste and/or odour of food
- learned preferences and aversions
- Psychological factors (fear, depression and social interactions)
- Ambient temperature
What happens to food intake depending on temperature?
- Cold = increase food intake
- Hot = decrease food intake
Give the gastrointestinal and post-absorptive factors that regulate appetite in the domestic species
- Gastrointestinal fill
- Stretch receptors send signals via vagus nerve to hypothalamus
- Full stomach and SI induce satiety
List some of the hormonal factors that regulate appetite in the domestic animal
- Grehlin- stimulator of appetite
- Nutrients concentration can change concentration of hormone, linked to satiety and hunger
- Glucose increases hormone levels in satiety centre
- VFAs inhibit food intake
- Amino acids stimulate the satiety centre
What is the effect of the sympathetic nervous system on gut motility and glandular secretion?
- Decreases both
What is the effect of the parasympathetic nervous system on gut motility and secretion and how does it carry out its effec?
- reach enteric NS directly
- Increase motility and secretion
- PH from vagus stimulates GI
What is the effect of neuropeptide Y (NPY) and orexin on appetite?
Stimulate appetite
What is the effect of melanocyte stimulating hormone (MSH) on appetite?
Reduces appetite
Explain how the concentration of nutrients affects appetite
- Nutrients such as glucose and amino acids absorbed
- Concentrations in blood rises
- Concentration of several hormones rises (CCK, insulin, glucagon)
- Linked to satiety
- Glucose increases activity in satiety centre
- VFAs inhibit food intake in ruminants
- Amino acids inhibit appetite centre
Describe the parasympathetic control of the internal anal sphincter
- PSNS nerves from S2-4 level of spinal cord
- Stimualtion causes sphincter to relax
- ALlows faeces to pass through when rectum and anal canal contract
Describe the sympathetic control of the internal anal sphincter
- Nerves from T11-L2
- Cause sphincter to contract
Describe the control of the external anal sphincter
- Spinal nerves from S2-4
- Skeletal muscle control (voluntary control)
Describe the control fo teh rectum and anal canal
- Smooth muscles controlled by PS nerves from S2-4
- Pelvic splanchnic nerve stimulates rectum and anal canal to contract
- Sympathetic nerves from T11-L2 form hypogastric nerve and stimulate rectum and anal canal to relax
What is the major parasympathetic nerve supply to the abdominal viscera?
- Vagus nerve
- travels down neck in vagosympathetic trunk
Describe the thoracic splanchnic nerves
- Arise from sympathetic trunk in thorax
- Provide sympathetic innervation to abdomen
- Contains preganglionic sympathetic and GVA fibres
- Can be divided into greater, lesser and least splanchnic
Describe the greater splanchnic nerve
- From T5-T9
- Though diaphragm enters abdominal cavity
- Synapses at coeliac ganglia - coelicaomesenteric plexus
- Modulates foregut
- Sympathetic innervation to adrenal medulla
Describe the lesser spanchnic nerve
- T9-10
- Travels lateral to greater splanchnic nerve
- Modulates midgut
- Synapses at superior mesenteric ganglia
Describe the least splanchnic nerve
- T11-12
- Abdomen to renal ganglia
Describe the pelvic splanchnic nerves
- Paired visceral nerves
- Carry fibres of ANS as well as sensroy from organs
- Arise from anerior rami of sacral spinal nerves
- Enter sacral plexus
- Travel to side’s inferior hypogastric plexus, bilaterally on walls of rectum
- From there innervate pelvic and genital organs
- AKA pelvic nerve
What is the function of the pelvic splanchnic nerves
- regulate emptying urinary bladder
- Opening and closing of internal urethral sphincter
- Influence motility in rectum
- Sexual functions
Describe the enteric nervous system
- Can operate independently of brain and spinal cord
- Within wall of digestive tract
- Submucosal and myenteric
- Sensroy cells react to different stimuli
- Motor cells connect to either smooth muscle cells or epithelial cells
- Short reflex arc occurs within ENS (most stimulatory, some inhibitory)
- Input from SNS and PSNS
- Respond and control loca conditions
What stimuli do the sensory cells of the ENS respond to?
- pH
- Stretch
- Irriation
- Changes in nutrition concentration
What is the function of the submucosal plexus?
- Local secretion
- Absorption
- Muscle movements
Where is the submucosal plexus located?
- In lamina submucosa
- Best developed in small intestine
What is the function of the myenteric plexus?
- Motor innervation to circular and longitudinal muscle layers
- Secretomotor innervation of mucosa
- Projections to gall bladder, pancreas, sympathetic ganglia
Where is the myenteric plexus located?
Between the longitudinal and circular muscle
Describe the short reflex arc
- 2 nerve plexuses on wall of GI tract
- Plexuses synapse with each other - attaching with either/both smooh muscles or glandular cells
- Excitatory fibres cholinergic (oral direction), inhibitory fibres nitric oxide (aboral direction)
- Short reflex arcs let GI tract self-control activities
- Involved in promotion of peristaltic contractions
Describe teh long reflex arcs between the ENS and ANS
- ENS independent from rest of NS
- Connections modulate secretion adn motility within GI tract
- Influence glands/smooth muscles (PSNS) whereas SNS inhibits PSNS and controls local blood flow
- Interneurons between ENS and ANS relay info between sensory neurons and PSNS
- Attaches to post-ganglionic PSNS pathways
- Can be excitatory or inhibitory dependent on cell type
What is the action os the SNS in the gut?
- Acts on PSNS pathway (gland and smooth muscle cell) to inhibit secretion/contracion
- Also directly to cause vasoconstriction in local blood supply
- Blood directed to vital organs for fight and flight response
What is meant by “electrical coupling” of the smooth muscle cells in the GIT?
- Gap junctions between smooth muscle cells = electrical coupling
- Regulated connection between cells, allows movement of molecules, ions and electrical impulses because cytoplasms are connected
What is the importance of electrical coupling in the GIT smooth muscle?
- Allows synchronised contraction as is seen in peristalsis
- Distension of hollow organs - contract simultaneously and allow diameter of hollow organ to change more or less in one motion
Breifly outline the key features of smooth muscle
- Involuntary
- Single nucleus
- Spindle shaped
- Arranged in sheets
- Cells contain actin and myosin filaments
- Not striated - myofilaments not arracnged in sarcomeres
- Lack T-tubules
- Poorly developed sarcoplasmic reticulum (compared to skeletal muscle)
Where is smooth muscle located in the GI tract?
- Distal oesophagus (some species)
- Stomach
- Small intestine
- Large intestine
What are slow waves in GIT smooth muscle?
- Waves generated and propagated by nterstitial cells of Cajal
- Spread through gap junctions between smooth muscle cells similar to myocardial auto-rhythmic cells
- Control motility
- Can’t cause a smooth muscle contraction
- Prepare smooth muscle for when chyme arrives
Explain how slow waves prime the smooth muscle for contractions
- Does not cause contractions, just prepares
- Higher amplitude of slow wave, higher frequency of spike potentials
- Means more contractions are possible
Describe the interstitial cells of Cajal
- Modified smooth muscle cells
- Central to GI motility regulation
- Function as pacemaker for gut contraction
- Different frequencies in different parts of GI tract
- Modulated by nervous andhormonal input
- Amplify neuronal input
Explain how chyme influences the cells of Cajal
- Activates the cells of Cajal
- Frequency independent of neuronal and hormonal inputs
- Frequency and strength of contraction stimulated by PSNS and SNS
- Increased vagal fibre activity increases contraction strength
- Chyme enters intestine - short reflex activates cells of Cajal
- More chyme = stronger contractions
Describe segmentation peristalsis in the small intestine
- Only occurs here
- Mixing of chyme with digestive juices
- Brings particles of food into contact with wall where nutrients can be absorbed
- Localised, rhythmic contraction of circular smooth muscles that constrict intestine into segments
- Can move chyme in both directions = greater mixing
Describe propulsive peristalsis
- Series of wave like muscle contractions, move food along tract
- Begins in oesophagus when food bolus swallowed
- Alternatig contractions between longitudinal and circular
- One way movement (aboral)
- Stretch receptors open as bolus down oesophagus, longitudinal contract, circular relax (opens space)
- Behind bolus C contract, L relax
- Chyme shunted forwards
What are the stages of swallowing?
- Oral phase
- Pharyngeal phase
- Oesophageal phase
What occurs in the oral phase of swallowing?
Bolus formed, transported to pharynx
What occurs in the pharyngeal phase of swallowing?
- Receives bolus
- Pushes to oesophagus
- Mechanoreceptors present here
What occurs in the oesophageal phase of swallowing?
- Bolus in oesophagus
- Peristalsis starts
Describe the motility in the upper stomach
- Low frequency sustained contractions
- Generates basal pressure within stomach
- Pressure gradient from stomach to SI and thus responsible for gastric emptying
- Swallowing of food and consequent gastric distension inhibits contraction of this region of stoamch
- Allows ballowing out and formation of reservoir without significant increase in pressure
Describe the motility in the lower stomach
- Strong peristaltic waves of contraction
- Increase in amplitude nearer to pylorus
- Effective gastric grinding
- Distension stimulates this type of contraction
- Accelerate liquefaction and hence gastric emptying
- In pylorus, contractions obliterate lumen
- So chyme delivered in spurts to SI
Briefly outline the control of gastric motility
- Complex set of neural and hormonal signals
- Nervous control: ENS (and PSNS and SNS)
- Hormonal - lots, gastrin CCK
- Patterns of gastric motility arise from smooth muscle cells integrating large number of inhibitory and stimulatory signals
List the factors affecting monogastric stomach emptying
- Pacemaker cells and rate of contraction
- The stomach
- Hormonal
- The duodenum
- Solids empty in linear fashion
- Neural
- Emptying of liquids
- Presence of fat in small intestine
Explain how pacemaker cells and rate of contraction affect monogastric stomach emptying
- Cells of Cajal in cranial portion of stomach
- Spontaneous oscillations in membrane potential of Cajal cells transferred to smooth muscle cells surrounding them via gap junctions = synchronicity of muscle cells
- Cells influenced by neural and hormonal factors
- No action potential in Cajal = no contraction of gastric muscle
Explain how conditions within the stomach affect stomach emptying
- Stimulatory signals in the stomach (inhibitory in the duodenum)
- Expansion of stoamch after meal - activation of mechanoreceptors initiates reflexes
- Short reflexes in stomach wall and long reflexes in vagal nerves lead to ACh release
- Larger meal, greater expansion of stomach stronger contractionof stomach = more rapid emptying of first part of stomach
Explain the hormonal control of stomach emptying
- Gastrin
- Secreted by epithelial cells when food present (peptides in stomach) and in dilation of stomach
- Increase contraction of smooth muscle cells
- Dilate pyloric sphincter
- High gastrin stimulates emptying
Explain hwo the duodenum affects stomach emptying
- Has greatest effect on emptying
- Can act to inhibit contractions from stomach and increases tension on the pyloric sphincter
- Increased pressure in duodenal lumen, low pH (high acid content reduces stomach emtyping), high fat concentration, high peptide concentration, high osmolarity
- If fat present in proximal duodenum, stomach will not empty further
Explain the neural control of stomach emptying
- Long reflexes - inhibit stomach emptying by inhibiting activity in SNS fibres, inhibiting vagal nerves
- Short reflexes inhibit cells of Cajal
What is the enterogastric reflex?
A reflex stimulated by the duodenum and stomach, that inhibits gastric motility and secretion of gastric acid
What are the major factors that inhibit stomach emptying?
- Low pH in duodenum
- High fat content in duodenum
- High pressure in duodenum
- High osmolarity in duodenum
- High concentration of peptides in duodenum
What are the hormones that inhibit gastric emptying and where are they released?
- Released from proximla part of SI
- Secretin
- CCK
- Gastric inhibitory peptide
