Lecture 5 - Motility Flashcards
Structures involved in prehension and chewing
Lips, tongue, incisors = procure the feed
Molars = grind the feed
How do molars vary in carnivores, omnivores and herbivores
Carnivores + omnivores = vertical movement
Herbivores = lateral movement (side to side)
Indications that a horse should be floated
Dropping feed
Losing weight
Head tossing
Balls of chewed hay coughed out
Two phases of swallowing
Oropharyngeal phase
Oesophageal phase
Describe the oropharyngeal phase of swallowing
Food pushed back by tongue, tongue prevents return to mouth
Uvula blocks nose
Glottis/epiglottis block access to lungs
Swallowing center in medulla coordinates and inhibits respiratory center
Pharyngoesophageal sphincter opens
Describe the oesophageal phase of swallowing
Peristaltic wave every 5-9s
Skeletal muscle
Oesophagogastric/cardiac sphincter opens
What is peristalsis
Waves of contraction
Rapid propulsion
Relaxation and contraction of longitudinal and circular muscles
What is the peristaltic reflex programmed by
The enteric nervous system
What are the three parts of the stomach
Fundus
Corpus
Antrum
What is the job of the fundus
Receptive relaxation (makes room when food enters)
What molecules are involved in relaxation of the fundus? What induces them?
Nitric oxide and vasoactive intestinal polypeptide
Induced by ACh
What is the role of the corpus
Mixing vat for saliva, food and gastric secretions
What is the role of the antrum
Propulsion of food through the pyloric sphincter to the duodenum
Size discrimination (small pieces only)
What is the antrum controlled by
Distention
Parasympathetic NS
Where/how does mixing occur in the stomach
In the corpus
Peristalsis acts against closed pyloric sphincter
HCl and pepsin stirred in
Protein digestion begins and lipid droplets form
How does gastric emptying occur
Peristalsis
Pyloric shincter opens
Chyme (digesta) enters duodenum
What regulates gastric emptying
Force of contraction
Signals from duodenum
What is gastric emptying
Empty the stomach into the SI
What factors increase gastric emptying
Neural control (distension of stomach wall, increased parasympathetic)
Endocrine control (gastrin)
What factors decrease gastric emptying
Neural control (chemoreceptors, osmoreceptors, mechanoreceptors at duodenum, sympathetic NS)
Endocrine control (CCK, secretin, gastric inhibitory peptide)
Why does increased sympathetic NS activity decrease gastric emptying
In fight or flight mode, do not want to waste energy on digestion
What is segmentation
Most common form of motility (contraction of circular muscle)
Alternating contractions = mixing (not directional)
Freq decreases distally duodenum -> colon
Motility in the colon
Haustration (segmentation) = mixing and movement
Clearing
Stimulus, sensor, signal, effector, response and effect of motility regulation in the duodenum
Stimulus = chyme distend walls
Sensor = stretch receptors in wall
Signal = nervous
Effector = smooth muscle
Response = vigorous segmentation
Effect = removal of stimulus
Stimulus of motility in the ileum/colon
Protein/digestion in stomach
Gastrin released from stomach
Two responses to stimulus in ileum/colon
- Gastro-ileal reflex (stimulates segmentation in ileum, inhibits (opens) ilea-caecal sphincter)
- Gastro-colic reflex (mass movement in colon)
What are tonic contractions? Examples?
Sustained (long-term) contraction of muscle
Divide GIT into functional segments
e.g. esophageal sphincter, pyloric sphincter, ileocecal sphincter
What do the interstitial cells of cajal do
set a constant pulse in smooth muscle cells and are considered the pacemakers of the GIT
What smooth muscle layers make up the GIT
Circular and longitudinal muscle layers
Smooth muscle cells have an intrinsic rhythm of ________________________________________
depolarization and repolarization
When do contractions occur in smooth muscle
When depolarization is high enough (reaches the threshold) to cause action potential
Which hormones regulate depolarization for contraction
Acetylcholine (parasym) increase resting basal membrane potential = stimulates AP and contraction
Norepinephrine (sym) decrease resting basal membrane potential = prevent AP and contraction
Small intestine motility in the fed state
Segmentation reflex (mixes contents, regulated by ENS)
Small intestine motility in fasted state (post meal)
Migrating myoelectric complex
- peristaltic activity
- uninterrupted in herbivores
Three phases of migrating myoelectric complex
- Quiescent period (nothing)
- Intermittent contraction (cross threshold periodically)
- Powerful propulsion sweeps intestinal contents out (contract every time)
Functions of large intestine motility
Microbial digestion
Reabsorption of water/electrolytes
What kind of motility is there in the large intestine
Stationary haustral contractions (mixing)
Peristaltic contractions
Antiperistaltic movement (fill the cecum)
Aboral mass movement (evacuate entire length of colon = defecation)
Aboral vs oral
Aboral = away from mouth
Oral = towards mouth
What can alter GIT motility
Stomach worms = increase gastric emptying
-> MMC reduced and replaced with migrating action potential complex (faster)
Where is the vomiting centre
Medulla
What is the vomiting centre activated by
Visceral afferents anywhere in GIT (nerves to brain) due to blockage, mucosal irritation
Vestibular stimulation (motion sickness)
Efferent nerves activate upper GIT and diaphragm and abdominal muscles
Slide 27**
vomiting
What does the vomiting centre do pre-vomit
Close soft palate, close glottis
Respiration stops
Abdominal pressure (diaphragm contracts)
Major concerns surrounding vomiting
Significant fluid and electrolyte loss (dehydration)
Acid-base imbalance
Plexuses of the enteric nervous system
Myenteric (Auerbach) plexus
Submucosal (Meissner) plexus
Characteristics of the myenteric plexus
Between circular and long smooth muscle
Extensive, long interneurons
Sympathetic and parasympathetic
Characteristics of the submucosal plexus
In submucosa
Interneurons minor
Only parasympathetic
Afferent neurons aka? Efferent neurons aka?
Sensory, motor
Slide 32, 33
Afferent and efferent neurons
What nervous system do efferent and afferent neurons fall under
Peripheral NS
What nervous system do the sympathetic and parasympathetic pathways fall under
Peripheral NS
Autonomic neurons
Two types of efferent neurons
Autonomic
Somatic
What kind of activity do the autonomic effector tissues have
Intrinsic (spontaneous or tonic) activity
What is the sympathetic NS
Inhibit digestion
Fear, fight or flight responses
Postganglionic neurons release norepineephrine (stop enteric NS)
What is the sympathetic NS
Inhibit digestion
Fear, fight or flight responses
Postganglionic neurons release norepinephrine
What does the sympathetic NS decrease
GI tract smooth muscle motility
Exocrine secretion (into gut)
Exocrine secretion
What is the parasympathetic NS
Promotes digestion
Postganglionic neurons release acetylcholine
What does the parasympathetic NS increase
GI tract smooth muscle motility
Exocrine secretion
Endocrine secretion
Slide 36
Symp and parasymp
What is an ileus
Caused by inhibition of motor activity
Increased activity upstream, decreased downstream
Blocked long time = distention, sympathetic reflex enters CNS = pain
Common after surgery
Slide 38**
Summary of ENS
