Motility Flashcards
Layers GI tract
Serosa
Muscularis externa: circular, longitudinal
Submucosa: muscularis mucosae
Mucosa
Mucosal glands
Innervation GI tract
S fibers = postganglionic
PS fibers = preganglionic
Afferent fibers
Myenteric plexus (Auerbach)
Submucosal plexus (Meissner)
Myenteric plexus location
Between circular and longitudinal layers of muscularis externa (interconnected by fibers that convey the AP)
Myenteric plexus modulators
Mechanoreceptors through ANS
Behavior of GI smooth muscle cells
Spike depolarization
Plateau depolarization
Slow waves - at rest
Depolarization: Ca influx // Hyperpolarization: K efflux
Causes of depolarization
Mechanic distension
Ach
Hormones
Causes for hyperpolarization
NA
A+NA
Patterns of contraction
Slow waves —> rhythmic contractions (rest)
Tonic contractions —> sustained, non rhythmic, segmental
S vs PS innervation
S inhibits peristalsis —> A, NA
PS stimulates peristalsis —> Ach
Functions myenteric and submucosal plexus
Myenteric p: motility
Submucosal p: activates gland secretion + vasoconstr / dilation of vessels
Effects myenteric plexus
Excitation (Ach) in muscularis externa
Relaxation (VIP) os sphincters and iliocecal valve
Effects submucosal plexus
(Local, region-specific effect)
Contraction submucosal plexus
Local secretion and absorption
Types of movement
Mixing contractions (segmentation)
Propulsive contractions
Mixing contraction (segmentation)
(Also propulsive but less effective)
Mix and spread the chyme —> increase absorption
Segmentation = subchymes
Alternative contractions = chymes are mixed
Propulsive contractions
Mediated by PS nerves in myenteric plexus
Upstream contraction and down stream relaxation
Displace the chyme (prox tube rapidly, distal tube slower)
Oral phase
Chewing / mastication
Homogenization (chemical and mechanical)
Taste-sensory inputs
Muscles of the oral phase and innervation
Jaw elevators
- temporal
- masseter
Jaw depressors
- lat and med pterygoideus
- digastric
- mylohyoid
- geniohyoid
Innervation: V3 (mandibular nerve)
Chemical homogenization occurs due to
Salivary glands —> amylase
- sublingual and submaxilary glands (VII CN)
- parotid gland (IX CN)
Mechanical homogenization occurs due to
Intrinsic lingual muscles (XII CN)
- sup longitudinal
- vertical
- transverse
- inf longitudinal
Extrinsic lingual muscles
- Genioglossus
- Hyoglossus
- Styloglossus
- Geniohyoid
Touch innervation (tongue)
Post 1⁄3 → IX CN
Ant 2⁄3 → branch of the mandibular nerve V3 (Lingual nerve)
Taste innervation (tongue)
Beginning of the pharynx → X CN, vagal branch (int laryngeal n)
Post 1⁄3 → IX CN
Ant 2⁄3 → branch from VII CN (Chorda tympani)
Pharyngeal phase
Movement of the bolus oropharynx —> esophagus.
Involuntary
3 functions:
1. Coordination of the passage of the bolus.
2. Preventing food from entering the nasopharynx
3. Preventing food from entering the lungs
Pharyngeal phase I: events
Elevation of the uvula
Elevation of the palate
Tension of the palate
Result: nasopharynx is sealed off
Pharyngeal phase I: muscles
Palate elevator (Levator veli palatini)
Tensor veli palatini.
Pharyngeal phase I: innervation
Sensory fibers: IX CN —> senses food presence = reflex starts
Motor fibers: X, V3 CN —> stimulates muscles
Pharyngeal phase II: events
Elevation pharynx + larynx
(due to muscles longitudinal layer)
Pharyngeal phase II: muscles
Constrictors/outer circular muscles (X CN)
- Superior constrictor
- Middle constrictor
- Inferior constrictor
Elevators/inner longitudinal layer:
- Stylopharyngeus (IX CN)
- Salpingopharyngeus (X CN)
- Palatopharyngeus (X CN)
Pharyngeal phase III: events
Adduction of the vocal cords
Deglutition apnea
Epiglottis closure
Elevation and anteversion of the larynx
Pharyngeal phase IV: events
Pharyngeal peristalsis: propulsion of the bolus
Relaxation of the upper esophageal sphincter
Pharyngeal phase IV: muscles
Outer circular muscles: superior, middle and inferior pharyngeal constrictors (pharyngeal peristalsis)
Esophageal phase
Bolus is forced inferiorly from pharynx —> esophagus
Esophageal phase I: EVENTS
Primary esophageal peristalsis
Secondary esophageal peristalsis
Primary esophageal peristalsis: muscles
Striated muscle in upper thirds (IX and X CN)
Smooth muscle in inferior third (myenteric plexus, X CN)
Primary esophageal peristalsis: innervation
CNS is required for activation of primary peristalsis, BUT peristalsis can also occur independently from it
There is central but also peripheral neurogenic control.
Secondary esophageal peristalsis: definition
If the bolus cannot be moved through primary esophageal peristalsis.
Activated through the firing of local mechanoreceptor (distension —> bolus)
Secondary esophageal peristalsis: events
Above the bolus level: circular layer contraction, longitudinal layer relaxation
Below the bolus level: circular layer relaxation, longitudinal layer contraction
Secondary esophageal peristalsis: neurotransmitters
Relaxation: VIP (vasoactive intestinal polypeptide) + NO
Activators: Ach and substance P.
Esophageal phase II: events
Relaxation + opening of lower esophageal sphincter (cardias)
Esophageal phase II: important structures
Costal diaphragm
Phreno-esophageal membrane
(Closure of cardias: physiological sphincter)
Gastric motility: innervation
PS: hypothalamus - dorsal vagal nucleus - X CN
S: greater splanchnic nerves - synapse in celiac ganglion
PS stimuli: touch and taste (tongue), smell, sight of food,…
S stimuli: stress, fight-flight response
Stomach functions
Storage
Mixing of food with gastric secretions
- Propulsion
- Grinding
- Retropulsion
Emptying
Functions of each stomach portion
Oral portion (fundus) -> receives the bolus, storage and some digestion.
Caudal portion (body and antrum) -> digestion + propulsion.
Mixing vs propulsion waves
Mixing waves = slow waves. From body to antrum
Propulsion waves = spikes. From antrum to pylorus
Vagal reflex
- Bolus = distension of stomach walls -> mechanoreceptors Myenteric plexus -> info to CNS
- PS reflex through X CN -> slow waves, segmentation movement → mixing
- Antrum: propulsion waves (spike) push the chyme from antrum to pylorus = closed -> allows + mixing and chyme formation
- Relaxation in the pylorus (X CN) = chyme continue its way (depends on hormones)
Gastric emptying - stimulus
Gastric factors:
- Gastric food volume.
-↑ gastrin secretion (by G cells in antrum)
Effects of gastric factors
Pyloric sphincter relaxation
- Pyloric pump activation
- Secretion of gastric acidic juices (HCl) (by stomach gastric glands)
Inhibition of gastric emptying
Duodenal factors (too much chyme -> regulate pyloric function to protect itself)
- Duodenal distension
- Toxic components
- Chyme acidity
- Osmolarity
- Protein/fat rich chyme
Mechanism of gastric emptying
Entero-gastic reflexes:
- Myenteric plexus
- S nervous system (extrinsic nerves)
- Inhibitory reflex of the vagus
- Hormonal mechanisms
- CCK (jejunum)in response to fat rich chyme
- Secretin (duodenal mucosa) in response to acid
- Gastric inhibitory peptide (GIP) in response to fat & carbohyd rich chyme
Effects inhibition of entero-gastric reflexes
Increase tone (contraction) of the pyloric sphincter
Inhibit pyloric pump (therefore inhibit emptying)
Neuro-hormonal mechanism of entero-gastric axis (tabla)
Cells that protect gastric mucosa
Principal/Mucus cells —> bicarbonate mucus
Parietal cells —> chloridric acid
Endocrine cells
Protection mechanism of the gastric mucosa
Bicarbonate mixes with the protons of the acid content of the stomach and becomes water, neutralizing and protecting the lining.
Motility small intestine
Segmentation movements
Propulsion
Migrating motor complex
Ileocecal valve
Segmentation movements of the small intestine
Mixing of the chyme with bile, pancreatic and intestinal fluids
Increases the absorption surface
Antrograde and retrograde movements
Propulsion in small intestine
Only anterograde
Stimulated by the PS nerves (stretching and irritants)
Inhibited by S nerves (fight-flight response)
Migrating motor complex
During fasting
Cleans the remaining food, dead cells, bacteria,…
Slow waves induced by motilin (endocrine cells in intest crypts)
Iliocecal valve (motility of the small intestine)
Always closed.
Opened by the presence of Gastrin -> gastroileal reflex -> activates the ileocecal reflex
Motility of large intestine
Austrae contractions
Peristalsis de Masa
Austrae contractions
Stimulus = stretching / distension
Segmentation movements by tinae coli when there is food.
Ascendent and transverse colon
Increases the absorption surface
Peristalsis de Masa
Stimulated by stretching + irritants and gastro-colic reflex
Small propulsion movements, continuous and directed towards the rectum.
Transverse and descending colon
During and after meals
Increase the absorption surface
Defecation reflex
Stimulus = distension of the sigmoid colon + rectum.
Pathway:
Distension of the rectum -> sensory neurons through pelvic splanchnic nerves -> dorsal horn of the spinal cord - reflex -> efferent fibres of these same S2-S4 PS nerves
from the ventrolateral horn -> propulsion (contractions)
Conscious relaxation:
- External muscles of the sphincter at the anus
- Regular, voluntary motor neurons (ventral horn)
