Gut Motility

Layers of the GI Tract

mucosa – generally consists of three distinct layers:

epithelium – modulates secretion and absorption

lamina propria – contains connective tissues, glands, and capillaries

muscularis mucosa – thin smooth muscle layer that helps to create folds

submucosa – contains loose connective tissue, glands, and blood vessels

muscularis externa – highly developed smooth muscle layer – primary modulator of motility

lines entire GI tract except for upper esophagus and anal sphincter – inner circular layer, outer longitudinal layer

serosa (adventitia) – outer connective tissue layer

Neural Control of GI Motility

Enteric Nervous System (ENS) – as many neurons (10⁸) as spinal column and has every neurotransmitter identified in CNS

consists of two networks:

(1) submucosal plexus (Meissner’s plexus) – lies between submucosa and muscularis externa

involved in internal processing – modulates secretion and blood flow

stimulatory secretomotoneurons stimulate glands and epithelium (via ACh and VIP)

sensory neurons respond to chemical or mechanical stimuli to produce secretomotor reflexes

submucosal vasodilator neurons dilate blood vessels in the submucosa

(2) myenteric plexus (Auerbach’s plexus) – linear sheath within muscularis externa (between circular and longitudinal)

control of muscularis externa – integrates different parts of gut (continuous esophagus to rectum)

excitatory motoneurons contract muscle by ACh to muscarinic receptors; inhibitory use VIP and NO

sensory neurons respond to chemical or mechanical stimuli – receptors in all layers of GI tract

many reflex arcs involve interneurons that use ACh to activate nicotinic receptors on motoneurons

filling Þ reflex that contracts muscles in oral direction and relaxes muscles in anal direction

Extrinsic Innervation – by Autonomic Nervous System (ANS)

sympathetic – post-ganglionic adrenergic (NE) – both ENS plexuses; also glands, blood vessels, muscularis mucosa

inhibits most gastric functions – causes vasoconstriction, reduced secretion, and muscle relaxation
stimulates contraction of certain sphincters (e.g., pyloric)

parasympathetic – pre-ganglionic cholinergic (ACh) – terminate in ganglia of both ENS plexuses

stimulates most gastric functions

vagus nerve innervates tract from esophagus to transverse colon, pelvic nerve from descending colon to anus

GI Smooth Muscle Cells

elongated bundle cells – coupled mechanically (dense bodies) and electrically (gap junctions)

unitary smooth muscle – smooth muscle system so rich in gap junctions that it contracts as a unit
more gap junctions in circular than longitudinal layer of muscularis externa, so contractions spread rapidly and efficiently around lumen, more slowly along lumen

contraction activated by influx of Ca⁺⁺ which causes calmodulin to activate myosin light chain kinase (MLCK)

MLCK phosphorylates myosin, which enables it to bind actin (MLCK activates "myosin motor function")
after Ca⁺⁺ is removed, some muscles relax (phasic tone, as peristalsis); others do not (tonic tone, sphincters)

neurotransmitters can cause contraction (ACh) or relaxation (VIP, NO)

no synapses – must diffuse 20-80 um across diffuse junctions from varicosities (swellings) in neurons

muscles also respond to hormones (can cause contraction or relaxation) or stretch (stretch-activated Ca⁺⁺channels)

Slow Waves – all muscle cells have a negative resting potential (-40 to –80 mV) produced by the Na⁺/K⁺ pump

variations in Na⁺/K⁺ pump activity produces intrinsic rhythmic oscillations in the resting potential
this zone of partial depolarization migrates down the GI tract, and can cause contraction if the peak of the slow wave is greater than the electrical threshold at which voltage-gated Ca⁺⁺ channels open
neurons can change amplitude and frequency of slow waves – ACh and substance P increases, NO and VIP decreases
frequency and shape of waves differ in different layers and levels (e.g., 3 waves per minute in stomach, 12 in duodenum)

The Swollowing Reflex – Three Phases

(1) oral/voluntary phase

(2) pharyngeal phase – soft palate rises and trachea closes; pharynx contracts rapidly

both upper esophageal sphincter (UES) and lower esophageal sphincter (LES) open

(3) esophageal phase – UES closes (by reflex), peristalsis carries bolus down esophagus; LES closes after bolus passes

if bolus is not cleared by primary peristalsis, secondary peristalsis may occur by reflex
all of this is controlled by the vagus nerve

upper 1/3 of esophagus and UES (striated muscle): direct motor endplate from the vagus nerve
lower 1/3 of esophagus and LES (smooth muscle): diffuse junctions from myenteric plexus (has vagal synapses)

UES – resting tone prevents air from entering esophagus
LES – resting tone prevents gastroesophageal reflux – if tone is too high, food gets stuck in esophagus (called achalasia)

Motility of the Stomach – gastric motility is used to accomplish three things:

(1) storage of ingested food by distention – fundus and body relax reflexively as food enters – capacity increased

called vasovagal reflex – mediated by parasympathetic inhibition of myenteric motoneurons (VIP from vagus n.)

(2) fragment food and mix with secretions by churning via slow waves (approx. 3/min) from pacemaker (in body, just below fundus)

slow waves move down body and increase in intensity as they approach the antrum – food separated into layers
end of antrum and pylorus contract simultaneously (systolic contraction of antrum)

small amount of liquid gets through pylorus before it closes – thus liquids clear faster than solid particles

antrum contents are forced back (retropulsion) to mix antrum contents with secretions and disrupt food particles

(3) transfer chyme to duodenum by contraction – called migrating myoelectric complex (MMC)

10-20 minutes of intense antral contractions every 1-2 hours during fasting – clear particles from the stomach

pyloric sphincter – between stomach and duodenum – substantial circular smooth muscle allows tight closure

regulates transfer of chyme to duodenum and prevents regurgitation of duodenal contents to stomach
mucosa and submucosa of pylorus and duodenum are separate, but the myenteric plexus is continuous
neurohormonally regulated – sympathetic contracts the sphincter, parasympathetic can relax or contract

regulation of gastric emptying primarily occurs via the duodenum (Ý dextrose, ß rate of gastric emptying), and also somewhat through gastric effects

duodenum and jejunum have receptors for distention, acid, fats, peptides, and osmolality

neuronal – reflex through intrinsic and extrinsic

hormonal – different receptors cause release of different hormones (acid Þ secretin, fats Þ CCK)

Vomiting – reflex oral expulsion of gastric (and sometimes intestinal) contents – reverse peristalsis and abdominal contraction

controlled by medullary centers in brain, and triggered by receptors in several locations:

labyrinthine receptors of ear (motion sickness)

overdistention receptors of duodenum and stomach – emetics (e.g., ipecac) act on duodenal receptors

trigger zone of CNS – many drugs (e.g., opiates) act here

touch receptors in throat

sensory innervation rich, both extrinsic and intrinsic

Motility of the Small Intestine

The small intestine is more than 5 m long, and makes up 75% the length of the GI tract

three regions: duodenum = 25 cm, jejunum = 2 m, ileum = 3 m – duodenum and jejunum are primary sites of absorption

Segmentation – dominant form of motility – different regions contract differently to mix contents without net movement

Peristalsis – short (5-10 cm) waves triggered by local stimulus – called myenteric or peristaltic reflex

a bolus in the lumen produces a peristaltic wave behind it – this is the "Law of the Intestine"

Migrating Myoelectric Complex (MMC) occurs as in the stomach – 10 minute bursts in any one area every 90 minutes

continues from stomach to ileum over about 100 minutes – new MMC enters at stomach as old one leaves ileum

Reflexes – occur over considerable distances

(1) intestinointestinal – overdistention in one segment causes relaxation in other

(2) ileogastric – distention of ileum decreases gastric motility

(3) gastroileal – gastric motility stimulates movement through ileocecal sphincter

Obstructions – occur from cancer, ulcers, hernia, or paralytic ileus (loss of motility following abdominal trauma or surgery)

type of vomit indicates location above pylorus is acidic, below pylorus is basic
high obstruction produces intense vomiting, low obstruction leads to constipation and delayed vomiting

Emptying – ileocecal sphincter is relaxed by ileal peristalsis, distention of terminal ileum, or gastroileal reflex

Motility of the Colon

The colon absorbs water and electrolytes (in ascending colon), stores fecal matter, and eliminates waste

longitundinal muscle is gathered into three narrow bands (teniae coli)
two anal sphincters – internal consists of smooth muscle, external of striated

Innervation – parasymp. innervation is from the vagus nerve above transverse colon and from the pelvic nerve below

sympathetic decreases motility, parasymp. increases segmental movements and produces sustained contractions
external anal sphincter is innervated by somatic motor fibers – both voluntary and reflex control

Haustration – segments (haustra) mix and knead contents

Mass Movement – periodic (1-3 per day) sustained contractions that move material forward

can be triggered by reflex from stomach (gastrocolic reflex) or duodenum (duodenalcolic reflex)

Defecation – mass movement of feces into rectum causes internal sphincter relaxation and external sphincter contraction

voluntary relaxation of external sphincter initiates defecation – control of muscle is learned behavior
if no motor nerves to external sphincter (babies or lower spinal damage), defecation is automatic upon filling of rectum