• The GIT is a tube, specialized along its length for the sequential processing of
food
• Assimilation of substrates from food requires both digestion and absorption
• Digestion requires enzymes, which are secreted in various parts of GIT
• Food ingestion triggers complex whole-body responses (endocrine, neural,
paracrine)
• GIT plays an important role also in homeostasis (absorption vs. excretion,
izovolemia, izoionia, etc.) and immunity
GASTROINTESTINAL TRACT
Mechanical and chemical processing of food
Absorption and excretion of products
Protection of internal environment (toxins, microbes…)
GIT motility – mainly nervous control
Secretion in GIT – mainly humoral control
Transport mechanisms, liver function
+
PARASYMPATICUS
(preganglionic cholinergic fibres)
n.VII, n.IX, n.X, nn.pelvici (S2-S4)
-
SYMPATICUS
(postganglionic adrenergic fibres)
Th5-L2
(tonus and motility –)
(vasoconstriction)
(musc.mucosae, sphincters +)
Motility, secretion, digestion, absorption, storing, excretion
serosis (adventicia)
epithelium
muscularis mucosae
longitudinal
layer
circular layer
plexus
submucosus
(Meissner)
plexus
myentericus
(Auerbach)
ENS
muscularisexterna
submucosa
Modified according to: J. Švíglerová: Fyziologie gastrointestinálního traktu, LF UK Plzeň, 2012
coordination of motility
secretion and absorption
+ glands
+ lymphatic tissue
Circular muscle layer: inhibitory fibers, contraction – gut is longer and smaller in diameter
Longitudinal muscle layer : no inhibitory fibers, contraction – gut is shorter and bigger in diameter
ENTERIC NERVOUS SYSTEM
(plexuses + endings of sympathetic and parasympathetic nervous system + other GIT neurons)
Local (short) reflexes
Control of:
•GIT motility
•GIT secretion
•GIT vasomotor control
Mediators and modulators: Ach, peptides and bioactive amines
Ach, VIP, NOR, DOPA, serotonin, histamine, AT II, PG
somatostatin, enkephalin, GABA, TRH, neuropeptide Y, substance P
secretin, GIP, glucagon, gastrin, CCK, G-releasing peptide
(Secretin group)
(Gastrin group)
Chemoreceptors, mechanoreceptors, thermoreceptors…
(mucosa, musc. externa)
Central reflexes
Continuous tonus of
S, PS
FORWARD SIGNALS : SPEED UP, OPEN THE WAY
BACKWARD SIGNALS: SLOW DOWN, CLOSE THE WAY
GIT INNERVATION
ENTERIC NERVOUS SYSTEM
SYMPATICUS PARASYMPATICUS
Local (short) reflex
Central (long) reflex
CNS
+-
mechanoreceptors
chemoreceptors
osmoreceptors
thermoreceptors
Vagovagal reflex
ELECTROPHYSIOLOGY OF GI SMOOTH MUSCLE
Resting potential: from - 40 to - 80mV ( gNa : gK)
Lower activity of Na+/K+-ATPase
Slow waves (oscillation of rest.MP) 3 (stom.) – 12(duod.)/min – basal electric rhythm
Spike (AP) low voltage, depolarisation – Na+ and Ca2+, 1-10/sec
Pacemaker cells in ENS automacy
Variability neurohumoural regulation
Innervations: nexus, innervations of circular muscle >> longitudinal muscle
No motor endplate Ach, ENS, exceptions
0
-40
ICa direct activation of contraction
(binding to calmodulin)
0
-40
5s
Basal muscle tonus, basal rhythm (time summation)
Spike -10 -20ms
F
E
5s
Contractions triggered by slow waves (no AP!!!)
F
E
Stomach
Modulation of slow waves – amplitude (less by frequency)
GIT MOTILITY
CONTRACTIONS tonic (stomach, colon)
rhythmic
MOVEMENTS propulsive (peristalsis, myenteric reflex)
mixing
Receptive relaxation.
These contractions and movements are responsible for churning, peristalsis and reservoir action
in GIT.
SWALLOWING•Oral phase (voluntary)
•Pharyngeal phase (reflex)<1s
•Oesophageal phase (peristaltic)
Food – chewing (voluntary and reflex)
Saliva (1.5 litres / day)
Frequency of swallowing – approx. 600x / day
SWALLOWING CENTRE
(oblongata, pons)
X., ….
REC
(touch)
Plexus myentericus
Parasympathetic NS (VIP)
Sympathetic NS
V.X.
Proximal sphincter
(somat.motoneurons – X.
striated muscles)
Peristalsis – 3-5cm/s
(primary - swallowing centre,
secondary - ENS)
Mouth
Pharynx
Centr.
Junction
Distal sphincter (cardia)
(smooth muscle) – opened by secondary peristalsis
Local
reflexes
Reflex relaxation of cardia (PS)
Achalasia (cardiospasmus)
Gastrooesophageal reflux
IX.
Oesophagus
GASTRIC MOTILITY
Reflex relaxation of distal oesophageal sphincter
Receptive relaxation of fundus and body
(X. – VIP) (Laplace: P = T . R)
Pacemaker zone (3/min)
Motoric gradient
(F <<< A)
Chyme stratification
1,5 l
A
B
F
D
P
C
O
Migrating myoelectric complex
(„hungry“ contractions)
Reservoir
Mixer (3/min)
1-2 hour: rest
10-20 min: activity, during fasting is stronger
PYLORUS = sphincter ???
Common ENS with bulbus duodeni
Smooth muscle
sympaticus +++, n.X. --- (VIP)
N. vagus +
Plexus cealicus prominent
circular layer
EMPTYING OF STOMACH
CNS
PACEMAKER
CONTRACTIONS OF ANTRUM
CONTRACTIONS OF PYLORUS
DUODENUM:
pH < 3,5
Lipids
Peptides
secretin
CCK, GIP
gastrin
Oddi
gastrin
chemoreceptors
osmoreceptors
slowed emptying
(entorogastric reflex)
Symp. Paras.
FA, H+, osmotically
active substances,
tryptofan
Modulation:
•Peristaltic
movements
•Continuous
tonus
(summation
of
contractions
of antrum –
relaxation)
pressure, distension, pH, pain
VIP
Ach
SYMP.
x.
Coordination of contractions of antrum
and relaxation of bulbus
Duodenogastric reflux
A/D
reciprocal
activity
-
VOMITING (PROTECTION)
HIGHER CENTRES
CENTRE FOR VOMITING
IN MEDULLA OBLONGATA
Afferentation: X., symp.
Taste
Smell
Vision
Labyrinths
Central
Mechanoreceptors
of pharynx
Peripheral
Mechano- a chemoreceptors
of stomach and duodenum
Peripheral
Trigger zone – bottom of 4.chamber
Area postrema – circ. ventric.
organisation
•Antiperistalsis in jejunum and duodenum
•Relaxation of pylorus and antrum
•Contractions of diaphragm (increased intraabdominal pressure)
•Inverse Valsalva manoeuvre (decreased intrathoracal pressure)
•Contractions of pylorus and antrum
•Relaxation of cardia
•Relaxation of upper pharyngeal sphincter
Emetics: central
peripheral
Antiemetics
MOTILITY OF SMALL INTESTINE
•Slow waves – approx.11-13/min in duodenum, 8-9 - ileum
•„Minute“ rhythm (jejunum) – salvos approx. every minute
•Hour rhythm (migrating myoelectric complex)
hours
100%
1 2 3
fasting food
5
10
12/min
duodenum jejunum ileum
Distance from pylorus
in vitro 7/min
in situ
Segmentation >>> peristalsis (up to 10 cm)
LAW OF INTESTINE
INTESTINO-INTESTINAL REFLEX
GASTRO-ILEAL R.
GASTRO-COLIC R.
MOTILITY OF COLON
• Slow waves with frequency 4 – 6 / min
•Segmentation = haustra; 5-10 cm/hour–
pendulum movements
•Mass peristalsis; 1-3/day – „sweeping“
•Reverse peristalsis – in proximal colon („delay“ –
absorption of water and ions)
•Control of anal sphincter: int. – reflex, ext. –
voluntary (+reflex)
•Defecation: abdominal muscles +++, muscles of
pelvic bottom –
•Reflex: colono-colonic, gastro-colic
•Parasympaticus + (X. till FL)
•Sympaticus – (L2 – L4)
4-8 hod
12-72 hod
VIC
PS.
SAKR.
PARAS
Int.
Ext. sfinkter
GI REFLEXES
SYMP.
Voluntary control
Voluntary control
pharynx
oesophagus
stomach
small intestine
colon
rectum
A
Continuous tonus
S, PS
a1
10´´
2-3 hrs
2-4 hrs
10-20 hrs
Signalling:
relax, move on!
slow down!
PARASYMP.
VI
P
Achswallowing
lower oesophagus
local
pyloric
gastroileal
enterogastric
internal anal sphincter
gastrocolic
receptive relaxation of fundus
colonoileal
colonocolonic
GI REFLEXES Voluntary control
Voluntary control
F
E
G
D
J
I
C
A
T
D
R
Superposed on basal tonus
PS and S (sphincters S PS)
swallowing r.
Receptive relaxation of fundus (X.)
Intestino-intestinal r.
(distension)
R. lower oesophagus
Pyloric r. (X.)
Colono-colonic r.
Colono-ileal r. (+)
R. of internal anal sphincter
Enterogastric r.
(chemoreceptors)
10´´
2-3 hrs
2-4 hrs
10-20 hrs
bolus
+
PS and S
-
-
-
-
-
-
-
-
+
+
Gastro-colic r.
Gastro-ileal r.
Local r.
Reciprocal
function of long.
and circ. muscle
Innervations of only circ. muscle
Motility is increased:
•CCK
•Substance P
•Gastrin
•Mass peristaltic movements
•Haustra, segmentation
•Reverse peristalsis