Transcript Gastrointestinal Motility, Propulsion and Digestion

Gastrointestinal Motility,
Propulsion and Digestion
Learning Objectives
GIT anatomy and the movement of food through the
alimentary tract.
• Know the basic anatomy of the GI tract, including the
circulation and structure of the intestinal wall.
• Understand how peristalsis provides GI motility and
mixes food.
• Understand neuronal control of the GI tract and the
types of electrical signals used by those neurons.
• Know the basic processes of digestion as food passes
from the beginning to the end of the alimentary tracts.
Alimentary Tract
• The general purpose of the alimentary tract is
to absorb H2O, electrolytes and nutrients from
ingested food and liquid.
• This requires:
- Propelling of contents
- Digestion of contents
- Absorption of nutrients
- Large supply of blood flow (~ 25% of total)
Alimentary Tract Anatomy
• From beginning to end:
Mouth
Esophagus
Stomach
Small intestine
Duodenum (26 cm = 9.8’’)
Jejunum (2.5 m = 8.2 ft)
Ileum (3.5 m = 11.5 ft)
Large intestine (1.5 m = 4.9 ft)
Cecum
Ascending colon
Transverse colon
Descending colon
Anus
Overview of GI Blood Flow
Arterial Blood Supply to the Intestines
• As you would expect,
there is extensive blood
flow for nutrient
absorption
• We will revisit the GI
circulation and
absorption of nutrients.
Microvasculature of Intestines
Intestinal Wall
GI Motility
• 2 Types of movement in the GI tract.
1. Propulsive movement of food forward at an
appropriate rate for digestion and
absorptiion.
2. Mixing movements – caused by peristalsis
and local intermittent contractions.
Peristalsis
• The propulsive movement in the GI is
achieved by peristalsis.
Muscle Contraction for Peristalsis
The smooth muscle of the gut is a syncytium.
Thus, stimulation at 1 point spreads to adjacent areas, creating a peristaltic spread.
Peristalsis also occurs in the bile ducts, glandular ducts, and ureters.
Stimulation of Peristalsis
• Stretching of the gut wall, such as occurs with
a large amount of food, stimulates contraction
2-3 cm behind the stretched wall.
• This initiates the peristalsis that propels the
food forward.
• Other stimuli include parasympathetic signals
and physical or chemical irritation of the
epithelial lining.
Neurons Controlling Peristalsis
The myenteric plexus primarily controls peristalsis. It is an interconnected chain of neurons
between the longitudinal and circular layers of muscle that extends the entire length of the
GI tract.
The submucosal plexus mainly controls GI secretion and local blood flow.
Myenteric Plexus
• Stimulation of the myenteric plexus causes:
- Increased tonic contractions
- Increased intensity of contractions
- Slight increase in rhythm of contractions
- Increased velocity of excitatory waves along the gut wall
• Blocking the myenteric plexus or its
parasympathetic stimulation greatly decreases
peristalsis.
Directional Movement in GI Tract
• The forward movement of food is achieved by:
- The initiation or peristalsis 2-3 cm behind the distended
wall.
- Relaxation of the gut wall in from of the distended wall.
• This pattern requires the myenteric plexus and
is called the meyenteric or peristaltic reflex.
Electrical Activity in GI Smooth Muscle
• Excited by fairly constant,
slow electrical activity that
consists of 2 types of
electrical activity:
1. Slow waves
2. Spikes
Note: the resting membrane
potential can be adjusted.
This is important, as spikes
occur once a threshold
resting membrane
potential is reached.
Slow Waves
• Slow waves are undulating
changes in the resting membrane
potential caused by the entry of
Na+.
• The frequency of the slow waves
sets the rhythm of contractions.
• However, the slow waves
themselves do not usually cause
the contractions. They drive the
membrane to threshold, which
causes spikes to occur.
• Spikes cause the contraction
Spikes
• Spikes are APs, caused by the
influx of primarily Ca2+ and some
Na+.
• They occur once the resting
membrane potential depolarizes
to ~ 40 mV.
• The more depolarized the resting
membrane potential becomes
during the slow waves, the
greater the frequency of spikes.
• The Ca2+ that enters the muscle
during the spike causes the
contraction.
Changes in Resting Membrane Potential (RMP)
• Factors that depolarize
the RMP and increase the
excitability are:
- Stretching
- Parasympathetic stimulation
(acetylcholine)
- Some GI hormones.
• The RMP hyperpolarizes
and becomes less
excitable by sympathetic
stimulation
(norepinephrine and
epinephrine).
Tonic Contractions
• Tonic contractions are continuous instead of
rhythmical and last several minutes-to-hours.
• They are caused by:
- Repetitive spikes
- Continuous depolarization
- Continuous entry of Ca2+
Transition Slide
• So far, we have discussed the general anatomy
of the alimentary tract and how food is
propelled via peristaltic contractions.
• Now, we will describe broadly the events that
occur as food passes through the alimentary
tract.
Chewing and Swallowing
Briefly,
• Mastication breaks apart indigestible components
and greatly increase the surface area exposed to
digestive enzymes.
• During swallowing, the trachea closes, the esophagus
opens, and a peristaltic wave moves the food into
the upper esophagus.
• In the esophagus, peristaltic waves propel the food
into the stomach.
Stomach
The stomach has a capacity of 0.81.5L and is divided into 2
sections: the body (upper 2/3)
and the antrum (lower 1/3).
Food processed in the stomach is
ultimately forced through the
pyloris into the duodenum.
The stomach performs 3 functions:
1. Storage of food
2. Mixing with gastric secretions to
form a semifluid mixture called
chyme.
3. Emptying of chyme at suitable
rate for digestion.
Mixing and Propulsion in the Stomach
• When food is present, peristaltic constrictor waves or
mixing waves begin in the body and move toward the
antrum. The rhythm of the constrictor waves is set by
the slow waves.
• In addition, the constrictor waves dig deep into the
food contents of the antrum. This mixes the food and
exposes more food to digestive juices that are secreted
by the walls of the stomach.
• At the pyloris, some of the processed food (chyme) is
passed into the duodenum; however, most of it is
pushed back upstream by strong contractions
(increasing the mixing).
Stomach Emptying
• Most of the stomach contractions mix the
food with gastric juices.
• Strong contractions occur ~ 20% of the time.
These propel larger amounts of chyme
through the pyloris (pyloric pump).
• The pyloris is tonically contracted to a point
where fluid easily passes, but unprocessed
food does not.
Regulation of Stomach Emptying by
the Stomach
• Stomach emptying is regulated somewhat by the
stomach itself. Stretching stimulates myenteric
reflexes that enhance the pylorc pump, relaxes
the pyloris and stimulates the release of the
hormone, gastrin.
• Gastrin causes the secretion of acidic gastric
juices and moderately stimulates the pyloric
pump.
• The digestive products of meat also stimulate
gastrin release.
Regulation of Stomach Emptying by
the Duodenum
• A more important regulator of stomach
emptying is the duodenum.
• Control by the duodenum prevents the
movement of excess food into intestines. Too
much food would limit the ability to reabsorb
nutrients.
• The inhibition of stomach emptying by the
duodenum is mediated by nervous reflexes
and hormones.
Inhibition of Stomach Emptying by the
Duodenum Nerve Reflexes
• Factors that inhibit stomach emptying by the
duodenum are:
- Distension of the duodenum
- High acidity of the chyme
- Hypertonic and hypotonic chyme (particularly
hypertonic)
- Breakdown products of proteins and to a lesser
extent, fat
- Irritation of the duodenum
Inhibition of Stomach emptying by the
Duodenum Hormones
• Fats in the duodenum inhibit stomach
emptying by extracting hormones from the
epithelium.
• One important hormone is cholecystokinin
(CCK). Other hormones will be discussed later.
• CCK inhibits stomach motility induced by
gastrin.
Movement in the Small Intestine
• Stretching of the intestine causes
peristaltic contractions that cause
forward movement and mixing.
• Movement is slow to allow
greater absorption of nutrients (1
cm/min). At this rate, it takes 3-5
hr for food to pass through the
small intestine.
• Hormones that increase
peristaltic activity are gastrin,
CCK, insulin, motilin, and
serotonin.
• Hormones that inhibit intestinal
motility are secretin and
glucagon.
Peristatic Rush
• Normally, peristalsis in the small intestine is
weak. As indicated by the slow movement of
food.
• However, irritation, as occurs with infectious
diarrhea, causes rapid and powerful
peristalsis.
• This involves the autonomic nervous system
and intrinsic control of the myenteric plexus.
Movements in the Colon
• The colon absorbs water and
nutrients, progressively
making solid feces.
• It also stores the solid feces
until it can be expelled.
• Movement is slow, requiring 815 hr to move material
through the colon.
• Movement and mixing are
achieved by peristaltic-like
contractions.
Haustrations
• The combination of constricted and relaxed
portions of the colon create haustrations (baglike pouches).
• Haustrations mix the chyme or fecal material
and provide slow persistent forward
movement.
Propulsive (Mass) Movements
• Occasionally (1-3 times/day), a modified
contraction will propel the contents forward
en masse.
• This occurs by a peristaltic contraction pushing
the contents through 20 cm or more of colon
in which the haustrations are relaxed.
• A series of mass movements persist for 10-30
min; then cease. They may recur ~ 12 hr later.
• Irritation can cause mass movements.
Defecation
• Feces entering the rectum
stretches the walls and stimulates
the myenteric plexus to initiate
peristaltic contractions in the
descending colon downward.
• The myenteric plexus relaxes the
internal sphincter.
• If the external sphincter is
voluntarily relaxed, defecation
occurs.
• This is augmented by
parasympathetic activity.