Gastrointestinal

M otility and Secretory Functions of the Alimentary Trac 10

M astication (Chewing)

Functions

Lubricate bolus with salivary secretion

Breakdown bolus to small particles

Begin digestion of carbohydrate ( α-amylase)

especially important for fruits and raw vegetables becauseindigestible cellulose membranes around their nutrient
portions

T eeth organization

Anterior teeth (incisors)

Posterior teeth (molars)

Chewing muscles are innervated by 5th
cranial nerve.

Chewing process is controlled by nuclei
in the brain stem.

Much of the chewing process is caused
by chewing & stretch reflexes

Presence of a bolus of food in the mouth initiates
reflex inhibition of muscles of mastication

causing
lower jaw to drop.
initiates a stretch reflex of jaw muscles
leading to rebound contraction.

which automatically raises the jaw to cause closure
of the teeth, and compresses the bolus against the
linings of the mouth

inhibits the jaw muscles
once again

allowing the jaw to drop and rebound
another time; this is repeated again and again.

S wallowing (Deglutition)

is the ordered sequence of events that
propels food from mouth to stomach

It is initiated voluntarily in the mouth, but thereafter is
under involuntary or reflex control.

The reflex portion is controlled by the swallowing
center in the medulla.

Stages of Swallowing:

Oral stage (voluntary)

Pharyngeal stage (involuntary)

Sensory impulses from the mouth

received by
nucleus tractus solitarius (NTS)

via medulla oblongata through trigeminal and glossopharyngeal Nerves.

The motor impulses to the pharynx and upper
esophagus are transmitted

from the swallowing
center by the 5th, 9th, 10th, and 12th cranial nerves

Esophageal stage (involuntary)

T he Swallowing process

Fu nction of Gastroesophageal Sphincter

This sphincter remains tonically constricted
(protects the esophagus from the stomach acidic
juices)

until the peristaltic swallowing wave passes
down the esophagus and causes a “receptive
relaxation” of the sphincter

Physiological Anatomy of the GIT Wal 1

Longtitudinal.m it's contraction causes shorting.

Circular.m it's contraction causes an increase in luminal pressure

Neural Control of Gastrointestinal Function 6

Enteric Nervous System (ENS):

Is the nervous system of GI
tract + lies entirely in the wall of
the gut

has as many neurons
(about 100 million).

composed mainly of two plexuses

Outer

Lying between
longitudinal and circular
muscle layers

Called myenteric or
Auerbach’s plexus

Controls mainly GI
movements.

Inner

Lying in the submucosa

• called the submucosal
or Meissner's plexus

controls mainly GI
secretion, absorption
and local blood flow.

SMOOTH MUSCLE CLASSIFICATIONS 2

Unitary type (single unit):

Numerous smooth muscle fibers
that contract together as a single
unit.

Cells are electrically coupled via
gap junctions

Multiunit type:

Composed of separate smooth muscle fibers.
•Each fiber operates
independently with single nerve ending

Contract in response to neural
input (such as in esophagus &
gall bladder)

Types of Neurotransmitters Secreted byENs 7

Excitatory motor neurons:

evoke muscle contraction &
intestinal secretion

Neurotransmitters of motor neurons

Substance P

Ach (acetylcholine)

Neurotransmitters of secreto-motor neurons

Release of water, electrolytes and mucus from crypts of Lieberkuhn

Ach

VIP (vasoactive intestinal peptide)

Histamine

Inhibitory motor neurons

suppress muscle contraction

ATP

NO (nitric oxide)

VIP

TYPES OF CONTRACTION 3

Phasic contractions

Periodic contractions
followed by relaxation

Such as in gastric
antrum, small intestine
and esophagus

Associated with slow
waves

Tonic contractions

Continuous contraction without
relaxation

Such as in Orad region of stomach,
lower esophageal, ileocecal and
internal anal sphincter

Caused by:

repetitive spike
potentials

hormones

entry of
Ca ions (not associated with
changes in membrane potentials)

main smooth muscle layer 4

Longitudinal (A)

• Contraction shortens length

expands lumen of longitudinal.

• Innervated by enterics
nervous system (ENS), mainly by excitatory
motor neurons.

Circular (B)

Thicker and more
powerful

Contraction reduces
lumen and increases
length.

• Innervated by ENS, both Excitatory and inhibitory
motor neurons.

More gap junctions.

Types of Movements in the GI 9

propulsive

cause food to move
forward along the
tract at an appropriate
rate

to accommodate
digestion and
absorption

Basic movement in gitis peristalsis

Usual stimulus is distention

Other stimuli

chemical

physical irritation of
epithelial lining in gut.

Atropine (cholinergic blocker) depresses propulsion

Myenteric plexus is important for peristalsis

Parasympathetic stimulation

mixing(segmentation

keep the intestinal
contents thoroughly
mixed at all times

Blend different juices
with the chyme.

Bring products of
digestion in contact
with absorptive
surfaces.

Hormonal control

Specific Characteristics of Smooth Muscle in the Gut 5

Functions as syncytium

Individual smooth muscle fibers are arranged in bundles

each bundle, muscle fibers are electrically connected
through large numbers of gap junctions.

Each muscle layer functions as a syncytium

Start of A.P anywhere within muscle it travels to all directions in muscle

Electrical Activity of it

Smooth muscle of the GIT is excited by continual slow
intrinsic

It has 2 basic types of electrical waves:

Slow waves

Most GI contractions occur rhythmicall

determined by ferquency of the “slow waves“ of s.m membrane potential

Are not A.P

They are oscillating depolarization and repolarization in
the resting membrane potential with unknown cause.

Their intensity varies (5 -15 mv)

Frequency (from 3 to 12 /min):

3 in body of stomach

12
in duodenum

8 terminal ileum

O rigin of slow waves

From interstitial cells of Cajal

ICC, the GI pacemaker), which are abundant in the myenteric
plexuses.

ICCs form a network with each other

interposed
between the smooth muscle layers

with synaptic-like
contacts to smooth muscle cells.

spikes

are true action potentials

Occur automatically

when resting membrane potential
of GIT smooth muscle becomes more positive

(-40mV) (normal resting membrane potential is
between -50 and -60 mV).

slow wave potential rises there is a greater
frequency of spike potentials

potentials excite the muscle contraction.

Autonomic Control of the Gastrointestinal Tract 8

Parasympathetic

Stimulates GIT
Activity.

Innervation

C ranial

entirely In vagus nerve.

esophagus, stomach, pancreas and the intestines down to the first half of the large intestine.

Sacral

pelvic nerves

distal half of the large intestine up to anus (to execute defecation reflexes).

Sympathetic

Inhibits GIT Activity.

fibers originate in spinal cord
between segments T-5 and L-2 To git

Innervate essentially all of the GI tract.

It's ending secrets norepinephrine