Gastrointestinal

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Gastrointestinal

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Autonomic Control of the Gastrointestinal Tract 8

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Sympathetic

It's ending secrets norepinephrine

Innervate essentially all of the GI tract.

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

Inhibits GIT Activity.

Parasympathetic

Innervation

Sacral

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

pelvic nerves

C ranial

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

entirely In vagus nerve.

Stimulates GIT Activity.

Specific Characteristics of Smooth Muscle in the Gut 5

Electrical Activity of it

It has 2 basic types of electrical waves:

spikes

potentials excite the muscle contraction.

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

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 waves

O rigin of slow waves

From interstitial cells of Cajal

ICCs form a network with each other

interposed between the smooth muscle layers

with synaptic-like contacts to smooth muscle cells.

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

Frequency (from 3 to 12 /min):

8 terminal ileum

12 in duodenum

3 in body of stomach

Their intensity varies (5 -15 mv)

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

Are not A.P

Most GI contractions occur rhythmicall

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

Smooth muscle of the GIT is excited by continual slow intrinsic

Functions as syncytium

Each muscle layer functions as a syncytium

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

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

Individual smooth muscle fibers are arranged in bundles

Hormonal control

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Types of Movements in the GI 9

mixing(segmentation

Bring products of digestion in contact with absorptive surfaces.

Blend different juices with the chyme.

keep the intestinal contents thoroughly mixed at all times

propulsive

The gut can cause a contractile ring to appear in the circular muscle, and this ring then spreads along the gut tube.

Parasympathetic stimulation

Myenteric plexus is important for peristalsis

Atropine (cholinergic blocker) depresses propulsion

Usual stimulus is distention

Other stimuli

physical irritation of epithelial lining in gut.

chemical

Basic movement in gitis peristalsis

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

to accommodate digestion and absorption

main smooth muscle layer 4

Circular (B)

More gap junctions.

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

Contraction reduces lumen and increases length.

Thicker and more powerful

Longitudinal (A)

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

expands lumen of longitudinal.

• Contraction shortens length

TYPES OF CONTRACTION 3

Tonic contractions

Continuous contraction without relaxation

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

Caused by:

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

hormones

repetitive spike potentials

Phasic contractions

Periodic contractions followed by relaxation

Such as in gastric antrum, small intestine and esophagus

Associated with slow waves

Types of Neurotransmitters Secreted byENs 7

Inhibitory motor neurons

suppress muscle contraction

VIP

NO (nitric oxide)

ATP

Excitatory motor neurons:

evoke muscle contraction & intestinal secretion

Neurotransmitters of secreto-motor neurons

Histamine

VIP (vasoactive intestinal peptide)

Ach

Release of water, electrolytes and mucus from crypts of Lieberkuhn

Neurotransmitters of motor neurons

Ach (acetylcholine)

Substance P

SMOOTH MUSCLE CLASSIFICATIONS 2

Subtopic

Multiunit type:

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

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

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Unitary type (single unit):

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Cells are electrically coupled via gap junctions

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

Neural Control of Gastrointestinal Function 6

Enteric Nervous System (ENS):

On GIT wall

composed mainly of two plexuses

Inner

controls mainly GI secretion, absorption and local blood flow.

Lying in the submucosa

• called the submucosal or Meissner's plexus

Outer

Controls mainly GI movements.

Lying between longitudinal and circular muscle layers

Called myenteric or Auerbach’s plexus

has as many neurons (about 100 million).

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

Physiological Anatomy of the GIT Wal 1

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

Longtitudinal.m it's contraction causes shorting.

.

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M otility and Secretory Functions of the Alimentary Trac 10

S wallowing (Deglutition)

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

T he Swallowing process

1. Oral phase 2. Pharyngeal phase 3. Esophageal phase

Stages of Swallowing:

Esophageal stage (involuntary)

Pharyngeal stage (involuntary)

The motor impulses to the pharynx and upper esophagus are transmitted

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

Sensory impulses from the mouth

received by nucleus tractus solitarius (NTS)

via medulla oblongata through trigeminal and glossopharyngeal Nerves.

Oral stage (voluntary)

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

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

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

M astication (Chewing)

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

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

inhibits the jaw muscles once again

Chewing process is controlled by nuclei in the brain stem.

T eeth organization

Chewing muscles are innervated by 5th cranial nerve.

Posterior teeth (molars)

Anterior teeth (incisors)

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

Functions

Begin digestion of carbohydrate ( α-amylase)

Breakdown bolus to small particles

Lubricate bolus with salivary secretion