Exam 3 (Lecture 6) - GI Tract Motility, Deglutition, Motility Patterns of Esophagus

Question 1

Define GI motility and the 3 types of GI motility. What are the 4 functions of GI motility?

Answer 1

Contraction and relaxation of the smooth muscle layers of the GI tract; has direct actions on ingesta in the gut lumen.

3 Types:
1) Propulsive (peristalsis)
2) Mixing (segmentation)
3) Retentive

4 Functions:
1) Move ingesta from esophagus to anus
2) Retention of ingesta at a given site for digestion, absorption, or
storage
3) Mechanical digestion in stomach and mix with gastric
secretions
4) Circulate ingesta for maximal contact with the absorptive
surface

Question 2

Describe how the Interstitial Cells of Cajal (ICC) influence the electrical properties of GI smooth muscle and motility (slides 5-12).

Answer 2

First level of control of GI motility lies in the intrinsic electrical properties of the smooth muscle mass.

Interstitial Cells of Cajal (ICC) cause spontaneously undulating waves of partial depolarization and repolarization that stimulates the GI smooth muscle to contract and relax.
- interstitial cells: morphological term denoting a variety of cells
of differing origins and phenotypes occupying spaces within the
interstitium between the cells in smooth muscle tissue
- fibroblasts, mast cells, macrophages, and ICCs
- ICCs are categorized as fibroblast-like interstitial cells

ICCs are the autorhythmic electrical pacemakers of the gut
- fluctuations in intracellular calcium concentrations are
responsible for the spontaneous changes in membrane
polarization

ICCs are similar in structure and function to the Purkinje cells of the heart.

They form an interconnecting lattice of cells that surrounds most layers of the GI tract along the entire length (circular and longitudinal muscle)

ICC are connected to each other and to the smooth muscle mass by gap junctions. (Gap junctions allow for the flow of ions from cell to cell).

Movement of ions leads to the propagation of waves of partial cell membrane depolarization and repolarization across large numbers of smooth muscle cells.

ICCs determine the origin and direction of propagation of slow waves.
- changes in membrane potential begin high in the esophagus
and are propagated aborally along the length of the GI tract
- stomach and colon slow waves occur at a slow rate (5x/min dog)
- slow waves occur frequently in the SI (20x/min dog)
- frequency of slow waves varies among domestic species

Question 3

What is the membrane potential, frequency of slow waves and/or action potentials, and muscle tone of the GI tract at rest, during parasympathetic influence, and sympathetic influence? (slide 10)

Answer 3

During Parasympathetic Influence:
- Slow waves superimposed with action potentials
- Increased frequency (increased amplitude)
- Increased muscle tone

During Sympathetic Influence:
- Slow waves
- Decreased frequency
- Decreased (zero) muscle tone

Question 4

What causes GI smooth muscle to contract? What causes GI smooth muscle to relax?

Answer 4

To contract:

Excitatory molecules (Ach) elevate the baseline (closer to zero) > depolarization stage of action potential > smooth muscle contraction
- increased smooth muscle stretch (tension)
- ENS Neurocrine peptides and non-peptides
- parasympathetic nervous system (Ach)

To relax:
Inhibitory molecules (NE) lower the baseline (more negative) > repolarization and/or hyperpolarization > smooth muscle relaxation
- decreased smooth muscle stretch (tension)
- ENS neurocrine peptides and non-peptides
- sympathetic nervous system (NE)

Question 5

Describe the voluntary and involuntary phases of deglutition.

Answer 5

Voluntary Phase:
- Bolus is pushed back caudally into the pharynx by the tongue
- Sensory nerve endings in the pharynx detect the bolus and
initiate the involuntary portion of the swallow reflex

Involuntary Phase:
- Caudal part of pharynx and esophagus
- Pharynx is the common opening of the respiratory/digestive
tract
- Major physiological function of the pharynx is to ensure that air
enters the trachea and that only food and water enter the
digestive tract
- Involuntary portion of swallow reflex is the action that directs
food into the esophagus and away from the trachea
- Reflex involves nightly coordinated actions

  - Breathing stops momentarily
  - Soft palate is elevated dorsally, closing the nasopharyngeal 
    opening and preventing food from entering the nasopharynx
  - Tongue is presses against the hard palate, closing off the 
    oropharynx
  - Hyoid bone and larynx are pulled forward; this action pulls the 
    glottis under the epiglottis, blocking the laryngeal opening
  - Arytenoid cartilages constrict, further closing the opening of the
    larynx and preventing the movement of food into the trachea
  - When all openings to the pharynx are closed, a wave of
    muscular contraction passes over the walls of the pharynx,
    pushing the bolus of food caudally to the esophagus 
  - Upper esophageal sphincter (UES) relaxes to accept bolus

Question 6

How does aspiration pneumonia occur?

Answer 6

Aspiration pneumonia occurs when a food bolus enters the trachea instead of the esophagus.
- due to the failure of the hyoid bone/larynx and glottis/epiglottis
to close off the laryngeal opening

Question 7

Describe the motility of the esophagus.

Answer 7

Peristaltic contraction begins at the cranial end of the esophagus and progresses toward the stomach.

Contraction of inner circular muscle.
- reduces or obliterates the esophageal lumen size

Contraction of outer longitudinal muscle
- increases esophageal lumen size to accommodate the
advancing food bolus
- aboral to the ring of the circular muscle contraction

Striated muscle portions of esophagus are under control of somatic motor neurons in the vagus nerve (X)
- Smooth muscle portions are under direct control of the ENS and
indirect control of the autonomic NS