05: Relaxation, Stomach, Vomiting

How is relaxation of the GIT part of moving food through

Relaxation runs ahead of the peristaltic wave, giving the bolus a path of least resistance to move into

What controls the receptive relaxation ahead of peristalsis

Myenteric inhibitory neurons

What causes the lower esophageal sphincter (LES) to relax

Receptive relaxation

Achalasia

Failure of the LES to relax and let food into the stomach

Two causes of megaesophagus

• Obstruction

• Esophageal dysfunction

How does Chaga’s disease affect receptive relaxation

Destroys myenteric neurons → achalasia → megasophagus

Gastroesophageal reflux disease (GERD)

Failure of the LES to close when it should

Clinical procedure that can cause GERD

Improper positioning and fasting during anesthesia

Two theories for why blowing in an animal’s nose causes them to swallow

• Triggers a pharyngeal swallow via direct pharyngeal stimulus

• Laryngeal closure reflex: tricks the body into thinking there is a foreign body in the pharynx, so it closes the airways

Three functions of the stomach

• Storage

• Mixing to form chyme

• Slow rate of emptying

How does the stomach account for greater storage needs

Stretch activates the vagovagal reflex to relax the stomach wall and allow greater stretch

Pattern of mixing movements in the stomach

Mixing waves that start at the top and get stronger as they move towards the antrum

How does the stomach empty chyme slowly into the stomach

Mostly constricted pyloric sphincter and strong constrictive ring from the mixing waves push most of the contents up, and only a little gets pushed down

What parts of the GIT does vomiting empty

Esophagus, stomach, and upper duodenum

Triggers for vomiting

• Over distention

• Excessive irritation

• Too excited

Source of the initial signals for vomiting

Pharynx, esophagus, stomach, or upper duodenum

What signal actually initiates vomiting

Neural response from the area postrema in the medulla that overrides normal myenteric polarity

Location and purpose of retro/antiperistaltic waves

Starts as far down as the ileum to push things up in preparation for vomiting

What signal slams the gas on the vomiting process

The distension in the duodenum from all the food pushed up

What sphincters have to open for vomiting to occur

• Pyloric sphincter

• LES

• UES

What signals open the sphincters for vomiting

Neural and muscular input

Muscular processes in vomitin

• Diaphragm contracts

• Abdominal muscles contract

• Stomach is squeezed as intrabdominal pressure increases

• LES opens

• Up comes the contents :)

Part of the area postrema of the medulla that has the receptors that initiate vomiting

Chemoreceptor trigger zone (CRTZ)

Common triggers for the CRTZ

• Drugs (opioids!)

• Motion sickness

How does Cerenia work

Substance P competitive antagonist, inhibiting the CRTZ

Two reasons some species can’t vomit

• Missing the neural component (rats)

• Problematic anatomy (horses)

What anatomical features make vomiting impossible in horses

• VERY strong LES

• Lower attachment angle of the esophagus means that stomach contents also push the LES close

Evolutional explanation for the very strong LES in horses

They have VERY acidic gastric liquid, and if it splashed up while running there would be problems

Regurgitation

Passive and voluntary expulsion of contents from the esophagus (NO stomach involvement)

Difference in the contents from vomiting vs regurgitation

Vomiting has stomach liquid, regurgitation has mucus and that’s about it

In what animals is regurgitation a normal process

• Ruminants

• Some birds