Why Rats Cannot Vomit

[Noir 19]

Rats can't vomit.

They can't burp either.

And they don't experience heartburn.

See rat image - Rats can't vomit for several related reasons: Rats have a powerful barrier between the stomach and the esophagus. They don't have the esophageal muscle strength to overcome and open this barrier by force, which is necessary for vomiting. Vomiting requires that the two muscles of the diaphragm contract independently but rats give no evidence of being able to dissociate the activity of these two muscles. Rats don't have the complex neural connections within the brain stem and between brain stem and viscera that coordinate the many muscles involved in vomiting.

One of the main functions of vomiting is to purge the body of toxic substances. Rats can't vomit but they do have other strategies to defend themselves against toxins. One strategy is super-sensitive food-avoidance learning. When rats discover a new food, they taste a little of it, and if it makes them sick, they scrupulously avoid that food in the future, using their acute senses of smell and taste. Another strategy ispica, the consumption of non-food materials , in response to nausea. Clay binds some toxins in the stomach,which helps dilute the toxin's effect on the rat's body.

Looking deeper into the scientific literature,

I found a complex story about why a rat is unable to vomit.

See another "rat" image - Rats have a powerful and effective gastroesophageal barrier, consisting of the crural sling, the esophageal sphincter, and the centimeters of intraabdominal esophagus. The pressure at the two ends of this barrier is much higherthan the pressure found in the thorax or abdomen during any phase ofthe the breathing cycle. The strength and pressure of this barrier make reflux in rats nearly impossible under normal conditions.

In order to vomit, the rat would have to overcome this powerful barrier. Evidence suggests that rats cannot do this because they can't open the crural sling at the right time, and they can't wrench open the esophageal sphincter. In addition, rats lack the necessary neural connections to coordinate the muscles involved invomiting.

(1) Rats can't relax the crural sling while contracting therest of the diaphragm. The diaphragm is has two muscles: the crural and the costal. The esophagus passes through the crural sling, so when the crural diaphragm contracts the esophagus is pinched closed.

During the expulsive phase of vomiting in humans, the activity ofthese two diaphragm muscles diverges. The costal section contracts, putting pressure on the stomach while the crural section relaxes, allowing stomach contents to pass through the esophagus. Rats, however, do not dissociate theactivity of these two parts of their diaphragm: they do not relax the crural section while contracting the costal section. Instead, both muscles contract or relax together. The rat's inability to separately and selectively control its two diaphragmatic muscles therefore plays an important role in its inability to vomit: the rat can't put the necessary pressure on the stomach and open the crural sling to allow the contents to escape at the same time.

(2) Rats can't wrench open their esophageal sphincter. In humans, the esophageal sphincter is opened during vomiting with the help of the longitudinal muscle of the esophagus. This allows the expulsion of stomach contents during vomiting. Rats, however, have only a thin, weak longitudinal musclewhich is unstriated where it joins the stomach. It is too weak towrench open the sphincter and permit the evacuation of stomach contents.

(3) Rats lack the necessary neural connections within the brain and between brain and viscera. Animal species that vomit have a"vomiting center" in the brainstem, consisting of severalinterconnected nuclei that coordinate all the many muscles involvedin vomiting. Animals that don't vomit,like rats and rabbits, have the brainstem nuclei and the musclesystems used in vomiting, but they don't have the complex connectionsbetween the nuclei or between the brainstem and the viscera that arerequired for such a coordinated behavior.

Source: http://www.ratbehavior.org

Interesting. I just knew that rats don't actually vomit. Well, now that they mention it, I never have heard the statement about a rat vomiting. I guess that does make sense. >_< I also like the scientific explanation. I do know some stuff about the esophagus but I never knew that it's related to vomiting (or maybe I wasn't listening to our teacher).

[CarpetCrawler 280]

Lucky rats. It'd be nice to not feel hearburn, and to not vomit... >_> Fascinating stuff, btw.

[Guest Hajime Kindaichi]

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