When the Iguana Bites

     Okay, it's Spring Break this week, so I've been occupying my time with more lit searches. Todays piece is on ventilation in iguanas and the muscle involved in this. The study found that only four hypaxial muscles are involved in iguana breathing. (A hypaxial muscle is one located on the ventral trunk or the limbs i.e. the muscles of the abs, pecs, arms, and legs, but not those in the back.) The four muscles are the transversalis, the retrahentes costarum, and the external and internal intercostals. The first two are the muscles that control expiration, while the second two control inspiration, however, lizard respiration is a little different than normal (Carrier, 1989).     Iguanas bring air into their lungs by changing the shape of the thoracic cavity to create a sub-atmospheric pressure. If this sounds uncomfortable (the whole deforming of your body in order to breathe), it may be. Recent studies have shown that lizards cannot run and breathe at the same time. In most mammals, the more arduous the activity, the more oxygen we attempt to bring into our bodies, hence the panting that often accompanies running. But lizards can't do this. The author of this article, David Carrier, believes that this is caused by the opposing demands that the ventilatory and locomotive muscle place on the thorax. Both sets of muscles are pulling in different directions, thus reducing the effectiveness of both. Oddly enough, aspiration in iguanas begins with expiration followed immediately by inspiration, but this cycle is usually followed by a period of breath holding. Many humans do the same thing when they become aware of the sounds of their own breathing, particularly if they are standing close to someone else. It's odd how many of those parallels you find.
     So the transversalis and the retrahentes costarum are responsible for expiration. Pretty straight forward. In an iguana, the transversalis is located on the ventral side of the ribcage deep to the intercostals and obliques, while the retrahentes costarum is located on the dorsal side of the ribcage deep to the intercostals and obliques. When these muscle contract, they increase the pressure in the thoracic cavity and aid in the exhalation of the air in the lungs.
     The intercostals are responsible for inspiration, sort of. The internal intercostal lies superficial to the transversalis and the external intercostal lies superficial to the retrahentes costarum. When either of these muscles are stimulated ventrally, thoracic pressure is decreased. But if they are stimulated dorsally, thoracic pressure is increased. Thus these muscles are partially responsible for both parts of breathing.
     Also, these muscles are all composed of both slow twitch fibers and fast twitch fibers. Slow twitch fibers are what allow a marathoner to run 26.5 miles, or a weight lifter to hold a weight in the air for five minutes. They are the endurance fibers. The fast twitch fibers are the sprinters, the ones that produce a large boost of speed or power and then have to wait to be reactivated. The muscles that were responsible for respiration in iguanas relied mainly on slow twitch fibers to accomplish this. That means that there was a significant delay between the muscle activation (the thought to breathe) and the actual beginning of a breath.

AA

Carrier, David R. "Ventilatory Action of the Hypaxial Muscles of the Lizard Iguana Iguana: A Function of Slow Muscle." Journal of Experimental Biology (1989) 143: 435-457. Web. 22 Mar 2012.