MORPHOLOGY OF THE PIT ORGAN

The pit organs of pit vipers are located in between the eye and the nostril on each side of the head (Krochmal, 2011). While all pit organs serve the same function, the superficial structure will vary depending on the preferred behavior of the pit viper (arboreal, terrestrial, or aquatic) (Goris, 2011). The pit organ in pit vipers consists of several main parts: the outer chamber, the inner chamber, a thin membrane that separates the two, and a pore (Figure 2) (Moon et al.,2011). Contained within the thin membrane separating the chambers are the actual infrared receptor terminal nerve masses (TNMs) (Amemiya et. al, 1995). The pore serves as an equalizer of atmospheric pressure similar to the Eustachian tube in mammals (Goris, 2011).The pit organ in pit vipers is located away from the body giving it a greater sensitivity to infrared radiation while avoiding heat given off the body that may distort the stimuli aimed at the infrared receptors (Amemiya et. al, 1995).  Along the outer and inner chambers are oberhauchten cells covered in tiny pores. The inner chamber also contains domed structures covered in these pores (Amemiya et. al, 1995). These pores serve to reflect wavelengths in the visual spectrum that may inadvertently increase the temperature of the receptors and blur the infrared image (Amemiya et. al, 1995).

(Figure 1. Trimeresurus stejnegeri--arboreal. Arrow indicates pit organ--Image courtesy of Richard Goris)

(Figure 2. Cross section of a crotaline pit organ--Image courtesy of Richard Goris)

NERVE INNERVATION

The thin membrane of the pit organ is innervated by the opthamalic ganglion and maxillary ganglion of the trigeminal nerve, which is also connected to the medulla oblongata (Moon et al., 2011). Along with these nerve fibers, the thin membrane is also innervated by nociceptive fibers (pain sensors) and autonomic nerve fibers that control the blood flow to the thin membrane (Goris, 2011). The thin membrane is also filled with mitochondria and oxidative enzymes (Meszler, 1970). The mitochondria and oxidative enzymes, such as adenosine triphosphatase and lactic acid, serve to produce the energy needed to generate a potential as a result of infrared stimulation (Meszler, 1970).

(Innervation of the crotaline pit organ. The pit membrane (red) is innervated by three branches of the trigeminal nerve (yellow).
Abbreviations: C, cerebellum; E, eyeball (blue); IC, inferior colliculus; MO, medulla oblongata; N, nostril (green); OB, olfactory bulb; OT, optic
tectum; P, pit; SC, spinal cord; V, trigeminal ganglion--Click here for larger image--Image courtesy of Richard Goris.)