PHYSIOLOGY OF THE PIT ORGAN
The translation of infrared stimuli into an infrared image is a fairly straight forward process. The neurons of the terminal nerve masses (TNMs) are constantly firing at irregular intervals (spontaneous discharge) due to the constant infrared radiation being given off by various surrounding objects (Goris, 2011). As the stimuli from these spontaneous discharges travels from the pit organ to the telencephalon, however, the stimuli decreases in strength because the central nervous system has adapted the ability to filter out "extraneous background information" except in the case of objects in motion (Goris, 2007). The temperature or infrared radiation of the stimulus in relation to the background will determine the frequency of the neurons' firing rate in the pit (Goris, 2011). A stimulus of higher temperature or radiation than the background will increase the firing rate, while cooler objects suppress the firing rate (Van Dyke et al., 2009). Temperature changes as small as 0.003ºC can be detected within 0.01 seconds by these infrared receptors (Buning, 1983).
(A) Spontaneous discharge. Infrared neurons
constantly produce action potentials at irregular intervals. (B)
Stimulation by an infrared (830-nm) laser. The neuron responds with a
burst of firing, with a varying degree of latency depending on the
individual neuron. This is part of the encoding, which eventually
produces a conscious image in the central nervous system. The solid
line shows the onset, duration, and cessation of the radiation. A and B are
from the same neuron. (C) Response to a cold object (a popsicle).
Arrows indicate the points at which the popsicle entered and left the
pit's field of view. The spontaneous discharge at left ceases abruptly
and responds with a strong burst when the stimulating object leaves
the field of view. This shows that the infrared neuron can respond to an
object whose temperature is lower than the background radiation, for
example, a wet frog. The scale refers to all records. Image courtesy of Richard Goris
The pathway of infrared stimuli through the snakes nervous system is outlined in the diagram and steps below (Goris , 2011):
(The pathway of cell signaling due to infrared stimuli received in the pit organ. Image courtesy of Richard Goris)
- Infrared radiation activates the infrared receptors within the pit organ.
2. The signal is passed through the trigeminal ganglia to the medulla oblongata. The medulla oblongata contains the lateral descending tract of the trigeminal nerve (LTTD) only found in pit vipers. Here, the initial processing of the infrared information is carried out.
3. The processed information then passes through the reticularis caloris, which is believed to increase the resolution of the infrared image.
4. The processed information then continues to the contralateral optic tectum where visual information is also routed. Here the first images are formed by neurons that respond to visual and infrared stimuli. The image is "stereoscopic" because the visual field of the eyes and the pits overlap.
5. This visual information then proceeds to the nucleus rotundus in the thalamus followed by the anterior dorsal ventricular ridge in the telencephalon where distinctions of "color, form, motion, and looming" occurs and a final image is created (Goris, 2011).
BLOOD FLOW
Blood flow during this process is a vital element to the precision of the infrared image. Variations in blood flow help to prevent an afterimage (Goris et al., 2007). Infrared radiation focused on the infrared receptors results in the terminal nerve masses signaling for an increase in blood flow in the microvasculature of the thin membrane (Goris et al., 2007). It is postulated that the TNMs use nitric oxide (a neuromediator) to signal to the pericytes in the pit organ (Goris et al., 2007). The pericytes are responsible for mechanically regulating the blood flow to the thin membrane (Goris et al., 2007). The decrease in blood flow at the termination of the infrared stimuli helps to cool the membrane, which in turn prevents an afterimage allowing the pit viper to maintain its precise vision (Goris et al., 2007).
(Graph of pit capillary blood flow before, during, and after infrared stimulation of the pit.
Velocity is significantly higher during stimulation at p<.05 in both cases.--Figure courtesy of Richard Goris)