In
pit vipers, the heat pit consists of a deep pocket in the rostrum with a
membrane stretched across it. Behind the membrane, an air-filled chamber provides air contact on either side of the membrane. The pit membrane is highly vascular and heavily innervated with numerous heat-sensitive receptors formed from terminal masses of the
trigeminal nerve (terminal nerve masses, or TNMs). The receptors are therefore not discrete cells, but a part of the trigeminal nerve itself. The labial pit found in
boas and
pythons lacks the suspended membrane and consists more simply of a pit lined with a membrane that is similarly innervated and vascular, though the morphology of the
vasculature differs between these snakes and
crotalines. The purpose of the vasculature, in addition to providing oxygen to the receptor terminals, is to rapidly cool the receptors to their thermo-neutral state after being heated by thermal radiation from a stimulus. Were it not for this vasculature, the receptor would remain in a warm state after being exposed to a warm stimulus, and would present the animal with afterimages even after the stimulus was removed.
Neuroanatomy In all cases, the facial pit is innervated by the trigeminal nerve. In
crotalines, information from the pit organ is relayed to the nucleus reticularus caloris in the
medulla via the lateral descending trigeminal tract. From there, it is relayed to the contralateral
optic tectum. In
boas and
pythons, information from the labial pit is sent directly to the contralateral optic tectum via the lateral descending trigeminal tract, bypassing the nucleus reticularus caloris. It is the optic tectum of the brain which eventually processes these infrared cues. This portion of the brain receives other sensory information as well, most notably
optic stimulation, but also motor,
proprioceptive and
auditory. Some
neurons in the tectum respond to visual or infrared stimulation alone; others respond more strongly to combined visual and infrared stimulation, and still others respond only to a combination of visual and infrared. Some neurons appear to be tuned to detect movement in one direction. It has been found that the snake's visual and infrared maps of the world are overlaid in the optic tectum. This combined information is relayed via the tectum to the forebrain. The sensitivity of the nerve fibers is estimated to be <0.001 °C. The pit organ will adapt to a repeated stimulus; if an adapted stimulus is removed, there will be a fluctuation in the opposite direction. For example, if a warm object is placed in front of the snake, the organ will increase in firing rate at first, but after a while will adapt to the warm object and the firing rate of the nerves in the pit organ will return to normal. If that warm object is then removed, the pit organ will now register the space that it used to occupy as being colder, and as such the firing rate will be depressed until it adapts to the removal of the object. The latency period of adaptation is approximately 50 to 150 ms. In structure and function it resembles a biological version of warmth-sensing instrument called a
bolometer. This is consistent with the very thin pit membrane, which would allow incoming infrared radiation to quickly and precisely warm a given ion channel and trigger a nerve impulse, as well as the vascularization of the pit membrane in order to rapidly cool the ion channel back to its original temperature state. While the molecular precursors of this mechanism are found in other snakes, the protein is both expressed to a much lower degree and much less sensitive to heat. == Behavioral and ecological implications ==