After repeated encounters with a friendly rattlesnake last week I have snakes on the brain. Serpents are a storehouse of fascinating neuroscience. Infrared vision, venom, fast-twitch muscles to energize its "warning buzzer" and more....
The western diamondback rattlesnake can rattle its tail at frequencies of 90 Hz and do this continuously for hours. This is about the frequency of sound handled by the subwoofer in your fancy home theater system. Even a piano virtuoso can’t begin to approach this feat of turbocharged muscle contraction executed with ease by the lowly cold-blooded viper. Try it. You’ll find you can tap your finger at a feeble maximum rate of about five taps/sec (5 Hz), and you’ll quickly poop out.
The tailshaker muscle is absolutely packed with energy producing mitochondria. This enables the tailshaker muscle to use oxygen at a much faster rate than muscles of other warm or cold-blooded animals. Hummingbirds have shared this design principle to flutter their wings so fast they can hover in place.
Infrared night-vision cameras are the ultimate imaging technological that can peer through walls and see clearly in complete darkness. Recall the ghostly glowing image of the Boston Bomber hidden inside the fiberglass boat shot from a police helicopter’s infrared camera as the SWAT team closed in? Cool technology, but snakes have had this stealthy equipment for eons.
Have a look at the lovely face of the rattler who greeted us repeatedly last week in the backcountry of Nevada. Notice those tiny openings below the eyes? Snakes do have nostrils, but snakes “smell” with their tongues (that’s another story). They flick their tongue in the direction of a warm blooded prey item just before striking--something I observed, but my trigger finger was too slow to catch it on camera. The second set of pits on the reptile’s face are unique sense organs that give pit vipers infrared vision and also give them their name “pit vipers.”
In 1937 Noble and Schmidt put blindfolds on rattlesnakes and found that the blindfolded snakes could magically strike at moving objects very accurately, such as a dead rat or a cloth-wrapped light bulb. Moreover, the snakes had the ability to distinguish between identical warm and cold objects. In 1952, Bullock and Cowles took an electrophysiological approach to understand the function of these pits and uncover how they worked. The scientists surgically exposed the superficial branch of the superior maxillary division of the trigeminal nerve that connects the pit organs to the brain. They suspended the slender nerve on a pair of wire electrodes that were connected to an electronic amplifier powering a loudspeaker. (Try that sometime. I’m leaving out some thrilling procedural details.) What they heard on the loud speaker was a constant barrage of nerve impulses shooting from the sense organ to the brain. The researchers found that when a warm or cold object was held in front of the snake’s face, the firing rate of nerve impulses either suddenly increased or decreased depending on whether the object was slightly warmer or colder than room temperature. An ice cube, for example, held in front of the snake caused the firing rate to instantaneously slow--within 50 ms (5/100s of a second). This response was much too fast to be explained by actual heating or cooling of tissue in the snake’s sense organ. They concluded that these sense organs had to be detecting infrared radiation emitted by warm objects-- but how?
(I am fortunate to have had Ted Bullock as one of my mentors when I was a graduate student. His stories of delivery men and secretaries unwittingly walking into the lab and being perplexed by the sudden cacophony of buzzing surrounding them, emanating from burlap bags suspended from the ceiling, are precious. “Rattlesnakes,” he would explain offhandedly to the wide-eyed visitors who immediately departed, propelled by a jolting primal response embedded in the amygdala of human brains by evolution.)
It is difficult to imagine a sensory ability that we humans do not have, but these pit organs likely give the viper a visual sense. The neural pathways from the pit organs connect to the same brain structure as pathways from the snake’s eyes, the optic tectum. Behavioral studies by Bakken and Krochmal in 2007 indicate that the pit organ must be able to respond to temperature changes as minute as 0.001 degree C or less! This is sensitive enough to provide a detailed gray-scale image of objects from the emitted infrared radiation.
In 2002 Terashima and Ogawa reported that capsaicin, the fiery ingredient in hot peppers, caused the nerve terminals in the infrared receptors of snakes to degenerate. This is a clue that the molecular mechanism of detecting warmth somehow shares affinity with how we sense the burn of hot sauce. Indeed, in 2010, Grachevia et al., reported that the molecular basis of infrared detection by pit vipers was provided by an ion channel TRPA1 (transient receptor potential channels). This is a member of a large family of ion channels that give our own heat-sensing neurons the ability to respond to temperature changes and also give us the painful sensation of heat from hot sauce. The researchers discovered this by analyzing the genes expressed in sensory cells in the pit organs. There are many members of the TRP channel family, but more recent studies show that the same TRPA1 ion channel operates as a thermoreceptor in a wide range of animals from mosquitos to rats.
Fighter jets use thermal decoys to confuse heat seeking missiles, but ground squirrels have been using the same cleaver decoy against their venomous predators long before the DoD stumbled upon the same countermeasure. Rundus et al, found that California ground squirrels add an infrared component to their shaking tail when confronted by infrared-sensitive rattlesnakes, but squirrels don’t emit strong infrared signals from their wagging tails when confronted by gopher snakes, which lack the infrared receptors. Using a robotic squirrel to test the rattle snake’s response, the researchers found that when an infrared component was added to the flagging robotic tail, the rattlesnakes shifted from predatory to defensive behavior. This did not happen when the tail was flagged without the added infrared component. That behavior provoked the snake to strike.
Geez, already 1000 words and I’ve hardly gotten started. This story will have to be continued as a sequel--“Snakes on the Brain, Part II.”
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