Venom Expulsion in Rattlesnakes

Mechanics of Venom Expulsion in Crotalus, with special reference to the role of the fang sheath – Young et al. 2001

• Abstract: Seems the venom delivery system of rattlesnakes are capable of more regulation in venom expulsion than previously assumed.
• Introduction: pterygoid walk is when a snake eats something bigger than its head and it needs to works its prey down by independently moving each side of its jaw. 4 groups of snakes with venom viperidae, elapidae, atractaspidae, and rear fanged. Little is known about the delivery system b/t the fang and accessory venom gland, known as the secondary venom duct and the fang sheath. Also contentious (controversial) whether there is an open chamber at the base of the fang. They know that external pressure on the muscles are not enough to release venom and that a sphincter is not present which means that there may be a facilitator at the distal portion of the delivery system. This study attempts to explain this barrier to venom flow.
• Methods: n=6, (wild)western diamondback rattlesnake, Crotalus atrox, and n=1 (commercial) black tailed rattlesnake C. molussus. To ensure that snakes had a normal venom supply they waited 14 days of no feeding prior to the experiment.
  Morphology: took various samples of the fang sheath and stuff
  Demarcation of the venom chamber: This was done to explore the dimensions of the venom chamber, the duct was severed immediately distal to the accessory venom gland. Ink was dripped into it with no pressure and through gravity exited the sheath.
  Manipulation of anesthetized specimens: manipulations included applying pressure when retracted, applying electrical stimulation through a small slit, and applying pressure when the fangs are erect.
  Differential Pressure and Velocity: Read for specifics, but seems like they connected a tube to the fang in a manner that allowed for venom to be collected in a tube. Also part of the fang was surgically modified with a probe.
  Radiopaque injection: check for specifics, they also did a couple more tests like fang erection stuff, check for specifics.
• Results:
  Morphology: Found that the venom gland coalesces in tubules to form the primary venom duct, and all these tubules follow a convoluted path with a 180 degree turn at one point. The accessory venom gland is seen on the cranial margin of the primary venom duct, specifically noted by the slight expansion in this area. The secondary venom duct arises from the cranial surface of the accessory venom gland and is bound by connective tissue to the maxilla, but there is no direct contact between this duct and the and the musculature. In terms of fangs, sheaths cover all types including functional ones and replacements, they also adhere to the fangs via weak attachments at roughly 1/3 of the length. When the fangs are retracted the inner fang membrane covers to the tip and entrance orifice for venom expulsion, which then becomes exposed when the fangs rotate.
  

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  Demarcation of venom chamber: given that the ink was able to fill the venom gland with no pressure and purely through gravity, shows there is a reasonable space within the gland. the venom chamber is located at the base of the fang.
  Manipulation of anesthetized specimens: they found that it was possible to expel venom even when the fangs were retracted, however this required a lot of force and sometimes it was impossible given the species.
  Differential Pressure and Velocity: Pressure tracing in a semi-errect fang after electrical stimulation produces a unique pattern observable through the initial pressure rise that occurs after stimulations is applied and the initial peak then decreases as sharply as would be expected from a twitch contraction kinda like how your muscle twitches for a little bit when shocked. Venom pressure is higher in the primary venom duct than the fang and decreases along the length of the venom duct. Also the venom flow was retrograde after the initial anterograde which means the venom has a backflow after release which may also spill into fangs. This may also be present in the fangs which means when making the venom apparatus try to keep the venom we catch using a pressure difference in our favor or through automated suction.
• Discussion:
  Regulation of venom flow in the distal portion is controlled by 4 factors:
  1. Restriction in the secondary venom duct along the anterior surface of the maxilla
  2. volume of the venom chamber
  3. position of the inner fang membrane
  4. suction produced in the venom gland
  The restriction of venom at the secondary venom duct explains why its so difficult to expel venom when retracted. Also there seems to be no structural evidence for a pumping mechanism, rather the venom is expelled when the venom chamber dimensions are altered. The inner fang membrane also works as a fang plug that makes sure there is a physical block at the entrance of the fang. The retrograde movement of the venom may function to decrease chance of venom crystallizing in the fangs.
  Have provided an explanation for why manual extraction of venom while the fangs are retracted is so difficult. Basically the inner fang membrane acts like a plug and is released when the fangs are erected, thus the venom leaks through the sheath instead.
• What have we learned, different snakes expel venom in different ways, rattlesnakes in particular are capable of expelling venom even when the fangs are retracted. It should be noted that this was only see through the sheath though, which made sense given the fangs inner membrane which acts like a plug and the fact that it was only possible through a large amount of manual force. Also the presence of a venom chamber that expels venom through the alteration in dimensions of the chamber is pretty cool, also the fact that there is no active pumping system. The snake applies force through the movements of the head which compresses and relieves pressure on the chamber in turn pumping venom. Also there is a bit of retrograde at the end which ensures that there is no venom remaining in the fang cavity. -> this basically just reconfirms what we already knew with western hognose snakes, except for the fact that rattle snakes have stronger jaw adductor muscles than western so venom delivery may not be as efficient. Also take into account that its fixed fang, there is no retraction for a better delivery system either. Maybe electrical stimulation is the best way to do the venom extraction?