George,
The
amygdala is part of the Limbic System and according to the 3-brain model you espoused is therefore NOT part of the reptilian brain. So what is it doing in a Cobra’s head?
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The 3-part brain model you expounded is known as the Triune Brain was proposed by the American physician and neuroscientist Paul D. MacLean in the 1960s. It was hypothesised by MacLean to explain the evolution of the vertebrate forebrain and related behaviours. It was popularised through inclusion in a Pulitzer Prize winning book but gained very limited academic support, especially with comparative neuroscientists.
From the late 1980s on, advances in techniques that allowed the mapping of neurotransmissions in the brains of animals, revealed that the basis of MacLean’s hypothesis was incorrect. The basal ganglia structures from which MacLean’s reptilian brain were derived have been shown to exist in amphibians and fish as well i.e. they are present in all extant vertebrates which pushes the origin back to a common vertebrate ancestor more than 500 million years ago. This has further been supported by strong recent evidence that the neocortex was already present in the earliest mammals and that a homologous structure derived from the same tissues and with similar connections within the telencephalon, like those made by the neocortex, exist in reptiles and birds.
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Striated muscle is under voluntary control. It involves nerve pathways which involve the cerebrum.
You need to look up the nerve pathways involved in a reflex arc.
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From: Davidson College, Davidson, North Carolina US - Animal Physiology Class. (The college has graduated 23 Rhodes Scholars)
Excerpt: Rattlesnakes are also known to alter their behaviour according to the prey type, holding onto prey that are more mobile (birds) while releasing prey (mice) that may be easily located using the chemosensory system even if they are allowed to travel after being envenomated (Hayes, 1992a).
From: Miami University, Oxford, Ohio US - Anatomy of Venomous Snakes notes
Excerpt:The accessory gland was noted by Mitchell to prevent wasteful flow of the secretions. This gland lacks smooth muscle and is strictly regulated by striated muscle. Implications are that the accessory gland is under voluntary control and that snakes can and do control the amount of venom that is released during a bite.
From: Handbook of Venoms and Toxins of Reptiles
Excerpt: [p75-p76] The venom glands. of viperids (Kardong and Lavin-Murcio, 1993), elapids (Rosenberg, 1967), and atractaspidids (Kochva, 2002) are part of high-pressure delivery systems. The venom bolus is quickly expelled; rattlesnakes can deliver venom in less than half a second (Kardong and Bels, 1998). Although the specific gland compressor is different in each family (Jackson, 2003), all of these venom systems exhibit notably direct striated muscle insertion. When the gland compressor muscle contracts, the main venom gland is pressurized, producing expulsion of a presynthesized, stored, venom bolus. From venom gland to exit orifice at the tip of the tubular fang, this system is closed when activated, not open to ambient pressures, and therefore can develop, under striated muscle action, a sustained high-pressure head until venom enters the prey or predator (cf. Rosenberg, 1967).
From: Young et al. Functional Bases of the Spatial Dispersal of Venom during Cobra “Spitting”. Physiological and Biochemical Zoology, 2009; 82 (1): 80 DOI: 10.1086/595589
Excerpt: The name "spitting cobra" is a bit of a misnomer. Cobras don't actually "spit" venom, says the study's lead author Bruce Young, director of the Anatomical Laboratory in the Department of Physical Therapy at the University of Massachusetts, Lowell. Muscle contractions squeeze the cobra's venom gland, forcing venom to stream out of the snake's fangs. .
From: The Journal of Experimental Biology 213, 1797-1802 © 2010. Published by The Company of Biologists Ltd
doi:10.1242/jeb.037135.
Target tracking during venom ‘spitting’ by cobras. Guido Westhoff1, Melissa Boetig2, Horst Bleckmann1 and Bruce A. Young3,* 1 University of Bonn, Germany, 2 Washburn University, Topeka, KS USA and 3University of Massachusetts Lowell, MA USA *Author for correspondence (
[email protected]).
Excerpt: In the present study we show that spitting cobras can accurately track the movements of a potentially threatening vertebrate, and by anticipating its subsequent (short-term) movements direct their venom to maximize the likelihood of striking the target’s eye. Unlike other animals that project material, in spitting cobras the discharge orifice (the fang) is relatively fixed so directing the venom stream requires rapid movements of the entire head. The cobra’s ability to track and anticipate the target’s movement, and to perform rapid cephalic oscillations that coordinate with the target’s movements suggest a level of neural processing that has not been attributed to snakes, or other reptiles, previously. [Take note Longqi]
From: Venom flow in rattlesnakes: mechanics and metering. Bruce A. Young* and Krista Zahn.
Department of Biology and Program in Neuroscience, Lafayette College, Easton, PA 18042, USA. Accepted 28 September 2001
Excerpt: The functional morphology of venom injection in Crotalus atrox was explored using high-speed digital videography combined with direct recording of venom flow using perivascular flow probes. Although venom flow was variable, in most strikes the onset of venom flow was coincidental with fang penetration, and retrograde flow (venom suction) was observed prior to fang withdrawal. The duration of venom flow was consistently less than the duration of fang penetration. The occurrence of retrograde flow, ‘dry bites’ (which accounted for 35 % of the strikes) and unilateral strikes all support a hypothesis for venom pooling in the distal portion of the venom delivery system. No significant difference in temporal or volumetric aspects of venom flow were found between defensive strikes directed at small and large rodents. With the species and size of target held constant, the duration of venom flow, maximum venom flow rate and total venom volume were all significantly lower in predatory than in defensive strikes.
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To say venom has "two primary purposes" is not quite correct. This indicates that either both are of
equal importance in their functions or that both are
fundamental in what they do. A venomous snake can digest prey that has not been envenomated, so clearly assisting digestion is not a fundamental function if it can be done without. In terms of importance, procuring a meal in the first place is more important than gaining assistance with its digestion. You cannot digest a meal that you have been unable to procure.
I suspect that nearly all venoms provide some assistance to digestion but this varies from minimal, as would seem to be the case with our Australian elapids, to significant with many of the Viperidae, in particular those with strongly cytotoxic venom.
I also think it important to add “defence” to the list of functions.
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I stopped researching because I had too much material. As it is I have only used a portion of what I did collect due to length. The information is out there and accessible – there is no question about that.
I don’t think I can convince you of anything. I am simply putting some hard data on the table for you to consider if you so wish. If it still clashes with your schema then it is only sensible that we agree to disagree and leave it at that.
Blue
