Classic neurotoxins are generally considered to be paralysing toxins. These
are the typical snake neurotoxins, causing progressive muscle weakness. Most
achieve this effect by blocking nerve signal transmission at the neuromuscular
junction. However, a few snake neurotoxins effectively cause weakness by overstimulating
the neuromuscular junction; an example is the class of mamba toxins known as
dendrotoxins.
This effective paralysis by neurotoxins that excite the nervous system is not
the only possible effect such toxins may have. Some excitatory neurotoxins from
spiders and scorpions cause much more widespread excitation of the nervous system.
In doing so they stimulate many types of nerves, resulting in a diversity of
effects, which can include pain, sweating, salivation, tearing, piloerection
(body hair standing on end), muscle fasciculation, high blood pressure, tingling
sensation, particularly around the lips, and flooding of the lungs with fluid
(pulmonary oedema). For most excitatory venoms, only some of these clinical
effects will be present. For some species, while the venom effects are distressing,
they are unlikely to prove lethal, but for a few species, such as some scorpions
and the Australian funnel web spiders, these excitatory effects can prove rapidly
lethal, especially in young children.
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