Neonicotinoids, like many of our insecticides, are neurotoxins that go after the central nervous system. We often refer to the nervous system as an organism’s wiring, but its workings are a lot more complicated because all physiological actions must take place at low temperatures that won’t damage the surrounding proteins. Any electrician will tell you that continuous wiring doesn’t always do that for you.
The wiring from the brain to the muscles is made up of transmitter cells that we call neurons placed end to end in a series; they’re not directly connected to each other. Each neuron ends with a structure called an arbor (from the Latin arbor meaning “tree” because it looks like tree branches) which intermeshes with the next cell through a receptor called a dendrite (from the Greek dendrites meaning “tree” because it looks like tree branches. In this way biology avoids confusion).
The impulse reaches the end of the neuron where there’s actually a tiny gap called a synapse between the transmitter and receptor. The charge isn’t strong enough to jump the gap so the tips (referred to as the pre synaptic axons) actually produce a compound called acetylcholine, and it’s a wave of this stuff that gets pushed across that gap.
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On the other side, the receptors on the tips of the dendrites (the post synaptic axons) have a series of protein gates, and the acetylcholine binds with them — almost like a key inserting into a lock — and this opens up a series of small channels into the next neuron. When these channels open, a whole collection of sodium ions (one-half of the formula for sodium chloride, which you would know as table salt) that hang around the gap then move into the next cell and, since they’re positively charged, the cell becomes positively charged and that starts moving down through the next neuron. This is how the impulse gets to the next gap and does it at a low enough temperature to keep from cooking the proteins that make up the body.
That’s half the story. Now that the impulse has jumped the gap, an enzyme called acetylcholinesterase moves in and destroys the acetylcholine, thereby removing the “key” and closing up the channel so the sodium ions can’t get in anymore. The ions in the cell are pumped back out into the gap, the system returns to a negative charge, and it relaxes until the next impulse activates the whole process again. This is all very tightly organized, and it promotes the overall co-ordination an organism requires so that its heart can beat and its legs will walk.
What the neonicotinoid does is to pretend to be acetylcholine and open up the gate so the sodium ions can move into the cell and fire up the positive charge that moves the impulse down the next cell. When the cleanup enzyme, acetylcholinesterase slips in to remove the key, it doesn’t actually recognize the neonic and leaves it alone. The key stays in, the ion channel won’t close, and the cell becomes, in the words of a neuroscientist “overstimulated.” What this leads to is a huge number of nerve cells firing in a completely unco-ordinated hodgepodge, so the insect goes into severe convulsions, burns out and dies.