Broadcasting’s use of abbreviations and obscure notation started with the original radio-men, when every character required some pain to transmit. Today’s tweeters and texters have invented their own slang for much the same reason, but without the long linguistic history that accompanies radio lingo. When you first enter the ham world, you have to cross the language barrier, even if it isn’t particularly high. You turn on the radio and connect to the digital stations and immediately see CQ CQ CQ DE K7BT K7BT PSE K streak across the screen, highlighted in red. What can that mean? Here is a translation and lexicography of some of the more interesting bits of ham lingo in no particular order, but starting with the message in red.
CQ – “Calling any station” comes about from the sounds of the letters C and Q spoken in French, and meaning “sécurité”, here translated roughly as “attention.” This is the universal “all call.” When my digital radio program sees CQ it turns the line red to alert me to the call.
DE – “from” from before English was the lingua franca, often followed by the sender’s call sign as in our message above. Usually the call sign is repeated more than once for clarity. This universal calling convention, along with call-sign naming conventions, allow automated listening programs to pick out a station’s origin from the jumble of text being received. The very useful program PSK Reporter utilizes this convention of ham lingo to amazing effect.
K7BT – a call sign, one of millions, the unique moniker of every amateur operator. The characters before the numeral are unique to the country granting the call sign.
PSE – “please” when every letter costs a good fraction of a second, you abbreviate everything.
K – “done transmitting – your turn – go” This is a curious one, since I can find barely a reference to it anywhere in the simple form of ‘K’. It seems to be so ingrained as a form of “over” that it goes without saying in the ham lingo guides. Its more formal cousin, “SK,” derives from the obsolete American Morse Code sound-alike for the Western Union code “30” which means “No More (end).” Shortening the very final “SK” to simply “K” seems to imply that this is just the end for now, your turn.
SK – “The End, No More, All Done” when the receiver should not expect another response from you. Derived as described above, but this term also has another meaning. SK is also short for “Silent Key,” meaning an operator who has passed away. After all, the “silent key” is indeed “no more – all done.”
73 – “best regards” is another of the Western Union telegraph codes that migrated into the ham world. Not to be confused with its cousin…
88 – “love and kisses” – Western Union again. This is would be a nice way to end a message to your YL or XYL.
YL – “young lady” or just about any woman, young or old, unless specifically your XYL.
XYL – “wife” as in ex-YL. If you think that too denigrating, the guys don’t have it much better.
OM – “old man” is just about any operator that is not a YL. If you found that funny, you could laugh like a ham.
Hi Hi – “ha ha” apparently because in the dits in the morse code “hi hi” translates into “he he he he he he he he he he he he” which sort of sounds like laughter. Which gets us to the confusing ones…
FB – “fine business” — Really? who says that? Everyone if you are a ham. But I think it translates better into “fabulous” or maybe just “ok.” Ambiguity continues with…
TU – “thank you” obviously, but less obvious when …
BTU – “back to you” is another form of “over.” So it seems like TU should just be “to you” as in sending “30W TU.” Am I thanking him or am I sending him my transmitting power – or both?
TEST – “contest” because one of the major pastimes of amateur radio operators is spent “contesting” for more contacts. It took me a while to realize that the ham sending CQ TEST wasn’t after help to fix his equipment.
DX – “long distance” usually outside of the country of the broadcaster.
There is an entire set of Q-codes. These codes seek to have specific meanings and can be questions or answers – pretty handy when it comes to reducing typing!
- QRZ / QRZ? – “who is calling me.”
- QSB / QSB? – “you are fading / am I fading?”
- QRM / QRM? – “I’m suffering from interference / are you suffering from interference?”
- QRN / QRN? – “I’m suffering from noise / are you suffering from noise?”
- QRP / QRP? – “I’m running low power / are you running low power?”
- QSY / QSY? – “I’m changing frequency / shall I change Frequency?”
- QSL / QSL? – “I acknowledge receipt / do you acknowledge receipt?”
- QSO/QSO? – “I shall contact ____ / shall I contact ____ ?”
- and many more
Not surprisingly, the Q-codes have lost their verbish nature and become nouns as well. QSL becomes the acknowledgement and can have physical form in a QSL card, commonly sent in the mail to confirm a radio contact. QSO is the contact, not the process of making a contact.
The more difficult and time-consuming it is to send characters, the more one sees the use of ham abbreviations. My experience has been mostly with the PSK digital modes where characters are transmitted at roughly my typing speed, so extreme abbreviations are not really necessary. Inexperienced Morse code operators may be only able to punch out 5 characters/sec, so speaking in sentences is prohibitive. The RTTY digital mode also has its own form of rapid fire idiosyncratic exchange syntax that I have yet to master completely. The RTTY experts seem to relish making an exchange of call signs an information take no more than a few seconds and then moving on. Despite the common tendency for transferring only the most basic information very quickly, if you spend some time and practice a bit of verbal diplomacy, you can discover that there are real people with real personalities on the other end of the connection.
So for now,
TU for web QSO, 73 de AF7NX sk
When I jumped into the radio game with my new old Icom IC-745 transceiver, I really had no idea what I was doing. I knew I needed an antenna, so I went to a 1990 ARRL Handbook and found the “Loop Skywire” antenna which seemed like the one for me. The one resource I have is tall Douglas Fir trees, and I thought that a “Skywire” made sense hung from those trees. The ARRL article gave the impression that it was hard to go wrong, so I spent a couple of days figuring out how to hang wire in trees and get the feed line down to my radio. Turns out, the unused chimney was perfect for both supporting one corner of the loop and as a cable feed-through into the new “radio room” in the house. So I strung the wire, connected it all up and then went to try it out. To my chagrin, the transmitter balked at the arrangement and would not allow me to plow through the standing waves that were on my antenna feed. The SWR meter on my transceiver indicated SWR > 3, at which point the output circuit would refuse to give more power. So much for “hard to go wrong!”
The solution was clear — I needed an antenna tuner that would make the impedance match to the antenna feed line. I found an old Heath Kit tuner which fit the bill and solved the problem. But it irked me that I didn’t understand enough to avoid this issue in the first place.
I recently decided I wanted to compare signals from my two rigs on two antennas, and having only one tuner, I was stuck again with how to match that loop. In the last couple of months I’ve become proficient using the NEC antenna codes, so I decided to see if they could provide guidance. I use the 4NEC2 implementation by Aire Voors which is a very nice version of these venerable programs. The code has the ability to model transmission lines as well as radiating elements; it seemed it should work to figure out the feed line.
Whenever trying to use a modeling program, the first thing I do is make sure I can get the program to give me answers to a problem to which I already know the answer. That gives me confidence that I am not wasting my time doing something wrong. For the transmission line problem, I set up a very simple line with a variable termination resistance and nothing else.
You set this up in NEC with a pair of “wires” that are the ends of the transmission line and not connected to one another. The transmission line definition includes the line impedance and its length. One end was fed with a voltage source; the other end included two more short segments to complete the circuit, one of which included a load resistance.
I know that if this line is terminated with a resistance that same as the characteristic impedance of the line, I should see no reflections; power should all go from source to load and the standing wave ratio (SWR) should be one. If I run a frequency sweep in NEC, that is pretty much what I see.
The short wires I used to connect the load show up as reactance at the higher frequencies, but that is to be expected. The model shows SWR close to one with everything close to the 50 ohms I specified for the line impedance and the load resistance.
Now we are ready to have fun. In real life I expect the antenna to have about 150 ohms radiation resistance, and the big spool of co-ax I have is 75 ohm CATV cable. So lets run that simulation with line that is 3/4 wavelength long at 14MHz. (We will get into why I picked that length shortly.)
The cable is no longer matched to a 50 ohm source, so the SWR is no longer 1. But as you can see it is closer to 1 at some frequencies than at others. The impedance seen at by the source is now complex, with the real part varying between 150 and 37 ohms, depending upon the frequency.
Let’s back up and recall the physics of the anti-reflective lens coatings. This idea is simply that if you have a coating that is exactly 1/4 of a wavelength thick, then the reflections from the two sides of the 1/4 wave coating will be exactly 180 degrees out of phase with each other and will tend to cancel, so reflection from the surface will be diminished because of the coating. Any odd integer multiple of 1/4 wavelength will have this effect.
We want to do the same thing with our feed line, minimize reflections by making the feed line an odd integer multiple of 1/4 of the wavelength. I chose the length of the line to be 3/4 of a wavelength at 14 MHz – the 20m band that is arguably the most important ham band. My antenna is too far away to use just a 1/4 wavelength line. If my antenna impedance really was 150 ohms, using the 3/4 wavelength 75 ohm line would bring the SWR down from 3 to a more tolerable 1.3 by just cutting the feed cable the right length.
But what about the other bands? In general an odd integer quarter wavelength multiple for one band will not be and odd integer multiple on another band. The antenna resonances, and the feed line resonances are likely to interact in ways that are hard to predict. This is where using the NEC model really helps to extend our design beyond the single band case.
First, let us look at a square 40m loop antenna with NEC. The rendition on the right is produced by the very nice 3D viewer in 4NEC2. We can look at the impedance sweep for this geometry and get an idea of what to expect when we build something like this. The curves shown below are the SWR for a 50 ohm driver and the antenna impedance versus frequency. The resonances at the loop fundamental, 7MHZ, and its harmonics, 14, 21, & 28 MHz, are clear to see.
It is also clear why my first attempt to get this antenna to work failed. No-where does the 50 ohm SWR drop below 3. The advice to just “connect it up and go” from the ARRL article was just plain wrong. At the time, I also read some advice about making the feed line an integer number of half-wavelenghts in order to perfectly transfer the impedance characteristics at the antenna back to the other end of the line. If you put those two pieces of advice together, you get the following SWR sweep – much like my antenna behaved when first I connected it to my transceiver.
The curve above was generated with the model, shown in the figure to the right that includes both the loop antenna and the transmission line feed. The feed line is modeled as a radiating wire (the outside of the coax cannot be neglected) as well as the ideal transmission line. If you look at the resonances at 7, 14, 21, and 28 MHz, you will see that indeed the half-wavelength feed (also half-wave harmonic to the antenna harmonics) does transfer the SWR at the antenna to the other end of the line. Both the model for the antenna alone and the model that includes the half-wave feed line have SWR-50 of ~3.5 at 7 MHz and ~5 for the higher bands. The antenna impedance is just too high to match well, so we are back at square one.
We’ve recreated the problem I was having with the model. Now let’s see what happens when we use the odd quarter-wave line length instead of the half-wave length.
Low and behold, we have tolerable SWR on all of the main harmonic bands with nothing more than chopping the feed line in the right place! Note that doing the arithmetic for all of the other bands gets complicated quickly with the interacting sets of harmonics. The simulations come into their own as the complexity exceeds your intuition about what should be happening.
So modeling and simulation are one thing. Proof is in the pudding! I put this together and measured SWR of 1.5 at 7 MHz, 2.2 at 14 MHz, and about 3 at 21 MHz, so I can easily operate on the 20 and 40m bands. However, after I went through the excercise to write this up for this blog post, I also realized how it should be done.
If you look back at the 40m Loop impedance plot, you will notice that at the antenna resonances, the real part of the impedance is between ~150 ohms and ~240 ohms for the upper bands. The best solution is to use a 4:1 impedance transformer at the antenna and 50 ohm cable to the transmitter. The reflected 200 ohm impedance will match admirably on all of the resonant bands without resorting to tricks with a quarter-wave feed as shown in the simulation below.
But back to the 1990 ARRL handbook where the construction directions for the Loop Skywire state, “connect the coaxial feed line ends directly to the wire ends. Don’t do anything else. Baluns or choke coils at the feed point are unnecessary. Don’t let anyone talk you into using them.”
What can I say — I just plain disagree! The model results don’t support such a statement nor does my experience. Using a 4:1 current balun I manged to get SWR of <1.4 on all of the resonant ham bands on the antenna. That’s the way it is supposed to work!
What can I say, I really am a nerd! All the gardening and beekeeping are just there to keep the tech guy in check. It got away from me lately. It all started when I ran into my old “Electroluminescent Receiver,” a four band ham kit radio I found on the internet that I built a decade ago. I had modified that radio to cover all the shortwave listening bands as well, and learned a lot about radio circuitry in the process. At the time I never thought about getting a ham licence because there was a Morse Code requirement that I was unwilling to put the time in to master. In 2007 the FCC abandoned the code requirement. This time, when bitten by the radio bug, there was no reason not to go for the license. I studied a little, took the exams, and was awarded an amateur radio operating license in February as an Amateur Extra with call sign AF7NX. Little did I know what a club I was joining! It comes complete with its own lingo, special handshakes, and hangouts. Much of ham on-air culture consists of making brief contacts with other hams and exchanging information. The HF bands were given to the hams long ago because they were never “reliable” for communication. Radio signals must reflect off of the ionosphere to propagate any distance, which they are very prone to do given the right circumstances, but then again conditions vary and there will be no signal path where there was one just minutes before. This “worthless” band is perfect for hobbyists because it naturally rations the airwaves to only those path over which propagation is possible, yet provide the allure of world-wide communications with just a few watts of power. But the fickle nature of HF communications also means that you want to say your piece quickly, clearly and efficiently before the tenuous connection path with the other party disappears. Hence the handshake — call sign to call sign, signal reception reports and location exchanged, then say good-bye. Minimally, all that can be done with about 20 characters and only take a few seconds, if you know the special handshakes and ham lingo!
After I got my license, I needed to get a radio. The radio world is turning upside down with the advent of fast digital electronics in the past couple of decades. But I learned electronics when analog was king, and that is where I feel comfortable, so I picked up a couple of thirty year old radios on eBay to become my station. The uncomfortable paring of radio that-knew-no-computer with laptop sound card was accomplished to let the radio talk on-the-air in modern digital modes. The digital modes are largely replacing Morse code as the preferred method of on-air low-power discourse, and is part of the reason that there is no longer the code requirement for the license. Although voice/phone remains an important part of ham radio, I find myself more attracted to the keyboard than the microphone. Voice also requires more bandwidth and consequently more power to be reliably intelligible over long distance. For now I am content with a few watts to make connections around the globe. Challenges arise regularly as I become proficient at operating the radio. I quickly realized the need for a good antenna. A good antenna can make any old radio great, whereas a poor antenna will ruin a fine radio. The technical and cultural aspects of this fascinating hobby will be the subjects of a few future posts, as the blog steps out of retirement for a while.
What started out as a bit of curiosity about the time-dependent toxicity of insecticides led to a blog piece I did a little over a year ago titled Time-dependent Toxicity of Imidacloprid in Bees and Ants. I thought my results were interesting enough to get a comment from other scientists that were looking at the time-dependent toxicity question so I sent out the link to a few. With the encouragement of especially Dr.Fransico Sanchez-Bayo at the University of Sydney in Austrailia, I went ahead expanded the research and we turned that blog post into a paper. I am especaily grateful to my co-authors, especially Fransico Sanchez-Bayo and Nicolas Desneux, who shepparded the manuscript through the journal submission and review process.
So please take a look at the real thing. We were published in Nature’s online publication Scientific Reports.
Delayed and time-cumulative toxicity of imidacloprid in bees, ants and termites,
Gary Rondeau, Francisco Sanchez-Bayo, Henk A. Tennekes, Axel Decourtye,
Ricardo Ramırez-Romero & Nicolas Desneux, Scientific Reports 07/2014; 4(5566):8. DOI: 10.1038/srep05566
It is with dismay that I must report on another bumblebee kill, this time only about a mile away from the bees in my yard. All of the details are not in yet, but the basic picture is clear. Insecticide was sprayed early Monday morning, 6/16, on blooming linden trees in an apartment complex in northwest Eugene. Residents reported that the sidewalks were littered with dead and dying bees. Apparently officials from the Oregon Department of Agriculture (ODA) and Oregon State University were on the scene to collect samples and initiate an investigation. I’m sure we will know soon what was sprayed and who did it. [We know now that the chemical sprayed was imidacloprid.]
Recall that about this time last year more than 50,000 bumblebees were killed in Wilsonville. That prompted the ODA to issue restrictions on the neonicotinoids dinotefuran and imidacloprid, banning their use on linden trees. It is also against label directions to spray pesticides on blooming plants when there bees present.
I went to survey the bee kill scene this evening. The ODA and OSU investigators had gone, but the dying bees were still falling from the trees. Walking around the complex I identified eleven blooming linden trees that had been sprayed and had dead and dying bees on the ground under the trees. [I located two more sprayed lindens, bringing my total to 13 trees; the official report is that 17 trees were sprayed.] Under one set of three trees I counted about 300 bees on the side-walk. Easily there was 200 more in the grass and shrubbery. I later learned from one of the residents that she and others had swept the bees up already once and she was dismayed to continue to see them fall from the trees. My brief survey was apparently just the more recent victims. The eventual body count will depend significantly on what was sprayed. If the insecticide is one of the neonics, the chemical is designed to be taken up in the plant tissues, and the trees will be lethal to bees until after they stop blooming. If they were sprayed with a pyrethrin, then the worst toxicity will probably be gone in about a week.
That such a thoughtless act falls in the middle of Pollinator Week just illustrates how far we still have to go. Eugene may be the first city to ban the use of neonicotinoids on city property, but that action helps little when these chemicals are actively promoted and used by thousands of homeowners, landscapers, residential maintenance companies and pesticide applicator on private lands.
All this matters to me because my bees are in range of those trees. Right now the blackberry honey flow is still keeping most of the bees occupied, but the blackberry flowers are beginning to dry up and the bees will be looking further afield. Bees heading west from my house that find their way across the industrial railroad yard will come upon the very attractive linden blossoms.
The residents mentioned that the trees in their neighborhood often dripped a sticky mist from aphids in the summer. Apparently, this is not the first time that the trees have been sprayed, but this year they sprayed earlier than in previous years.
Although bee kills like this one will make headlines, it is the less dramatic impact of pesticides that are even much more troubling. Have these trees been poisoning my bees for years at sub lethal levels? How many pounds of these toxic pesticides are spread about in my bees’ foraging range? Is residential beekeeping doomed by thoughtless policy and people? The pollinators are weak this week.
[On a return trip Thursday afternoon, I wanted to see if more bees were finding the site during prime foraging hours. Indeed, I saw both bumblebees and honeybees visiting the trees, though not heavily. Curiously, there were no honeybees on the ground, just the bumblebees. I passed several more blooming lindens along River Road on my bike trip home. These trees were abuzz with activity, sporting ample numbers of both bumblebees and honeybees. ]
Agricultural pesticides have become part of the chemical landscape that we all live in. To be able to make intelligent decision about the use and regulation of these chemicals, it’s important to understand how they work. Almost all modern pesticides are chemicals that interfere in some way with the nervous system. The characteristics of the chemical interaction with the nervous system function can shed light on the effectiveness of the pesticide and on its physiological effects at residual levels. We will start by looking at how some of the normal processes of the nervous system work, because it will be disruption of those processes that lead to toxic effect. Then we will look at the mode of action for three major classes of pesticides and how they specifically interfere with normal function. In a future article we will look at how the specific mechanisms of action can effect dose scaling relationships.
Normal Neuron Function – Neurons, action potentials, sodium and potassium voltage gated ions channels, and ion pumps
The nervous system of insects and humans share many common features, starting with the basic structure of the neuron.
There are many variations on the same theme in different parts of the organism. Terminal branches can attach to dendrites of other neurons at synapses, or through motor synapses to muscle cells. Individual neurons are connected in complex, interacting networks by the synaptic connections. Information processing involves summing the inputs from many neurons and generating an output. When the summed stimulus is high enough, the neuron will generate an electrical pulse that is sent along the axon and which will, in turn, stimulate multiple downstream neurons connected through synapses to the axon branch terminals.
Neuronal signalling is accomplished by way of “action potentials”, which are short electro-chemical pulse that travel along the neuron axon. The short pulse-like nature of the nerve signals are generated and maintained by way of “voltage-gated” ion channels and ion pumps. Ion pumps use the cellular energy store, ATP, to move sodium and potassium ions across the cell membrane, setting up a concentration gradient across the membrane that establishes a “resting potential” of about -70mV from the inside to the outside of the nerve cell. Once this gradient is established, then merely opening ion channels in the cell wall allows the sodium or potassium ions to move back across the membrane and move the potential closer to zero. Nature’s trick, that turns this process into a useful information processing network, is to open the ion channels which depolarize the neuron with a positive feedback action associated with the membrane potential. Once the membrane potential rises from its resting potential to a “threshold” the voltage gated channels open, steepening the rising edge into the action potential nerve pulse. The figure below is a nice schematic of the ‘anatomy’ of the action potential.
Signaling happens by way of the action potentials, which propagate along the axons and terminate at the synapse. There are several ways the action potential can be interact with cellular structures. We will concentrate on the acetylcholine mediated synaptic response because this is the target of several pesticide chemicals.
Normal Synapse Function – acetylcholine-mediated transmission
Acetylcholine (ACh) is a molecular neurotransmitter that conveys information across the synapse. In the figure above, the basic steps of the interaction are illustrated. Action potentials, those pulses of neural activity, cause synaptic vesicles containing ACh to release the ACh molecules into the synaptic cleft, the junction region between the two cells. The ACh quickly diffuses across the narrow junction region and is captured by acetylcholine receptors (AChRs) that are part of ion channel molecules. The AChRs that have captured an ACh molecule open the ion channel and allow Na+ ions to enter the post-synaptic neuron. The binding is transitory, however; the ion channels rapidly open and close as the ACh molecules latch and unlatch from the AChR channel. Meanwhile, another ACh receptor is also present in the synaptic junction called acetylcholinesterase (AChE). This molecule is an enzyme which rapidly breaks apart the acetylcholine into choline and acetate, effectively ridding the synaptic cleft of the neurotransmitter almost as fast as it is made available. The result of all of this chemical activity is that the AChRs, as an ensemble, are open only for a few milliseconds. During this time, ions flood into the post-synaptic dendrite, depressing the potential in the down stream neuron, making it more likely to generate its own action potential.
This simplified discussion leaves out many details. There are many more specialized molecules that are part of cell membranes. Often molecules that are specific for one important function also are involved in unrelated functions. Nerve cells can be specialized and synaptic details can vary. Nevertheless, the basic picture we are painting is valid across much of the animal kingdom. These same basic process happen in the nervous systems of humans and bees alike. Now let us move on to discuss ways to interrupt these normal processes for insecticidal effect.
Insecticides targeting axonal voltage-gated ion channels
Two major classes of insecticides target the voltage gated ion channels shown in our cartoon. The organochlorines (e.g. DDT, dieldrin, chlordane) and pyrethroids (e.g. deltamethrin) act by opening these voltage gated ion channels. The molecules hold open the channels and allow ions into the axon that depolarizes the neuron. In the depolarized state the neuron is non functional, characterized by paralysis. In between the normal state and paralysis there is a range where the depolarization of the neuron is only partial. Partial depolarization leaves the neuron susceptible to “false triggering”. A small stimulus that would normally not trigger an action potential will produce one more easily as the resting potential gradually climbs to the threshold required to launch an action potential. Organisms in this state typically exhibit twitching and uncontrolled movements as the uncontrolled nerve impulses trigger muscles to move.
Nothing is static at the molecular scale. As organic molecules interact with one another, they can latch onto each other either very loosely or with tenacity depending upon the exact shape of the molecules involved and type of binding that happens. Binding that occurs via the covalent sharing of electrons is usually very strong, essentially permanent and irreversible. In contrast, many biological molecules interact through polar or Van Der Walls forces that are much weaker. Such interactions may last for a fleeting amount of time before thermal fluctuations pull them apart. Weak binding is reversible and can be characterized by a dissociation time, how long it takes to break the bond due to random and thermal fluctuations.
When dealing with pesticide chemicals, stronger bonds mean the insecticide is spending more time at the active site, so its potency is higher. Frequently it is just how tenacious the binding that determine the potency of the insecticide.
Chemical scavengers known as cytochrome P450 enzymes are always on the lookout for foreign chemicals which these enzymes break down into smaller parts in the process of metabolizing and eliminating unwanted molecules. Often, within a few hours much of a foreign chemical will be metabolized and eliminated from the organism. Bound molecules are not as easily digested by the cytochrome P450s so once toxins are bound to their site of action, they are more immune to detoxification.
Insecticides targeting the acetylcholine pathway
There are several classes of pesticides that disrupt the acetylcholine pathway. We will start by looking at the neonicotinoids because they have the simplest mechanism, similar to the “direct action” of the pyrethroids discussed above.
The neonicotinoids bind strongly to the AChRs. Binding causes he ion channels to open so Na+ ions can flow into the neuron. Unlike the normal acetylcholine response where the channel is only open for about a millisecond, when the neonicotinoid binds the receptors never close. Hence, it takes only a relatively few open channels to eventually depolarize the neuron. If the ion pumps cannot keep up with the leakage through the nicotinoid-bound AChRs the cell will depolarize. Partial depolarization will make the neuron more excitable; complete depolarization leads to paralysis.
This situation is more complicated with acetylcholinesterase inhibitors such as the organophosphate and carbaryl insecticides. For these chemicals, the insecticide does not directly bind to neuronal receptors that open ion channels. Instead the chemicals bind to the acetylcholinesterase (AChE) enzymes which rid the synaptic junction of the ACh neurotransmitter than is released with normal activity. However, without the AChE to clear the junction, the ACh continues to bind with AChR ion channels. The figure below shows schematically what happens with these AChE inhibitors.
Insecticide molecules bind to the acetylcholinesterace (AChE) sites in the synaptic junction, preventing the naturally released ACh for being removed and recycled from the junction. The acetylcholine continues to activate receptors, keeping their channels open thereby depolarizing the post synaptic neuron. Again, poisoning symptoms begin with an over-excitable nervous system, characterized by uncontrolled twitching, similar to the other classes of neurotoxins we have looked at.
Neurotoxins are among the most potent biological chemicals known. The chemicals are targeted to interact with specific receptor molecules that are crucial for nervous system function. This means that very few pesticide molecules are required to have a large biological effect. Chemicals used as pesticides need to effectively poison target species while remaining benign to non-target organisms and humans. However, much of the cellular machinery is shared across the animal kingdom, so differentiating between target and non-target organisms is a challenge. Often only space and time are used to separate target and non-targets creatures from chemical exposure. The environmental effects of pesticide chemicals depends upon the success of various strategies to limit harmful exposure to non-target species. In many cases dilution is the solution, but as industrial agriculture and residential uses of potent chemicals become even more widespread, minute residual levels of toxins is inevitable. Next time we will see why this is more likely to be a problem with some classes of chemicals more than others.