Ten key points:#1) In some cases, such as between one heart cell and other heart cells, action potentials can propagate directly from cell to cell, by direct permeability, so depolarization of one cell will cause depolarization of the adjacent cell. There are also cases of electrical synapses between certain nerves.
#2) The other method is a "chemical synapse" [Don't bother to memorize the next bit of information about the chemical structure of choline: BUT, if you take a serine, let the carboxyl group fall off as a CO2, then attach 3 methyl groups onto the amino nitrogen (so that it has 4 bonds! yes!) then that's choline. Then if you attach an acetic acid to the hydroxy group at the other end (the hydroxy group that phosphates get put on by kinases) then that's acetyl-choline] #3) Motor nerves stimulate muscles to contract by synapses that secrete small amounts of acetylcholine at places where the plasma membrane of the muscle cell has sodium channels that open in response (acetylcholine-gated sodium channels). So that sets off an action potential in the muscle cell; and its calcium channels also open, and the calcium ions stimulate the contraction (active sliding of myosin past actin) This is not the only example of synapses that use acetylcholine as their neurotransmitter substance. In some other cases, (the heart) acetylcholine inhibits depolarization. This is because the heart cells' channel proteins respond differently.
#4) Why doesn't the muscle cell keep on leaking sodium and therefore contracting?
#5) Nerve gas works by inactivating acetylcholinesterase.
#6) Atropine (the nerve gas antidote)
But it also is poisonous by itself. It is belladonna. Among other effects, atropine causes widening of pupils of your eyes which women used to make themselves more attractive! (a chemical analog of eye make-up!) You can tell whether people like you, if your eyesight is good enough to see whether their pupils dilate or constrict when they look at you. Dilation means they like you. Advertising agencies mount small telescopic cameras to detect pupil diameter changes. Subconsciously, to the extent that you can detect other people's pupils dilating when they look at you, then they will seem more attractive to you. Hence the belladonna. bella = beautiful donna = lady #7) Succinylcholine is a chemical analog of acetylcholine (with succinic acid where acetic acid would be) It causes mild paralysis by blocking the acetylcholine receptors.
When wild animals are shot with drug-injecting darts, the "tranquilizer" substance used is really not a tranquilizer, it is succinylcholine, and paralyzes the animal (but not too much).
Succinic acid is a 4 carbon chain, with carboxy groups on each end. Curare is a plant poison that also blocks acetylcholine receptors and is used in poison arrows in South America, and is now used in surgery to prevent muscle spasms.
So if you get simultaneously attacked by poisoned arrows and nerve gas, they may cancel out!
#8) Acetylcholine is one of about 20 different neurotransmitter substances. Instead of being chemically split, like acetylcholine, other neurotransmitter substances are re-absorbed.
#9) Many psychologically-active drugs by mimicking or by blocking reabsorption of some of these neurotransmitters. #10) Our central nervous system does its calculations by having both stimulatory and inhibitory neurotransmitters secreted at the same synapses. Although any one nerve cell will only secrete one neurotransmitter, a given nerve can be sensitive to several different neurotransmitters, some of which stimulate (depolarize) while others inhibit (by hyperpolarization, generally by increased permeability to chloride ions, which leak in like sodium, but have negative charges, and therefore cause hyper-polarization (make the resting potential larger). Such synapses are somewhat analogous to transistors (functionally, in that voltage in one wire changes conduction of signals in another wire; that's what transistors do by changing conductivity by means of voltages in another circuit) One nerve cell in the brain may have millions of synapses. Lasting changes in sensitivities of synapses have long been a favorite hypothesis about how memory works. (And would be somewhat like computer RAM) But are instincts like ROM chips? Can they be caused by genes? Are instincts inherited behaviors? Based on what: genes causing nerves to connect in certain patterns? Are languages "hard wired" into nerve connection patterns? Why are they so easy to learn before you are 12, but so difficult to learn after that? (especially Russian!) -----
Questions that might be on an exama) Do heart cells have action potentials? [yes]b) Can an action potential in one heart cell cause adjacent cells to depolarize and become temporarily more permeable to sodium? How? c) How does this mechanism differ from the method by which an action potential in a motor nerve causes an action potential in the muscle it innervates? d) It so happens that this heart cell-heart cell mechanism is sometimes called an "electrical synapse", in contrast to "chemical synapses" (the nerve-muscle connection being a chemical synapse. You should understand the reason for this terminology. e) What happens to the vesicles of acetylcholine when a motor nerve undergoes an action potential? f) Acetylcholine-gated sodium channels produce what effect in this kind of chemical synapse? g) Suppose that a cell had either acetylcholine-gated potassium channels or acetylcholine-gated chloride channels: what effect would acetylcholine have on the resting potential of that cell? h) Why do cells normally depolarize only very briefly in response to secretion of acetylcholine at a synapse? [hint: what is the function of acetylcholinesterase?] i) How does nerve gas kill people and animals? *j) Would nerve gas also kill plants? Why not? k) What is the name of an antidote to nerve gas? l) How does this antidote work? Why will it only protect you from a small amount of nerve gas? m) Is this antidote also poisonous, even though less so than nerve gas? n) Why is another name for this antidote "belladonna"? o) What is the "tranquilizer" chemical used to paralyse wild animals, such as bears that wander into town? p) If you knew the chemical structure of a neurotransmitter substance, then how might you use this information to design a synthetic drug? q) About how many different neurotransmitter substances are normally used in the human brain? r) What are at least two specific examples of neurotransmitters, in addition to acetylcholine? s) How do amphetamines produce their effects? (for example, decreasing sleep and appetite? *t) If one neurotransmitter substance tends to cause depolarization of a certain nerve, but a different neurotransmitter substance causes hyperpolarization of that nerve, then will the effects cancel each other out, or what? u) Compare synapses in the brain to transistors in a computer, in terms of how they work as well as what functions they serve. **v) Are instincts due to genetic control of synaptic connections? (Maybe? How could one find out?) w) True or false: Re-polarization of the membrane after passage of a nerve impulse must depend on the sodium pump bailing the sodium ions back out of the cytoplasm, and the potassium ions inward? x) Compare burning of a fuse to propagation of an action potential along a nerve? *y) In order for a skin cell or a liver cell (or any cell other than a nerve or muscle), then what specific kind of protein would you need to introduce into the plasma membranes of these skin cells, etc. ? z) What kind of membrane protein needs to be concentrated in the part of a muscle cell's plasma membrane in the area where acetylcholine is secreted onto it? -----
Some widely-believed fallacies about nerve impulses(that are even taught as fact in many or most textbooks)
i) That resting potentials occur only in nerves, or only in nerves and muscles. Actually, almost all cells have potassium potentials in the range of 50-100 millivolts, negative inside/positive outside. ii) That some kind of negative ions inside cells, especially ionized proteins, must be what causes the resting potential; because they can't handle the paradox of positive potassium ions making the inside negative, by being more concentrated here, and leaking out. The textbook we previously used in this course said that. iii) That all the sodium and potassium leak across the membrane every time an action potential occurs. Really, the fraction is as low as a millionth of the concentration differences. iv) That the repolarization of nerves after each action potential results from the sodium pump, bailing all those sodium ions out again, etc. The previous textbook even had a CD with a computer simulation showing this happening! Very misguided. Lower-level textbooks, and "educators" (when teachers or professors start calling themselves educators", then beware!) are tempted to change the "facts" to make them closer to what people might expect. This makes them "easier to learn". But imagine that you asked someone along the road for directions about how to get to Pittsboro, and they told you to take the wrong road (maybe the road to Durham), and thought they were doing you a favor because the road to Durham was better paved or wider, or something. (Or because they just didn't know how to get to Pittsboro, and didn't want to admit that.)
About immunology, allergy and autoimmunity, textbooks tend to be especially bad.
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