Eleven key points:#1) Voltage is electrical pressure . (But zero voltage is not definable: there are only differences) A voltage difference = "a potential" A voltage difference between one side of a membrane and the other side of that membrane is a "membrane potential" #2) Nerves are sort of like wires (but also somewhat different). Nerves DO carry electrical signals, but not by simple conduction of an electrical current. Nerve signals ("impulses") are somewhat more like burning of a fuse, instead of like electric currents along a wire. A change at one location causes more changes in the adjacent areas, like the heat of a flame causing the next part of the fuse to burn #3) Each nerve fiber is a strand of cytoplasm, and its plasma membrane undergoes changes in permeability to ions.
Nerves carry signals of touch, pain, heat, etc. from the skin;
A QUESTION FOR CLASS DISCUSSION:
#4) What are ions? When potassium chloride dissolves in water, it produces jillions of potassium ions each with a positive charge, " K+ " and also jillions of chloride ions , each with a negative charge. Cl- There are also such things as calcium ions = Ca++ and aluminum ions = Al+++, etc. etc. #5) Ion pumps: Membrane proteins that use ATP energy to pull particular ions from one side of that membrane to the outside.
"The sodium-potassium pump" uses almost half the ATP supply of cells In a healthy cell, the concentration of potassium inside is about 30 times higher than outside. Sodium is 10 or more times more concentrated outside. Calcium ions are strongly pumped out by different ATPases, and kept at VERY low concentrations inside cells. #6) Resting potentials : occur in nearly all the differentiated cells of your body, including nerve cells and muscle cells . (also all the other cells; but most people only know about nerves)
The resting potential consists of a voltage difference between the cytoplasm and the outside of the cell, = about 70 thousandths volt. In other words, about seventy millivolts . #7) The cause of the resting potential is that potassium ions leak out through the plasma membrane . (but the other kinds of enzymes can't leak; not nearly as much)
Notice that this seems somewhat paradoxical! ????
As your first step toward understanding why this is, The explanation is that these leaking ions carry their positive charge outward, as they leak. It's sort of like osmosis is caused by the leakage of water from high to low concentration, and creating a higher water pressure in the place to which it diffuses. If sodium ions could leak through the membrane, then the result would be to make the inside positively charged. #8) Nerve impulses are caused by the plasma membrane becoming temporarily more permeable to sodium than potassium. This change in permeability causes the voltage to become about 15 millivolts negative outside (the reverse direction from before). The nerve impulse is one example of an action potential. #9) Nerve cells and muscle cells (and a very few others) will propagate action potentials because their plasma membranes have special voltage gated sodium channels that let sodium diffuse through (for a very short time >1 thousandth second). These channels open (faster than a camera shutter, and briefer) whenever the resting potential becomes smaller than, say, 65 mv. (the threshold potential ) So if something makes the resting drop below the threshold at one location, that induces opening of sodium channels there, which lets sodium diffuse inward, which makes that whole area depolarize, opening more channels. #10) "Voltage-gated channels" means channels that open in response to certain changes in voltage. Guess what change causes opening of "temperature-gated sodium channels" . Guess what would cause opening of "chemical-gated sodium channels" . What about glycine-gated chloride channels? Imagine that certain plants synthesize a certain chemical that would somehow cause opening of heat-gated sodium channels? What would that plant taste like? Do you like Mexican food? Have you wondered how it works? What would happen if somebody injected a concentrated solution of a potassium salt next to nerve or muscle cells? #11) Refractory period (= insensitive period) The voltage-gated sodium channels open only briefly, for less than a thousandth of a second. After that, these channels close, and remain closed for about thousandth of a second, during which they are NOT able to re-open, even though the voltage difference across the plasma membrane is still less than the threshold voltage. This allows time for repolarization = the reestablishment of the resting potential (which is achieved by potassium leaking out, and does NOT require functioning of the sodium pump! Contrary to what many elementary textbooks claim. A given nerve fiber can propagate a hundred thousand or more nerve impulses even after the sodium pump is poisoned somehow. A thought question, if the refractory period were 5 milliseconds, then what would be the maximum number of action potentials that could be propagated per second along that nerve? (for this calculation, assume that 5 milliseconds is the sum of the refractory period and the time of the depolarization itself) Another thought question: What would happen if repolarization were slowed down enough that a nerve wasn't able to re-establish its resting potential (above the threshold for opening Na+ channels) by the end of the refractory period of those channels? (DDT does that) Please Guess The Following : When a nerve fiber sends a stronger signal, such as to report more heat, or more pain, or to increase the contraction of a muscle, etc. do you guess this is done by sending BIGGER nerve impulses, or by sending MORE nerve impulses of the same size? ALSO GUESS: is the same mechanism of nerve impulses used for sensory nerves as for motor nerves (that stimulate muscles to contract)? What about the waves of stimulation that sweep across heart tissue, to cause the heart beat? Could/should that also work by voltage-gated sodium channels? -------
Questions that you should be able to answer on an exam:a) Is the concentration of potassium ions higher inside cells, in their cytoplasm, or outside in the surrounding fluid?b) How to the concentrations of sodium ions differ, inside versus outside the plasma membrane of a healthy cell? c) How are voltages related to electricity? d) What is another word for a voltage difference? What phrase refers to a voltage difference between the inside of a cell, as compared with the outside? e) If a cell's plasma membrane were much more permeable to sodium ions than to any other ions, then what voltage difference would that create across the plasma membrane? f) In an ordinary cell, or a resting nerve or muscle cell (when it isn't currently propagating a signal), which ion is its plasma membrane most permeable to? g) Explain the seeming paradox that the outside of a cell is given a positive voltage by diffusion of a positive ion that has a higher concentration inside the cell. h) Why do potassium ions tend to diffuse from the inside to the outside of cells? i) If the concentration of potassium ions were higher on the outside of cells, then in which direction would these ions tend to diffuse? j) What prevents more than just a millionth of the potassium ions from diffusing out of cells (even if the sodium pump were turned off)? k) The propagation of nerve impulses is caused by temporary opening of what kind of channels through the plasma membrane that let which kind of ion diffuse through them? l) What stimulates the temporary opening of these channels? m) What is the positive feedback that causes these channels to open in one area, and then in nearby areas, etc. n) For what medical purpose could you use a drug that prevents this temporary opening of sodium channels? o) Why does there have to be a refractory period in order for a nerve fiber to repolarize after passage of an action potential (=nerve impulse)? p) If the refractory period were a hundredth of a second (actually, it is less) then how many impulses could that nerve propagate per second? q) Which kinds of cells have resting potentials? Which other kinds also? r) Which kinds of cells also propagate action potentials? s) Why do cells have to have resting potentials in order to propagate action potentials? (in other words, why can't you have nerve impulses unless you have resting potentials?) t) If somebody injected a water solution with potassium salts dissolved in it under your skin: Would it hurt? Would it cause muscles to contract? u) What about injecting water with a high concentration of sodium ions dissolved in it? What effect would you expect, if any? Explain. v) In what sense is a pay phone "money gated"? w) If a chemical blocked nerve cells from becoming permeable to sodium, then how could it be used by dentists as a local anaesthetic? x) Would you expect nerve impulses (action potentials) to be able to propagate in both directions along nerve fibers, even though they normally only propagate in one direction? Explain why or why not? y) If two nerve impulses were propagating in opposite directions along a nerve fiber, then they would both be destroyed where they "collided", rather than passing through each other. Can you figure out why that happens? *z) Can you invent a way that would make action potentials behave as if they "bounced off" part of a cell, as if they were being reflected so that an action potential comes back in the other direction. **!) If our bodies didn't use electrical signalling mechanisms, then would electric shocks kill us? (would you expect?) **@) Conversely, explain why "magnetic chairs" might be used to execute people, if we used propagating magnetic fields to send signals from one part of the body to another? **#) Let's invent some different (science fiction) mechanism by which signals could propagate rapidly along long-narrow cells! For example, suppose that the cell pumps water inward, creating a pressure, and that holes then open whenever this pressure falls below a certain threshold amount... then what?
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