Cell Discussion Questions
Q1. I am having a bit of trouble understanding the difference between osmolarity and tonicity. I understand that osmolarity is the total CONCENTRATION of solute in water, and that tonicity is the total NUMBER of solute in EC fluid, but I guess I don't understand the difference in the definitions. If a cell with a semi-permeable membrane is in a solution, how do you know to call it a hyper, hypo, or iso-osmotic or -tonic solution? Does this make sense?
A1. The difference in the definitions is subtle. Osmolarity is the total concentrations of solutes, penetrating and non-penetrating. Tonicity is only the concentration of non-penetrating solutes. In addition, tonicity always is in reference of the extracellular non-penetrating solute concentration to the cell's non-penetrating solutes. I will give some examples.
Assume normal osmolarity is 300 mOsm.
A cell contains 300 mOsm of non-penetrating solutes (the cell is isosmotic) and
1. the extracellular fluid also contains 300 mOsm of non-penetrating solutes, the extracellular solution is isotonic and the cell will not change volume.
2. the extracellular solution contains 250 mOsm non-penetrating solutes, the extracellular fluid is hypotonic and the cell will swell.
3. the extracellular solution contains 350 mOsm concentration of non-penetrating solutes, the extracellular solution is hypertonic and the cell will shrink.
A cell contains 300 mOsm of non-penetrating solutes and 20 mM of penetrating solute such as glucose. The cell is hyperosmotic (320 mOsm).
1. the extracellular fluid also has 300 mOsm of non-penetrating solutes, the extracellular solution is isotonic and the cell will not change volume. The tonicity is always the relationship of the non-penetrating solutes. Even thought the cell's osmolarity is 320 mOsm (300 mOsm non-penetrating and 20 penetrating solutes) the extracellular solution is isotonic because it has the same concentration of non-penetrating solutes as the cell.
2. the extracellular solution contains 250 mOsm non-penetrating solutes, the extracellular fluid is hypotonic and the cell will swell. The intracellular penetrating solute concentration does not affect water movement because the penetrating solutes will cross the membrane and reach chemical equilibrium.
3. the extracellular solution contains 350 mOsm concentration of non-penetrating solutes, the extracellular solution is hypertonic and the cell will shrink. The intracellular penetrating solute concentration does not affect water movement because the penetrating solutes will cross the membrane and reach chemical equilibrium.
All that being said, I will not ask you to differentiate between penetrating solutes and non-penetrating solutes for the exam. We will discuss non-penetrating solutes only.
Q2. I'm having trouble wrapping my mind around the relationship between the intra and extracellular concentrations of ions and their respective equillibrium potentials.
Ion [Extra] [Intra] Eq. Poten.
Na+ 150mM 15mM +62mV
K+ 5mM 140mM -90mV
Cl- 110mM 10mM -64mV
So, as I understand it, to keep Na+ from wanting to follow its concentration gradient into the cell I would need to increase the intracellular "electricity" to +62mV to provide sufficient repulsive force to keep the Na+ out. Conversely, since K+ wants to exit the cell due to its conc. grad. I would have to decrease the intracellular "electricity" to -90mV to provide enough attractive force to keep K+ in the cell. Now, if these assumtions are correct, what correlation can I make between them (the assumptions) and an action potential and the associated refractory period?
A. You have the correct concept of the equilibrium potential. Now let's discuss the relationship between the equilibrium potentials and the action potential. Under resting conditions the membrane is 50x more permeable to K than Na thus the membrane potential is around -60 mV. When there is a depolarization that activates the voltage-gated Na channels, the permeability to Na increases to levels that far exceed the resting K permeability. The membrane potential is -60 mV, but the Na equilibrium potential is +62 mV. Thus, Na enters the cell and tries to drive the membrane potential to +62 mV. The channels close before the membrane potential reaches +62 mV and it only depolarizes to +30 mV. The depolarization caused by the Na influx activates the K channels. The permeability of K increases above the Na permeability and above its resting permeability. K leaves the cell to try to bring the membrane potential to its equilibrium potential of -90 mV. This causes a repolarization, and for a brief period the membrane potential goes below -60 mV (hyperpolarization). When the K channels close, K permeability returns to the resting levels and membrane potential goes back to the resting potential of -60 mV.
There are two components of the refractory period, absolute and relative refractory periods. The absolute refractory period is the time when no amount of stimulus can cause an action potential. This is due to the Na channels being in an "inactive" state. When the channels close they are "inactive" and cannot open until they are reset to the "resting" state. Once the channels are in the resting state they can be activated. The time that the channels are in the "inactive" state is the time of the absolute refractory period.
The relative refractory period occurs after the Na channels return to the resting state. For a brief time after the Na channels have reset the K channels are still open and the membrane potential is repolarizing. During the relative refractory period you can stimulate another action potential, but it takes a much larger stimulus because any depolarization must overcome the hyperpolarizing K current. The efflux of K will short circuit any depolarizing stimulus. The end of the relative refractory period occurs when all of the K channels are closed.
Q3.1. Can tetanus be thought of as a series of rapid summations? In other words, whats the difference between tetanus and summation?
A. Yes, tetanus is the summation of contratures in muscle. Tetanus is temporal summation in muscle. Remember there is no spatial summation in muscle contraction because there is only one neuron per muscle fiber.
Q3.2. If you bathed muscle in Ca++ how would the contraction differ? Would it be different for skeletal and smooth muscle?
A. For skeletal muscle, contracture does not require influx of extracellular Ca. So, you can eliminate the extracellular Ca and still have contracture of skeletal muscle. This is not the case in smooth muscle.
If you raise the extracellular Ca in skeletal muscle it will inhibit contraction by stabilizing the membrane and shielding the negative charges on the outside of the membrane. This will prevent the generation of an action potential. This is because the voltage-gated Na channels do not "sense" the depolarization.
Q3.3. Could you clarify the mechanism for the latch state? In the syllabus it says, "if myosin is attached to actin when it is dephosphorylated, then the myosin remains attached to actin and a sustained contraction occurs (latch state)." What controls whether myosin is attached to or detached from actin when its dephosphorylated?
A.The myosin is being phosphorylated and depohphorylated by the kinase and phosphatase during stimulation. The dephosphorylation can occur at any time during the cross bridge cycle. If the dephosphorylation occurs when the mysosin is detatched from actin, then the myosin will not attatch to actin until it is phosphorylated again by light chain kinase. If the myosin is dephosphorylated when the myosin is still attached to the actin, then the cross bridge cycling will slow down and the myosin will remain attached to actin and form the "latch state". There is still ATPase activity and eventually the myosin will detatch from actin.
Q4. Do we need to know what the conduction system is for the receptors in the ANS? For example, the alpha- adrenergic receptors are phosopholipase C conduction systems. I can't seem to find anywhere what kind of receptors the muscarinic and nicotinic cholinergic receptors are.
A. No you don't need to know what signal transduction systems are associated with the ANS receptors. The Nicotinic cholinergic rceptors are ligand-gated channels just like those at the neuromuscular junction. The only difference is in the selectivity to certain antagonist.
Q4.1. Also, does the membrane potential of EPP's oscillate like the membrane potential of EPSP's and the IPSP's?
A. No, the resting membrane potential of muscle is much more stable than the neuronal membrane potentials because there is no inhibitory input.
Q5. I had a couple of quick questions as I was doing the practice problems on your web page. On question 2 about osmolarity, b) Why is the solution isoosmotic, when the osmolarity is 300.5(over 300).
A. 300.5 is so close to 300 that I said that it is isosmotic. If you want to be entirely accurate it would be hyperosmotic.
Q. Also, why is the solution, in question c. hypotonic when, again, the osmolarity is 300.5? I understand the whole concept, and I know that tonicity refers to only non-penetrating, and the osmolarity refers to both penetrating and non-penetrating. The non-penetrating, penetrating factor might play a role in this problem, but you don't specify non-penetrating or penetrating in the problem, and you said we wouldn't have to be able to distinguish between penetrating and non-penetrating either.
A. You have the correct concept of penetrating vs. non-penetrating solutes. The problem comes in the definition of tonicity vs. osmolarity. Tonicity is the concentration of the non-penetrating solutes in the extracellular fluid in reference to the intracellular fluid. So in this situation, the extracellular fluid is isosmotic or slightly hypertonic, but the concentration of non-penetrating solutes is lower than the intracellular concentration, so it is hypotonic. This is very picky. These are questions I wrote several years ago and I don't go into as much detail now as I have in the past. It is helpful for you to know the difference between osmolarity and tonicity and this question highlights the difference. I will not ask a question that is this picky on the exam. As I said before and you also mentioned, on the exams all solutes will be salts and all solutes are assumed to be non-penetrating.
Q6.1. When K is 50x more pearmable than Na, does that mean that the cell is still pearmable to Na? for example when K+ channels are closed, K+ doesn't leak out the cell, but Na can leak in any way because its pearmability is not affected, then the cell becomes depolarized?
A. Yes, the cell is still permeable to Na. That is why there is some Na leak into the cell which causes the membrane potential to be -60 mV. In some situations, such as when the extracellular solution becomes acidic, K leak channels will close. When that happens there is still Na leak and the cells depolarize.
Q6.2 Are Na/K channels in the end plate open ion channels all the time not ligand or voltage gated ones?
A. The only channels in the end plate are the nicotinic cholinergic receptors which are ligand-gated channels. The voltage-gated Na and K channels are not in the end plate, but in the membrane surrounding the end plate. That is why the current from the end plate potential must spread from the end plate to the adjacent region to depolarize the membrane surrounding the end plate and generate an action potential.
Q7. Could you please explain what will happen to the membrane potential and the action potential if the extracellular potassium concentration is increased, while there is no change in the intracellular K concentration.
A. When extracellular K increases the equilibrium potential will become more positive. If you are not clear why that happens, look at the Nernst equation. Put some numbers in and calculate the equilibrium potential. If your calculator does not calculate natural logs (ln), 2.303*log 10 = ln. Since the permeability of K is 50x greater than Na, the equilibrium potential of K will has a dominant role in determining the resting membrane potential. As the K equilibrium potential becomes more positive, so will the membrane potential. Thus, the membrane potential will depolarize.
As the resting membrane potential depolarizes, it will be closer to threshold and it will take less stimulus to cause an action potential.