Even More Muscle and Membrane Physiology

Hello All,

As many of you have noticed, Membrane potentials keep popping up and are truly KEY to understanding much of what happens in physiology.  Therefore, there have been a few questions regarding this subject that we should address first:

1. Please can you explain to be what is the difference between these 2 statements you made in the handout for membrane potentials?

STATEMENT I: The total net charge inside the cell is more negative than the total net charge outside of the cell.  this is due to a high proportion of negatively charged proteins inside the cell.

Typically at rest, there are ions of both types - and + that balance each other.  Therefore, their NET charge is balanced.  However, the proteins, which are negatively charged, do not necessarily have any + charged ions that balance their charge.  If, at rest, the cell was not permeable to anything (which as you can see below is not accurate), then you would have a slightly negative overall charge inside of the cell with respect to the outside.  Therefore, this negativity accounts for a VERY SLIGHT amount of negativity at rest, but only about 1-5mV.

STATEMENT II:  The cell membrane potential is most preferentially permeable to K+ (but is also permeable to oda ions) thus resulting in the very NEGATIVE resting membrane potential inside the cell with respect to the outside.

Membrane potential is, overall, a difference in charges across the membrane.  This is due to the concentration differences of a variety of ions with respect to their relative permeabilities.  What that means is that each ion that the membrane is permeable to have a membrane potential where THEY want the membrane to be at their 'rest' or equilibrium and that is due to their concentration differences.  Remember that the equilibrium potential is the membrane potential where the concentration gradient for a given ion is directly balanced by an electrical gradient (the difference in charges measures a number of this electrical gradient).  If you make the membrane permeable to any of these ions the membrane potential will reside close to their equilibrium potential.  The equilibrium potential for K+ is very negative, therefore as the membrane IS permeable to K+ at rest, the membrane potential (the balance of the concentration differences for a variety of ions with respect to their relative permeabilities) will be close to that equilibrium potential.

2. Is the resting membrane potential the same as the "sum of the individual equilibrium potential of the different ions across the ICF and ECF"? ( i got this idea from the Chord Conductance equation)

YES.  The resting membrane potential is the sum of the individual equilibrium potentials for the different ions across the ICF and the ECF but WITH respect to THEIR relative permeabilities.  Therefore, for a given ion that the membrane is more permeable to, its equilibrium potential will have more 'weight' in that of the membrane potential, while those that the membrane is less permeable to will have less effect.

Additionally, there is still some trouble that people are having regarding Synaptic Transmission and the NMJ.  Therefore, here are a few of those questions:

3. I had a question concerning the practice questions posted at the end of the NMJ lecture.

8.  Periodic hyperkalemic paralysis is characterized by high potassium concentration and muscle weakness.  Which of the following is likely to cause muscle weakness as a result of increased extracellular potassium concentration?
                A.  Hyperpolarization of muscle cells
                B.  Inactivation of sodium channels in muscle cells
                C.  Increased release of neurotransmitters from alpha motorneurons
                D.  Decreased potassium conductance in muscle cells
                E.  Increased duration of action potentials produced by alpha motorneurons

I know that in your blog you already explained why D is not the answer. But can you please explain to me why B is the correct answer. I don't understand why the high concentration of potassium would inactivate the sodium channels. I was thinking that if the potassium concentration is high outside the cell then it would prevent repolarization from occurring and that depolarization would last longer, thus exhausting the muscle.

Please let me know where I'm wrong. 

Remember that Na+ channels will INACTIVATE if they are open too long, therefore depolarization happens and the membrane could REMAIN depolarized, but that without repolarization the Na+ channels cannot RECOVER from inactivation and they an get stuck in their inactivated phase, thereby not allowing any further action potentials to occur.

4.My first question deals with definitions, specifically with the motor end plate, is it another way of saying the post synaptic plate, or are they two different structures?

The motor endplate IS the postsynaptic cell in the neuromuscular junction.  It serves the same role as the postsynaptic cell in other chemical synapses in that it receives the neurotransmitters (in this case ACh) and the receptors respond to that and produce a response in the postsynaptic cell.

And here are some more muscle physiology questions:

5. Could you help me better understand concentric and eccentric isotonic contraction.  I understand what their definitions say (concentric --> muscle shortens while eccentric --> muscle lengthens) but I can't quite picture it.  Muscle shortening is easy to visualize since that's when you're simply trying to/ and is able to  lift say a 10lb dumbbell but I don't quite get eccentric isotonic contraction.

How do you put down the 10lb dumbbell?  What work of the muscles are occurring as you are retracting that muscle that you have just lifted?  THAT is eccentric contraction!  Yes, if you lift a dumbbell with your biceps brachii and then put it back down there is a bit of concentric contraction on the way down by your triceps, but MOST of the work is an eccentric contraction (as the biceps brachii is getting longer).

When it comes to exercise: strength training and aerobic exercise, I'm a tad confused on what you have in the notes.  under the "long duration but low intensity" type exercise, you had it under the title of "susceptibility to fatigue" but then the notes that are found under the pictures states that these types of exercises have minimum fatigue? 

The GOAL of these types of exercises is to reduce your susceptibility to fatigue.  By doing long duration but low intensity exercises you stimulate the muscle to create more mitochondria and therefore allowing for more oxidative phosphorylation, thereby reducing the muscle's susceptibility to fatigue. 

6. Could you explain why low extracellular [Calcium] would cause an increase in the opening of sodium channels that would ultimately lead to tetany?

GREAT question and a really difficult one.  This seems counter-intuitive that a decrease in extracellular [Ca2+] would lead to an increased excitability of the membrane.  However, this seems to be another one of Ca2+'s mysteries.  Ca2+ is probably THE MOST important intracellular signalling molecule and here it is causing havoc on the extracellular membrane.  Research is unclear as to the reason for this, but somehow there is a bit of communication between Ca2+ on the extracellular membrane and the Na+ channels such that without the Ca2+, the Na+ channels appear to be more excitable.  You know that if THEY are more excitable, you will get more action potentials ultimately leading to a potential of tetany.

7. I'm having some issues understanding what passive tension and active tension are.  I understand you need ATP for active and not for passive, but I do not understand what you mean when you say the "recoil generated by stretching the connective material is called passive tension" and adding additional active tension.  Can you provide a real-life example of this movement? These always help me!

Active tension is the tension that a muscle produces due to the interaction between mysoin and actin during the cross-bridge cycle.  This is a kinetic energy and is what you typically think of as the tension that occurs during a muscle contraction and you are correct to make that assumption.  However, in a muscle fiber in order for the contractile components (thick and thin filaments) to be located where they are, they are kept there by proteins (titin and nebulin) that when stretched have a potential energy to recoil back to their rest length.  This potential energy is uncontrolled physiologically and is called passive tension.

8. For smooth muscle, it has two ways of bring calcium into the cell and uses it to bind to calmodium. It uses the processes such as L-Ca 2t, TRP, and G-protein coupling (extracellular), and it also using the Sacroplasmic Reticulum as a source. Is there a specific method that the cell prefers to bring calcium intracellularily? If one method is damaged, either extracelluarily or the Sacroplasmic Reticulum, does the cell compensate in any way? Does this question make any sense to you?

GREAT questions.  It would be nice and 'simple' if there was a single way that smooth muscles work, but because smooth muscles are located in a variety of body systems and designed to contribute in a variety of different ways it is important that they have a variety of ways to bring Ca2+ into the cell both from the extracellular and intracellular sources.  Each system may have a 'preferred' way to bring Ca2+ into the sarcoplasm, but that is specific to each organ system.  Details of this (as discovered and considered medically important) will be covered.  I will caution you, however, original research wanted all smooth muscle to be 'the same', and research is still currently discovering the variety of differences and will presumably continue.

9. This is probably an unnessary detail but its bothering me so I will ask - the ADP + Pi that is released from the myosin upon binding with G-actin, reform the ATP and is the "same" ATP that will bind to myosin to unbind it from actin? Or is it a different ATP?

Also, how many ATP's are used in one cycle? 1 or 1.5?

I understand this question and where it is coming from as many people teach the cross-bridge cycle differently.  It is, indeed, not entirely necessary that you know 'how many' ATP are used in the cycle, but I understand the need to 'simplify' this event down into numbers.  If you study the cycle, you can see that the SAME ATP that bound to myosin in a previous cycle is the SAME hydrolyzed ATP that is then released in the next cycle.  Looking at this cycle linearly allows you to make a judgement of 1.5 ATP, but truely in one cycle you are using the hydrolyzed ATP from the previous cycle and allowing a new ATP molecule to bind.

10. I was wondering if you could clarify why the answer is C, in the following question (aside from stating that it is the only logical answer, which I got):

3. During the process of excitation-contraction coupling in smooth muscle, intracellular [Ca2+] is increased through all of the following methods except which?

  A.  Ca2+ influx from extracellular stores through voltage-activated Ca2+ channels

  B.  Ca2+ influx from the sarcoplasmic reticulum through IP3 receptors

  C.  Ca2+ influx from extracellular stores through ryanodine receptors

  D.  Ca2+ influx from extracellular stores through ligand-gated Ca2+ channels

On slide 15 notes, you state: Calcium entering from the extracellular space (through voltage-gated or ligand-gated ion channels) activates ryanodine receptors (RyR) in the SR membranes, opening calcium channels.  

Indeed, RyR ARE opened upon the SR membranes, but this release calcium from INTRACELLULAR stores, not the extracellular stores that are identified in the question.  RyR are located on the SR membrane not the plasma membrane and therefore allow for intracellular Ca2+ from the SR to be released into the sarcoplasm.

11. Can you explain the difference between fused tetanus and unfused tetanus?

A twitch is the increase in tension that corresponds to a single action potential on the skeletal muscle that releases a solid concentration of [Ca2+] from the sarcoplasmic reticulum (SR) that is then sequestered back into the SR (which causes the decrease in tension).  Tetanic contraction, however, is what happens when you have more than one action potential (AP) close together in time so as the concentration of [Ca2+] within the sarcoplasm accumulates.  Unfused tentanus is when there are AP close together in time so that Ca2+ is coming out of the SR and accumulates but a bit of Ca2+ is resequestered prior to the next AP releasing Ca2+ again.  Fused tetanus, on the other hand, is when the APs are coming so close together in time that for each molecule that is sequestered back into the SR, another molecule of Ca2+ is released through the RyR from the SR.  Therefore, the result is a constant concentration of [Ca2+] in the sarcoplasm and a constant steady-state level of tension seen within the muscle.

12. So I have tied my best to comprehend the concept of eccentric contraction, but in my head contraction is always associated with shortening as thats what contraction of the sarcomers does - it shortens the muscle. I have, since grade 9, never really understood how eccentric contraction actually leads to lengthening of a muscle - unless it is relaxation (which it is not) or opposite of contraction (ie elongation of sarcomeres because the Z-lines are pushed past each other and now being pulled to stretch the opposite way). 

I understand that visualizing the way in which the myosin head group allows for the lengthening of the muscle is difficult.  This is difficult to scientists as well which is why the theories are only theories.  What is hypothesized is that instead of the myosin head group tugging on actin when it releases the ADP and Pi, it tugs a bit, but then allows for the actin to move in the other direction while the tugging is being done by another myosin head group from a different sarcomere in a different place.  You know that eccentric contractions occur, however, as stated in #5 above, because you need to control the lengthening of a muscle while putting down a weight. 

Also, while I understand isometric and isotonic, the use of the words force (generated by muscle) and tension (generated by load?) throws me off and I am even more lost with regards to the velocity-force curve....I don't understand what is going on in the eccentric/lengthening velocity region of the curve...with an increasing weight, the lengthning becomes faster? 

The general idea of the force-velocity curve is this:  The heavier the load of an object (therefore the more force or tension needed to be produced by the muscle in order to equal that of the load), the more difficult it is to do work on that object and move it (thereby changing the length of the sarcomere).  To pull that object (concentric isotonic contraction), the velocity will decrease as the load (or force/tension) increases.  To lengthen the muscle (put down the object), gravity works in assistance.  It helps the movement of the object to an extent that the muscle can still control the movement of that object until the muscle no longer has enough ability to lengthen moving the muscle.

EXAMPLE: if you have 100 cross-bridge cycles possible.  If it takes 90 cross-bridge cycles to equal a load that you are moving (ex. box of books), then there are only 10 cross-bridge cycles available to move the load and it will move very slowly (low shortening velocity).  If, however, it only took 10 cross-bridge cycles to equal that of a load (ex. box of feathers), then there would be 90 cross-bridge cycles available to actually do the work on the load and it could move much faster.  Alternatively, to put down the box of books gravity would help move the box of books while the muscle simply controls the movement, but to put down the box of feathers your muscle needs to do a lot more of the movement as gravity is not helping as much.

12. No matter how much this is related to a rubber band analogy I am still getting confused...I understand that the active tension goes down due to the decrease in cross bridge formation.  Why does passive tension not do the same?  I know that as the muscle becomes longer its tension will increase but what is the molecular basis for this if we are not talking actin and myosin binding in this instance....is it elasticity that we are dealing with?  Thefore the total tension at extremely large lengths will only be the passive tension since cross bridge assembly is inhibited?  

The molecular basis for the increase in tension at longer lengths is that you are stretching the connective proteins and they have potential energy that creates tension at these long lengths.  Therefore, YES the total tension at extremely long lengths IS due to that of only passive tension as the myosin head groups no longer have the ability to interact with actin (even if Ca2+ is around) because the actin is moved away from the myosin head groups by stretch.  Please see question #7 above for further help.

If two muscles have a constant force upon them but one muscle is longer then the other how would that cause the longer muscle to have a greater shortening velocity?  This is where my problem lies with the last question because in this case I am also thinking "ok so the longer muscle may have more tension therefore it would recoil in a sense quicker then the shorter muscle" or "ok is one or the other at an optimal length with optimal overlap of actin and myosin?  how could I determine which muscle then would have more efficient actin myosin cross bridge formation?" 

The question I am thinking through is this:

6) Assuming two skeletal muscle fibers have the same myosin ATPase activity, and both are producing an equal force, but one fiber is longer than the other one . Which of the following best describes their shortening velocities?

A) Long fiber has a higher velocity during isotonic contraction than the short one

B) long fiber has a higher velocity during isometric contraction than the short one

C) long fiber has a lower velocity during isotonic contraction than the short one

D) long fiber has a lower velocity during isometric contraction than the short one

E) Both fibers have the same velocity during isotonic contraction 

the answer was the longer muscle will have a greater shortening velocity in an isotonic contraction...wasnt the force constant?  If force is constant you cant have an isotonic contraction correct??

For this question you are assuming that the longer muscle IS longer because it has more sarcomeres in series (not that it is the same number of series but it is just stretched to a longer length).  BOTH muscles are assumed to be at their 'rest' length (that you know is close to the optimal length or "1" on the length graph).  You seem to be combining two concepts: increasing the length of the muscle by adding more sarcomeres and increasing the length of the muscle by stretching.  A LONGER muscle is assumed to have more sarcomeres not to just be stretched to be longer.  For question 6 please refer to slide #69 in the lecture.  Increasing sacromeres increase the shortening velocity of a muscle (for a longer spring to shorten to the same length as a short spring is shortening to it has to move faster). Additionally, slide #69 states that two muscles (short vs. long where the long one has additional sarcomeres in series) produce the same amount of total force (not that their forces are constant all the time, but their total amount of force they can produce stays constant through the change in increasing sarcomeres in series).  Therefore the longer fiber has a higher velocity of shortening while the muscle is actually shortening (the isotonic contraction).