Cardiac Cell / Electrolyte Questions

[systemet]

after depolarization does the cell become more positive inside than outside?

Depends on what you mean by "after depolarisation", i.e. at what point in the actional potential. If you look at the top half of the second image in the previous post it shows the changes in membrane potential with time.

So if you mean, does it become more positive inside than outside at the end of phase 0, when the sodium channels have opened, and sodium has entered in large amount, then yes. You can see than on the graph where the membrane potential is > 0 mV.

and i guess potassium would out because of this

If the inside of the cell becomes more positive, it will increase the tendency of positively charged species to leave the cell.

Potassium does indeed move out after sodium entry in phase 0. But the leak channels only contribute to a small amount of this movement (in fact, some become less permeable as membrane voltage increases, a property called rectification).

We have a series of different potassium currents, including Ito (phase 1), and a series of conductances, such as IKr and IKs that are responsible for potassium movement during plateau (phase 2), and ultimately contribute to repolarisation in phase 3.

These currents are illustrated in the lower panel on the third image.

but would more sodium ions leak out through leak channels also than normal because the cell is more positive at this time?

You would have less of a force pushing sodium in. It's going to depend on the membrane potential, and the concentration of sodium inside and outside of the cell. So you'd probably have a smaller "sodium leak current", if you will. But any "sodium leak" is going to be a very minor player, and have very little effect on the membrane potential, or the intra/extra cellular sodium compared to the large conductance of the V-gated sodium channels in phase 0. There are simply bigger currents, and other channels that are more important for determining membrane potential at this point.

[If you want to get into more complicated math, you can use the Nernst equation to predict an equilibrium potential for sodium.

E(Na) = 61 log ([Na]e / [Na]i)

If the membrane potential is less than the equilibrium potential for sodium, then sodium will tend to move into the cell. If membrane potential is above this sodium will tend to move out. But any actual movement will depend also in the permeability of the cell to sodium. ]

or is the leaking of potassium enough to repolarize cell?

No. We have specific potassium currents, see IKr, IKs, etc. that are involved in repolarisation. The leak current is too small to do this in a timely manner, and if these channels lose function we see delayed repolarisation and long QT syndrome. If repolarisation takes too long, we risk another opening of sodium channels, and an afterdepolarisation, which can cause an ectopic beat, or ectopic arrhythmia, e.g. V tach.

also the sodium potassium exchange is not needed to repolarize cell?

No, but it's a minor determinant of the resting membrane potential. It does exert an influence on the membrane potential, but other processes are more important for repolarisation -- primarily the previously described potassium currents.

its needed to maintain concentation gradient?

Ultimately, yes.

 

[systemet]

does each skeletal muscle cell have its own neuron or motor neuron,

I'm not doing a homework assignment here, am I? It's ok if I am, I'd just like to know.

Each skeletal muscle cell is connected to a motor neuron afferent at a neuromuscular junction. But one motor neuron may innervate more than one muscle fiber. A motor neuron and the associated fibers are called a "motor unit". Stimulation of the motor neuron will result in contraction of all muscle fibers in a given motor unit. Motor units with a small number of fibers are used to produce fine movements, larger motor units are recruited to produce gross movements.

So, I guess, each motor fiber is connected to one motor neuron, but each fiber doesn't usually have "it's own" motor neuron, normally it shares it with many others. The number of fibers innervated by a given motor neuron varies.

what is the size of skeletal muscle cell to neurons in muscle?

Not sure if I understand the second question. The size of a muscle fiber varies. It's length is determined by the number of myocytes that fused to form the mature polynucleate fiber. The size of the motor neuron depends on the distance it must travel from the spinal cord to the muscle. An alpha motorneuron coming from the anterior horn to the big toe is going to be pretty long. Typically their diameter reflects the size of the motor unit they're innervating, with larger motor units have wider motor neuron axons.

what is the size of neurons in the body compared to neurons in brain or spinal cord?

Which neurons are we comparing? Are we comparing alpha motor neurons innervating skeletal muscle fibers? Or are we thinking about neurons in the stomach wall or the intestines? Is this just neurons in the periphery in general? There's lots of neurons. You could even argue that the alpha motor neuron is a central neuron, as its cell body is within the anterior horn of the spinal cord and could be considered part of the CNS, even if its axon constitutes most of its volume and is in the periphery innervating a peripheral target.

You could certainly state that a motor neuron innervating muscle in a toe or digit is going to be larger than say, a pyramidal cell.

It might be worth finding a copy of a good physiology text like Guyton or Boron & Boulpaep. All this information is in there. Most of it's available free on the internet through Google Books, Wikipedia, NCBI bookshelf, etc.

 

[phillybadboy]

so when people say muscle cell they mean muscle fiber,(not the individual myocytes that form a muscle fiber? one axon terminal controls one muscle fiber, one axon terminal is not needed for each individual myocyte in the muscle fiber? and thanks no not homework just a gigh school graduate

 

[phillybadboy]

so its the leak channels does some but the voltage gated ion channels doe most of repolarization?

 

[phillybadboy]

so the cell bodies of motor neurons and sensory neurons are located in the spinal or brain and only the axons are located in the skeletal muscles? on the sensory neurons there are these receptors, are they potreins or cells, and if they are cells are they still considered as a part of the whole neuron or an individuals working with neurons? the cell bodies of motor neurons and cell bodies of sensory neurons are they about the same size?

 

[systemet]

so when people say muscle cell they mean muscle fiber,(not the individual myocytes that form a muscle fiber?

Yes, or at least, I'd hope so.

Muscle fibers are polynucleate. They originate from the fusion of numerous progenitor cells or myoblasts. After these myoblasts join together, the membrane separating them is removed. So you can consider a muscle fiber to be a very large cell with multiple nuclei.

one axon terminal controls one muscle fiber, one axon terminal is not needed for each individual myocyte in the muscle fiber?

There are no individual myocytes in a muscle fiber. Individual cells no longer exist, as the physical boundaries between them have been removed. A muscle fiber is an individual cell.

Each muscle fiber has a structure called a "neuromuscular junction" that enables the nerve cell to communicate chemically with the muscle fiber. If you trace the muscle fiber back to the spinal cord it's only innervated by one alpha motor neuron.

and thanks no not homework just a gigh school graduate

No problem answering some questions! I hope this helps.

 

[systemet]

so its the leak channels does some but the voltage gated ion channels doe most of repolarization?

If I understand you correctly, yes, repolarisation is mostly accomplished by the different populations of voltage-gated potassium channels that make up the IKs and IKr conductances.

There's a common misconception that the sodium-potassium pump fulfills this role. It doesn't. While it's continuously active, its contribution to membrane potential is more important in the resting period.

Even after repolarisation much of the potassium returns to the cell through the delayed inward rectifier current IK1. I can't remember if I showed that in earlier posts of not.

 

[systemet]

so the cell bodies of motor neurons and sensory neurons are located in the spinal or brain and only the axons are located in the skeletal muscles?

Where the cell bodies are located depends on the type of sensory or motor neurons we're discussing. When we talk about alpha motor neurons, they have cell bodies located in the anterior horn of the spinal cord. The axons extend from the spinal cord to the muscle, so they can be quite long.

on the sensory neurons there are these receptors, are they potreins or cells,

Not sure what the question is? Are you asking if there are receptors present on sensory neurons? If so, yes, there are receptors. And it's probably safe to think of them as proteins.

and if they are cells are they still considered as a part of the whole neuron or an individuals working with neurons?

Again, not sure what you're asking. Can you clarify this a little?

the cell bodies of motor neurons and cell bodies of sensory neurons are they about the same size?

Depends on which sensory neurons and which motor neurons we're comparing. For example there's a lot of different types of neurons:

 

[phillybadboy]

thanks for all the help, u r awsome, if u ever need some schoolwok help just ask me, just kiddin,

 

[Handsome Robb]

What a refreshing discussion.

I'm looking for a resource, I'm sure someone has put this together before:

I want a side by side comparison of : stages of depolarization, electrolyte in/outflux, and the ECG wave.

Anyone seen anything like that?

I have one similar to what your describing that was handed out to us in school. Let me see if I can find it and a scanner, my table and floor are currently covered in cardiology notes. This could be a search haha.

 

[systemet]

thanks for all the help, u r awsome, if u ever need some schoolwok help just ask me, just kiddin,

No problem. Glad I could help. Feel free to post or pm if you have any other questions.