Lecture hand-outs for cardiac muscle physiology

I have put up the hand-outs for the lectures on cardiac muscle physiology at resources page in PhysioBytes. Those of you who could not get the photocopies can download it from there.

A few objective-type questions...

I have posted some objective-type questions on nerve-muscle physiology at PhysioBytes (click to visit).
Give them a try. Objective -type questions are a great way to review a topic.
Besides, it pays to solve these type of questions - all postgraduate entrance tests are MCQ based.

Answers to the crosswords on membrane physiology

To check out the answers for the crosswords on membrane physiology, visit PhysioBytes (click here)

Sodium-Potassium pump and action potentials

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Regulation of ACh release at neuromuscular junction

An interesting question was raised a few days ago by a couple of you regarding the neuromuscular junction (NMJ).
Is there any mechanism that regulates the amount of ACh released at the NMJ?
One of the most important factors that determine the amount of ACh release at the NMJ is off course the ECF Ca++ concentration. A fall in ECF [Ca++] will decrease ACh release as the Ca++ influx that occurs during a presynaptic potential is dependent on the out-to-in gradient for Ca++.
Other than that, in many of the central neural synapses there are additional presynaptic mechanisms that influnce neurotransmitter release. Receptors for the very neurotransmitter released at a synapse are often present on the presynaptic membrane. Such receptors are refered to as autoreceptors. The nerutransmitter released during synaptic transmission, in addition to its primary action on the postsynaptic side, also binds to these presynaptic autoreceptors and brings about changes in the presynaptic mechanisms thus influencing further release of neurotransmitters. For instance, consider the synapses in the sympathetic nervous system. Nor-adrenaline is a neurotransmitter employed by sympathetic nerve endings and acts upon different types of nor-adrenergic post synaptic receptors. A particular type of adrenergic receptor, the alpha2 adrenergic receptor, is found on the presynaptic membrane of these nerve terminals. The nor-adrenaline released during sympathetic activation acts on these presynaptic autoreceptors to reduce further release nor-adrenaline. Thus this is an example for a feed back regulation of neurotransmitter release. In some cases such feedback regulation may increase neurotransmitter release rather than decrease it. Such mechanisms are believed to be important in maintaining the sensitivity of the post-synaptic emmbrane to neurotransmitters and regulating the efficiency of synaptic transmission.
Coming back to the NMJ, it was traditionally believed that such feedback regulation of neurtransmitter release does not occur at the NMJ. But recent studies suggest that the release of ACh at NMJs may also be influenced by a feedback effect of ACh on presynaptic autoreceptors which belong to the muscarinic class of cholinergic receptors.It remains to be seen how important such regulation is and whether it has clinical significance.

Sloppy work

I must admit that I have been a little sloppy of late. Errors in the crossword, mix-ups in the problems and missing -ve signs in the solutions to problems...! Well I have corrected whatever that was brought to my notice. Nice to know that some of you have looked at these things carefully and have taken the time to give me some feedback.I hope this continues and more of you join in the action.

Hodgkin & Huxley

I simply can't stop admiring the contribution of Dr. Allan Hodgkin and Dr. Andrew Huxley to the field of Neuroscience. Their amazing work with squid axons is an classical example of the success of the experimental approach in biology. It is also illustrative of the multidisciplinary approach to neurscience that has yielded rich dividends. Their description of the ionic basis of action potentials in neuronal membranes is indeed a major landmark in the history of Neuroscience. The quantitative model (a mathematical description) of the action potential that they developed marks the birth of computational neurobiology which has added an exciting element to the field of neuroscience. It is indeed very rare that such a precise and convincing explanation is discovered for a fundamental phenomenon. The early 1950s were indisputably some of the most exciting yeras for science in general and bilology in particular. Even as Hodgkin and Huxley where discovering the basis of the electrical signals that the brain uses to perform its amazing feats, Watson and Crick were cracking the structure of DNA - the very essence of life. More than 50 years after these seminal discoveries, scientists still continue to expolre their implications for the understanding of life and all its mysteries. For those who are interested in reading more about these great men of science, may I recommend a visit to the following sites...
More about Hodgkin
More about Huxley

Crossword corrections

Just realized that there are a few errors in the croosswords - thanks to few blessed souls who have actually tried it!
1. (1) Across was meant to be "A change in the membrane potential that makes it less NEGATIVE". But it does'nt really matter - it does'nt mess up the rest of puzzle anyway...
2. The answer to (3)Down has two alternate spellings...that may create some trouble...Heres a clue - the fifth letter in (3)Down is "A"

I have made the corrections in the version on PhysioBytes

Answers? we'll wait for another couple of days to give the lazybones another chance...

Crosswords

I hope there are some crossword enthusiasts amongst you. When I was in the hostel during mu under-graduate years, the only reason I used to get up early in the morning was to lay my hands first on the crosswords in the newspaper that was delivered to the hostel reading room. There were quite a few crossword nerds in our hostel and so there was always stiff competition!
Try this simple crossword on membrane potentials - it may even help you review the topic. Visit PhysioBytes (you'll find links at the top as well as on the side bar of this page)for the crossword.

Membrane potential - Answers to problems

Hi!
I am glad that many (if not most)of you have tried to solve the problems. One common error is that while calculating the change in RMP, you have calculated (Old RMP - New RMP) instead of (New RMP - Old RMP). As a result, many of you have concluded that increase in ECF K+ makes the RMP more negative, which uis of course wrong. Here are some suggestions for solving the problems...

Problem No. 1
RMP = -90 mV

Problem No. 2
Part 1
Normal Vrest= -90mV
If [K+]o = 9mMol/L; [Na+]o = 160mMol/L
Then RMP = -75mV (Use Goldman-Hodkin-Katz Equation)
Change in RMP from normal = -75 - (-90)= 15mV
ie Vm becomes LESS negative

Increase in ECF [K+] makes RMP LESS negative
Brings it closer to firing (Threshold level)
Increases excitability
Increases risk of abnormal heart beats - arrhythmias

Part 2
If [Na+]o is corrected to normal level
RMP = -76mV
Change in Vrest from previous = -76 - (-75)= -1mV
In resting membrane , PK+>>PNa+
Therefore, changes in [K+] affect RMP but not changes in [Na+]

Increase in ECF [Na+] does not affect RMP much, but it will affect the amplitude of the action potential - greater the Na+ concentration gradient (out-to-in) - bigger the action potential spike.

Membrane potential - Problems

Here are e few problems to work out as an assignment
Assignments due by 9.30 AM, 8th Sept 2005

Problem No.1
Calculate the equilibrium potentials for each of the ions and the resting membrane potential for a cardiac muscle cell from the data given below. Is the calculated value in the normal range for a cardiac muscle

Na+ in ECF: 145mMol/L; Na+ in ICF: 12mMol/L, Permeability for Na+:1; K+ in ECF:4; K+ in ICF: 155mMol/L;Permeability for K+:200; Cl- in ECF: 120mMol/L; Cl- in ICF: 3.8mMol/L, Permeability for Cl: 100

Problem No.2
A person develops a severe renal disease. As a result he is unable to excrete the normal amounts of and Na+ and K+ from her plasma and they accumulate to levels of 160mMol/L (Na+) and 9.0mMol/L (K+).
Assuming that all other values remain the same as in Problem No.1, predict the change in resting membrane potential from normal. Explain your observations.
She was treated to correct the Na+ concentration normal to levels. However the K+ level remained at 9mMol/L. What is the new RMP? Has it changed much? Explain your observations

Hello!

PhysioCentral is intended to provide additional interactive learning oppurtunities for the students of K. S. Hegde Medical Academy (KSHEMA)who take the Physiology course at the Institute's Department of Physiology. There is not always sufficient time in the lectures / practicals to discuss some of the interesting questions that are raised. Also, providing feedback on assignments, tests etc takes up a lot of time in the regular classes. So we will try to make use of this forum to enhance our learning experience. Feel free to post your queries / comments here.
The views expressed by the author of this blog do not necessarily reflect those of the Department or the Institution. To put it in simple words... There's nothing official about this!
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