1. (30 points) Electrical activity stimulates the muscular contractions needed f
ID: 3479254 • Letter: 1
Question
1. (30 points) Electrical activity stimulates the muscular contractions needed for the heart to function as a pump.
a. Describe the roles of K+, Na+, and Ca2+ in the production of a pacemaker potential and an action potential in a contractile cardiocyte.
b. How does electrical activity in the pacemaker cells stimulate electrical activity in the contractile cells?
c. How does the plateau phase in the contractile cells enable normal heart function?
d. Explain how hyperkalemia and hypokalemia would affect heart function and why those effects would be observed.
e. Digoxin is a positive inotropic agent. What does that mean and by what mechanism does it influence heart function?
f. Metoprolol is a negative chronotropic agent. What does that mean and by what mechanism does it influence heart function?
Explanation / Answer
Sinoatrial Nodal Action Potential
SA nodal action potential is divided in 5 phase i.e. Phase 4 --> 0 --> 1 --> 2 --> 3 --> 4
Phase 4: is the Resting phase
Phase 0: is the Depolarization phase
Phase 1: is the Early repolarization
Phase 2: is the Plateau Phase
Phase 3: is the Repolarisation
The answer for ...
a.) The change in movement of ions across the membrane via different ion channels (viz Na+, K+ & Ca ) at different phase causes the membrane potential in cardiac cells. Please note that cardiac membrane potential is different from rest of the body cells due to 1) Increased influx of Sodium (Na+) 2) Decreased efflux of Potassium (K+) 3)Ca ion flow which vital in muscle contraction.
Increased Sodium Influx
Na+ influx main driving factor that generates pacemaker potential towards the threshold. During depolarization phase (Phase 0) the cardiac cells possess a unique Na+ ion channels known "funny channels" which opens in response to hyperpolarization from the neighbouring cardiac cells. After an action potential, these open and allow sodium to flow into the cell, pushing the membrane potential towards the resting potential for sodium (60mV).
Decreased Potassium Efflux
Due to Na+ influx, Potassium channels in the heart begin to close during hyperpolarisation (of the action potential). The potassium channels open briefly and an outward flow of K+ returns the membrane potential to approximately 0 mV. This limits the amount of potassium that can leave the cell, pushing the membrane potential up further forming Phase 1, early repolarization.
Calcium Channel Open
Calcium channels known as L Type Ca2+ channel, open causing a small but constant inward current of Ca2+., whereas, the K+ leaks lowering its concentration gradient through delayed rectifier K+ channels. This counterflow of Potassium & Calcium makes it electrically balanced maintaining a plateau, the Plateau Phase.
The Calcium channel gradually inactivates, whereas the K+ still maintains the outflow, at a certain point exceeding the Ca2+ which bring the membrane potential back repolarising to resting potential (-90mV) and preparing for the next depolarization.
b) The electrical action potential leads to contraction of cardiac muscle cells in the process known as Excitation-contraction coupling. In this process, the chemical signals are converted into mechanical energy via the action of contractile proteins (viz. Actin & Myosin).
c) Plateaus phase maintains the beating rate of the heart by causing a delay in re-depolarization.
d) The K+ contents play a vital role in action potential since it regulates the membrane potentials by keeping it in an optimal interval.
Hypokalemia causes quicker repolarisation as the K+ channel cannot be prolonged, resulting in heart beating fast "Tachycardia".
Hyperkalemia causes increased Plateaus phase in few causes the membrane potential no to depolarise, resulting in heart beating slower known as "Bradycardia", complicating to Cardiac Arrest.
e. Positive Inotropes are agents which increase the contractility of the Cardiac muscles by increasing the intercellular calcium concentration. By increasing the calcium concentration, they extend the action potential for a much longer duration which in turn increase the contractility. Used in conditions like Cardiomyopathy, Cardiogenic Shock, where the heart loses the contractility.
. Negative Inotropes : are agents which act opposite to that of positive weakening the muscle contractility. They shut the calcium channel faster they hastening the action potential quicker, resulting in decreased contractility.Used in a condition like Angina, Congestive Cardiac Failure, where relieving the cardiac mucle stress in vital .
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