week 5 Cardiac Cell Action Potential and Antiarrhythmics

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the role of calcium in the cardiac cell action potential?

Calcium comes into the cell and triggers the release of more calcium from the sarcoplasmic reticulum, which binds to Troponin C and induces contraction.

What property of pacemaker cells allows the heart to contract without neural innervation?

The property of automaticity.

What are the three main ions involved in the flow of electrical impulses across the cardiac cell membrane?

Sodium (Na+), Potassium (K+), and Calcium (Ca++).

What is the basis of myocardial excitation?

Electrical activity precedes mechanical activity. Signup and view all the answers

What is the function of Troponin C in the cardiac muscle cell?

It binds to calcium and induces a change in the shape of tropomyosin, exposing the active site between actin and myosin, leading to contraction. Signup and view all the answers

What is another electrolyte essential to the efficient functioning of ion channels?

Magnesium (Mg++). Signup and view all the answers

What is a unique characteristic of amiodarone compared to other antiarrhythmics?

relatively safe with little or no increase in mortality Signup and view all the answers

What is the effect of verapamil on the heart?

mostly acts on SA & AV node → ↓ HR Signup and view all the answers

What is the mechanism of action of digoxin?

inhibits the Na/K ATPase pump and the Ca exchanger Signup and view all the answers

What is the effect of diltiazem on the heart?

acts as a calcium channel blocker, with a moderate negative inotropic effect and a strong effect on the SA & AV nodes Signup and view all the answers

What is a major difference between amiodarone and other antiarrhythmics?

contains iodine in the chemical structure Signup and view all the answers

What is the indication for digoxin?

SVT, diagnostic aid Signup and view all the answers

What is the effect of amlodipine on blood vessels?

vasodilation Signup and view all the answers

What is the outcome of using amiodarone in 'out of hospital arrest'?

increases survival to hospital admission Signup and view all the answers

What is the main function of ion channels in the cell membrane?

To allow ions to move through the cell membrane Signup and view all the answers

What are the two major forces that drive ion transport?

Concentration gradient and electrical gradient Signup and view all the answers

What is the refractory period in cardiac tissue?

A period of time when the cardiac cell cannot depolarize again Signup and view all the answers

What is the main effect of Class I antiarrhythmic drugs?

Blocks fast sodium channels Signup and view all the answers

What is the main effect of Class II antiarrhythmic drugs?

Blocks β-adrenergic receptors Signup and view all the answers

What is the main effect of Class III antiarrhythmic drugs?

Blocks potassium channels Signup and view all the answers

What is the main difference between Class I and Class III antiarrhythmic drugs?

Class I blocks fast sodium channels, while Class III blocks potassium channels Signup and view all the answers

What is the effect of β-blockers on the heart?

Reduces automaticity of pacemaker cells, increases the refractory period of the AV node, and has anti-ischaemic effects Signup and view all the answers

What is the main difference between selective and non-selective β-blockers?

Selective β-blockers only affect the heart, while non-selective β-blockers affect both the heart and peripheral vasculature, bronchi, and kidneys Signup and view all the answers

What is the main effect of amiodarone on the heart?

Prolongs the action potential duration (APD) and refractoriness of all myocardial tissue Signup and view all the answers

Study Notes

Cardiac Cell Action Potential

The cardiac cell action potential is the electrical activity of the heart muscle cells.
It involves the movement of ions (Na+, K+, Ca++) across the cell membrane, resulting in depolarization and contraction of the myocardium.
The action potential has three phases: resting state, depolarization, and repolarization.

Role of Ca, Na, and K in the Action Potential

Calcium (Ca++):
- Enters the cell and triggers the release of more Ca++ from the sarcoplasmic reticulum.
- Binds to Troponin C, inducing a change in tropomyosin shape, exposing the active site between actin and myosin, leading to contraction.
Sodium (Na+):
- Moves into the cell during depolarization, causing the cell to depolarize and propagate the impulse.
Potassium (K+):
- Moves out of the cell during repolarization, helping the cell return to its resting state.

Electrical-Mechanical Coupling

The electrical activity of the heart precedes mechanical contraction.
Pacemaker cells have the property of automaticity, meaning they can initiate electrical activity without neural innervation.

Ion Channels and Electrolytes

Ions move through specific channels in the cell membrane.
Voltage-gated channels open and close when the membrane potential reaches a certain value.
Two major forces drive ion transport: concentration gradients and electrical gradients.
The concentration of ions across the cell membrane:
- Na+: higher outside (145 mmol) than inside (15 mmol)
- K+: higher inside (150 mmol) than outside (5 mmol)
- Ca++: higher outside (2 mmol) than inside (10^-7 mmol)
- Cl-: higher outside (120 mmol) than inside (5 mmol)

Phases of the Action Potential

Resting state: no electrical activity takes place.
Depolarization: Na+ ions move into the cell, causing depolarization and propagation of an impulse.
Repolarization: the cell attempts to return to its resting state, involving a complex exchange of Na+, K+, and Ca++ ions.

Pacemaker Cells

Have no resting phase during phase 4.
Once repolarized, they begin to slowly spontaneously depolarize during phase 4.
Na+ ions slowly move into the cell, and once the threshold potential is reached, the cell depolarizes.

Refractory Period

A normal property of cardiac tissue, during which the cardiac cell cannot depolarize again until it has repolarized.
Divided into two periods: absolute refractory period (ARP) and relative refractory period (RRP).

Antiarrhythmic Drugs

Classes:
- Class I: block fast sodium channels (e.g., procainamide, lignocaine, flecainide).
- Class II: block β-adrenergic receptors (e.g., β-blockers).
- Class III: block potassium channels (e.g., sotalol, amiodarone).
- Class IV: block slow calcium channels (e.g., verapamil, diltiazem).

Class I Agents

Inhibit fast sodium channels, slowing depolarization and conduction.

Class II Agents (β-blockers)

Competitively inhibit β-adrenergic receptors, reducing sympathetic tone and automaticity.
Increase the refractory period of the AV node, interrupting reentry arrhythmias.

Class III Agents

Block potassium channels, prolonging the action potential duration and refractoriness.
May precipitate torsades de pointes, especially with low K+, low Mg2+, and bradycardia.

Class IV Agents

Block slow calcium channels, primarily used for supraventricular tachyarrhythmias.
Verapamil and diltiazem are commonly used.

Other Antiarrhythmic Agents

Adenosine: acts on the atrium, SA and AV nodes, increasing the refractory period and decreasing automaticity.
Digoxin: inhibits the Na+/K+-ATPase pump, increasing intracellular Ca++ and inotropy.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz covers the cardiac cell action potential, the role of Ca, Na, and K, and the relationship between the AP and the mechanical response of the myocardium. It also discusses frequently used antiarrhythmic medications.