Cardiac Muscle (Action Potential, Automaticity, ARP, RRP) | Heart Physiology
Cardiomyocyte QUIZ: • Cardiomyocyte QUIZ
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Content:
0:00 Introduction
0:47 You should already know
01:10 Automaticity
01:57 Types of Cardiomyocytes
02:28 What are Pacemaker Cells?
05:00 Transmission Speed and AV-node delay
10:35 What are Endocrine Cardiomyocytes?
11:23 What are Contractile Cardiomyocytes?
12:20 Action Potential in Pacemaker Cell
15:57 Action Potential in Contractile Myocardium
22:22 Phases
24:53 Refractory Periods of Cardiomyocytes
28:54 Properties of Cardiomyocytes
32:03 Next video
32:21 QUIZ
Types of Cardiac Muscle Cells
Cardiac muscle cells can be classified into three primary categories based on their function:
1. Pacemaker Cells (Nodal Cells):
Location: SA node, AV node, Bundle of His, RBB, LBB, Purkinje fibers.
Function: Generate and propagate electrical impulses. They exhibit automaticity, meaning they can spontaneously depolarize and initiate action potentials.
Action Potential: These cells lack a stable resting membrane potential due to "funny" sodium channels (If channels) that slowly allow Na+ influx, causing gradual depolarization. Upon reaching threshold, T-type and L-type Ca2+ channels open, leading to a rapid influx of Ca2+ and cell depolarization. Repolarization occurs as K+ channels open, allowing K+ efflux.
2. Contractile Cells:
Location: The myocardium, or the heart muscle layer.
Function: Generate the force of contraction to pump blood.
Action Potential: Contractile cells have a stable resting membrane potential at about -85 mV. Depolarization occurs due to Na+ influx through voltage-gated sodium channels. This is followed by a plateau phase (Ca2+ influx via L-type channels balanced by K+ efflux) and repolarization (closure of Ca2+ channels and continued K+ efflux).
3. Endocrine Cells:
Location: Both atria and ventricles.
Function: Secrete atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) in response to stretching due to increased blood volume. These hormones help regulate blood pressure and volume by promoting vasodilation and reducing sodium and water reabsorption in the kidneys.
Conduction System of the Heart
SA Node: Primary pacemaker located at the superior-lateral border of the right atrium. Sets the pace at 60-80 beats per minute.
AV Node: Secondary pacemaker located at the atria-ventricular junction. It slows the conduction to ensure atria contract before ventricles, pacing at 40-60 beats per minute if SA node fails.
Bundle of His and Purkinje Fibers: Conduct impulses rapidly through the ventricles, ensuring coordinated contraction.
Action Potentials: A Comparative View
Pacemaker Cells:
Phase 4: Slow depolarization due to "funny" Na+ channels.
Phase 0: Rapid depolarization from Ca2+ influx (L-type Ca2+ channels).
Phase 3: Repolarization due to K+ efflux.
Contractile Cells:
Phase 0: Rapid depolarization from Na+ influx.
Phase 1: Initial repolarization with K+ efflux.
Phase 2: Plateau phase with balanced Ca2+ influx and K+ efflux.
Phase 3: Rapid repolarization as Ca2+ channels close and K+ efflux continues.
Phase 4: Resting membrane potential maintained by K+ permeability.
Electrical Conduction Pathway
Impulse Generation at SA Node: Initiates depolarization.
Atrial Conduction: Impulses spread rapidly through atria (1 m/s), causing atrial contraction.
AV Node Delay: Slow conduction (0.1 m/s) ensures atria empty into ventricles before ventricular contraction.
Ventricular Conduction: Fast conduction through Bundle of His and Purkinje fibers (1-4 m/s).
Refractory Periods
Absolute Refractory Period (ARP): The cell cannot respond to another stimulus, ensuring the heart muscle relaxes before the next contraction.
Relative Refractory Period (RRP): The cell can respond to a strong stimulus, which may cause premature contractions.
Effective Refractory Period (ERF): ARP with the beginning part of RRP
Properties of Cardiomyocytes:
Automaticity
All-or-none law
Long refractory periods
Sources:
University lectures and notes
Barrett, K. E., Barman, S. M., Boitano, S., & Brooks, H. L. (2016). Ganong's review of medical physiology (25th ed.). McGraw-Hill Education.
Hall, J. E. (2016). Guyton and Hall textbook of medical physiology (13th ed.). Elsevier.
Mohrman, D. E., & Heller, L. J. (2014). Cardiovascular physiology (11th ed.). McGraw-Hill Education.
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