lecture 7 Cardiac Muscle and Electrical Conduction Quiz

Cardiac Muscle Composition

• Myocardium is composed of two types of muscle cells
  • Contractile cells: 99% of total cells, responsible for contractions and require action potentials
  • Autorhythmic cells: modified non-contractile cells, concentrated in parts of the heart, capable of spontaneous action potential generations, act as pacemakers

Gap Junctions

• Adjacent cells are connected by water-filled pores forming open connections called gap junctions
• Allow ions to freely move between cells, facilitating the electrical conduction system

Electrical Conduction System

• Initiated by pacemaker cells (autorhythmic)
• Sino-atrial (SA) node acts as the command center, determining heart contractions, with rhythmic self-excitation
• Atrio-ventricular (AV) node possesses autorhythmic capability but at a slower pace, therefore controlled by the SA node, serves as the electrical gateway between atria and ventricles
• Bundle of His and Purkinje fibers facilitate swift electrical activity propagation from the AV node to the rest of the ventricles

Significance of the Electrical Activity

• The electrical system contributes to
  • Maintaining proper heart rate
  • Coordinating the contractions of the atria and ventricles
  • Synchronizing individual chamber contractions
• Electrocardiogram (ECG) can be used to determine heart rhythm
• Conduction problems can lead to abnormal rhythm, also known as arrhythmia

Sequence of Excitation

• The action potential sequence is as follows:
  • SA node generates its own action potential
  • Action potential spreads through atria, triggering atrial contraction
  • AV node receives the action potential, transmitting it to the bundle of His and Purkinje fibers with a slight delay (allows ventricles to fill with blood and AV valves to close)
  • Action potential reaches ventricles, inducing ventricular contraction

Generation of Action Potential in the SA Node

• Under normal circumstance:
  • SA node cells gradually depolarize due to sodium ion influx and reduced potassium ion diffusion
  • Once the threshold is reached, an action potential is generated
  • The cycle repeats itself, creating a pacemaker potential
  • Controlled by the autonomic nervous system
• Sympathetic fibers: reduce the time required to reach the threshold, resulting in faster pace
• Parasympathetic fibers: extend the time required to reach the threshold, leading to slower pace

Cardiac Muscle Contraction

• Autorhythmic cells action potentials propagate to contractile cells
• Contractile cell action potentials cause muscular contractions due to gap junctions
• The heart functions as a "functional syncytium" due to gap junctions, resulting in an all-or-none contraction rule

Action Potential of Contractile Cells

• Distinguishable from autorhythmic cell action potentials by a stable resting membrane potential
• Distinct from skeletal muscle action potentials in terms of
  • Duration: skeletal muscle action potentials are short (milliseconds), cardiac muscle action potentials are long (hundreds of milliseconds)
  • Refractory Period: Skeletal muscle's refractory period is shorter than cardiac muscle
• The functional impact of these differences is the prevention of rapid repeated contractions in the heart, preventing a tetanic state.

Excitation - Contraction Coupling

• Calcium-stimulated-calcium-release mechanism: - Depolarization occurs - Voltage-gated calcium channels open - Calcium ion entry triggers a surge in calcium release from the sarcoplasmic reticulum, resulting in significant intracellular calcium increase - The heightened intracellular calcium concentration triggers contraction - Calcium is pumped back to its pre-stimulation compartment

Control of Heart Contraction

• Sympathetic nervous system:
  • Stimulates heart rate by accelerating SA node firing and increasing AV node conduction velocity (shorter delay)
  • Increases force of contraction by enhancing calcium release from sarcoplasmic stores
  • Reduces contraction time through increased calcium transport speed (reducing plateau length)
  • These actions are mediated by epinephrine/norepinephrine binding to beta-adrenergic receptors present in all cardiac cells
• Parasympathetic nervous system (Vagus nerve):
  • Decreases heart rate by slowing down SA node firing and lowering AV node conduction velocity (longer delay)
  • Actions are mediated by acetylcholine binding to muscarinic receptors in autorhythmic cells
  • Increases potassium permeability
  • Hyperpolarization increases the time needed to reach the action potential threshold

Electrocardiogram (ECG)

• ECG measures the collective heart membrane potential during the cardiac cycle
• Multiple heart cells fire action potentials synchronously, generating a measurable electrical current
• Electrodes placed on the skin detect the electric current wave progression
• Standard ECG consists of three waves:
  • P wave: atrial depolarization
  • QRS wave: ventricular depolarization, appears larger due to bigger muscle mass
  • T wave: ventricular repolarization

Use of ECG

• Assess heart rate and rhythm:
  • Heart rate: measured by intervals between cycles
  • Contraction force: determined by wave amplitude
  • Rhythm: measured by intervals between waves
• Identification of abnormalities:
  • Heart rate (under resting condition):
    • Bradycardia: slower rate
    • Tachycardia: faster rate
  • Rhythm:
    • Abnormally long P-Q interval indicates an AV conduction problem
    • "Ectopic" beats (extrasystole): action potentials generated independently of the SA node, resulting in extra contractions

Test your knowledge on the composition of cardiac muscle, focusing on the roles of contractile and autorhythmic cells. Understand the importance of gap junctions and the electrical conduction system dominated by the SA and AV nodes. This quiz will help clarify how these components work together to maintain heart rhythm.