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THE HEART

Cardiac Muscle Contraction

  • Action potential triggers Ca²⁺ influx from both sarcoplasmic reticulum and extracellular fluid.

  • Long refractory period prevents tetanus.

  • Contraction mechanism similar to skeletal muscle (sliding filaments).

  • Depolarization and Contraction of heart muscle is intrinsic even though ANS can affect basic rhythm 

  • Intrinsic Cardiac Conduction System 

    • Noncontractile cardiac cells that initiate and distribute impulses throughout the heart to coordinate depolarization and contraction 

  • Resting membrane potential is different from skeletal muscle and unstable

1. Pacemaker Potential (Slow Depolarization)

  • What happens:
    The membrane slowly depolarizes due to slow Na⁺ (sodium) inflow through “funny” channels (If channels) and reduced K⁺ outflow.

  • Goal:
    This slow rise in voltage brings the cell to threshold.

  • Key ion: Na⁺ (inward leak)

2. Depolarization (Action Potential)

  • What happens:
    Once threshold is reached (around –40 mV), Ca²⁺ channels open, and Ca²⁺ rushes in, causing rapid depolarization.

  • Key ion: Ca²⁺ (influx)

3. Repolarization

  • What happens:
    Ca²⁺ channels close    , and K⁺ channels open, allowing K⁺ to flow out, bringing the membrane potential back down (around –60 mV).

  • Key ion: K⁺ (efflux)

Intrinsic Cardiac Conduction System (Basic Rhythm)

  • Network of autorhythmic cells coordinating depolarization.

  1. Sinoatrial (SA) node: right atrium; pacemaker (~75 bpm).
    → initiates heartbeat, spreads through atria.

  2. Atrioventricular (AV) node: delays impulse (~0.1 s) for atrial contraction to complete.

  3. AV bundle (Bundle of His): only electrical connection between atria & ventricles.

  4. Right and left bundle branches: carry impulse through interventricular septum.

  5. Purkinje fibers: spread through ventricles; cause coordinated ventricular contraction.

⚡ 1. Sinoatrial (SA) Node

  • Location: Right atrium (near superior vena cava)

  • Function: The pacemaker — generates the electrical impulse (about 75 times per minute).

  • Effect: The impulse spreads across both atria, causing atrial contraction.

⚡ 2. Atrioventricular (AV) Node

  • Location: Lower right atrium (near the tricuspid valve).

  • Function: Delays the impulse for about 0.1 second so the atria finish contracting before the ventricles begin.

⚡ 3. Atrioventricular (AV) Bundle (Bundle of His)

  • Location: In the interatrial septum, continuing into the interventricular septum.

  • Function: The only electrical connection between the atria and ventricles.

⚡ 4. Right and Left Bundle Branches

  • Location: Along the interventricular septum.

  • Function: Carry the impulse toward the apex (bottom) of the heart through the septum.

⚡ 5. Purkinje Fibers

  • Location: Spread throughout the ventricular walls.

  • Function: Distribute the electrical impulse to ventricular muscle cells, causing ventricular contraction from apex upward (pumping blood out).

Extrinsic Regulation of Heart Rate

  • Autonomic Nervous System (ANS)

    • Sympathetic: increases heart rate & contractility via norepinephrine-  speeds up heart rate 

    • Parasympathetic (vagus nerve): decreases rate via acetylcholine-  slows down the heart rate 

  • Cardiac Centers (in medulla oblongata):

    • Cardioacceleratory center: activates sympathetic neurons to speed up heart via cardiac nerve 

Cardioinhibitory center: activates parasympathetic fibers in the vagus nerve to slow down the heart