The Heart and Cardiac Physiology

Overview of Heart Function

• The heart is a muscle that contracts to pump blood.
• Contains specialized muscle cells: cardiac myocytes.
• Contraction initiated by action potentials (electrical impulses).
• Unlike skeletal muscles, the heart has intrinsic electrical stimulation.

Cardiac Electrical System

• Pacemaker Cells: Generate and conduct action potentials.
  • SA Node: Primary pacemaker, initiates all heartbeats.
  • Damaged SA node: Other conduction parts can take over.
• Spontaneous firing of SA node cells results in an average heart rate of 80 BPM.

Conduction Pathway

• Action potentials start in the SA node and spread through atrial myocytes.
• Gap Junctions: Enable ionic flow and electrical coupling between cells.
• Impulses reach AV Node, slow for atrial contraction, and spread through ventricles.

Action Potentials in Cardiac Cells

• Different action potentials in pacemaker and contractile myocytes.

Pacemaker Cells

• No true resting potential; spontaneous action potentials.
• "Funny" Currents: Na+ influx below -40mV leads to gradual depolarization.
• Calcium influx at threshold leads to further depolarization.
• Potassium efflux returns voltage to -60mV, cycle repeats.

Contractile Myocytes

• Stable resting potential of -90mV.
• Depolarization triggered by neighboring cells, at -70mV threshold.
• Rapid Na+ influx, followed by Ca++ influx (plateau phase).
• Plateau Phase: Balances K+ efflux and Ca++ influx, lasting ~200 msec.
• Calcium-induced calcium release triggers contraction.

Muscle Contraction

• Contraction initiated by "sliding filament mechanism".
• Contraction lasts through plateau phase for blood expulsion.

Refractory Period

• Long absolute refractory period (~250 msec) prevents tetanus.
• Ensures relaxation before new stimulus; prevents summation like skeletal muscle.

Ionic Regulation

• Sodium/Potassium Pump: Restores ionic balance post-action potential.
• Calcium is transported out and back into SR after contraction.

Significance of Plateau Phase

• Prolonged contraction compared to skeletal muscle is crucial for heart function.
• Ensures effective blood expulsion from the heart chambers.