cardiac physiology

Introduction to Cardiac Physiology

  • Overview of contractile cell functions and their relationship with the heartbeat.
  • Emphasis on the significance of action potentials in cardiac muscle function.

Action Potentials in Cardiac Muscle

  • Definition of Action Potential: An action potential is a temporary shift in the electrical membrane potential of a cell, measured through voltage changes over time, resulting in cellular functions such as contraction.
  • Cells Involved: In cardiac physiology, the primary focus is on contractile cells which generate the heartbeat.

Major Phases of Cardiac Action Potential

  1. Depolarization Phase:

    • Initial influx of sodium ions through voltage-gated sodium channels.
    • Sodium influx leads to a rapid change in electrical charge, causing depolarization.
    • Peaks at a membrane potential of approximately +30mV.

  2. Plateau Phase:

    • Unique to cardiac myocytes, marked by the opening of calcium channels.
    • Calcium influx occurs while sodium influx continues, preventing immediate repolarization.
    • This phase is crucial for sustaining contraction (myocardial contraction).

  3. Repolarization Phase:

    • Initiated by the opening of potassium channels.
    • Potassium ions exit the cell, returning the membrane potential towards its resting state.
    • Unlike neurons, no hyperpolarization occurs; the potential returns to resting potential.

Characteristics of Cardiac Action Potentials

  • No hyperpolarization phase as seen in typical action potentials.
  • Presence of a prolonged refractory period during which the heart muscle cannot contract again.
  • Importance of refractory period in preventing tetany in cardiac muscle, which could lead to life-threatening conditions like arrhythmias.

Contractile Cells vs. Autorhythmic Cells

Contractile Cells

  • Responsible for the contraction of the heart muscle.
  • Exhibit action potentials characterized by long refractory periods to ensure proper heart rhythms.

Autorhythmic Cells

  • Define the basic pace of the heartbeat.
  • Utilize funny channels (leak channels) that allow sodium to flow in, generating spontaneous depolarization leading to rhythmic heartbeats.
  • Structures include:
    • Sinoatrial (SA) Node: Primary pacemaker generating about 70-100 beats per minute.
    • Atrioventricular (AV) Node: Secondary pacemaker that can modify pacing.
    • AV Bundle (Bundle of His): Transmits impulses from the AV node to the ventricles.
    • Purkinje Fibers: Distribute electrical impulses throughout the ventricles for effective contraction.

Electrocardiogram (ECG/EKG)

  • Definition: A test that measures the electrical activity of the heart over time, providing insights into heart rhythm and potential pathologies.
  • PQRST Waveforms:
    • P Wave: Represents atrial depolarization and contraction.
    • QRS Complex: Indicates ventricular depolarization and occurs as the ventricles contract (atrial repolarization also occurs here).
    • T Wave: Indicates ventricular repolarization and relaxation.
    • U Wave: (rarely seen) Correlates to repolarization of Purkinje fibers.
  • Normal heart rate ranges from 60 to 100 beats per minute.

Pathological Conditions Related to ECG

  • Arrhythmias: Abnormal heart rhythms that can manifest in various forms:
    • Bradycardia: Slow heart rate.
    • Tachycardia: Fast heart rate.
    • Atrial Fibrillation (AFib) and Ventricular Fibrillation (VFib) are critical conditions requiring monitoring and treatment.
  • Distinction between symptoms of heart conditions versus other issues like gastritis or panic attacks.

Cardiac Cycle and Heart Function

  • Systole: Refers to contraction phase of the heart.
  • Diastole: Refers to relaxation phase of the heart.
  • Blood pressure readings taken during systole and diastole, indicating overall cardiovascular health.

Cardiac Output and Stroke Volume

  • Cardiac Output (CO): Total volume of blood pumped by the heart per minute, calculated as:
    ext{CO} = ext{Heart Rate} imes ext{Stroke Volume}
  • Stroke Volume (SV): Volume of blood pumped from the heart per beat.
  • Factors impacting heart rate and stroke volume include contractility and blood volume.

Heart Transplants and Post-operative Care

  • Post-transplant patients may experience elevated heart rates due to loss of autonomic regulation from vagus nerve severance.
  • Importance of using medication to manage heart rate and support recovery.

Summary of Key Hormonal Influences on Cardiac Function

  • Various hormones (not specified in the transcript but generally include norepinephrine, epinephrine, and others) play crucial roles in modulating heart rate and contractility during stress responses.

  • Review complex topics through practical case studies in lab sessions.