Lecture Outline - Chapter 19 Heart.docx

19.4a Microscopic Structure of Cardiac Muscle

• Myocardium: Composed of cardiac muscle tissue.

• Cardiac Muscle Cells:

  • Striated, short, and branched.

  • Typically possess one or two central nuclei.

• Support Structure:

  • Supported by areolar connective tissue known as endomysium.

• Sarcolemma:

  • Plasma membrane that invaginates to form T-tubules extending into the sarcoplasmic reticulum.

  • T-tubules invaginate once per sarcomere, positioned over Z-discs.

• Sarcoplasmic Reticulum:

  • Surrounds myofilaments grouped as myofibrils and denotes the striated appearance.

• Optimal Length:

  • Maximum overlap of filaments is observed when the myocardium is stretched by blood filling its chambers, contributing to a more forceful contraction.

• Intercellular Structures:

  • Sarcolemma folds at connections between cells to enhance structural stability and facilitate communication.

  • Intercalated Discs connect cells at junctions:

    • Desmosomes: Mechanically join cells, preventing separation.

    • Gap Junctions: Electrically connect cells, allowing ion flow, forming a functional syncytium.

19.4b Metabolism of Cardiac Muscle

• Cardiac muscle exhibits high energy demand.

  • Supported by:

    • Extensive blood supply.

    • Numerous mitochondria.

    • Presence of myoglobin and creatine kinase.

The Heart's Conduction System (19.5a)

• Function: Initiates and conducts electrical events for synchronized contractions.

• Composed of specialized cardiac muscle cells that develop action potentials but do not contract.

• SA Node:

  • Located in the right atrium, acts as the pacemaker, setting rhythm for heartbeat.

• AV Node:

  • Positioned in the floor of the right atrium (near right AV valve).

• AV Bundle (Bundle of His):

  • Extends through the interventricular septum, splitting into left and right bundles.

• Purkinje Fibers:

  • Emanate from bundle branches at the heart's apex and extend into ventricular walls.

Innervation of the Heart (Pages 3-4)

• Cardiac Center: Located in the medulla oblongata, regulates heart function via the autonomic nervous system.

  • Parasympathetic Innervation:

    • Cardioinhibitory center decreases heart rate via vagus nerve (CN X).

  • Sympathetic Innervation:

    • Cardioacceleratory center increases heart rate and force of contraction via cardiac nerves (T1-T5).

• Baroreceptors and Chemoreceptors:

  • Send sensory input to the cardiac center; influence heart rate and contraction strength.

Stimulation of the Heart (Page 6)

• Contraction Process:

  1. Action potential arises at SA node.

  2. Spreads to the atria, causing contraction, then travels to AV node where slight delay occurs.

  3. Propagates through AV bundle to Purkinje fibers, leading to ventricular contraction.

• SA Nodal Cells:

  • Exhibit autorhythmicity, have a resting membrane potential of -60 mV and do not maintain a stable RMP.

Electrical Events at the SA Node (Pages 5-6)

• Action Potential Generation:

  1. Threshold Reach: Slow voltage-gated Na+ channels open, causing depolarization from -60 mV to -40 mV.

  2. Depolarization: Fast voltage-gated Ca2+ channels open, pushing potential above 0 mV.

  3. Repolarization: K+ channels open, restoring the -60 mV resting membrane potential.

  • The SA node generates impulses approximately every 0.8 seconds, equating to 75 beats/min.

19.7b Electrical and Mechanical Events of Cardiac Muscle Cells

1. Depolarization:

   • Fast Na+ channels open, Na+ enters, and potential changes from -90 mV to +30 mV.

2. Plateau:

   • Voltage-gated K+ and slow Ca2+ channels open. K+ exits while Ca2+ enters, creating a sustained contraction.

3. Repolarization:

   • Closing of Ca2+ channels and sustained K+ efflux returns the membrane potential to -90 mV.

Electrocardiogram (ECG/EKG) (19.7d)

• ECG Overview:

  • Diagnostic tool measuring heart electrical signal patterns.

  • Key contributors:

    • P wave: Atrial depolarization.

    • QRS complex: Ventricular depolarization.

    • T wave: Ventricular repolarization.

  • Segments:

    • P-Q segment: Duration of atrial contraction.

    • S-T segment: Duration of ventricular contraction.

Cardiac Cycle Overview (19.8a-b)

• Definition: All events from one heartbeat to the next, involving systole and diastole.

• Ventricular Activities:

  • Contraction increases pressure, moving blood forward through valves; relaxation decreases pressure, allowing filling.

• Key phases include:

  1. Isovolumic Contraction: Ventricles contract, pushing AV valves closed but not yet ejecting blood.

  2. Ventricular Ejection: Blood flows out into arteries as pressure exceeds arterial trunk pressure.

  3. Isovolumic Relaxation: Ventricles relax; all valves are closed, and no blood enters or exits.

• Equal Ejection: Both ventricles pump equal amounts to prevent volume disparity.

Introduction to Cardiac Output (19.9a)

• Calculation: Cardiac Output (CO) = Heart Rate (HR) × Stroke Volume (SV).

• Important for understanding cardiovascular system efficiency and response to activity demands.

• Example: 75 beats/min × 70 mL/beat = 5.25 L/min.

Variables Influencing Heart Rate and Stroke Volume (19.9b-c)

• Chronotropic Agents:

  • Positive agents (e.g., sympathetic stimulation) increase heart rate.

  • Negative agents (e.g., parasympathetic activity) decrease heart rate.

• Stroke Volume Factors:

  • Influenced by:

    • Venous return: Directly affects EDV.

    • Inotropic agents: Modulate contractility by altering Ca2+ levels.

    • Afterload: Resistance against ventricular ejection, impacted by arterial health.