Detailed Study Notes on Cardiac Muscle Physiology

Class Information

  • Class will be held in the same room (Casio 55, basement) as last time.
  • Activities planned for the class:
    • Practice Questions: Short answer questions will be tackled during the session.
    • Jeopardy Game: A quiz game to reinforce learning.
  • Class schedule:
    • Today’s class and Monday's class will precede the Jeopardy session.

Assessment Scores

  • Assessment scores for the previous exam were posted recently.
  • Initially required to access through the lockdown browser but this restriction has been lifted due to technical issues faced by students.
    • For those who took the test on paper: Copies are available for pick-up at the end of class.
  • Guidance:
    • If performance was unsatisfactory, students are encouraged not to be discouraged; focus on changing study strategies for future improvements.
    • Students are urged to visit the instructor or the Peer Learning Services (PLS) for additional help.

Course Intro and Assignment Deadline

  • Final call for submitting course introduction and Unit One assignments by tonight.
  • **Issue with Mastering AAP:
    • Bug affecting those who started assignments before the due date.
    • A workaround is available, and students should email the instructor if affected.**

Transition to Cardiac Physiology

  • Subject transition from previous topics to electrical activity within the heart.
  • Today’s Focus:
    • Overview of cardiac muscle tissue.
    • Understanding electrophysiology
    • Discussion of action potentials in cardiac muscles.

Cardiac Muscle Tissue Overview

  • Definition of Autorhythmicity:
    • Cardiac muscle exhibits autorhythmicity, meaning it generates its own rhythm through action potentials.
    • Different from skeletal and smooth muscle, which rely on nervous system stimulation for action potentials and contraction.
  • Although the nervous system affects cardiac activity, it is not necessary for the initiation of contraction.

Types of Cardiac Muscle Cells

  • 1. Pacemaker Cells:
    • Function to pace the heart and generate action potentials autonomously.
    • Comprise less than 1% of the cardiac muscle cells.
  • 2. Contractile Cells:
    • Most abundant cardiac muscle cells that require signals from pacemaker cells for action potentials and contraction.
    • Responsible for generating tension necessary for blood pumping.

Importance of Both Cell Types

  • Both pacemaker and contractile cells are critical:
    • Without pacemaker cells, no electrical activity would be set.
    • Without contractile cells, there would be no actual blood pumping.

Structure of Cardiac Muscle Cells

  • Similarities with Skeletal Muscle:

    • Both contain myofibrils and contractile filaments arranged in sarcomeres.
    • Contract via the sliding filament mechanism.
    • Have T-tubules and sarcoplasmic reticulum facilitating excitation-contraction coupling.

  • Differences from Skeletal Muscle:

    • Cardiac muscle cells are shorter, wider, and typically have one nucleus (occasionally two).
    • Cardiac muscle cells possess branched structures, allowing them to weave through the fibrous skeleton of the heart, providing structural integrity during contraction.

Energy Demand of Cardiac Muscle

  • Higher Energy Needs:
    • Cardiac muscle cells are always active (unlike skeletal muscle which can rest) and possess a constant higher energy demand.
    • They contain more mitochondria and myoglobin to meet these energy requirements.

Intercalated Discs in Cardiac Muscle

  • Function:
    • Intercalated discs connect cardiac muscle cells, allowing synchronization in contraction.
  • Types of Junctions in Intercalated Discs:
    • Desmosomes: Anchoring junctions preventing muscle cells from pulling apart during contraction.
    • Gap Junctions: Allow ions to flow between cells, facilitating rapid electrical signaling necessary for coordinated contraction.

Comparison Activity

  • An in-class activity will compare the structures of cardiac muscle to skeletal muscle.
  • Topics to include:
    • The presence of striations and sarcomeres in both.
    • The arrangement of T-tubules and sarcoplasmic reticulum being similar, but with notable differences as discussed.

Electrophysiology of the Heart

  • Focus shifts to understanding the electrical activity in the heart, particularly action potentials generated by pacemaker cells and utilized by contractile cells.

Membrane Potential Overview

  • Definition:

    • Membrane potential is the charge difference across the cell membrane, with typical resting values being negative.
    • Action potentials are temporary changes in membrane potential initiated by ion movement according to their electrochemical gradients.

  • Polarized cells have a negative membrane potential, while hyperpolarized cells are even more negative.

  • Ion Movement:

    • Movement is governed by both concentration gradients and electrical gradients, which determines overall electrochemical gradients.

Sodium-Potassium Pump

  • Active transport mechanism that maintains ion gradients necessary for action potentials.
  • Moves sodium out and potassium into the cell, ensuring proper cell function after action potentials occur.

Action Potential in Cardiac Muscle Cells

  • Initial Action Potential Phase:

    • Rapid depolarization occurs when sodium channels open, and sodium flows into the cell.
    • Followed by an initial repolarization phase due to the opening of potassium channels, leading to a return towards resting potential.

  • Contractile Cell Action Potential Phases:

    1. Rapid Depolarization: Sodium influx.
    2. Initial Repolarization: Potassium efflux begins; calcium channels open.
    3. Plateau Phase: Characterized by calcium influx balancing potassium efflux, preventing rapid repolarization and prolonging contraction.
    4. Full Repolarization: Following calcium channel closure, potassium channels open, bringing cells back to resting potential.

Comparison of Cardiac and Skeletal Muscle Action Potentials

  • Cardiac muscle action potentials differ due to the longer refractory period facilitated by the plateau phase; this inhibits rapid successive contractions and allows the heart chambers time to fill with blood.

Pacemaker Cells Action Potentials

  • Pacemaker cells initiate action potentials and are critical for heart rhythm. They do not have a stable resting membrane potential but undergo continuous depolarization due to unique ion channels.
  • Phases of Pacemaker Potentials:
    1. Slow Initial Depolarization: Triggered by initial sodium influx and potassium efflux through HCN channels.
    2. Full Depolarization: Calcium channels open, leading to further depolarization.
    3. Repolarization: Calcium channels close, and potassium channels open, moving the membrane potential negatively, usually reaching a hyperpolarized state before starting a new cycle.

Cardiac Conduction System

  • The conduction system connects pacemaker action potentials to contractile cells:

    • Sinoatrial (SA) Node: Primary pacemaker cell group in the heart setting the rhythm by generating the fastest action potentials.
    • Atrioventricular (AV) Node: Secondary pacemaker that slows down the heart rate when active.
    • Purkinje Fiber System: Collection of fibers spreading signals throughout the ventricles, capable but not effective as a long-term pacemaker.

  • Each section of the conduction system has its unique role and failure in one can lead to dependence on slower, less effective pacemaker cells, impacting heart function drastically.