Detailed Study Notes on Cell Membrane, Ion Distribution, and Cardiac Physiology

Cell Membrane and Ion Distribution

  • Cell Membrane: Refers to the plasma membrane surrounding the cell.

Ion Distribution Inside and Outside the Cell

  • Inside the Cell: High concentration of Potassium ions (K⁺).

  • Outside the Cell: High concentration of Sodium ions (Na⁺).

  • Presence of Calcium ions (Ca²⁺) both inside and outside of the cell.

    • Storage of Calcium: In muscle cells, calcium is primarily stored in the sarcoplasmic reticulum (SR).

Role of Calcium in Muscle Contraction

  • Calcium's Role: Binding to troponin, moving tropomyosin, and exposing active sites for myosin and actin interaction.

  • Calcium is essential for muscle contraction as it triggers the contraction process.

Resting Membrane Potential

  • Definition: The electrical potential of a cell when it is not actively sending signals, essentially when it is in a resting state.

  • Value for Neurons: Approximately -70 mV.

  • Specifics for Heart Cells: Varies between pacemaker cells and contractile cells; further details in Chapter 12 suggested for review.

Depolarization and Repolarization

  • Depolarization: Refers to the process of becoming less negative compared to resting membrane potential, e.g., from -70 mV to -50 mV.

  • Repolarization: Returning to resting membrane potential after depolarization.

  • Hyperpolarization: Going below the resting membrane potential, leading to values less negative than the resting state.

Types of Cardiac Muscle Cells

  • Skeletal vs. Cardiac Muscle Cells: Both are muscle cells but behave differently.

    • Pacemaker Cells: Autorhythmic cells that initiate the heartbeat.

    • Contractile Cells: Responsible for muscle contraction in heart tissues.

    • Normal Heart Function: Atria contract first, followed by ventricles in a coordinated sequence.

Electrical Systems in the Heart

  • Conducting System: Specialized cells responsible for initiating and propagating electrical impulses.

    • Components:

    • Pacemaker Cells (e.g., SA node, AV node).

    • Conducting Cells (e.g., AV bundle (Bundle of His), bundle branches, Purkinje fibers).

Functions of Neurons and Hormones on Heart Rate

  • Sympathetic Nervous System: Increases heart rate.

  • Parasympathetic Nervous System: Decreases heart rate.

  • Hormones: (e.g., Adrenaline) can also influence heart rate.

Action Potentials in Pacemaker Cells

  • Pacemaker Potential: Refers to the gradual depolarization in pacemaker cells due to ion permeability changes.

    • Resting Membrane Potential: Not stable; undergoes spontaneous depolarization.

    • Ion Flow: Na⁺ influx leads to gradual depolarization, followed by Ca²⁺ influx during depolarization, and K⁺ efflux for repolarization.

EKG (Electrocardiogram)

  • Definition: A recording of electrical events in the heart that can identify abnormalities.

  • Components of EKG:

    • P Wave: Atrial depolarization.

    • QRS Complex: Ventricular depolarization (and simultaneous atrial repolarization, masked by the QRS).

    • T Wave: Ventricular repolarization.

  • Intervals:

    • PR Interval: Duration from the start of atrial depolarization to the start of ventricular depolarization (measures the conduction through AV node).

    • QT Interval: Duration for ventricular depolarization and repolarization cycle.

Heart Rate Abnormalities

  • Bradycardia: Abnormally slow heart rate, potentially due to conduction blocks.

  • Tachycardia: Abnormally fast heart rate when resting; important to measure appropriately after some time of rest.

  • Ectopic Pacemakers: Abnormal cells that generate action potentials disrupting the normal rhythm can lead to erratic contractions, resulting in heart inefficiency.

Contractile Cells Characteristics

  • Resting Membrane Potential for Contractile Cells:

    • Atrial Cells: -80 mV.

    • Ventricular Cells: -90 mV.

  • Intercalated Discs:

    • Feature desmosomes for adhesion and gap junctions for ionic communication between cells.

Summary of Action Potential Phases in Contractile Cells

  • Phases:

    • Depolarization: Fast influx of Na⁺.

    • Plateau Phase: Influx of Ca²⁺ while K⁺ efflux occurs, maintaining a longer depolarization.

    • Repolarization: Opening of K⁺ channels as Ca²⁺ channels close, allowing the cell to return to resting potential.

  • Important for Understanding: Differences in action potential between pacemaker and contractile cells have distinct physiological implications, notably in heart function regulation.