Cardiac Muscle and the Heart
Cardiac Muscle & the Heart
Chapter 18
Systemic and Pulmonary Circuits
The heart functions effectively as two pumps arranged in a figure eight design:
Right side: Dedicated to the pulmonary circuit (responsible for blood flow to the lungs for oxygenation).
Left side: Dedicated to the systemic circuit (responsible for oxygenated blood delivery to the rest of the body).
Layers of the Heart Wall
The heart wall consists of several distinct layers, each with specific characteristics:
Pericardium: The fibrous outer layer that encapsulates the heart.
Superficial fibrous layer: Provides protective support.
Serous layer: Divided into two sections:
Parietal pericardium: Lines the inner surface of the fibrous pericardium.
Visceral pericardium (epicardium): Covers the outer surface of the heart.
Myocardium: The thick, muscular layer composed of cardiac muscle that facilitates contraction.
Cardiac skeleton: A fibrous structure that supports the myocardium and provides electrical insulation between atria and ventricles.
Endocardium: The inner lining of the heart chambers and valves.
Cardiac Muscle Features
Cardiac muscle is characterized by the following properties:
Striated, indicating a banded appearance due to organized myofilaments.
Short and branched fibers that interconnect to allow coordinated contraction.
Each cell typically contains one central nucleus, although there may be up to two.
Contains numerous large mitochondria for energy production.
Intercalated discs: Specialized connections between cardiac muscle cells that include:
Desmosomes: Connections that hold cells together and prevent separation during contraction.
Gap junctions: Allow ions to pass between cells, contributing to electrical coupling and synchronous contractions, enabling the heart to act as a functional syncytium (a mass of cells acting collectively).
Structure of Cardiac Muscle Cells
Components of a cardiac muscle cell include:
Intercalated disc: Facilitates communication between cells.
Nucleus: Central feature of the cell.
Sarcoplasm: Cytoplasm of muscle cells.
Mitochondria: Abundant for energy production.
T-tubules: Extensions of the sarcolemma that help transmit action potentials into the cell.
Sarcoplasmic reticulum: Stores calcium ions necessary for contraction.
Myofibrils: Contain contractile proteins organized into sarcomeres, responsible for muscle contraction.
Physiology of Skeletal vs. Cardiac Muscle
Muscle contraction is initiated by action potentials (AP):
An AP travels down the T-tubules, causing the sarcoplasmic reticulum (SR) to release calcium ions (Ca²+).
Excitation-contraction coupling: The process where Ca²+ binds to troponin, leading to the sliding of actin and myosin filaments.
Similarities
Both skeletal and cardiac muscle cells exhibit contraction in response to action potentials.
Differences
Contractile cells: In cardiac muscle, these cells are responsible for contractions.
Pacemaker cells: Non-contractile cells that spontaneously depolarize to initiate heart contractions.
Cardiomyocytes (heart muscle cells) contract as a coordinated unit due to the presence of gap junctions and functional syncytium formation.
Calcium influx from extracellular fluid is critical for the triggering of Ca²+ release from the SR.
Refractory period: The absolute refractory period in cardiac muscle cells is longer than in skeletal muscle cells, preventing sustained contraction (tetanus).
Key Differences between Skeletal and Cardiac Muscle (Table 18.1)
Feature | Skeletal Muscle | Cardiac Muscle |
|---|---|---|
Structure | Striated, long, cylindrical, multinucleate | Striated, short, branched, one or two nuclei per cell |
Gap Junctions | No | Yes |
Contracts as a unit | No, motor units must be stimulated individually | Yes, due to functional syncytium |
T-tubules | Abundant and well-developed | Fewer and wider |
Sarcoplasmic Reticulum | Elaborate; has terminal cisterns | Less elaborate; no terminal cisterns |
Source of Ca²+ for contraction | Sarcoplasmic reticulum only | Sarcoplasmic reticulum and extracellular fluid |
Ca²+ binding to troponin | Yes | Yes |
Presence of Pacemaker Cells | No | Yes |
Tetanus Possible | Yes | No |
Supply of ATP | Aerobic and anaerobic (fewer mitochondria) | Aerobic only (more mitochondria) |
Syncytiums of the Heart and Action Potentials
The heart has two main syncytiums that are separated by fibrous tissue:
Atrial syncytium: Group of cells that contracts together to facilitate atrial contraction.
Ventricular syncytium: Controls ventricular contraction, allowing for efficient pumping of blood.
Action Potentials in Cardiac Muscle
Phases of Cardiac Action Potentials
Phase 0: Depolarization occurs as fast sodium (Na+) channels open.
Phase 1: Initial repolarization as fast sodium channels close.
Phase 2: Plateau phase; L-type calcium (Ca²+) channels open slowly while fast potassium (K+) channels close, maintaining depolarization for approximately 0.2 seconds.
Phase 3: Rapid repolarization occurs as calcium channels close and slow potassium channels open, restoring the resting potential.
Phase 4: The heart returns to its resting membrane potential, averaging -85 to -90 mV.
Figure 9-4 Guyton & Hall, 13th Ed. illustrates these phases of action potentials.
Cardiac Muscle Excitation-Contraction Coupling
Calcium's Role in muscle contraction:
The action potential triggers the influx of Na+ through T-tubules, which in turn causes the release of Ca²+ from the sarcoplasmic reticulum.
Contraction Process:
Calcium ions interact with contractile proteins, leading to muscle contraction.
Contraction Signal Pathway
Steps involved:
Excitation reaching the sarcolemma triggers open channels for calcium ions.
Calcium signals contraction through the release of Ca²+ sparks from the sarcoplasmic reticulum, leading to muscle contraction.
Calcium Pumping and Relaxation in Cardiac Muscle
At the end of the action potential, calcium ions are pumped out of the cytoplasm to terminate the contraction and allow for relaxation.