Cardiac Muscle: Structure, Contraction, and Exercise Effects
Introduction
Also called "Myocardium."
Cardiac muscle is striated and utilizes the same fundamental contractile mechanisms as skeletal muscle, though it exhibits numerous distinctions.
It consists of cardiomyocytes, each possessing one pale, oval, centrally located nucleus.
Excitation is facilitated by rhythmically active, modified cardiac muscle cells.
Cardiac muscle is involuntarily innervated by the autonomic nervous system, which modulates both the force generated by the muscle cells and the heart's beat frequency.
Types of cardiac muscle fibers include the contractile unit and the pacemaker conductive system.
Comparison of Cardiac, Skeletal, and Smooth Muscle
Character | Skeletal Muscle | Smooth Muscle | Cardiac Muscle |
|---|---|---|---|
Striation | Striated | Non-striated | Striated |
Location | Biceps, triceps, postural muscles, etc. | Digestive tract | Heart |
Function | Locomotion, mastication, phonation | Move food along | Heart contracts and relaxes |
Cells | Long, cylindrical, multinucleated, peripherally placed nuclei | Fusiform, a central nucleus | Elongated, branched, single centrally placed nucleus, intercalated discs at the ends |
Contraction | Voluntary, quick & vigorous | Involuntary, slow & long-lasting | Involuntary, vigorous rhythmic |
Cardiac Muscle Tissue Structure
Muscle Fibers Forming the Contractile Unit
General Characteristics:
Striated, resembling skeletal muscle fibers.
Muscle fiber is enclosed by sarcolemma, contains a centrally placed nucleus.
Myofibrils are embedded within the sarcoplasm.
Sarcomere comprises all contractile proteins: actin, myosin, troponin, and tropomyosin.
Sarcotubular System Differences (Cardiac vs. Skeletal Muscle):
Feature | Cardiac Muscle | Skeletal Muscle |
|---|---|---|
Location of T Tubules | At Z line | At A-I junction |
Diameter of T Tubules | More (5 times) | Less |
L Tubules | Narrow Tubules | Cistern |
Association of T Tubules | Diad (1 Tubule & 1 Cistern) | Triad (1 Tubule & 2 Cisterns) |
Sarcomeric organization | Less regular | More regular |
Cellular Details:
Composed of roughly cylindrical cells, typically single.
One or, occasionally, two nuclei located at its center.
Cell ends are often uneven or forked.
A cardiac muscle cell usually connects with two other cardiac muscle cells at its ends, giving the tissue a branched appearance.
Cross-Sectional View:
Exhibits irregular profiles with variable sizes of muscle fibers and nuclei.
Centrally located nucleus (appearing as white spots).
Intercalated Discs:
Anchored to the cell membranes at the cell ends by an \alpha-actinin (Z-line protein)/vinculin complex, specifically to the fascia adherens part of the intercalated disc.
Visible as a distinct line "between" cells in light microscopy (LM).
Primarily observed in longitudinal sections.
Connect individual muscle cells end-to-end.
Crucially permit the conduction of electrical impulses between cells.
Junctional Components within Intercalated Discs:
Macula Adherens Junctions (Desmosomes):
Specialized and highly organized membrane domains.
Possess both transverse (within intercalated discs) and lateral (along the sides of the cells) components.
Form strong adhesive junctions between adjacent cardiac muscle cells.
Prevent cellular separation during contraction by binding intermediate filaments, thereby joining the cells together.
Gap Junctions:
Also have both transverse (primarily on intercalated disks) and lateral (extending along cell sides) components.
Enable action potentials to propagate between cardiac cells via the passage of ions.
Facilitate depolarization of the heart muscle.
Allow the muscle to function as a syncytium.
Heart Function as a Syncytium:
When one cardiac cell undergoes an action potential, the electrical impulse rapidly diffuses to all other cells interconnected by gap junctions.
This collective excitation causes all connected cells to contract as a single, functional syncytium.
The heart contains two functional syncytia: the atrial syncytium and the ventricular syncytium, which enable the heart to contract as a coordinated unit.
Orientation of Cardiac Muscle Fibers:
Unlike skeletal muscles, cardiac muscles must contract in multiple directions.
Although cardiac muscle cells are striated and primarily contract along their long axis, the fibers are wrapped around each other to achieve contraction in two axes.
Muscle Fibers Forming the Pacemaker Structure
The muscle fibers in the heart's Sinoatrial ($ ext{SA}$) node are modified into a specialized structure that functions as the pacemaker.
These fibers are termed nodal fibers, pacemaker cells ($ ext{P cells}$).
They are smaller than typical cardiac muscle cells.
They possess a significant amount of "unoccupied" cytosol between myofibrils.
They are deficient in \alpha-actinin (Z-line protein).
Contraction of cardiac muscle is initiated by the periodic depolarization of these nodal fibers.
Muscle Fibers Forming the Conductive System
The heart's conductive system is composed of modified cardiac muscle fibers.
Impulses from the $ ext{SA}$ node are directly transmitted to the atria.
Impulses are transmitted to the ventricles through various components of the conducting system.
Purkinje Fibers:
Are modified cardiac muscle cells.
Compared to ordinary cardiac muscle cells, they are: larger, deficient in \\alpha-actinin, possess fewer myofibrils, contain substantial glycogen, and are thicker.
Extend from the atrioventricular ($ ext{AV}$) node, penetrate the fibrous body, divide into left and right bundles, and travel beneath the endocardium towards the apex of the heart.
Conduct stimuli significantly faster than ordinary cardiac muscle cells (2-3 \text{ m/s} vs. 0.6 \text{ m/s}).
Discovered in 1839 by Jan Evangelista Purkyně.
While they are cells, Purkinje fibers are never referred to as "Purkinje cells."
Contraction of Cardiac Muscle: Properties
Properties of Cardiac Muscle
Electrical Properties (Electrophysiology):
Excitability (Bathmotropic action): Ability to respond to a stimulus.
Autorhythmicity: Ability to spontaneously generate impulses.
Conductivity (Dromotropic action): Ability to conduct impulses.
Mechanical Properties (Contractility):
Contractility (Inotropic action): Ability to contract.
Refractory period: Period during which the muscle cannot be re-stimulated.
Treppe effect (Bowditch effect, Staircase phenomenon): Gradual increase in contraction strength with repeated stimulation.
Autorhythmicity
Definition: The inherent ability of the heart to initiate its beat continuously and regularly without external neural or hormonal stimulation.
Nature: It is myogenic (independent of nerve supply), attributed to the heart's specialized excitatory & conductive system.
Mechanism: This intrinsic ability for self-excitation leads to waves of depolarization and subsequent cardiac impulses.
Autorhythmic Fibers:
Constitute approximately 1\% of the cardiac muscle fibers.
Functions:
Act as a pacemaker, establishing the rhythm of electrical excitation.
Form the conductive system, a network of specialized cardiac muscle fibers that provides a pathway for each cycle of cardiac excitation to traverse the heart.
Locations: Sinoatrial ($ ext{SA}$) node, Atrioventricular ($ ext{AV}$) node, Bundle of His (atrioventricular bundle), and Purkinje fibers.
Pacemaker Potential (Diastolic Potential, Prepotential):
Autorhythmic cells lack a stable resting membrane potential ($ ext{RMP}$).
Their unstable $ ext{RMP}$ is termed the pacemaker potential, which is a gradual increase in the electrical charge of cardiac pacemaker cells leading to a heartbeat.
This occurs due to natural leakiness to $ ext{Na}^+$ and $ ext{Ca}^{2+}$, causing spontaneous and gradual depolarization until the threshold (approx. -40 \text{ mV}) is reached, leading to spontaneous action potential generation.
Excitability
Definition: The ability of cardiac muscle to respond to a stimulus of adequate strength and duration by generating an action potential ($ ext{AP}$).
Action Potential ($ ext{AP}$) - Contraction Relation:
Skeletal Muscle: The $ ext{AP}$ in skeletal muscle is very short-lived and is typically over before a measurable increase in muscle tension occurs.
Cardiac Muscle: The $ ext{AP}$ in cardiac muscle is significantly longer-lived. It possesses an additional component (influx of $ ext{Ca}^{2+}$), which prolongs its duration. Consequently, the contraction is nearly complete before the $ ext{AP}$ has fully subsided.
Propagation: An $ ext{AP}$ initiated by the $ ext{SA}$ node travels along the conductive pathway, thereby exciting both atrial and ventricular muscle fibers.
Phases of the Cardiac Action Potential:
Rapid Depolarization: Triggered by the influx of $ ext{Na}^+$ through voltage-gated $ ext{Na}^+$ channels.
The Plateau: Characterized by the opening of slow $ ext{Ca}^{2+}$ channels and simultaneous closing of $ ext{K}^+$ channels, sustaining depolarization.
Repolarization: Occurs as slow $ ext{Ca}^{2+}$ channels close.
Refractory Period: Includes an absolute refractory period (no new $ ext{AP}$ possible) and a relative refractory period (stronger stimulus can elicit a new $ ext{AP}$). Cardiac muscle's long refractory period prevents tetanus.
Contractility
Definition: The ability of cardiac muscle to contract in response to stimulation.
All Or None Law:
The response to a threshold stimulus is maximal. If the stimulus is below threshold, no response occurs, assuming physiological conditions remain constant.
Crucially, the cardiac muscle follows the all-or-none law as a whole organ/syncytium.
In contrast, in skeletal muscle, the all-or-none law applies only to a single muscle fiber.
Mechanism of Contraction (Similar to Skeletal Muscles, with key differences):
Depolarization of the T-Tubule membrane.
Opening of Dihydropyridine Receptors ($ ext{DHPRs}$, which are voltage-gated cation channels).
$ ext{Na}^+$ and $ ext{Ca}^{2+}$ flow into the cell.
Increased intracellular $ ext{Na}^+$ facilitates the opening of more $ ext{DHPRs}$.
Increased intracellular $ ext{Ca}^{2+}$ opens Ryanodine Receptors ($ ext{RyRs}$) located on the sarcoplasmic reticulum ($ ext{SR}$) terminal cisternae.
This triggers a large release of $ ext{Ca}^{2+}$ from the $ ext{SR}$.
Key Distinction: Unlike in skeletal muscle, ryanodine receptors in cardiac muscle are not mechanically linked to the dihydropyridine receptors; the $ ext{Ca}^{2+}$ influx itself triggers further $ ext{Ca}^{2+}$ release (calcium-induced calcium release).
Calcium Dependence:
The cardiac muscle stores significantly more calcium ($ ext{Ca}^{2+}$) in its tubular system than skeletal muscle and is considerably more dependent on extracellular calcium.
An abundance of calcium is bound by mucopolysaccharides inside the T-tubules.
This calcium is essential for the contraction of cardiac muscle, and the strength of contraction is directly dependent on the calcium concentration surrounding the cardiac myocytes.
At the initiation of the action potential, fast sodium channels open first, followed by the opening of slow calcium channels.
Excitation-Contraction Coupling in Cardiac Contractile Cells:
Action potential in a cardiac contractile cell.
Travels down T tubules.
Entry of a small amount of $ ext{Ca}^{2+}$ from the extracellular fluid ($ ext{ECF}$). This $ ext{Ca}^{2+}$ acts as a trigger.
Release of a large amount of $ ext{Ca}^{2+}$ from the sarcoplasmic reticulum ($ ext{SR}$), due to calcium-induced calcium release.
Increased cytosolic $ ext{Ca}^{2+}$ concentration.
Troponin-tropomyosin complex in thin filaments is pulled aside.
Cross-bridge cycling occurs between thick and thin filaments.
Thin filaments slide inward between thick filaments.
Contraction of the cardiac muscle cell.
Conductivity
Definition: The property by which excitation is conducted through the cardiac tissue.
Conducting System Pathway:
SA node depolarizes, initiating the heartbeat.
Electrical activity rapidly travels to the AV node via internodal pathways.
Depolarization spreads more slowly across the atria.
Conduction slows down through the AV node, allowing time for atrial contraction before ventricular contraction.
Depolarization moves rapidly through the ventricular conducting system (Bundle of His, bundle branches, Purkinje fibers) to the apex of the heart.
The depolarization wave then spreads upward from the apex, allowing for efficient ejection of blood.
Exercise and Cardiac Muscle
Principal Effect: Exercise primarily increases blood circulation to the cardiac muscle.
Short-Term Effects: Circulation is increased by elevated blood pressure and dilatation of arteries and myocardial capillaries.
Repeated Endurance Exercise (20 \text{ minutes} or more): Causes permanent enlargement of arteries and an increase in the already numerous capillaries.
High-Intensity Endurance Exercise (heart rate approaching maximum, 150-180 \text{ beats per minute}): Leads to myocardium hypertrophy (increase in size of myofibrils, more mitochondria, and more myoglobin).
Sustained High-Intensity Endurance Exercise (several weeks): Telocytes are hypothesized to induce cardiac stem cells to multiply and differentiate into new cardiac muscle cells.
Exercise and Skeletal Muscle
The response of skeletal muscle fibers to exercise is dependent on the type of fibers involved and the nature of the exercise.
Four Types of Skeletal Muscle Cells in Humans:
Red & Intermediate Fibers: Contain large amounts of the red oxygen-storage protein myoglobin.
White & Extraocular Fibers: Possess little myoglobin.
Comparison of Red (Slow Twitch) and Pale (Fast Twitch) Muscle
Feature | Red (Slow Twitch) Muscles (Type I fibers) | Pale (Fast Twitch) Muscles (Type II fibers) |
|---|---|---|
Examples | Back muscles, gastrocnemius muscles | Hand muscles, ocular muscles |
Twitch contraction | Long $\implies$ designed for sustained contraction | Short $\implies$ designed for short contraction |
Myoglobin content | More $\implies$ red | Less $\implies$ pale |
Blood vessels | More | Less |
Mitochondria | More $\implies$ aerobic glycolysis for ATP production $\implies$ less liable to fatigue | Less $\implies$ anaerobic glycolysis for ATP production $\implies$ more liable to fatigue |
Sarcoplasmic reticulum | Less extensive $\implies$ contraction is less powerful | More extensive $\implies$ contraction is more powerful |
Response | Slow with long latent period | Rapid with short latent period |
Summary of Cardiac Muscle Properties
Cardiac muscle consists of cells containing one or two nuclei, which are electrically coupled by gap junctions.
Contraction of cardiac muscle occurs through the ATP-driven sliding of thick myosin filaments along thin actin filaments.
Contraction is initiated by the depolarization of nodal fibers, which then depolarize Purkinje fibers, which in turn depolarize cardiac muscle cells through gap junctions.
Practice Questions
Which of the following statements about cardiac muscle is most accurate?
B. The strength and contraction of cardiac muscle depends on the amount of calcium surrounding cardiac myocytes.
How is cardiac muscle primarily controlled?
B. On an involuntary basis via the autonomic nervous system.
Which of the following statements is false concerning gap junctions?
C. They are found in both skeletal muscle and cardiac muscle.
Intercalated discs have gap junctions which:
C. Allow free movement of ions, molecules, and electrical signals.
In what way are cardiac muscles and skeletal muscles similar?
B. both have striations.
A calcium influx triggers $\text{Ca}^{2+}$ release.
A. True.
Contractions in cardiac muscle are by sliding filament mechanism.
A. True.