4. Cardiac Muscle Physiology: Structure, Electrophysiology, and Excitation-Contraction Coupling

Anatomical Organization of the Heart

Location and Anatomical Position

The heart resides within the thoracic cavity, specifically situated in the central compartment known as the mediastinum.
It is positioned between the lungs and protected by the thoracic cage, spanning approximately from the third to the sixth intercostal spaces.

Structural Chambers and Valves

The mammalian heart is a four-chambered muscular pump organized into left and right sides:

Right Atrium (RA): Receives deoxygenated blood returning from the systemic circulation via the superior vena cava and inferior vena cava.
Right Ventricle (RV): Receives blood from the right atrium through the tricuspid valve and pumps it into the pulmonary trunk through the pulmonary valve to the lungs for oxygenation.
Left Atrium (LA): Receives oxygenated blood returning from the lungs via four pulmonary veins.
Left Ventricle (LV): Receives oxygenated blood from the left atrium through the mitral (bicuspid) valve and pumps it into the systemic circulation through the aortic valve into the aorta under high pressure.

Heart Wall Layers and Pericardial Cavity

The structural wall of the heart consists of three distinct layers surrounded by a protective fibrous sac:

Epicardium (Visceral Pericardium): The outermost protective layer covering the cardiac surface. It consists of mesothelium and underlying areolar connective tissue.
Myocardium: The thick, middle muscular layer comprising the bulk of the heart wall. It is composed primarily of specialized cardiac muscle cells (cardiomyocytes) organized in a complex basket-weave architectural pattern to facilitate efficient coordinated chamber contraction.
Endocardium: The innermost continuous lining covering the inner surfaces of the chambers, papillary muscles, and heart valves. It consists of a simple squamous endothelium and an underlying layer of areolar connective tissue.
Pericardium: The heart is enclosed within the parietal pericardium, which consists of an outer dense fibrous layer and an inner mesothelial layer. Between the parietal pericardium and the epicardium lies the pericardial cavity, containing serous fluid that lubricates the heart surfaces and reduces friction during cardiac cycles.

Functional Cell Types in the Myocardium

The heart contains three specialized populations of cardiac cells, each fulfilling distinct functional roles:

Working Myocardial Cells

Located in both the atrial and ventricular walls.
Consists of striated muscle specialized for forceful contraction and mechanical work, as well as impulse transmission across working tissues.
Constitutes the vast majority of the cardiac mass.

Pacemaker Cells

Specialized autorhythmic cells that lack stable resting membrane potentials.
Exhibit spontaneous, automatic, and rhythmic electrical discharge in the form of action potentials.
Responsible for initiating the primary action potential that triggers each cardiac cycle.
Chiefly concentrated in the Sinoatrial (SA) node and Atrioventricular (AV) node.

Conduction Cells

Highly specialized cells (such as those in the Bundle of His, bundle branches, and Purkinje fibers) optimized for rapid electrical transmission.
Propagate action potentials rapidly throughout the heart to coordinate synchronous atrial and ventricular excitation.

Structural Comparisons: Cardiac vs. Skeletal Muscle

Histological and Molecular Similarities

Striations: Cardiac muscle fibers display cross-striations similar to skeletal muscle due to organized repeating units called sarcomeres.
Sarcomere Architecture: Composed of interdigitating thin filaments (actin) and thick filaments (myosin).
Contractile Isoforms: Utilize cardiac-specific isoforms of actin, myosin, troponin, and tropomyosin.
Sarcoplasmic Reticulum and T-Tubules: Possess a network of sarcoplasmic reticulum (SR) for calcium storage and transverse tubules (T-tubules) for transmitting membrane electrical signals into the cell interior.

Unique Structural Differences

Involuntary Regulation: Cardiac muscle contraction is regulated involuntarily by intrinsic pacemakers and the autonomic nervous system, whereas skeletal muscle is under voluntary somatic control.
Cellular Morphology: Cardiac myocytes are shorter, broader, and branched/bifurcated at their ends, forming an interconnected meshwork, unlike the long, unbranched cylindrical fibers of skeletal muscle.
Nucleation: Cardiomyocytes are typically uninucleate (containing a single centrally located nucleus per cell), whereas skeletal muscle fibers are multinucleated syncytia.
Cellular Independence: Cardiac myocytes remain distinct individual cells bounded by their own sarcolemma; they do not fuse during embryonic development.

The Concept of Functional Syncytium

Etymology: Derived from Greek (Syn = together; Kytos = cell).
Morphological vs. Functional: Skeletal muscle is a morphological syncytium formed by the embryonic fusion of myoblasts into a continuous multinucleated cytoplasm. In contrast, cardiac muscle is composed of discrete individual cells separated by plasma membranes.
Mechanism: Because cardiac cells are interconnected end-to-end through low-resistance ion channels, electrical depolarization spreads rapidly from cell to cell. Consequently, the myocardium behaves functionally as a single synchronized unit—a functional syncytium—where stimulation of one cell leads to contraction of the entire tissue mass.

Intercalated Disk Architecture

Intercalated disks appear histologically as dark, dense transverse bands at the junctions between adjacent myocardial cells, running continuous with the sarcolemma. They contain two critical types of specialized intercellular junctions:

Gap Junctions:
- Protein channels composed of connexons that create direct cytoplasmic connections between adjacent cells.
- Allow rapid diffusion of ions ( $Na^+$ , $Ca^{2+}$ , $K^+$ ) between cells with minimal electrical resistance.
- Enable rapid propagation of action potentials from one myocyte to the next across the functional syncytium.
Desmosomes (Maculae Adherentes):
- Strong mechanical junctions that anchor intermediate filaments of adjacent cell membranes.
- Provide high mechanical tensile strength, holding the cells tightly together during intense mechanical stretching and forceful contractions.

Cardiac Muscle Microstructure and Intercalated Disks

Cardiac Electrical Conduction and Autonomic Control

Primary Pacemaker Origin

Action potentials normally originate spontaneously in the Sinoatrial (SA) node (known as the "sinus node"), situated in the wall of the right atrium near the opening of the superior vena cava.
The SA node generates rhythmic action potentials at a higher intrinsic frequency than any other cardiac tissue, making it the primary pacemaker of the heart.
Electrical activity propagates from the SA node through atrial muscle to the Atrioventricular (AV) node, through the AV bundle (Bundle of His), into the right and left bundle branches, and throughout the ventricular myocardium via Purkinje fibers.

Autonomic Nervous System Regulation

While the heart generates its own rhythmic electrical impulses intrinsically, the autonomic nervous system modulates heart rate (chronotropy) and contractile force (inotropy):

Parasympathetic System

Center & Nerve: Originates in the cardioinhibitory center (vagal nucleus) within the medulla oblongata and travels via the Vagus nerve (Cranial Nerve X) to synapse in the cardiac plexus.
Neurotransmitter: Releases Acetylcholine (ACh), which binds to $M_2$ muscarinic receptors on nodal cells.
Effects:
- Decreases the rate of SA node firing (slows heart rate / negative chronotropy).
- Decreases conduction velocity through the AV node.

Sympathetic System

Center & Pathway: Originates in the cardioacceleratory center in the medulla oblongata, passes down the spinal cord, and projects through preganglionic fibers to sympathetic chain ganglia (cervical and superior thoracic ganglia $T_1 - T_4$ ). Postganglionic fibers travel via cardiac nerves to the heart.
Neurotransmitter: Releases Norepinephrine (NE), which binds to $\beta_1$ -adrenergic receptors on nodal and myocardial cells.
Effects:
- Increases SA node firing rate (speeds up heart rate / positive chronotropy).
- Enhances AV node conduction speed.
- Increases myocardial contractility (force of contraction / positive inotropy).

Electrophysiology of Pacemaker Cells

Definition and Role of Pacemaker Potential

The pacemaker potential is a slow, automatic, spontaneous depolarization of the membrane potential during Phase 4, gradually shifting the voltage from its most negative resting level (approximately $-60\,mV$ ) toward the threshold potential (approximately $-40\,mV$ ).
Because autorhythmic cells lack a fixed, stable resting potential, this continuous background depolarization guarantees automatic action potential generation without neural stimulation.
The slope of the pacemaker potential directly determines the firing rate of the cell and thus sets the overall heart rate.

Ionic Currents and Sequential Channel Gating

The pacemaker potential and subsequent action potential upstroke involve a coordinated cascade of four primary ion channels:

Funny Channels ( $I_f$ Channels):
- Voltage-gated channels that open hyperpolarization-dependently when the membrane reaches its most negative value (around $-60\,mV$ ).
- Permit a slow inward flux of sodium ions ( $Na^+$ ).
- Initiate the initial phase of slow spontaneous depolarization toward threshold.
Transient Calcium Channels (T-Type $Ca^{2+}$ Channels):
- Voltage-gated channels that open briefly as the membrane potential approaches threshold (around $-50\,mV$ ).
- Allow a brief influx of calcium ions ( $Ca^{2+}$ ).
- Provide the final depolarizing boost required to push the membrane to threshold (around $-40\,mV$ ).
- Upon reaching threshold, $I_f$ channels close completely.
Long-Lasting Calcium Channels (L-Type $Ca^{2+}$ Channels):
- Open at threshold ( $-40\,mV$ ).
- Allow a major influx of extracellular $Ca^{2+}$ , producing the upward action potential spike (Phase 0 depolarization upstroke).
- Unlike skeletal muscle or working myocardial cells (where Phase 0 is driven by fast $Na^+$ channels), pacemaker cell depolarization is mediated primarily by $Ca^{2+}$ influx.
Potassium Channels ( $K^+$ Channels):
- Open near peak depolarization (around $+10$ to $+20\,mV$ ), while L-type $Ca^{2+}$ channels close/inactivate.
- Inward $Ca^{2+}$ flow ceases, and outward $K^+$ flux repolarizes the cell back toward $-60\,mV$ .
- Once the membrane hyperpolarizes back to $-60\,mV$ , $K^+$ channels close, and funny ( $I_f$ ) channels reopen, initiating the next cycle.

Ion Channel Cascade Summary

$\text{Funny } Na^+ \text{ channels } (I_f) \rightarrow \text{T-type } Ca^{2+} \text{ channels} \rightarrow \text{L-type } Ca^{2+} \text{ channels (Action Potential)} \rightarrow K^+ \text{ channels (Repolarization)} \rightarrow \text{Repeat}$

Electrophysiology of Working Myocardial Cells

Action potentials in working ventricular and atrial myocardial cells differ significantly from pacemaker cells. They exhibit a distinct five-phase shape characterized by a sharp Phase 0 depolarization and a long Phase 2 plateau.

Phase 4: Resting Membrane Potential ("Ready to fire")

Voltage: Maintained at approximately $-90\,mV$ .
Mechanism: High membrane permeability to potassium ( $K^+$ ) via open inward rectifier $K^+$ channels ( $K^+$ leaks outward). Voltage-gated $Na^+$ and $Ca^{2+}$ channels are closed.
State: The myocyte rests stably until stimulated by depolarizing currents from adjacent cells.

Phase 0: Rapid Depolarization ("Na+ rushes in")

Trigger: Depolarizing current arrives from adjacent myocytes through gap junctions, driving the membrane potential to threshold.
Mechanism: Fast voltage-gated $Na^+$ channels open rapidly, resulting in a massive, rapid influx of $Na^+$ into the cytosol.
Result: Membrane potential spikes rapidly from $-90\,mV$ to positive values (approximately $+20$ to $+30\,mV$ ).

Phase 1: Initial Repolarization ("Brief recovery")

Mechanism: Fast $Na^+$ channels rapidly inactivate. Transient (fast) outward $K^+$ channels open briefly, permitting a small outflow of $K^+$ ions.
Result: A slight, brief drop in membrane potential back toward $0\,mV$ .

Phase 2: Plateau Phase ("Contraction phase")

Mechanism: Voltage-gated slow L-type $Ca^{2+}$ channels open, allowing extracellular $Ca^{2+}$ to enter the cytosol. Simultaneously, delayed rectifier $K^+$ channels open, allowing $K^+$ to exit the cell.
Equilibrium: Inward $Ca^{2+}$ influx and outward $K^+$ efflux are nearly equal and balanced.
Result: Membrane potential remains sustained near $0\,mV$ for an extended duration (total action potential duration $\approx 300\,ms$ ).
Excitation-Contraction Link: The entering $Ca^{2+}$ triggers Calcium-Induced Calcium Release (CICR) from the sarcoplasmic reticulum, initiating mechanical muscular contraction.

Phase 3: Repolarization ("Return to resting state")

Mechanism: L-type $Ca^{2+}$ channels close, halting inward $Ca^{2+}$ flux. Delayed rectifier $K^+$ channels remain open, allowing continued $K^+$ efflux.
Result: Outward $K^+$ current exceeds inward currents, rapidly driving membrane potential back down to $-90\,mV$ .

Absolute vs. Relative Refractory Periods

Refractory Period	Timeframe & Phases	Channel State	Physiological Significance
Absolute Refractory Period (ARP)	Spans Phase 0, Phase 1, Phase 2, and early Phase 3.	Fast $Na^+$ channels are completely inactivated and closed.	The cell is completely unexcitable; no new stimulus, regardless of strength, can trigger an action potential.
Relative Refractory Period (RRP)	Occurs during late Phase 3 as voltage returns toward $-90\,mV$ .	Fast $Na^+$ channels transition from inactivated to closed (resting) state and can be reopened.	An action potential can be triggered, but requires a stronger-than-normal depolarizing stimulus.

Physiological Significance of the Long Plateau

Prevention of Tetanus: The prolonged action potential duration ( $\approx 300\,ms$ ) causes the absolute refractory period to last nearly as long as the mechanical contraction itself. This prevents summation of contractions and tetanic spasm, ensuring the heart alternates between distinct contraction and relaxation phases.
Complete Ventricular Filling: Ensures sufficient time for full ventricular contraction, blood ejection, and subsequent complete relaxation for ventricular chamber refilling.

Regional Differences in Myocardial Action Potentials

Atrial vs. Ventricular Myocardial Action Potentials

Atrial myocardial action potentials are significantly shorter in duration than ventricular myocardial action potentials.

Ionic Basis for Shorter Atrial Action Potentials

L-Type $Ca^{2+}$ Channels: In atrial cells, slow $Ca^{2+}$ channels remain open for a shorter duration, reducing total $Ca^{2+}$ influx during Phase 2.
$K^+$ Channels: In atrial cells, $K^+$ channels stay closed for a shorter time (opening earlier and more robustly), increasing $K^+$ efflux during Phase 2.
Net Result: $\text{Less } Ca^{2+} \text{ influx} + \text{More } K^+ \text{ efflux} \rightarrow \text{Shorter plateau} \rightarrow \text{Shorter action potential duration} \rightarrow \text{Shorter contraction time.}$

Functional Importance

Because atrial action potentials and contractions are shorter, the atria complete their contraction and enter relaxation before ventricular contraction reaches peak pressure. This timing difference allows efficient atrial pumping to complete ventricular filling prior to ventricular systole.

Excitation-Contraction Coupling in Cardiac Muscle

Excitation-contraction coupling (ECC) is the sequence of physiological processes connecting cardiac cell membrane electrical depolarization to mechanical muscle fiber contraction.

Excitation-Contraction Coupling and Calcium Cycling

Sources of Calcium for Cardiac Contraction

Unlike skeletal muscle (which relies almost entirely on intracellular sarcoplasmic reticulum calcium), cardiac muscle relies on two distinct sources of $Ca^{2+}$ :

Extracellular Fluid (ECF): Contributes $20\% - 30\%$ of the total $Ca^{2+}$ needed for contraction. T-tubules in cardiac muscle are larger in diameter than in skeletal muscle, filled with ECF, and allow direct inward movement of extracellular $Ca^{2+}$ .
Sarcoplasmic Reticulum (SR): Contributes $70\% - 80\%$ of the total $Ca^{2+}$ required for contraction.

Calcium-Induced Calcium Release (CICR) Sequence

Propagation: An action potential travels along the sarcolemma and down into the cell interior via the T-tubules.
Extracellular $Ca^{2+}$ Influx: T-tubule depolarization opens voltage-gated L-type $Ca^{2+}$ channels (DHP receptors), allowing extracellular $Ca^{2+}$ to flow down its electrochemical gradient into the cytosol.
Ryanodine Receptor Activation: This initial influx of extracellular $Ca^{2+}$ binds to Ryanodine Receptor 2 (RyR2) channels on the sarcoplasmic reticulum membrane.
$Ca^{2+}$ Spark Generation: RyR2 opening triggers a massive release of stored $Ca^{2+}$ from the SR into the cytoplasm, producing a localized, transient burst called a $Ca^{2+}$ spark.
$Ca^{2+}$ Signal Summation: Multiple localized $Ca^{2+}$ sparks summate to create a cellular $Ca^{2+}$ signal, dramatically elevating cytosolic $Ca^{2+}$ concentration.
Cross-Bridge Activation: Intracellular $Ca^{2+}$ binds to Troponin C on thin filaments, causing a conformational shift in tropomyosin that uncovers myosin-binding sites on actin. Myosin heads bind actin, initiate cross-bridge cycling, slide filaments, and contract the sarcomere.

Cardiomyocyte Relaxation Mechanics

To allow cardiac relaxation (diastole), cytosolic $Ca^{2+}$ concentrations must drop rapidly so calcium unbinds from troponin.

Unbinding of Calcium from Troponin

As cytosolic $Ca^{2+}$ levels drop, $Ca^{2+}$ unbinds from Troponin C.
Tropomyosin shifts back into its blocking position over actin's myosin-binding sites, terminating cross-bridge cycling and relaxing the myofibril.

Active Re-sequestration via SERCA2a ( $70\% - 80\%$ )

Approximately $70\% - 80\%$ of cytosolic $Ca^{2+}$ is actively pumped back into the sarcoplasmic reticulum lumen for storage by the SERCA2a pump (Sarco-Endoplasmic Reticulum $Ca^{2+}$ -ATPase).

Extracellular Calcium Removal via NCX and $\text{Na}^+/\text{K}^+\text{-ATPase}$ ( $20\% - 30\%$ )

The remaining $20\% - 30\%$ of $Ca^{2+}$ is extruded across the cell membrane into the extracellular fluid.
Sodium-Calcium Exchanger (NCX): Primary extrusion mechanism; an antiporter that exchanges $1\,Ca^{2+}$ ion outward against its concentration gradient for $3\,Na^+$ ions inward down their electrochemical gradient.
Maintenance of Sodium Gradient: The inward $Na^+$ gradient driving NCX function is maintained by the sarcolemmal $\text{Na}^+/\text{K}^+\text{-ATPase}$ pump, which uses ATP to pump $Na^+$ out and $K^+$ into the cell.
Minor extrusion also occurs via sarcolemmal plasma membrane $Ca^{2+}\text{-ATPase}$ (PMCA) pumps.

Based on Source 1 (the PowerPoint presentation Cardiac Muscle Physiology: Structure, Electrophysiology, and Excitation-Contraction Coupling by Clara Camargo, DVM), here are the answers to the 12 learning objectives:

Organization of the heart (chambers and layers):
- Location: Resides in the thoracic cavity within the mediastinum, protected by the ribs between the 3rd and 6th intercostal spaces.
- Chambers: Consists of four chambers: Right Atrium (receives deoxygenated blood from systemic circulation), Right Ventricle (pumps blood to the lungs), Left Atrium (receives oxygenated blood from lungs), and Left Ventricle (pumps oxygenated blood to systemic circulation under high pressure).
- Layers: Composed of three wall layers—Endocardium (inner lining), Myocardium (middle muscular layer), and Epicardium (outer visceral layer)—surrounded by the parietal pericardium containing lubricating fluid.
Three main types of cardiac cells and their functions:
- Working Myocardial Cells: Found in atrial and ventricular walls; striated muscle cells specialized for forceful contraction and mechanical work.
- Pacemaker Cells: Autorhythmic cells (mainly in SA and AV nodes) that spontaneously generate action potentials without neural stimulation.
- Conduction Cells: Specialized cells (e.g., Bundle of His, Purkinje fibers) optimized for rapid impulse transmission to coordinate heart contractions.
Similarities and differences between cardiac and skeletal muscle fibers:
- Similarities: Both are striated, share similar sarcomere arrangements (actin and myosin filaments), and contain T-tubules and sarcoplasmic reticulum.
- Differences: Cardiac muscle contraction is involuntary (skeletal is voluntary); cardiac myocytes are shorter, branched, and usually uninucleate (skeletal fibers are long, unbranched, and multinucleated syncytia); cardiac myocytes are interconnected end-to-end by intercalated disks.
Concept of functional syncytium:
- Unlike skeletal muscle, which forms a morphological syncytium through cell fusion during embryonic development, cardiac myocytes remain separate individual cells bounded by their own sarcolemma.
- Because they are connected electrically through low-resistance gap junctions, an electrical depolarization in one cell rapidly spreads throughout the entire tissue, allowing the myocardium to function as a single synchronized unit—a functional syncytium.
Intercalated disks:
- Dark, dense transverse bands at the junctions of adjacent myocytes, continuous with the sarcolemma. They contain two main types of intercellular junctions:
  - Gap Junctions: Protein channels that allow rapid ion diffusion and fast electrical impulse propagation.
  - Desmosomes: Strong mechanical junctions that anchor cell membranes together to withstand mechanical stress during contraction.
Generation of APs in the heart and factors affecting contraction:
- Action potentials normally originate spontaneously in the Sinoatrial (SA) node (the primary pacemaker located in the right atrium).
- Autonomic Regulation:
  - Sympathetic System: Releases Norepinephrine (NE) binding to $\beta_1$ receptors, increasing heart rate (positive chronotropy) and contractile force (positive inotropy).
  - Parasympathetic System: Travels via the Vagus nerve (CN X) and releases Acetylcholine (ACh) binding to $M_2$ receptors, decreasing heart rate (negative chronotropy).
Definition of pacemaker potential:
- A slow, spontaneous depolarization that gradually moves the membrane potential from its resting level (around $-60\,mV$ ) toward threshold (around $-40\,mV$ ), guaranteeing automatic, continuous action potential generation.
Sequence of events of the action potential in pacemaker cells:
- Funny Channels ( $I_f$ ): Open at negative potentials ( $-60\,mV$ ), allowing slow $Na^+$ influx to initiate depolarization.
- T-type $Ca^{2+}$ Channels: Open briefly near $-50\,mV$ , permitting transient $Ca^{2+}$ influx to push the potential to threshold ( $-40\,mV$ ).
- L-type $Ca^{2+}$ Channels: Open at threshold ( $-40\,mV$ ), allowing a major $Ca^{2+}$ influx that forms the rapid action potential upstroke.
- $K^+$ Channels: Open near peak depolarization; $K^+$ efflux repolarizes the cell back to $-60\,mV$ , restarting the cycle.
Sequence of events of the action potential in myocardial cells:
- Phase 4: Resting membrane potential maintained near $-90\,mV$ by $K^+$ leak channels.
- Phase 0: Fast voltage-gated $Na^+$ channels open, causing rapid $Na^+$ influx and depolarization.
- Phase 1: Fast $Na^+$ channels inactivate and transient $K^+$ channels open for a brief initial repolarization.
- Phase 2 (Plateau): L-type $Ca^{2+}$ channels open ( $Ca^{2+}$ enters) while delayed rectifier $K^+$ channels allow $K^+$ exit, creating an electrical balance.
- Phase 3: L-type $Ca^{2+}$ channels close; continued $K^+$ efflux repolarizes the membrane back to $-90\,mV$ .
The plateau phase:
- Sustained membrane potential near $0\,mV$ caused by the balance of inward $Ca^{2+}$ and outward $K^+$ currents.
- Importance: Extends the absolute refractory period so it coincides with mechanical contraction, preventing tetanic contractions, allowing full contraction/ejection, and ensuring time for full relaxation and chamber refilling.
Main sources of calcium for cardiac contraction:
- Extracellular Fluid (ECF): Supplies $20\% - 30\%$ of the required $Ca^{2+}$ entering via T-tubules.
- Sarcoplasmic Reticulum (SR): Supplies $70\% - 80\%$ of the required $Ca^{2+}$ released intracellularly.
Excitation-contraction coupling in cardiac muscle:
- Sequence: AP travels down T-tubules —→ Opens L-type $Ca^{2+}$ channels —→ Extracellular $Ca^{2+}$ enters —→ Triggers RyR2 channels on SR (Calcium-Induced Calcium Release / CICR) —→ SR releases a large burst of $Ca^{2+}$ (calcium spark/signal) —→ $Ca^{2+}$ binds Troponin C —→ Cross-bridge cycling and muscle contraction.
- Relaxation: Cytosolic $Ca^{2+}$ is pumped back into the SR via the SERCA2a pump ( $70\% - 80\%$ ) and extruded across the sarcolemma via the $NCX$ exchanger ( $20\% - 30\%$ ), maintained by the $\text{Na}^+/\text{K}^+\text{-ATPase}$ pump.