Cardiac Muscle, Conduction System, and Cardiac Cycle
Cardiac Muscle Histology
Cardiac muscle fibers, compared to skeletal muscle, are:
- Shorter in length
- Less circular in transverse section
- Branching with a "stair-step" appearance
- Typically 50–100 μm long and about 14 μm in diameter
- Connected by gap junctions, allowing the myocardium to contract as a single, coordinated unit
The Action Potential
The action potential is an electrical current that travels across the cell membranes of the heart, causing contraction. It is caused by the interaction of three ions: potassium (K+), calcium (Ca2+), and sodium (Na+).
Key Terms
| Term | Definition |
|---|---|
| Polarized | Inside of cell is more negative than the outside |
| Depolarization | Movement of charged particles across the membrane making the inside positive |
| Repolarization | Movement of ions restoring the inside of the cell to its negative charge |
| Resting Membrane Potential (RMP) | Electrical charge difference across the fibers of the heart cell at rest (~ -90 mV) |
Five Phases of the Action Potential
| Phase | Name | Key Events |
|---|---|---|
| Phase 0 | Depolarization | Threshold reached at -70 mV; Na+ moves rapidly into the cell through voltage-gated fast Na+ channels, causing a spike; Ca2+ moves in slowly through Ca2+ channels (causes contraction); K+ is leaving the cell |
| Phase 1 | Early repolarization | Na+ channels partially close, slowing Na+ inflow; voltage-gated K+ channels open; decreased positive ions create a negative deflection |
| Phase 2 | Plateau phase | Sustained contraction; Ca2+ slowly enters through Ca2+ channels (shifts graph positive); K+ continues to slowly leave through K+ channels (shifts graph negative) |
| Phase 3 | Final repolarization | K+ flows out rapidly through K+ channels; Na+ and Ca2+ channels close, stopping entry |
| Phase 4 | Resting state | RMP returns to -90 mV; voltage-sensitive ion channels return to pre-depolarized permeability; excess Na+ inside returns to normal; K+ inside returns to normal; additional Na+/Ca2+ pump removes extra Ca2+ |
Refractory Periods
| Period | Description | Phases |
|---|---|---|
| Absolute Refractory Period (ARP) | Cell will not respond to further stimulation | Phase 0, 1, 2, and part of 3 |
| Relative Refractory Period (RRP) | Some cells repolarized to threshold; can respond to a stronger than normal stimulus | Late phase 3 |
| Effective Refractory Period (ERP) | Includes ARP and first half of RRP; a conducted action potential cannot be generated | Through first half of RRP |
| Supranormal Period (SNP) | Weaker than normal stimulus can cause depolarization; extends from end of phase 3 to beginning of phase 4; can cause R on T phenomenon and dysrhythmias | End of phase 3 to beginning of phase 4 |
Cardiac Conduction System Terminology
| Term | Definition |
|---|---|
| Automaticity | Ability of SA node cells to generate an action potential without being stimulated |
| Excitability | Ability of a cell to reach threshold potential and respond to a stimulus (irritability) |
| Conductivity | Ability of a cardiac cell to receive and conduct an electrical impulse to an adjoining cell |
| Contractility | Ability of myocardial cells to shorten, causing contraction in response to electrical stimulus |
Cardiac Conduction System
The conduction system is composed of autorhythmic fibers that are self-excitable and repeatedly generate action potentials to trigger heart contractions.
Pathway and Components
| Component | Location | Function | Intrinsic Rate |
|---|---|---|---|
| Sinoatrial (SA) Node | Right atrial wall, inferior and lateral to SVC opening | Lead pacemaker; repeatedly depolarizes to threshold spontaneously (pacemaker potential) | 60–100 bpm |
| Bachmann's Bundle | From SA node to LA | Conducts impulses directly to the left atrium, causing simultaneous atrial contraction | — |
| Internodal Tracts (anterior, middle, posterior) | Right atrium | Conduct electrical impulse from SA node to AV node | — |
| Atrioventricular (AV) Node | Interatrial septum, anterior to coronary sinus opening | Delays impulse to allow atria to empty blood into ventricles, increasing stroke volume | 40–60 bpm |
| AV Bundle (Bundle of His) | Fibrous skeleton insertion | Only site where action potentials can conduct from atria to ventricles | — |
| Right and Left Bundle Branches | Interventricular septum toward apex | Conduct impulses to Purkinje fibers | — |
| Purkinje Fibers | From apex upward to ventricular myocardium | Rapidly conduct action potential from apex upward, causing contraction to begin at the apex | 20–40 bpm |
AV Node Functional Regions
- Atrionodal (AN) — transitional zone
- Nodal (N) — midportion
- Nodal-His (NH) — lower region merging with Bundle of His
The Cardiac Cycle
One cardiac cycle includes the systolic and diastolic phases of both the atria and ventricles. At a heart rate of 75 bpm, one cycle lasts 0.8 seconds.
Key Acronyms
| Acronym | Definition |
|---|---|
| EDV | End-diastolic volume (~130 mL) — blood volume at end of relaxation period |
| SV | Stroke volume (~70 mL) — volume ejected per beat from each ventricle |
| ESV | End-systolic volume (~60 mL) — blood remaining after contraction |
SV = EDV - ESV
Atrial Systole (0.1 sec)
- SA node depolarization causes atrial depolarization (P wave on ECG)
- Atria contract, forcing blood through open AV valves into ventricles
- Contributes a final ~25 mL to the ventricular volume
- At the end: each ventricle has ~130 mL (EDV)
Ventricular Systole (0.3 sec)
- AV node depolarization causes ventricular depolarization (QRS complex)
- Ventricles contract; pressure rises, forcing AV valves closed
- Isovolumetric contraction (~0.05 sec) — all four valves closed; muscle fibers contract but don't shorten
- When LV pressure exceeds aortic pressure (~80 mmHg) and RV pressure exceeds pulmonary trunk pressure (~20 mmHg), SL valves open
- Ventricular ejection (~0.25 sec): LV reaches ~120 mmHg, RV reaches ~25-30 mmHg
- Each ventricle ejects ~70 mL into aorta/pulmonary trunk
- T wave marks ventricular repolarization
Ventricular Diastole (0.4 sec)
- Ventricles relax; pressure falls
- Blood in aorta/pulmonary trunk flows backward, closing SL valves
- Dicrotic notch — rebound of blood off closed aortic valve cusps
- Isovolumetric relaxation — brief period with all four valves closed
- When ventricular pressure drops below atrial pressure, AV valves open
- Ventricular filling begins; majority of blood rushes in rapidly
Cardiac Output
Cardiac output (CO) is the volume of blood ejected from the ventricle per minute.
CO = SV × HR
In a typical resting adult male:
- SV = 70 mL/beat, HR = 75 bpm
- CO = 70 mL × 75 bpm = 5,250 mL/min = 5.25 L/min
Cardiac Reserve
The difference between maximum CO and resting CO. Average person has a cardiac reserve of 4–5 times resting value. Top endurance athletes may have 7–8 times. Severe heart disease may result in little to no cardiac reserve.
Regulation of Stroke Volume
| Factor | Description |
|---|---|
| Preload | Degree of stretch on the heart before contracting (at end of diastole). Frank-Starling law: greater stretch = larger volume = stronger contraction |
| Contractility | Forcefulness of contraction of individual ventricular muscle fibers. Positive inotropic agents (e.g., increased Ca2+) increase contractility; negative inotropic agents (e.g., calcium channel blockers) decrease it |
| Afterload | Pressure that must be exceeded before ejection can occur. RV overcomes pulmonary artery pressure (~20 mmHg); LV overcomes aortic pressure (~80 mmHg). Increased afterload (hypertension, atherosclerosis) decreases stroke volume |