Question 2: Effect of Sympathetic and Parasympathetic Nervous System on Pacemaker Potentials

• Increases the slope of phase 4 depolarization - threshold is reached faster
• Result: increased heart rate (positive chronotropy)
• Also increases contractility (positive inotropy) and conduction velocity (positive dromotropy)
• In denervated hearts (no parasympathetic tone), the natural intrinsic rate is ~100 bpm

• Opens K⁺ channels (IKACh) - hyperpolarizes the cell
• Decreases the slope of phase 4 depolarization - takes longer to reach threshold
• Also decreases the funny current (If)
• Result: decreased heart rate (negative chronotropy)
• At baseline, parasympathetic dominance keeps resting HR at 60-100 bpm (below the intrinsic 100 bpm)

"Removal of parasympathetic tone or an increase in sympathetic innervation leads to an increase in rate of depolarization." - Harrison's Principles of Internal Medicine 22E

Question 3: Frank-Starling's Law - Definition and Clinical Importance

• The relationship between ventricular stroke volume and end-diastolic volume is called the Frank-Starling curve
• When regulated by changes in fiber length, this is called heterometric regulation
• In the heart, the preload = end-diastolic volume (EDV); afterload = resistance against which blood is expelled

"Starling pointed this out when he stated that the 'energy of contraction is proportional to the initial length of the cardiac muscle fiber'" - Ganong's Review of Medical Physiology, 26th Ed.

1. Cardiac compensation in heart failure: When the ventricle dilates (increased EDV), the heart initially maintains stroke volume by using the Frank-Starling mechanism - the basis of compensated heart failure
2. Matching output of left and right ventricles: If the right ventricle pumps more, more blood returns to the left, increasing its preload and thus automatically increasing its output to match
3. Response to exercise: Increased venous return during exercise stretches the ventricle, boosting cardiac output
4. Basis of preload optimization in ICU: Volume resuscitation targets optimizing the preload on the Frank-Starling curve
5. Valvular disease: In aortic or mitral regurgitation, ventricular dilation uses this mechanism for compensation until the heart decompensates (moves down the descending limb of the curve)

Question 4: Significance of Long Refractory Period in Cardiac Muscle - Why Cardiac Muscle Cannot Be Tetanised

• During phases 0 to 2 and approximately half of phase 3 (until membrane potential reaches ~-50 mV during repolarization), cardiac muscle is in its absolute refractory period - it cannot be excited again regardless of stimulus strength
• It remains in the relative refractory period until phase 4
• This refractory period lasts nearly as long as the entire contraction (~200-250 ms), in contrast to skeletal muscle where the refractory period (~1-2 ms) is far shorter than the twitch duration

"During phases 0 to 2 and about half of phase 3, cardiac muscle cannot be excited again; that is, it is in its absolute refractory period. It remains relatively refractory until phase 4. Therefore, tetanus of the type seen in skeletal muscle cannot occur." - Ganong's Review of Medical Physiology, 26th Ed.

• Prevents tetanic cardiac arrest: If the heart could tetanize, it would remain in sustained contraction with no relaxation phase - making ventricular filling impossible and causing immediate death
• Ensures rhythmic pumping: The long refractory period guarantees alternating contraction and relaxation, which is essential for filling (diastole) and ejection (systole)
• Basis of the "vulnerable period": Near the end of the relative refractory period (corresponds to the peak of the T wave on ECG), a stimulus can trigger a premature beat. This is the "R-on-T phenomenon" - delivering a shock during this window can precipitate ventricular fibrillation
• Explains why antiarrhythmics work by modifying refractory periods: Class Ia, Ic, and III drugs prolong the effective refractory period to terminate re-entrant arrhythmias

"Of course, tetanization of cardiac muscle for any length of time would have lethal consequences." - Ganong's Review of Medical Physiology, 26th Ed.

Regulation of Cardiac Output: Heterometric vs. Homometric

1. Heterometric Regulation (Starling's Mechanism)

Basis

"When cardiac output is regulated by changes in cardiac muscle fiber length, this is referred to as heterometric regulation." - Ganong's Review of Medical Physiology, 26th Ed.

Mechanism (molecular)

The Length-Tension Curve

Key features

Physiological examples

• Exercise: Increased venous return stretches the ventricle → greater SV
• Lying down: More venous return than standing → SV increases
• Balancing RV and LV output: If RV pumps more, LV receives more → LV stretches more → LV automatically pumps more to match

Clinical relevance

• Basis of compensated heart failure - dilated ventricle uses this mechanism to maintain CO temporarily
• Basis of fluid challenge/volume resuscitation in ICU - we are trying to move the patient up their Frank-Starling curve
• When the heart overdilates (beyond optimal length), the mechanism fails - this is decompensated heart failure (descending limb of the curve)

2. Homometric Regulation

"Regulation due to changes in contractility independent of length is sometimes called homometric regulation." - Ganong's Review of Medical Physiology, 26th Ed.

Mechanism

Key features

Factors that increase contractility (positive inotropes - upward curve shift)

1. Sympathetic stimulation / catecholamines (norepinephrine, epinephrine) - act via β₁ receptors → ↑ cAMP → ↑ intracellular Ca²⁺ → stronger contraction
2. Digitalis / cardiac glycosides - inhibit Na⁺/K⁺ ATPase → ↑ intracellular Na⁺ → ↓ Na⁺/Ca²⁺ exchanger activity → ↑ intracellular Ca²⁺
3. Xanthines (caffeine, theophylline) - inhibit phosphodiesterase → ↑ cAMP
4. Increased heart rate (Bowditch/Treppe effect) - more Ca²⁺ accumulates intracellularly with faster rates
5. Post-extrasystolic potentiation - the contraction following an ectopic beat is stronger (due to increased Ca²⁺ availability); this is independent of ventricular filling

Factors that decrease contractility (negative inotropes - downward curve shift)

1. Vagal stimulation - negative inotropic effect on atrial and (to a lesser extent) ventricular muscle
2. Heart disease / myocardial infarction - loss of contractile tissue
3. Beta-blockers, calcium channel blockers
4. Hypoxia, acidosis

Comparison Summary