Heart rate

2026

📘 Comprehensive Heart Rate Regulation Lecture Notes

🖇 I. Central Regulation: The Cardiovascular Centers

📌 Location & Components

• Central control of heart rate and blood pressure resides in Cardiovascular Centers located in Medulla Oblongata

🟣 Cardio Inhibitory Center (CIC)

• Source of Vagus nerve (Parasympathetic system)
• Function: Cardiac inhibition → decreases all properties of heart
• Activation of CIC → causes Bradycardia (decreased heart rate)

🟣 Vasomotor Center (VMC)

• Includes:
• • Vasoconstrictor Center (VCC)
  • Vasodilator Center
• VCC = origin of Sympathetic nerve system
• Actions leading to Tachycardia (increased heart rate) → involve stimulating VCC

🖇 II. Reflex Regulation from the Circulatory System

📌 A. Vagal Tone and Basic Heart Rate Control

SA Node Intrinsic Rate:

• SA node inherently discharges at 100-110 bpm
• Normal resting heart rate: 75 bpm (range: 60-90 bpm)

Mechanism of Reduction:

• Reduction from 110 bpm to 75 bpm due to SA node under influence of both:
• • Sympathetic nerve
  • Vagus nerve
• Vagus nerve dominates at rest (Vagal Tone)

Vagal Tone Definition:

• Continuous inhibitory discharge from Vagus nerve
• Operates constantly to rest the heart
• Prevents chronic hypertension (Exam Point)

📌 B. Mery's Law / Arterial Baroreflex (High Pressure Response)

Alternative Names:

• Also called Hering-Mayer's Law
• Also called Arterial Baroreflex

Basic Principle:

• Describes reflex response to increased blood pressure
• Stimulus: Increased Blood Pressure (e.g., Hypertension)
• Response: Reflex Bradycardia (decreased heart rate)

🟠 Reflex Pathway (Exam Point)

1. Receptor:
2. • Arterial Baroreceptors (pressure receptors)
   • Located in: Carotid Sinus and Aortic Arch
   • These areas contain highest blood volume
2. Afferent:
3. • Cranial nerves 9 and 10
   • Glossopharyngeal nerve (CN IX)
   • Vagus nerve (CN X)
3. Center:
4. • Afferent signal stimulates Cardio Inhibitory Center (CIC)
4. Efferent:
5. • Vagus nerve
5. Effect:
6. • Bradycardia → reducing heart rate

Significance:

• Protective reflex
• Prevents heart from overworking
• Conserves energy when blood pressure is high

Key Relationship:

• Inverse relationship between blood pressure and heart rate
• Increased BP → Decreased HR

📌 C. Hypotension Response (Low Pressure Response)

Stimulus:

• Decreased Blood Pressure (Hypotension)
• Example: standing up quickly → blood pooling in lower limbs

Response:

• Heart rate increases (Tachycardia)

Mechanism:

• Decreased BP → reduces afferent impulses that normally stimulate CIC
• This inhibits the CIC
• Allows Vasoconstrictor Center (VCC) to become dominant
• Results in:
• • Tachycardia
  • Vasoconstriction

Significance:

• Tachycardia accelerates blood flow
• Ensures adequate perfusion to tissues during hypotension

📌 D. Bainbridge Reflex (Volume Response - Right Side)

Key Concepts:

• Increased Venous Return (VR) and Tachycardia (Exam Point)
• Relates increased blood volume and venous return to heart rate

Stimulus:

• Increased Blood Volume OR Increased Venous Return
• Examples: intravenous fluids, exercise

🟠 Reflex Mechanism:

1. Increased VR → raises pressure in Right Atrium
2. Receptor: Stimulates Stretch Receptors (Type A Baroreceptors) in Right Atrium
3. Afferent: Via Vagus nerve
4. Center: Afferent signals stimulate Vasoconstrictor Center (VCC)
5. Efferent: Sympathetic nerves
6. Effect: Tachycardia

🟠 Bainbridge Effect (Local Mechanism)

• Alternative explanation
• Increased flow/volume of blood directly impacts and stimulates SA Node
• Causes Tachycardia WITHOUT full reflex pathway
• Local Effect

🟠 Gauer Reflex (Reverse Bainbridge)

• When Venous Return OR Right Atrial Pressure decreases
• Causes: hemorrhage/bleeding
• Result: Sympathetic system stimulated
• Effects:
• • Tachycardia
  • Vasoconstriction

📌 E. Ventricular and Coronary Reflexes (Left Side)

🟣 Ventricular Stretch Reflex

• High blood volume in Left side of heart
• Results in:
• • Bradycardia
  • Hypotension

🟣 Coronary Chemoreflex

Clinical Context:

• Occurs in patients with Myocardial Infarction (MI)

Mechanism:

• Dying cardiac muscle tissue → releases chemical substances (e.g., Serotonin)
• Chemicals stimulate receptors
• Send impulses to stimulate CIC
• Results in:
• • Bradycardia
  • Hypotension

🚨 Clinical Note

• Bradycardia and Hypotension often observed in patients presenting with MI

📌 F. Vagal Escape and Ventricular Autonomy

Vagus Nerve Distribution:

• Vagus nerve primarily influences Atria (SA and AV nodes)
• Does NOT supply the Ventricle
• Lack of vagal supply to ventricle = protective feature

Ventricular Autonomy:

• If primary pacemakers (SA/AV nodes) fail (e.g., heart block)
• Ventricle functions independently via Idioventricular Rhythm
• Impulse generated by Purkinje fibers

Idioventricular Rhythm Characteristics:

• Operates at very low rate: 25-40 bpm
• While low, sufficient to:
• • Maintain circulation
  • Allow patient to reach medical care
  • Prevent sudden death

Important Principle:

• Vagal stimulation CANNOT affect ventricular pumping power (contractility)

🖇 III. Nervous Regulation: Supra-Spinal Control and Other Sensory Inputs

📌 A. Supra-Spinal Control (Brain Influence)

Definition:

• Nervous signals originating above spinal cord
• Affect cardiac centers → regulate heart rate
• Do NOT affect heart directly

🟣 Cortex / Conditioning Reflex

Positive Stimuli:

• Seeing, hearing, or smelling (e.g., loved one, good news)
• Triggers signals from Cortex to VCC
• Stimulates Sympathetic system
• Causes Tachycardia (Exam Point)

Negative Stimuli:

• Meeting disliked person
• Triggers signals to CIC
• Causes Bradycardia

🟣 Voluntary Control

• Experts (e.g., yoga practitioners) can voluntarily control (slow down) heart rate
• Some people feign illness by voluntarily inducing slow heart rates (Bradycardia)

🟣 Hypothalamus and Emotions

Function:

• Hypothalamus and Limbic System control emotional responses
• Some emotions → Tachycardia
• Other emotions → Bradycardia

Extreme Emotional Shock:

• Example: sudden bad news
• Stimulates Hypothalamus → activates CIC
• Causes severe Vagal stimulation
• Results in Bradycardia
• Potentially leads to:
• • Shock
  • Syncope (due to inadequate cerebral perfusion)

📌 B. Respiratory Influence (Respiratory Sinus Arrhythmia)

Basic Effects:

• Inspiration (breathing in) → causes Tachycardia (increased heart rate)
• Expiration (breathing out) → causes Bradycardia

🟠 Mechanisms for Tachycardia During Inspiration (Exam Point)

1. Direct Radiation:
2. • Respiratory Center directly stimulates VCC
   • Inhibits CIC
2. Lung Inflation:
3. • Stretch receptors in expanded lungs
   • Send signals to stimulate VCC
3. Bainbridge Reflex:
4. • Increased venous return during inspiration contributes

📌 C. Muscle Reflexes

🟠 Al-Am-Smirk Reflex

• Occurs during exercise
• Proprioceptors (receptors in muscles) send afferent impulses
• Impulses stimulate VCC
• Results in Tachycardia

🟠 Tachycardia During Exercise - Combined Effects:

1. Al-Am-Smirk reflex
2. Bainbridge reflex
3. Sympathetic stimulation
4. Respiratory center stimulation

Anticipatory Heart Rate Increase:

• Heart rate increases BEFORE exercise starts (e.g., before race)
• Due to Conditioning Reflex (Cortex activation)

📌 D. Visceral and Somatic Inputs

🟣 Pain Effects (Skin and Viscera)

Mild or Chronic Pain:

• Leads to Vagal stimulation
• Results in Bradycardia

Severe Acute Pain:

• Causes Sympathetic stimulation
• Results in Tachycardia

🟣 Needle/Injection Fear

• Extreme fear (emotion) of injection
• Stimulates Vagus nerve (via Hypothalamus → CIC)
• Causes severe Bradycardia
• Potentially leads to syncopal attacks

🚨 Trigger Zones (Clinical Note)

Definition:

• Four areas in body highly sensitive to trauma
• Due to rich Parasympathetic innervation
• Hitting these zones → severe Vagal stimulation and shock

Four Trigger Zones:

1. Larynx
2. Epigastrium
3. Pericardium
4. Testes

🚨 Oculocardiac Reflex (Clinical Note)

Mechanism:

• Applying pressure on eye
• Transmits signals that stimulate Vagus nerve

Response:

• Severe Bradycardia

Clinical Use:

• Used to temporarily slow heart rate in conditions like:
• • Paroxysmal Atrial Tachycardia

🖇 IV. Physical and Chemical Regulation

📌 A. Temperature (Physical Regulation)

Temperature-Heart Rate Relationship:

• For every 1°C rise in body temperature
• Heart rate increases by 10 beats per minute (Exam Point)

Mechanism:

• Direct stimulation (increased metabolic activity) of SA Node
• OR activation of heat-regulating center in Hypothalamus

Cold Temperature Effect:

• Decreased temperature (cold) → causes Bradycardia

📌 B. Chemical Regulation (Gases)

🟣 Oxygen (Hypoxia)

Mild to Moderate Hypoxia:

• Causes stress condition
• Results in Tachycardia
• Mechanism: direct SA Node stimulation OR VCC stimulation

Severe Hypoxia:

• Causes Bradycardia
• Due to: central nervous system depression/damage

🟣 Carbon Dioxide (CO₂) (Exam Point)

• Typical response:
• • Initial Bradycardia
  • Followed by Tachycardia

📌 C. Hormones

🟠 Adrenaline (Epinephrine) (Exam Point)

• Effect depends on dose

Small Dose:

• Causes Tachycardia

Large Dose:

• Causes severe Hypertension (increased BP)
• Per Mery's Law: elevated BP triggers reflex Bradycardia

🟠 Noradrenaline (Norepinephrine)

• Causes severe Hypertension
• Results in:
• • Initial Bradycardia (via Mery's Law)
  • Followed by strong Tachycardia

🟠 Thyroxine (Thyroid Hormone)

• Causes Tachycardia

Mechanisms:

1. Increases sensitivity of heart to catecholamines (Adrenaline/Noradrenaline)
2. Enhances metabolic effects
3. Increases venous return
4. All contribute to Tachycardia

📌 D. Bile Salts

🚨 Clinical Note

• Patients with increased bile salts commonly exhibit Bradycardia
• Example causes: Cholecystitis (inflammation of gallbladder)

Mechanism:

• Bile salts exert direct inhibitory (depressant) action on SA Node

🖇 V. Summary of Key Exam Concepts

🟠 Most Common Exam Questions Cover:

1. Vagal Tone mechanism (Mery's Law/Arterial Baroreflex)
2. Bainbridge Reflex
3. Mery's Law (effect of hypertension)
4. Effect of Respiration (Inspiration/Expiration)
5. Effect of Adrenaline/Noradrenaline dose
6. Effect of Temperature
7. Effect of CO₂ and Oxygen

🖇 Quick Reference: SA Node Rates

• SA Node intrinsic rate: 100-110 bpm
• Normal resting HR: 75 bpm (range 60-90 bpm)
• AV Node rate: 60 bpm
• Idioventricular rhythm: 25-40 bpm