Neural control of the heart

2026

📘 Innervation of the Heart: Regulation of Heart Rate and Blood Pressure

🖇 Central Regulation & Basic Innervation

📌 Cardiovascular Center Components

• Central regulation of heart rate involves the Cardiovascular Center
• Nerve supply consists of:
• • Sympathetic system → causes stimulation of heart rate
  • Parasympathetic system (Vagus nerve) → causes inhibition of heart rate

🖇 The Two Main Centers of Cardiovascular Regulation

📌 Cardiac Inhibitory Center (CIC)

Location & Function:

• Located in the Vagal Nucleus (Vagus Nucleus)
• Transmits signals via Vagus nerve (Parasympathetic system)

Effects:

• Stimulation of CIC → inhibition and decreased heart rate (Exam Point)
• Inhibition of CIC → increased heart rate

📌 Cardiac Accelerator Center (CAC)

Effects:

• Stimulation of CAC → increased heart rate (Exam Point)
• Inhibition of CAC → decreased heart rate
• Closely related to the Vasomotor Center

🖇 Heart Rate, Cardiac Cycle, and Pulse Relationship

📌 Key Correlations:

• Every Cardiac Cycle (depolarization and action potential) → produces one heartbeat
• Heartbeat → what is heard with stethoscope
• Every heartbeat → correlated with one arterial pulse
• Arterial pulse (pulsation) → what is felt
• Pulse serves as indicator of heart rate → if no pulse felt, there is no heart rate

🖇 The Vasomotor Center (VMC)

📌 Three Primary Areas:

🟠 1. Vasoconstrictor Center (VCC)

Mechanism:

• Causes vasoconstriction (narrowing of blood vessels)
• Narrowing increases pressure → increases heart rate/pulse (like narrowing end of hose)
• Vasoconstriction ultimately increases blood pressure (BP)

Cardiac Effect:

• Stimulation of VCC → leads to Tachycardia in the heart

🟠 2. Vasodilator Center (VDC)

Mechanism:

• Causes vasodilation
• Achieved primarily by inhibiting the Vasoconstrictor Center (VCC)
• VCC is considered the fundamental center
• VDC generally has no direct relationship with vasodilatory nerves (Exam Point)

🟠 3. Vasopressor Area

🚨 Clinical Application: Vasoconstriction & Catecholamines

📌 Atherosclerosis and Hypertension

• Patients with Atherosclerosis (hardening of arteries):
• • Complain of high blood pressure
  • Mechanism: narrowed arteries → VCC stimulation → increased BP
  • Treatment: medication to lower pressure

📌 Catecholamines

• Vasoconstriction + Adrenaline release → secretion of Catecholamines
• Catecholamines include:
• • Adrenaline (Epinephrine)
  • Noradrenaline (Norepinephrine)
• Both cause Tachycardia and increase heart rate

Contrast:

• Cholinergic drugs (like Acetylcholine) → reduce heart rate

🚨 Emergency Treatment

• Intracardiac Adrenaline → administered to patients who die in emergency room

🖇 Sensory Areas & Higher Center Control

📌 Sensory Areas (SA)

• Receive signals (sensory signals)
• Signals received from VCC or VDC
• Relate to blood pressure
• Influence: VCC, VDC, and CIC

📌 Hypothalamic Control

Anterior Hypothalamus:

• Affects Parasympathetic system
• Factors causing inhibition of heart rate (negative effects) associated with Anterior Hypothalamus

Posterior Hypothalamus:

• Affects Sympathetic system ("S" in Posterior relates to "S" in Sympathetic)
• Factors causing stimulation of heart rate associated with Posterior Hypothalamus (Exam Point)

🖇 Sympathetic vs Parasympathetic Actions on the Heart

🟣 Sympathetic System Effects:

• Increases heart rate
• Stimulates CAC
• Inhibits CIC
• Increases all cardiac properties
• Causes vasodilation
• Increases Cardiac Output

🟣 Parasympathetic System Effects:

• Decreases heart rate
• Stimulates CIC
• Decreases all cardiac properties
• Causes constriction of arteries
• Reduces Cardiac Output

🖇 Vagal Escape Phenomenon (VEP)

📌 Definition & Mechanism

• VEP = escape of ventricle from strong vagal stimulation
• Vagus nerve usually acts on ventricle to reduce heart rate

🚨 Clinical Relevance

• Occurs during conditions like Heart Block
• Ventricle operates independently through Idioventricular Rhythm
• Ventricle NOT supplied by Vagus nerve in this context
• NOT dependent on impulses from SA node or AV node

📌 Idioventricular Rhythm Characteristics

• Ventricle works at low rate: approximately 25 beats per minute
• Significantly lower than:
• • SA node: 90 bpm
  • AV node: 60 bpm

🟠 Protective Mechanism

• Low rate (Idioventricular Rhythm) = divine protective measure
• Allows patient to maintain circulation enough to move from home to hospital
• Provides safeguard against sudden death from angina or heart block

🚨 Clinical Example: MI and Sildenafil

📌 Clinical Presentation

• 40-year-old married man presents to ER late at night (3-4 AM)
• Complaint: gastric pain
• Doctor should immediately suspect Myocardial Infarction (MI)
• Perform ECG immediately

🟠 Important Note

• Many patients with cardiac issues complain of gastric symptoms
• Case cited: patient with stomach pain treated superficially → died next day due to MI

📌 Mechanism with Sildenafil (Viagra)

• Patient taking Sildenafil (Viagra) → causes vasoconstriction
• Vasoconstriction + stress of sexual intercourse → can precipitate:
• • MI
  • Heart block

🖇 Cardiac Vagal Tone

📌 Definition

• Vagal tone = presence of continuous inhibitory discharge from Vagus nerve
• Vagus nerve supplies ventricle
• Constantly sends inhibitory signals

🟠 Importance

• Essential during rest (Exam Point)
• Prevents diseases:
• • If SA node continuously active at high rates → hypertension or intracranial hemorrhage

📌 Mechanism During Rest

• Vagus nerve stimulates Cardiac Inhibitory Center
• Constantly works to:
• • Reduce pressure on heart
  • Lower heart rate
• Prevents:
• • Hypertension
  • Sudden catastrophic events (e.g., intracranial hemorrhage)

🖇 Vagal Tone Reflex Pathway (Exam Point)

📌 Mnemonic: "صراصير" (SARASIR)

1. S - Stimulus: Rest
2. R - Receptor: Baroreceptors located on:
3. • Carotid Sinus
   • Aortic Arch
3. A - Afferent: Nerve carrying signal:
4. • Sinus nerve
   • Aortic nerve
4. S - Center: Cardiac Inhibitory Center (location receiving signal)
5. E - Efferent: Vagus nerve (returns signal to heart)
6. R - Response: Decreased heart rate (or Organ)

🖇 Physiological Variations of Vagal Tone

📌 Vagal Tone is Higher in:

• Males > Females
• Athletes > Non-athletes
• Adults > Children

🖇 Sympathetic Tone (Pumping Power)

📌 Function & Mechanism

• Heart functions as pumping power
• Sympathetic system increases heart's pumping power by approximately 25%
• Increase exceeding 25% → can lead to hypertension (Exam Point)

📌 Mechanism of Action

• Sympathetic system achieves increased pumping power by:
• • Increasing heart rate
  • Stimulation of Vasomotor Center
  • Inhibition of Cardiac Inhibitory Center