Surface anatomy of heart

SA node → internodal pathways → AV node → Bundle of His → RBB & LBB → Purkinje fibres → ventricles

• Location: Interatrial wall (between right & left atrium)
• Function: Rapid conduction from SA node (Sinoatrial node) to AV node (Atrioventricular node)
• Cells: Modified cardiac muscle cells + Purkinje-like fibres

Types
• Anterior internodal pathway
• Passes in front of SVC (Superior Vena Cava) opening
• Conducts to AV node
• Gives Bachmann bundle → conducts impulse to left atrium
• Middle internodal pathway (Wenckebach bundle)
• Passes behind SVC (Superior Vena Cava)
• Conducts to superior part of AV node
• Posterior internodal pathway (Thorel bundle)
• Passes behind SVC (Superior Vena Cava)
• Conducts to posterior part of AV node

• Site: Membranous IVS (Interventricular Septum)
• Function: Only electrical connection between atria and ventricles
• Divides into:
• RBB (Right Bundle Branch) → right ventricle
• LBB (Left Bundle Branch) → left ventricle

• Origin: AV bundle in membranous IVS (Interventricular Septum)
• Structure: Narrow fascicles
• Course:
• Runs within myocardium
• Moves to subendocardial layer
• Travels along right side of IVS
• Passes through moderator band (septomarginal trabecula)
• Supplies anterior papillary muscle & right ventricular wall

• Origin: AV bundle in membranous IVS (Interventricular Septum)
• Structure: Flattened sheets of fascicles
• Course:
• Left half of muscular IVS
• Subendocardial space
• Toward ventricular apex
• Trifurcates into:
• Anterior division
• Septal division
• Posterior division
• Supplies left ventricular wall, IVS & papillary muscles

• Location: Subendocardial layer of the ventricles (just beneath the endocardium)
• Cell size: Much larger than surrounding ventricular muscle cells
• Arrangement: Run as bundles and branches throughout the ventricular walls
• Course:
• Reach the cardiac apex
• Then curve upward and backward through ventricular walls
• Cell connections:
• Contain many more gap junctions than:
• Atrioventricular nodal cells
• Ordinary ventricular myocytes
• Conduction speed:
• 6× faster than ventricular muscle fibres
• 150× faster than atrioventricular nodal fibres
• Function:
• Rapid, synchronized activation of both ventricles
• Ensures coordinated ventricular contraction from apex → base

• Supplied by Autonomic Nervous System
• Sympathetic fibres
• Parasympathetic fibres
• Nerves form cardiac plexuses
• Located below the arch of the aorta

• Origin: Cervical and upper thoracic segments of the sympathetic trunks
• Effect:
• Increases heart rate
• Increases conductivity
• Increases contractility

• Origin: Vagus nerve
• Effect:
• Decreases heart rate
• Decreases conductivity
• Decreases contractility

network of autonomic nerves that supplies the heart.
Two cardiac plexuses
1. Superficial cardiac plexus
2. Deep cardiac plexus

• Location:
• Below the arch of the aorta
• In front of the right pulmonary artery
• Sympathetic input:
• Left sympathetic chain
• Superior cervical cardiac branch
• Parasympathetic input:
• Left vagus nerve
• Inferior cervical cardiac branch

• Location:
• In front of the bifurcation of the trachea
• Behind the arch of the aorta
• Sympathetic input:
• All cervical and thoracic ganglia of sympathetic chains
• Cardiac branches
• Parasympathetic input:
• Vagus nerve
• Recurrent laryngeal nerve (branch of vagus nerve)
• Distribution:
• Divides into right and left halves
• Supplies:
• Corresponding coronary plexuses
• Pulmonary plexuses

Autonomic nerves → cardiac plexuses (superficial & deep) → coronary vessels, myocardium & conducting system