Rhythmical Excitation of the Heart

Specialized excitatory and conductive system of the heart

Automatic electrical rhythmicity of sinus fibers

Mechanism of sinus nodal rhythmicityiK: delayed rectifier

current slow voltage-gated K+

channels (activated by depolarization)

if: funny current Voltage-gated Na+

channels activated at hyperpolarization beyond -50 mV

iCa2+: L-type Ca2+

channel activated at Em higher

than -55 mV

Transmission of cardiac impulse through atria and internodal pathways

Ends of SA nodal fibers connect (gap junctions) direclty with:

1. Surrounding atrial muscle fibers (0.3 m/s)

2. Conduction fibers (1 m/s): Anterior interatrial band Anterior, lateral and posterior internodal pathways

Atrioventricular nodeThe only pathway of

cardiac impulse entry into ventriculi.

Delays the impuls transmission – slow conduction.

transmission in only one direction.

Transmission of cardiac impulse in ventriculi Fast transmission

through Purkinje fibers (1.5-4 m/s).

Subendocardial position of left and right bundle branch.

Penetration of their branches to 1/3 thickness of LV.

Then, connection with cardiac muscle fibers and slower conduction (0.3-0.5 m/s).

Sinus node- cardiac pacemakerDischarge rates: SA node: 70-80 /min AV node: 40-60 /min Purkinje fibers: 15-40 /min

Abnormal “ectopic” pacemakers increased discharge rate most often in AV-node

and Purkinje fibers. Or, blockage in A-V transmission.

Stokes-Adams syndrome sudden A-V bundle block synkope

Control of heart rhythmicity and impulse conductionRegulation by autonomic nervous system:

Parasympathetic innervation Sinus node, AV-node, lesser atria, ventriculi no supresseion of sinus node rhythmicity slows down conduction through AV-node (can cause

complete AV-block – ventricular escape) mechanism: hyperpolarization (-65 to-75 mV)

Sympathetic innervation whole heart (conduction and contractile fibers) β1-receptors: increase permeability to Ca2+ and Na+

Modulation of pacemaker rhythmicity