Specialized excitatory and conductive system of the heart.

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