specialized excitatory and conductive system of the heart
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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
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