Ch 9.2B -- Electrical Activity of the Heart -- VTPP 435

Ch 9.2B -- Electrical Activity of the Heart -- VTPP 435

Interatrial Pathway

-from SA node in right atrium to left atrium
-rapidly transmits action potential from SA node to left atrium
-allows both atria to contract simultaneously

Internodal Pathway

-from SA to AV node
-directs spread of action potential to make sure ventricles contract after atria

AV Nodal Delay

delay in impulse transmission between atria & ventricles at AV, which allows time for atria to depolarized and contract before ventricles contract

Cardiac Contractile Cells Action Potential Steps

1) Rising Phase -- Na+ channels activated at threshold
2) Peak Potential -- transient K+ channels open for quick brief K+ efflux; Na+ channels close
3) Plateau Phase -- L-type channels open for slow Ca2+ entry and transient and leaky K+ channels close
4) Falling Phase -- L-Ca2+ channels inactivated & delayed activation of regular K+ channels for rapid K+ efflux
5) Resting Phase -- K+ ordinary gate closes & K+ leaky channels open

Ca2+-Induced Ca2+ Release

when in cardiac muscle cells, excitation-induced entry of small amount of Ca2+ from ECF thru voltage-gated membrane receptors triggers opening of Ca2+ release channels in Sarcoplasmic reticulum

Excitation-Contraction Coupling in Cardiac Contractile Cells Steps

1) Action potential in cell travels down T-tubules
2) small ECF Ca2+ entry
3) ECF Ca2+ triggers Sarcoplasmic Reticulum to release a lot of Ca2+
4) Troponin-tropomyosin complex in thin filaments pulled aside
5) Cross-bridge cycling
6) Contraction occurs

Electrocardiogram (ECG) (EKG)

-graphic record of electrical activity that reaches surface of body as a result of cardiac depolarization & repolarization
-represents overall spread of activity throughout heart
-recording of that part of electrical activity present in body fluids from cardiac impulse that reaches body surface

3 Distinct ECG Waves (letters only indicate sequence of waves)

1) P-wave
2) QRS Complex
3) T-wave

P-wave

-represents atrial depolarization
-when wave of depolarization spreads across atria

QRS Complex

-represents ventricular depolarization
-ventricular depolarization
-big spike
-R wave deflection is dominant

T-wave

-represents ventricular repolarization

PR Segment

-during AV nodal delay
-time between end of P and onset of QRS

ST Segment

-time between QRS & T
-ventricles completely depolarized and plateau phase occurs before repolarization

TP Segment

-time after T wave and before next P wave
-heart muscle completely repolarized
-coincides with ventricles at rest and filling

Heart Rate (HR)

figured out form distance between 2 consecutive QRS complexes

Rhythm

regularity or spacing of ECG waves

Arrhythmia

any variation from normal rhythm & sequence of excitation of heart

Atrial Flutter (2:1 or 3:1 rhythm)

-rapid, but regular sequence of atrial depolarizations
-high atrial rate, but normal ventricular rate or above normal
-AV node can't respond
-ventricle can't keep pace with atria
-can lead to unconsciousness or death
-can cause pulse deficit

Atrial Fibrillation

-rapid, irregular, uncoordinated depolarization of atria with no definite P waves
-QRS shape is normal but occurs sporadically
-can cause pulse deficit

Pulse Deficit

-difference in HR and pulse rate when HR > Pulse rate
-heart beats faster than pulse felt

Ventricular Fibrillation

-multiple impulses travel erratically in all directions around ventricles
-ventricles become ineffective pumps
-ECG irregular with no pattern
-restore circulation or else irreversible brain damage or death
-treat with external cardiac compression or electrical defibrillation

Heart Block (2:1 or 3:1 block)

-arises from defects in cardiac conducting system
-atria rate normal, but ventricular rate low

Complete Heart Block

-complete dissociation between atrial and ventricular activity
-P waves normal
-QRS & T waves are regularly but independent of P waves

Cardiac Myopathies

damage of heart muscle

Myocardial Ischemia

inadequate oxygenated blood supply to heart tissue

Necrosis in Heart

occurs when blood vessel suppyling area of heart becomes blocked or ruptured

Acute Myocardial Infarction (Heart Attack)

-when part of area of heart's vessels supplying it are blocked or ruptured
-abnormal QRS have forms appear when heart tissue dies