Exercise 31Conduction System of

the Heart

&

Electrocardiography

Objectives

• Parts of the intrinsic conduction system of the heart

• P, QRS, T waves on an ECG

• Tachycardia, bradycardia, fibrillation

Intrinsic Conduction System

• Intrinsic = heart beats without nervous system sending impulses to initiate contractions

• Heartbeat is partially regulated by autonomic nervous system (ANS) (↑↓ heartbeat), and partially by this intrinsic conduction system

Intrinsic Conduction System

• Nodal system

• Makes sure heart muscle beats as a coordinated unit and in orderly fashion (atria to ventricles)

• As the signal passes through chambers of the heart, causes contraction…once it’s passed, chamber relaxes

SA Node (SinoAtrial)– In right atrium– Just below

superior vena cava entrance

– Provides stimulus for contraction, sets rate for heart as a whole

– “pacemaker”

Fig. 20-13

AV Node (AtrioVentricular)

–In lower atrial septum

–At atria/ventricle junction

Fig. 20-13

Atrioventricular (AV) bundle

– (Bundle of His)

– Within the interventricular septum

Fig. 20-13

Bundle Branches

• Right and left– Also within the

interventricular septum

Fig. 20-13

Purkinje Fibers

– Within ventricular walls

– More dense in left ventricle (larger chamber)

Fig. 20-13

Intrinsic Conduction System: Impulse Propagation

SA Node

Signal to Atria (R, L; pushes blood pushes blood

into into

ventriclesventricles)

AV bundle/Bundle of His

AV node (bridge);

pause to allow ventricles to

fill

Right bundle branch

Left bundle branch

Purkinje fibers

Ventricular Ventricular contraction pushes contraction pushes

blood out of the blood out of the heartheart

Fig. 20-11

Electrocardiography (ECG)

• Electrical currents eventually spread throughout whole body

• Detected by ECG on body’s surface

• 3 recognizable waves

Electrocardiography (ECG)

• 3 recognizable waves– P wave

• Atrial depolarization (change in charge across cell membrane…action potential…)

• Atrial contraction

– QRS complex• Ventricular depolarization & contraction

– T wave• Ventricular repolarization & relaxation

Fig. 20-14

Electrocardiography (ECG)

• TachycardiaHeart rate >100 bpm (beats per minute)

• FibrillationFrom prolonged tachycardiaRapid uncoordinated heart contractionsMakes the heart useless as a pump

• BradycardiaHeart rate <60 bpmAthletes—good thing—more efficient