Transcript V. Electrocardiogram (EKG or ECG

Heart Contraction Overview
V. Electrocardiogram (EKG or ECG)- record impulses by
detecting electrical currents in heart; measures voltage (mV),
not contractions. Impulses lead to contractions.
1. Unique characteristics of cardiac muscle:
1.Stimulation- autorhythmicicity; depolarize spontaneously
2.Organ contraction- heart cells function as a unit as ions pass through
gap junctions
3.Absolute refractory period- inexcitable period
2. Events of Contraction:
1.Sodium ions rush into cells creating action potential from –90 mV to
+30 mV  depolarization.
2.Sarcoplasmic reticulum (SR) release calcium.
3.Sarcomere contracts by sliding filaments of actin and myosin.
4.Calcium channels close + potassium enters the cells + sodium leaves =
repolarization
5.resting potential back at –90 mV.
Sequence of Excitation: (takes .22 sec)
Sequence of the Heart Beat
1. SA node (AKA pacemaker)
– initiates sinus rhythm to
determine heart rate/
depolarization; atrial
excitation 
2. AV node – depolarization
spreads through gap
junctions 
3. AV Bundle of His –
ventricular excitation 
Right and Left Bundle
branches alone
interventricular septum 
4. Purkinje fibers- into
ventricular walls; excitation
complete
Sequence of excitation
Mechanism and events of a
contraction
Sequence of events is
similar to skeletal
muscle
1. Influx of sodium 
cells get +
2. Depol. moves down ttubules
3. SR releases Ca
4. Ca allows myosinactin interaction
5. Yada-yada
Electrocardiogram---ECG or EKG
• EKG
– Action potentials of all
active cells can be detected
and recorded
• P wave
– atrial depolarization
• P to Q interval
– conduction time from atrial
to ventricular excitation
• QRS complex
– ventricular depolarization
• T wave
– ventricular repolarization
Electrocardiography (EKG)
• 3 deflection waves
– 1st P wave: atrial
depolarization
– 2nd QRS complex:
Ventricle depol.
– 3rd T wave: ventricle
repolarization
P-Q normal 0.16 sec
Q-T normal 0.38 sec
Sample EKGs
Summary of events during cardiac
cycle