Download BIOL 245 Heart 2 I. Conduction System

BIOL 245
Heart 2
I. Conduction System - specialized cardiac muscle fibers
A. Sinoatrial node (SA)
1. “roof” of right atrium
2. pacemaker
3. impulses segregated from ventricles by “fibrous skeleton”
B. AP generated in SA node (spontaneously)
1. begins following complete ventricular relaxation
a. pacemaker potential
- slow, spontaneous depolarization
- triggered by slow influx of Na+
b. fast calcium channels open at threshold (-40 mV)
c. repolarization via outflow of K+ (-60mV)
d. new pacemaker potential initiated
e. AP’s from SA node depolarize myocardial cells to threshold
2. SA block--> AV node can become pacemaker
3. ectopic beats: AP’s initiated in other cardiac cells
C. Atrioventricular node (AV)
1. “floor” of rt. atrium near interatrial septum
2. AP’s come from atria
3. spead slowly through AV node
4. spread from node to AV bundle (of His)
D. Bundle branches (rt & left)
1. travel in interventricular septum to apex
2. form Purkinje fibers at their tips --> cardiac muscle
II. Contractile Myocytes
A. Depolarization
1. similar to neurons
2. RMP about –90mV
3. Na+ influx depolarizes to threshold
4. Na+ influx for rising phase of action potential
5. Ca++ released from SR
additional Ca++ through slow calcium channels (from ECF)
B. Action potential
1. prolonged
- continued Ca++ influx after closing of Na+ channels
2. more sustained contraction than skeletal muscle twitch
3. absolute refractory period of 250 msec (1-2 msec in skeletal muscle)
III. Electrocardiogram
A. Electrical currents of heart
1. electrical events
2. correlates with mechanical event
B. Wave patterns
1. P wave
a. depolarization of atrial myocardium
- reflects potential differences between depolarized and
unstimulated regions of the atria
- spread of AP from SA node
b. initiated just prior to contraction
c. atrial systole during P-Q segment
2. QRS complex
a. depolarization of the ventricles
b. initiated just prior to contraction
c. atrial repolarization occurs here
d. 1st heart sound at end of QRS
e. systole during S-T segment
3. T wave: repolarization of ventricles
2nd heart sound at end of T wave
C. Heart rate
1. bradycardia (< 60 bpm)
2. tachycardia (> 100 bpm)
a. both usually a normal occurrence
b. V-tac: ventricles beat rapidly and independently of atria
3. flutter: rapid but coordinated contractions (200-300 bpm)
4. fibrillation: uncoordinated contractions
a. atrial:
b. ventricular:
c. causes:
- enlarged heart
- shock during T wave
- damage to muscle tissue
d. defibrillation
5. heart block: (conduction slowed at AV node - damage)
a. only a portion of impulses conducted
b. complete
- ventricles paced by ectopic pacemakers
- beat at abnormally slow rate, dissociated from SA node