Ch. 28 - A. Stimulation of Cardiac Muscle Contraction

action potentials

• electrical signals created by changes in ion movement across cardiac cell membranes

  • ion movement: Na⁺, Ca²⁺, K⁺

• in the heart, action potentials:

  • trigger contraction of cardiac muscle cells

  • spread in a specific pathway

  • coordinate the timing of atria and ventricles

sinoatrial (SA) node

• 1st part of the cardiac conduction system through which the cardiac action potentials travel

• sinoatrial (SA) node: called the pacemaker because it sets the pace of cardiac action potential generation

• located in the wall of the right atrium just inferior to the opening of the vena cava

• action potential travels through both atria via gap junctions, resulting in simultaneous contraction of atria

atrioventricular (AV) node

• 2nd part of the cardiac conduction system through which the cardiac action potentials travel

• receives action potentials from the atrial muscle fibers

• located in the lower interatrial septum anterior to the opening of the coronary sinus

• slows the action potential so atrial contraction ends before ventricular contraction begins

atrioventricular (AV) bundle (bundle of His)

• 3rd part of the cardiac conduction system through which the cardiac action potentials travel

• electrical connection located in a membranous septum between the atria and ventricles superior to the interventricular septum

• only site where action potential can pass from atria to ventricles

• sends action potentials to the bundle branches

right and left bundle branches

• 4th part of the cardiac conduction system through which the cardiac action potentials travel

• sends action potentials to the Purkinje fibers

• located in the interventricular septum

• conducts action potential to apex of heart

subendocardial conduction network (Purkinje fibers)

• 5th part of the cardiac conduction system through which the cardiac action potentials travel

• sends action potentials to the ventricular cardiac muscle fibers and papillary muscles, then stimulates them to contract

• located in the apex of the myocardium, as well as in the lateral walls of the right and left ventricles

• action potential is conducted upward

• ventricular contraction pushes blood up toward semilunar valves

ECG

• records electrical changes in the entire heart muscles

• ECG records only voltage changes over time and not the force of cardiac muscle contraction

• ECGs are recorded by placing electrodes (leads) on the body surface

P wave

• first wave; small, curved upward deflection

• represents atrial depolarization that spreads from the SA node just before the atria contract

QRS complex

• short downward deflection (Q), tall upward reflection (R), medium downward deflection (S)

• represents ventricular depolarization that spreads from the AV node to the AV bundle, to right and left bundle branches, and to the subendocardial conduction network just before the ventricles contract

T wave

• medium, curved upward deflection

• represents ventricular repolarization and occurs just before the ventricles relax

P-Q (P-R) interval

• interval between the beginning of the P wave until the beginning of the Q wave

• represents the time it takes for the electrical conduction (excitation) to travel through the atria and AV node to the subendocardial conduction network

S-T segment

• segment from the end of the S wave to the beginning of the T wave

• represents the time the ventricular fibers are fully depolarized

Q-T interval

• interval between the beginning of the Q wave and the end of the T wave

• represents the time from the beginning of ventricular depolarization until the end of ventricular repolarization

normal sinus rhythm (NSR)

• in an adult, a heart rate of 60-100 beats/min is a normal sinus rhythm

tachycardia

• a heart rate above 100 beats/min is called tachycardia

  • in young children, this rate would be considered normal

bradycardia

• adults with heart rates below 60 beats/min are called bradycardia

  • normal for highly conditioned individuals

ventricular fibrillation

• prolonged tachycardia can develop into ventricular fibrillation:

  • rapid, uncoordinated heart contractions that do not pump blood

heart rate calculations using ECG

• standard ECG paper speed: 25 mm/sec

  • 5mm (one large square) = 0.2 sec

  • 1mm (one small square) = 0.04 sec

• heart rate (beats/min)

  • R wave # in 30 large squares (6 sec) x 10

• heart rate (beat/min)

  • 300 / # of large squares between two beats (R-R)