Chapter 17: The Cardiovascular System: The Heart (PART 5)

The independent, but coordinated, activity of the heart is a function of what 2 things?

(1) the presence of gap junctions (2) the activity of the heart's "in-house" conduction system

Describe the electrical event in the heart that occurs during each of the following: (a) the QRS complex of the ECG (b) the T wave of the ECG (c) the P-R interval of the ECG

(a) The QRS wave occurs during ventricular depolarization. (b) The T wave of the ECG occurs during ventricular repolarization. (c) The P-R interval of the ECG occurs during atrial depolarization and the conduction of the action potential through the rest of the intrinsic conduction system.

What are Sinoatrial Nodes?

1. Specialized myocardial cell in the walls of the right atrium 2. Pacemaker of the heart

What is a second-degree heart block?

1. The AV node fails to conduct some SA node impulses 2. There are more P waves than QRS waves 3. In the tracing, there are usually 2 P waves for every QRS wave

What are arrhythmias?

Irregular heart rhythms, often caused by defects in the intrinsic conduction system

What is pacemaker potential?

Slow depolarization due to opening of sodium channels and closing potassium channels.

What is the atrioventricular (AV) node?

Specialized mass of conducting cells located at the atrioventricular junction in the heart.

Pacemaker cells have unstable resting potential that do what?

continuously depolarizes, drifting slowly toward threshold

The bulk of heart muscle is composed of what?

contractile muscle fibers

What are the right and left bundle branches?

courses along the interventricular septum toward the heart apex

The AV bundle connects the what?

atria to the ventricles

The impulses pause (0.1 sec) at the what?

atrioventricular (AV) node

From the AV node, the impulse sweeps to the what?

atrioventricular bundle

The healthy heart is supplied with what?

autonomic nerve fibers that alter its basic rhythm

What does the subendocardial conducting network do?

depolarizes the contractile cells of both ventricles

The sinus rhythm does what?

determines heart rate

The AV bundle is the only what?

electrical connection between the atria and ventricles

The crescent-shaped sinoatrial node is located where?

in the right atrial wall, just inferior to the entrance of the superior vena cava

What are pacemaker potentials (prepotentials)?

initiate the action potentials that spread throughout the heart to trigger its rhythmic contractions

The depolarization wave spreads via gap junctions throughout the atria and via the what?

internodal pathway

Ventricular contraction ejects some of the contained blood superiorly to what?

into the large arteries leaving the ventricles

What is the P-R interval?

is the time (about 0.16 s) from the beginning of atrial excitation to the beginning of ventricular excitation

. The pace set by the AV node is called what?

junctional rhythm

What is the P wave?

lasts about 0.08 s and results from movement of the depolarization wave from the SA node through the atria

Where is the AV node located?

located in the inferior portion of the interatrial septum immediately above the tricuspid valve

What is the cardioacceleratory center?

projects to sympathetic neurons in the T1-T5 level of the spinal cord

What is the QRS complex?

results from ventricular depolarization and precedes ventricular contraction

The short AV bundle splits into two pathways such as what?

right and left bundle branches

Long strands of barrel-shaped cells with few myofibrils are called what?

the subendocardial conducting network

What is the S-T segment?

when the action potentials of the ventricular myocytes are in their plateau phases, the entire ventricular myocardium is depolarized

What is a junctional rhythm?

1. 40 to 60 beats per minute 2. slower than sinus rhythm but still adequate to maintain circulation 3. A pace set by the AV node

What is depolarization?

1. Action potential begins when the pacemaker potential reaches threshold. 2. Due to Ca2+ influx through Ca2+ channels

Explain depolarization

1. At threshold (approximately −40 mV), Ca2+ channels open 2. Allows explosive entry of Ca2+ from the extracellular space. 3. In pacemaker cells, it is the influx of Ca2+ (rather than Na+) that produces the rising phase of the action potential and reverses the membrane potential

What is the atrioventricular bundle?

1. Bundle of specialized fibers that conduct impulses from the AV node to the right and left ventricles 2. Also called the bundle of His

Explain repolarization

1. Ca2+ channels inactivate 2. The falling phase of the action potential and repolarization reflect the opening of K+ channels and K+ efflux from the cell.

What is the intrinsic cardiac conduction system?

1. Consists of noncontractile cardiac cells specialized to initiate and distribute impulses throughout the heart 2. Depolarizes and contracts in an orderly, sequential manner

What is repolarization?

1. Due to Ca2+ channels inactivating and K+ channels opening. 2. Allows K+ efflux, which brings the membrane potential back to its most negative voltage

What is ventricular fibrillation?

1. Electrical activity is disorganized 2. Action potentials occur randomly throughout the ventricles 3. Results in chaotic, grossly abnormal ECG deflections Seen in acute heart attack and after an electrical shock

The long plateau in cardiac muscle has what 2 consequences?

1. It ensures that the contraction is sustained so that blood is ejected efficiently from the heart. 2. It ensures that there is a long absolute refractory period 3. Tetanic contractions cannot occur and the heart can fill again for the next beat

In skeletal muscle, every muscle fiber must be innervated so it can contract. In cardiac muscle, the intrinsic conduction system does not connect to every cardiac myocyte. 1. Why is this not necessary? 2. Which part of the intrinsic conduction system directly excites ventricular myocardial cells? 3. In which direction does the depolarization wave travel across the ventricles?

1. It is not necessary for the intrinsic conduction system to contact each individual cardiac myocyte. 2. This is because cardiac myocytes are connected to each other by gap junctions and so are depolarized when an adjacent myocyte depolarizes. 3. The subendocardial conducting network excites ventricular muscle fibers. 4. The depolarization wave travels upward from the apex toward the atria.

What is the subendocardial conducting network?

1. Modified ventricular fibers of the conduction system of the heart. 2. Also called Purkinje fibers

What are slow Ca2+ channels?

1. Na+-dependent membrane depolarization occurs 2. the voltage change also opens channels that allow Ca2+ to enter from the extracellular fluid

What's a junctional rhythm?

1. The SA node is nonfunctional 2. P waves are absent 3. The AV node paces the heart at 40-60 beats per minute

Let's say you're referring to graphs of action potentials. Two of these occur in the heart, and one occurs in skeletal muscle. 1. Which one comes from a contractile cardiac muscle cell? 2. A skeletal muscle cell? 3. A cardiac pacemaker cell? 4. For each one, state which ion is responsible for the depolarization phase and which ion is responsible for the repolarization phase.

1. The action potential (AP) is in a contractile cardiac muscle cell 2. The AP in a skeletal muscle cell 3. Represents an AP in cardiac pacemaker cells 4. The depolarization phase is due to Na+ influx in skeletal muscle and contractile cardiac muscle cells, and it is due to Ca2+ entry in cardiac pacemaker cells. K+ efflux is responsible for the repolarization in all action potentials.

explain pacemaker potential

1. The pacemaker potential is due to the special properties of the ion channels in the sarcolemma. 2. Hyperpolarization at the end of an action potential both close K+K+ channels and opens slow Na+Na+ channels. 3. The Na+Na+ influx alters the balance between K+K+ loss and Na+Na+ entry 4. The membrane interior becomes less and less negative (more positive).

What is an ectopic focus?

1. an abnormal pacemaker 2. may appear and take over the pacing of heart rate, or the AV node may become the pacemaker

What is the T wave?

1. caused by ventricular repolarization 2. lasts about 0.16 s

What is the Q-T interval?

1. lasting about 0.38 s 2. is the period from the beginning of ventricular depolarization through ventricular repolarization

What does the cardioinhibitory do?

1. sends impulses to the parasympathetic dorsal vagus nucleus in the medulla 2. sends inhibitory impulses to the heart via branches of the vagus nerves

Typical cardiac pacemaker cells are found in the what?

1. sinoatrial nodes 2. atrioventricular nodes

What are the 3 deflection waves of an EKG?

1. the P wave 2. the QRS complex 3. the T wave

Damage to any of these structures interferes with the ability of the ventricles to receive pacing impulses, and may cause what?

A heart block

What is a plateau?

A hump

The Ca2+Ca2+ surge across the sarcolemma prolongs the depolarization, producing a what in the action potential tracing?

A plateau

Defects in the intrinsic conduction system can cause irregular heart rhythms or what?

Arrhythmias

Cardiac muscle cannot go into tetany. Why?

Cardiac muscle cannot go into tetany because the absolute refractory period is almost as long as the contraction.

What is a fibrillation?

Condition of rapid and irregular or out-of-phase heart contractions

What do branch bundles do?

Conduct the impulses through the interventricular septum

The vagus nerve (parasympathetic) does what?

Decreases heart rate

1. Is due to Na+ influx through fast voltage-gated Na+ channels 2. A positive feedback cycle rapidly opens many Na+ channels 3. Reverses the membrane potential 4. Channel inactivation ends this phrase. What type of phase is this?

Depolarization

The electrical currents generated in and transmitted through the heart spread throughout the body and can be detected with a device called a what?

Electrocardiograph

A condition of rapid and irregular or out-of-sync contractions in which control of heart rhythm by the SA node is disrupted by rapid activity in other heart regions. What type of heart conditions describes this?

Fibrillation

What do sinoatrial (SA) nodes (pacemakers) do?

Generates impulses

What is an electrocardiogram (ECG)?

Graphic record of the electrical activity of the heart

What is a heart block?

Impaired transmission of impulses from atrium to ventricles resulting in abnormally slow heart rhythms

What does the sympathetic cardiac nerves do?

Increase heart rate and force of contraction

What is a normal sinus rhythm?

Normal ECG trace (sinus rhythm)

If the Q wave is visible (which is often not the case), it marks the beginning of ventricular excitation, and for this reason this interval is sometimes called the what?

P-Q Interval

1. Due to Ca2+ influx through slow Ca2+ channels 2. Keeps the cell depolarized because most K+ channels are closed What type of phase is this?

Plateau phase

The subendocardial conducting network is also called what?

Purkinje fibers

1. Due to Ca2+ channels inactiviating and K+ channels opening 2. Allows K+ efflux 3. Brings the membrane potential back to its resting voltage What type of phase is this?

Repolarization

An ECG is a composite of all the action potentials generated by nodal and contractile cells at a given time—not what?

a tracing of a single action potential

Pacemaker cells trigger what?

action potentials throughout the heart

About 1% of cardiac fibers are autorhythmic what?

cardiac pacemaker cells

Depolarization opens a few what in the sarcolemma, allowing extracellular Na+ to enter?

fast voltage-gated Na+Na+ channels

What are cardiac pacemaker cells?

having the special ability to depolarize spontaneously and so pace the heart

The sinoatrial (SA) node is what?

heart's pacemakers

The action potential has a what?

plateau

What is extrasystole?

premature heart contraction