A&P: The Heart Part B

Electrocardiogram (ECG or EKG):

1. A graphic recording of electrical activity. 2. Composite of all action potentials at given time; not a tracing of a single AP. 3. Electrodes are placed at various points on body to measure voltage differences and the most typical are 12 lead ECG's.

Difference between contractile muscle fiber and skeletal muscle fiber contractions:

1. AP in skeletal muscle lasts 1-2 ms; in cardiac muscle it lasts 200 ms. 2. Contraction in skeletal muscle lasts 15-20 ms; in cardiac contraction lasts over 200 ms.

Heart contracts as a unit:

1. All cardiomyocytes contract as unit (functional syncytium), or on contract. 2. Contraction of all cardiac myocytes ensures effective pumping action. 3. Skeletal muscles contract independently.

Tetanic contractions cannot occur in cardiac muscles:

1. Cardiac muscle fibers have longer absolute refractory period than skeletal muscle fibers. 2. Absolute refractory period is almost as long as contraction itself. 3. Prevents tetanic contractions. 4. Allows heart to relax and fill as needed to be an efficient pump.

The heart relies almost exclusively on aerobic respiration:

1. Cardiac muscle has more mitochondria than skeletal muscle so has greater dependence on oxygen (cannot function without oxygen) 2. Skeletal muscle can go through fermentation when oxygen not present. 3. Both types of tissues can use other fuel sources and cardiac is more adaptable to other fuels, including lactic acid, but must have oxygen.

Action potential initiation by pacemaker cells:

1. Cardiac pacemaker cells have unstable resting membrane potentials called pacemaker potentials or prepotentials.

Cardiac muscle cells:

1. Contains numerous large mitochondria (25-35% of cell volume) that afford resistance to fatigue. 2. Rest of cell volume composed of sarcomeres (Z discs, A bands, and I bands are all present). 3. T tubules are wider, but less numerous (enter cell only at Z disc). 4. SR simpler than in skeletal muscle; no triads.

Steps involved in AP:

1. Depolarization opens fast voltage-gated Na+ channels; Na+ enters cell. Positive feedback influx of Na+ causes rising phase of AP (from -90 mV to +30 mV). 2. Depolarization by Na+ also opens slow Ca2+ channels. -At 30+ mV, Na+ channels close, but slow Ca2+ channels remain open, prolonging depolarization which is seen as a plateau. 3. After about 200 ms, slow Ca2+ channels are closed, and voltage-gated K+ channels are open. - Rapid efflux of K+ repolarizes cell to RMP and Ca2+ is pumped both back into SR and out of cell into extracellular space.

Contractile muscle fibers make up bulk of heart and are responsible for pumping action:

1. Different from skeletal muscle contraction; cardiac muscle action potentials have plateau.

Microscopic anatomy of cardiac muscle fibers:

1. Intercalated discs which are connecting junctions between cardiac cells that contain desmosomes and gap junctions. 2. Intercellular space between cells has connective tissue matrix (endomysium) which contains numerous capillaries and connects cardiac muscle to cardiac skeleton, giving cells something to pull against.

Similarities of skeletal and cardiac muscle

1. Muscle contraction is preceded by depolarizing action potential. 2. Depolarization wave travels down T tubules; causes sarcoplasmic reticulum (SR) to release Ca2+. 3. Excitation-contraction coupling occurs. Ca2+ binds troponin causing filaments to slide.

Three parts of action potential:

1. Pacemaker potential: K+ channels are closed, but slow Na+ channels are open, causing interior to become more positive. 2. Depolarization: Ca2+ channels open (around -40 mV), allowing huge influx of Ca2+, leading to rising phase of action potential. 3. Repolarization: K+ channels open, allowing efflux of K+, and cell becomes more negative.

Coordinated heartbeat is a function of:

1. Presence of gap junctions. 2. Intrinsic cardiac conduction system. - Network of non-contractile (autorhythmic) cells. - Initiate and distribute impulses to coordinate depolarization and contraction of heart.

Differences between skeletal and cardiac muscle:

1. Some cardiac muscle cells are self-excitable. 2. Two kinds of myocytes: *Contractile cells which are responsible for contraction. *Pacemaker cells which are noncontractile cells that spontaneously depolarize; initiate depolarization of entire heart and don't need nervous system stimulation, in contrast to skeletal muscle fibers.

Benefit of longer AP and contraction:

1. Sustained contraction ensures efficient ejection of blood. 2. Longer refractory period prevents tetanic contractions.

Influx of Ca2+ from extracellular fluid triggers Ca2+ release from SR:

1.Depolarization opens slow Ca2+ channels in sarcolemma, allowing Ca2+ to enter cell. 2. Extracellular Ca2+ then causes SR to release its intracellular Ca2+. 3. Skeletal muscles do not use extracellular Ca2+.

What structure collects the depolarization wave from the atria to pass it onto the ventricles?

AV node

Which part of the intrinsic conduction system delays the impulse briefly before it moves on to the ventricles?

AV node

Contraction of the atria results from which wave of depolarization on the ECG tracing?

P wave

Which portion of the ECG represents the depolarization wave received by the atria from the SA node?

P wave

Which portion of an ECG represents the time during which the ventricles are in systole?

Q-T interval

During which portion of the ECG do the AV valves close, correlating with the first heart sound ("lub")

R-S waves

What part of the intrinsic conduction system generates the original stimulus that reaches the AV node, allowing it to conduct impulses to the AV bundle?

SA node

Which part of the conduction system initiates the depolarizing impulse, which spreads throughout the heart?

SA node

The order of impulse conduction in the heart, from beginning to end, is

SA node, Av node, bundle of His, bundle branches, Purkinje fibers

Adding a chemical that reduces Na+ transport near the SA node would have what effect on the heart's intrinsic conducting system?

The SA node would depolarize less quickly, reducing the HR

T/F: Myofibrils of cardiac muscle tissue vary in diameter and branch extensively.

True

Suppose a patient develops a myocardial infarction that disables the SA node. Would the heart still pump blood to the aorta and the pulmonary trunk?

Yes, because the atrioventricular node will still stimulate ventricular systole.

gap junctions

allow ions to pass from cell to cell; electrically couple adjacent cells. allows heart to be a functional synctytium, a single coordinated unit.

The P wave on an ECG represents

atrial depolarization

action potential

brief, large, all-or-none change in membrane potential that occurs in muscle and nerve cells. Regenerative signals that can propagate long distances along the membrane of a cell. also called impulses or spikes.

What's not a difference between cardiac and skeletal muscle?

cardiac muscle doesn't use the sliding filament mechanism for contraction; skeletal muscle does.

Depolarization

change in membrane potential such that the cell interior becomes less negative (more positive). characteristic of the initial phase of an action potential.

What can detect electrical currents generated by the heart?

electrocardiograph

When threshold is reached at the SA node (an autorhythmic cell), what channels open causing further depolarization of the membrane?

fast calcium

Action potentials generated by the autorhythmic cells spread to the contractile cells through what structures in the membrane?

gap junctions

Why are gap junctions a vital part of the intracellular connection of cardiac muscles?

gap junctions allow action potentials to spread to connected cells.

Desmosomes

hold cells together; prevent cells from separating during contraction

What structures connect the individual heart muscle cells?

intercalated discs

Cardioinhibitory center:

parasympathetic signals via vagus nerve to decrease rate. Inhibits SA and AV nodes via vagus nerves.

One of the changes that occurs in the pacemaker potential (unstable resting membrane potential) in the SA node (an autorhythmic cell) is a decreased efflux of what ion?

potassium

Cardioacceleratory center:

sends signals through sympathetic trunk to increase both rate and force. Stimulates SA and AV nodes, heart muscle, and coronary arteries.

What feature is found by both cardiac and skeletal muscle?

striations

In order to cause cardiac muscle contraction, the contractile cells must also depolarize. What causes the depolarization of the contractile cells?

the flow of positive ions from adjacent cells

What does the QRS complex represent in the ECG wave tracing?

ventricular depolarization

Repolarization of an autorhythmic cell is due to the opening of which channels?

voltage-gated potassium channels