anatomy 2 modules 2

What is the myocardium?

The cardiac muscle tissue that makes up the heart wall and pumps blood.

What type of muscle is found only in the heart?

Cardiac muscle.

What unique property does cardiac muscle have that skeletal muscle does not?

Autorhythmicity (can generate its own electrical impulses).

Define autorhythmicity.

The ability of cardiac muscle cells to spontaneously depolarize and generate action potentials.

Can cardiac muscle contract without nervous stimulation?

Yes, because of autorhythmicity.

Which body systems modify heart rate despite autorhythmicity?

The autonomic nervous system and endocrine system.

What percentage of heart cells are contractile cells?

99%

What percentage of heart cells are conducting cells?

1%

Function of myocardial contractile cells?

Produce force to pump blood.

Function of myocardial conducting cells?

Generate and conduct electrical impulses.

Which specialized conducting cells rapidly distribute impulses through the ventricles?

Purkinje fibers.

Which heart cells resemble neurons in function?

Myocardial conducting cells.

What is the contractile unit of cardiac muscle?

Sarcomere.

A sarcomere extends from what structure to what structure?

Z line to Z line.

What happens to the Z lines during contraction?

They move closer together.

What causes the striated appearance of cardiac muscle?

Alternating A bands and I bands.

What are T-tubules?

Invaginations of the sarcolemma that rapidly conduct action potentials.

Where are cardiac T-tubules located?

At the Z lines.

Where are skeletal muscle T-tubules located?

At the A-I junction.

Which muscle has fewer T-tubules?

Cardiac muscle.

Why does cardiac muscle contract more slowly than skeletal muscle?

It depends on extracellular calcium entering the cell.

Where does most calcium come from during cardiac contraction?

Extracellular fluid.

How much of the cell volume is occupied by mitochondria in cardiac muscle?

About 25%.

Why are cardiac muscle cells packed with mitochondria?

They require continuous ATP production.

How many nuclei do cardiac muscle cells usually have?

One central nucleus.

How many nuclei do skeletal muscle fibers have?

Many (hundreds).

Describe the shape of cardiac muscle cells.

Short, branched cells.

What connects adjacent cardiac muscle cells?

Intercalated discs.

Function of intercalated discs?

Hold cells together and coordinate contraction.

Two major structures found in intercalated discs?

Desmosomes and gap junctions.

Function of desmosomes?

Prevent cells from pulling apart during contraction.

Function of gap junctions?

Allow ions to pass directly between cells.

Why are gap junctions important?

They synchronize cardiac contraction.

Cardiac muscle metabolism is primarily…?

Aerobic.

Primary fuels used by cardiac muscle?

Fatty acids and glucose.

Cardiac muscle stores oxygen in what protein?

Myoglobin.

Why is myoglobin important?

Provides oxygen during increased demand.

Main energy molecule for contraction?

ATP.

Where is most ATP produced?

Mitochondria.

Which metabolic pathway requires oxygen?

Oxidative phosphorylation.

Which metabolic pathway produces acetyl-CoA from glucose?

Glycolysis.

Which pathway breaks down fatty acids?

Beta oxidation.

Where does the Krebs cycle occur?

Mitochondria.

What is produced by the Krebs cycle?

NADH and FADH2.

Where is ATP produced from NADH and FADH2?

Electron transport chain.

What happens if oxygen supply decreases?

ATP production falls and heart function declines.

Cardiac muscle relies on aerobic metabolism because…?

It requires continuous ATP production.

Which muscle fatigues first: skeletal or cardiac?

Skeletal muscle.

Can cardiac muscle undergo tetanus?

No.

Why can't cardiac muscle undergo tetanus?

Long refractory period prevents sustained contractions.

What triggers contraction in skeletal muscle?

Somatic motor neurons.

What triggers contraction in cardiac muscle?

Autorhythmic pacemaker cells.

What ion enters from outside the cell to trigger cardiac contraction?

Calcium.

Cardiac muscle uses calcium from where?

Extracellular fluid AND sarcoplasmic reticulum.

Skeletal muscle uses calcium from where?

Sarcoplasmic reticulum only.

Why is blood supply to cardiac muscle extensive?

Continuous oxygen demand.

Cardiac muscle contractions are…?

Rhythmic with brief relaxation periods.

Cardiac muscle depends heavily on what organelle?

Mitochondria.

What protein does calcium bind to during contraction?

Troponin.

What happens when calcium binds troponin?

Tropomyosin moves away from actin binding sites.

What protein blocks actin binding sites at rest?

Tropomyosin.

What forms after myosin binds actin?

Cross bridges.

What powers the power stroke?

ATP hydrolysis.

What causes myosin to detach from actin?

Binding of a new ATP molecule.

What happens when intracellular calcium falls?

Muscle relaxes.

Which band shortens during contraction?

H band.

Which structure anchors thick filaments?

M line.

Which structure anchors thin filaments?

Z line.

What is excitation-contraction coupling?

The process linking an action potential to muscle contraction.

What percentage of contraction calcium enters through calcium channels?

About 20%.

Where does the remaining calcium come from?

Sarcoplasmic reticulum.

What is the function of the cardiac conduction system?

Generate and conduct electrical impulses that coordinate heart contractions.

What are the components of the cardiac conduction system in order?

SA node → AV node → Bundle of His → Right & Left Bundle Branches → Purkinje Fibers.

What is the normal pacemaker of the heart?

Sinoatrial (SA) node.

Where is the SA node located?

Superior posterior wall of the right atrium near the opening of the superior vena cava.

Why is the SA node called the pacemaker?

It has the fastest spontaneous depolarization rate.

Normal intrinsic firing rate of the SA node?

60–100 beats/min.

What rhythm does the SA node establish?

Sinus rhythm.

What happens after the SA node fires?

The impulse spreads through both atria.

What electrical event does the SA node initiate?

Atrial depolarization.

What mechanical event follows atrial depolarization?

Atrial contraction (atrial systole).

What are internodal pathways?

Specialized conduction pathways that carry impulses from the SA node to the AV node.

How long does it take an impulse to travel from the SA node to the AV node?

Approximately 50 milliseconds.

Why doesn't the impulse spread directly from the atria into the ventricles?

The cardiac skeleton electrically insulates the atria from the ventricles.

What is the only normal electrical connection between the atria and ventricles?

The AV node.

Where is the AV node located?

Inferior right atrium within the atrioventricular septum.

Function of the AV node?

Delays electrical conduction before sending impulses to the ventricles.

Why is the AV node delay important?

It allows the atria to finish emptying blood into the ventricles before ventricular contraction.

How long is the AV node delay?

Approximately 100 milliseconds.

What causes the AV node delay?

Small conducting cells and slower conduction between cells.

Maximum conduction rate of the AV node?

Approximately 220 impulses/min.

What happens after the AV node?

The impulse enters the Bundle of His.

Another name for the AV bundle?

Bundle of His.

Where is the Bundle of His located?

In the interventricular septum.

What does the Bundle of His divide into?

Right and left bundle branches.

Which bundle branch is larger?

Left bundle branch.

Why is the left bundle branch larger?

The left ventricle is much larger.

What do the bundle branches conduct?

Electrical impulses toward the apex of the heart.

Where do the bundle branches terminate?

Purkinje fibers.

Function of Purkinje fibers?

Rapidly distribute impulses throughout ventricular myocardium.