Module 2 Review 2

Intercalated discs in cardiac muscle contain which two types of junctions that serve different functions?

• Desmosomes for mechanical coupling and gap junctions for electrical coupling

• Tight junctions for mechanical coupling and adherens junctions for electrical coupling

• Gap junctions for mechanical coupling and desmosomes for electrical coupling

• Tight junctions and gap junctions, both serving electrical coupling functions

A key difference between cardiac and skeletal muscle is that cardiac muscle:

• Contains actin and myosin arranged in sarcomeres

• Requires calcium for contraction to occur

• Has no motor units and not every cell is directly innervated by a nerve

• Uses ATP as its primary energy source

In healthy cardiac muscle, what percentage of ATP comes from oxidative metabolism versus glycolysis?

• 50% oxidative, 50% glycolysis

• ~98% oxidative, <2% glycolysis

• 75% oxidative, 25% glycolysis

• 85% oxidative, 15% glycolysis

Compared to peripheral tissues, myocardial tissue extracts what percentage of arterial oxygen?

• ~30%, the same as peripheral tissues

• ~25%, less than peripheral tissues

• ~40%, slightly more than peripheral tissues

• ~50%, significantly more than peripheral tissues

The mechanism of calcium-induced calcium release in cardiac muscle involves:

• Extracellular calcium entering through L-type channels triggering SR calcium release via ryanodine receptors

• Sodium entry triggering direct release from the sarcoplasmic reticulum

• Potassium efflux causing calcium channels to open automatically

• Gap junctions directly transferring calcium between adjacent cells

Epinephrine increases heart rate in the SA node by:

• Directly opening more HCN channels through membrane depolarization

• Activating β1 receptors → adenylate cyclase → increased cAMP → more HCN channel opening

• Inhibiting potassium channels and preventing repolarization

• Blocking acetylcholine receptors and removing parasympathetic inhibition

During the effective refractory period (ERP) of cardiac muscle:

• Some Na+ channels have recovered but cannot conduct an action potential to the next site

• All Na+ channels are completely closed and none have begun to recover

• The cell is more excitable than normal and requires a smaller stimulus

• Na+ channels are fully recovered and ready for normal depolarization

If the R-R interval on an ECG is 0.75 seconds, the heart rate is approximately:

• 75 beats per minute

• 90 beats per minute

• 60 beats per minute

• 80 beats per minute

During Phase 2 (plateau phase) of the ventricular action potential:

• Fast Na+ channels are the primary contributors to membrane potential

• Only K+ channels are active, causing rapid repolarization

• Ca2+ influx balances K+ efflux, maintaining a stable depolarized state

• The membrane potential rapidly returns to resting levels

In a parallel resistance circuit (like organ blood flow from the aorta):

• Total resistance equals the sum of individual resistances

• Blood flow is inversely proportional to resistance, so lower resistance vessels receive more flow

• All vessels receive equal blood flow regardless of their resistance

• Total resistance is always higher than any individual vessel resistance