cardiac pressure-volume cycle

Question 1

Why is the lipid bilayer impermeable to ions?

Answer 1

The lipid bilayer of the cell membrane is impermeable to ions due to its hydrophobic interior.

Question 2

What are the most common ions that pass through ion channels, and what is the specificity of most ion channels?

Answer 2

The most common ions that pass through ion channels are potassium (K+), sodium (Na+), and calcium (Ca2+). Most ion channels are specific for only one type of ion.

Question 3

What are voltage-gated channels, and how do they open?

Answer 3

Voltage-gated channels are ion channels that open when the membrane voltage (Vm) becomes positive. This opening is triggered by depolarization of the membrane.

Question 4

What are time-dependent channels, and how do they open?

Answer 4

Time-dependent channels are ion channels that open with a delay or close after a set time. These channels may open in response to a stimulus, such as depolarization of the membrane.

Question 5

What are inward rectifier K+ channels, and what is their function?

Answer 5

Inward rectifier K+ channels open when Vm goes below -60 mV, and they are more open when cells are at rest. Their function is to clamp membrane potential (Vm) at rest and let K+ out of the cell, which repolarizes it.

Question 6

What are delayed rectifier K+ channels, and how do they open?

Answer 6

Delayed rectifier K+ channels open when the membrane depolarizes. All gating takes place with a delay.

Question 7

What is the resting membrane potential, and what is the dominant current at rest?

Answer 7

The resting membrane potential of a cell is around -70 mV, and the dominant current is the outward flow of potassium (K+) ions through inward rectifier K+ channels.

Question 8

What are the events that can trigger depolarization of a cell?

Answer 8

Depolarization of a cell can be triggered by intrinsic depolarization, depolarization of a nearby cell, or synaptic transmission through a neurotransmitter.

Question 9

What happens when a cell becomes depolarized, and how does this lead to an action potential?

Answer 9

When a cell becomes depolarized, a few sodium (Na+) channels open, leading to an increase in Na+ permeability and a flow of Na+ ions into the cell. This further depolarizes the cell and creates a positive feedback loop. When the voltage reaches a threshold voltage of around -50 mV, the cell is committed to an action potential, which is an all-or-nothing event.

Question 10

What happens when the membrane potential of a cell becomes positive during an action potential?

Answer 10

When the membrane potential of a cell becomes positive during an action potential, the inward flow of Na+ ions through open sodium channels continues, which further depolarizes the cell and drives the membrane potential closer to 0 mV.

Question 11

What is the threshold voltage for an action potential, and why is it important?

Answer 11

The threshold voltage for an action potential is around -50 mV. It is important because when the voltage reaches this level, the cell is committed to an action potential, and the positive feedback loop of increasing Na+ channel conductance continues.

Question 12

What are the two time-dependent events that occur during repolarisation of the basic action potential?

Answer 12

The two time-dependent events that occur during repolarisation of the basic action potential are Na+ channel inactivation, which results in a decrease of Na+ current going into the cell, and the opening of delayed rectifier K+ channels, which increases the outflow of K+ from the cell.

Question 13

What happens during the after hyperpolarisation phase of the basic action potential?

Answer 13

During the after hyperpolarisation phase of the basic action potential, the voltage goes below -60 mV, causing inward rectifier K+ channels to open again and stay open until the next depolarisation. This results in an increase of K+ permeability and a decrease of Na+ permeability, which causes the membrane potential (Vm) to move closer to the equilibrium potential for K+ (EK) and clamp Vm toward EK.

Question 14

What is the refractory period in the basic action potential?

Answer 14

The refractory period is the period during which the cell cannot reinitiate an action potential. It is the amount of time it takes for the cell membrane to be ready for a second stimulus once it returns to its resting state. The refractory period occurs mostly during the after-hyperpolarisation phase of the action potential.

Question 15

What ion is responsible for the depolarization of the cardiac action potential in Phase 0?

Answer 15

Sodium (Na+) ions.

Question 16

What ion channels are responsible for the transient outward current in Phase 1 of the cardiac action potential?

Answer 16

Delayed rectifier K+ channels.

Question 17

What ion channels open during the plateau phase of the cardiac action potential in Phase 2?

Answer 17

Calcium (Ca2+) channels.

Question 18

What is the main ion responsible for the rapid repolarization in Phase 3 of the cardiac action potential?

Answer 18

Potassium (K+) ions.

Question 19

What is the role of the inward rectifier K+ current in Phase 4 of the cardiac action potential?

Answer 19

It helps maintain the resting potential by allowing K+ to move out of the myocyte.

Question 20

How long does a nerve cell action potential last?

Answer 20

A nerve cell action potential lasts approximately 1 millisecond.

Question 21

What follows a skeletal muscle action potential?

Answer 21

Contraction follows a skeletal muscle action potential.

Question 22

What is tetany?

Answer 22

Tetany occurs with repeated stimulation of muscles, resulting in sustained and involuntary muscle contractions.

Question 23

How does the duration of cardiac muscle action potential compare to nerve and skeletal muscle action potentials?

Answer 23

The duration of cardiac muscle action potential varies in duration and can last up to 500 milliseconds, whereas nerve and skeletal muscle action potentials last much shorter, around 1-5 milliseconds.

Question 24

Is there tetany during the refractory period of cardiac muscle action potential?

Answer 24

No, there is no tetany during the refractory period of cardiac muscle action potential due to its long duration.

Question 25

What are pacemaker tissues in the heart?

Answer 25

Pacemaker tissues in the heart are specialized tissues that have spontaneous depolarization and no inward K+ rectifier current. They are not stable at rest and their depolarization is due to Ca2+ rather than Na+. The repolarization in pacemaker tissues is due to delayed rectifier K+.

Question 26

What is If in the context of cardiac auto-rhythmicity?

Answer 26

If is the “funny current” that leads to a net inward current and involves a large Na+ current inward and a tiny K+ current outward. It increases upon hyperpolarization and is responsible for cardiac auto-rhythmicity.

Question 27

What is the role of If in the SA node cells?

Answer 27

If makes the SA node cells spontaneously active, leading to cardiac auto-rhythmicity. It depolarizes the cell toward 0 mV by conducting a net inward current.

Question 28

What is the function of intercalated discs in cardiac muscle?

Answer 28

Intercalated discs join adjacent cardiac myocytes and allow action potentials to pass easily from cell to cell through gap junctions, thereby facilitating coordinated cardiac contraction.

Question 29

What is a functional syncytium?

Answer 29

A functional syncytium is a multinucleated cell resulting from the fusion of multiple cells that function together as a single unit. In cardiac muscle, there are two functional syncytia: the atria and the ventricles.

Question 30

What is the AV node conduction?

Answer 30

The AV node conduction is the slowest velocity, with a 120-200 ms delay. It allows atrial contraction to complete before ventricular contraction. The autonomic nervous system controls the velocity, which acts via effects on phase 0 depolarisation. The sympathetic system stimulates b1 receptors, increasing cAMP, while the vagal nerve stimulates muscarinic receptors, decreasing cAMP. Drugs such as Bisoprolol (b1), Verapamil (L-type Ca channels), and Digoxin (↑ vagal tone) slow the conduction.

Question 31

What is Frank Starling’s law?

Answer 31

Starling’s laws refer to the relationship between the amount of blood filling the heart (preload) and the force of the heart’s contraction (stroke volume). The laws state that the more the cardiac muscle is stretched (within physiological limits), the greater the force of contraction and the larger the stroke volume will be.

Question 32

What is the sarcomere?

Answer 32

The sarcomere is the basic contractile unit of muscle fiber. It is made up of two types of filaments, thin (actin) filaments and thick (myosin) filaments, which overlap in the dark regions. The Z-line is where thin filaments are anchored, while the M-line is where thick filaments are anchored.

Question 33

How does myocyte stretch affect the contractile force of the heart?

Answer 33

Myocyte stretch increases the overlap of thick and thin filaments, which leads to an increase in actin/myosin cross-bridging, resulting in an increase in the force generation and duration of contraction.

Question 34

What is the intrinsic regulation of contractile force in the heart?

Answer 34

The intrinsic regulation of contractile force is the Starling mechanism, which leads to an increase in the force and duration of contraction. This is achieved through an increase in cross-bridges between thick and thin filaments due to myocyte stretch.

Question 35

How does extrinsic regulation of contractile force differ from intrinsic regulation?

Answer 35

Extrinsic regulation, such as sympathetic stimulation, leads to an increase in force but with a similar duration. This is achieved by working harder with the same number of cross-bridges, unlike intrinsic regulation where more cross-bridges are formed, resulting in a longer and stronger contraction.

Question 36

What happens to the overlap of thick and thin filaments during myocyte stretch?

Answer 36

During myocyte stretch, the overlap of thick and thin filaments increases, which leads to an increase in actin/myosin cross-bridging, resulting in an increase in force generation.

Question 37

What is the role of cross-bridges in the regulation of contractile force in the heart?

Answer 37

Cross-bridges play a crucial role in the regulation of contractile force in the heart by forming bonds between the thick and thin filaments, leading to the contraction of the heart muscles. An increase in cross-bridges results in an increase in force generation and duration of contraction.

Question 38

What does the Wiggers diagram show?

Answer 38

The Wiggers diagram shows the events of the cardiac cycle and the pressure/volume changes that occur in the heart during each phase.

Question 39

What does the “a” wave in the Wiggers diagram represent?

Answer 39

The “a” wave in the Wiggers diagram represents atrial contraction and the increase in left atrial pressure that occurs as blood is pumped into the left ventricle.

Question 40

What does the “c” wave in the Wiggers diagram represent?

Answer 40

The “c” wave in the Wiggers diagram represents the bulging of the mitral valve into the left atrium during isovolumic contraction of the left ventricle.

Question 41

What does the “x” descent in the Wiggers diagram represent?

Answer 41

The “x” descent in the Wiggers diagram represents the drop in left atrial pressure that occurs as the left ventricle begins to contract and the mitral valve opens.

Question 42

What does the “v” wave in the Wiggers diagram represent?

Answer 42

The “v” wave in the Wiggers diagram represents the increase in left atrial pressure that occurs as blood is passively filling the left atrium during systole.

Question 43

What is the LV pressure: volume loop?

Answer 43

The LV pressure: volume loop is a graphical representation of the cardiac cycle that shows the relationship between left ventricular volume and pressure over one cardiac cycle. It is divided into four phases: isovolumic contraction (a), ejection (b), isovolumic relaxation (c), and diastolic filling (d). It is a useful tool for understanding cardiac function and can provide information on stroke volume, ejection fraction, and cardiac output.

Question 44

What is the function of the Troponin complex in thin filaments?

Answer 44

The Troponin complex is where cross-bridging between thick and thin filaments occurs. Tn-T binds the complex to tropomyosin, Tn-C binds Ca2+ during excitation-contraction coupling, and Tn-I inhibits cross-bridging to myosin heavy chains.

Question 45

What is the role of creatine kinase?

Answer 45

Creatine kinase moves high-energy phosphate from ATP in the mitochondria to ADP in the cytoplasm.

Question 46

What is CK-MB?

Answer 46

CK-MB is a subtype of creatine kinase that is more specific to cardiac muscle.

Question 47

What are the markers of myocardial damage?

Answer 47

Troponin I and T, creatine kinase, and CK-MB are all markers of myocardial damage.