Lesson 1 - cardiac action potential Flashcards
What activates cardiac cells?
Electrical currents across the plasma membrane by means of opening and closing ion channels
What are the major ions involved in the action potential?
- Sodium
- Potassium
- Calcium
- Chloride
What is the term for the change in membrane potential that decreases electronegativity within a muscle cell?
Depolarization
What does depolarization lead to in muscle cells?
Muscle contraction
What electrical activity corresponds with the relaxation of contracted muscles?
Repolarization
What is referred to as the inward current?
The flow of positive ions into a cell
What is the charge inside cardiac cells during the resting state?
Negatively charged
Which ions cause depolarization by flowing into the cell?
- Sodium
- Calcium
What does outward current refer to?
The flow of positive ions out of a cell
What happens as sodium and calcium leave the cell?
Repolarization occurs
Which ions are considered negative ions within the cell?
Chloride ions
What are voltage-gated ion channels?
Cellular pores formed by membrane proteins that open and close to allow the passage of specific ions.
They facilitate the movement of ions intracellularly or extracellularly.
Why are they called ‘voltage-gated’?
They are activated by changes in the electrical membrane potential.
This activation is crucial for various cellular processes, including action potentials in neurons.
What is the mechanism that controls the state of ion channels?
The gating mechanism.
This mechanism determines whether the channel is open or closed.
List the four types of voltage-gated ion channels.
- Sodium channels
- Potassium channels
- Calcium channels
- Chloride channels
Each type of channel plays a specific role in cellular excitability and signaling.
What are sodium channels?
Membrane proteins that allow the passage of sodium ions.
What are the two types of sodium ion channels?
- Fast sodium channels
- Slow sodium channels
What role do fast sodium channels play?
They play a major role in the depolarization of non-pacemaker cardiac cells.
Where are slow sodium channels present?
In the nodal tissues of the SA-node and AV-node.
How many states do sodium channels have?
Three states: deactivated (closed), activated (open), and inactivated (closed).
What causes sodium channels to open?
Depolarization causes sodium channels to open transiently.
Which state of sodium channels is activated faster than inactivation?
Activated (open) state.
What type of amino acids make up sodium channels?
Negatively charged amino acids.
What do negatively charged amino acids in sodium channels do?
Repel negative ions and attract positively charged sodium ions.
Can potassium pass through sodium channels?
No, potassium is too large to pass through the sodium channels.
What role do potassium channels play in the action potential?
They are crucial for the generation and regulation of action potentials.
What is the function of the inward-rectifier channel?
Allows the passage of positively charged potassium ions into the cell.
How does the inward-rectifier channel affect the membrane resting potential?
It maintains the membrane resting potential.
What happens to the inward-rectifier channel during repolarization?
It closes to prolong phase 2 of the action potential.
What does the outward-rectifier channel do?
Allows the passage of positively charged potassium ions out of the cell.
What are the two types of outward-rectifier channels?
- Transient-rectifier channels
- Delayed-rectifier channels
When do transient-rectifier channels open?
They open briefly during depolarization.
What is the role of transient-rectifier channels in action potential?
They regulate the action potential duration.
When do delayed-rectifier channels open?
They open during early phase 2 of the action potential.
What is the function of delayed-rectifier channels?
They stay open to initiate repolarization.
Fill in the blank: The inward-rectifier channel maintains the _______.
[membrane resting potential]
True or False: The delayed-rectifier channels close quickly after opening.
False
What are the two types of calcium channels?
L-type (long lasting) and T-type (transient) calcium channels
L-type channels contribute to phase 2 of the action potential, while T-type channels initiate the action potential.
What role do L-type calcium channels play in the action potential?
They allow calcium ions into cells, contributing to phase 2 of the action potential and excitation-contraction coupling
L-type channels are crucial for sustaining the action potential.
What is the main function of T-type calcium channels?
They slowly allow a small amount of calcium ions into cells and initiate the action potential
T-type channels do not significantly contribute to depolarization.
How do T-type calcium channels contribute to signaling?
They generate small calcium signals that participate in proliferative signaling
Calcium ions act as second messengers, utilized by nonsteroidal hormones to stimulate regeneration.
What is the importance of calcium in cell upkeep?
Calcium is vital for maintaining resting tension as cells are constantly dying or getting damaged
Proper calcium signaling helps in cellular regeneration and upkeep.
Fill in the blank: L-type calcium channels contribute to _______ of the action potential.
phase 2
True or False: T-type calcium channels play a major role in depolarization.
False
What are chloride channels?
Membrane proteins that allow the passage of chloride ions.
What do chloride ions mainly contribute to?
Volume regulation.
What are gap junction channels?
Intercellular pathways that are insulated and offer low resistance to impulses.
How do gap junctions facilitate impulse propagation?
They are aligned and connected to form a continuous cell to cell passageway of impulses.
What factors affect the conductance of gap junctions?
Cardiac diseases, inactivation of protein kinase A, and low pH.
What is the role of protein kinase A in relation to gap junctions?
It regulates several cellular functions; its inactivation decreases junctional conductance.
What is a contributing factor to slow conduction during acute ischemia?
Decreased junctional conductance due to factors like low pH and protein kinase A inactivation.
True or False: Gap junctions have high resistance to impulses.
False
Fill in the blank: The conductance of gap junctions is affected by _______.
[cardiac diseases]
What is the the sodium-potassium pump (Na-K Pump)?
A primary active transport mechanism that
pumps intracellular sodium ions outward and extracellular potassium ions inward.
How is the sodium-potassium pump activated?
The pump is activated by magnesium
Since active transport is the movement of ions against a
concentration gradient, what is needed for the sodium-potassium pump to function?
Energy in the form of ATP (adenosine triphosphate) is required for the Na-K pump to function.
Intracellularly, the sodium pump opens, and what happens?
3 sodium ions bind to the sodium channel and are released extracellularly.
Extracellularly, the potassium pump opens, and what happens?
2 potassium ions bind to the potassium channel and are released intracellularly.
What is the sodium-calcium exchanger (Na-Ca exchanger or NCX)?
A secondary active transport
mechanism that removes calcium ions from the cell.
The exchanger allows the removal of 1 calcium ion in exchange for 3 sodium ions
What is the normal resting membrane potential in the ventricular myocardium?
About -90 mV
At this phase, the membrane is more permeable to potassium and impermeable to other ions.
Explain the permeability of the membrane at resting potential.
At resting potential, the membrane is more permeable to potassium and impermeable to other ions.
How does the concentration gradient of potassium affect its movement across the membrane?
Since potassium concentration is higher intracellularly than extracellularly, this gradient causes potassium to leave the cell through a potassium channel.
Define the role of negative ions in the resting membrane potential.
Negative ions are unable to leave the cell, contributing to the cell’s intracellular negativity.
What happens to potassium that leaves the cell?
Potassium that leaves the cell is attracted back into the cell by the negative intracellular resting potential.
Explain the balance between potassium efflux and influx.
The potassium gradient causes potassium efflux, which is countered by the electrical gradient that causes potassium influx.
How is the equilibrium potential of the cell maintained?
The equilibrium potential of the cell is maintained by the balance between potassium efflux due to concentration gradient and potassium influx due to electrical gradient.
Describe the role of the SA-node in cardiac function.
The SA-node generates electrical impulses that stimulate the ventricular myocardial muscle cells, initiating the process of depolarization.
Explain the process of depolarization in ventricular myocardial muscle cells.
Depolarization begins when electrical impulses from the SA-node cause sodium channels to open, allowing sodium ions to flow into the cell.
Define the threshold potential for ventricular muscle cells.
The threshold potential for ventricular muscle cells is approximately -70 mV, which must be reached to initiate full depolarization.
How does the all or none principle apply to action potentials in cardiac muscle cells?
The all or none principle states that if the threshold potential is not met, an action potential will not be initiated; once the threshold is reached, a full action potential occurs.
What happens to sodium channels during the depolarization phase?
Sodium channels begin to open when the muscle cell is stimulated, and upon reaching the threshold potential, more sodium channels open, leading to a rapid influx of sodium ions.
Explain the significance of sodium ion influx during depolarization.
The influx of sodium ions during depolarization causes the intracellular environment of the cell to become more positive compared to the extracellular environment.
Describe the relationship between sodium channels and threshold potential in cardiac cells.
Sodium channels must satisfy a threshold potential to fully open; if the threshold is not reached, the channels will not open sufficiently to trigger an action potential.
Describe the role of sodium ions during the initial repolarization phase.
Sodium ions enter the cell rapidly, causing an initial depolarizing current that leads to the action potential overshooting 0 mV before the sodium channels close.
Explain the significance of the plateau phase in myocardial cells.
The plateau phase is significant because it balances the inward movement of calcium ions with the outward movement of potassium ions, sustaining contraction in myocardial cells through excitation-contraction coupling.
Define the changes in ion movement during the rapid repolarization phase.
During rapid repolarization, L-type calcium channels close while delayed-rectifier channels remain open, allowing more potassium ions to exit the cell, making the intracellular environment more negative.
How does the action potential behave during the initial repolarization phase?
The action potential initially rises due to sodium influx, overshooting 0 mV, followed by a short downward deflection caused by the transient outward current of potassium and chloride ions.
What happens to the delayed-rectifier channels during the rapid repolarization phase?
The delayed-rectifier channels remain open during rapid repolarization, allowing potassium ions to move outward until the membrane potential is restored to about -90 mV.
Describe the effect of calcium ions during the plateau phase.
Calcium ions move inward through L-type calcium channels, which counteracts the closure of sodium channels and helps maintain the action potential during the plateau phase.
Explain the transition from the plateau phase to rapid repolarization.
The transition occurs when L-type calcium channels close, leading to a decrease in calcium influx and allowing delayed-rectifier channels to continue potassium efflux, resulting in rapid repolarization.
What is the outcome of the initial depolarizing current of sodium?
The outcome is a rapid rise in the action potential that briefly overshoots 0 mV before sodium channels close.
How does the closure of sodium channels affect the action potential?
The closure of sodium channels leads to a continuous downward deflection of the action potential, which is then balanced by calcium and potassium movements during the plateau phase.
Define excitation-contraction coupling in the context of myocardial cells.
Excitation-contraction coupling in myocardial cells refers to the process by which electrical excitation leads to muscle contraction, sustained during the plateau phase by calcium influx.
