RES 111 anatomy chapter 18 Electro cardio
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81 Terms
What is a membrane potential?
The electrical (voltage) difference across the myocardial cell membrane.
What is the charge pattern across a resting cardiac cell membrane?
The cell interior is electronegative; the cell exterior is electropositive.
What is the resting membrane potential (RMP) of a myocardial cell?
Approximately −85 to −90 mV.
Which major ions are located outside the cardiac cell?
Sodium (Na⁺) and calcium (Ca²⁺).
Which ions or charged particles are emphasized inside the resting cardiac cell?
Potassium (K⁺) and large protein ions with negative charges.
Why does the inside of a resting cardiac cell become electrically negative?
K⁺ diffuses out, while large negatively charged proteins remain inside the cell.
How permeable is the resting cardiac cell membrane to K⁺ compared with Na⁺?
It is very permeable to K⁺ and almost completely impermeable to Na⁺.
Why does Na⁺ gradually diffuse into a resting cardiac cell?
Na⁺ has a large concentration gradient and can enter through membrane leak channels.
Why can large protein anions not follow K⁺ out of the cell?
The cell membrane is impermeable to the large protein anions.
What is the role of the sodium-potassium pump in resting membrane potential?
It uses ATP to restore the original Na⁺ and K⁺ concentration gradients.
How many Na⁺ and K⁺ ions does the sodium-potassium pump move per cycle, according to the slides?
It pumps 3 Na⁺ out of the cell and 2 K⁺ into the cell.
Does the sodium-potassium pump directly cause repolarization?
No. K⁺ leaving the cell causes repolarization; the pump restores the ion gradients.
What is depolarization in the cardiac cell?
A change in membrane potential associated with changing membrane permeability, especially rapid Na⁺ entry.
What do voltage-gated fast Na⁺ channels do during an action potential?
They open rapidly and allow Na⁺ to enter, causing rapid depolarization.
What is the main role of K⁺ channels during the cardiac action potential?
K⁺ leaves the cell, contributing to repolarization.
How do Ca²⁺ channels differ from fast Na⁺ channels in their response?
Ca²⁺ channels respond more slowly and help produce the action-potential plateau.
In the instructor’s analogy, what is the sodium channel like?
A light switch.
In the instructor’s analogy, what is the calcium channel like?
A dimmer switch.
What event brings a cardiac muscle cell to threshold potential?
A stimulus from a neighboring cardiac cell brings the membrane to threshold.
What happens when a cardiac muscle cell reaches threshold during Phase 0?
Fast voltage-gated Na⁺ channels open and Na⁺ rapidly enters the cell.
What membrane potential does the cardiac cell reach during Phase 0 in the slides?
Approximately +30 mV, rising from an RMP of about −90 mV.
What is Phase 0 of the cardiac action potential called?
Depolarization.
What happens to fast Na⁺ channels during Phase 1?
They inactivate, reducing Na⁺ influx.
What happens to K⁺ channels during Phase 1?
They begin to open, allowing K⁺ to leave the cell.
What happens to Ca²⁺ channels at the start of Phase 1?
Slow Ca²⁺ channels begin to open.
What is the result of the ion-channel changes during Phase 1?
Brief initial repolarization.
What is Phase 2 of the cardiac action potential called?
The plateau phase.
What happens to slow Ca²⁺ channels during Phase 2?
They fully open and allow Ca²⁺ to enter the cell.
What happens to K⁺ during Phase 2?
K⁺ continues to leave the cell.
Why does the cardiac action potential have a plateau during Phase 2?
Ca²⁺ influx balances K⁺ efflux, producing little net change in membrane potential.
Why is the plateau phase important for the heart?
It allows time for effective ventricular contraction.
What happens to Ca²⁺ channels during Phase 3?
They close.
What happens to K⁺ channels during Phase 3?
They remain open, allowing K⁺ to rush out of the cell.
Why does membrane potential move back toward −90 mV during Phase 3?
Positive K⁺ ions leave the cell, making the membrane potential more negative.
What is Phase 3 of the cardiac action potential called?
Repolarization.
What happens during Phase 4 of the cardiac action potential?
The cell returns to its resting membrane potential and is ready for another stimulus.
What restores the original Na⁺ and K⁺ gradients after an action potential?
The ATP-dependent sodium-potassium pump.
Which ion movement is chiefly responsible for repolarization in the slides?
K⁺ leaving the cell.
How does an action potential spread from one myocardial cell to another?
Positive charge spreads through gap junctions, bringing neighboring cells to threshold and triggering their action potentials.
What does the combined electrical activity of many cardiac cells produce at the body surface?
An electrocardiogram (ECG) signal.
What is automaticity in cardiac tissue?
The ability to self-depolarize or self-excite intrinsically.
What is rhythmicity in cardiac tissue?
Regular, cyclical depolarization.
What is excitability in cardiac tissue?
The ability to respond to a stimulus by reaching threshold and generating an action potential.
What is contractility in cardiac tissue?
The ability to contract forcefully when depolarized.
Which cardiac tissue is identified as the heart’s natural pacemaker?
The sinoatrial (SA) node in the right atrium.
Why is the SA node especially likely to depolarize?
Its RMP is closer to threshold than that of other cardiac tissue, giving it the highest automaticity.
What RMP is listed for the sinus node?
Approximately −55 mV.
What RMP is listed for ventricular fibers?
Approximately −90 mV.
Why does an RMP closer to threshold make depolarization easier?
The membrane requires a smaller change in potential to reach threshold.
What is sinus rhythm according to the slides?
A rhythm of about 70–80 beats per minute when the SA node initiates each beat.
What is an ectopic beat?
A heartbeat whose depolarization is initiated by muscle cells other than SA node cells.
What is an ectopic focus?
A non-SA-node pacemaker.
What may happen if the SA node fails to depolarize?
Cells with the next-highest intrinsic self-excitation rate may become the pacemaker.
What does downward displacement of the pacemaker refer to in the slides?
Pacemaker activity originating from a site other than the SA node.
What is the function of the internodal pathways?
They transmit impulses through the atria from the SA node toward the AV node.
What does Bachmann’s bundle connect, according to the slides?
The SA node region to the left atrium.
Why is there an impulse delay at the AV node?
The delay allows time for ventricular filling.
Which fibers are described as producing the greatest electrical resistance and impulse delay?
Junctional fibers between the AV node and bundle.
What is the role of the AV bundle in conduction?
It penetrates the fibrous skeleton and provides one-way conduction from atria to ventricles.
What is a key characteristic of Purkinje system conduction?
The Purkinje system penetrates into heart muscle and conducts impulses at greater velocity.
How long is the total conduction time from the SA node to the last ventricular fiber in the slides?
Approximately 0.22 second.
What effect does parasympathetic (vagal) stimulation have on SA node firing rate?
It decreases the SA node firing rate.
What effect can parasympathetic stimulation have on AV nodal excitability?
It decreases AV nodal excitability and can lead to AV block.
How does acetylcholine affect K⁺ permeability in cardiac muscle fibers?
It greatly increases K⁺ permeability, allowing K⁺ to leave the cell.
How does increased outward K⁺ diffusion affect the cardiac cell’s internal charge?
The inside becomes more negative, making depolarization more difficult.
What overall effect does the parasympathetic system have on the heart in the slides?
A quieting effect, including reduced SA firing and reduced AV nodal excitability.
What effect does sympathetic stimulation have on SA node firing?
It increases SA node firing rate.
What effect does sympathetic stimulation have on conduction rate and excitability?
It increases conduction rate and excitability.
Which substances are identified with sympathetic stimulation in the slides?
Epinephrine and norepinephrine.
How do epinephrine and norepinephrine affect membrane permeability, according to the slides?
They increase membrane permeability to Na⁺ and Ca²⁺.
Compare the listed RMPs of the SA node and ventricular fibers.
SA node: about −55 mV; ventricular fibers: about −90 mV.
Compare Phase 0 and Phase 3 of the cardiac action potential.
Phase 0: rapid Na⁺ entry causes depolarization. Phase 3: K⁺ leaves while Ca²⁺ channels close, causing repolarization.
Compare Phase 1 and Phase 2 of the cardiac action potential.
Phase 1 is brief initial repolarization; Phase 2 is the plateau as Ca²⁺ entry balances K⁺ exit.
A cardiac cell is receiving rapid Na⁺ influx and its voltage rises toward +30 mV. Which phase is occurring?
Phase 0: depolarization.
A cell has Ca²⁺ entering while K⁺ continues to leave, with little net voltage change. Which phase is occurring?
Phase 2: the plateau phase.
Ca²⁺ channels close while K⁺ continues to leave the cell. Which phase is occurring?
Phase 3: repolarization.
A stimulus spreads through gap junctions and brings a neighboring myocardial cell to threshold. What happens next?
The neighboring cell generates its own action potential.
Why can the SA node normally set the pace for the heart?
It has the highest automaticity because its resting potential is closer to threshold than that of other cardiac tissue.
What is the purpose of the AV nodal conduction delay?
To provide time for ventricular filling before ventricular activation.
What distinguishes an ectopic beat from a sinus beat?
An ectopic beat begins outside SA node cells; a sinus beat is initiated by the SA node.