lecture 8 Flashcards
List the parts of the electrical conduction system of the heart in the correct sequence for one contraction
o First it goes to the sinoatrial (SA) node (is the pacemaker of the heart) this section functions to initiate the heart’s rhythm. The impulse of this causes the atria to contract and pushes blood into the ventricles.
o Second it goes to the atria for atrial contraction which functions by causing the atria to contract and push blood into the ventricles.
o Third it goes to the atrioventricular (AV) node which receives the impulse from the atria and slows it down slightly to allow for the ventricles to fill with blood before contraction occurs.
o Fourth it goes to the bundle of his (atrioventricular bundle) which connects to the next part of the conduction. It sends the electrical impulse down the bundle and goes down to the right and left bundle bridges. This is where the impulses are carried down to the right and left ventricles respectively.
o Then the impulse goes to the Purkinje fibers which are in the walls of the ventricles. This allows for contraction of the ventricles to pump blood out of the heart and into the lungs or the body.
explain how the electrical conduction system functions.
The system functions by initiating an electrical impulse to spread across to the atria to cause contraction and pushing to the ventricles. This causes a delay in allowing the ventricles to fill with blood before they contract. The impulse after the delay will then travel down the bundle of his to then spread through the Purkinje fibers and thus cause the ventricle to contract.
Explain why the SA node normally paces the heart.
The SA node normally paces the heart because if has the fastest intrinsic firing rate of all of the parts of the heart because of it automaticity, and it is in the best position in the heart to be able to effectively coordinate the heart’s rhythm through generating the impulses first which then has the ability to later send if to the rest of the conduction system.
- Automaticity, fastest firing rate (it depolarizes the fastest so it reaches the threshold for triggering an action potential before any other cell), location (is at the top of the right atrium which allows it to quickly spread the electrical impulse through the heart), ability to override suppression to prevent other cells from firing at the wrong time, is able to respond to physiological changes
Explain how the cardiac conduction system produces coordinated heart chamber contractions.
By generating the electrical impulses that travel through specialized pathways within the heart, the atria is caused to contract first which is followed by a slight delay so the the ventricles can fill before the atria allows the blood to be synchronized to go hourglass the ventricles. This is done by transmitting impulses to the bundles to allow for this and to allow the purkinje fibers to force the ventricles to contract simultaneously. This ensures the sequential and efficient pumping action of the heart chambers and allows the heart to Chen relax and recover.
Is initiated by the sinoatrial node which is the pacemaker that allows for signals to go to the AV node and down to the purkinje fibers
Name the waveforms in a normal electrocardiogram (ECG or EKG) and explain the electrical events represented by each waveform.
o P wave – the first small upward deflection, represents depolarization of the atria
o QRS complex – Q is a small negative deflection, R is a large positive deflection, and S is a small negative deflection following the R wave. This all symbolizes the depolarization of the ventricles.
o T wave – is a smooth asymmetrical, upward deflection that occurs after the QRS complex, is the repolarization of the ventricles
Explain the significance of autorhythmic or pacemaker cells.
o The significance of autorhythmic is to ensure that the heart can maintain the ability to generate electrical impulses on its own while ensuring that it is continuous and intrinsic.
o The pacemaker potential allows for the cells to initiate potentials at regular intervals, without requiring signals to trigger the actions. The pacemaker is driven by the influx of sodium and calcium ions and the efflux of potassium to allow depolarization and the continuing of muscle contractions.
Compare and contrast the action potentials in cardiac pacemaker cells, in cardiac contractile cells, and in skeletal muscle cells and how they relate to contraction and refractory periods.
pacemaker cells spontaneously generate action potentials, while cardiac contractile cells and skeletal muscle cells require external stimulation to trigger an action potential; this results in a longer refractory period in cardiac muscle, essential for coordinated heart contractions, compared to the shorter refractory period in skeletal muscle, allowing for rapid muscle contractions.
Initiation
- Pacemaker cells: Generate action potentials spontaneously due to “self-rhythmic activity” that gradually depolarizes the membrane potential, setting the heart rhythm.
- Cardiac contractile cells: Action potentials are triggered by electrical signals from pacemaker cells, propagating through gap junctions.
- Skeletal muscle cells: Action potentials are initiated by neurotransmitter release from a motor neuron at the neuromuscular junction.
Refractory
- Long refractory period to prevent tetany and ensure coordinated contractions
- The pacemaker cells trigger the electrical signals to coordinate the contractions of the cardiac muscles, cardiac contractile cells trigger the sliding filament mechanism, and skeletal muscle cells trigger the release of calcium and initiate muscle contraction.
Explain the significance of the plateau phase in the action potential of a cardiac contractile cell.
The plateau phase is an extended period of depolarization which occurs curing the action potential stage. It is characterized by a prolonged period of stable membrane potential. It is significant because if allows for prolonged contraction (the ability to force enough blood to pump out of the heart), allows for the prevention of tetany (allows the heart to relax), allows the heart to continue to function effectively (contributes to the timing and coordination of contractions), ensures that the heart will fill properly, and enhances the strength of the contractions.
- it is maintained by continuing to bring calcium into the muscle cell
Compare and contrast the role of autonomic innervation in the depolarization of cardiac pacemaker cells.
Autonomic innervation does not initiate the depolarization of cardiac pacemaker cells, but rather acts to modulate the rate of depolarization by influencing the ion channels involved in the pacemaker potential, effectively speeding up (sympathetic stimulation) or slowing down (parasympathetic stimulation) the heart rate without changing the inherent rhythmicity of the pacemaker cells themselves; essentially, the autonomic nervous system acts as a “fine-tuner” for the heart rate set by the pacemaker cells.
the SNS and PNS specific roles in depolarization of cardiac pacemaker cells
Sympathetic Nervous System: Increases heart rate by increasing the rate of depolarization of pacemaker cells, primarily via norepinephrine and β₁ receptors. This is beneficial during active states like exercise or stress.
Parasympathetic Nervous System: Decreases heart rate by slowing depolarization, primarily through acetylcholine and M₂ receptors, allowing the heart to beat slower during restful states.
- essentially the SNS acts as the “accelerator” to increase the rate and force of the heart beat, and the PNS acts as the “brakes” to slow the heart rate
what does the calcium plateau allow for
a long contraction phase so that there is sustained contraction through the ejection of blood continuing.
- also prevents the tetanic contractions to allow the heart to fill
- a better smoother contraction phase
action potential in the cardiac cycle
1) the rapid depolarization allows for sodium to enter, lasts 3-5 seconds and ends with the closure of voltage-gated fast sodium channels
2) the plateau causes Calcium to enter lasts -175 msec, and enacts the closure of slow calcium channels
3) the repolarization causes potassium loss, lasts for 75 msec, and ends with the closure of slow potassium channels
electrical conduction system
generating electrical impulses in the sinoatrial (SA) node, which then spread through the atria, causing them to contract, before reaching the atrioventricular (AV) node where the signal is slightly delayed to allow the atria to fully empty; the signal then travels down the bundle of His, divides into bundle branches, and finally reaches the Purkinje fibers which rapidly distribute the electrical impulse throughout the ventricles, causing them to contract and pump blood out to the body; this coordinated sequence ensures efficient blood circulation throughout the body
what does the SA node do
The SA (sinoatrial) node normally paces the heart because it has the inherent ability to generate electrical impulses at a faster rate than any other tissue in the heart, making it the “natural pacemaker” that sets the rhythm for cardiac contractions; essentially, it initiates the electrical signal that causes the atria to contract, thus starting the cardiac cycle
