Section 3 Flashcards
What is unique about the heart as an organ?
The heart is an electro-mechanical organ, possessing both electrical activity, similar to nerves, and the ability to contract as a large muscle.
How many types of cardiac muscle cells exist, and what are their primary functions?
There are two types of cardiac muscle cells: primarily contractile cells responsible for mechanical work, and electrical cells responsible for generating and propagating action potentials.
What is the primary function of primarily contractile cells in the heart?
Primarily contractile cells perform the mechanical work of the heart.
What are some situations where the SNS or PNS could affect heart rate?
Physical and emotional stress can cause changes in heart rate. This stress can be due to exercise, fear,
injury, and even illness.
- Exercise increases heart rate to bring additional blood, and therefore oxygen, to working muscles and to rid them of CO2 and other waste.
- Fear increases heart rate due to the “fight or flight” response which releases epinephrine from the adrenal glands. Epinephrine stimulates the SNS and raises the heart rate.
- Illness and injury cause an increase in blood flow to peripheral tissues, which increases heart rate via the SNS.
Does the heart require the CNS or PNS to generate or propagate electrical signals?
No
What makes the heart interesting in terms of electrical activity is that it generates and propagates electrical signals independently. That is, the heart is not dependent upon either the CNS or PNS for its function.
What are specialized cardiac muscle cells capable of generating action potentials known as?
Autorhythmic cells.
How do autorhythmic cells differ from other cells in terms of resting membrane potential?
Autorhythmic cells have a slowly depolarizing resting membrane potential until the threshold is reached and an action potential is fired.
What is the suggested mechanism for depolarization in autorhythmic cells, and what channels are thought to be involved?
Autorhythmic cells may contain If channels, possibly activated by the hyperpolarization-activated cyclic nucleotide-gated channel (HCN) family or the T-type Ca2+ channel.
What is unique about the upstroke of the action potential in autorhythmic cells compared to neurons and cardiac contractile cells?
The upstroke of the action potential in autorhythmic cells is due to the L-type Ca2+ channel, unlike neurons and cardiac contractile cells where it is typically due to Na+ channels.
Where are autorhythmic cells localized in the heart?
Autorhythmic cells are localized in specific regions of the heart, including the sinoatrial (SA) node.
Where is the sinoatrial (SA) node located?
The sinoatrial (SA) node is a very small area located in the right atrial wall near the opening of the superior venae cavae.
What is the If channel?
A channel with unusual properties that allow current (I) to flow
Where are sinoatrial (SA) nodes located?
Sinoatrial (SA) nodes are located in a very small area in the right atrial wall near the opening of the superior venae cavae.
Where is the atrioventricular (AV) node located, and how is it often described?
The atrioventricular (AV) node is a small area located in the right atrium, where the right atria and right ventricle come together. It is often described as being located in the interatrial septum due to its central position in the heart.
What does the bundle of His consist of, and where does it arise from?
The bundle of His consists of specialized cells that arise from the AV node.
Where do the bundle branches of the bundle of His travel, and what is their path in the heart?
They travel down each side of the septum to the bottom of the heart, where they curve around and travel back towards the atria.
What is the path and location of the Purkinje fibers?
Purkinje fibers are small fibers that branch off the bundle of His and spread along the inner (endocardial) surface of the ventricles.
What is the role of autorhythmic cells in the SA node?
pacemaker cells of the heart, as they have the fastest rate of depolarization, controlling heart rate at around 70-80 beats/minute for the average person.
Why are SA node cells considered pacemaker cells?
because they control heart rate and have the fastest rate of depolarization, reaching threshold the fastest.
What happens once an action potential is generated in SA node cells?
it conducts through the rest of the cardiac conduction system, overriding the pacemaker activity of other autorhythmic cells.
What is the significance of the pacemaker activity of the SA node?
As long as the SA node is functioning fine, it controls the rate at which the heart beats. Without pacemaker activity, the heart would not beat at all
Why should atrial excitation and contraction be complete before the onset of ventricular contractions?
Atrial excitation and contraction should be complete before the onset of ventricular contractions to allow the ventricles to fill with blood completely. If ventricular contraction occurred simultaneously with atrial contraction, incomplete ventricular filling would occur.
What percentage of total ventricular filling is accounted for by passive blood flow, and when does it occur?
Passive blood flow accounts for around 80% of total ventricular filling, occurring during the relaxation of the heart when the AV valves are open.
Why is it crucial for excitation of cardiac muscle fibers to be coordinated?
Coordination of excitation is crucial for efficient pumping, as uncoordinated depolarization (ventricular fibrillation) can hinder the heart’s ability to eject blood.
What is ventricular fibrillation, and why is it problematic?
Ventricular fibrillation is uncoordinated depolarization of different regions of a ventricular wall, hindering the heart’s ability to eject blood.
Why must the pair of atria and the pair of ventricles be functionally coordinated?
The pair of atria and the pair of ventricles must be functionally coordinated to ensure both pumps work together, moving the same amount of blood at the same time for efficient blood circulation.
What do you think would happen if the ventricles contracted out of sync?
If the right and left ventricles contract at different times, such as is the case with a bundle branch block (a block in one of the branches of the bundle of His), the blood pumping to the lungs to be oxygenated,and the blood pumping through the aorta would occur at different times.
These blocks can cause unnecessary stress on the ventricular walls and may require a pacemaker to re-coordinate the
contraction of the ventricles.
Alone, these blocks are generally not dangerous, however they can be a symptom of a much larger
problem such as a heart failure, a valve problem, lung disease, or other cardiac conditions.
What are the two main mechanisms by which the wave of excitation travels throughout the atria?
The wave of excitation travels throughout the atria by gap junctions and pathways (interatrial and internodal pathways).
What are gap junctions, and how do they contribute to atrial excitation?
Gap junctions are connections between atrial cells that allow the wave of excitation to spread through the atria.
What is the function of the interatrial pathway in atrial excitation?
The interatrial pathway extends from the right atrium to the left atrium, ensuring that the wave of excitation spreads across both atria at the same time, allowing them to contract simultaneously.
What is the role of the internodal pathway in atrial excitation?
The internodal pathway connects the SA node to the AV node, facilitating the transmission of the excitation wave between these two nodes.
What separates the muscle cells of the atria from the muscle cells of the ventricles, and what is its significance?
The muscle cells of the atria are separated from the muscle cells of the ventricles by a dense region of connective tissue. This separation limits the means by which an electrical signal can move from the atria to the ventricles, allowing only the AV node and the bundle of His to transmit the signal.
