Dr Gawad Physiology Course Online Exam - CVS – Course Lecture 1

The correct answer is "The fastest impulse discharge." This means that the S.A.N. (sinoatrial node) is responsible for initiating the electrical impulses that regulate the heart's rhythm at the highest rate compared to other pacemakers in the heart.

The correct answer is that Na is important to initiate the pacemaker action potential, but not important for its maintenance. This means that the initial depolarization of the pacemaker cells is primarily driven by the influx of Na ions. However, once the action potential is initiated, the maintenance of the pacemaker potential and subsequent action potentials are mainly regulated by the movement of K ions, rather than Na. This is because the repolarization phase of the action potential is primarily due to the outflow of K ions in exchange for Na ions.

The main cause of plateau in cardiac muscle action potential is the balance in influx of and outflux of positive ions along the membrane. This balance is crucial for maintaining the prolonged depolarization phase, which characterizes the plateau phase. It is the result of the coordinated activity of various ion channels, such as the L-type calcium channels and delayed rectifier potassium channels, which allow for the influx of calcium and efflux of potassium respectively. This balance helps sustain the depolarization and allows for proper contraction and relaxation of the cardiac muscle.

Cardiac muscle fibers form a true syncytium, meaning that they are interconnected by gap junctions that allow for the rapid spread of electrical impulses throughout the entire muscle. This synchronized contraction ensures that the heart beats as a single unit, allowing for efficient pumping of blood. The presence of gap junctions also allows for the coordinated contraction of neighboring cardiac muscle fibers, leading to the characteristic rhythmic contractions of the heart.

The given answer is correct because it accurately describes the difference between pacemaker action potential and cardiac muscle action potential. Pacemaker action potential refers to the electrical activity in the cells of the sinoatrial (SA) node, which is responsible for initiating the heartbeat. This action potential is unstable because it does not have a stable resting membrane potential and can spontaneously depolarize. On the other hand, cardiac muscle action potential in the working myocardium is stable and follows a predictable pattern of depolarization and repolarization. Therefore, the statement correctly states that pacemaker action potential is unstable while cardiac muscle potential is not.

The pacemaker potential refers to the spontaneous depolarization of certain cells, such as those in the sinoatrial node of the heart, that initiate action potentials. The un-stability of this potential is primarily due to hyperpolarization stimulating voltage gated Na channels. Hyperpolarization refers to a more negative membrane potential, and when this occurs, it stimulates the opening of voltage-gated Na channels, allowing Na ions to enter the cell and depolarize it. This depolarization leads to the generation of an action potential and the continuation of the pacemaker potential.

The sinoatrial node (SAN) is responsible for initiating the electrical impulses that regulate the heart's rhythm. It is often referred to as the "natural pacemaker" of the heart. The SAN sets the pace for the rest of the heart by generating electrical signals that cause the atria to contract. These signals then travel through the atrioventricular node (AVN), bundle of His, and Purkinje fibers to stimulate the contraction of the ventricles. The SAN has the steepest prepotential, meaning it has the fastest rate of depolarization, making it the correct answer.

The left ventricle has a thicker wall than the right ventricle because it ejects blood against a higher pressure. The left ventricle pumps oxygenated blood to the rest of the body, which requires a higher pressure to overcome the resistance of the systemic circulation. In contrast, the right ventricle pumps deoxygenated blood to the lungs, which have lower resistance, so it does not need to generate as much pressure. Therefore, the left ventricle has a thicker wall to support the higher pressure needed for systemic circulation.

The absolute refractory period (ARP) in the heart is longer than in skeletal muscles. During the ARP, the cardiac muscle is unable to respond to any further stimulation, regardless of the strength of the stimulus. This is important for the proper functioning of the heart as it ensures that the muscle has enough time to fully relax and reset before it can be stimulated again. In contrast, skeletal muscles have a shorter ARP, allowing for more rapid contractions and movements.

The prepotential of a pacemaker is due to the leaky nature of the SAN (sinoatrial node) membrane to Ca ion, which affects repolarization. This prepotential coincides with the cardiac diastole and the action of the Na-K pump on the cardiac muscle action potential. Therefore, both option C and D are correct, as they both contribute to the prepotential of a pacemaker.

Idioventricular rhythm refers to a condition where an ectopic pacemaker, rather than the normal pacemaker or the atrioventricular node (AVN), is responsible for generating the heart's rhythmic contractions. In this rhythm, the ventricles of the heart initiate the electrical impulses, causing them to contract independently of the atria. This can result in a slower heart rate and may be seen in certain pathological conditions, such as heart block or damage to the normal pacemaker cells.

The correct answer is "heart rate that is produced due to the permeability of SAN to Na." Intrinsic heart rate refers to the heart rate that is generated by the sinoatrial node (SAN), which is the natural pacemaker of the heart. The SAN is able to generate electrical impulses and initiate the contraction of the heart muscle due to its unique property of being permeable to sodium (Na) ions. This permeability allows for the spontaneous depolarization of the SAN cells, leading to the generation of electrical signals that regulate the heart rate.

Purkinje fibers are specialized cardiac muscle fibers that conduct electrical impulses rapidly through the ventricles of the heart. They have the slowest rhythmicity among the options provided. The sinoatrial node (SAN) is responsible for initiating the electrical impulses, the atrioventricular (AV) bundle and AV node help in transmitting the impulses between the atria and ventricles, and the ventricular muscle contracts in response to the impulses. However, the Purkinje fibers have the slowest intrinsic rate of firing, making them the correct answer.

The pacemaker potential refers to the gradual depolarization of the membrane potential in certain cells, such as cardiac cells. It is responsible for initiating the action potential and generating rhythmic contractions. The amplitude of the pacemaker potential is not fixed and can vary depending on the specific cell and its conditions. Therefore, it is incorrect to state that its amplitude is -80 to -90 mV. The other statements are true: the pacemaker potential is unstable during rest, its slope is decreased by increased permeability to K, and the firing level occurs at a potential difference of about -45 mV.

The correct answer is "there is no vagal autonomic fibers supplying the ventricles." Vagal autonomic fibers are responsible for slowing down the heart rate by releasing acetylcholine. When these fibers are not present in the ventricles, it means that there is no parasympathetic control over the ventricles, leading to an increase in heart rate. This condition is known as ventricular escape.

The AVN (atrioventricular node) is considered as an ectopic pacemaker. An ectopic pacemaker is an abnormal site within the heart that initiates electrical impulses and takes over the role of the sinoatrial node (SAN) as the primary pacemaker. The AVN is located between the atria and the ventricles and can generate electrical impulses when the SAN is not functioning properly. This can result in a slower heart rate and irregular heart rhythm. The other options listed (SAN, Bundle of His, and Ventricular muscle) are not typically considered as ectopic pacemakers.

The complete repolarization is not at the end of phase 3 when all K is effluxed. During phase 3 of the cardiac muscle action potential, the majority of K ions are indeed effluxed, leading to repolarization. However, repolarization is not complete until the membrane potential returns to its resting state, which occurs after phase 3. Therefore, the statement is not true.