Dr Gawad Physiology Course Online Exam - CVS – Course Lecture 2 - Conductivity, Contractility

1.

The most rapid conduction velocity is present:

A.

The SAN.
B.

The AVN.
C.

The ventricular muscle.
D.

The Purkinje fibres.

Correct Answer
D. The Purkinje fibres.

Explanation
The Purkinje fibers have the most rapid conduction velocity among the options provided. These specialized cardiac muscle fibers are responsible for rapidly transmitting electrical impulses throughout the ventricles, allowing for coordinated contraction and efficient pumping of blood. Their unique structure, with a large diameter and rich network of gap junctions, enables fast conduction of electrical signals. In contrast, the sinoatrial node (SAN) and atrioventricular node (AVN) have slower conduction velocities to allow for appropriate timing and coordination of the cardiac cycle. The ventricular muscle also has a slower conduction velocity compared to the Purkinje fibers.

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2.

About the cardiac conductivity, all the following is true except:

A.

It is slowest in the AV node.
B.

It is maximal in the Purkinje fibres.
C.

It is slowest in the ventricular muscle.
D.

It is decreased by vagal stimulation.
E.

It occurs through modified cardiac muscle fibres.

Correct Answer
C. It is slowest in the ventricular muscle.

Explanation
Cardiac conductivity refers to the ability of the heart to transmit electrical impulses. The statement "It is slowest in the ventricular muscle" is incorrect because the ventricular muscle has a fast conduction velocity. The correct answer is that cardiac conductivity is slowest in the AV node, which acts as a delay point allowing for coordinated contraction of the atria and ventricles. The Purkinje fibers have the fastest conduction velocity, allowing for rapid spread of the electrical signal throughout the ventricles. Vagal stimulation decreases cardiac conductivity, while modified cardiac muscle fibers are responsible for conducting the electrical impulses.

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3.

In the AV conduction all the following is true except:

A.

Conduction is the slowest.
B.

Conduction is delayed.
C.

Conduction is usually difficult in the ventriculo-atrial direction.
D.

Conduction is enhanced by sympathetic stimulation.

Correct Answer
C. Conduction is usually difficult in the ventriculo-atrial direction.

Explanation
In AV conduction, the electrical signals travel from the atria to the ventricles through the atrioventricular node (AV node). The conduction is delayed in order to allow the atria to fully contract before the ventricles contract. Conduction is enhanced by sympathetic stimulation, which increases the conduction speed. However, conduction is usually difficult in the ventriculo-atrial direction because the AV node acts as a barrier and prevents electrical signals from traveling back up to the atria.

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4.

The fibres of the A-V bundle & its branches :

A.

Conduct impulses very slowly.
B.

Are highly contractile.
C.

Are modified muscle fibres.
D.

Are nerve fibres
E.

A & c

Correct Answer
E. A & c

Explanation
The fibres of the A-V bundle and its branches conduct impulses very slowly and are modified muscle fibres. This means that they have the ability to transmit electrical signals at a slower rate compared to other nerve fibers, and they have been adapted to perform specific functions within the heart muscle.

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5.

The main function of the cardiac Purkinjie system is to :

A.

Increase the speed of impulse conduction through the cardiac muscle.
B.

Prevent premature ventricular beats.
C.

Coordinate the valve movements with myocardial contraction.
D.

Enable all parts of the ventricle to contract simultaneously.
E.

Delay the systole until the ventricles fill.

Correct Answer
D. Enable all parts of the ventricle to contract simultaneously.

Explanation
The main function of the cardiac Purkinje system is to enable all parts of the ventricle to contract simultaneously. The Purkinje fibers are specialized cardiac muscle fibers that rapidly conduct electrical impulses throughout the ventricles, ensuring coordinated and synchronized contraction of the ventricular muscle. This allows for efficient pumping of blood out of the heart and ensures that all parts of the ventricle contract together, maximizing the effectiveness of each heartbeat.

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6.

About cardiac contractility all the following is true except:

A.

It depends on the interaction between actin and myosin.
B.

It increases when the serum K+ rises above normal.
C.

It is lost if the bathing medium is made calcium-free.
D.

It needs ATPase activity during systole and diastole

Correct Answer
B. It increases when the serum K+ rises above normal.

Explanation
Cardiac contractility refers to the ability of the heart to contract and pump blood effectively. It is influenced by various factors, including the interaction between actin and myosin, the availability of calcium ions, and the activity of ATPase during systole and diastole. However, an increase in serum potassium (K+) levels does not enhance cardiac contractility. In fact, high levels of potassium can disrupt the normal electrical conduction in the heart and lead to arrhythmias, which can impair cardiac function. Therefore, the statement that cardiac contractility increases when serum K+ rises above normal is not true.

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7.

Within physiological limits, if the EDV is increased:

A.

The force of cardiac contraction is increased.
B.

The cardiac output is increased.
C.

Starling's law is applicable.
D.

All of the above.

Correct Answer
D. All of the above.

Explanation
When the end-diastolic volume (EDV) is increased within physiological limits, it leads to an increase in the force of cardiac contraction. This is because the heart muscle fibers are stretched more, allowing for a stronger contraction. Additionally, the increased EDV also results in an increase in cardiac output, which is the amount of blood pumped by the heart per minute. Finally, the increased EDV falls under Starling's law, which states that the more the heart muscle fibers are stretched, the stronger the contraction will be. Therefore, all of the statements mentioned in the options are correct.

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