L18-cardiac Properties

Question 1

What is meant by myocardial excitability

Answer 1

Ability of cardiac muscle to be excited by adequate stimulus generating an action potential and contraction

Question 2

What is the refractory period

Answer 2

Interval of time during which a normal cardiac impulse cannot reexcite and already excited area of card muscle
Time from phase zero until the next possible depolarization of a Myocyte

Question 3

Refractory period for atrial and ventricular muscle

Answer 3

Atrial about 0.15 second and Ventricles 0.25 to 0. 30

Question 4

Why do cardio myocytes have a longer refractory period than other muscle cells

Answer 4

Given the long plateau from the slow calcium ion channels in phase 2

Question 5

What does the degree of refractoriness indicate

Answer 5

The number of fast sodium ion channels that have recovered from the inactive state and are capable of reopening

Question 6

What is meant by absolute refractory period

Answer 6

Cardiac muscle completely insensitive to further stimulation and sodium channels are closed therefore stimulation of the muscle during this phase cannot produce further action potential

Question 7

When is the absolute refractory period

Answer 7

Depolarization and the first 2/3 of re-polarization phases 0,1,2 and beginning of phase 3

Question 8

What is the duration of the absolute refractory period

Answer 8

0.25 to 0.3 seconds

Question 9

What does the absolute refractory period occupy mechanically

Answer 9

Whole period of systole and early diastole

Question 10

What does the longer factory period prevent

Answer 10

Tetanic contractions

Question 11

What is the relative refractory period

Answer 11

A greater than normal stimulus and cause an action potential



Question 12

What is the time for the relative refractory period and its duration

Answer 12

The last 1/3 of repolarization the rest of phase three
mechanically it occupies the middle of diastole
the duration is 0.05 seconds in ventricles and 0.03 seconds in atria

Question 13

What is the significance of a longer refractory period of the cardiomyocytes

Answer 13

Allows sufficient time for the ventricles to empty and refill prior to the next contraction

Question 14

What are the factors affecting myocardial excitability

Answer 14

Cardiac innervation

Effect of ions concentration in ECF

Question 15

Define myocardial conductivity

Answer 15

Ability of cardiac muscle to transmit the cardiac impulse which is originated in the SAN to all parts of the heart

Question 16

 describe the pathway for myocardial conductivity

Answer 16

Action potential proceeds along the internodal pathways in the atria and finally reaches the AV node on top of the septum the wave of the depolarization spreads in the rapidly conducting purkinjie fibers to all parts of the ventricles

Question 17

What stops the transmission of the action potential from the atria to the ventricles

Answer 17

The region of fibrous tissue called the annulus fibrosis

Question 18

What does the wolf Parkinson white or pre-excitation syndrome

Answer 18

If there is an electrical leak in the ring of fiber tissue separating the atria and ventricles this would be an alternative direct route for excitation to spread from atria to ventricles which will generate arrhythmia

Question 19

Where is the atrioventricular node

Answer 19

Right atrium of the posterior part of the interatrial septum close to the opening of the coronary sinus

Question 20

Functions of atrioventricular node

Answer 20

▪️Receives impulses originating from the SAN and transmitted to the ventricles to the AV bundle
▪️can initiate the cardiac impulses but at a slower rate(40-60)
▪️Delays the passage of cards of impulse from the atria to the ventricles which allows time for the atria to empty their contents into the ventricles before ventricular contraction begins

Question 21

What are the functional characteristics of the AV node

Answer 21

Cells within the AVN structure you resemble those of the SAN and the shape of the action potential is similar except:
Initial resting potential is about -80 mV and depolarization does not normally exceed + 5 to +10 mV
Depolarization is in phase 4 is slower because of the absence of the population of sodium channels hence the slower intrinsic rate of firing
And I was the very short delay of O one second cause by the small diameter of the noodle cells and slow conductive fibers

Question 22

What is the importance of the AVN delay

Answer 22

Allows a trio time to contract and fully empty prior to ventricular systole
Allows the AVN to act as a gatekeeper limiting the transmission of ventricular stimulation during abnormally rapid atrial rhythms

Question 23

Describe the bundle of His

Answer 23

Consists of perKenje fibers:
Modified ventricular muscle cells that are grouped together in left and right bundles
they have a relatively large diameter and they are the largest of all the cells in the heart which means they have a high conduction velocity

Question 24

Sequence of depolarization in ventricles

Answer 24

Starting with the papillary muscles and the septum and then spreading to the endocardial part of the ventricular muscle and out towards the epicardial surface
Responsible for shape of ECG