L18-cardiac Properties Flashcards
What is meant by myocardial excitability
Ability of cardiac muscle to be excited by adequate stimulus generating an action potential and contraction
What is the refractory period
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
Refractory period for atrial and ventricular muscle
Atrial about 0.15 second and Ventricles 0.25 to 0. 30
Why do cardio myocytes have a longer refractory period than other muscle cells
Given the long plateau from the slow calcium ion channels in phase 2
What does the degree of refractoriness indicate
The number of fast sodium ion channels that have recovered from the inactive state and are capable of reopening
What is meant by absolute refractory period
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
When is the absolute refractory period
Depolarization and the first 2/3 of re-polarization phases 0,1,2 and beginning of phase 3
What is the duration of the absolute refractory period
0.25 to 0.3 seconds
What does the absolute refractory period occupy mechanically
Whole period of systole and early diastole
What does the longer factory period prevent
Tetanic contractions
What is the relative refractory period
A greater than normal stimulus and cause an action potential

What is the time for the relative refractory period and its duration
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
What is the significance of a longer refractory period of the cardiomyocytes
Allows sufficient time for the ventricles to empty and refill prior to the next contraction
What are the factors affecting myocardial excitability
Cardiac innervation
Effect of ions concentration in ECF
Define myocardial conductivity
Ability of cardiac muscle to transmit the cardiac impulse which is originated in the SAN to all parts of the heart
 describe the pathway for myocardial conductivity
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
What stops the transmission of the action potential from the atria to the ventricles
The region of fibrous tissue called the annulus fibrosis
What does the wolf Parkinson white or pre-excitation syndrome
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
Where is the atrioventricular node
Right atrium of the posterior part of the interatrial septum close to the opening of the coronary sinus
Functions of atrioventricular node
▪️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
What are the functional characteristics of the AV node
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
What is the importance of the AVN delay
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
Describe the bundle of His
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
Sequence of depolarization in ventricles
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
