Guyton Chapter 9 - Cardiac Muscle Flashcards
What 2 channels open to cause action potential in cardiac muscle?
- The same fast sodium channels as those in skeletal muscle
- Another entirely different population of slow calcium channels which are also called calcium-sodium channels
What helps to account for both the prolonged action and its pleateu in cardiac muscle?
Immediately after the onset of the action potential, the permeability of the cardiac muscle membrane for potassium ions decreases about fivefold, an effect that does not occur in skeletal muscle. The decreased potassium permeability greatly decreases the outflux of positively charged potassium ions during the action potential plateau and thereby prevents early return of the action potential voltage to its resting level.
The mechanism of excitation-contraction coupling is quite the same as for skeletal muscle but there is a second effect that is quite different - what is this?
In addition to the calcium ions that are released into the sarcoplasm from the cistarnae of the sarcoplasmic reticulum - calcium ions also diffuse into the sarcoplasm from the T tubules themselves at the time of the action potential which opens voltage dependent calcium channels in the membrane of the T tubules. Calcium entering the cell then activates calcium release channels also called ryanodine receptor channels in the sarcoplasmic reticulum membrane triggering the release of calcium into the sarcoplasm. Calcium ions in the sarcoplasm then interact with troponin to initiate cross bridge formation and contraction by the same basic mechanism as described for skeletal muscle.
What ion in the ECF does the strength of contraction of cardiac muscle depend on?
Calcium ions in the ECF
What happens in the end of the plateau of the cardiac action potential?
The influx of calcium ions to the interior of the muscle fibre is suddenly cut off and the calcium ions in the sarcoplasm are rapidly pumped back out of the muscle fibers into both the sarcoplasmic reticulum and the T tubule extracellular fluid space. Transport of calcium back into the sarcoplasmic reticulum is achieved with the help of a calcium ATP:ase pump. Calcium ions are also removed from the cell by a sodium-calcium exchanged.
Where is the sinus node located?
In the superior lateral wall of the right atrium near the opening of the superior vena cava.
The action potential travels from here rapidly through both atria and then through the AV bundle into the ventricles.
What is the P was caused by?
Spread of depolarisation through the atria.
What is the period of isovolumic of isometric contraction?
It’s the short time that is required for the ventricle to build up sufficient pressure to push the aortic and pulmonary valves open against the pressures in the aorta and pulmonary artery. Therefore during this period, contraction is occurring in the ventricles but there is no emptying.
What is the ejection fraction?
The fraction of the end diastolic volume that is ejected.
What is the function of the papillary muscles?
They pull the vanes of the valves inward toward the ventricles to prevent their bulging too far backward toward the atria during ventricular contraction.
A so called incisura occurs in the aortic pressure curve when the aortic Valera closes - what causes this?
This is causes by a short period of backward flow of blood immediately before closure of the valve followed by sudden cessation of the back flow.
What sound is heard in the first heart sound?
When the ventricles contract, a sound is hey when the A-V valves are closed.
What is the second heart tone?
When the aortic and pulmonary valves close at the end of systole, a rapid snap can be heard because these valves close rapidly. This sound is called the second heart sound.
What is the preload?
The degree of tension on the muscle when it begins to contract. The preload is usually considered to be the end-diastolic pressure when the ventricle has become filled.
What is the afterload?
The load against which the muscle excerts its contractile force. The afterload of the ventricle is the pressure in the aorta leading from the ventricle.
