Lecture 06 Cardiac Muscle Tissue Flashcards
What are the characteristics of the cardiac muscle tissue syncytium?
Sarcomeric arrangement (striated) Mononucleated Central nuclei Syncytium Intercalated discs Cells may branch
What is true of cardiac muscle cells but not skeletal muscle fibers?
Cells are mononucleated
What are the changes of mV in a ventricular fiber during action potential?
Averages about 105 mV
Rises from -85 to +20mV
Remains depolarized for about .2 sec following initial spike
Plateau
Sudden repolarization at the end of the plateau
Where are T tubules found in skeletal muscles fibers compared to cardiac muscle fibers?
T tubules are found at the ends of the thin filaments in skeletal muscle and along the z line in cardiac muscle
How many T tubules per sarcomere in skeletal muscle fibers?
2
What do T tubules form with the sarcoplasmic reticulum in the cardiac muscle fiber?
Diads
What do T tubules form with the SR in skeletal muscle fiber?
Triads
The SR is less extensive in which type on muscle fiber?
Cardiac
What do cardiac muscle cells form?
Syncytium
In the motor unit arrangement in the skeletal muscle fiber, how many nerve fiber synapses with one or more muscle fibers?
1
Why does cardiac muscle fibers only form diads?
T tubules are at the very end so there is room for only one cisternae on one side of the tubules
What are the two main types of cardiac action potentials?
Fast and slow
Fast action potentials are found where?
Atria, ventricles and conduction system (purkinjie)
Where do the very rapidly conducting but non-contractile action potentials occur?
purkinje fibers
Where do the rapidly conducting and contractile action potentials occur?
Atrial and ventricular fibers
Where are slow cardiac action potentials found?
SA and AV nodal tissues
What are the characteristics of Slow cardiac action potentials?
Conduct slowly and automatically depolarizes during resting phase
What are the peaks of amplitude in fast and slow cardiac action potentials?
Fast 100mV
Slow 60 mV
What does it mean when slow cardiac action potentials automatically depolarizes during resting phase?
Start to creep back up and its automatic. Its leaky and reach threshold and fires again. Doesnt need any extrinsic signal
The resting potential of -85 mV is characteristic of which of the following phases of the cardiac fast action potential?
Stage 4
What are the phases of fast action potentials?
Phase 4: resting potential
Phase 0: Rapid depolarization
Phase 1: Initial, incomplete repolarization
Phase 2: Plateau or slow decline of membrane potential
Phase 3. Reploarization
Fast action potentials are due to changes in conductance of what ions?
potassium, sodium, and calcium ions
What is the conductance pattern mostly due to?
Voltage dependent gates
What results in a fast conduction velocity
Greater AP amplitude
More rapid rate of rise of phase 0
Larger cell diameter
Slow action potentials do not have what type of gates?
Fast sodium ion gates
What is the upstroke (depolarization) of action potential due to?
Slow action potential
Calcium
Why does Calcium proceed slowly?
Because Ca concentration is much lower outside of the cell than the sodium concentration
Resting phase potential 4 is close to what mV?
Slow action potential
-60mV
True/False
Change in slow action potential (amplitude) is less than that for fast action potentials
True
SA and AV nodal tissue spontaneoulsy do what to reach threshold during phase 4?
Depolarize slowly
What are the characteristics of fast type contractile myocytes?
Large diameter
High amplitude
Rapid onset of action potential
What are teh characteristics of fast type non-contractile myocytes?
Very large diameter
Very rapid upstroke
What are the characteristics of slow type non-contractile myocytes?
Small diameter
Low amplitude
Slow rate of depolarization
What channels cause the action potential in ventricular fibers?
Fast sodium channels and slow calcium-sodium channels
What is responsible for the initial spike of action potential in ventricular fibers
Fast sodium channels
What ion is necessary for electrical-mechanical coupling?
Calcium
What is the source of calcium necessary for electrical-mechanical coupling
T tubules via diffusion through voltage-dependent dihydrophyridine receptors
And cisternae through channels called ryanodine recpeptors
What type of calcium channels are found in the T tubule membrane in cardiac muscle?
L - type and they are referred to as dihydropyridine receptors and they open in response to the action potential
Once calcium is flowing through the dihydropyridine receptors, they cause what channel to also open and release what ion?
Open the calcium release channels in the SR and they are referred to as ryanodine receptors
Why is it impossible to generate another action potential during absolute refractory period?
During the period the sodium channel gates are closed and nothing will get them open
Why during the relative refractory period a stronger than normal stimulus can generate an action potential?
Towards the end of the repolarization this period occurs where some of the gates can now open at that point.
What makes a refractory period either shorter or longer?
The faster the ion channels and gates return to phase 4 stage the short the RP
The slower the ion channels and gates return to phase 4 the longer the RP
What is involved in gradual depolarization?
SA AV nodes and the purkinje fibers
Does the SA node or the AV node usually depolarizes more rapidly than the others and reaches threshold first?
SA
What becomes the normal “pacemaker” of the heart’s rhythmicity?
SA node
What determines the rhythmicity of the cell?
Rate of depolarization
Gradual depolarization during phase 4 is cause by what?
Special sodium channels which open following phase 3
What is responsible for the plateau that characteries a cardiac muscle action potential?
A high concentration of calcium ions in the intracellular fluids
What is the difference between the repolarization in skeletal muscle vs cardia muscle?
In both the skeletal muscle and the cardiac muscle the sodium channels close rapidly, however the calcium channels open slowly and stay open for a longer period of time in cardiac muscle
What ions are responsible for the plateau in action potential in cardiac muscles?
Calcium ions - Channels open slowly and Ca slowly comes in
Potassium - channels open slowly
So with Ca slowly coming in and adding to the positivity and on top of that the K is not leaving quickly then it plateaus
In what tissue are there far fewer ca-induced calcium release channels? what does this allow?
In cardiac muscle and this allows fine control over sarcoplasmic calcium concentration and contractility.
Can fine control over sarcoplasmic calcium concentration and contractility happen ins skeletal muscle? why or why not?
No, excitation always triggers maximum release of calcium from the SR
What is the electromechanical coupling
The coupling which transforms an electrical impulse into a mechanical action
What are the two ways Ca is removed from the intracellular fluid?
It is sequestered into the SR using ATP.
It is pumped out of the cell using antiporters secondary transport. Sodium calcium pumps
The sodium-calcium exchanger in the sarcolemma of the cardiac muscle cell is an example of what mechanism?
Secondary active transport involving antiporter
Relaxation is the result of which two transporters?
SERCA-Sarcoplasmic reticulum calcium ATPase
Sodium-calcium exchanger in the sarcolemma
How does SERCA work?
Stimulated by phosphorylation via an integral SR protein called phospholambian. Reduces its ability to inhibit the SERCA pump.
Returns calcium to the SR during diastole
What does SERCA allow?
Even greater calcium release on the next beat and for fast clearance of calcium from the sarcoplasm
Left ventricular isovolumic contraction occurs immediately following what events?
Closure of AV valve
What is diastole?
When ventricles are relaxed
What is systole?
When ventricles are contracted
On the electrocardiogram, what does the P section represent?
Represents atrium contracting and there is depolarization of the atrial muscle
On the electrocardiogram what does the QRS section represent?
Represents the ventricular contraction and depolarization
The T curve in the electro cardiogram represents what?
The reploarization of the ventricle
Why is the repolarization of the atrium not represented in the electrocardiogram?
Because it is hidden by the QRS wave
What are the three section of diastole?
Rapid inflow: Blood into the atrium
Diastasis: Slow inflow of blood into the atrium
Atrial systole: Atrium contracts
Is there any electrical activity happening in Diastole?
Not until the last step of atrial systole and the atrium contracts
What period of diastole becomes compressed when the heart rate goes up rapidly?
The diastasis
When does the Ventricular volume increase?
During the Diastole when the AV valve opens and blood flows into the ventricle
When the AV valve closes what opens? Does this start the systole or diastole stage?
The aortic valves open and it starts the systole. The atrium has contracted and closed
When does the ventricular pressure and aortic pressure increase?
During the systole stage and the blood is being ejected.
If atrium does not contract do the ventricles still fill with blood?
Yes
When does the majority of blood flow from the atria to the ventricles?
80% of blood flows before the atria contract
How much additional blood flow does the contraction of atria contribute?
20%
What valves are closed during systole?
AV valves
When do the AV valves open? And why?
At the end of systole because of increased pressures in the atria
True/False
Blood is always flowing into the atriums
True
During what period are the ventricles contracting?
Isovolumic contraction
During the isovolumic periods are the valves open or closed
All where closed
What happens during the Isovolumic relaxtation?
Ventricles are relaxing
Why are AV valves closed during systole?
To keep the blood from going back into the atrium
What is the purpose of the chordae tendineae and papillary muscle?
To maintain proper tension and keep the flaps from going backwards on the tricuspids and mitral (this would cause blood to flow back into the atrium)
Which of the following statements is true for the period of rapid ejection?
a. It occurs when the left ventricular pressure is above 80mmHg.
b. The semilunar valves are closed during this phase.
c. It occurs during the last two - thirds of ejection
d. It results in the ejection of about 90% of the total volume of blood
A.
During diastasis a small amount of blood flows into the ventricles, what does this represent?
Blood that continues to flow into atria during diastole. Blood is continuously flowing into the atria
During atrial systole how much blood is pushed into the ventricles?
20%
What are Semilunar valves?
Half moon like crescent valves that are the pulmonary and aortic valves
What are the AV valves?
Mitral and tricuspid
What happens during the isometric contraction?
Isometric is the same as isovolumic
Ventricles contract but the semilunar valves do not open for .02 to .03 seconds
What does the A on artial pressure represent?
After depolarization/contraction of atria (p) the a on the atrial pressure graph rises.
A represents pressure wave caused by atrial contraction
What does the C represent on the atrial pressure?
Pressure wave caused by backflow of blood into atria at beginning of ventricular contraction
What does the V represent on the atrial pressure?
Pressure wave caused by slow flow of venous blood into atrium while AV valves are closed
The slow flow of venous blood into the atrium causes the pressure in the atrial pressure to increase. What does this pressure allow?
This allows for when the AV valves open for most of the blood to flow through before the atrial contraction
When does the period of rapid ejection occur?
Occurs when left ventricular pressure is a little above 80 mm Hg and right ventricular pressure is slightly above 8 mm Hg
What valves are open during rapid ejection?
Semilunar valves open
Why is the pressure higher during rapid ejection in the left ventricular pressure (80 mm Hg) than the right ventricular pressure (8mm Hg)?
High pressure is in the aorta vs in the low pressure in the pulmonary artery. Mechanics are the same but left has higher pressure going into the systemic system.
Are the volumes in each ventricle the same?
Yes - unless you are increasing heart rate then there is one cycle period necessary to readjusting the volume equilibrium between the two ventricles
Why is the pressure on the left side 80 mm Hg?
Has to slightly surpass the back pressure in the proximal aorta (ascending aorta) only way to force the aortic valve open
How much blood is ejected during the rapid ejection?
70% of the blood that will be ejected. We are not going to eject all of the blood that is in the ventricle.
What period of systole is there rapid ejection?
The first third of ejection
What is the period of slow ejection?
Remaining 30% of blood is ejected from the ventricles and occurs during the last two-thirds of ejection
What is the Frank-Starling Law?
The greater the heart muscle is stretched during filling, the greater the force of contraction and the greater the quantity of blood pumped into the aorta
What does the stretching of the cardiac muscle do to actin and myosin filaments?
Brings the actin and myosin filaments to a more nearly optimal degree of overlap for force generation. So the myosin and actin are not completely pulled apart but and some overlap is there, then there is more room to create a contraction
Stroke volume output can be increased by which of the following mechanisms? A. Decreasing EDV B. Increasing EDV C. Increasing ESV D. By decreasing EDV and increasing ESV
B
What is EDV?
End Diastolic volume. Referring to the ventricle
Volume at the end of diastole - how much blood has come into the ventricle before the ventricles are going to contract
110-120 mL volume per cycle but can be increased to 180 mL
What is SV?
Stroke volume - The amount of blood that will be pumped out of the ventricle per cycle. 70 mL
If there are about 70 beats/min and 70mL of blood pumped out per beat, how much blood is pumped out of the heart per minute?
(70kg male)
5L is the average
What is ESV?
End systolic volume, Volume of blood in the ventricle at the end of systole. When the ventricle contracts ( no matter how strong) there will be some blood left in the ventricle. Blood left is usually 40-50mL but can be as little as 10ml
What is ejection fraction?
SV/EDV % of blood that was originally in the ventricle that was ejected. SV = 70 EDV = 110 70/110 = 64%
How can stroke volume output be increased to more than double?
By decreasing the amount of blood still left in the ventricle (ESV)
Increasing the amount of blood in the ventricle before contraction
Increase EDV from 110 to 180 in the ventricle before
Decrease ESV from 50 to 10 what is left
180-10 = 170 ejected
The norm was 70 SV now its 170 SV so more than doubled
Why would the heart increase the amount of blood per cycle?
Tissue in need of more oxygen
When the ventricle fills up why does the pressure not increase by much?
Because the AV valve is open which relieves some of the pressure
What are the phases of EW
Phase I: Period of filling
Phase II: Isovolumic contraction
Phase III: period of ejection
Pase IV: Isovolumic relaxation
What happens to volume and pressure during the isovolumetric contraction?
The volume remains the same (AV just shut and the aortic is shut) but the ventricle contracts so the pressure goes up
When isovolumetric contraction end, why does the Aortic valve opens?
The pressure is a little bit more than the back pressure in the aorta.
What happens to the pressure during the period of ejection?
the pressure continues to increase because the ventricle continues to contract and pushes the blood out of the ventricle. As blood leaves the ventricle the pressure in the ventricle begins to drop
What causes the aortic valve to close?
There is a lot of blood in the aorta now so the high pressure in the aorta, there is enough back pressure to have it close
What is EW?
The area that shows the net external work by left ventricle during the cardiac cycle. Shows how much work is done by the heart
What is the flow and velocity in the proximal aorta?
Mean velocity = 40 cm/s
Flow is phasic
Velocity 120 cm/s (systole) to negative value before aortic valves close in diastole (backflow)
What is phasic?
Flows up and down in concert with the pulsation of the heart itself
When is velocity greater in the distal aorta and arteries?
Greater in systole than diastole
Why does the blood continue to flow in a forward flow once it is pumped out of the heart?
The blood vessel walls during diastole are elastic and when expanded they will begin to contract down and that will cause the blood to flow foward
What is the rate of blood flow to each tissue controlled by?
Usually precisely controlled in relation to tissue need
How much more blood flow with active tissues need compared to at rest?
20 to 30 times more
Cardiac output cannot exceed ______ x greater than at rest
4-7
What monitors tissue needs?
Microvessels
What do the needs of tissues act directly on?
Local blood vessels
What else can help control tissue blood flow?
Nervous control and hormones
What does the parasympathetic system primarily focused on?
Controlling the heart rate, controlling the pacemakers of the heart
Sympathetic nervous system is primarily focused on what?
the force of contraction
At any given right atrial pressure, the cardiac output increases during increased _________ stimulation
Sympathetic
Note that at any given right atrial pressure, the cardiac output decreases during increased _______ stimulation
parasympathetic
Starting at what pressure does the heart start to decrease the amount of blood pumped from 5L/min?
175 arterial pressure (mm Hg)
Excess potassium in the extracellular fluid would have what effect on the heart activity?
Heart will become dilated. More blood is coming in that cant be pumped out. Heart becomes weaker. Harder to get an action potential because K cant leave the intracellular fluid to repolarize
What will an increase in the concentration of Ca in the extracellular matrix result in?
Heart rate will increase
Heart will go toward spastic contraction
Heart action potential will increase
