Force generation by the heart Flashcards
What causes the striation of cardiac muscle?
regular arrangement of contractile protein within the cardiac muscle cells
What ensures that the electrical excitation reaches all of the cardiac myocytes?
Gap junctions
What are desmosomes?
structures within the intercalated discs provide mechanical adhesion between adjacent cardiac cells.
-They ensure that the tension developed by one cell is transmitted to the next
what are myofibrils?
Intracellular organelles in the muscle fibres. They are the contractile units of the muscle
What is a sarcomere?
it is the functional unit of the muscle. Actin and myosin are arranged into sarcomeres within each myofibril
What is the functional unit of a cell?
The smallest unit that can do all the functions of the tissue
How do muscles contract?
Sliding of the actin filaments on the myosin filaments.
What has to be present for cross bridges to generate?
ATP
What two molecules must be present for contraction and relaxation of muscle?
ATP and Ca++
How does calcium switch on cross bridge formation?
- myosin binding sites on actin are covered by regulatory proteins (troponin and tropomyosin)
- when calcium binds to these regulatory proteins, it causes a conformational change causing the proteins to release from the myosin binding site on actin.
- Thereby allowing myosin to form a cross bridge
Where is calcium stored in cardiac muscle cells?
in the lateral sacs of the sarcoplasmic reticulum
What is required for SR to release calcium?
The presence of extra cellular Ca++
What happens when there is not enough intracellular calcium?
Muscles relax
What happens during the plateau phase of action potential?
Calcium will flow through the L type Ca++ channels into the cardiac myocytes. This will then cause SR to release calcium
What happens when the action potential has passed?
, Ca++ influx ceases, Ca++ re-sequestered in SR by Ca++-ATPase, heart muscle relaxes
True/ False. During the plateau phase of ventricular action potential, the Na+ channels are in the hyperpolarised, open state
False,
During the plateau phase of ventricular action potential the Na+ channels are in the depolarised closed state i.e. they are not available for opening
What is the benefit of the long refractory period?
To protect the heart muscle via preventing generation of tetanic contractions.
What is the long refractory period?
is a period following an action potential in which it is not possible to produce another action potential
True/False. During the descending phase of action potential the K+ channels are open and the membrane can not be depolarised
True
What is stroke volume?
This is defined as “the volume of blood ejected by each ventricle per heart beat”
What is the equation for Stroke volume?
SV = End Diastolic Volume (EDV) – End Systolic Volume (ESV
What is the stroke volume regulated by>
Intrinsic and extrinsic mechanisms.
- intrinsic- within the heart muscle itself
- extrinsic- nervous, hormonal control
What causes changes in stroke volume?
changes in the DIASTOLIC LENGTH of MYOCARDIAL FIBERS
What is the end diastolic volume?
the volume of blood within each ventricle at the end of diastole.
-It is determined by the venous return to the heart
What is the cardiac preload?
How much the heart is loaded with blood before it contracts
What is the Starling’s law of the heart?
“the more the ventricle is filled with blood during diastole (END DIASTOLIC VOLUME), the greater the volume of ejected blood will be during the resulting systolic contraction (STROKE VOLUME)
What does Afterload mean?
Resistance into which the heart is pumping
How does the body respond to increased afterload?
- increased afterload leads to decreased stroke volume and therefore EDV increases to compensate
- hypertrophy is afterload increases over time.
What is a positive inotropic effect?
Increase in the force of contraction
What are the effects of noradrenaline on ventricular contraction?
Leads to a stronger and faster contraction.
- force of contraction increases by activation of Ca channels
- peak ventricular pressure rises
- rate of pressure change during systole increases
- reducing the duration of systole
- rate of ventricular relaxation increases
- this reduces the duration of diastole
Any end diastolic value will lead to an increased stroke volume if sympathetic stimulation occurs. T/F?
True
What is the extrinsic control of stroke volume?
Hormones adrenaline and noradrenaline are released from adrenal medulla and have inotropic and chronotropic effects
What is cardiac output?
The volume of blood pumped by each ventricle per minute
What is the equation for cardiac output?
CO=SV x HR
