Force Generation by the Heart Flashcards
What is the general structure of cardiac muscle?
Striated
What does striated mean?
Regular arrangement of contractile proteins
What kind of junction is present in cardiac muscle?
Gap junctions
What is the purpose of desmosomes?
Provide mechanical adhesion
What is the contractile unit of cardiac muscle?
Myofibril
Describe the structure of myofibrils
Alternate between thick and thin protein filaments (myosin and actin)
What is the functional unit of cardiac muscle?
Sarcomere
How is muscle tension produced?
Sliding of actin filaments on myosin
What chemical is responsible for formation of cross-bridges?
ATP
What will inhibit contraction?
Absence of Ca
What is the importance of calcium?
When it binds to troponin the myosin binding site on actin is exposed and the bridge can form in the presence of ATP
What needs to be present for calcium to be released from the SR?
Extra-cellular calcium
Describe the roll of calcium in the plateau phase of ventricular muscle action potential?
Calcium will flow through L-type channels and enter myocytes, stimulating the release of more calcium from within the SR within myocytes
What happens to CICR after contraction
Calcium influx will cease and so will CICR
When is it not possible to generate another action potential?
During the refractory period
What state are the Na channels in during the plateau phase?
Closed depolarised state
What state are the K channels in during the descending phase?
Open
What is stroke volume?
Volume ejected per ventricle per minute
What can change the stroke volume?
Changes in diastolic length of myocardial fibres (volume of blood within the ventricles at the end of the relaxation period)
What is EDV dependant on?
Venous return to the heart
What does the frank starling curve tell us about the relationship between EDV and SV?
The more the EDV the higher the SV, unless we strain the heart too much which is unlikely in a normal heart
What does stretch do to Ca affinity for troponin?
Decreases it
How is optimum length of cardiac muscle achieved?
By stretching it within the FS curve
How does the FS curve compensate for decreased SV caused by increased after load?
Increasing the force of contraction until eventually ventricular hypertrophy occurs
What is after load?
resistance into which the heart is pumping
imposed AFTER contraction
What can extrinsically control SV?
Nerves and hormones
How is ventricular muscle innervated?
sympathetic nerve fibres
What does sympathetic stimulation cause?
Increased HR and force of contraction
how does sympathetic stimulation cause a positive inotropic effect?
by activating Ca channels, mediated by cAMP, raising peak ventricular pressure and dP/Pt during systole increases, increasing the duration of systole and reduced diastole duration
What shifts the FS curve left?
Sympathetic stimulation
What shifts the FS curve right?
Heart failure
What affect do adrenaline and noradrenaline have?
Positive inotropic and positive chronotropic effect
