Flashcards in Force Generation by the Heart Deck (53)
Describe cardiac muscle?
No neuromuscular junctions
Protein channels which form low resistance electrical communication pathways between neighboring myocytes
Intercalated discs - desmosomes
Contains gap junctions
What causes striations?
Regular arrangement of contractile proteins
Why are there no neuromuscular junctions in the heart?
Because cardiac muscle does not require nervous stimulation- it is myogenic
What do protein channels in cardiac muscle ensure?
Ensure that each electrical excitation reaches all of the cardiac myocytes (All or none law of the heart)
What do the desmosomes between intercalated provide?
Provide mechanical tension between adjacent cardiac cells
they ensure the tension developed by one cell is transferred to the next cell
What does each muscle contain?
Myofibrils - contractile units of muscle
What do myofibrils have?
Alternating segments of thick and thin protein filaments (actin and myosin) actin is lighter appearance, myosin is darker
How are actin and myosin arranged in a myofibril?
Into sarcomeres (smallest functional unit of a muscle)
How is muscle tension produced?
Sliding of actin filaments on myocin filaments
What is the sliding filament theory?
Explanation of how muscle shortens and produces force
What does force generation depend on?
ATP dependant interaction between actin and myocin filaments
ATP is required for both contraction and relaxation. True or false?
What is Ca2+ required for?
Switching on cross bridge formation
Where is Ca2+ released from?
What is the release of Ca2+ from SR dependent on in cardiac muscle?
Presence of extracellular Ca2+
Systole- what happens with calcium?
Ca2+ influx during plateau phase
Ca2+ induced Ca2+ release from sarcoplasmic
Ca2+ influx activates contractile machinery
What happens when an AP has passed after systole?
Ca++ influx ceases
Ca++ is re-sequestered in sarcoplasmic reticulum by Ca++ ATPase
Heart muscle relaxes
Describe the muscle fibre when relaxed?
No cross-bridge binding site because the cross-bridge binding site on actin is physically covered by the troponin-tropomyosin complex
What does the bindning of actin and myosin cross-bridge trigger?
Power stroke that pulls actin in during contraction
Describe the muscle fibre when excited?
Ca++ binds with troponin
Pulling troponin-tropomyosin complex aside to expose cross-bridge binding site
Cross-bridge binding occurs
What is the importance of a refractory period in the cardiac cycle?
The long refractory period is protective for the heart preventing generation of tetanic contractions in the cardiac muscle
What triggers contraction?
The ventricular muscle AP
What is a refractory period?
Period following an AP in which it is not possible to produce another AP
Theory of a refractory period?
During plateau phase of a ventricular AP: The Na+ channels are in the depolarised closed state- not available for opening
During the descending phase of AP- The K+ channels are open and membrane cannot be depolarized
Define Stroke volume?
Volume of blood ejected from each ventricle per heart beat
Stroke volume equation?
SV= End diastolic volume - end systolic volume
What is SV regulated by?
Intrinsic and extrinsic mechanisms
Intrinsic= within the heart itself
Extrinsic= Nervous and hormonal control
What are changes in stroke volume brought about by?
Changes in the diastolic length/diastolic stretch of myocardial fibers
What is the diastolic length/stretch (CARDIAC PRELOAD) determined by?
The volume of blood within each ventricle at the end of diastole - The END DIASTOLIC VOLUME