Force Generation by the Heart Flashcards
Describe cardiac muscle?
Striated 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?
True
What is Ca2+ required for?
Switching on cross bridge formation
Where is Ca2+ released from?
Sarcoplasmic reticulum
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
reticulum
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
What is the end diastolic volume determined by?
Venous return to the heart
Another name for the Frank-starling mechanism ?
Starling’s law of the heart
What does the Frank starling mechanism state?
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 relationships does Starling’s law of the heart outline?
Relationship between venous return, end diastolic volume and stroke volume
What does diastolic stretch increase?
Affinity of troponin for Ca++
How is optimal length achieved in cardiac muscle?
Stretching the muscle. In skeletal muscle it is achieved at resting muscle length
What happens if venous return to right atrium increases?
EDV of right ventricle increases which leads to increased SV into pulmonary artery
Venous return to left atrium from pulmonary vein increases?
EDV of left ventricle increases= larger SV into aorta
Define afterload?
The resistance into which the heart is pumping
When is the afterload imposed?
After the heart has contracted
What happens if the afterload increases?
-At first the heart is unable to eject full SV so the end diastolic volume increases
If it continues-Eventually the ventricular muscle mass increases to overcome resistance
What is an increase in ventricular muscle mass called?
Ventricular hypertrophy
What do you call increased afterload existing continuously?
Hypertension
What type of nerves is the ventricular muscle supplied by
Sympathetic nerve fibers
What is extrinsic neurotransmitter?
Noradrenaline
What does stimulation of sympathetic nerves do to the heart?
Increases the force of contraction - positive inotropic effect
Positive chronotropic effect- - increases the HR
What does extrinsic control involve?
Nerves and hormones
What does heart failure do to Frank-Starling curve?
Shifts it to the right
What does sympathetic nerve stimulation do to Frank-Starling curve?
Shifts it to the left
Effect of parasympathetic nerves on ventricular contraction ?
Very little innervation of ventricles by vagus in man- little/no effect on SV
Vagal stimulation has influence on rate not force of contraction
Hormones that control stroke volume?
Adrenaline and noradrenaline
These are released from adrenal medulla and have inotropic and chronotropic effects
The effects are normally minor compared to effects of noradrenaline from sympathetic nerves
Define cardiac output?
The volume of blood pumped by each ventricle per minute is known as the Cardiac Output (CO)
CO equation?
CO= SV x HR
What is the resting CO in a healthy adult?
5 litres per minute
