Force generation by heart - lecture 4 Flashcards
What type of muscle is the cardiac muscle,what causes it and what is absent from it?
- Striated
- caused by regular arrangement of contractile protein
- no neuromuscular junctions in the cardiac muscle
What is the all or none law of the heart and how is this carried out?
- either electrical activity is initiated in the whole heart or none at all
- Cardiac myocytes are electrically coupled by gap junctions which forms low resistance electrical communication pathways between neighbouring myocytes and so all electrical excitation reaches all cardiac myocytes
What provides mechanical adhesion between cardiac cells and why is this important?
Desmosomes within the intercalated discs and this ensures that the tension developed by one cell is transmitted to the next
What causes muscle tension ?
Produced by sliding of actin filaments (thin) on myocine filaments (thick)
What does force generation depend on?
ATP- dependant interaction between myosin and actin filaments (without ATP- will not occur)
What is troponin?
During a relaxed stat, binding sites of myosin to actin are covered by troponin
Why is Ca++ required?
Required to bind with troponin to uncover binding site and to switch on cross bridge binding which allows contraction
What is released from the sacroplasmic reticulum ands what is it dependant on?
Ca++ - this release is dependant on presence of extra-cellular Ca++
Where is the concentration of Ca++ high when ventricular muscle relaxes?
High in sacroplamisc reticulum but low in cytoplasm
What happens to Ca++ during ventricular muscle contracts ?
Induced Ca++ release from sarcoplasmic reticulum
What is the refractory period?
A period following an action potential in which it is not possible to produce another action potential
Why is long refractory period important?
Protective for the heart by preventing generation of tetanic contractions in the cardiac muscle
What is stroke volume?
The volume of blood ejected by each ventricle per heart beat
How can Stroke volume be calculated?
SV= end diastolic volume -end systolic volume
What does intrinsic mean?
Within the heart muscle itself
What does extrinsic mean?
Nervous and hormonal control
What brings about the change in stroke volume?
Changes in diastolic length of my myocardial fibres
What determines cardiac preload?
The diastolic length of myocardial fibers is determined by “the volume of blood within each ventricle at the end of diastole”. This is called the end diastolic volume. It determines the cardiac PRELOAD
What determines end diastolic volume?
Venous return to the heart
What Is the frank-starling mechanism ?
It states that the more the ventricle is filled with blood during diastole (end diastolic volume), the greater the volume of ejected blood will be during systolic contraction( stroke volume)
What is afterload?
The resistance into which heart is pumping
What happens if after load increases and continuous to?
This is untreated hypertenion and eventually the ventricular msucle mass increase (ventricular hypertrophy) to overcome the resistance
What does sympathetic nerves do to force of contraction and what is this called?
Increases it - positive inotropic effect
Describe the effects of sympathetic stimulation on ventricular contraction.
- Force of contraction increases (activation of Ca++ channels - greater Ca++ influx)
- The effect is cAMP mediated
- The peak ventricular pressure rises
- Rate of pressure change (dP/dt) during systole increases
- This reduces the duration of systole
- Rate of ventricular relaxation increases (increased rate of Ca++ pumping)
- This reduces the duration of diastole
What is the effect of parasympathetic nerves on ventricular contraction?
- Very little enervation of ventricles by vagus nerves there is very little effect on stroke volume
- Vagal stimulation hasm major influence in heart rate, but not force of contraction
What are the extrinsic control of stroke volume?
Adrenaline and noradrenaline have inotropic and chronotropic effects
What is cardiac output?
The volume of blood pumped by each ventricle per minute
CO=SV x HR
What is the resting cardiac output in a healthy adult?
-approx 5 litres
70ml (SV) x 70bmp = 4900ml
