2: Force generation by the heart Flashcards
Cardiac muscle has a streaky appearance - what are these streaks called?
Striations
So cardiac muscle is described as ‘striated’
Why is cardiac muscle striated?
Alternating actin and myosin proteins
(Actin has a lighter colour and myosin has a darker colour)
Do muscles have a nerve supply?
Why?
No
Autorhythmicity - heart can generate its own nerve impulses
What allows for cell-to-cell conduction of impulses between cardiac myocytes?
Gap junctions
What property of gap junctions allows for electrical communication between cardiac myocytes?
Low resistance
What is the All-or-none Law of the Heart?
All cardiac myocytes will respond to an action potential, no matter its strength
The low resistance of gap junctions ensures that excitation reaches ___ cardiac myocytes.
all
What is the function of desmosomes?
Provide mechanical adhesion - sticks cardiac myocytes together so they all tense up together
Within a muscle fibre, fascicles contain ___ which are made up of sarcomeres.
myofibrils
Myofibrils have alternating __ and __ protein segments.
thick , thin
What is the smallest unit of contractile muscle?
Sarcomere
What are the thin, lighter segments of myofibril made of?
Actin
What are the thicker, darker sections of myofibril called?
Myosin
What are the smaller functional units found within myofibrils called?
Sarcomeres
Actin filaments ___ along myosin filaments to cause muscle ___.
slide along
muscle contraction
The interactions between actin and myosin are dependent on what two molecules?
ATP
Ca2+
ATP binds to myosin to trigger what?
Detachment of myosin cross bridge from actin
Cross bridge then slides along actin and rebinds
Ca2+ triggers (binding / detachment) of myosin-actin.
Binding
What ion is required to switch on myosin cross-bridge formation?
Ca2+
So ATP binds to myosin to detach it from actin, it then slides forward and if calcium is present, it rebinds (and the muscle has contracted)
Name the two regulatory proteins found on actin filaments.
Tropomyosin
Troponin
What is the name of the rope-like protein structure found on actin filaments?
Tropomyosin
What protein blocks myosin binding sites on actin filaments until it is disabled by calcium ions?
Troponin
Calcium binds to troponin causing a _____ change, moving the protein away and exposing the ___ binding sites on the ___ filaments.
conformational change
myosin-binding sites on actin filaments
Ca2+ is released from the ____ ____ depending on the extracellular presence of Ca2+.
sarcoplasmic reticulum
What triggers the release of Ca2+ ions from the sarcoplasmic reticulum of myofibrils?
Extracellular calcium - i.e Calcium-dependent Calcium release
During systole…
- Contractile machinery activated
- Ca2+ influx during plateau phase of action potential
- Ca2+ released from the sarcoplasmic reticulum
- Ca2+ binds to troponin and allows myosin-actin cross-bridge binding
Arrange these statements in the correct order.
2 → 3 → 4 → 1
What is the refractory period?
The period following an action potential in which it is not possible to produce another action potential
During the plateau phase, how do Na+ channels ensure that another action potential (i.e depolarisation) does not take place?
They close
In the descending phase, how do K+ channels ensure that another depolarisation does not take place?
They open
bringing membrane back down to K levels
Why is the refractory period of the heart important?
It prevents generation of tetanic contractions of cardiac muscle (i.e constant heartbeats)
What are tetanic contractions of the heart?
Sustained muscle contraction when the rate of action potentials in cardiac muscle is too high
Contraction of ventricular muscle ejects the ___ volume.
stroke
What is the definition of stroke volume?
The volume of blood ejected by each ventricle per heart beat
What is the equation for stroke volume?
Stroke volume (SV) = End diastolic volume (EDV) - End systolic volume (ESV)
SV = EDV - ESV
Which two types of mechanism regulate the stroke volume of the heart?
Intrinsic and extrinsic mechanisms
What is intrinsic control of stroke volume?
Controls found within the heart muscle itself
What is extrinsic control of stroke volume?
Control by the nervous system and hormones
Intrinsic changes in stroke volume are brought about by…
end diastolic length of myocardial fibres (preload)
Which volume determines the preload of myocardial fibres?
End Diastolic Volume (EDV)
What is the cardiac preload?
The stretching of myocardial fibres at the end of diastole (which determines the stroke volume of the next heart beat)
What determines the end diastolic volume?
Venous return
- End diastolic volume
determines
- Diastolic length of myocardial fibres
determines
- Venous return
determines
- Cardiac preload
Arrange the numbers 1-4 in the correct order.
3 → 1 → 2 → 4
What is Starling’s Law of the Heart?
The more the ventricle fills with blood during diastole (greater end diastolic volume) the greater the volume of ejected blood will be (greater stroke volume) during the resulting systolic contraction
i.e greater EDV = greater stroke volume
Which law does this graph show?
Starling’s Law of the Heart
(increased EDV = increased stroke volume)
Increased stretch has an effect on a regulatory protein found on actin.
What is this effect?
Increased stretch > increased affinity of troponin for Ca2+
According to Starling’s Law of the Heart, optimum length in cardiac muscle is achieved by _____ the muscle.
stretching
- Increased stroke volume into the pulmonary artery by Starling’s Law
- Increased venous return to left atrium from pulmonary vein
- Increased venous return to right atrium
- EDV of left ventricle increases
- EDV of right ventricle increases
…causing increased stroke volume into the aorta by Starling’s Law.
Arrange 1-5 in the correct order.
3 → 5 → 1 → 2 → 4
What is afterload?
The resistance into which the heart is pumping, imposed after the heart has contracted
Determined by TPR
What happens if afterload increases?
1. The heart is unable is eject its full stroke volume, so some is left over
2. Thus EDV of the ventricles increases
3. So force of contraction increases by Starling’s Law
4. If chronic, ventricular hypertrophy may occur to overcome the resistance
Which autonomic nervous division supplies ventricular muscle?
Sympathetic
(remember that parasympathetic impulses have no effect on contractility)
Which post-synaptic neurotransmitter acts on ventricular muscle?
Noradrenaline (sympathetic division supplies heart muscle)
Stimulation of sympathetic nerves (increases / decreases) force of contraction.
increases
Sympathetic stimulation increases the force of contraction of ventricular muscle. What name can be given to this effect?
Positive inotropic effect
Stimulation of the sympathetic nerves in the heart produces two different effects, what are they called?
Positive chronotropic effect (increased heart rate)
Postivie inotropic effect (increased force of contraction)
Remember sympathetic stimulation also reduces AV nodal delay
Why does sympathetic stimulation increase the force of contraction of ventricular muscle?
Activation of Ca2+ channels causes greater Ca2+ influx
What molecule mediates the activation of calcium channels associated with sympathetic stimulation of the heart?
cAMP
The activation of calcium channels associated with sympathetic stimulation of the heart also reduces the durations of ___ and ___.
systole and diastole
(Thus increasing heart rate)
What effect does sympathetic stimulation have on the Frank-Starling curve and why?
Shifts curve to the left
Contractility of the heart at a given EDV increases
Explain the trends seen on this Frank-Starling curve.
Positive inotropic effect is an increase in the force of contraction so stroke volume increases
and the opposite
Heart failure causes a shift to the ___ on a Frank-Starling curve.
right
What effect does parasympathetic stimulation have on the force of contraction?
No effect
Parasympathetic stimulation influences rate, not force, of contraction
Where in the body are adrenaline and nonadrenaline released?
Adrenal medulla
What is cardiac output?
The volume of blood pumped by each ventricle per minute
Write an equation for cardiac output.
Cardiac output = Stroke volume x Heart rate
CO = SV x HR
What is the resting cardiac output in a healthy adult?
Approx. 5L
70ml SV x 70bpm = 4900ml CO
