Force generation by heart - lecture 4

Question 1

What type of muscle is the cardiac muscle,what causes it and what is absent from it?

Answer 1

• Striated
• caused by regular arrangement of contractile protein
• no neuromuscular junctions in the cardiac muscle

Question 2

What is the all or none law of the heart and how is this carried out?

Answer 2

• 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

Question 3

What provides mechanical adhesion between cardiac cells and why is this important?

Answer 3

Desmosomes within the intercalated discs and this ensures that the tension developed by one cell is transmitted to the next

Question 4

What causes muscle tension ?

Answer 4

Produced by sliding of actin filaments (thin) on myocine filaments (thick)

Question 5

What does force generation depend on?

Answer 5

ATP- dependant interaction between myosin and actin filaments (without ATP- will not occur)

Question 6

What is troponin?

Answer 6

During a relaxed stat, binding sites of myosin to actin are covered by troponin

Question 7

Why is Ca++ required?

Answer 7

Required to bind with troponin to uncover binding site and to switch on cross bridge binding which allows contraction

Question 8

What is released from the sacroplasmic reticulum ands what is it dependant on?

Answer 8

Ca++ - this release is dependant on presence of extra-cellular Ca++

Question 9

Where is the concentration of Ca++ high when ventricular muscle relaxes?

Answer 9

High in sacroplamisc reticulum but low in cytoplasm

Question 10

What happens to Ca++ during ventricular muscle contracts ?

Answer 10

Induced Ca++ release from sarcoplasmic reticulum

Question 11

What is the refractory period?

Answer 11

A period following an action potential in which it is not possible to produce another action potential

Question 12

Why is long refractory period important?

Answer 12

Protective for the heart by preventing generation of tetanic contractions in the cardiac muscle

Question 13

What is stroke volume?

Answer 13

The volume of blood ejected by each ventricle per heart beat

Question 14

How can Stroke volume be calculated?

Answer 14

SV= end diastolic volume -end systolic volume

Question 15

What does intrinsic mean?

Answer 15

Within the heart muscle itself

Question 16

What does extrinsic mean?

Answer 16

Nervous and hormonal control

Question 17

What brings about the change in stroke volume?

Answer 17

Changes in diastolic length of my myocardial fibres

Question 18

What determines cardiac preload?

Answer 18

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

Question 19

What determines end diastolic volume?

Answer 19

Venous return to the heart

Question 20

What Is the frank-starling mechanism ?

Answer 20

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)

Question 21

What is afterload?

Answer 21

The resistance into which heart is pumping

Question 22

What happens if after load increases and continuous to?

Answer 22

This is untreated hypertenion and eventually the ventricular msucle mass increase (ventricular hypertrophy) to overcome the resistance

Question 23

What does sympathetic nerves do to force of contraction and what is this called?

Answer 23

Increases it - positive inotropic effect

Question 24

Describe the effects of sympathetic stimulation on ventricular contraction.

Answer 24

• 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

Question 25

What is the effect of parasympathetic nerves on ventricular contraction?

Answer 25

• 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

Question 26

What are the extrinsic control of stroke volume?

Answer 26

Adrenaline and noradrenaline have inotropic and chronotropic effects

Question 27

What is cardiac output?

Answer 27

The volume of blood pumped by each ventricle per minute

CO=SV x HR

Question 28

What is the resting cardiac output in a healthy adult?

Answer 28

-approx 5 litres

70ml (SV) x 70bmp = 4900ml