Mechanical Properties 1

Question 1

List the 3 steps for the contraction of a ventricular cell

Answer 1

A single ventricular cell

CALCIUM IS NEEDED FOR CONTRACTION

They are rod shaped - they can be stimulated to contract

Process:

Electrical event

Calcium Transient (the amount of calcium in the sarcoplasm has increased for a short period of time)

Contractile event

Question 2

Answer 2

IMPORTANT: THE HEART WILL NOT BEAT WITHOUT EXTERNAL CALCIUM

This is DIFFERENT to skeletal muscle which can contract without external calcium

Question 3

What is the important ion channel in a cardiomyocyte?

Answer 3

Important ion channel in a cardiomyocyte is the L-type calcium channel

Question 4

Describe the process of Excitation-Contraction Coupling in the Heart ?

Answer 4

Important ion channel in a cardiomyocyte is the L-type calcium channel

Depolarisation is sensed by the L-type calcium channel and calcium from outside enters the cell

Some of this calcium can directly cause contraction

The rest of the calcium binds to Ryanodine Receptors (also called Sarcoplasmic Reticulum Calcium Release Channel) which causes release of calcium from the sarcoplasmic reticulum

After it has had its effect, some of the calcium is taken back up into the SR by Ca ATPase channels (also called SERCA - SARCO/ENDOPLASMIC RETICULUM CALCIUM ATPase)

Same amount of calcium that came into the cell is effluxed by a Sodium-Calcium Exchanger

This does NOT need energy - it uses energy from the concentration gradient of sodium to expel calcium form the cell

SR Ca release channel= ryanodine receptor

Question 5

What shape is the graph showing the relationship between force production and intracellular {Ca}

Answer 5

The force-calcium relationship is SIGMOIDAL

Around a 10 micromolar intracellular concentration of calcium is sufficient to produce maximum force

Question 6

An increase in muscle length causes an ……………. in force

Answer 6

An increase in muscle length causes an increase in force

Question 7

As you keep stretching the muscle, you get to a point where further stretching DOES NOT generate more force?

Why does this happen?

Answer 7

As you keep stretching the muscle, you get to a point where further stretching DOES NOT generate more force - this is because there is not enough overlap between the filaments to produce force

Question 8

Answer 8

This is ISOMETRIC CONTRACTION so the muscle doesn’t shorten - it is just pulling on the force transducer

An increase in muscle length causes an increase in force

As you keep stretching the muscle, you get to a point where further stretching DOES NOT generate more force - this is because there is not enough overlap between the filaments to produce force

Passive force- Think of the muscle as Stretching an elastic band. It is the recoil after stretching. Also increase as muscle length increases

Active force- caused by the cross bridges forming and …..

Question 9

What is more resilient to stretch and why?

Cardiac muscle or Skeletal muscle

Answer 9

Cardiac muscle is more resilient to stretch

Due to properties of the extracellular matrix and cytoskeleton

Question 10

Which part of the graph is important in physiological circumstances in cardiac muscle?

Answer 10

ONLY THE ASCENDING LIMB OF THE LENGTH-TENSION CURVE IS IMPORTANT IN PHYSIOLOGICAL CIRCUMSTANCES IN CARDIAC MUSCLE

The descending limb doesn’t happen in physiological conditions because the pericardium restricts the stretching

Question 11

What two forms of contraction does the heart use and explain each one?

Answer 11

Question 12

What is preload?

Answer 12

PRELOAD - the weight that stretched the muscle BEFORE it is stimulated to contract (i.e. the filling of the ventricles with blood makes it stretch before it is stimulated to contract)

Question 13

Why can’t you overstretch the heart muscle?

Answer 13

The descending limb doesn’t happen in physiological conditions because the pericardium restricts the stretching

Question 14

What is afterload?

Answer 14

AFTERLOAD - weight that is NOT APPARENT to the muscle in the resting state - only encountered ONCE MUSCLE HAS STARTED TO CONTRACT

Question 15

Explain the graph below?

Answer 15

The preload causes the stretching of the cardiac muscle. Because the cardiac muscle is now longer, it can generate more force when it contracts.

If you have a large load you have less shortening- think of a heavy weight- you won’t be able to lift it all the way up

Question 16

What is preload dependant on?

Answer 16

•Preload is dependent upon venous return to the heart

Question 17

What determines the preload on the ventricles before ejection?

Answer 17

•The stretch or filling determines the preload on the ventricles before ejection

Question 18

Describe preload inside the heart

Answer 18

•As blood fills the ventricles during the relaxation phase of the cardiac cycle (diastole) it stretches the resting ventricular wall

Question 19

List some measures of preload?

Answer 19

•Measures of preload: end-diastolic volume, end diastolic pressure, right atrial pressure

Question 20

Name a simple measure of afterload?

Define afterload in the context of the heart?

Answer 20

• The load against which the left ventricle ejects blood after opening of the aortic valve
• A simple measure of afterload is the diastolic arterial blood pressure

Question 21

•Any increase in afterload ………………….. the amount of isotonic shortening that occurs and ………………….. the velocity of shortening

Answer 21

•Any increase in afterload decreases the amount of isotonic shortening that occurs and decreases the velocity of shortening

Question 22

More afterload= ……….. shortening

Answer 22

MORE AFTERLOAD = LESS SHORTENING

Question 23

MORE PRELOAD = …………. FORCE (up to a certain point)

Answer 23

MORE PRELOAD = MORE FORCE (up to a certain point)

Question 24

MORE AFTERLOAD = ………….. VELOCITY OF SHORTENING

Answer 24

MORE AFTERLOAD = LOWER VELOCITY OF SHORTENING

Question 25

Answer 25

Preload will govern the amount of force the muscle is capable of producing Afterload is the weight or mass or pressure that the muscle is trying to overcome - it only becomes apparent after contraction has been stimulated So, for the same afterload if you stretch the muscle prior to excitation it will generate more force

Question 26

What is the sequence of events linking cardiac muscle excitation with contraction, then relaxation? Use the terms below and put them in order T-Tubule Ca2+ influx L-type Ca channel activation SR Ca channel (RyR) activation SR Ca Release Myofilament Activation Ca binds to troponin Action potential SERCA (sarco/endoplasmic reticulum Ca²⁺-ATPase) and Na/Ca2+ exchange Relaxation

Answer 26

Action potential T-Tubule L-type Ca channel activation Ca2+ influx SR Ca channel (RyR) activation SR Ca Release Ca binds to troponin Myofilament Activation SERCA (sarco/endoplasmic reticulum Ca²⁺-ATPase) and Na/Ca2+ exchange Relaxation

Question 27

What is the Frank-Starling relationship?

Answer 27

Definition: Increased diastolic fibre length increases ventricular contraction The Frank–Starling law of the heart (also known as Starling's law and the Frank–Starling mechanism) represents the relationship between stroke volume and end diastolic volume.[1] The law states that the stroke volume of the heart increases in response to an increase in the volume of blood in the ventricles, before contraction (the end diastolic volume), when all other factors remain constant.

Question 28

What is the consequence of the Frank-Starling Relationship?

Answer 28

Consequence: Ventricles pump greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return In other words: the amount of blood coming in to the ventricles determines the strength of the ventricular contraction and hence determines the amount of blood leaving the ventricles Definition: Increased diastolic fibre length increases ventricular contraction

Question 29

What 2 factors cause the Frank-Starling relationship

Answer 29

•Due to two factors: –Changes in the number of myofilament cross bridges that interact –Changes in the Ca sensitivity of the myofilaments

Question 30

The mechanism(s) responsible for the increase in calcium sensitivity with increased length remain(s) elusive Name 2 possible mechanism

Answer 30

•At longer sarcomere lengths the affinity of troponin C for Ca is increased due to conformational change in protein With decreasing myofilament lattice spacing, the probability of forming strong binding cross- bridges increases

Question 31

What is stroke work and how is it calculated?

Answer 31

Question 32

What is the law of laplace?

Answer 32

•This Law states that: when the pressure within a cylinder is held constant, the tension on its walls increases with increasing radius

Question 33

What is the Physiological relevance of Laplace’s Law?

Answer 33