Mech of Heart (1) 2

Question 1

What is needed for contraction?

Answer 1

Calcium

Question 2

What is the difference between cardiac and skeletal muscle cells?

Answer 2

Skeletal muscle cells can contract without extracellular calcium

Question 3

Describe cardiac cell structure:

Answer 3

• length = 100 micrometres
• width = 15 micrometres
• ttubule diameter is 200 nm
• ttubules spread about 2 micrometres apart

Question 4

Most important ion channel in the T-tubule?

Answer 4

L type Calcium Channel

Question 5

Describe excitation-contraction coupling in the cardiomyocytes

Answer 5

1) depolarisation is sensed by the L type Calcium channels and calcium entres the cell
2) Some calcium binds directly to cause contraction
3) some binds to ryanodine receptors on the sarcoplasmic reticulum which causes a further release of calcium
4) This causes further contraction
5) the same amount of calcium that entered the cell then leaves the cell by the Sodium - Calcium Exchanger

Question 6

How would you describe the relationship between force of contraction and intracellular calcium?

Answer 6

Sigmoidal

logarithmic scale on x axis

Question 7

What type of contraction occurs in the heart?

Answer 7

Isometric (muscle doesnt shorten). Therefore, an increase in muscle length = increased force. This occurs when the heart fills up and initially contracts

Isotonic = muscles shortens for force production. This occurs when the heart is full and ventricular pressure > arterial pressure

Question 8

Why can you not overstretch the heart?

Answer 8

The pericardium restricts the stretching

Question 9

What is preload?

Answer 9

The weight that stretched the muscle before it was stimulated to contract. More preload causes more stretching of the muscle so increased force (to a point)

Question 10

What is afterload?

Answer 10

Weight that is not apparent to the muscle in the resting state. More afterload = less shortening. More afterload = lower velocity of shortening

Question 11

What are the in vivo correlates of preload?

Answer 11

• End-diastolic Volume (EDV)
• End-diastolic Pressure (EPV)
• Right atrial pressure

Question 12

What is preload dependent on?

Answer 12

Venous return to the heart

Question 13

Afterload definition

Answer 13

The load that the left ventricle ejects against after opening of the aortic valve

Question 14

Measure of afertload?

Answer 14

Diastolic arterial blood pressure

Question 15

What is Starlings Law?

Answer 15

Increased diastolic fibre length increases ventricular contraction

Question 16

What is the frank-starling relationship dependent on?

Answer 16

• Changes in the number of cross-bridges in myofilaments that contract
• changes in the calcium sensitivity of the myofilaments

Question 17

What happens at shorter than ideal lengths in myofilaments?

Answer 17

Actin filaments overlap and therefore there is a reduced number of myosin cross bridges that can be made

Question 18

What are the two possible causes for myofilament changes in calcium sensitivity?

Answer 18

1) At longer sarcomere lengths, the affinity for calcium of Troponin C is increased due to a conformational change
2) At longer the lengths, the space between actin and myosin filaments decreases so there is a stronger possibility of forming good cross-bridges. This is called lattice spacing.

Question 19

What is stroke work?

Answer 19

Work done by the heart to eject blood against the pressure of the pulmonary artery and aorta

Question 20

How is stroke work calculated?

Answer 20

Stroke Work = Volume of blood ejected (SV) multiplied by the pressure at which the blood is ejected (P)

Question 21

What is the Law of Laplace?

Answer 21

When the pressure in a cylinder is constant, the pressure on the walls increases with increasing radius.

Question 22

What is the physiological relevance of the law of laplace?

Answer 22

The radius of curvature of the left ventricle is less than that of the right ventricle. This allows the LV to generate more force with the same wall stress (tension) to the RV.

Question 23

What happens in failing hearts related to the law of laplace?

Answer 23

Hearts become dilated and this increases the wall stress ( tension). Therefore, heart needs to work harder to maintain the same pressure