Cardiac Muscle Structure & Function

Question 1

Troponin - 3 subunits

Answer 1

TN-C: Calcium-binding troponin subunit

TN-I: Inhibitory subunit; contains N-terminal extension that is highly regulated by PKA phosphorylation,

TN-T: Tropomyosin-binding subunit

Question 2

Properties of cardiac muscle cells

Answer 2

Striated
Mono-nucleated
Coupled via gap junctions & intercalated discs
Not under direct neural control
Rich in mitochondria

Question 3

How is Ca2+ removed from the active myocyte?

Answer 3

70% via SR Ca2+ ATPase (SERCA)

30% via Na+-Ca2+ exchanger (NCX)

Question 4

3 molecular factors underlying the length-tension relationship

Answer 4

1. Extent of overlap - peak tension development occurs at sarcomere lengths of ~2.2uM
2. Changes in Ca2+ sensitivity - at longer lengths, more of the cross-bridges become activated by the same change in intracellular Ca2+
3. Increased Ca2+ release - due to stretch-sensitive ion channels in the cell membrane

Question 5

Cardiac Index

Answer 5

CO normalized to body size

Question 6

What determines CO?

Answer 6

CO = HR x SV

HR can increase by a larger % than SV and so can produce larger changes in CO

SV is determined by pre-load, afterload, and contractility

Question 7

Stroke Volume - Definition

Answer 7

SV = EDV - ESV

Question 8

Ejection Fraction - Definition

Answer 8

EF = (EDV-ESV) / EDV

EF = SV / EDV

Question 9

Stroke Work - Definition

Answer 9

Energy expended per beat (J), corresponds to the area inside the PV loop diagram

Question 10

Pulse Pressure - Definition

Answer 10

PP = peak systolic pressure - end diastolic pressure