Cellular aspects

Question 1

Membrane of heart muscle cell

Answer 1

Normally only permeable to k+.
Potential only determined by ions that can cross the membrane.

Question 2

Negative membrane potential

Answer 2

K+ ions diffuse outwards.
Anions can’t follow.
Excess of anions inside cell.
Generates negative potential inside the cell.

Question 3

Myocyte membrane pumps

Answer 3

K+ pumped into cells.
Na+ and Ca2+ pumped out.
Against their gradients.
Active transport - needs ATP.

Question 4

Action potential

Answer 4

Na+ is leaving the cell via leak channels and pumps - cell negative.
Voltage gated Na+ channels open - lots enter - cell positive - depolarisation.
Transient outward current of K+ causes a small repolarisation.
Ca2+ channels open - enters cell - maintains depolarised state.
K+ leaving - repolarisation back to resting potential.

Question 5

Resting potential

Answer 5

-90 mV

Question 6

Action potential propagation

Answer 6

Local depolarisation - causes nearby VG Na+ channels to open.
This is the wave of depolarisation.

Question 7

Why?

Answer 7

Contraction of the heart muscle requires Ca2+.
‘Excitation- contraction coupling’

Question 8

Calcium

Answer 8

1) Ca2+ influx through surface ion channels.
2) Amplification of Ca2+ with NaCa.
3) Calcium-induces calcium release.
4) Binds to troponin.
5) Change in tropomyosin reveals myosin binding sites.
6) Myosin head links with actin - pivots to cause muscle contraction.

Question 9

SAN

Answer 9

In the right atrium.
Phase 4 is upsloping - leak of charge.
Phase 0 is less rapid.
No discernible phase 1/2.
Phase 4 - If - Funny current.
Phase 0 - Ica,T - transient calcium current.

Question 10

SAN phase 4

Answer 10

Slope at the start.
Affected by - drugs, hypoxia, age and electrolytes.

Question 11

Autonomic control

Answer 11

Sympathetic = increases heart rate (positively chronotropic), increases force of contraction (positively inotropic) and increases cardiac output.
Parasympathetic = decreases heart rate (negatively chronotropic), decreases force of contraction (negatively inotropic) and decreases cardiac output.

Question 12

Sympathetic stimulation

Answer 12

Controlled by: adrenaline, noradrenaline - increase adenylyl cyclase - more cAMP.

Question 13

Parasympathetic stimulation

Answer 13

Controlled by: acetylcholine - M2 receptors - inhibit adenylyl cyclase - less cAMP.

Question 14

AV node

Answer 14

Transmits cardiac impulse between atria and ventricles.
Delays impulse - atria can empty.

Question 15

Automaticity

Answer 15

Spontaneous discharge rate of heart muscle cells decreases down the heart.
SAN fastest.
Ventricular myocardium slowest.