Initiation of the heart beat

Question 1

How does the duration of a neuronal action potential differ with cardiac

Answer 1

Neuronal-500microsecond-1msec

Cardiac-200-400ms

Question 2

What is different about the cardiac refractory period

Answer 2

very long action pot->very long refractory potential but rather short relative refractory period

Question 3

what is the duration of cardiac action potential roughly the same as

Answer 3

QT interval of ECG

Question 4

What happens to the action pot duration as the rate increases

Answer 4

decrases to fit in more beats per sec

Question 5

why is the cardiac action pot so long

Answer 5

to protect from tetany hence long refractory period

skeletal muscle can contract with temporal summation, fused and unfused tetanic contraction

Question 6

sketch action pot for atria, ventricles, SAN, AVN. How do they differ

Answer 6

ref. notes. SAN and AVN show diastolic depolarisation. No stable resting memb pot-can depolarise between beats giving pacemaker function

Question 7

Does the SAN have the highest or lowest intrinsic rate

Answer 7

highest

Question 8

What is the membrane clock theory

Answer 8

repetitive pacemaker is generated by ion channels in membrane

Question 9

What is the calcium clock theory

Answer 9

cyclical release of Ca from intracellular stores drive the memb pot up and down in diastole and hence regulates the pacemaker

Question 10

When does the funny current occur, what is it carried by

Answer 10

inward current activated when the memb pot gets more negative
carried by Na and K

Question 11

what is the funny current: stimulated by, inhibited by, blocked by

Answer 11

stimulates: adrenaline
inhibits: acetylcholine
blocked: ivabradine

Question 12

Why is the conduction through AVN slow

Answer 12

1) allows ventricular filling to occur before electrical impulse is passed to the ventricle
2) also prevents transmission of high rates from the atria

Question 13

Why is conduction through ventricular conduction system fast

Answer 13

allows apex to contract before the base

Question 14

What do intercalated discs contain

Answer 14

gap junctions (connexons) form low resitance pathway coupling adjacent cells

Question 15

PQRST what does it represent

Answer 15

P atria depolarisaton
Q depolarisation of septum towards atria
R depolarisation ventricle towards apex
S depolarisation ventricles towards aria
T repolarisation of the ventricles

Question 16

What do the following represent and list e.g. pathologies:
P-Q interval
QRS duration
Q-T interval
S-T segment

Answer 16

P-Q interval: atrial condution and A-V nodal delay (AV block)
QRS duration: ventricular conduction velocity (bundle branch block)
Q-T interval: ventricular action potential duration (long QT syndrome)
S-T segment: heterogeneity of ventricular polarisation (myocardial infarction)

Question 17

Why is the relative height of the ST seg important

Answer 17

because ST occurs once all ventricles depolarised, if parts of ventricles more or less depolarised, the segment becomes elevated or depressed

Question 18

How does calcium induce cardiac muscle contraction

Answer 18

Calcium enters through voltage gated L-type Ca channel in t tubules and binds to Ryanodine receptor. Receptor opens and lets out lots of calcium to cytoplasm from SR. Calcium binds to muscle filament and induce contraction

Question 19

How does cardiac muscle relax

Answer 19

active transport of Ca out of cell by SERCA. SERCA returns Ca released from SR back to SR. Small amount of Ca entering through L type calcium channels removed from cell by Na/Ca exchanger

Question 20

What do the following mean: chronotropy, inotropy, lusitropy, positive, negative

Answer 20

chronotropy: heart rate
inotropy: strength of contraction
lusitropy: rate of relaxation
positive: increase
negative: decrease

Question 21

What are positive chronotropic agents and what do they do

Answer 21

agents: sympathetic stimulation (adrenaline, noradrenaline)

increases funny current, faster rate of diastolic depolarisation, faster heart rate)

Question 22

What are negative chronotropic agents and what do they do

Answer 22

Agent: parassympathetic (ACh)

decreases funny current, opens KACh channels, slower rate of diastolic depolarisation, slower heart rate

Question 23

Draw how lines of positive inotropic and lusitropic will differ from control

Answer 23

ref. notes

Question 24

Draw a flow chart of beta 1 adrenoreceptor stimulation

Answer 24

ref. notes

Question 25

where does the PKA phosphorylation and cAMP act on in cardiac muscle cycle

Answer 25

L type Ca channels: increase channel opening for positive chronotropy, positive inotropy
Ryanodine receptor: increase SR Ca release (positive inotropy)
ATPase subunits: phodpholamban and phospholemman o increase SR Ca uptake and cellular Na extrusion (positive lusitropy)
Pacemaker: membrane and calcium clock (positive chronotropy)
myofilaments: troponin I and myosin binding protein C and increase rate of crosss bridge cycling (positive inotropy and lusitropy)