Electrical activation heart

Question 1

Where is the Sinoatrial node sound?

Answer 1

Crista terminalis of right atrium

Question 2

What generates rhythm of heart (pacemaker)?

Answer 2

Sinoatrial node (SAN)

Question 3

How is heart activated?

Answer 3

1) Action potential generated
2) Reaches -60mV leads to opening HCN
channels (hyperpolarisation gated cyclic
nucleoside channels)
3) HCN allows Na+ ions move into cell-
depolarisation from -60mV to -40mV
4) Voltage gated Ca2+ channels open- Ca2+
enters
- lead depolarisation cell membrane +20mV
5) +20mV results in opening K+ channels
6) K+ move out cell, loss of +ve charge results
repolarisation
7) Hyperpolarisation threshold reached
- reopens HCN and Na+ moves into cell

This is a cycle, no need activation

Question 4

What increases/decreases HR?

Answer 4

Catecholamine- increases
- faster repolarisation
- reach threshold faster
- HR increased

Acetylcholine- decreases
- slows opening HCN

Question 5

Draw graph show pacemaker action potential?

Answer 5

Question 6

How does action potential spread?

Answer 6

Exit SAN- spread cells through gap junctions- connexins
Activate other myocytes- Na+ depolarise adj cells

Question 7

What is the pathway of heart conduction?

Answer 7

1) SA Node
2) AV Node
3) Bundle of His
4) R/L Bundle branches
5) Purkinje Fibres

Question 8

Label?

Answer 8

Question 9

Outline process myocardial action potential?

Answer 9

Cardiac myocyte polarised, no HCN channels Activation of myocardium results:
1) Resting potential -90mV
2) Depolarisation cell- reach threshold -70mV 3) Leads Na gated fast channels opening
- Na+ enters the cells, depolarises +20mV
4) K+ channels open
- K+ leaves cell -15mV
5) Voltage gated Ca+2 channels open
- Ca+2 move in
- counters K channels
- result plateau for 200msec
6) Entry Ca+2 into cell cause contraction
myocyte
7) Ca+2 closes, K+ channel opens leads
repolarisation
8) Reaches resting potential -90mV

Question 10

What 4 steps in myocardial action potential?
What ion responsible?

Answer 10

Question 11

Draw graph cardiac myocyte action potential (label)

Answer 11

Question 12

What occurs when calcium enters cardiac myocytes?

Answer 12

Cardiac myocytes have T-tubules
Entry Calcium
Filaments contract- actin-myosin coupled movement

Question 13

Function of SAN?

Answer 13

Determine HR
60-100 bpm

Question 14

What is normal HR?

Answer 14

60-100 BPM

Question 15

What is resting membrane potential?

Answer 15

-60mV
Hyperpolarisation results HCN opening
Calcium enter
Depolarisation cell

Question 16

Function AVN?

Answer 16

Exit electrical impulse from atria to ventricle
Slow impulse 100-200msec
Allows atria contract and ventricles to fill

Question 17

Why is there a slow down in impulse?

Answer 17

Allow atria to contract and ventricles fill

Question 18

Function of his-purkinje system?

Answer 18

Allow rapid electrical conduction into ventricles
Bundle his- large fibres, gap junctions
Rapid spread conduction in ventricles

Question 19

Effect sympathetic input on HR?

Answer 19

Increases HR (positive chronotropic)
Increase force contraction (positive inotropic)
Increase CO 200%

Question 20

What controls sympathetic input?

Answer 20

Adrenaline and Noradrenaline
Type 1 beta adrenoreceptors
Increased adenyl cyclase and cAMP

Question 21

Effect parasympathetic input on HR?

Answer 21

Decrease HR (by 30-40bmp)- negative chrontropic
Decrease force contraction- negative inotropic
Decrease CO (50%)

Question 22

What is parasympathetic input controlled by?

Answer 22

Acetylcholine- M2 receptors inhibit adenyl
cyclase
- less cAMP

Question 23

How is there a difference in activation between myocardial and skeletal?

Answer 23

Myocardial- activated adjacent cell
Skeletal- activated nervous system

Question 24

In what activation is there a plateau phase? Why?

Answer 24

Myocardial- Ca channels open
- maintain plateau phase
- allow refractory period
- allow heart fill

Skeletal- no plateau
- rapid entry Na
- repolarised K+
- no plateau- tetany- spasm occur

Question 25

Which contraction is longer myocyte or skeletal?

Answer 25

Myocyte- 1.5x longer