L8 - Electrical Acitivity of the Heart

Question 1

By which cells is electrical activity spread?

Answer 1

Nodal cells, conducting cells and muscle cells

Question 2

Overview of how the heart is excited method

Answer 2

1. Initiated at sino-atrial node (dominant pacemaker)
2. Conduction to right atria
3. Conduction to left atria and atrioventricular node
4. Conduction to atrioventricular ring
5. Passage through bundle of His, depolarizing septum from left to right
6. Depolarization spreads upwards through the Purkinje system distributing to ventricular muscle cells

Question 3

Where is the sino atrial node found?

Answer 3

Found in the posterior aspect of the heart

Junction between superior vena cava and right atrium

Question 4

What is the speed of conduction via nodal tissue?

Answer 4

0.05 m/s

Question 5

What is the speed of conduction via atrial myocardium

Answer 5

1.00 m/s

Question 6

Where does electrical activity spread after the SAN?

Answer 6

AVN and left atrium

Question 7

Where is the atrio-ventricular node found?

Answer 7

Found in the posterior aspect of the heart

Right side interatrial septum

Question 8

What are the structural zones of the AVN?

Answer 8

Atrial nodal, nodal, nodal ventricular

Question 9

What is the speed of conduction via atrial node to nodal?

Answer 9

0.05 m/s

Question 10

What is the role of the AVN?

Answer 10

AVN delay allows atrial contraction to finish
AVN refractoriness
- Prevents excess ventricular contraction
- Increases at high heart rate

Question 11

What is sub-nodal conduction?

Answer 11

Conduction via His - 1 m/s
Septal activation
Conduction via Purkinje fibres - 4 m/s
Conduction via ventricular muscle - 1 m/s
Spiral muscle contraction evokes a torsion

Question 12

What are the two types of cardiac action potential?

Answer 12

Nodal cells - initiate contraction of heart
- They are able to fire action potentials without any outside stimulation
Contractile cells - atrial and ventricular muscle cannot fire action potentials without a signal

Question 13

What are pacemakers cells and what is their role?

Answer 13

Pacemaker cells show automaticity and rhythmicity
They do not have a static resting membrane potential
- As soon as they reach -60mV they start slow depolarisation reaching threshold, causing an action potential
- Do not need a signal to stimulate this
AV node and Purkinje fibres can also be pacemakers

Question 14

What are the 4 stages of a pacemaker action potential?

Answer 14

Pre-potential (pacemaker potential)
- Decrease K+ efflux and increase Na influx (If current)
Threshold
- - 40 to - 50mV
Depolarisation 
- Increase Ca2+ influx
Repolarisation 
- Increase K+ efflux

Question 15

What is an If current?

Answer 15

If current – activated by hyperpolarization, inactive when positive membrane potential

• Mediates slow Na influx
• Cation selective channel

Question 16

What are pacemaker action potential regulated by?

Answer 16

Innervation, temperature and other pacemakers

Question 17

What 2 things are pacemaker cells controlled by?

Answer 17

Parasympathetic vagal fibres (ACh)

Sympathetic (NAdr)

Question 18

How do sympathetic (NAdr) fibres control pacemaker cells?

Answer 18

Increased prepotential slope
Increased firing rate
This increases the heart rate

Question 19

How do parasympathetic vagal fibres( ACh) control pacemaker cells?

Answer 19

Hyperpolarisation
Decreased prepotential slope
This slows the heart rate

Question 20

What is the action of ACh and NAdr on pacemaker cells?

Answer 20

If current regulated by cAMP
- Sympathetic stimulates increased by β1 and β2
- Parasympathetic stimulates decreased M2
Calcium clock oscillations (rhythmic alterations of sarcoplasmic reticulum Ca2+ release)

Question 21

Cardiac muscle action potential characteristics?

Answer 21

No automaticity
Propagated and prolonged action potential with long plateau phase
- Plateau phase means muscle unable to show summation
Fast depolarisation and overshoot

Question 22

Cardiac muscle action potential method?

Answer 22

1. Voltage gated Na channels (Nav 1.5) open and Na influx depolarises the membrane
2. Triggers opening of more Na channels creating positive feedback loop, increasing membrane voltage
3. Na channels close when the cell depolarises and voltage reaches +30 mV
4. Ca entering through slow Ca channels (Cav 1.2) prolongs depolarisation creating a plateau
   a. Plateau falls slightly because of some K leakage
5. Ca channels close and Ca transported out of cell
6. K channels open and rapid K outflow returns membrane to resting potential