CVS Cardiac Action Potentials and Conduction Pathway

Question 1

How does the heart generate electrical activity?

Answer 1

The heart generates electrical activity and contraction
without external stimulation, e.g., from nerves or hormones
Termed : Myogenic

Question 2

How does skeletal muscle generate electrical activity?

Answer 2

Skeletal muscle: needs motor nerve input

Question 3

How do smooth muscle generate electrical activity?

Answer 3

Smooth muscle: needs ANS input

Question 4

What does the sinoatrial node(SAN) generate?

Answer 4

SAN generates action potentials called pacemaker potentials
in absence of any external stimulation, e.g. nerves or hormones

Question 5

What does the firing rate of the action potentials generated from the SAN equate to?

Answer 5

Firing rate of these action potentials equates to heart rate
Rhythm of these action potentials equates to heart rhythm

Question 6

How does the SAN produce pacemaker potentials?

Answer 6

1. Starts at resting membrane potential of -60mv
2. Phase 4: If channels are activated. These cause hyperpolarization by activate Na+ channels. Na+ influx produces depolarising slope
3. Phase 3: Voltage gated K+ channels open which cause a K+ efflux. This results in repolarisation
   4 . Phase 0: Voltage gated Ca2+ channels open, causing depolarization by a Ca2+ influx
4. K+ efflux continues until cardiac resting membrane potential threshold is met

Question 7

What do intercalated discs allow in conduction of electrical activity in heart?

Answer 7

Low resistance pathways
between atrial and ventricular cells called
intracalated discs
- Allows fast conduction -

Question 8

What are the steps involved in conduction of electrical activity from SAN through heart?

Answer 8

(1) Conduction across both atria to atrial-ventricle node (AVN)
(2)AVN is non-contractile tissue which slows electrical conduction
between atria and ventricles
- This allows atrial ejection to correctly fill ventricles before electrical activity
is conducted to ventricles and contraction occurs -
(3)Bundle of His to purkinje fibres is a fast conduction pathway which
allows both ventricles to be stimulated and contracted together

Question 9

What does the P wave in an ECG represent in the conduction pathway?

Answer 9

atrial depolarisation

Question 10

What does the QRS complex in an ECG represent in the conduction pathway?

Answer 10

ventricle depolarisation

Question 11

What does the ST interval in an ECG represent in the conduction pathway?

Answer 11

Interval between ventricle depolarisation and repolarisation

Question 12

What does the T wave in an ECG represent in the conduction pathway?

Answer 12

Ventricle repolarization

Question 13

Electrical-contraction coupling through the heart

Answer 13

(1)Electrical activity generated in SA
node (non-contractile tissue)
spreads out into (2) atria to produce
contraction
Electrical activity then enters AVN
which delays conduction before
stimulating ventricles
- allows ventricles to fill properly
during atrial contraction -
(3) Excitation conducts rapidly through
bundle of His into purkinje fibres
(4) Excitation through purkinje fibres is
conducted throughout the ventricular
contractile tissue producing contraction

Question 14

Where does ventricular contraction begin and spread?

Answer 14

Ventricular excitation and contraction begins at the apex then spreads to base of heart

Question 15

What are action potentials in atrial/ventricular cells generated by?

Answer 15

Action potentials in atrial/ventricular cells are ultimately generated from
electrical activity arising in SAN
atrial/ventricular cells have a stable resting membrane potential

Question 16

What are the phases involved in atrial/ventricular action potential?

Answer 16

1. Phase 4 is resting memrbane potential which starts at -90mv
2. Phase 0 is where voltage gated Na+ channels open causing depolarisation by a Na+ influx
3. Phase 1 is where voltage gated calcium channels open resulting in sustained Ca2+ influx causing depolarisation
4. phase 2 is the plateau phase lasting 200-400 ms
5. When going form phase 2 into phase 3, VGCCs switch off and K+ channels switch on
6. Phase 3 is where voltage gated K+ channels open causing repolarization by a K+ efflux

Question 17

What is the function of plateau phase?

Answer 17

Tissue is unexcitable or REFRACTIVE – CANNOT fire more action potentials
Voltage-gated Na+ channels are inactivated
No twitching - One action potential produces ONE contraction
ESSENTIAL for proper ejection of blood from heart

Question 18

What is contraction of atria and ventricles caused by?

Answer 18

Contraction caused by an INCREASE in cytosolic Ca2+ levels

Question 19

How much does Ca2+ rise in health resting individuals?

Answer 19

In a healthy resting individual Ca2+ rises from about 0.1 μM to about 1 μM

Question 20

How do atrial and ventricular action potentials couple to contraction?

Answer 20

1. Voltage gated Ca2+ channel open which causes a Ca2+ influx
2. Some Ca2+ will bind to ryanodine receptors (RyR) on sarcoplasmic reticulum which results in calcium induced calcium release(CICR)
3. This increases intracellular Ca2_ which interacts with contractile proteins(Troponin system)
4. This leads to contraction