4 – Impulse Conduction System of the Heart

Question 1

Conduction system in heart: autorhythmic cells

Answer 1

1. Pacemaker cells
2. Conduction fibers
   *AP spreads through gap junctions
   *work as a unit: syncytium

Question 2

Impulse conducting system in heart:

Answer 2

-autorhythmic cells generate AP spontaneously
-conduction fibers spread impulse throughout the heart
-impulse travels via gap junctions (intercalated discs)

Question 3

Intercalated discs:

Answer 3

-connections that contain anchoring proteins and gap junctions
>allows the spread of AP from cell to cell

Question 4

Pacemaker cells:

Answer 4

-SA and AV node
-generate APs spontaneously
-faster rate sets the pace (SA>AV)

Question 5

Conduction fibers:

Answer 5

-spread throughout the heart
-generate APs spontaneously but at a much slower rate

Question 6

Depolarization/impulse wave (‘steps/pathway’):

Answer 6

-SA node
-across atria
-AV node
-AV bundle/L/R/bundle branches (in the interventricular septum)
-Purkinje fibers
-recovery (repolarization)

Question 7

Why is there a time delay in the conduction system of the heart?

Answer 7

-the electrical signal needs to be converted to a mechanical signal
*L to R depolarization of the interventricular septum

Question 8

AV node momentary delay:

Answer 8

-APs transmitted more slowly in these cells than other cells in conduction system
*allows time for atria contraction to occur (fill the ventricles before they contract)

Question 9

Why does the SA node set the pace?

Answer 9

-fires more frequently than AV node
>after cells in AV node are stimulated they have a refractory period

Question 10

What happens if SA node fails to fire or if the impulses are blocked?

Answer 10

-the AV node can take over

Question 11

Why does the impulse travel to apex (via septum) and then through ventricles to the base?

Answer 11

-enables heart to pump efficiently
>contraction starts at apex and travels toward base where blood is ejected from ventricles

Question 12

Autonomic nervous system control of the heart:

Answer 12

-SNS: fight/flight
-PNS: rest/digest

Question 13

SNS and PNS have opposite effects on the heart:

Answer 13

-chronotropy (heart rate changes)
-dromotropy (conduction speed)
-lusitropy (rate of relaxation)
-inotropy (muscle contraction) (less from PS)

Question 14

SNS on the heart: increases

Answer 14

-rate of depolarization
-conduction velocity
-contractility
-rate of relaxation

Question 15

Sympathetic nerves release and act via:

Answer 15

-NE
-beta-adrenergic receptors (beta-1, beta-2)

Question 16

PNS on heart: decreases

Answer 16

-rate of depolarization
-conduction velocity

Question 17

Parasympathetic nerves release and act via:

Answer 17

-ACh
-muscarinic receptors (M2)

Question 18

High disease states leads to more SNS or PNS on heart?

Answer 18

-more SNS

Question 19

Aerobic training leads to more SNS or PNS on heart?

Answer 19

-more PNS (vagus nerves)

Question 20

When cardiac impulse spreads across heart:

Answer 20

-portion of electrical current also spreads along surface of body
>can detect this with electrodes placed on surface of skin (ex. ECG)

Question 21

P wave:

Answer 21

-atria depolarize and contract

Question 22

QRS complex:

Answer 22

-ventricles depolarize and contraction
-atrial repolarization is masked)

Question 23

T wave:

Answer 23

-ventricles repolarize

Question 24

PR interval:

Answer 24

-time between atrial and ventricular depolarization (P-Q)
>wave spreads through AV node and AV bundle
>conduction velocity between SA and AV nodes

Question 25

QT interval:

Answer 25

-time required for ventricles to depolarize and repolarize

Question 26

Less time intervals between R-R=

Answer 26

-HR increased

Question 27

Excitation contraction coupling is the:

Answer 27

-conversion of electro-chemical signal to a mechanical event
>autorhythmic cells fire impulses/Aps spontaneously
>Aps spread to and between adjacent cardiomyocytes through gap junctions
*cardiac syncytium (work as a unit)

Question 28

Excitation contraction coupling steps in cardiac muscle:

Answer 28

1. Current spreads through gap junction to contractile cell
2. AP travels along PM and T-tubules
3. Ca2+ channels open in PM and SR
4. Ca2+ induces Ca2+ release from SR
5. Ca2+ binds to troponin (exposes myosin-binding sites)
6. Crossbridge cycle begins (muscle fiber contracts)
7. Ca2+ is actively transported back into SR and ECF
8. Tropomyosin blocks myosin-binding sites (muscle fiber relaxes)

Question 29

What might cause heart failure in an animal?

Answer 29

-calcium re-uptake isn’t great=can’t relax well=can’t contract well

Question 30

What would happen to a cardiomyocyte treated with NE?

Answer 30

-more calcium release=great deflection=increase contractility

Question 31

How does SNS effect the depolarization of the heart?

Answer 31

-increases rate of depolarization

Question 32

Does LTCC (L-type Ca2+ channel) enable or disable Ca entry into the cell?

Answer 32

-enables Ca entry