WEEK 8 - Pacemaker Cells

which types of cardiac myocytes facilitate the one-way flow of blood

• contractile cardiomyocytes

• autorhythmic cells (SA node pacemaker cells)

pacemaker cells vs. contractile cardiac myocytes

autorhythmic cells / pacemakers (makes up only 1% of total cardio cells)

• few myofibrils → does not contribute to contraction force

• electrically self-excitable → spontaneously and rhythmically
  generate and conduct AP (no neural input)

• Facilitate coordinated contraction and relaxation of cardiac chambers

contractile cardiac myocytes

• many myofibrils → allows for contraction

• electrically excitable → conduct AP only if depolarized to threshold

• responsible for the contraction of cardiac chambers → required for propulsion of blood

Unstable membrane potential

the pacemaker’s membrane potential which is never in a resting phase, hence they are spontaneously depolarizing to threshold

what facilitates the spontaneous depolarization of the pacemaker cells

additional special voltage gated ion channels

• Na+ “funny“ channels (slow)

• T-type (T = transient) Ca2+ channels

Process of Pacemaker AP Production

1. Depolarization: K+ channels close and Na+ channels open spontaneously; slow Na+ influx → less negative interior membrane

2. At threshold / rising phase: Ca2+ channels open and Ca2+ rapidly influxes from extracellular space → interior membrane is even more positive

3. Repolarization: Ca2+ channels close; K+ channels open and K+ effluxes

4. Complete repolarization: K+ channels close which reduces the K+ efflux

5. cycle repeats

how do pacemakers modulate the HR

by changing the speed of which pacemaker cells depolarize

SA node

• a collection of autorhythmic cells (pacemaker cells) located in the right atrium inferior to the SVC

• functions as the pacemaker of the heart

Cardiac Conduction System Importance

this system ensures that the cardiac chambers contract and relax in the correct sequence and at the right time

• atriums contract FIRST and once the ventricles are filled up with blood, they contract SECOND

Sequence of the Cardiac Conduction System

1. SA node generates AP by depolarizing the connected cardiac myocytes through gap junctions

2. AP is rapidly propagated to the LA to ensure spontaneous atrium contractions

3. AP travels to AV node through the internodal pathway

4. AP node depolarizes SLOWLY for delayed conduction to give time for optimal ventricular filling

5. AP travels along AV bundles of His

6. which branches into Left Bundle and Right Bundle Branches to conduct AP through the interventricular septum (layer in between the ventricles)

7. AP reaches apex first and conducts depolarization, then reaches the purkinje fibers located subendocardially to depolarize the ventricles

what contributes to faster conduction velocity

• larger diameter

• more gap junctions

• less myofibrils

what happens when the SA node is damaged?

AV node will take over as the pacemaker but it will have slower AP firing which causes decreased HR = less blood output = less oxygenation, hence, an artificial pacemaker is required as the AV node is insufficient as a pacemaker

what modulates HR

• autonomic nervous system (emotional/physical stressors that secrete norepinephrine → rapid AP fires)

• chemical regulation (hormones)

• ions (electrolyte imbalance affects heart’s function)

Bachman’s Bundle Function

connects the right and left atria to the SA Node allowa for conduction of electrical impulses