Day 4 - Cardiac Electrophysiology

Question 1

What are the four properties of the heart? What do they do?

Answer 1

Automaticity
* intiating / creating an impulse

Excitability
* respond to an impulse

Conductivity
* transmit an impulse to other cells

Contractility
* respond to an impulse with mecahnical contraction

Question 2

Explain the pathway of normal electro-conduction in the heart

Answer 2

1. Impulses generated from the SA node travel through the intra-atrial tracts to the AV node
2. Through the inter-nodal tract to the bundle of his
3. Through the left and right bundle branches, purkinje fibers and throughout the ventricular muscle cells

Question 3

Explain cardiac cell action potential with ion movement. Phases 0-4

Answer 3

Action potential = electrical activity of the cell

Resting cell (polarized) phase 4
* the cell is negative with minimal ionic movement

Depolarization, phase 0
* cell becomes more semi-permeable after receiving a stimulus from SA node
* sodium ions rush into the cell
* ^ cell is positive

Repolarization, phases 1-3
Phase 1
* Potassium moves out of the cell d/t positive overload from sodium entry

Phase 2
* Calcium enters the cell via Na-Ca channels w/ Na
* mechanical contraction!

Phase 3
* Na-Ca channel closes
* the cell is negative
* the Na-K pump opens to return cells to resting state by exchanging Na for K inside

Question 4

Compare absolute and relative refractory periods

Answer 4

Refractory periods = refilling the toilet

Absolute
* cells are unable to respond to a second stimulus during this time
* cells are not excited to respond, you just flushed toilet

Relative
* cells can respond to a stimulus, but only a strong one
* cells are somewhat excited to respond
* toilet still refilling, but if you hold the lever down itll go

Question 5

Compare the action potential of a pacemaker cell vs nonpacemaker cell

Answer 5

Pacemaker cell (SA node)
* makes its own electrical impulse
* slow and automatic depolarization
* NaCa channels always open, require continuous ATP and O2 to create energy
* RMP -60 TP -40

Non-pacemaker cell (ventricular muscle cell)
* requires action potential to depolarize then contract
* rapid depolarization
* NaCa channels only open with influx of Na
* RMP -90 TP -60

RMP - resting membrane potential
TP - threshold potential, electricity

Question 6

What is the Singh Vaughan Williams Classification System?

Answer 6

Classifies medications according to their effect on the action potential

Class I - interferes with Na channel
Class II - anti-sympathetic nervous system agents (beta blockers)
Class III - interfere with K+ movement out of cell
Class IV - interfere with calcium movement into cell CCB
Class V - unknown mechanism

correlates with action potential of cardiac cells

Question 7

What is the action of a class III medication?

Answer 7

Class III - interferes with K+ movement out of cell
* Lengthens the repolarization time by slowing movement of K+

Question 8

What are key structures and functions in a cardiac muscle cell?

Answer 8

Sarcomere - the whole muscle cell including all the components for contraction
Sarcolemma - cell membrane of the muscle cell
Sarcoplasmic reticulum - yellow storage areas for Ca+
Transverse tubule - passageways that allow Ca to enter the cell
Actin & myosin - filaments that combine / engage during contraction

Question 9

Explain the roles of actin, myosin, troponin, tropomyosin, and calcium during muscle cell contraction and relaxation

Answer 9

Contraction
* Calcium enters the cell and binds to troponin, moving tropomyosin
* the myosin is now touching the actin causing contraction

Relaxation
* without calcium, troponin and tropomyosin form cross bridges that prevent contraction (keeps actin and myosin from touching)

Requires ATP!