Download Electrophysiology Part 1 worksheet answers

Electrophysiology Part 1 worksheet answers
1. Na+, K+, Ca++, Cl2. A cardiac cell is polarized when the inside of the cell is more negative than the
extracellular environment. The difference in charges across the membrane is the
membrane potential.
3. ATP/ volts
4. The movement of charged particles across the cell membrane to change the
polarity of the inside of the cell (from negative to positive)
5. P- atrial depolarization QRS- ventricular depolarization (with hidden atrial
repolarization) T- ventricular repolarization
6. Phase 0- rapid depolarization phase- cell receives an impulse. Na+ moves into
cells through Na+ channels, Ca++ ,moves into cells through Ca++ channels, and K+
leaves the cells. The cell becomes positive, depolarizes, and contraction begins. This
is the QRS. Phases 1-3 are repolarization.
Phase 1- Na+ channels partially close to slow Na+ influx. K+ continues to leave and
Cl- begins to enter the cell, making it less positive.
Phase 2- Plateau phase- Ca++ continues to enter cells through slow channels and K+
continues to leave. This phase is important in the maintenance of sustained
contraction and is part of the absolute refractory period, represented by the ST
segment and the first part of repolarization of the ventricles
Phase 3- Na+ and Ca++ channels close and K+ continues to rush out of cells, quickly
making the cell electrically negative until it reaches baseline. This is represented by
the T wave and ventricular repolarization, which is complete by the end of phase 3.
Phase 4- resting membrane potential- lots of Na+ inside the cell and lots of K+
outside the cell. The Na+-K+ pump is activated to actively pump K+ into and Na+ out
of the cell, polarizing the heart until another stimulus occurs. This is when the
cardiac cell is fully polarized.
Phases 1-3 are electrical systole and phase 4 is electrical diastole
7. Automaticity, excitability (irritability), conductivity, contractility, refractoriness
8. The absolute refractory period involves phases 0-part of 3 and on the EKG ranges
from the onset of the QRS to the peak of the T wave. At this point the cell will not
respond to any additional stimulus. This is the reason tetanic contractions can’t
happen in cardiac cells.
The relative refractory period is also known as the vulnerable period. Some of the
cells have repolarized and will respond to a strong stimulus. On the EKG this is the
downslope of the T wave. It lasts around 15 microseconds and stimulus occurring at
this time can lead to a fatal dysrhythmia.
The supernormal period ranges from the end of phase 3 to the beginning of phase 4,
on the EKG the end of the T wave. The cells have repolarized and will respond to a
weaker than usual stimulus. This period is associated with many dysrhythmias.
9. From the SA node, across the atria, through the interatrial septum to the left atria.
It is delayed at the fibers separating the atria from the ventricles. The impulse
travels down the three internodal pathways, eventually reaching the AV node. This
area is slow to depolarize/repolarize because the fibers are smaller than those in
the atria and they don’t have as many gap junctions (it can easily develop blocks in
conduction, but allows the atria time to empty blood into the ventricles and allows
impulses that come by too frequently to be blocked- like AF!). The AV node merges
with the Bundle of His at the AV junction. If for some reason the impulse doesn’t go
through the AV junction- it bypasses it- the route the impulse takes is called an
accessory pathway. The bundle of His, at the upper part of the interventricular
junction, has an intrinsic rate of 40-60. The bundle of His receives a dual blood
supply so it is much less prone to ischemia. The impulse travels from the bundle of
His down the right and left bundle branches. The right bundle branch innervates the
right ventricle. The left bundle branch is divided into three fascicles (anterior,
posterior, and septal) which innervate respectively the anterior left ventricle,
posterior left ventricle, and midseptum. The bundle branches split up into tiny
Purkinje fibers, which go from the interventricular septum to the papillary fibers to
the apex of the heart, entering into to heart muscle itself. Impulses going through
these fibers travel from the endocardium to the epicardium, which creates a
twisting motion in the ventricles that allows the heart to more completely pump
blood. The intrinsic rate of the Purkinje fibers (the ‘ventricular rate’) is 20-40 bpm.
10. Fast response action potentials include cells in the atria, ventricles, and Purkinje
fibers. These fibers have lots of fast Na+ channels that allow quick movement of Na+
into the cell when the channel is open. This allows these parts of the heart to
conduct at very rapid rates. The SA and AV nodes do not have fast Na+ channels.
Instead, they have slow Na+ and slow Ca++ channels. The speed of conduction is
slower and impulses in slow areas are more likely to be blocked. Slow channels
occasionally occur in other parts of the heart as part of a pathological process.