contractile

conductile

What characterizes the pacemaker cells?

• pacemaker cells that fire spontaneously and repetitively
• cells in the specialized conduction pathway

• "working myocardium"
  • generate pressures within atrium and ventricle that propels blood out of the chambers

Definition of autorhymicity

heart's intrinsic ability to beat spontaneously and repetitively in the absence of input from the nervous system (due to pacemaker cells)

1. SA node
   1. Right and left atria
   2. AV node
2. Bundle of His
3. Purkinje fibers
4. Right and left ventricles

• steady influx of calcium and sodium through the If channel
• dont have K1 channels present in ventricular cells that would act to hyperpolarize and inhibit the cell

• If channels
• voltage gated calcium channels (depolarization causes them to open rapidly)
• voltage gated potassium (TEA type) channels (depolarization opens slowly)

1. I(f) channels allow sodium and potassium into cell, depolarizing it
2. reaches threshold, leading to the rapid opening of voltage gated calcium channels
3. voltage gated potassium channels will be opened up slowly by the depolarization as well
4. AP end as potassium channels open, leading to repolarizing membrane

• phase 0- opening of voltage gated sodium channels (depol. open very rapidly)
  • causes closure of K1 channels (depolarization closes these channels)
• phase 1- inactivation of sodium channels, activation of voltage gated potassium channels
  • slight dip in the membrane potential do to sodium channels overshooting electrochemical equilibrium
• phase 2- plateau phase
• phase 3- inactivation of voltage gated calcium channels and activation of voltage gated potassium channels (cause dip in membrane potential
• phase 4- resting state (K1 activated)

• alter autorhythmicity
  • abnormal activity of pacemakers
  • ectopic pacemakers (cells are acting as pacemakers that shouldn't be)
• abnormality in conduction pathway

• Na channel blockers (lidocain, TTX)- no effect on refractory period
• class III antiarhythmias (block voltage gated K channels)- prolong refractory period

it will not affect pacemaker cells since pacemaker cells dont have sodium channels

• ventricular purkinje cell or contractile cell fire spontaneously at a rate faster than the normal rate set by the pacemaker, thereby making it an ectopic pacemaker

• If a group of cells is damaged, it will be in refractory state and unable to fire during normal period of activation
• Sometimes, they can recover from their refractory period before the wave of depolarization ends fully (transient block wears off)
• This causes these damaged cells to be depolarized from an abnormal direction as well as their neighboring cells. This can cause it to circle indefinitely.

• atria depolarizes so frequently that atrial contraction is ineffective in movin blood into the ventricle

The ventricular rate becomes rapid and irregular due to AV node responding irregularly to atrial depolarization.

How is defibrillation of ventricle accomplished?

• slowest in SA, AV nodes
• increases in speed in the atria and ventricle muscles
• it increases in speed as you go through the system

• decreases as we progress through the system
• atria, ventricles only spontaneously discharge if damaged

depolarization approaching

repolarization receding

depolarization receding

repolarization approaching

• Lead I- left arm (+) to right arm (-)
• Lead II- right arm (-) to left leg (+)
• Lead III- left arm (-) to left leg (+)

1. first wave is a downward deflection (depolarization receding) is the branching of purkinje fibers in left ventricle beginning at the apex going left to right
2. second wave is an upward deflection as depolarization moves from endocardium to epicardium
3. third wave making a slight downward deflection depolarization movin toward the base of the heart to depolarize ventricular cells in the upper part of septum

• definition- normal fluctuation in HR that occurs with respiratory cycle
• cause
  • during inspiration, inspiratory neurons in medulla inhibit parasymp. cardiac neurons
  • leads to HR increase with each inspiration

• -30-110 is normal
• it represents the general direction of depolarization of the heart (usually toward the left leg)

left bundle branch block will lead to depolarization going from right to left instead of going endocardium to epicardium, so it the mean cardiac vector will show as going toward the right arm

• EKG show prolonged interval between P and QRS wave
• caused by abnormal delay between atrial depolarization and ventricular depolarization

• EKG shows P wave not always proceeded by a QRS wave (could be 2:1, 3:1, 4:1)

• EKG shows complete dissociation between atrial and ventricular depolarization
• AV node is completely blocked and unable to conduct impulses

prolonged and atypically shaped QRS (due to wave of conduction not proceeding normally)