lecture 19

Question 1

what is the resting membrane potential of cardio myocytes

Answer 1

-90mV

Question 2

How is AP initiated in cardiac muscle

Answer 2

• a wave of depolarization caused by a pacemaker cell enters from adjacent cell
• made possible by intercalated discs and gap junctions

Question 3

Steps of a cardiac muscle potential

Answer 3

1. a wave of depolarization started from a pacemaker cell enters from adjacent cell (made possible w gap junction and intercalated disks)
2. rapid upstroke is due to activation of voltage gated Na+ channels
   - increase permeability of Na+
   - Na+ rushes in
3. Notch —> Na+ channels inactive and fast K+ channels open which causes a little repolarization then they close
4. plateau—> medium K+ channels open (K+ out) and Ca2+ channels open (Ca2+ in)
5. eventually K+ wins because slow K+ channels open and Ca2+ channels inactivate
6. slow K+ channels close and K+ leak channels continue to leak returning to resting membrane potential

Question 4

Name the phases of cardiac muscle action potential

Answer 4

0. rapid upstroke/depolarization
1. notch
2. plateau
3. rapid repolarization
4. rest

Question 5

why are cardiac muscle cells action potentials much longer

Answer 5

• because they prevent summation because the heart must contract then relax before contracting again so that heart can act as a pump
• we must repolarize to activate voltage gated Na+ channels to intimate an action potential

Question 6

what ensures the heart acts as a pump

Answer 6

• long refractory period
• no summation

Question 7

pacemaker cell AP

Answer 7

• we hit threshold and voltage gated Ca2+ channels activate
• Ca2+ voltage gated channels cause rapid upstroke/depolarization (Ca2+ in)
• Ca2+ voltage gated channels inactivate and K+ channels open causing repolarization
• K+ channels close and HCN channels open
• HCN channels are non selective and hyperpolarization gated, conduct both K+ and Na+ but since we’re farther from Ena Na+ will have a greater driving force and cause depolarization
  until we reach threshold to activate Ca2+ channels again and then HCN close

Question 8

where are pacemaker cells located

Answer 8

in the sino-atrial (SA) node where they are spread through gap junctions found at intercalated disks

Question 9

where does the SA node lies where

Answer 9

the sino-atrial node lies at the junction of superior vena cava with the right atrium

Question 10

the internodal pathways….

Answer 10

spread across both atria

Question 11

what are purkinje fibers

Answer 11

they are specialized muscle cells that conduct electrical signals allowing for contraction

Question 12

excitation-contraction coupling throughout the heart

Answer 12

• action potential and continually and spontaneous initiated in SA node
• action potential is spread through electrical conducting system of the heart
• as action potentials pass across ventricles and atria depolarization spreads across myosyte
• action potential depolarize myocyte membrane
• myocyte contracts

Question 13

QRS wave

Answer 13

ventricular depolarization and atrial repolarization

Question 14

T wave

Answer 14

ventricular repolarization

Question 15

P wave

Answer 15

atrial depolarization

Question 16

common arythmias

Answer 16

tachycardia: heart rate is faster than usual
bradycardia: heart rate is slower than usual

Question 17

what is fibrillation and give 2 examples

Answer 17

when the electrocardiograph is disorganized
1. atrial fibrillation: heart still acts as a pump
2. ventricular fibrillation: heart does not function as an effective pump