week 1 lecture 3: excitation-contraction coupling

• action potentials differ according to where they are in the heart
• how many phases make ventricular action potentials?


  five (0, 1, 2, 3, and 4)
• what phase is the true resting membrane potential of a ventricular action potential in?


  phase 4
• what is the Ek⁺?


  the electrochemical equilibrium; the voltage where the chemical and electrical driving forces balance each other with no net movement of ions
• when the true resting membrane potential is near the Ek⁺, the potassium channels open (high gk)
• action potentials from the SA node are passed rapidly through the conduction system to trigger myocytes
• action potentials are passed between myocytes via gap junctions
• what happens during phase 0 of the ventricular action potential?

  rapid depolarisation is initiated, due to the transient opening of Na⁺ channels, and fast inward N⁺ current
• what happens during phase 1 of the ventricular action potential?


  there is a transient outward flow of K⁺ current, which is counteracted by the inward Ca²⁺ current (L-type channels)
• what happens during phase 2 of the ventricular action potential?


  there is a Ca²⁺ potential
• what happens during phase 3 of the ventricular action potential?


  repolarisation; the K⁺ channels open, whereas the Ca²⁺ channels close
• what happens during phase 4 of the ventricular action potential?


  ATPase restores Na⁺ and K⁺ to their resting levels
• what is the refractory period?


  the recovery phase after a neuron has fired, during which it cannot fire again
• how long is the average refractory period?


  250ms
• what happens during the absolute refractory period?


  Na⁺ channels are closed (gated) and cannot be opened again, and there is prolonged Ca²⁺ entry
• what happens during the relative refractory period?


  there is a large efflux of potassium
• what is an efflux?


  outward flow
• why is it not possible to generate an action potential following a relative refractory period?


  the inward current is too small, especially compared to the large efflux of potassium
• which kind of refractory period is fast?


  absolute refractory period
• which kind of refractory period is slow?


  relative refractory period
• which phases experience the absolute refractory period?


  phases 1, 2, and part of 3
• which phases experience the relative refractory period?


  phase 3
• what is ARP?


  the absolute refractory period
• what can prolong the absolute refractory period (ARP)?


  sodium and potassium channel blockers
• what uses sodium/potassium channel blockers to prolong ARP?

  (class I) antiarrhythmics
• what are antiarrhythmics?


  medications that prevent and treat arrhythmias (i.e., heart rhythms that are too fast or irregular)
• give examples of sodium channel blockers

  • quinidine
  • procainamide
  • lidocaine
  • phenytoin
  • flecainide
• give examples of potassium channel blockers

  • amiodarone
  • sotalol
  • ibutilide
• the ventricular action potential is much longer than the neuronal action potential
• what is ECG stand for?

  electrocardiogram
• what do ECGs do?

  monitor/record cardiac activity
• where are electrodes placed when conducting an ECG?


  chest, arms, and legs (i.e., limbs)
• how are electrodes connected when conducting an ECG?


  electrodes are connected by wires to the ECG recording machine
• what are waveforms?


  graphic representations showing the shape of a wave
• what does the P wave represent?


  atrial depolarisation
• what does the QRS complex (waveform) represent?


  ventricular depolarisation
• what does the T wave represent?


  ventricular repolarisation
• how is the U wave shaped?


  as a small plateau; what it represents is unknown
• which heart waveform has the largest peak?


  the QRS complex
• what is the order of waves in an ECG?


  P, QRS, T, (then U)