3: Cardiac muscle, pacemakers, and EKG

• Ventricular muscle contractions generate the pressure which drives flow through the circulatory system.
• Cardiac muscle cells are striated, single nucleus, but post-mitotic, meaning they cannot divide to create new cells and thus cannot replace damage upon cell death.
• Ventricular muscle fiber action potential:

  1. Resting potential, hyperpolarized b/c of open Kir K+ channels
  2. V-gated Na+ channels open, depolarization occurs
  3. At peak: V-gated L-type Ca2+ channels open and V-gated Na+ channels inactivate
  4. Plateau occurs... then L-type Ca2+ channels inactivate AND delayed-rectifying V-gated K+ channels open, repolarization occurs
  5. V-gated Na+ and L-type Ca2+ channels close, as well as delayed K+ channels
• Calcium-indueced calcium release, or CICR, uses modified DHP receptors called L-type Ca2+ channels that are noy physically connected to RyR on the SR membrane.

  When Ca2+ enters the cytoplasm, it binds to RyR and induces more release of Ca2+.
• Order of the conducting system of the heart:

  • SA node > internodal pathways > AV node > Bundle of His > right/left bundle branches > Purkinje fibers
• The SA node sets the rhythm of the heart and will drive all other pacemakers to the fastest possible rhythm.
• Cardiac pacemaker cell action potentials:

  1. V-gated F-type (funny) cation channels open upon hyperpolarization, Na+ enters
  2. V-gated T-type Ca2+ channels open, depolarize to threshold
  3. V-gated L-type Ca2+ channels open, depolarization to peak
  4. L-type Ca2+ channels inactivate and V-gated K+ channels open, repolarization occurs
• The AV node slows down action potential conduction so that the right/left atria can repolarize while the ventricles start to depolarize.
• Atrial contraction does not cause most of ventricular filling; the process is mostly passive.
• The long refractory period of cardiac muscle cells means that cardiac muscle cannot undergo fused tetanus (i.e. maintained contractions), so there is always relaxation to allow for ventricular filling.

  If the ventricle could not be relaxed, refilling of the ventricle could not occur and CO would fall (not good).
• On an EKG, depolarizing current flowing towards the positive direction translates to an upward deflection.
• In summary and summation:

  • P wave: depolarization of the atrium
  • R wave of QRS complex: depolarization of the ventricle
  • T wave: repolarization of the ventricle
• The P-R interval length shows how long it takes AP to conduct from the SA through the AV node; typically, this lengthens with AV node dysfunction.
• The Q-T interval notates the time when the ventricle is in its depolarized state until repolarized. Lengthening or shortening of this interval means that repolarization is occuring later or sooner than normal, respectively.
• The R-R interval shows time between heartbeats and is used to measure heart rate.