Electrophysiology of the heart

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Conducting system

Compare general electric properties of the heart and skeletal muscle

  • skeletal muscle cells are electrically isolated from one another, but cardiac muscle cells act essentially as a single unit (functional syncytium)
  • electrically coupled by low-resistance areas (intercalated discs; gap junctions)

List the specialized conducting tissues in the heart. How do they differ from contractile cells?

  • Sinoatrial (SA) node
  • Atrioventricular (AV) node
  • internodal tracts
  • Atrioventricular (His) bundle
  • branches of His bundle
  • terminal Purkinje fibers
  • capable of automaticity and rhythmicity (i.e., not dependent on innervation to contract)

What are pacemaker cells?

  • Spontaneously undergo depolarization in rhythmical manner
  • independent of external (e.g., neural) stimulus
  • spontaneously active cells with highest frequency of repetitive depolarization determines what the heart rate will be
  • usual pacemaker for the heart = SA node
  • any cell may become a pacemaker due to drugs or pathological conditions
  • this would cause cardiac arrhythmias, some of which suck ass.

Excitation

List the order and time sequence for excitation of the heart

  • SA node -> AV node - 0.05 sec
  • delay in the AV node - 0.15 sec
  • activation of ventricles - 0.05 sec
  • ventricles activated by Purkinje fibers
  • depolarization proceeds by cell-to-cell communication

How does the delay of conduction at the A-V node influence heart function?

  • Gives the atria enough time to contract before ventricular contraction

Describe the intracellular action potential recorded from a non-pacemaker cell.

  • See CVS-8 diagram

What is the ionic basis for the various phases of the action potentials?

  • Resting membrane potential (4) - due to high K+; stable
  • depolarization phase (0) - rapid increase in Na+ conductance
  • plateau (2) - Ca++ channels hold membrane potential depolarized for several hundred ms

How do these action potentials differ from those recorded from nerve or skeletal muscle?

  • 4,0, and 3 are relatively similar to skeletal muscle and nerve potentials
  • 2 is the different one (the plateau). It differs "markedly" from other action potentials.

How do intracellular action potentials recorded from pacemaker cells differ from those of non-pacemaker cells? What mechanisms are involved?

  • differences
    • See CVS-8 diagram
    • Lower membrane potential
    • unstable phase 4 (prepotential)
    • slow rising phase
    • lack of a prominent plateau phase
  • mechanisms
    • increase in Na+ conductance or decrease in K+ conductance
    • most textbooks say it?s K+ conductance
    • evidence that Ca++ is also linked to this effect

What is the significance of the long refractory period of cardiac muscle cells?

  • Prevents summation and tetanization of cardiac muscle
  • permits region to act as a ?rate filter?
  • quiescent period is needed so that chambers can fill with blood

Control of heart rate

What changes in the electrophysiological characteristics of pacemaker cells could influence heart rate? What other factors could be involved?

  • Factors that can change heart rate
  • alterations in threshold
  • slope of phase 4
  • resting membrane potential
  • body temperature (7 bpm per °C)

Describe the innervation of the heart by the ANS

  • Atria and nodal tissue are highly innervated by sympathetic and parasympathetic fibers
  • parasympathetic innervation of ventricles present, but role unclear
  • sympathetic innervation of ventricles primarily influences strength of contraction


How is heart rate controlled? Which division of the ANS normally exerts tonic influence on heart rate?

  • ACh presence keeps spontaneous rate of pacemaker cells lower than when it is not present
  • mechanism: increase in K+ conductance hyperpolarizes membrane; decreases slope of phase 4.
  • vagus nerves exert tonic influences on the heart