Heart Muscle

Question 1

Learning outcomes

Answer 1

• To emphasise the importance to the
modern healthcare professional of a
knowledge of cardiovascular physiology
• To show how the properties of cardiac
muscle fit the hearts function as an
essential blood pump.
• To fully describe the
• Morphologic
• Electrical
• Mechanical
• Metabolic
• Properties of cardiac muscle

Question 2

Cardiac muscle morphology

Answer 2

• Myocardium composed of cardiac myocytes
• Similar to skeletal muscle
• Striated
However
• Branched - heart muscle needs to electrical signal can travel throughout heart and pull in all directions
• Automatic in nature, as is smooth muscle
• Intercalated disc- desmosomes and tonofilaments- gap junctions provide electrical connections
• Mechanical (folds)
• Electrical (gap junctions)- MYOGENIC tissue
• Functional “syncitium”- fused mass of cells
• Single nucleus
• Many mitochondria
• Aerobic metabolism
• Efficiency and economy

Question 3

Cardiac electrophysiology

Answer 3

• RMP (resting membrane potential)
• Large negative ions on the inside of cell
• K+ leaks out, but Na+ doesn’t leak in
• Na+/K+ exchanger is electrogenic
• Excitable cells activated by allowing positive ions
in, making the cell temporarily positive (Depolarisation)
• Action potential

Question 4

Cardiac action potentials

Answer 4

• Non Pacemaker
  • -92 mV RMP
  • In atrial and ventricular myocardium
  • Force development
  • Channels
  • Phase 0; Na+ channel opens at -70mV, has a slower closing gate at –30 - -40 mv
  • Phase 1; Closure of Na+ channels, opening of K+ channel (ITO) and Cl channel (ICl).
  • Phase 2; Slow prolonged opening of Ca++ channels
  (plateau V important)
  • Phase 3; 2K+ channels , channels (IKr, and IKs- inwardly rectifying channel, inwardly )
  • Phase 4; Resting membrane potential

- Pacemaker
• Varying prepotential
• In SA and AV nodes
• Setting and carrying the rhythm
• Channels
• Channels
• Phase 0; Long lasting Ca++ channels. The current is called ICaL. They open at – 40 mV  msec
• Channels
• No phase 1 or phase 2 here
• Channels
• Channels
• Phase 3; K+ channel
• Phase 4; Slow rise in Ca+ permeability and fall in K+
permeability (Creep potential)
• Also Ih and If (funny- sodium in potassium out, net effect of these is more Na+ enters cell than K+ leaves, phase 4 begins)

Question 5

Refractory period

Answer 5

• “That time after an initial stimulation during which a muscle or nerve is unstimulable”
• Absolute and relative refractory periods
• Long plateau, so no summation and tetanus like in skeletal muscle (tetanus would be lethal in c muscle). Special design feature.

Question 6

Mechanical properties of heart muscle

Answer 6

• Contraction of heart muscle when calcium
concentrations go from 10-7 to 10-4 inside cells.
• Mechanical response is 15 times as long as the electrical one.
• Skeletal muscle can produce a wide range of
contraction strengths by recruiting varied numbers of motor fibres.
• Cardiac muscle can not because all cells are linked (“functional syncitium”)
• Long refractory period prevents tetanic contractions (see previous slide)
• Cardiac muscle has a similar length tension
relationship to skeletal muscle. Can you guess
how this might be useful?

Question 7

Clinical relevance- cardiomyopathies

Answer 7

• Hypertrophic cardiomyopathy. Hypertrophy is a normal muscular response to increased load – Physiologic response
• Pathology when genes coding for contractile apparatus mutate, leading to a weakened contraction. Can be lethal
• Enlarged heart without hypertrophy in Duchenne or Becker Muscular dystrophy. Dilated myopathies
• Channelopathies, arrythmogenic, causing long QT
syndrome. Problems with the K+ channels underlying
repolarisation.

Question 8

Metabolic properties of the heart

Answer 8

• 99% of the hearts energy through aerobic means (almost obligatory aerobic respiration unlike skeletal
muscle).
• Lots of mitochondria
• Myoglobin
• Basal Caloric needs
• 60% provided by fats
• 35% oxidation of carbohydrates
• 5% oxidation of amino acids and ketone bodies