Properties of Heart Muscle

Question 1

Cardiac Morphology

Answer 1

• myocardium composed of cardiac myocytes
• striated
• branched
• autonomic in nature
• intercalated discs - mechanical (folds) and electrical (gap junctions)
• functional - ‘syncitium’
• single nucleus
• many mitochondria - aerobic metabolism and efficiency and economy

Question 2

Cardiac action potentials - Non Pacemaker

Answer 2

• -92 mV RMP
• in atrial and ventricular myocardium
• force development

Question 3

Cardiac action potentials - Pacemaker

Answer 3

• varying prepotential
• in SA and AV nodes
• setting and carrying the rhythm

Question 4

Channels - Non Pacemaker

Answer 4

• phase 0: Na+ channel opens at -70mV, has a slower closing gate at -30- -40mV
• phase 1: closure of Na+ channels, opening of K+ channel and Cl channel
• phase 2: slow prolonged opening of Ca2+ channels (plateau very important)
• phase 3: 2K+ channels (Ikr and Iks)
• phase 4: resting membrane potential

Question 5

Channels - Pacemaker

Answer 5

• phase 0: long lasting Ca2+ channels. The current is called ICaL. They open at -40mV
• No phase 1 or 2 here
• Phase 3: K+ channel
• Phase 4: Slow rise in Ca+ permeability and fall in K+ permeability (creep potential)
• also Ih and If

Question 6

Refractory Period

Answer 6

• the 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 cardiac muscle)

Question 7

Mechanical Properties

Answer 7

• contraction of heart muscles when calcium concentrations goes from 10^-7 to 10^-4 inside cells
• mechanical response is 15 times as long as the electrical one
• cardiac muscle cannot produce a wide range of contraction strengths like skeletal muscle due to “functional syncitium” (all cells are linked)
• long refractory periods prevents tetanus contractions
• cardiac muscle has a similar length tension relationship to skeletal muscle

Question 8

Metabolic Properties

Answer 8

• 99% of hearts energy is through aerobic respiration
• lots of mitochondria
• myoglobin
• basal caloric needs: 605 provided by fats, 35% oxidation of carbohydrates, 5% oxidation of amino acids and ketone bodies