Cardiovascular System - Properties of Cardiac Muscle

Question 1

Describe the cardiac morphology.

Answer 1

Myocardium composed of myocytes
Striated
Branched and autonomic in nature
Intercalated discs - mechanical folds, electrical gap junctions
Functional syncitium
Single nucleus
Many mitochondria - aerobic metabolism and efficiency

Question 2

Describe the Turbo Electrophysiology Primer.

Answer 2

Resting membrane potential
Negative ions inside the cell
Potassium ions leak out but sodium ions don’t leak in
Na+/ K+ exchanger is electrogenic
Excitable cells activated by allowing influx of positive ions temporarily depolarising membrane leading to an action potential

Question 3

Describe the non pacemaker cardiac action potential.

Answer 3

• The non-pacemaker cardiac action potential is the electrical activity generated by cardiac muscle cells in the heart’s atria, ventricles, and specialized tissues.
• Triggered by external electrical stimuli, primarily propagated by pacemaker cells.
• The action potential has four phases: Rapid Depolarization, Early Repolarization, Plateau Phase, Repolarization, and Resting Membrane Potential.
• Rapid depolarization occurs when sodium ions influx through fast voltage-gated sodium channels, leading to a sharp upstroke.
• Early repolarization occurs when sodium channels close and potassium channels open, partially restoring the membrane potential to its resting state.
• The plateau phase prolongs the action potential duration, allowing for sustained contraction of cardiac muscle cells.
• After repolarization, the membrane potential returns to its resting state, typically around -85 to -90 mV.

Question 4

Describe the pacemaker cardiac action potential.

Answer 4

• The pacemaker cardiac action potential is the electrical activity generated by specialized cells within the sinoatrial (SA) node, which serves as the heart’s natural pacemaker.
• The SA node generates rhythmic electrical impulses that propagate throughout the heart, triggering cardiac muscle contraction.
• The pacemaker action potential is unique in its spontaneous generation and unique ion channel characteristics.
• Its phases include a gradual depolarization phase, followed by rapid depolarization, repolarization, and a diastolic interval.
• This automaticity allows the SA node to set the heartbeat rhythm and rate independently of external stimuli.
• However, the rate of pacemaker firing can be influenced by factors like autonomic nervous system activity, hormones, and certain medications.

Question 5

Define the refractory period

Answer 5

Time after initial stimulation during which a nerve or muscle is unexcitable
Absolute and relative refractory periods
Long plateau therefore no summation

Question 6

Describe the mechanical properties of cardiac muscle.

Answer 6

Contraction of heart muscle when calcium concentration increases from 10−7 to 10−4
Skeletal muscle can produce a wide range of contraction strengths by recruiting varied numbers of motor fibres
Cardiac muscle can’t as all cells are linked in a functional syncitium.

Question 7

Describe the metabolic properties of cardiac muscle.

Answer 7

Lots of mitochondria
Myoglobin
Basal caloric needs - 65% provided by fat and 35% by carbs

Question 8

What is the importance of the refractory period ?

Answer 8

Ensures unidirectionality of the wave of excitation
Prevention of tetany
Synchronisation of contraction
Protection against overload