G1iii: Describe the structure and functional significance of contractile elements of the heart

1. Atrial — Like skeletal m. but contract longer
2. Ventricular — Like skeletal m. but contract longer
3. Conduction → few contractile fibres; they exhibit automatic rhythmic electrical D/C in the form of AP which controls rhythmical beating of the heart

Consist of:

• Sarcomeres → with actin & myosin filaments
  • The functional unit of cardiac muscle
• Intercalated discs → cell membranes that separate muscle cells but join the ends of myocytes together
  • Fusion of the cell membrane via IC discs allows fast communication between myocytes
  • Enables rapid propagation of AP
• Gap Junction → a low resistance pathway within an intercalated disc
  • Cell connection which lets adjacent cytoplasm communication & ions pass v. quickly
• T-tubules → extension of cell membrane that penetrates cell
  • Bigger & wider cf. T-tubule of skeletal m., which allows greater Ca²⁺ influx
  • AP passes over cardiac myocyte PM
  • Causes SR to release Ca²⁺
  • Extra Ca²⁺ enters myocyte via T-tubule
  • Mass Ca²⁺ release allows non-bridge formation & contraction
  • SR of cardiac myocyte less developed skeletal m. → requires extra Ca²⁺ from T-tubule to generate an appropriate muscle contraction

Strength of contraction ∝ [Ca²⁺ ECF]

• Syncytium
  • Cardiac m. performs as a unit of atrial & ventricular syncytium
  • When suprathreshold stimulus applied → AP → depolarisation → contraction of entire syncytium
  • Atrial & ventricular syncytium separated by fibrous tissue, which prevents conduction from atrial → ventricular syncytium directly
  • AV bundle conducts the message ∴allows the atria to contract before ventricles