1. Apart from the size of the fibres, what are the structural differences between skeletal and cardiac muscle?

structural differences:

• Cardiac cells are mononuclear, skeletal muscle cells are multinuclear

• The cardiac cell (myocyte) nucleus is centrally located, but peripherally located for skeletal cells

• Cardiac muscle fibres are branched, unlike skeletal fibres

• Cardiac cells are connected to one another by intercalated disks. Gap junctions at these discs allow excitation to pass from one cell to another. Therefore, cardiac myocytes contract as a syncitium

• The T tubule system (which spreads the action potential) is larger in cardiac muscle

• In cardiac muscle, such T tubules are located at the Z line. In skeletal muscle, it is located at the junction of the A and I bands

2. List some functional differences between skeletal and cardiac muscle.

• Skeletal muscle is voluntary

• Cardiac muscle contracts spontaneously (myogenic)

• In skeletal muscle, Ca2+ is released from the SR following spread of depolarisation through the T tubule network

• With cardiac muscle, Ca2+ -release from the SR is triggered by Ca2+ that already been released by the SR, and by Ca2+ that has influxed through membrane voltage channels. This is called Ca2+-induced Ca2+release

• Mechanical summation and tetanus do not occur with cardiac muscle because of the longer duration of cardiac action potential

• In the case of skeletal muscle, increases in force are generated by recruitment of motor units and mechanical summation (see ‘Skeletal muscle’)

• The force of cardiac muscle contraction is determined by the amount of intracellular Ca2+ generated. For example through the action of hormones

• […]