IB Biology Topic B2.3.9: Adaptations of Cardiac Muscle Cells and Striated Muscle Fibres Notes

Cardiac and Skeletal Muscle: Same Striations, Different Adaptations

1. Cardiac muscle cells and striated (skeletal) muscle fibres are both striated, because both are packed with myofibrils showing repeating sarcomeres.
2. Their differences in shape, nuclei, and control reflect their very different jobs in the body.
   1. Cardiac muscle powers the continuous, tireless pumping of the heart.
   2. Skeletal muscle powers voluntary movement of the body under conscious control.

Cardiac Muscle: Branched Cells Wired for Synchronised Beating

Branched Cells Joined by Intercalated Discs

1. Cardiac muscle cells are branched, so they interconnect into a continuous network.
2. Neighbouring cells are joined end to end by intercalated discs.
   1. Intercalated discs contain gap junctions that let electrical impulses pass directly between cells.
   2. This rapid electrical coupling makes the whole heart contract in a synchronised wave.

One Central Nucleus and Many Mitochondria

1. A cardiac muscle cell is uninucleate, usually with a single, centrally placed nucleus.
2. The cells are packed with many mitochondria to supply constant aerobic energy.
   1. This heavy aerobic supply means cardiac muscle does not fatigue easily.

Myogenic Contraction Needs No Nerve

1. Cardiac muscle is myogenic, meaning it contracts without nervous stimulation.
2. The signal starts in pacemaker cells and spreads through the gap junctions of the intercalated discs.

Myofibrils for Strong, Repeated Contraction

1. Cardiac cells contain myofibrils made of repeating sarcomeres.
2. These give the strong, controlled contractions needed to pump blood around the body.

Skeletal Muscle: Long, Multinucleate Fibres Under Voluntary Control

Long, Unbranched Fibres

1. Striated (skeletal) muscle fibres are very long and unbranched, extending across large parts of a muscle.
2. Many fibres contracting in parallel generate large forces for movement.

Multinucleate, Formed by Cell Fusion

1. Skeletal muscle fibres are multinucleate, containing many nuclei per fibre.
2. They form by the fusion of many separate cells during development.
   1. The nuclei sit at the periphery of the fibre.
   2. Multiple nuclei are needed because a single nucleus could not control such a long fibre.

Myofibrils and Motor Neuron Control

1. Skeletal fibres are packed with myofibrils of repeating sarcomeres, giving the striped appearance.
2. Contraction only happens when a motor neuron stimulates the fibre.
   1. This makes skeletal muscle neurogenic and under voluntary control.

Side by Side: The Key Differences That Earn Marks

Why Skeletal Fibres Are Multinucleate

1. A skeletal muscle fibre forms when many cells fuse into one long syncytium.
2. Each fused cell keeps its nucleus, so the fibre ends up multinucleate.
   1. By contrast, a cardiac cell stays as one cell and keeps its single nucleus.