The immediate step that occurs when muscle fibre is electrically excited is

The immediate step that occurs when muscle fibre is electrically excited is release of Ca²⁺ from sarcoplasmic reticulum.

1. Electrical Excitation: When a nerve impulse reaches the neuromuscular junction (the point of communication between a nerve and a muscle fiber), it triggers the release of the neurotransmitter acetylcholine.
2. Acetylcholine Binding: Acetylcholine binds to receptors on the muscle fiber's membrane, which leads to the propagation of an action potential (electrical signal) along the muscle cell's surface (sarcolemma).
3. Propagation of Action Potential: The action potential travels down the T-tubules (transverse tubules), which are invaginations of the sarcolemma that penetrate deep into the muscle fiber.
4. Sarcoplasmic Reticulum Response: The T-tubules are in close proximity to the sarcoplasmic reticulum, a specialized intracellular calcium storage organelle in muscle cells. The action potential causes the sarcoplasmic reticulum to release stored calcium ions into the cytoplasm (sarcoplasm) of the muscle fiber.
5. Calcium Binding to Troponin: The released calcium ions bind to troponin, a regulatory protein found on the thin filaments of the muscle (actin filaments). This interaction causes a conformational change in the troponin-tropomyosin complex.
6. Exposure of Binding Sites: The conformational change in the troponin-tropomyosin complex uncovers the binding sites on the actin filaments for myosin heads (thick filaments) to interact with.
7. Cross-Bridge Formation: Myosin heads bind to the exposed actin binding sites, forming cross-bridges.
8. Muscle Contraction: The cross-bridges undergo a series of chemical reactions, leading to the sliding of actin filaments over myosin filaments, which shortens the sarcomeres (the functional units of muscle fibers) and results in muscle contraction.
9. Relaxation: When the electrical excitation ceases, calcium ions are actively pumped back into the sarcoplasmic reticulum, and the troponin-tropomyosin complex returns to its original conformation, covering the actin binding sites. This process allows the muscle to relax.