how a skeletal muscle contracts

• Resting State: 

  • the actin and myosin filaments in the muscle fibres are not touching each other.

  • The muscle relaxes, and a tropomyosin protein covers the actin's active sites.
• Neural Stimulation:

  • When you move a muscle, a signal travels from your brain through nerves (motor neuron)to the muscle.

  • acetylcholine is released into the neuromuscular junction

  • ACT is then diffused across synaptic cleft

  • binds to motor end plate and triggers an action potential.
• Muscle Excitation:

  • acetylcholinesterase removes ATC to prevent another action potential until more acetylcholine is produced

  • action potential reaches the muscle and spreads along the muscle fibres through Transverse-tubules.

  • Ca2+ is released from the sarcoplasmic reticulum
• Calcium Release

  • The sarcoplasmic reticulum is a storage area for Ca2+.

  • When the action potential reaches it: channels open, releasing calcium into the muscle cell.
• Binding of Calcium:

  • Ca2+ binds to a protein called troponin attached to the tropomyosin.

  • this changes troponin's shape, and the tropomyosin moves away from the active sites on the actin.
• Cross-Bridge Formation:

  • myosin active sites on the actin are exposed,

  • the myosin heads from the thick filaments can attach to the active sites.

  • This forms cross-bridges between the actin and myosin.
• Power Stroke:

  • hydrolysis of myosin with ATP enables this to happen.

  • The myosin filament attaches to the actin filaments and exerts force to rotate them towards the centre of the sacromere and pulling it to the midline, releasing ADP (power stroke)

  • the muscle fibres become shorter and contract.
• Sliding Filament Mechanism:

  • myosin heads pulling on the actin and causing the muscle to contract.

  • It's like the actin and myosin filaments sliding past each other during muscle contraction.
• Muscle Contraction:

  • sliding filament mechanism happens in all the muscle fibres within the muscle

  • the entire muscle contracts and creates movement

  • This allows you to bend your arm, walk, run, or do any physical activity.
• Relaxation:

  • Ca2+ is pumped back into the sarcoplasmic reticulum after the action potential.

  • troponin returns to its original position of covering the active sites on the actin again.

  • This relaxes the muscle and, returns it to its original length.