Gross and microscopic structure of skeletal muscle. The ultrastructure of a myofibril. The roles of actin, myosin, calcium ions and ATP in myofibril contraction. The roles of calcium ions and tropomyosin in the cycle of actinomyosin bridge formation.

The roles of actin, myosin, calcium ions and ATP in myofibril contraction. The roles of calcium ions and tropomyosin in the cycle of actinomyosin bridge formation.

In a muscle cells join up to make muscle fibres, resulting in long strands of cells sharing nuclei and cytoplasm (aka sarcoplasm). Also inside these fibres are myofibrils: these are the structures that allow contraction. Myofibrils are made up of proteins.

Fibers are grouped together into bundles which are grouped further to create muscles.

Myofibrils are made from myofilaments. These are proteins which mean contraction can happen. They are arranged in such a ways as to create the appearance of different coloured bands: A-band and I-band (easily remembered by thing dArk band and lIght band). The distance between two z-lines is called a sarcomere.

(turned 90 degrees from previous diagram)

The two filaments are called actin and myosin.

Actin

Myosin

During contraction, the heads of myosin go into the dips in actin and then push it so that the two are propelled in opposite directions. This makes them move like they are crawling up one another, bringing the z-lines closer together and decreasing the size of myofibrils and therefore muscles overall.

For this to happen, Ca2+ binds to troponin, which changes its positioning moving tropomyosin out of the binding site of actin.
This allows myosin heads to bind there, as they do they change angle and release a ADP molecule. ATP then attatches to the head causing it to detach from the actin. The ATP turns to ADP providing energy for the head to return to where it was before so that it can repeat the process.