Muscles

Muscle Structure A very high resolution E.M reveals that each myofibril is made up of parallel filaments. There are 2 kinds of filament called thick & thin filaments. These 2 filaments are linked at intervals called cross bridges, which actually stick out from the thick filaments

Mechanism of muscle contraction The above micrographs show that the sarcomere gets shorter when the muscle contracts The light (I) bands become shorter The dark bands (A) bands stay the same length

The Sliding Filament Theory So, when the muscle contracts, sarcomeres become smaller However the filaments do not change in length. Instead they slide past each other (overlap) So actin filaments slide between myosin filaments and the zone of overlap is larger

What makes the filaments slide past each other? Energy for the movement comes from splitting ATP ATPase that does this is located in the myosin heads. The energy from the ATP causes the angle of the myosin head to change. The myosin heads can attach to actin. Movement of the myosin heads and them attaching and detaching from actin causes the filaments to slide relative to one another. This movement reduces the sarcomere length.

Repetition of the cycle One ATP molecule is split by each cross bridge in each cycle. This takes only a few milliseconds During a contraction 1000’s of cross bridges in each sarcomere go through this cycle. However the cross bridges are all out of synch, so there are always many cross bridges attached at any one time to maintain force.

The Cross Bridge Cycle

The cycle begins with ATP binding to the myosin head The cycle begins with ATP binding to the myosin head. This causes the myosin head to be released from actin.

The Cross Bridge Cycle

2. The ATP molecule is then hydrolysed while the myosin head is unattached. The ADP & Pi formed remain bound to the myosin head.

The Cross Bridge Cycle

3. The energy released by the hydrolysis of ATP is absorbed by the myosin This causes the myosin head to change shape (places it in energised state or cocked state – also called the recovery stroke) It then binds to the actin filament.

The Cross Bridge Cycle

4-5. The ADP and Pi are then released from the myosin head Result = Power stroke occurs (the myosin head changes shape) This draws the actin filament over the myosin filament.

The Cross Bridge Cycle

The cycle begins again when the next ATP binds to the myosin head The cycle begins again when the next ATP binds to the myosin head. Causing the myosin head to be released from actin.

Control of Muscle Contraction How is the cross bridge cycle switched off in a relaxed muscle? This is where the regulatory protein on the actin filament, tropomyosin is involved. Actin filaments have myosin binding sites. These binding sites are blocked by tropomyosin in relaxed muscle. When Ca2+ bind tropomyosin is displaced and the myosin binding sites are uncovered. So myosin & actin can now bind together to start the cross bridge cycle

Tropomyosin, Ca2+ & ATP Ca2+ causes tropomyosin to be displaced. So it no longer blocks the myosin binding site So myosin and actin can bind together allowing cross bridge cycling

Neuromuscular junction: Note Ach = Acetylcholine

Sarcoplasmic Reticulum

Sequence of events 1. An action potential arrives at the end of a motor neurone, at the neuromuscular junction. 2. This causes the release of the neurotransmitter acetylcholine. 3 This initiates an action potential in the muscle cell membrane (Sarcolemma). 4. This action potential is carried quickly into the large muscle cell by invaginations in the cell membrane called T-tubules.

Sequence of events 5. The action potential causes the sarcoplasmic reticulum to release its store of calcium into the myofibrils. 6. Ca2+ causes tropomoysin to be displaced uncovering myosin binding sites on actin. 7. Myosin cross bridges can now attach and the cross bridge cycle can take place. Relaxation is the reverse of these steps