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Muscle Action (Neuromuscular Junctions (Depolarisation wave travels down…
Muscle Action
Neuromuscular Junctions
- Depolarisation wave travels down the T-tubule.
- T-system depolarisation leads to a release a Ca^2+ from stores in sarcoplasmic reticulum.
- Acetylcholine binds to the receptors on muscle fibre membrane causing a depolarisation.
- Ca^2+ binds to proteins in the muscle, leading to contraction.
- Nerve impulse arrives at the neuromuscular junction causing the vesicles containing acetylcholine to fuse and release contents into the cleft.
Acetylcholinesterase rapidly breaks down acetylcholine so that contraction only occurs when an impulse arrives.
Theory of Contraction
- The myosin heads all have a molecule of ADP and a phosphate attached to them when not active. When the heads bind to the actin, they eject the ADP and phosphate, which releases energy, and they use this energy to bend their heads towards the thin filaments and pull the thin filaments toward them.
- After calcium moves into the cell and binds to troponin, revealing actin binding sites, the myosin heads attach to the surrounding actin, forming a cross-bridge.
- The binding sites on actin for the myosin heads are covered up by the molecule tropomyosin, but when the calcium ions enter the muscle cell and bind to the binding site on troponin, they shift shape slightly, revealing the binding site for the myosin heads on the actin - so when calcium ions are not present, the muscle can't contract but as calcium moves into the cell and binds to troponin, myosin is able to bind to the actin molecules.
- Then a molecule of ATP replaces the one lost before it on the myosin head, which breaks down the cross-bridge, and the ATP is hydrolysed to give ADP and phosphate (the myosin is returned back to its original state). This causes the myosin head to pull back down, releasing the thin filament, which slides back into place.
- So long as there is calcium present in the muscle cell, the actin binding sits will remain open, and so this cycle can continue for as long as the calcium remains.
Types of Muscle
Cells of skeletal muscle are large cells which are long and thin. They are multinucleated cells (have many nuclei), and under the light microscope skeletal muscle appears banded in repetitive strips.
Cells of smooth muscle appear spindle-shaped, with small gaps (fenestrations) in between them, and the cells are singly-nucleated, and not striated - they contain small bundles of actin and myosin and contract very slowly compared to the other two types of muscle.
All muscle types are composed of cells that are elongated to form fibres, but the three types (skeletal, smooth and cardiac) differ significantly in terms of their structure.
Cardiac muscle cells also appear striated(banded) but are not the same as skeletal muscle. The cells actually have connecting platforms between each other called intercalated discs.
Muscle and Joints
The elbow is a synovial joint where a lot of movement occurs. These muscles work as an antagonistic pair. Although they can't actively contract, they re-extend when pulled by opposite antagonistic muscle.
Ligament - holds the ones together to prevent dislocation.
Cartilage - pads where bones meet that reduce friction as they move.
Synovial membrane - produces synovial fluid.
Synovial fluid - lubricates the joint.
Muscles can work in antagonistic pairs. In order for smooth movement at a muscular junction, such as the elbow, two muscles must be involved. If there are two muscles, the bone at the joint can only move smoothly if one muscle contracts, and the other relaxes.