B - Muscle Contraction

• Skeletal muscles contract and relax to allow movement of the body, using antagonistic pairs consisting of an agonist and an antagonist.
• Antagonistic pairs consist of an agonist and an antagonist.
• The muscle that is relaxing in an antagonistic pair is called the antagonist.
• The muscle that is contracting in an antagonistic pair is called the agonist.
• Skeletal muscle consists of many bundles of muscle fibres, which are long, specialised cells.
• The membrane of muscle fibres is called the sarcolemma, which folds inwards to the sarcoplasm (muscle fibre cytoplasm) at certain points, forming transverse (T) tubules.
• The sarcoplasmic reticulum (SR) is an organelle in the sarcoplasm, storing calcium (Ca2+) ions, which is important in muscle contraction.
• Muscle fibres also have many mitochondria and nuclei, providing ATP to power muscle contraction.
• Myofibrils are cylindrical organelles in muscle fibres, made of multiple units that run end-to-end along the myofibril, these units are called sarcomeres.
• The end of a sarcomere is called the Z-line.
• Sarcomeres are made from two types of myofilaments, the two myofilaments slide past each other, this movement is what makes muscles contract.
• The two types of myofilaments are: Thick myofilaments made of myosin protein, Thin myofilaments made of actin protein.
• Myosin and actin filaments are arranged in an alternating pattern in sarcomeres, thick myosin filaments overlap with the thin actin filaments at each end, the overlapping region is called the A-band, the region with only myosin filament is called the H-zone.
• Thin actin filaments only overlap with myosin filaments in the middle of the sarcomere, the middle is called the M-line, the region with only actin filament is called the I-band.
• Sliding Filament Theory explains muscle contraction as the sliding of myosin and actin filaments due to globular heads on myosin filaments.
• Depolarisation of the sarcolemma causes the myosin and actin filaments to slide over each other, causing the sarcomeres to contract.
• There are multiple sarcomeres along the length of myofibrils, and as many sarcomeres contract simultaneously, the muscle fibres contract.
• Contraction of the muscle fibres causes the whole muscle to contract.
• After the muscle has contracted, the sarcomeres relax, causing the filaments to slide back over each other and the muscle relaxes.
• The Sliding Filament Theory takes place due to globular heads on myosin filaments.
• There are two binding sites on every myosin head: one site can bind to actin, and one site can bind to ATP.
• Tropomyosin is a protein that is located on actin filaments and plays an important role in muscle contraction by blocking the actin-myosin binding site when muscle fibres are at rest.
• When muscle fibres are stimulated, the tropomyosin protein is moved so that myosin heads can bind to the actin-myosin binding site.
• When actin and myosin bind, they can slide past each other to cause muscle contraction.
• Aerobic respiration makes ATP through oxidative phosphorylation and requires oxygen.
• Anaerobic respiration makes ATP by glycolysis and lactate fermentation, and lactate is produced by lactate fermentation.
• The build-up of lactate in the muscles can cause fatigue.
• Anaerobic respiration is mainly used for short periods of high-intensity muscle use.
• Phosphocreatine is a molecule that can supply ATP for muscle contraction, and during intense muscular effort, phosphocreatine donates phosphate to ADP to produce ATP.
• The ATP produced is used to sustain muscle contraction.
• During low periods of muscle activity, ATP can be used to phosphorylate creatine back to phosphocreatine, a process that is anaerobic and produces no lactate but phosphocreatine is in short supply.