Structure of Contractile proteins :
Thin filament or Actin :
- Each actin filament is made of two ‘F’ actins helically wound to each other.
- Each ‘F’ actin is made of polymer of monomeric ‘G’ (Globular) actin.
- Each ‘F’ actin associated with another protein, tropomyosin also run throughout its length.
- Another complex protein, Troponin is distributed at regular intervals on the tropomyosin.
- Each troponin has three component –
- Troponin-C binds with calcium.
- Troponin-M, binds with the tropomyosin.
- Troponin T, masks the active site on the ‘G’ actin (thin filament)
- In the resting state a sub-unit of Troponin (Tn-T), masks the active binding sites on the thin filaments for myosin.
Thick filament :
- Each myosin (thick) filament is consists of many monomeric protein called Meromyosins.
- Each meromyosin has two parts –
- Heavy meromyosin (HMM) - A globular head with a short arm.
- Light meromyosin (LMM) – a tail.
- The HMM component, i.e. the head and short arm projects outwards at regular distance and angle from each other from the surface of a polymerized myosin filament and is known as cross arm.
- The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.
Mechanism of muscle contraction :
- Mechanism of muscle contraction is explained by sliding filament theory which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments.
- Muscle contraction is initiated by a signal sent by the central nervous system via a motor neuron.
- A motor neuron along with the muscle fibres connected to it constitutes a motor unit.
- The junction between a motor neuron and the sarcolemma of the muscle fibre is called neuromuscular junction or motor-end plate.
- Neurotransmitter releases here which generates an action potential in sarcolemma.
- These causes release of Ca++ into sarcoplasm.
- These Ca++ binds with troponin, thereby remove masking of active site.
- Myosin head binds to exposed active site on actin to form a cross bridge, utilizing energy from ATP hydrolysis.
- This pulls the actin filament towards the centre of ‘A’ band.
- ‘Z’ lines also pulled inward thereby causing a shortening of sarcomere i.e. contraction.
- ‘I’ band get reduced, whereas the ‘A’ band retain the length.
- During relaxation, the cross bridge between the actin and myosin break.
- Ca++pumped back to sarcoplasmic cisternae.
- Actin filament slide out of ‘A’ band and length of ‘I’ band increases. This returns the muscle to its original state.
- Repeated muscle contraction causes accumulation of lactic acid, produced from anaerobic breakdown of glycogen leads to muscle fatigue.
- Muscle contains red coloured oxygen storing pigment called myoglobin.
- Muscle with myoglobin called red muscle fibres, they are also contain large number of mitochondria which can utilize large amount of oxygen stored in them for ATP production also called aerobic muscle.
- Some muscles possess very less quantity of myoglobin and less mitochondrion hence called white fibres. Amount of sarcoplasmic reticulum is high in these muscles. They depend on anaerobic process for energy.
SKELETAL SYSTEM :
- Human skeleton consists of 206 bones in adult.
- Axial skeleton – 80 bones
- Appendicular skeleton – 126 bones.
- Axial skeleton :
- Skull – 29 bones.
- Cranium – 8 bones forms the brain box.
- Facial – 14 bones forms the front part of the face.
- Hyoid – a single U-shaped bone at the base of the buccal cavity.
- Ear ossicles – 6 bones- 3 on either side (Malleus, Incus and stapes)
- The skull region articulates with the superior region of the vertebral column with the help of two occipital condyles hence called dicondylic skull.
- Vertebral column – 26 bones
- Cervical – 7 vertebrae.
- Thoracic – 12 vertebrae.
- Lumber – 5 vertebrae.
- Sacral – 1 vertebra. (fused five bone)
- Caudal – 1 vertebra (fused four bones)
- Sternum or breast bone – 1 bone in the middle line of the thorax.
- Ribs – 12 pairs – (24 bones)
- 1-7 are true ribs (connected to the sternum directly)
- 8th, 9th, 10th pairs are called false ribs they attached to the 7th ribs.
- 11th and 12th not connected ventrally hence called floating ribs.
- Ribs attaché dorsally to the vertebra and ventrally with the sternum by hyaline cartilage.
- Thoracic vertebrae, ribs and sternum together form the rib cage.
