10. Fill in the blank spaces:
(a) All mammals (except a few) have __________ cervical vertebra.
(b) The number of phalanges in each limb of human is __________
(c) Thin filament of myofibril contains 2 ‘F’ actins and two other proteins namely __________ and __________.
(d) In a muscle fibre Ca++ is stored in __________
(e) __________ and __________ pairs of ribs are called floating ribs.
(f) The human cranium is made of __________ bones.
6. Match Column I with Column II :
Column I Column II
(a) Smooth muscle (i) Myoglobin
(b) Tropomyosin (ii) Thin filament
(c) Red muscle (iii) Sutures
(d) Skull (iv) Involuntary
4. Write true or false. If false change the statement so that it is true.
(a) Actin is present in thin filament
(b) H-zone of striated muscle fibre represents both thick and thin filaments.
(c) Human skeleton has 206 bones.
(d) There are 11 pairs of ribs in man.
(e) Sternum is present on the ventral side of the body.
The process of muscle contraction includes the following events
1. Depolarisation of Sarcolemma- In this event, a signal is sent by the central nervous system through a motor neuron and it reaches the neuromuscular junction or motor end plate. The neuromuscular junction refers to the junction between a motor neuron and the sarcolemma of the muscle fibre. The receiving of the signal by neuromuscular junction causes the acetylcholine which is a neurotransmitter to release and set the action potential in the sarcolemma.
2. Release of Calcium Ions- This step includes the transmission of the action potential by sarcolemma to the sarcoplasmic reticulum to release calcium ions in the sarcoplasm.
3. Conformational Changes in Actin Filaments- The calcium ions thus released binds to the troponin and tropomyosin on active filaments changing the three-dimensional shape of the actin–troponin-tropomyosin complex. This causes the active site for myosin which is found on the actin filament to be exposed.
4. Activation of Myosin Heads- Later on, the myosin heads also get activated and they release energy by the hydrolysis of ATP resulting in the binding of myosin heads to the active sites present on the actin filaments. This leads to the formation of actin-myosin cross-bridge.
5. Sliding of Actin Filaments over Myosin- After the formation of cross-bridges, the myosin head rotates to pull the actin filament towards the centre of the A-band, i.e. the H-zone. The Z-line attached to the actin filaments is also pulled inwards resulting in the contraction of the sarcomere. During contraction, the I-band shortens, while the A-band retains its length. This causes the muscles to contract.