During contraction, actin filaments move toward the centre of the A-band, pulling Z-lines inward and shortening the sarcomere. A-band length remains unchanged while I-band and H-zone reduce.
Sliding filament theory states that muscle fibre contraction occurs by sliding of thin actin filaments over thick myosin filaments.
A motor neuron releases acetylcholine at the neuromuscular junction, generating an action potential in sarcolemma. This causes Ca++ release from sarcoplasmic reticulum. Calcium binds troponin and removes masking of myosin-binding sites on actin. Myosin head, using ATP energy, binds actin to form a cross-bridge and pulls actin toward the A-band centre. A new ATP breaks the bridge, and repeated cycles cause contraction. When Ca++ is pumped back, active sites are masked and the muscle relaxes.
Muscle contraction involves neural stimulation, calcium release, exposure of actin sites, cross-bridge formation, power stroke and relaxation.
Thin filaments contain actin. H-zone is the central non-overlap region of A-band. Human skeletal system has 206 bones. There are 12 pairs of ribs. Sternum is a flat bone on the ventral midline of thorax.
(a) True. (b) False; H-zone contains only thick myosin filaments not overlapped by thin filaments. (c) True. (d) False; humans have 12 pairs of ribs. (e) True.
(a) Actin forms thin filaments with F-actin, tropomyosin and troponin; myosin forms thick filaments with meromyosin heads having ATPase activity. (b) Red muscles have high myoglobin, many mitochondria and aerobic metabolism; white muscles have low myoglobin, fewer mitochondria, more sarcoplasmic reticulum and rely more on anaerobic metabolism. (c) Pectoral girdle has clavicle and scapula and articulates upper limbs; pelvic girdle has two coxal bones formed by ilium, ischium and pubis and articulates lower limbs.
Actin/myosin differ as thin/thick contractile filaments; red/white muscles differ in myoglobin, mitochondria and metabolism; pectoral/pelvic girdles attach upper/lower limbs.
Smooth muscle is involuntary. Tropomyosin is associated with thin actin filaments. Red muscles contain abundant myoglobin. Skull bones are joined by fibrous sutures.
(a)-(iv), (b)-(ii), (c)-(i), (d)-(iii).
Leucocytes and macrophages show amoeboid movement using pseudopodia. Ciliated epithelium shows ciliary movement, for example in trachea and female reproductive tract. Muscle cells show muscular movement for locomotion and body movements.
Human body cells exhibit amoeboid, ciliary and muscular movements.
Skeletal muscle fibres are long, cylindrical, unbranched and under voluntary nervous control. Cardiac muscle cells form branching networks, are involuntary, and contract rhythmically as heart muscle.
Skeletal muscle is striated, voluntary and attached to bones; cardiac muscle is striated, involuntary, branched and present in the heart.
Atlas-axis joint permits rotation, so it is pivot. Thumb carpometacarpal joint is saddle. Interphalangeal joints are hinge. Femur head with acetabulum is ball and socket. Cranial bones are joined by sutures. Pubic symphysis is a cartilaginous joint.
(a) Pivot joint. (b) Saddle joint. (c) Hinge joint. (d) Ball and socket joint. (e) Fibrous joint/suture. (f) Cartilaginous joint.
These values and terms come directly from vertebral column, limb skeleton, contractile protein and sarcoplasmic reticulum descriptions.
(a) seven. (b) fourteen. (c) tropomyosin and troponin. (d) sarcoplasmic reticulum. (e) 11th and 12th. (f) eight.