Chapter 17: Locomotion and Movement (Human Physiology)

(Draw the relaxed sarcomere of the figure on page 3.) Label: two Z lines at the ends; thin actin filaments attached to them forming the half I bands (light); the central A band (dark) of thick myosin filaments; the M line in the middle of the A band; the H zone, the central part of the A band not overlapped by actin.

The sliding filament theory states that contraction of a muscle fibre takes place by the sliding of the thin (actin) filaments over the thick (myosin) filaments, towards the centre of the A band; the filaments themselves do not shorten. As a result the Z lines come closer, the sarcomere shortens, the I band and H zone narrow and the A band stays the same.

(i) A signal from the CNS travels through a motor neuron to the neuromuscular junction, releasing acetylcholine. (ii) This generates an action potential in the sarcolemma, which spreads through the fibre. (iii) Ca²⁺ is released from the sarcoplasmic reticulum. (iv) Ca²⁺ binds a subunit of troponin, unmasking the active sites on actin. (v) Using energy from ATP hydrolysis, the myosin head binds actin forming a cross bridge. (vi) The cross bridge rotates and pulls actin towards the centre of the A band; the Z lines move in and the sarcomere shortens (I band shortens, A band unchanged). (vii) Myosin releases ADP and Pi; a new ATP binds and breaks the cross bridge; ATP is hydrolysed and the cycle repeats, giving further sliding. (viii) When Ca²⁺ is pumped back into the sarcoplasmic cisternae, actin is masked again and the muscle relaxes.

(a) True. (b) False: the H zone has only thick (myosin) filaments, not overlapped by thin filaments. (c) True. (d) False: there are 12 pairs of ribs. (e) True.

(a) Actin: thin filament in the I band; two F-actins (of G-actin) with tropomyosin and troponin; attached to Z line; has binding sites for myosin. Myosin: thick filament in the A band; polymer of meromyosins with heads (HMM, ATPase) and tails (LMM); held by M line; forms cross bridges. (b) Red: much myoglobin, many mitochondria, aerobic, for sustained work. White: little myoglobin (pale), few mitochondria, much sarcoplasmic reticulum, anaerobic, for quick bursts. (c) Pectoral: for upper limbs; each half = clavicle + scapula; glenoid cavity for humerus (shoulder joint). Pelvic: for lower limbs; two coxal bones, each of fused ilium, ischium and pubis; acetabulum for femur; halves meet at the pubic symphysis.

(a)–(iv); (b)–(ii); (c)–(i); (d)–(iii).

(i) Amoeboid: macrophages and leucocytes move by pseudopodia formed by protoplasmic streaming, with microfilaments. (ii) Ciliary: in tubular organs with ciliated epithelium; cilia of the trachea remove dust and foreign particles; cilia help ova move through the female reproductive tract. (iii) Muscular: contraction of muscles moves limbs, jaws, tongue and causes locomotion. (Flagellar movement is shown by sperms.)

Skeletal: attached to bones; fibres unbranched, long, multinucleate (syncytial); striated; voluntary; tire easily; for locomotion and posture. Cardiac: only in the heart; cells branched and joined in a network; striated; involuntary (not directly controlled by the nervous system); contract rhythmically all life without fatigue.

(a) Pivot (synovial). (b) Saddle (synovial). (c) Hinge (synovial). (d) Ball and socket (synovial). (e) Fibrous (sutures). (f) Cartilaginous (pubic symphysis).

(a) seven. (b) 14. (c) tropomyosin, troponin. (d) sarcoplasmic reticulum. (e) 11th, 12th. (f) 8.