Locomotion and Movement

A sarcomere is the region between two successive Z lines. It should be shown with:

- Z line at each end
- I band on either side of the Z line, containing thin (actin) filaments
- A band in the middle, containing thick (myosin) filaments
- H zone in the centre of the A band, where only thick filaments are present
- M line at the centre of the H zone

A simple labelled sketch would look like this:

`Z | I band | A band (H zone with M line) | I band | Z`

The thin filaments are attached to the Z line, and the thick filaments are held together at the M line.

The sliding filament theory states that muscle contraction occurs by the sliding of thin filaments over thick filaments.

The important steps in muscle contraction are:

1. A signal from the CNS reaches the muscle through a motor neuron.
2. At the neuromuscular junction, the neurotransmitter acetylcholine is released.
3. This generates an action potential in the sarcolemma.
4. The action potential causes release of **Ca$^{2+}$ from the sarcoplasmic reticulum** into the sarcoplasm.
5. Ca$^{2+}$ binds to troponin and removes the masking of active sites on actin.
6. The myosin head binds to actin, forming a cross bridge.
7. Using energy from ATP hydrolysis, the myosin head pulls actin towards the centre of the A band.
8. The Z lines move inward, the sarcomere shortens, and the muscle contracts.
9. A new ATP binds to myosin, the cross bridge breaks, and the cycle repeats.
10. When Ca$^{2+}$ is pumped back into the sarcoplasmic reticulum, the actin sites are masked again and the muscle relaxes.

The true/false answers are:

(a) True

(b) False — The H-zone of a striated muscle fibre represents only thick filaments.

(c) True

(d) False — There are 12 pairs of ribs in man.

(e) True

Actin is present in the thin filament.

The corrected statement is: H-zone of striated muscle fibre represents only thick filaments.

The human skeleton has 206 bones.

The corrected statement is: There are 12 pairs of ribs in man.

The sternum is present on the ventral side of the body.

The differences are given below:

### (a) Actin and Myosin
- Actin forms the thin filament.
- Myosin forms the thick filament.
- Actin has binding sites for myosin; myosin heads have ATPase activity and bind to actin.

### (b) Red and White muscles
- Red fibres contain more myoglobin and many mitochondria.
- White fibres contain very little myoglobin and fewer mitochondria.
- Red fibres are aerobic; white fibres depend more on anaerobic energy production.

### (c) Pectoral and Pelvic girdle
- Pectoral girdle helps articulate the upper limbs with the axial skeleton and consists of clavicle and scapula.
- Pelvic girdle helps articulate the lower limbs with the axial skeleton and consists of two coxal bones.
- The pectoral girdle forms the shoulder region; the pelvic girdle forms the hip region.

### Actin and Myosin
- Actin is the thin filament.
- Myosin is the thick filament.
- Actin has active sites for myosin; myosin heads have ATPase and bind to actin.

### Red and White muscles
- Red muscles have more myoglobin and many mitochondria; they are aerobic.
- White muscles have little myoglobin and fewer mitochondria; they depend mainly on anaerobic processes.

### Pectoral and Pelvic girdle
- Pectoral girdle: clavicle and scapula; supports upper limbs.
- Pelvic girdle: two coxal bones; supports lower limbs.

### Red muscles
- Rich in myoglobin
- More mitochondria
- Reddish in colour
- Use aerobic respiration

### White muscles
- Little myoglobin
- Fewer mitochondria
- Pale or whitish in colour
- Depend mainly on anaerobic respiration

### Pectoral girdle
- Helps in articulation of the upper limbs with the axial skeleton
- Formed by clavicle and scapula

### Pelvic girdle
- Helps in articulation of the lower limbs with the axial skeleton
- Formed by two coxal bones

The matching is:

- (a) Smooth muscle → (iv) Involuntary
- (b) Tropomyosin → (ii) Thin filament
- (c) Red muscle → (i) Myoglobin
- (d) Skull → (iii) Sutures

The cells of the human body exhibit three main types of movements: amoeboid movement, ciliary movement and muscular movement.