NCERT Solutions For Class 6 Science Chapter 8
Body Movements

According to the sliding filament theory of muscle contraction, contraction of muscle fibres to produce contractile force takes place by the sliding of the thin filaments (actin fibres) over the thick filaments (myosin) found in the sarcomere of the muscle cell.

[Explanation: Sliding filament theory best describes the molecular basis of muscle contraction. It explains how myofibrils (actin and myosin) interact to produce contractile force. In sarcomere, actin forms thin filaments and myosin forms thick filaments. Sarcomere consists of two bands- A-band (Dark band) and I-band (Light band). A-band is the region of myosin thick filaments while I-band is the region of actin filaments which are not superimposed with myosin filaments. Actin filaments of I-band are connected to an elastic fibre called Z-line. There is a region in the central part of sarcomere that does not overlap with actin fibres is called H-zone. During the process of muscle contraction, thick filaments (myosin fibres, A-band) remains constant while thin filaments (actin fibres, I-band) change their length and slide over the myosin filaments. Actin filaments interact with the myosin head that results in pulling the actin filaments towards the centre of the sarcomere. In this process, length of sarcomere decreases and Z-lines of sarcomere come closer.]

The diagram of a sarcomere of skeletal muscle showing different regions:

(a) Ball and socket joint is a type of joint that allows movement in all directions. It is a joint in which a rounded surface of bone moves within a cup-like cavity of another bone. Ball and socket joint is present in shoulder joint and hip joint.
(b) Lower jaw of the skull bone is movable.
(c) Our elbow cannot move backward because it has hinge joint which only allows movement in one plane.

Four different types of movements are exhibited by the cells of human body:

1. Amoeboid movement: This kind of movement is shown by specialised cells like macrophages and leukocytes in the blood which migrate from bloodstream to the site of injury to perform their function. These cells move by the formation of pseudopodia (streaming of cytoplasm) in the direction of movement. Microfilaments of the cytoskeleton are involved in this kind of movement.

2.  Ciliary movement: Ciliary movement is shown by cells of the ciliated epithelium lining internal tubular organs. Cells present in the lining of trachea perform ciliary movement to sweep the dirt and mucus out of the lung. In females, the ciliary movement shown by the cells lining the fallopian tubes helps in moving ova from ovary to uterus. Cilia are also found in the cochlear cells of the ear.

3. Muscular movement: Muscle cells exhibit contraction and relaxation movement. This kind of movement is involved in moving our limbs, jaws, tongue, etc.

4. Flagellar movement: Sperms swim by means of flagellum which shows flagellar movement. 

(a) True

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

(c) True

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

(e) True

Muscle contraction is explained by sliding filament theory. According to sliding filament theory, contraction of muscle fibers to produce contractile force takes place by the sliding of the thin filament (actin fibers) over the thick filaments (myosin) found in the sarcomere of the muscle cell. Important steps in muscle contraction are as follows:

1. Central nervous system sends signal for initiation of muscle contraction via a motor neuron.

2. Neural signal reaches motor end plate (the junction between a motor neuron and sarcolemma of the muscle fiber) and releases a neurotransmitter (acetylcholine) which generates an action potential in the sarcolemma that spreads through the muscle fiber and causes the release of calcium ions into the sarcoplasm.

3. Calcium ions bind to troponin that removes tropomyosin from the active sites of actin. These exposed active actin sites are now available to interact with myosin.

5. Heads of myosins form cross bridges by interacting with active sites on actin filaments and pull them towards the center of A-band by utilising the energy from ATP hydrolysis. Z line which is attached to actin filaments are also pulled inwards that shortens the sarcomere resulting in contraction. In this process, I-bands get shortened while the lengths of A-bands remain the same.

6. Myosin goes into relaxed state by releasing ADP and Pi. Cross bridge is broken. A new ATP binds to myosin and upon ATP hydrolysis next cycle of cross-bridge formation starts. This process continues till the calcium ions are sent back into sarcoplasmic cisternae that results into masking of active sites on actin filaments.

(a) The movement and locomotion of all the animals is exactly the same. (F)     
(b) The cartilages are harder than bones. (F)
(c) The finger bones do not have joints. (F)
(d) The fore arm has two bones. (T)
(e) Cockroaches have an outer skeleton. (T)

(a) Joints of the bones help in the movement of the body.
(b) A combination of bones and cartilages forms the skeleton of the body.
(c) The bones at the elbow are joined by a hinge joint.
(d) The contraction of the muscles pulls the bones during movement.

(a) All mammals (except a few) have seven cervical vertebra.

(b) The number of phalanges in each limb of human is 14.

(c) Thin filament of myofibril contains 2 ‘F’ actins and two other proteins namely troponin and tropomyosin.

(d) In a muscle fibre Ca⁺⁺ is stored in sarcoplasmic reticulum.

(e) 11^th and 12^th pairs of ribs are called floating ribs.

(f) The human cranium is made of eight bones.

1. Actin and Myosin

2. Red and White muscles
   

3. Pectoral and Pelvic girdle