Chapter 17: Locomotion and Movement

Ans: The diagrammatic representation of a sarcomere is as follows:

Ans: The sliding filament theory describes the process of muscle contraction in which the thick filaments (myosin) of muscle fibres slide past the thin filaments (actin), resulting in the shortening of the myofibril. Each muscle fibre consists of alternate light and dark bands. They contain a unique contractile protein called actin and myosin respectively. Myosin is a thick contractile protein found in the dark band and is known as the A-band or Anisotropic band. Actin is a thin contractile protein found in the light band and is known as the I-band or Isotropic band.  Each I-band is bisected by an elastic fibre known as the Z line. The thin filament (actin) is firmly attached to the Z line. The H-zone is the central portion of the thick filament (myosin) that is not overlapped by the thin filament. Sarcomere, the functional unit of contraction, is the portion of the myofibril between two successive Z lines.

During muscle contraction, the myosin heads come into close contact with the thin filaments. As a result, the thin filaments are pulled towards the centre of the sarcomere as well as the Z line attached to the actin filaments is also pulled, causing shortening of the sarcomere. The length of the A-band or anisotropic band remains constant as its original length whereas the I-band or isotropic band shortens and the H-zone disappears.

Ans: During skeletal muscle contraction, the thick filament slides past the thin filament (actin) via repeated binding, releasing myosin along the filament. This entire process takes place in sequential order. 

1. Step 1: Muscle contraction is initiated by a signal that is sent by a CNS (Central nervous system) through a motor neuron. A neuromuscular junction or motor-end plate is a junction between a motor neuron and the sarcolemma of the muscle fibre. When a signal reaches the neuromuscular junctions, Acetylcholine (a neurotransmitter) is released which results in the generation of an action potential in the sarcolemma. 

2. Step 2: This spreads through the muscle fibres, resulting in the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm.

3. Step 3: The increased calcium ions $\text{(C}{{\text{a}}^{\text{2+}}}\text{)}$ in the sarcoplasm leads to the activation of actin sites. Calcium ions $\text{(C}{{\text{a}}^{\text{2+}}}\text{)}$ bind with the subunit of troponin on actin filaments and thus remove the masking of active sites for myosin. Hence, active sites on actin are exposed and this allows myosin heads to attach to this site. 

4. Step 4: The myosin head now attaches to the exposed site of actin to form a cross-bridge by utilizing energy from ATP hydrolysis. The actin filaments are pulled towards the centre of the A-band. The Z line attached to the actin filaments is also pulled, causing shortening of the sarcomere, i.e., the contraction of the muscle occurs. It is clear from the above steps, that during contraction, the length of the A-band or anisotropic band remains constant as its original length whereas the I-band or isotropic band gets reduced.

5. Step 5: After muscle contraction, the myosin head goes back to its relaxed state, releasing ADP and inorganic phosphate (Pi). A new ATP molecule binds and detaches myosin, thus the cross-bridges are broken.

6. Step 6: This process of formation and breaking down is repeated causing further sliding. This process continues until the calcium ions are pumped back to the sarcoplasmic cisternae. Hence, the calcium ions concentration decreases. This results in masking the actin filaments and leading to muscle relaxation.

Ans: 

1. True

2. False. The H-zone of striated muscle fibre represents only a thick filament.

3. True

4. False. There are 12 pairs of ribs in a man.

5. True

Ans:

1. The difference between Actin and Myosin are: -

1. The difference between red and white muscle fibres are: -

1. The difference between pectoral and pelvic girdles are: -

Ans: 

Ans: Movement is a characteristic property of all living organisms. Cells in the human body exhibit various types of movement, which are as follows:

1. Amoeboid Movement: This type of movement is exhibited by some specialised cells like leucocytes (White blood cells) and macrophages (immune cells) present in the blood. These blood cells move from the circulatory system to the injury site during tissue damage to initiate an immune response.

2. Ciliary Movement: This type of movement is exhibited by reproductive cells such as sperms and ova. Ciliary movement facilitates the passage of the ova through the fallopian tube towards the uterus.

3. Muscular Movement: This type of movement is exhibited by muscle cells which are caused by the contraction of the myofibril of the muscle cells.

Ans: The difference between a skeletal muscle and a cardiac muscle are:

Ans.

1. Atlas/axis: Pivot joint is present in between atlas and axis.

2. carpal/metacarpal of thumb: The saddle joint is located between the carpal and metacarpal of the thumb.

3. between phalanges: Hinge joint is present in between phalanges.

4. femur/acetabulum: Ball and socket joint is present in between the head of femur and acetabulum.

5. between cranial bones: Fibrous joint is present in between cranial bones.

6. between pubic bones in the pelvic girdle: Ball and socket joint is present in between pubic bones in the pelvic girdle.

Ans.

1. All mammals (except a few) have seven cervical vertebrae. 

2. The number of phalanges in each limb of a human is 14. 

3. The thin filament of myofibril contains 2 ‘F’ actins and two other proteins namely troponin and myosin.

4. In a muscle fibre $\text{(C}{{\text{a}}^{\text{2+}}}\text{)}$ is stored in the sarcoplasmic reticulum.

5. $\text{1}{{\text{1}}^{\text{th}}}$ and $\text{1}{{\text{2}}^{\text{th}}}$ pairs of ribs are called floating ribs. 

6. The human cranium is made of eight bones.