Locomotion and Movement

There are twelve pairs of ribs which form the bony lateral walls of the thoracic cage. The first seven pairs are called true ribs; eighth, ninth and tenth pairs are called false ribs. The last two pairs of ribs are called floating ribs because their anterior ends are not attached either to the sternum or to the cartilage of another rib. The floating ribs protect the kidneys.

Collar bones (Clavicle) – 2 pairs
Salivary glands – 3 pairs
Cranial nerves – 12 pairs

Myosin is a contractile protein that interacts with actin to bring about contraction of muscle or cell movement. The type of myosin molecule found in muscle fibres consists of a tail, by which it aggregates with other myosin molecules to form socalled thick filaments and a globular head, which has sites for the attachment of actin and ATP molecule. Troponin, tropomyosin and $\alpha $-actinin are the actin in the thin filament.

Gliding joint permits sliding movements of two bones over each other. Hinge joint allows movements in one plane only. Knee joint, elbow joint, ankle joint are of this type. Cartilaginous joint is a slightly movable joint and is found between the centre of vertebrae, at the pubic symphysis and between ribs and sternum. Fibrous joint is an immovable joint which occur between the bones of cranium.

Each half of pectoral girdle has two bones i.e. clavicle and scapula. A spine like, acromian process is attached to scapula for articulation with clavicle bone.

When ligaments are torn, the stability of the joint they serve is compromised, leading to increased mobility beyond the normal range, which can cause pain and instability. Therefore, the correct answer is:

Option B: Bone less movable at joint and pain.

This is because ligaments are tough, elastic bands of connective tissue that surround a joint to give support and limit the joint's movement. When ligaments are damaged, it can lead to pain, swelling, and a reduced ability to move the joint, contrary to the joint becoming more movable without pain or the bones moving freely without any discomfort. Ligaments provide stability to the joints, so their injury typically results in instability and pain due to the loss of this support and limitation of movement.

Cartilage is an important component of skeleton. It consists of a firm matrix containing collagen and elastin fibres and cells in fluid-filled lacunae. Cartilage has many types. Elastic cartilage occurs in the pinna and external auditory canal of the ear, epiglottis, Eustachian tubes and tip of the nose to make these organs flexible. Fibrous cartilage is very strong yet has a degree of flexibility. It is found in the intervertebral discs where it acts as a cushion and in pubic symphysis where it allows parturition without damage to the girdle. Hyaline cartilage occurs in sternal ribs where it allows expansion of chest during inspiration. It also forms the tracheal and bronchial rings and supports larynx and nasal septum and also at the end of long bones.

Darks bands on the myofibril form A-band. Middle part of A-band contains light zones-Hensen line (H-lines). Light band on myofibril is called I-band. Middle part of I-band contains Z-lines. Sarcomere is part between two Z-lines.

The correct statement for muscle contraction is :

Option A: Length of H-zone decreases During muscle contraction, the sliding filament theory explains that the thin filaments (actin) slide over the thick filaments (myosin) toward the center of each sarcomere. This results in a decrease in the length of the H-zone, which is the region in the center of the sarcomere that is primarily composed of myosin and devoid of actin. To present the information more clearly: - Option A: Length of H-zone decreases - Correct.

- Option B: Length of A-band remains constant - This is also correct; however, it is not the choice that is being asked for confirmation. The A-band length, which corresponds to the length of the myosin filaments, indeed remains constant during contraction.

- Option C: Length of I-band increases - This is incorrect. The I-band, which contains thin filaments with no overlapping thick filaments, actually decreases during muscle contraction.

- Option D: Length of two Z-line increases - This is incorrect. The distance between two Z-lines, or the length of the sarcomere, decreases during muscle contraction as the Z-lines move closer together. The H-zone's change in length is a direct indicator of muscle contraction because it specifically refers to the action of the sliding filaments during the process.

Cartilaginous joint is present between ribs and sternum. It allows only limited movement. An angular joint allows movement in two directions - side to side and back and forth. Wrist and metacarpophalangeal joints are of this type. Gliding joint permits sliding movements of two bones over each other, e.g. joints between sternum and clavicles. Fibrous joints do not allow movement and are present between the bones of cranium.

Sternum is connected to ribs by hyaline cartilage (= giving a shiny glass like appearance and gives flexibility and support at the joints). Sternum is also called breast bone. It is a narrow, elongated and flattened structure, present just under the skin in the middle of front of the chest. It consists of three parts - manubrium, mesosternum and xiphoid process. Manubrium is the thickest, strongest part and articulates with the clavicle of pectoral girdle and first pair of ribs. Mesosternum provide articulation to second to sixth pairs of ribs and xiphoid process (also called metasternum) articulates with seventh pair of ribs in association with mesosternum.

Atlas is the first cervical vertebra. Coracoid is part of the pectoral girdle. Pterygoid is a small bone articulated with palatine. Arytenoids are cartilage present at the back of larynx.