Locomotion and Movement: Complete Guide for Class 11 NCERT Biology

Overview of the Human Skeletal System

The human skeletal system forms the rigid framework that supports the body and facilitates movement. It consists of 206 bones divided into two main parts:

• Axial skeleton: Includes 80 bones along the body's central axis such as the skull, vertebral column, ribs, and sternum.
• Appendicular skeleton: Comprises bones of the limbs and girdles (pectoral and pelvic) that attach limbs to the axial skeleton.

Key components:

• Skull: Made of 22 bones; 8 cranial bones protect the brain, and 14 facial bones form the face.
• Vertebral column: 26 vertebrae divided into cervical (7), thoracic (12), lumbar (5), sacral (1 fused), and coccygeal (1 fused) regions. It protects the spinal cord and supports the head.
• Rib cage: 12 pairs of ribs attached to the sternum protect vital organs like the heart and lungs.

Bones are hard due to calcium salts, while cartilage is flexible due to chondroitin salts. This combination provides strength and flexibility necessary for movement.

Structure and Functions of Bones and Cartilage

Bones are specialized connective tissues with a hard matrix mainly composed of calcium phosphate, which gives them strength. Cartilage, on the other hand, has a pliable matrix containing chondroitin salts, making it flexible.

Functions of bones:

• Provide support and shape to the body
• Protect vital organs (e.g., skull protects brain, rib cage protects heart and lungs)
• Act as levers for muscles to produce movement
• Store minerals like calcium and phosphorus
• Produce blood cells in bone marrow

Functions of cartilage:

• Provide smooth surfaces for joint movement
• Support soft tissues (e.g., trachea, nose)
• Reduce friction and absorb shock in joints

Comparison of Bone and Cartilage:

Muscle Structure and the Sliding Filament Theory

Movement in humans is possible due to muscles attached to bones. Skeletal muscles are made up of long fibers containing myofibrils, which have repeating units called sarcomeres — the basic contractile units.

Sarcomere structure:

• Z-line: Boundary of each sarcomere
• I-band: Light band with thin (actin) filaments
• A-band: Dark band with thick (myosin) filaments
• H-zone: Central region with only thick filaments
• M-line: Middle line of sarcomere

Sliding filament theory: Muscle contraction occurs when thin actin filaments slide past thick myosin filaments, shortening the sarcomere without changing filament lengths. This sliding is powered by ATP energy and involves the formation of cross-bridges between myosin heads and actin.

Steps of muscle contraction: 1. Nerve impulse triggers acetylcholine release at neuromuscular junction. 2. Acetylcholine causes depolarization and calcium release from sarcoplasmic reticulum. 3. Calcium binds troponin, exposing binding sites on actin. 4. Myosin heads attach to actin forming cross-bridges. 5. Using ATP, myosin heads pivot, pulling actin filaments inward. 6. Sarcomere shortens, causing muscle contraction.

Types of Joints and Their Role in Movement

Joints are connections between bones that allow movement and provide mechanical support. Based on their structure and movement, joints are classified as:

• Fibrous joints: Immovable joints where bones are joined by fibrous tissue (e.g., sutures of the skull).
• Cartilaginous joints: Slightly movable joints connected by cartilage (e.g., between vertebrae).
• Synovial joints: Freely movable joints with a synovial cavity filled with fluid (e.g., knee, elbow).

Types of synovial joints and movements:

Joints are supported by ligaments that connect bones and tendons that connect muscles to bones, enabling coordinated movement.

Pectoral and Pelvic Girdles: Connecting Limbs to the Body

The appendicular skeleton includes the limbs and the girdles that attach them to the axial skeleton.

Pectoral girdle:

• Composed of clavicle (collarbone) and scapula (shoulder blade).
• Connects the upper limbs to the axial skeleton.
• The scapula is a flat triangular bone with a spine and acromion process that articulates with the clavicle.
• Provides great mobility to the upper limbs.

Pelvic girdle:

• Formed by two coxal bones, each made by fusion of ilium, ischium, and pubis.
• Connects the lower limbs to the axial skeleton via the acetabulum (hip socket).
• The two halves meet ventrally at the pubic symphysis containing fibrous cartilage.
• Provides strong support and stability for weight-bearing and locomotion.

These girdles serve as attachment points for muscles and enable a wide range of limb movements essential for locomotion.

Worked Example: Calculating the Number of Bones in the Axial Skeleton

Problem: How many bones are present in the axial skeleton of the human body?

Solution: The axial skeleton includes:

• Skull: 22 bones
• Vertebral column: 26 bones
• Sternum: 1 bone
• Ribs: 24 bones (12 pairs)

Total bones in axial skeleton:

$$22 + 26 + 1 + 24 = 73$$

Therefore, the axial skeleton has 73 bones.

This calculation helps in understanding the distribution of bones and their role in supporting and protecting vital organs along the body’s central axis.