Updated NCERT Solutions for Class 11 Biology Chapter 17: Locomotion and Movement

A sarcomere is the basic functional unit of a striated muscle fibre. It is the region between two consecutive Z-lines.

Diagram Labelling and Explanation:

• Z-Line: An elastic fibre that bisects the I-band and anchors the thin filaments. A sarcomere extends from one Z-line to the next.
• I-Band (Isotropic Band): The lighter region containing only thin filaments (Actin). It is bisected by the Z-line.
• A-Band (Anisotropic Band): The darker, central region of the sarcomere which contains the entire length of the thick filaments (Myosin) and the overlapping parts of the thin filaments.
• H-Zone: A lighter region in the middle of the A-band where only thick filaments are present.
• M-Line: A thin fibrous line in the centre of the H-zone that holds the thick filaments together.

The sliding filament theory is the most widely accepted explanation for the mechanism of muscle contraction. It was proposed by Andrew Huxley and Rolf Niedergerke.

Key Points of the Theory:

1. Signal Initiation: Muscle contraction starts with a signal from the central nervous system (CNS) via a motor neuron.
2. Calcium Release: This signal causes the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum into the sarcoplasm.
3. Actin Site Exposure: Increased Ca²⁺ levels bind to troponin on actin filaments. This binding changes troponin's shape, which in turn moves tropomyosin, exposing the active sites on actin.
4. Cross-Bridge Formation: Using energy from ATP hydrolysis, myosin heads bind to these exposed active sites on actin, forming a cross-bridge.
5. Power Stroke: The myosin head then pulls the attached actin filament towards the centre of the sarcomere (A-band). This is called the power stroke.
6. Sarcomere Shortening: As the actin filaments slide, the Z-lines are pulled inwards, the I-bands shorten, and the H-zone disappears. The length of the A-band remains constant.
7. Cycle Repetition: This process of forming and breaking cross-bridges repeats, causing the thin filaments to slide over the thick filaments, resulting in muscle contraction.

Girdles are skeletal structures that connect the limbs to the axial skeleton.

(a) Pectoral Girdle (Shoulder Girdle):

Each pectoral girdle consists of two bones:

1. Clavicle (Collar Bone): A long, slender S-shaped bone. It articulates with the sternum and the scapula, helping to keep the shoulder joint away from the body for maximum mobility.
2. Scapula (Shoulder Blade): A large, triangular flat bone on the back of the thorax. It features:
   • Acromion: A process that articulates with the clavicle.
   • Glenoid Cavity: A shallow depression that articulates with the head of the humerus (upper arm bone) to form the ball-and-socket shoulder joint.

(b) Pelvic Girdle (Hip Girdle):

The pelvic girdle consists of two coxal bones (hip bones), each formed by the fusion of three bones:

1. Ilium: The large, wing-like upper part.
2. Ischium: The lower and posterior part you sit on.
3. Pubis: The anterior and lower part.

At the fusion point of these three bones is the acetabulum, a deep socket that articulates with the head of the femur (thigh bone). The two halves of the pelvic girdle meet at the pubic symphysis.

(a) Gout: Gout is a type of inflammatory arthritis caused by the accumulation of uric acid crystals in the joints. When the body produces too much uric acid or fails to excrete it, these crystals deposit in joints (commonly the big toe), causing intense pain, swelling, and inflammation.

(b) Osteoporosis: This is an age-related disorder where bone mass decreases, leading to weak, porous bones that are highly susceptible to fractures. It is common in post-menopausal women due to decreased estrogen levels, but can also be caused by calcium and vitamin D deficiency.

(c) Tetany: Tetany is a condition causing rapid muscle spasms and cramps due to low levels of calcium ions (Ca²⁺) in the body fluid (hypocalcemia). This hypocalcemia increases the excitability of nerves and muscles, leading to involuntary contractions. It is often linked to underactivity of the parathyroid glands.

Joints are structurally classified into three main types:

1. Fibrous Joints (Immovable): Bones are fused end-to-end with the help of dense fibrous connective tissue. They permit no movement.
   Example: Sutures of the skull.
2. Cartilaginous Joints (Slightly Movable): Bones are joined together by cartilage, allowing for limited movement.
   Example: Joints between adjacent vertebrae.
3. Synovial Joints (Freely Movable): Characterized by a fluid-filled synovial cavity between the articulating bones. These joints allow considerable movement.
   Examples: Ball and Socket (shoulder), Hinge (knee), Pivot (neck), Gliding (carpals), Saddle (thumb).

(a) Difference between Actin and Myosin:

(b) Difference between Red and White Muscles:

(c) Difference between Pectoral and Pelvic Girdle:

A motor unit consists of a single motor neuron and all the muscle fibres it innervates (supplies with nerves). When the motor neuron fires an impulse, all the muscle fibres in that unit contract together as a single functional unit.

Muscle fatigue after strenuous exercise is primarily due to the accumulation of lactic acid. During intense activity, muscles switch to anaerobic respiration, which produces lactic acid as a byproduct. This accumulation lowers the muscle's pH, which can interfere with enzyme activity and the ability of calcium ions to bind to troponin, leading to pain and a reduced capacity for contraction.

The three types of muscles are:

1. Skeletal Muscles: Voluntary, striated muscles attached to bones.
2. Visceral (Smooth) Muscles: Involuntary, non-striated muscles in internal organs.
3. Cardiac Muscles: Involuntary, striated muscles found only in the heart.

Calcium ions (Ca²⁺) are the primary trigger for muscle contraction. Their role involves these steps:

1. After a nerve signal, Ca²⁺ is released from the sarcoplasmic reticulum.
2. It binds to a specific subunit of the troponin complex on the actin filament.
3. This binding causes a conformational change in troponin, which in turn pulls the tropomyosin protein away from the myosin-binding sites on actin.
4. With the binding sites exposed, myosin heads can attach to actin and initiate the power stroke, leading to contraction.

The mechanism of muscle contraction is explained by the Sliding Filament Theory.

1. Initiation: A nerve impulse arrives at the neuromuscular junction, releasing acetylcholine. This generates an action potential in the sarcolemma.
2. Calcium Release: The action potential travels down the T-tubules and triggers the sarcoplasmic reticulum to release Ca²⁺ ions into the sarcoplasm.
3. Exposure of Active Sites: The Ca²⁺ ions bind to troponin on the actin filaments. This causes tropomyosin to shift, exposing the myosin-binding sites.
4. Cross-Bridge Formation: Energized myosin heads (with ADP + Pi) bind to the exposed sites on actin, forming a cross-bridge.
5. Power Stroke: The myosin head releases ADP and Pi and pivots, pulling the actin filament towards the M-line.
6. Detachment: A new ATP molecule binds to the myosin head, causing it to detach from actin.
7. Re-energizing: The ATP is hydrolyzed to ADP + Pi, re-energizing the myosin head for another cycle.

This cycle of binding, pulling, and detaching continues as long as Ca²⁺ and ATP are present, causing the sarcomere to shorten and the muscle to contract.

The axial skeleton forms the central axis of the body and consists of 80 bones. It is divided into:

1. Skull (22 bones + ear ossicles + hyoid):
   • Cranial Bones (8): Form the protective braincase (e.g., frontal, parietal).
   • Facial Bones (14): Form the face (e.g., maxilla, mandible).
   • Ear Ossicles (6): Malleus, Incus, and Stapes in each ear.
   • Hyoid Bone (1): U-shaped bone at the base of the tongue.
2. Vertebral Column (26 vertebrae): Protects the spinal cord and supports the head.
   • Cervical (7): Neck region.
   • Thoracic (12): Chest region, articulate with ribs.
   • Lumbar (5): Lower back.
   • Sacrum (1, fused): Part of the pelvis.
   • Coccyx (1, fused): Tailbone.
3. Sternum (1 bone): The breastbone on the ventral midline.
4. Ribs (12 pairs): Form the rib cage.
   • True Ribs (1-7): Attach directly to the sternum.
   • False Ribs (8-10): Attach to the 7th rib's cartilage.
   • Floating Ribs (11-12): Do not connect ventrally.

(a) Identify the disorder Rohan is suffering from.
The disorder is Muscular Dystrophy, a genetic condition causing progressive weakness and degeneration of skeletal muscles.

(b) Which type of muscle is primarily affected?
Skeletal muscles are primarily affected.

(c) Why is this disorder more common in boys?
It is more common in boys because it is an X-linked recessive trait. Males (XY) have only one X chromosome, so a single faulty gene on it will cause the disease. Females (XX) are usually carriers as they have a second, healthy X chromosome.