Notes Class 11 Chapter 16 Biology Maharashtra Board

Skeleton and Movement

Introduction

Organisms exhibit various movements, from protoplasmic streaming to locomotion like walking or running. Movements can be internal (e.g., peristalsis) or external (e.g., limb movement) and may be voluntary or involuntary. Locomotion involves the displacement of an organism’s entire body, driven by the need for food, shelter, mates, or escaping predators. This chapter explores the skeletal system, muscles, and their roles in movement and locomotion in humans.

16.1 Movements and Locomotion

Movements

Movements are classified as:

• Internal: Occur within the body, e.g., peristalsis in the alimentary canal, heartbeat.
• External: Involve body parts, e.g., limb movements, head rotation.
• Voluntary: Controlled consciously, e.g., walking, writing.
• Involuntary: Not under conscious control, e.g., heartbeat, peristalsis.

Types of Muscles and Their Roles:

1. Smooth Muscles: Involuntary, control internal movements like peristalsis, blood vessel constriction/dilation.
2. Cardiac Muscles: Involuntary, responsible for heart contraction and relaxation.
3. Striated (Skeletal) Muscles: Voluntary, control movements of limbs, head, trunk, etc.

Examples of Movements:

• Whorling Movement: Performed by flagella, e.g., sperm movement.
• Peristaltic Movement: Wave-like contractions in the alimentary canal.
• Eye Movement: Controlled by striated muscles for voluntary gaze shifts.

Locomotion

Locomotion is the movement of an organism from one place to another, driven by survival needs (e.g., finding food, escaping enemies). It involves coordinated action of bones, joints, and muscles.

Types of Locomotory Movements:

1. Amoeboid Movement: Via pseudopodia, e.g., leucocytes.
2. Ciliary Movement: Via cilia, e.g., ciliated epithelium in Paramecium aiding food passage.
3. Whorling Movement: Via flagella, e.g., sperm.
4. Muscular Movement: Via muscles, bones, and joints, e.g., walking, running.

Key Fact: All locomotions are movements, but not all movements result in locomotion.

Human Muscle Facts:

• Approximately 640 muscles in the human body (634 paired, 6 unpaired).
• Skeletal muscles are attached to bones via tendons (inelastic collagen fiber bands).

16.2 Location and Structure of Skeletal Muscles

Skeletal muscles are striated, voluntary muscles attached to bones, enabling movement. They are typically located on a bone proximal to the one they move (e.g., biceps/triceps in the upper arm move the forearm).

Structure

• Origin: The end of the muscle attached to a stationary bone.
• Insertion: The end attached to a movable bone.
• Belly: The thick, central part of the muscle, fusiform in shape due to the maximum number of fibers in the middle.

Types of Skeletal Muscles (Based on Movement)

1. Agonists (Prime Movers): Initiate movement, e.g., biceps for forearm flexion.
2. Antagonists: Produce opposite action, e.g., triceps for forearm extension.
3. Synergists: Assist prime movers, e.g., brachialis assists biceps.

Muscle Arrangement

• Muscles work in antagonistic pairs, where one muscle’s contraction opposes the other’s action (e.g., biceps flex the elbow, triceps extend it).
• One muscle in a pair is typically stronger (e.g., biceps stronger than triceps).
• Muscles can only pull (contract), not push, due to their contractile nature.

16.3 Working of Skeletal Muscles

Skeletal muscles produce movement by contracting and pulling bones at joints. They work in antagonistic pairs to enable opposing actions (e.g., flexion and extension).

Antagonistic Muscle Pairs

1. Flexor and Extensor:
   • Flexor: Bends a joint, e.g., biceps.
   • Extensor: Straightens a joint, e.g., triceps.
2. Abductor and Adductor:
   • Abductor: Moves a part away from the body axis, e.g., deltoid (shoulder).
   • Adductor: Moves a part toward the body axis, e.g., latissimus dorsi.
3. Pronator and Supinator:
   • Pronator: Turns the palm downward.
   • Supinator: Turns the palm upward.
4. Levator and Depressor:
   • Levator: Raises a body part.
   • Depressor: Lowers a body part.
5. Protractor and Retractor:
   • Protractor: Moves a part forward.
   • Retractor: Moves a part backward.
6. Sphincters: Circular muscles controlling openings, e.g., in the anus or stomach.

16.4 Mechanism of Muscle Contraction

Sarcomere: The Contractile Unit

The sarcomere is the functional unit of striated muscle, containing:

• Actin Filaments (thin): Anchored at Z-lines, with tropomyosin and troponin.
• Myosin Filaments (thick): Centrally located, with heads forming cross-bridges.

Structure of Contractile Proteins

1. Myosin Filament:
   • Composed of multiple meromyosin units.
   • Each unit has two heavy chains (forming a tail and head) and four light chains.
   • Myosin head: Has ATPase activity, splits ATP for energy, and forms cross-bridges with actin.
   • Tails point toward the sarcomere center, heads outward.
2. Actin Filament:
   • F-actin: Double-stranded polymer of G-actin molecules (each with ADP).
   • Tropomyosin: Covers myosin-binding sites in the resting state.
   • Troponin: Binds calcium, tropomyosin, and actin, initiating contraction when calcium is present.

Sliding Filament Theory

Proposed by H.E. Huxley and A.F. Huxley, this theory explains muscle contraction:

1. Action Potential: A nerve impulse triggers calcium release from the sarcoplasmic reticulum.
2. Calcium Binding: Calcium binds to troponin, shifting tropomyosin to expose actin’s binding sites.
3. Cross-Bridge Formation: Myosin heads, energized by ATP hydrolysis, bind to actin, forming an actomyosin complex.
4. Power Stroke: Myosin heads tilt, pulling actin filaments inward, shortening the sarcomere.
5. Detachment: ATP binds to myosin, releasing it from actin, and is hydrolyzed for the next cycle.

This sliding of actin over myosin shortens the muscle fiber, causing contraction.

16.5 Physiology of Muscle Relaxation

• In the relaxed state, tropomyosin and troponin cover actin’s binding sites, preventing myosin interaction.
• Relaxation Process:
  1. Stimulation ceases, stopping calcium release.
  2. Calcium is actively pumped back into the sarcoplasmic reticulum (using ATP).
  3. Troponin-tropomyosin complex reforms, covering actin’s binding sites.
  4. Myosin detaches from actin (using ATP), and actin filaments slide back, relaxing the muscle.
• Relaxation is an active process requiring ATP, similar to contraction.

16.6 Properties of Muscles on Electrical Stimulation

1. Single Muscle Twitch: A brief contraction from one stimulus, with three stages:
   • Latent Period: No contraction.
   • Contraction Period: Muscle shortens.
   • Relaxation Period: Muscle returns to original length.
2. Summation: Repeated stimuli before twitch completion increase contraction strength (staircase phenomenon).
3. Tetanus: Rapid, frequent stimuli prevent relaxation, maintaining contraction.
4. Refractory Period: A brief period (~0.02 seconds) after a stimulus where the muscle cannot respond to another.
5. Threshold Stimulus: Minimum stimulus strength required for contraction.
6. All or None Principle: A muscle fiber contracts fully or not at all, depending on whether the stimulus reaches the threshold.
7. Oxygen Debt: During strenuous exercise, muscles contract anaerobically, accumulating lactic acid. Post-exercise, extra oxygen (oxygen debt) oxidizes lactic acid and restores ATP/creatine phosphate.

Rigor Mortis: Post-death muscle stiffening due to ATP depletion, preventing myosin detachment from actin. It helps estimate time of death.

16.7 Skeletal System

The skeletal system provides structural support, protects organs, enables movement, stores calcium, and supports hematopoiesis. It consists of bones and cartilage, forming the endoskeleton (internal) and exoskeleton (external, e.g., nails, hair).

Human Skeleton

• Total Bones: 206 in adults, divided into:
  • Axial Skeleton: 80 bones (along the body’s longitudinal axis).
  • Appendicular Skeleton: 126 bones (limbs and girdles).
• Cartilage: Pliable, found in joints and certain skeletal parts (e.g., nose, ear).
• Bones: Rigid, with a hard matrix, providing shape and support.

Comparison with Simple Machines

Bones, muscles, and joints function like a lever:

• Joint: Acts as the fulcrum.
• Muscle: Provides the effort (force).
• Bone: Acts as the resistance (load).
• Examples:
  • Class I Lever: Head movement at the atlanto-occipital joint (fulcrum: joint, effort: back muscles, resistance: head).
  • Class II Lever: Standing on toes (fulcrum: toe, effort: calf muscles, resistance: body).
  • Class III Lever: Forearm flexion at the elbow (fulcrum: elbow, effort: biceps, resistance: radius/ulna).

16.8 Axial Skeleton (80 Bones)

Skull (22 Bones)

• Cranium (8 Bones):
  • Frontal Bone: Forehead, roof of orbits (unpaired).
  • Parietal Bones: Roof and sides of cranium (paired).
  • Temporal Bones: Lateral, above ears, with zygomatic process and mandibular fossa (paired).
  • Occipital Bone: Back and base, with foramen magnum and occipital condyles (unpaired).
  • Sphenoid Bone: Butterfly-shaped, holds cranial bones together, with sella turcica for pituitary (unpaired).
  • Ethmoid Bone: Spongy, supports nasal cavity, part of nasal septum (unpaired).
• Facial Bones (14 Bones):
  • Nasals: Bridge of nose (paired).
  • Maxillae: Upper jaw, house upper teeth (paired).
  • Palatines: Roof of buccal cavity (paired).
  • Zygomatic Bones: Cheekbones, form zygomatic arch (paired).
  • Lacrimal Bones: Medial orbit wall, house lacrimal sacs (paired).
  • Inferior Nasal Conchae: Lateral nasal cavity wall (paired).
  • Vomer: Inferior nasal septum (unpaired).
  • Mandible: Lower jaw, only movable skull bone (unpaired).
• Sutures: Immovable joints in the skull:
  • Coronal: Frontal to parietals.
  • Sagittal: Between parietals.
  • Lambdoidal: Parietals to occipital.
  • Lateral/Squamous: Parietal to temporal.
• Fontanelles: Soft spots in newborn skulls, ossify by age two, allowing flexibility during birth and brain growth.

Hyoid Bone (1 Bone)

• U-shaped, suspended from temporal bones, supports tongue and neck muscles, does not articulate with other bones.

Ear Ossicles (6 Bones)

• Malleus, Incus, Stapes: Three tiny bones in each middle ear, transmit sound vibrations.

Vertebral Column (26 Bones in Adults)

• Childhood: 33 vertebrae; in adults, 5 sacral fuse into sacrum, 4 coccygeal into coccyx.
• Types of Vertebrae:
  • Cervical (7): Neck, includes atlas (1st) and axis (2nd).
  • Thoracic (12): Chest, articulate with ribs.
  • Lumbar (5): Abdominal, robust for support.
  • Sacral (5, fused): Form sacrum, in pelvic cavity.
  • Coccygeal (4, fused): Form coccyx, vestigial tailbone.
• Curvatures:
  • Primary: Thoracic and sacral (concave, present at birth).
  • Secondary: Cervical and lumbar (convex, develop post-birth).
  • Functions: Balance, shock absorption, protect vertebrae.
• Typical Vertebra Structure:
  • Centrum: Central body, amphiplatyan (flat surfaces).
  • Neural Arch: Forms vertebral foramen (houses spinal cord).
  • Spinous Process: Posterior projection for muscle attachment.
  • Transverse Processes: Lateral projections for muscles.
  • Zygapophyses: Superior/inferior articular processes for vertebra articulation.
  • Intervertebral Foramina: Allow spinal nerve passage.

Special Vertebrae:

• Atlas: Ring-like, no centrum, forms “Yes joint” with occipital condyles.
• Axis: Has odontoid process, forms “No joint” (pivot) with atlas.

Thoracic Cage (25 Bones)

• Sternum (1):
  • Parts: Manubrium, body, xiphoid process.
  • Manubrium: Attaches to clavicles and first two rib pairs.
  • Body: Attaches to ribs via costal cartilages.
  • Xiphoid: Cartilaginous, ossifies in adults, attaches diaphragm.
• Ribs (24, 12 pairs):
  • True Ribs (1-7): Directly attach to sternum via costal cartilages.
  • False Ribs (8-10): Attach to sternum indirectly via rib 7.
  • Floating Ribs (11-12): No sternal attachment.
  • Structure: Head (articulates with vertebrae), tubercle (attaches to transverse processes), costal groove.
  • Intercostal Spaces: House intercostal muscles.

16.9 Appendicular Skeleton (126 Bones)

Pectoral Girdle (4 Bones)

• Clavicle (2): S-shaped, connects sternum to scapula, stabilizes shoulder.
• Scapula (2): Flat, triangular, with glenoid cavity (for humerus), coracoid, and acromion processes for muscle attachment.

Upper Limbs (60 Bones)

• Humerus (2): Upper arm, with head (forms shoulder joint), tubercles, bicipital groove, trochlea (articulates with ulna).
• Radius (2): Lateral forearm, with disc-like head and styloid process.
• Ulna (2): Medial forearm, with olecranon process (elbow joint) and radial notch.
• Carpals (16): Wrist, 8 per hand, arranged in two rows.
• Metacarpals (10): Palm, 5 per hand, form knuckles.
• Phalanges (28): Fingers, 14 per hand (3 per finger, 2 per thumb).

Pelvic Girdle (2 Bones)

• Coxal Bones (2): Each has ilium, ischium, pubis, fused at the acetabulum (hip joint).
• Pubic Symphysis: Cartilaginous joint joining pubis bones.
• Obturator Foramen: Space formed by pubis and ischium.

Lower Limbs (60 Bones)

• Femur (2): Thigh, longest bone, with head (hip joint), neck, trochanters, condyles.
• Patella (2): Knee cap, sesamoid bone.
• Tibia (2): Medial shank, stronger, articulates with femur and talus.
• Fibula (2): Lateral shank, slender, does not articulate with femur.
• Tarsals (14): Ankle, 7 per foot (e.g., calcaneum, talus).
• Metatarsals (10): Sole, 5 per foot.
• Phalanges (28): Toes, 14 per foot (3 per toe, 2 for big toe).

16.10 Types of Joints

Joints (articulations) are points where bones meet, enabling movement. They are classified by movement degree:

1. Synarthroses (Fibrous, Immovable)

• Held by fibrous connective tissue, no movement.
• Types:
  • Sutures: Thin fibrous layer, e.g., skull sutures (coronal, sagittal, lambdoidal).
  • Syndesmoses: Fibrous tissue as sheets/bundles, e.g., tibiofibular ligament.
  • Gomphoses: Cone-shaped bone in a socket, e.g., teeth in jaw.

2. Amphiarthroses (Cartilaginous, Slightly Movable)

• Held by hyaline or fibrocartilage, limited movement.
• Types:
  • Synchondroses: Hyaline cartilage, e.g., epiphyseal plate, rib-sternum junction.
  • Symphysis: Fibrocartilage disc, e.g., intervertebral discs, pubic symphysis.

3. Diarthroses (Synovial, Freely Movable)

• Feature a synovial cavity (filled with synovial fluid), hyaline cartilage on articulating surfaces, synovial membrane, and ligaments.
• Synovial Fluid: Lubricates, absorbs shocks, nourishes cartilage, removes waste.
• Types:
  • Pivot Joint: Rotation around one axis, e.g., atlas-axis (“No joint”).
  • Ball andзя- Socket Joint: Multiaxial movement, e.g., shoulder, hip.
  • Hinge Joint: Monoaxial (flexion/extension), e.g., elbow, knee.
  • Condyloid Joint: Biaxial (flexion/extension, abduction/adduction), e.g., metacarpophalangeal joint.
  • Saddle Joint: Biaxial, allows circumduction, e.g., thumb carpometacarpal joint.
  • Gliding Joint: Non-axial, flat surfaces, e.g., intercarpal joints.

16.11 Disorders Related to Muscles

1. Muscular Dystrophy:
   • Genetic, progressive muscle wasting, affects voluntary muscles.
   • Types: Duchenne (boys, lower limbs), limb-girdle (adults, shoulders/hips).
   • No cure, weakens muscles over time.
2. Myasthenia Gravis:
   • Autoimmune, weakens skeletal muscles by blocking acetylcholine receptors.
   • Symptoms: Ptosis, double vision, difficulty swallowing/speaking.
   • Causes progressive muscle weakness.

16.12 Disorders Related to Bones

1. Arthritis: Joint inflammation, painful, can lead to disability.
   • Osteoarthritis: Cartilage degeneration, affects hands, knees, spine, due to aging, obesity.
   • Gouty Arthritis: Uric acid deposits in joints, causes inflammation, affects feet.
   • Rheumatoid Arthritis: Autoimmune, synovial membrane swells, forms pannus, erodes cartilage, stiffens joints.
2. Osteoporosis:
   • Bones become porous and brittle, common in postmenopausal women.
   • Causes: Estrogen decline, low calcium/vitamin D, reduced bone formation.
   • Symptoms: Height loss, hunched back, bone pain, fractures.
   • Prevention: Calcium-rich diet, exercise, avoid smoking/alcohol.