Biology: Class 11 — Chapter 17: Locomotion and Movement

• Amoeboid Movement: Exhibited by specialized cells like macrophages and leucocytes in blood. It is effected by pseudopodia formed by the streaming of protoplasm, with cytoskeletal elements like microfilaments actively involved.
• Ciliary Movement: Occurs in internal tubular organs lined by ciliated epithelium. Examples include the coordinated movement of cilia in the trachea to remove dust particles, and the passage of ova through the female reproductive tract.
• Muscular Movement: Required for the movement of limbs, jaws, tongue, etc. It relies on the contractile property of muscles and requires perfect coordination between muscular, skeletal, and neural systems.

• Skeletal Muscles: Closely associated with skeletal components. They appear striped under the microscope (striated muscles) and are under voluntary control. They are primarily involved in locomotory actions and posture changes.
• Visceral Muscles: Located in the inner walls of hollow visceral organs (e.g., alimentary canal, reproductive tract). They are non-striated (smooth muscles) and function involuntarily.
• Cardiac Muscles: Muscles of the heart. They assemble in a branching pattern, are striated, and function involuntarily.

Each organized skeletal muscle is made of several muscle bundles or fascicles held together by a collagenous connective tissue layer called fascia. Each fascicle contains numerous muscle fibres.

• Sarcolemma & Sarcoplasm: The muscle fibre is lined by a plasma membrane called sarcolemma enclosing the sarcoplasm. It is a syncytium containing many nuclei.
• Sarcoplasmic Reticulum: The endoplasmic reticulum of muscle fibres, serving as the storehouse of calcium ions ($Ca^{2+}$).
• Myofibrils: Parallelly arranged filaments containing alternate dark and light bands. The striated appearance is due to two proteins: Actin and Myosin.
• Bands and Lines:
  • I-band (Isotropic band): Light band containing actin, bisected by an elastic fibre called the Z line.
  • A-band (Anisotropic band): Dark band containing myosin, held together in the middle by a thin fibrous membrane called the M line.
  • Sarcomere: The portion of a myofibril between two successive Z lines; it is the functional unit of contraction.
  • H-zone: The central part of the thick filament (A-band) not overlapped by thin filaments during the resting state.

• Actin (Thin Filament): Composed of two helically wound filamentous F-actins, which are polymers of monomeric G-actins. Two filaments of tropomyosin run along its length. A complex protein, troponin, is distributed at regular intervals, masking the active binding sites for myosin in a resting state.
• Myosin (Thick Filament): A polymerized protein made of monomeric units called meromyosins. Each meromyosin has:
  • Heavy Meromyosin (HMM): Comprises a globular head and a short arm projecting outwards as a cross arm. The head acts as an active ATPase enzyme with binding sites for ATP and active sites for actin.
  • Light Meromyosin (LMM): Forms the tail.

Contraction occurs when thin filaments slide over thick filaments, initiated by a neural signal from the central nervous system (CNS) via a motor neuron.

1. A neural signal reaches the neuromuscular junction, releasing acetylcholine, which generates an action potential in the sarcolemma.
2. The action potential spreads through the muscle fibre, triggering the release of calcium ions ($Ca^{2+}$) from the sarcoplasmic reticulum into the sarcoplasm.
3. $Ca^{2+}$ binds to a subunit of troponin on actin filaments, removing the mask from active myosin-binding sites.
4. Utilizing energy from ATP hydrolysis, the active myosin head binds to the exposed site on actin to form a cross bridge.
5. The myosin head pulls the attached actin filaments toward the center of the A-band, pulling the Z lines inward and shortening the sarcomere (contraction). During this, the I-band reduces while the A-band retains its length.
6. Myosin releases ADP and $P_i$, returning to its relaxed state. A new ATP molecule binds, breaking the cross bridge.
7. The ATP is hydrolyzed again, and the cycle repeats for continued sliding.
8. When $Ca^{2+}$ ions are pumped back into the sarcoplasmic cisternae, actin sites are masked again, and the muscle relaxes.

• Muscle Fatigue: Repeated activation of muscles leads to anaerobic breakdown of glycogen, causing the accumulation of lactic acid and resulting in fatigue.
• Red Fibres (Aerobic): Contain high levels of the oxygen-storing pigment myoglobin, giving them a reddish appearance. They have abundant mitochondria to utilize oxygen for ATP production.
• White Fibres (Anaerobic): Contain very little myoglobin, appearing pale or whitish. They have fewer mitochondria but a high amount of sarcoplasmic reticulum, relying on anaerobic processes for energy.

• Skull (22 bones + associated bones): Composed of 8 cranial bones (forming the protective cranium) and 14 facial bones. A single U-shaped hyoid bone lies at the base of the buccal cavity. The middle ear contains three tiny ear ossicles: Malleus, Incus, and Stapes. The skull is dicondylic, articulating via two occipital condyles.
• Vertebral Column (26 vertebrae): Formed by serially arranged units: Cervical (7), Thoracic (12), Lumbar (5), Sacral (1-fused), and Coccygeal (1-fused). The first vertebra is the atlas.
• Sternum (1 bone): A flat bone located on the ventral midline of the thorax.
• Ribs (12 pairs): Double-headed (bicephalic) bones:
  • True Ribs (1st-7th pairs): Attached dorsally to thoracic vertebrae and ventrally to the sternum via hyaline cartilage.
  • False/Vertebrochondral Ribs (8th-10th pairs): Do not articulate directly with the sternum; they join the 7th rib.
  • Floating Ribs (11th & 12th pairs): Not connected ventrally.

• Limbs (30 bones per limb):
  • Forelimb (Arm): Humerus, radius, ulna, carpals (wrist bones - 8), metacarpals (palm bones - 5), and phalanges (digits - 14).
  • Hindlimb (Leg): Femur (thigh bone), tibia, fibula, tarsals (ankle bones - 7), metatarsals (5), phalanges (14), and the cup-shaped patella (knee cap).
• Girdles: Help articulate the limbs with the axial skeleton.
  • Pectoral Girdle: Consists of two halves, each containing a clavicle (collar bone) and a scapula (large triangular flat bone). The scapula features the acromion process and the glenoid cavity, which articulates with the head of the humerus.
  • Pelvic Girdle: Consists of two coxal bones formed by the fusion of three bones: ilium, ischium, and pubis. The thigh bone articulates at a cavity called the acetabulum. The two halves meet ventrally at the pubic symphysis.

• Fibrous Joints: Do not allow any movement. Flat skull bones fuse end-to-end with dense fibrous connective tissues to form sutures.
• Cartilaginous Joints: Bones are joined together with the help of cartilages, permitting limited movement (e.g., joints between adjacent vertebrae).
• Synovial Joints: Characterized by a fluid-filled synovial cavity between articulating surfaces, allowing free movement. Examples include:
  • Ball and Socket Joint: Between humerus and pectoral girdle.
  • Hinge Joint: Knee joint.
  • Pivot Joint: Between atlas and axis.
  • Gliding Joint: Between carpals.
  • Saddle Joint: Between carpal and metacarpal of thumb.

• Myasthenia Gravis: An auto-immune disorder affecting the neuromuscular junction, leading to fatigue, weakening, and paralysis of skeletal muscle.
• Muscular Dystrophy: Progressive degeneration of skeletal muscle, mostly due to a genetic disorder.
• Tetany: Rapid spasms (wild contractions) in muscle due to low $Ca^{2+}$ levels in body fluid.
• Arthritis: Inflammation of joints.
• Osteoporosis: An age-related disorder characterized by decreased bone mass and increased chances of fractures, commonly caused by decreased estrogen levels.
• Gout: Inflammation of joints due to the accumulation of uric acid crystals.