Movement is a fundamental characteristic of living organisms, ranging from simple streaming of protoplasm (e.g., in Amoeba) to complex movements of limbs, jaws, and other body parts in humans. Some movements result in a change of location, known as locomotion (e.g., walking, running, flying, swimming).

Locomotory structures might also be involved in other types of movement (e.g., cilia in Paramoecium help with food movement and locomotion; tentacles in Hydra are used for capturing prey and locomotion; human limbs are used for changing posture and locomotion).

Movement and locomotion are interconnected: all locomotions are movements, but not all movements are locomotions (a beating heart moves but doesn't change location).

Methods of locomotion vary depending on habitat and situation, usually serving purposes like searching for food, shelter, mate, suitable breeding grounds, favorable climate, or escaping predators.

Types Of Movement

Cells of the human body exhibit three primary types of movement:

1. Amoeboid movement: Exhibited by specialized cells like macrophages and leucocytes in blood. This movement is effected by the formation of temporary cytoplasmic extensions called pseudopodia, driven by streaming of protoplasm. Cytoskeletal elements like microfilaments are also involved.
2. Ciliary movement: Occurs in internal tubular organs lined by ciliated epithelium. Coordinated beating of cilia helps in moving substances along the tube (e.g., removing dust/foreign particles from the trachea, facilitating passage of ova through the female reproductive tract).
3. Muscular movement: Required for movements of limbs, jaws, tongue, etc. Utilizes the contractile property of muscle tissue. This is the primary type of movement for locomotion and other actions in multicellular organisms, requiring coordination with skeletal and neural systems.

Movements also involve cilia and flagella, which are outgrowths of the cell membrane (Chapter 8). Flagellar movement aids in swimming of spermatozoa, maintaining water currents (e.g., in sponges), and locomotion of some protists (e.g., Euglena).

Muscle

Muscle is a specialized tissue of mesodermal origin, contributing about 40-50% of human body weight. Muscles possess unique properties:

• Excitability (responsiveness to stimuli)
• Contractility (ability to shorten)
• Extensibility (ability to be stretched)
• Elasticity (ability to return to original length after stretching)

Muscles are classified based on location, appearance, and nature of regulation:

1. Skeletal muscles: Closely associated with skeletal bones. Appear striped (striated) under microscope. Activities are under voluntary control of the nervous system. Primarily involved in locomotion and posture changes.
2. Visceral muscles: Located in the inner walls of hollow visceral organs (e.g., alimentary canal, reproductive tract). Do not show striations, appearing smooth (nonstriated). Activities are involuntary (not under conscious control). Assist in transport (e.g., food through digestive tract, gametes through genital tract).
3. Cardiac muscles: Muscles of the heart. Appear striated and are involuntary. Cardiac muscle cells branch and assemble in a branching pattern, connected by intercalated discs.

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Structure of a skeletal muscle:

An organized skeletal muscle is made of muscle bundles (fascicles) held together by a connective tissue layer called fascia (Figure 20.1). Each fascicle contains numerous muscle fibers (muscle cells).

Structure of a muscle fiber (Figure 20.2 a):

• Lined by plasma membrane called sarcolemma.
• Contains cytoplasm called sarcoplasm.
• Multinucleated (a syncitium) due to many nuclei in sarcoplasm.
• Endoplasmic reticulum is called sarcoplasmic reticulum and stores calcium ions (Ca$^{++}$).
• Contains a large number of parallelly arranged filaments called myofilaments or myofibrils.

Structure of Myofibril and Sarcomere:

Myofibrils show alternate dark and light bands due to the distribution of contractile proteins, Actin and Myosin.

• I-band (Isotropic band): Light band, contains only thin filaments (Actin). Bisected by an elastic fiber called the Z line, to which thin filaments are attached.
• A-band (Anisotropic band): Dark band, contains thick filaments (Myosin) and overlapping portions of thin filaments. Thick filaments are held together in the middle by a thin fibrous membrane called the M line. The central part of the A band, not overlapped by thin filaments in the resting state, is the H zone.

Sarcomere: The portion of a myofibril between two successive Z lines. It is the functional unit of muscle contraction (Figure 20.2 b).

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Structure Of Contractile Proteins

Actin (thin) filaments: Made of two filamentous ('F') actins twisted helically. Each 'F' actin is a polymer of monomeric 'G' (globular) actins. Two filaments of tropomyosin run along the length of 'F' actins. A complex protein, troponin, is distributed at regular intervals on tropomyosin. In the resting state, a subunit of troponin covers the active binding sites for myosin on the actin filaments (Figure 20.3 a).

Myosin (thick) filaments: Polymerized proteins consisting of many monomeric proteins called Meromyosins (Figure 20.3 b). Each meromyosin has two main parts: a globular head with a short arm (Heavy Meromyosin - HMM) and a tail (Light Meromyosin - LMM).

The HMM components (head and short arm) project outwards as cross arms. The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.

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Mechanism Of Muscle Contraction

The Sliding Filament Theory explains muscle contraction: Thin filaments (actin) slide over thick filaments (myosin), shortening the sarcomere.

Steps in Muscle Contraction (Figure 20.4):

1. Neural signal: A signal from CNS reaches the neuromuscular junction (motor-end plate), the junction between a motor neuron and muscle fiber sarcolemma.
2. Neurotransmitter release: Acetylcholine is released, generating an action potential in the sarcolemma.
3. Ca$^{++}$ release: The action potential spreads through the muscle fiber, causing sarcoplasmic reticulum to release Ca$^{++}$ into the sarcoplasm.
4. Activation of actin: Increased Ca$^{++}$ levels bind to a subunit of troponin on actin filaments, causing a conformational change that moves tropomyosin and unmasks the active binding sites for myosin on actin.
5. Cross-bridge formation: Myosin head, using energy from ATP hydrolysis, binds to the exposed active sites on actin, forming a cross bridge.
6. Power stroke (Sliding): The bound myosin head pivots, pulling the attached actin filaments towards the center of the A band. This causes the Z lines to be pulled inwards, shortening the sarcomere and causing contraction. During contraction, I bands reduce, H zone may disappear, but A bands retain length (Figure 20.5).
7. Cross-bridge detachment: A new ATP molecule binds to the myosin head, causing it to detach from the actin cross bridge.
8. Myosin re-cocking: ATP is hydrolyzed by the myosin head (ATPase), storing energy and returning the myosin head to its high-energy (cocked) state, ready to bind again if actin binding sites are still exposed.
9. Cycle repetition: The process of cross-bridge formation, power stroke, detachment, and re-cocking repeats as long as Ca$^{++}$ is available, leading to continued sliding and contraction.

Relaxation: Occurs when the neural signal stops, and Ca$^{++}$ ions are pumped back into the sarcoplasmic reticulum cisternae. This reduces Ca$^{++}$ levels in the sarcoplasm, allowing tropomyosin to cover the myosin binding sites on actin again. Cross bridges detach, and the muscle fiber returns to its resting length.

Muscle fatigue: Repeated muscle activation can lead to accumulation of lactic acid (from anaerobic glycogen breakdown), causing fatigue.

Muscle fiber types:

• Red fibres: High myoglobin content (oxygen storing red pigment), giving a reddish appearance. Numerous mitochondria. Utilize stored oxygen for aerobic ATP production. Can sustain prolonged activity.
• White fibres: Low myoglobin content, appearing pale or whitish. Fewer mitochondria, but high amount of sarcoplasmic reticulum. Rely on anaerobic processes for energy. Generate faster, shorter bursts of contraction.

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Skeletal System

The skeletal system provides a framework of bones and cartilage, crucial for movement, support, and protection. Bone is a hard connective tissue (calcium salts), cartilage is a pliable connective tissue (chondroitin salts).

Adult human skeleton: Consists of 206 bones and a few cartilages.

Divisions:

1. Axial Skeleton: 80 bones along the body's main axis. Includes skull, vertebral column, sternum, and ribs.
   • Skull: 22 bones (8 cranial forming brain case, 14 facial forming the front). Includes a single U-shaped hyoid bone at the base of the buccal cavity. Three tiny ear ossicles (Malleus, Incus, Stapes) in each middle ear. Skull articulates with the vertebral column via two occipital condyles (dicondylic skull) (Figure 20.6).
   • Vertebral column: Dorsally placed, extends from skull base, forms trunk framework. Composed of 26 serially arranged vertebrae (Figure 20.7). Protects spinal cord (passes through neural canal), supports head, provides rib/back muscle attachment. Differentiated regions: Cervical (7), Thoracic (12), Lumbar (5), Sacral (1 fused), Coccygeal (1 fused). First vertebra is Atlas, articulating with occipital condyles.
   • Sternum: Flat bone on ventral midline of thorax.
   • Ribs: 12 pairs (Figure 20.8). Thin flat bones connected dorsally to thoracic vertebrae (bicephalic articulation). Ventral connection varies:
     • True ribs (1-7 pairs): Ventrally attached directly to sternum via hyaline cartilage.
     • False ribs (8-10 pairs): Do not attach directly to sternum, join the 7th rib with hyaline cartilage (vertebrochondral).
     • Floating ribs (11-12 pairs): Not connected ventrally.

   Rib cage: Formed by thoracic vertebrae, ribs, and sternum.
2. Appendicular Skeleton: Bones of limbs and girdles (total 126 bones).
   • Upper limb (Hand/Fore limb): Humerus (upper arm), radius and ulna (forearm), Carpals (8 wrist bones), Metacarpals (5 palm bones), Phalanges (14 digit bones) (Figure 20.9).
   • Lower limb (Leg/Hind limb): Femur (thigh bone - longest bone), tibia and fibula (lower leg), Tarsals (7 ankle bones), Metatarsals (5 foot bones), Phalanges (14 digit bones) (Figure 20.10). Patella (knee cap) covers the knee ventrally.
   • Girdles: Attach limbs to the axial skeleton.
     • Pectoral girdle: Attaches upper limbs. Two halves, each with clavicle (collar bone) and scapula (shoulder blade) (Figure 20.9). Scapula has a spine projecting as the acromion (articulates with clavicle) and glenoid cavity (articulates with humerus head to form shoulder joint).
     • Pelvic girdle: Attaches lower limbs. Two coxal bones (Figure 20.10). Each coxal bone formed by fusion of ilium, ischium, and pubis. Acetabulum cavity (at fusion point) articulates with femur head. Two halves meet ventrally at the pubic symphysis (fibrous cartilage).

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Joints

Joints are essential for movement involving bony parts. They are points of contact between bones, or between bones and cartilages. Muscles generate force used for movement at joints (joints act as fulcrums).

Joints are classified by structural form and movability:

1. Fibrous joints: Do not allow any movement. Bones are joined by dense fibrous connective tissue (e.g., sutures between skull bones forming the cranium).
2. Cartilaginous joints: Bones joined by cartilage. Permit limited movement (e.g., joint between adjacent vertebrae).
3. Synovial joints: Characterized by a fluid-filled synovial cavity between articulating bone surfaces. Allow considerable movement, playing a significant role in locomotion and various movements. Examples:
   • Ball and socket joint (humerus and pectoral girdle - shoulder, femur and pelvic girdle - hip)
   • Hinge joint (knee, elbow)
   • Pivot joint (atlas and axis in neck)
   • Gliding joint (between carpals/tarsals)
   • Saddle joint (between carpal and metacarpal of thumb)

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Disorders Of Muscular And Skeletal System

Disorders affecting muscles and bones include:

• Myasthenia gravis: Autoimmune disorder affecting neuromuscular junction, causing fatigue, weakness, and paralysis of skeletal muscles.
• Muscular dystrophy: Progressive degeneration of skeletal muscles, often due to genetic disorders.
• Tetany: Rapid muscle spasms (wild contractions) caused by low levels of calcium ions ($\textsf{Ca}^{++}$) in body fluid.
• Arthritis: Inflammation of joints.
• Osteoporosis: Age-related disorder characterized by decreased bone mass and increased fracture risk. Common causes include decreased estrogen levels.
• Gout: Inflammation of joints due to accumulation of uric acid crystals.

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