Comprehensive Guide to Locomotion and Movement

Introduction to Locomotion and Movement

• Definition of Movement: Movement is a significant feature of all living beings. It encompasses a wide range of activities in both plants and animals.
• Examples in Different Organisms:
  • Amoeba: Exhibits simple movement through the streaming of protoplasm.
  • Cilia, Flagella, and Tentacles: These structures are utilized by many organisms for movement.
  • Human Movement: Humans move limbs, jaws, eyelids, and the tongue.
• Locomotion: Voluntary movements that result in a change of place or location are termed locomotion. Examples include walking, running, climbing, flying, and swimming.
• Functional Overlap: Structures used for locomotion can also serve other purposes.
  • In Paramoecium, cilia facilitate the movement of food through the cytopharynx and are also used for locomotion.
  • In Hydra, tentacles are used for capturing prey and for locomotion.
  • Humans use limbs for both body posture changes and locomotion.
• The Link Between Locomotion and Movement: All locomotions are movements, but all movements are not locomotions.
• Factors Influencing Locomotion: The method of locomotion depends on the animal's habitat and the specific demand of the situation. Animals generally locomote to search for food, shelter, a mate, suitable breeding grounds, or favorable climatic conditions, and to escape from enemies or predators.

Types of Movement

Cells in the human body exhibit three main categories of movement:

• Amoeboid Movement: Exhibited by specialized cells like macrophages and leucocytes in the blood. It occurs through pseudopodia formed by protoplasmic streaming (as seen in Amoeba). Cytoskeletal elements, specifically microfilaments, are involved.
• Ciliary Movement: Occurs in internal tubular organs lined with ciliated epithelium.
  • In the trachea, coordinated cilia movements help remove dust and foreign substances inhaled from the air.
  • In the female reproductive tract, ciliary movement facilitates the passage of ova.
• Muscular Movement: Involved in the movement of limbs, jaws, and the tongue. It relies on the contractile property of muscles. Effective locomotion requires the coordinated activity of the muscular, skeletal, and neural systems.
• Flagellar Movement: Mentioned as an outgrowth of the cell membrane (related to Chapter 8). It assists in:
  • Swimming of spermatozoa.
  • Maintenance of water currents in the canal system of sponges.
  • Locomotion in Protists like Euglena.

Muscle: Properties and Classification

• Origin and Composition: Muscle is a specialized tissue derived from the mesoderm. In a human adult, muscles contribute approximately $40\%-50\%$ of the total body weight.
• Special Properties: Muscles possess four key characteristics:
  • Excitability
  • Contractility
  • Extensibility
  • Elasticity
• Classification Criteria: Muscles are classified based on location, appearance, and the nature of their regulation.
• Types Based on Location:
  1. Skeletal Muscles: Closely associated with skeletal components. They appear striped (striated) under a microscope and are under the voluntary control of the nervous system. They are primarily used for locomotion and posture changes.
  2. Visceral Muscles: Located in the inner walls of hollow internal organs (e.g., alimentary canal, reproductive tract). They lack striations and appear smooth (nonstriated). They are involuntary and assist in tasks like transporting food through the digestive tract or gametes through the genital tract.
  3. Cardiac Muscles: The muscles of the heart. Cells assemble in branching patterns. They are striated in appearance but involuntary, as the nervous system does not control them directly.

Structure of Skeletal Muscle

• Organization: Each skeletal muscle consists of multiple muscle bundles called fascicles, which are held together by a collagenous connective tissue layer known as fascia.
• Muscle Fibers: Each fascicle contains many muscle fibres.
• Anatomical Components of Muscle Fibre:
  • Sarcolemma: The plasma membrane lining the muscle fibre.
  • Sarcoplasm: The cytoplasm within the muscle fibre. It is a syncitium, meaning it contains many nuclei.
  • Sarcoplasmic Reticulum: The endoplasmic reticulum of the muscle fibre, which serves as a storehouse for calcium ions ($Ca^{++}$).
• Myofibrils: Parallel filaments in the sarcoplasm. They exhibit alternate dark and light bands caused by two proteins: actin and myosin.
  • I-band (Isotropic band): The light band containing actin.
  • A-band (Anisotropic band): The dark band containing myosin.
• Sarcomere Structure:
  • Z-line: An elastic fibre in the center of each I-band that bisects it; actin filaments are attached here.
  • M-line: A thin fibrous membrane in the middle of the A-band that holds thick filaments together.
  • Sarcomere: The portion of a myofibril between two successive Z-lines, defined as the functional unit of contraction.
  • H-zone: The central part of the thick filament in the A-band that is not overlapped by thin filaments during the resting state.

Structure of Contractile Proteins

• Actin (Thin) Filament: Composed of two 'F' (filamentous) actins helically wound together. Each 'F' actin is a polymer of monomeric 'G' (Globular) actins.
  • Tropomyosin: Two filaments of this protein run close to the 'F' actins.
  • Troponin: A complex protein distributed at regular intervals on tropomyosin. In a resting state, a subunit of troponin masks the active binding sites for myosin on actin.
• Myosin (Thick) Filament: A polymerised protein made of monomeric units called Meromyosins.
  • Meromyosin Structure: Consists of a globular head with a short arm (Heavy Meromyosin or HMM) and a tail (Light Meromyosin or LMM).
  • Cross Arm: The HMM component (head and short arm) projects outwards at specific angles and distances.
  • Active Sites: The globular head acts as an active ATPase enzyme and has binding sites for ATP and active sites for actin.

Mechanism of Muscle Contraction

• Sliding Filament Theory: States that muscle contraction occurs as thin filaments slide over thick filaments.
• Steps of Contraction:
  1. Neural Signal: The Central Nervous System (CNS) sends a signal via a motor neuron (a motor neuron and its connected muscle fibres form a motor unit).
  2. Neurotransmitter Release: At the neuromuscular junction (motor-end plate), Acetyl choline is released.
  3. Action Potential: An action potential is generated in the sarcolemma and spreads through the fibre.
  4. Calcium Release: $Ca^{++}$ ions are released from the sarcoplasmic reticulum into the sarcoplasm.
  5. Binding and Exposure: $Ca^{++}$ binds to a subunit of troponin on the actin filament, removing the mask from the active sites for myosin.
  6. Cross Bridge Formation: Utilizing energy from ATP hydrolysis ($ATP \rightarrow ADP + P_i$), the myosin head binds to the exposed site on actin.
  7. Sliding: The myosin head pulls the actin filaments toward the center of the A-band. The Z-lines are pulled inward, shortening the sarcomere (contraction).
  8. Changes during Contraction: The I-bands reduce in length, the H-zone disappears or reduces, while the A-bands retain their original length.
  9. Resetting: Myosin releases $ADP$ and $P_i$, a new $ATP$ binds, the cross-bridge breaks, and the cycle repeats.
• Relaxation: Contraction continues until $Ca^{++}$ is pumped back into the sarcoplasmic cisternae, causing troponin to re-mask the actin filaments and the Z-lines to return to their original positions.

Muscle Fiber Types and Fatigue

• Fatigue: Repeated muscle activation leads to the accumulation of lactic acid due to the anaerobic breakdown of glycogen.
• Red Fibres (Aerobic Muscles):
  • Contain a high amount of myoglobin (a red oxygen-storing pigment).
  • Possess many mitochondria for ATP production.
  • Highly oxygenated and appear reddish.
• White Fibres:
  • Contain very little myoglobin and appear pale or whitish.
  • Have fewer mitochondria but a high amount of sarcoplasmic reticulum.
  • Depend primarily on anaerobic processes for energy.

The Skeletal System

• Components: A framework of 206 bones and a few cartilages.
• Tissues:
  • Bone: Hard matrix due to calcium salts.
  • Cartilage: Pliable matrix due to chondroitin salts.
• Divisions:
  1. Axial Skeleton ($80$ bones): Distributed along the main axis. Includes the skull, vertebral column, sternum, and ribs.
  2. Appendicular Skeleton: Includes the bones of the limbs and their girdles.

The Axial Skeleton

• Skull ($22$ bones):
  • Cranial bones ($8$): Form the cranium (brain box).
  • Facial bones ($14$): Form the front part of the skull.
  • Hyoid bone ($1$): U-shaped bone at the base of the buccal cavity.
  • Ear Ossicles: Three tiny bones in each middle ear (Malleus, Incus, Stapes).
  • Articulation: The skull is dicondylic, possessing two occipital condyles that articulate with the first vertebra (atlas).
• Vertebral Column ($26$ units):
  • Protects the spinal cord (which passes through the neural canal).
  • Regions: Cervical ($7$), Thoracic ($12$), Lumbar ($5$), Sacral ($1$ fused), and Coccygeal ($1$ fused).
  • Nearly all mammals have seven cervical vertebrae.
• Sternum: A flat bone on the ventral midline of the thorax.
• Ribs ($12$ pairs): All are bicephalic (two dorsal articulation surfaces).
  • True Ribs ($1st-7th$ pairs): Attached dorsally to thoracic vertebrae and ventrally to the sternum via hyaline cartilage.
  • Vertebrochondral/False Ribs ($8th-10th$ pairs): Join the $7th$ rib rather than the sternum.
  • Floating Ribs ($11th$ and $12th$ pairs): Not connected ventrally.
• Rib Cage: Formed by thoracic vertebrae, ribs, and the sternum.

The Appendicular Skeleton

• Limb Bones: Each limb contains $30$ bones.
  • Forelimb (Hand): Humerus, radius, ulna, carpals (wrist bones - $8$), metacarpals (palm bones - $5$), and phalanges (digits - $14$).
  • Hindlimb (Leg): Femur (longest bone), tibia, fibula, tarsals (ankle bones - $7$), metatarsals ($5$), phalanges ($14$), and patella (knee cap).
• Girdles:
  • Pectoral Girdle: Consists of two halves. Each half contains a clavicle (collar bone) and a scapula (triangular flat bone).
  • The scapula has a ridge called the spine ending in the acromion.
  • The glenoid cavity below the acromion articulates with the humerus.
  • Pelvic Girdle: Consists of two coxal bones. Each coxal bone is a fusion of the ilium, ischium, and pubis.
  • Acetabulum: The cavity where the thigh bone articulates.
  • Pubic Symphysis: The ventral meeting point of the two halves, containing fibrous cartilage.

Joints

• Function: Act as points of contact between bones or bone/cartilage; the joint serves as a fulcrum for muscular force.
• Structural Classifications:
  1. Fibrous Joints: No movement allowed (e.g., sutures in the skull).
  2. Cartilaginous Joints: Permit limited movement (e.g., between adjacent vertebrae).
  3. Synovial Joints: Characterized by a fluid-filled synovial cavity; allow considerable movement.
• Examples of Synovial Joints:
  • Ball and Socket: Humerus and pectoral girdle; femur and acetabulum.
  • Hinge: Knee joint; between phalanges.
  • Pivot: Between atlas and axis.
  • Gliding: Between carpals.
  • Saddle: Between carpal and metacarpal of the thumb.

Disorders of the Muscular and Skeletal System

• Myasthenia gravis: An autoimmune disorder affecting the neuromuscular junction; results in fatigue, weakening, and skeletal muscle paralysis.
• Muscular dystrophy: Progressive skeletal muscle degeneration, typically due to a genetic disorder.
• Tetany: Rapid spasms (wild contractions) caused by low $Ca^{++}$ in body fluids.
• Arthritis: Inflammation of joints.
• Osteoporosis: An age-related disorder with decreased bone mass and increased fracture risk; often caused by decreased estrogen levels.
• Gout: Joint inflammation resulting from the accumulation of uric acid crystals.