Locomotion and Movement: Exhaustive Academic Study Notes

Introduction to Locomotion and Movement

• Movement is a fundamental characteristic of living organisms. Both plants and animals demonstrate a diverse range of movements.

• Examples of movement in various organisms:

  • Unicellular Organisms (e.g., Amoeba): Display simple movement through the streaming of protoplasm.

  • Cilia, Flagella, and Tentacles: Utilized by many organisms for movement.

  • Human Beings: Can move specific body parts including limbs, jaws, eyelids, and the tongue.

• Locomotion: Refers specifically to voluntary movements that result in a change of place or location.

  • Examples include walking, running, climbing, flying, and swimming.

• Relationship Between Movement and Locomotion:

  • Locomotory structures are often the same as those used for other movements.

  • In Paramoecium, cilia serve a dual purpose: moving food through the cytopharynx and facilitating locomotion.

  • Hydra utilizes tentacles for capturing prey and for locomotion.

  • Humans use limbs for changing body postures and for locomotion.

  • Key Principle: All locomotions are movements, but all movements are not locomotions.

• Purpose of Locomotion: Animals perform locomotion based on their habitats and situational demands, generally to search for food, shelter, a mate, suitable breeding grounds, favorable climatic conditions, or to escape enemies and predators.

Types of Movement in Human Cells

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

• Amoeboid Movement:

  • Exhibited by specialized cells such as macrophages and leucocytes in the blood.

  • It is achieved by the formation of pseudopodia, produced by the streaming of protoplasm.

  • Cytoskeletal elements, specifically microfilaments, are involved in this movement.

• Ciliary Movement:

  • Occurs in internal tubular organs lined with ciliated epithelium.

  • In the trachea, coordinated ciliary movements help remove dust particles and foreign substances inhaled from the air.

  • In the female reproductive tract, ciliary movement facilitates the passage of ova.

• Muscular Movement:

  • Required for the movement of limbs, jaws, and the tongue.

  • Relies on the contractile property of muscles.

  • Effective locomotion requires the perfectly coordinated activity of three systems: muscular, skeletal, and neural.

• Flagellar Movement:

  • Helps in the swimming of spermatozoa.

  • Maintains water currents in the canal system of sponges.

  • Facilitates locomotion in Protists like Euglena.

Muscle: Properties and Classification

• Origin: Muscle is a specialized tissue of mesodermal origin.

• Mass: Muscles contribute approximately $40-50\%$ of the total body weight in a human adult.

• Key Properties:

  • Excitability: Ability to respond to a stimulus.

  • Contractility: Ability to shorten forcibly.

  • Extensibility: Ability to be stretched.

  • Elasticity: Ability to recoil to resting length.

• Classification Based on Location:

  1. Skeletal Muscles:

     • Closely associated with skeletal components.

     • Appear striped under a microscope, therefore called striated muscles.

     • Under voluntary control of the nervous system (voluntary muscles).

     • Primarily involved in locomotory actions 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; smooth in appearance (smooth muscles or non-striated muscles).

     • Not under voluntary control (involuntary muscles).

     • Assist in activities like transporting food through the digestive tract or gametes through the genital tract.

  3. Cardiac Muscles:

     • Muscles of the heart.

     • Cells assemble in a branching pattern.

     • Striated in appearance but involuntary in nature, as the nervous system does not control them directly.

Structure of Skeletal Muscle

• Organization:

  • Each organized skeletal muscle consists of multiple muscle bundles or fascicles.

  • These bundles are held together by a common collagenous connective tissue layer called fascia.

  • Each fascicle contains many muscle fibres (muscle cells).

• Muscle Fibre Anatomy:

  • Sarcolemma: The plasma membrane lining the muscle fibre.

  • Sarcoplasm: The cytoplasm enclosed by the sarcolemma.

  • Syncitium: The muscle fibre is multinucleated (contains many nuclei in the sarcoplasm).

  • Sarcoplasmic Reticulum: The endoplasmic reticulum of the muscle fibre; it serves as a storehouse for calcium ions ($Ca^{++}$).

• Myofibrils/Myofilaments:

  • Parallelly arranged filaments in the sarcoplasm.

  • Feature alternate dark and light bands caused by the distribution of two proteins: Actin and Myosin.

  • I-band (Isotropic band): The light band containing actin.

  • A-band (Anisotropic band): The dark band containing myosin.

• Filament Arrangement:

  • Both actin and myosin are arranged as rod-like structures parallel to each other and the longitudinal axis.

  • Thin Filaments: Actin filaments.

  • Thick Filaments: Myosin filaments.

Functional Unit of Contraction: The Sarcomere

• Z line: An elastic fibre in the center of each I-band that bisects it. Thin filaments are firmly attached to the Z line.

• M line: A thin fibrous membrane in the middle of the A-band that holds thick filaments together.

• Sarcomere: The portion of the myofibril between two successive Z lines. It is the functional unit of muscle contraction.

• H Zone: In a resting state, thin filaments only partially overlap thick filaments. The central part of the thick filament transitions that is not overlapped by thin filaments is the H zone.

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 run close to the 'F' actins along their length.

  • 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 the actin filaments.

• Myosin (Thick Filament):

  • A polymerized protein composed of monomeric proteins called Meromyosins.

  • Meromyosin Structure:

    1. Heavy Meromyosin (HMM): Consists of a globular head and a short arm. It projects outward at specific angles to form the cross arm.

    2. Light Meromyosin (LMM): The tail portion.

  • Globular Head Features: Acts as an active ATPase enzyme. It has binding sites for ATP and active sites for actin.

Mechanism of Muscle Contraction: Sliding Filament Theory

• The Theory: States that muscle contraction occurs by the sliding of thin filaments over thick filaments.

• Steps of Contraction:

  1. Initiation: A signal is sent by the Central Nervous System (CNS) via a motor neuron.

  2. Motor Unit: Consists of a motor neuron and the muscle fibres it connects to.

  3. Neuromuscular Junction (Motor-end plate): The junction between the motor neuron and the sarcolemma.

  4. Neurotransmitter Release: A neural signal triggers the release of Acetyl choline, generating an action potential in the sarcolemma.

  5. Calcium Release: The action potential spreads and causes the release of $Ca^{++}$ into the sarcoplasm.

  6. Unmasking: $Ca^{++}$ binds to a subunit of troponin, removing the mask from the active sites on actin.

  7. Cross-bridge Formation: Utilizing energy from ATP hydrolysis, the myosin head binds to the exposed active sites on actin.

  8. Sliding/Power Stroke: The myosin pulls the attached actin filaments toward the center of the A-band. The Z line is pulled inward, shortening the sarcomere (contraction).

• Changes during Contraction:

  • The I-bands reduce in length.

  • The A-bands retain their original length.

  • The H-zone diminishes.

• Cycle Continuation:

  • Myosin releases ADP and $P_i$ to go back to a relaxed state.

  • A new ATP binds, breaking the cross-bridge.

  • ATP is hydrolysed again, and the cycle repeats for further sliding.

• Relaxation: Occurs when $Ca^{++}$ is pumped back into the sarcoplasmic cisternae, causing troponin to mask the actin filaments again. The Z lines return to their original positions.

Muscle Fatigue and Fibre Types

• Fatigue: Repeated activation leads to the accumulation of lactic acid due to the anaerobic breakdown of glycogen.

• Red Fibres (Aerobic Muscles):

  • High content of myoglobin (red-colored oxygen-storing pigment).

  • Plentiful mitochondria to utilize oxygen for ATP production.

• White Fibres:

  • Low myoglobin content; appear pale or whitish.

  • Fewer mitochondria.

  • High amount of sarcoplasmic reticulum.

  • Depend on anaerobic processes for energy.

The Human Skeletal System

• Composition: Consists of a framework of 206 bones and a few cartilages.

• Tissue Types:

  • Bone: Hard matrix due to calcium salts.

  • Cartilage: Slightly pliable matrix due to chondroitin salts.

• Divisions: Divided into the Axial and Appendicular skeleton.

The Axial Skeleton ($80$ bones)

Distributed along the main axis of the body:

• Skull ($22$ bones):

  • Cranial bones ($8$): Form the protective cranium for the brain.

  • 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 ($3$ per ear): Malleus, Incus, and Stapes.

  • Dicondylic Skull: The skull articulates with the vertebral column via two occipital condyles.

• Vertebral Column ($26$ units):

  • Units are called vertebrae; includes a central hollow neural canal for the spinal cord.

  • Regions:

    1. Cervical: $7$ (First is the atlas, which articulates with occipital condyles).

    2. Thoracic: $12$.

    3. Lumbar: $5$.

    4. Sacral: $1$ (fused).

    5. Coccygeal: $1$ (fused).

• Sternum: Flat bone on the ventral midline of the thorax.

• Ribs ($12$ pairs):

  • Each rib is bicephalic (two articulation surfaces dorsally).

  • True Ribs (Pairs 1-7): Attached dorsally to thoracic vertebrae and ventrally to the sternum via hyaline cartilage.

  • Vertebrochondral (False) Ribs (Pairs 8-10): Do not attach to the sternum; join the 7th rib.

  • Floating Ribs (Pairs 11-12): Not connected ventrally.

• Rib Cage: Formed by thoracic vertebrae, ribs, and sternum.

The Appendicular Skeleton

Comprises the bones of the limbs and their girdles. Each limb contains $30$ bones.

• Fore Limb (Hand):

  • Humerus (upper arm).

  • Radius and Ulna (lower arm).

  • Carpals ($8$ wrist bones).

  • Metacarpals ($5$ palm bones).

  • Phalanges ($14$ digits).

• Hind Limb (Leg):

  • Femur (thigh bone - longest bone).

  • Tibia and Fibula (lower leg).

  • Patella (knee cap; cup-shaped bone covering the knee ventrally).

  • Tarsals ($7$ ankle bones).

  • Metatarsals ($5$ bones).

  • Phalanges ($14$ digits).

• Pectoral Girdle:

  • Consists of two halves, each with a Clavicle (collar bone) and a Scapula.

  • Scapula: Large triangular flat bone (between 2nd and 7th ribs). Features a ridge called the spine and a process called the acromion.

  • Glenoid Cavity: A depression below the acromion that articulates with the humerus head (shoulder joint).

• Pelvic Girdle:

  • Consists of two coxal bones.

  • Each coxal bone is a fusion of the ilium, ischium, and pubis.

  • Acetabulum: Cavity where the thigh bone articulates.

  • Pubic Symphysis: Where the two halves meet ventrally; contains fibrous cartilage.

Joints and Disorders

• Joints: Points of contact between bones or bone and cartilage. They act as a fulcrum for muscle-generated force.

• Structural Classifications:

  1. Fibrous Joints: Allow no movement (e.g., sutures in the skull).

  2. Cartilaginous Joints: Joined by cartilage; permit limited movement (e.g., between adjacent vertebrae).

  3. Synovial Joints: Feature a fluid-filled synovial cavity; allow considerable movement.

     • Ball and Socket: Humerus and pectoral girdle.

     • Hinge: Knee joint.

     • Pivot: Between atlas and axis.

     • Gliding: Between carpals.

     • Saddle: Between carpal and metacarpal of thumb.

Disorders of Muscular and Skeletal System

• Myasthenia gravis: Autoimmune disorder of the neuromuscular junction; causes fatigue and paralysis.

• Muscular dystrophy: Progressive genetic degeneration of skeletal muscle.

• Tetany: Rapid muscle spasms due to low $Ca^{++}$ in body fluids.

• Arthritis: Inflammation of joints.

• Osteoporosis: Age-related decrease in bone mass; often caused by decreased estrogen levels.

• Gout: Inflammation of joints due to uric acid crystal accumulation.