Locomotion and Movement - Detailed Notes

Locomotion and Movement

Types of Movement

Movement can be divided into:

• Movement: Change in position of body parts or the whole body.
• Examples: Swimming, walking, movement of jaws, limbs tongue etc.
• Locomotion: Voluntary movement from one place to another.
• Structures causing movement can also aid in locomotion such as flagella, limbs, etc.
• Locomotion helps in:
  • Search for food.
  • Shelter.
  • Mate.
  • Escape from enemies.

Types of Muscles

• Amoeboid: Uses pseudopodia and microfilaments. Examples include leucocytes, macrophages, and Amoeba. Facilitates processes like phagocytosis
• Ciliary: Uses cilia to move particles. Examples include removing dust from the trachea and passage of ova through the female reproductive tract.
• Flagellar: Uses flagella for movement. Examples include maintaining water current in sponges and locomotion in Euglena, and swimming of spermatozoa.
• Muscular: Uses muscles for movement such as running, walking, climbing, and flying.

Muscles

• Muscles constitute approximately $40-50$ percent of body weight.
• Properties include excitability, contractility, elasticity, and extensibility.

Muscle Types Based on Location

• Skeletal Muscles: Located close to skeletal components, are striated and voluntary. Functions in locomotion, voluntary actions, and changes in body posture.
• Smooth Muscles (Visceral Muscles): Located in the inner walls of hollow organs like the alimentary canal and digestive tract, are unstriated and involuntary. Functions in the transport of gametes and food.
• Cardiac Muscles: Located in the heart, are striated and involuntary. Functions in heartbeats.

Skeletal Muscle

• Composed of muscle bundles (fascicles) held together by connective tissue.
• A single muscle cell contains:
  • Sarcolemma: Plasma membrane.
  • Sarcoplasm: Cytoplasm, containing:
    • Sarcoplasmic reticulum.
    • Myofilaments (actin and myosin).
  • Syncytium: A multinucleated condition.
  • Striations: Alternating light and dark bands.

Red vs. White Muscle Fibers

Myofilaments

• Actin (Thin Filament): Appears lighter.
  • I-band (Isotropic Band)
• Myosin (Thick Filament): Appears darker.
  • A-band (Anisotropic Band): Contains overlapping regions of actin and myosin.
  • H-zone: Central part of the thick filament not overlapped by thin filaments.

Key Structures within Myofibrils

• Z-line: Bisects the I-band; defines the sarcomere boundary.
• M-line: Located in the middle of the A-band.

Sarcomere

• Functional unit of contraction in myofibrils.
• Defined as the region between two successive Z-lines.
• During resting state, thin filaments partially overlap thick filaments, leaving the H-zone in the center.

Contractile Proteins

• Actin: Thin filament.

• Myosin: Thick filament

  • Composition: Each myosin filament is composed of numerous monomeric proteins called Meromyosins.
  • Meromyosin Subunits
    • Heavy meromyosin (HMM): Projects outwards at a regular distance and angle from the surface; includes the head and cross arm.
    • Light meromyosin (LMM): Forms the tail.

Actin Filament Structure

• Made of two 'F' (filamentous) actins helically wound around each other.
• Each 'F' actin is a polymer of monomeric 'G' (globular) actin subunits.
• Two filaments of tropomyosin run close to the 'F' actins.
• Troponin is distributed at regular intervals on the tropomyosin.

Mechanism of Muscle Contraction

Sliding Filament Theory

1. Signal Transmission: The central nervous system (CNS) sends a signal via a motor neuron to the muscle.
2. Neuromuscular Junction: At the neuromuscular junction (motor end plate), the neurotransmitter acetylcholine is released, generating an action potential in the sarcolemma.
3. Calcium Release: The action potential causes the sarcoplasmic reticulum (SR) to release $Ca^{2+}$ into the sarcoplasm, increasing the $Ca^{2+}$ levels.
4. Active Site Exposure: $Ca^{2+}$ binds to troponin subunits, causing a conformational change that unmasks the active sites on actin filaments for myosin binding.
5. Cross-Bridge Formation: Energized myosin heads (Myosin-ADP + Pi) bind to the exposed active sites on actin, forming a cross-bridge (Actin-Myosin-ADP + Pi).
6. Sliding: The power stroke occurs, pulling the thin filaments toward the center of the sarcomere.
7. Sarcomere Shortening: Results in:
   • Shortening of the sarcomere.
   • Reduction in the length of the I-band.
   • Retention of the A-band length

Cross-Bridge Cycle

1. ADP and Pi Release: ADP and Pi are released from the myosin head.
2. ATP Binding: A new ATP molecule binds to the myosin head, causing the cross-bridge to break.
3. ATP Hydrolysis: ATP is hydrolyzed (broken down) on the myosin head, re-energizing it.
4. Cycle Repetition: The cycle repeats as long as $Ca^{2+}$ is present and active sites are exposed.
5. Termination: The process continues until $Ca^{2+}$ is pumped back into the sarcoplasmic reticulum, leading Z-lines to return to their original position.

Skeletal System

Components

• Bones
• Cartilages

Divisions

• Axial Skeleton (80 bones):
  • Skull (Cranial and Facial bones)
  • Vertebral column (26 bones)
  • Sternum (1 bone)
  • Ribs (12 pairs)
• Appendicular Skeleton (126 bones):
  • Girdles (Pectoral and Pelvic)
  • Limb bones (Bones of forelimbs and hindlimbs)

Skull

• Cranial Bones (8):
  • Frontal (1)
  • Parietal (2)
  • Temporal (2)
  • Occipital (1)
  • Sphenoid (1)
  • Ethmoid (1)
• Facial Bones (14):
  • Nasal (2)
  • Maxilla (2)
  • Zygomatic (2)
  • Mandible (1)
  • Lacrimal (2)
  • Inferior nasal concha (2)
  • Palatine (2)
  • Vomer (1)
• Other Skull Features *Hyoid bone - Present at the upper part of the neck and attached to the tongue
  • Ear ossicles (Malleus, Incus, Stapes)
  • Occipital condyles articulate the skull with the vertebral column.
  • The skull articulates with the atlas vertebra.

Vertebral Column

• Serially arranged vertebrae with the following functions:
  • Protects the spinal cord.
  • Supports the head.
  • Serves as the point of attachment for ribs and musculature to the back.
• Regions:
  • Cervical (7 vertebrae)
  • Thoracic (12 vertebrae)
  • Lumbar (5 vertebrae)
  • Sacral (1 fused vertebra)
  • Coccygeal (1 fused vertebra)

Ribs

• Articulate dorsally with vertebrae and ventrally with the sternum.
• Ribs have two articulation facets dorsally (bicephalic).

Types of Ribs

• True Ribs: First 7 pairs; attached directly to the sternum via costal cartilage.
• False Ribs: Remaining 5 pairs.
• Floating Ribs: Last 2 pairs of false ribs; not attached ventrally.

Bones of the Limbs and Girdles

Pectoral Girdle

• Clavicle
• Scapula

Forelimb Bones

• Humerus
• Radius
• Ulna
• Carpals (wrist bones)
• Metacarpals (palm bones)
• Phalanges (digits)

Pelvic Girdle

• Coxal bones consists of:
  • Ilium
  • Ischium
  • Pubis

Hindlimb Bones

• Femur
• Tibia
• Fibula
• Patella (knee cap)
• Tarsals (ankle bones)
• Metatarsals
• Phalanges

Scapula

• Large triangular flat bone in the dorsal part of the thorax between the second and seventh ribs. Spine: ridge on posterior surface *Acromion: expansion over the shoulder joint *Glenoid cavity: articulates with head of humerus*
  • Clavicle articulates with the Acromion.

Pelvic Girdle

• Consists of two coxal bones.
• Each coxal bone is formed by the fusion of three bones:
  • Ilium
  • Ischium
  • Pubis
• Acetabulum: Is a depression formed at the point of fusion of the Ilium, Ischium and Pubis, where the femur articulates.
• Pubis symphysis: Where the two halves of the pelvic girdle meet ventrally; contains fibrous cartilage.

Joints

• Points of contact between bones or between bones and cartilage.
• Force generated by muscles is used to carry out movement through joints.

Types of Joints

1. Fibrous Joints: Do not allow movement; found in flat skull bones forming the cranium.
2. Cartilaginous Joints: Bones held together by cartilage; permit limited movements such as those found in vertebrae.
3. Synovial Joints: Characterized by a fluid-filled synovial cavity allowing considerable movement. Examples:
   • Ball and socket joint (e.g., shoulder).
   • Hinge joints (e.g., knee).
   • Pivot joints (e.g., between atlas and axis vertebrae).
   • Gliding joints (e.g., between carpals).
   • Saddle joint.

Disorders of the Muscular and Skeletal System

1. Myasthenia Gravis: Autoimmune disorder affecting the neuromuscular junction, leading to fatigue, weakening, and paralysis.
2. Muscular Dystrophy: Genetic disorder causing degeneration of skeletal muscles.
3. Osteoporosis: Decreased bone mass in old age, increasing the risk of fractures; often due to decreased estrogen levels.
4. Gout: Inflammation of joints due to uric acid crystal accumulation.
5. Arthritis: Inflammation of joints.