Skeleton: Movement & Locomotion - A Comprehensive Guide

This note provides a detailed and intuitive understanding of the human skeleton, its functions, the division into axial and appendicular parts, and the various types of joints, as outlined in the syllabus.

1. Functions of the Human Skeleton

The human skeleton is a vital organ system that provides the body with its fundamental structure and enables numerous biological processes. Its major functions include:

• Support: The skeleton forms the rigid framework that supports the body's soft tissues, maintaining its shape and posture. For example, the vertebral column supports the head and torso, and the rib cage supports the thoracic organs.
• Protection: Bones encase and shield delicate internal organs from mechanical injury. The skull protects the brain, the rib cage protects the heart and lungs, and the vertebrae protect the spinal cord.
• Movement: Bones act as levers, and joints serve as fulcrums. Muscles attach to bones via tendons, and when muscles contract, they pull on the bones, causing movement at the joints. This coordinated action allows for locomotion and manipulation of objects.
• Mineral Storage: Bones serve as a significant reservoir for essential minerals, primarily calcium and phosphorus. These minerals are crucial for various physiological processes, and the skeleton can release them into the bloodstream to maintain mineral homeostasis when needed.
• Blood Cell Production (Hematopoiesis): The red bone marrow, found within certain bones (e.g., flat bones like the sternum, pelvis, and ends of long bones), is the site of hematopoiesis, the process of producing red blood cells, white blood cells, and platelets.
• Fat Storage: Yellow bone marrow, primarily found in the medullary cavity of long bones, stores fat (triglycerides), which can serve as an energy reserve.

2. Axial and Appendicular Skeleton

The human skeleton is divided into two main functional and anatomical parts:

2.1 Axial Skeleton

• Definition: Forms the central axis of the body, providing support and protection for the head, neck, and trunk.
• Components (80 bones):
  • Skull: Includes the cranial bones (protecting the brain) and facial bones.
  • Auditory Ossicles: Small bones of the inner ear (malleus, incus, stapes) involved in hearing.
  • Hyoid Bone: A U-shaped bone in the neck that supports the tongue.
  • Vertebral Column (Spine): Composed of 33 vertebrae (cervical, thoracic, lumbar, sacrum, coccyx), protecting the spinal cord and providing flexibility.
  • Thoracic Cage: Consists of the sternum (breastbone) and 12 pairs of ribs, protecting the heart and lungs.
• Functions: Primarily involved in protecting vital organs, supporting the body's upright posture, and providing attachment points for muscles of the head, neck, and trunk.

2.2 Appendicular Skeleton

• Definition: Comprises the bones of the limbs (appendages) and the girdles that attach them to the axial skeleton.
• Components (126 bones):
  • Pectoral (Shoulder) Girdle: Consists of the clavicle (collarbone) and scapula (shoulder blade), connecting the upper limbs to the axial skeleton.
  • Upper Limbs: Bones of the arm (humerus), forearm (radius, ulna), wrist (carpals), hand (metacarpals), and fingers (phalanges).
  • Pelvic (Hip) Girdle: Formed by the two coxal (hip) bones, connecting the lower limbs to the axial skeleton and protecting pelvic organs.
  • Lower Limbs: Bones of the thigh (femur), kneecap (patella), lower leg (tibia, fibula), ankle (tarsals), foot (metatarsals), and toes (phalanges).
• Functions: Primarily responsible for movement, locomotion, and manipulation of objects, allowing for a wide range of activities like walking, running, grasping, and lifting.

3. Types of Joints

Joints (articulations) are the points where two or more bones meet. They are classified based on the amount of movement they allow (functional classification) or the type of material connecting the bones (structural classification).

3.1 Classification by Movement (Functional Classification)

• Immovable Joints (Synarthroses):
  • Description: These joints allow no movement between the bones.
  • Location & Example: Sutures of the skull, which tightly interlock the cranial bones, providing maximum protection to the brain.
• Slightly Movable Joints (Amphiarthroses):
  • Description: These joints allow a limited amount of movement, often due to the presence of cartilage between the bones.
  • Location & Example:
    • Vertebral Joints: The joints between adjacent vertebrae in the spinal column, connected by intervertebral discs, allowing for bending and twisting of the spine.
    • Pubic Symphysis: The joint connecting the two pubic bones in the pelvis, which allows slight movement, particularly important during childbirth.
• Freely Movable Joints (Diarthroses / Synovial Joints):
  • Description: These are the most common type of joint in the body, characterized by a joint capsule containing synovial fluid, which lubricates the joint and allows for a wide range of motion. They are further categorized based on the shape of their articulating surfaces and the types of movement they permit.
  • Types and Examples:
    • Hinge Joint:
      • Movement: Allows movement primarily in one plane, like the hinge of a door (flexion and extension).
      • Location & Example: Elbow joint (between humerus and ulna), knee joint (between femur and tibia), and finger/toe joints (interphalangeal joints).
    • Ball and Socket Joint:
      • Movement: Allows for the greatest range of motion, including flexion, extension, abduction, adduction, rotation, and circumduction (movement in all directions).
      • Location & Example: Shoulder joint (between humerus and scapula) and hip joint (between femur and pelvic bone).
    • Pivot Joint:
      • Movement: Allows for rotational movement around a central axis.
      • Location & Example: Atlantoaxial joint (between the atlas and axis vertebrae in the neck, allowing head rotation) and the radioulnar joint (allowing pronation and supination of the forearm).
    • Gliding (Plane) Joint:
      • Movement: Allows for flat, sliding, or gliding movements between two relatively flat bone surfaces.
      • Location & Example: Carpals of the wrist, tarsals of the ankle, and facet joints between vertebrae.
    • Condyloid (Ellipsoid) Joint:
      • Movement: Allows for movement in two planes (flexion/extension, abduction/adduction, and circumduction), but no axial rotation.
      • Location & Example: Radiocarpal joint (wrist joint, between radius and carpal bones) and metacarpophalangeal joints (knuckles of the hand).
    • Saddle Joint:
      • Movement: Allows for back-and-forth and side-to-side movements, providing a wide range of motion but limited rotation. The articulating surfaces are saddle-shaped.
      • Location & Example: Carpometacarpal joint of the thumb (between the trapezium carpal bone and the first metacarpal), which allows the thumb's unique opposable movement.

3.2 Classification by Structure (Brief Overview)

Joints can also be classified by the type of connective tissue that binds the bones together:

• Fibrous Joints: Bones are joined by dense fibrous connective tissue (e.g., sutures of the skull).
• Cartilaginous Joints: Bones are united by cartilage (e.g., pubic symphysis, intervertebral discs).
• Synovial Joints: Bones are separated by a fluid-filled joint cavity, allowing for free movement (all freely movable joints fall into this category).

This detailed explanation aims to provide a clear and comprehensive understanding of the human skeleton, its divisions, and the various types of joints, serving as a thorough study guide.