The Skeletal System

Objectives of the Lecture

• Upon completion, students should be able to identify:
  • Skeletal organization
  • Parts of the skeletal system
  • Types of bones
  • Structure of bones
  • Different types of vertebra
  • Structure of the of vertebral column Structure of the skull
  • Bones of the shoulder girdle, upper extremity, pelvic girdle, and lower extremity
  • Different types of joints and muscles

Bone Function

• Bones provide support for the body's weight
• Bones protect vital organs such as lungs
• Bones and muscles work together to enable movement of limbs or body parts
• Bones are involved in blood cell formation through red marrow
• Bones store inorganic salts:
  • Calcium
  • Phosphate
  • Magnesium
  • Sodium
  • Potassium

Introduction to the Skeletal System

• Axial skeleton:
  • Forms the long axis of the body
  • Composed of 80 bones
  • Major regions include the skull, vertebral column, and bony thorax
• Appendicular skeleton:
  • Includes bones of the upper and lower extremities and girdles
  • Composed of 126 bones
  • Major regions include girdles (shoulder and pelvic), upper extremity, and lower extremity

Skeletal Organization

• The axial skeleton consists of:
  • Skull, vertebral column, and thoracic cage
• The appendicular skeleton consists of:
  • Upper and lower limbs, pectoral girdle, and pelvic girdle

Parts of the Skeletal System

• Key parts of the skeletal system include:
  • Bones (skeleton)
  • Joints
  • Cartilages
  • Ligaments (bone to bone)
  • Tendons (bone to muscle)

Types of Bones

• Bones are classified into types based on shape

The Skull

• Composed of two sets of bones:
  • Cranium and facial bones (sense capsules and jaws)
• Bones are joined by sutures, except for the mandible
• Only the mandible is attached by a freely movable joint

Cranial Bones

• There are 8 cranial bones:
  • Frontal bone (1)
  • Parietal bones (2)
  • Sphenoid bone (1)
  • Temporal bones (2)
  • Occipital bone (1)
  • Ethmoid bone (1)

Facial Bones

• There are 14 facial bones

Bones of the Skull

• Cranial bones are divided into paired and unpaired:
  • Paired: Temporal and Parietal
  • Unpaired: Frontal, Occipital, Sphenoid, and Ethmoid
• Facial bones are divided into paired and unpaired:
  • Paired: Maxillae, Zygomatic, Nasal, Lacrimal, Palatine, and Inferior nasal conchae
  • Unpaired: Mandible and Vomer

The Bony Thorax

• Consists of three main parts:
  • Sternum, ribs & costal cartilages, and thoracic vertebrae

Ribs

• All ribs attach to the vertebral column posteriorly
• True ribs (1-7):
  • The superior seven pairs attach to the sternum in the front
  • They are known as true or sternal ribs
• False ribs (8-12):
  • Do not directly connect to the sternum
  • The upper three connect to the costal cartilages of the ribs just above
  • The last two (11-12) usually have no ventral attachment and are called floating, fluctuating, or vertebral ribs

Vertebral Column

• Formed from 26 irregular bones in adults
• Provides axial support for the trunk
• Transmits the weight of the trunk to the lower limbs
• Protects the spinal cord
• Serves as an attachment site for ribs and muscles
• Separated by intervertebral discs
• Number of individual vertebrae:
  • 24 vertebrae
  • Sacrum (5 fused)
  • Coccyx (4 fused)

Regional Characteristics of the Vertebral Column

• Cervical (C1-C7):
  • Located in the neck and is movable
• Thoracic (T1-T12):
  • Forms the rib cage and has limited movement
• Lumbar (L1-L5):
  • Located in the low back and is movable
• Sacral:
  • Consists of 5 fused vertebrae that join the pelvis
• Coccyx:
  • Consists of 4 fused vertebrae and forms the terminus

Normal Curvatures of the Spine

• Cervical and lumbar curves are concave
• Thoracic and Sacral curves are convex
• Primary curvatures: Thoracic and sacral
• Secondary curvatures: Cervical and lumbar

Structure of a Vertebra

• Vertebral Body
• Vertebral Arch: -Formed by:
  • Spinous processes
  • Transverse processes
  • Pedicles
  • Lamina
  • Articular processes

Differences Between Vertebrae Types

• Cervical (3-7):
  • Small, wide body
  • Short bifid spinous process
  • Triangular vertebral foramen
  • Contains foramina within the transverse process
• Thoracic:
• Larger, heart-shaped body
• Long, sharp, inferior-projecting spinous process
• Circular vertebral foramen
• Bear facets for ribs (except T11-T12) within the transverse process
• Lumbar:
• Kidney shaped body
• Short, blunt spinous process that projects directly posteriorly
• Triangular vertebral foramen
• Thin and tapered transverse process

Atlas (C1)

• Articulates with the occipital bone of the skull and allows for flexion and extension of the neck

Axis (C2)

• Forms the pivot upon which the atlas rotates
• Has a large vertical process called the dens

Sacrum

• Bone formed from five vertebrae fused together near the base of the spinal column

Bones of the Shoulder Girdle

• Includes the scapula and clavicle
• The Glenoid cavity articulates with the humerus
• Acromion articulates with the clavicle
• The Coracoid process projects anteriorly

Bones of the Upper Limb

• Arm: Formed by a single bone, the humerus
• Forearm: Contains two bones, the ulna and radius
• Hand:
  • Carpals (wrist)
  • Metacarpals (palm)
  • Phalanges (fingers)

Pronation and Supination

• Refers to the movements of the forearm
• Supination: Palm facing upwards
• Pronation: Palm facing downwards
• Involved joint:
  • Proximal radioulnar joint

The Pelvis and Pelvic Girdle

• Bony pelvis includes:
  • Two hip bones (pelvic bones)
  • Sacrum
  • Coccyx
• Four articulations within the pelvis;
  • Sacroiliac joints (ilium of the hip bones and the sacrum)
  • Sacrococcygeal symphysis (sacrum and the coccyx)
  • Pubic symphysis (pubis bodies of the two hip bones)

Bones of the Lower Limbs

• Thigh:
  • Contains one bone, the femur (thigh bone)
• Leg:
  • Contains two bones, the tibia and fibula
• Foot:
  • Tarsus (ankle)
  • Metatarsals (sole)
  • Phalanges (toes)

Joints

• Joints are classified based on their function and structure:
  • Functional classification:
    • Immovable
    • Slightly movable
    • Freely movable
  • Structural classification:
    • Fibrous joints (generally immovable)
    • Cartilaginous joints (immovable or slightly moveable)
    • Synovial joints (freely moveable)

Introduction to the Muscular System

• Muscles are tissues that can contract, enabling movement
• Muscle cells are called myocytes
• Functions of the muscular system:
  • Attaches to bones of the skeletal system
  • Moves food through the digestive system
  • Passes blood through the circulatory system
  • Expels fluids through the excretory system

Skeletal Muscle

• Skeletal muscles produce movements of skeleton
• They are voluntary and made of striped muscle fibers
• These muscles have at least two attachments (origin & insertion)
• The fleshy part of muscle is called belly
• The ends of muscles are attached to bone, cartilage, or ligaments

Muscle Attachments

• Muscles attach to bone in one of following three shapes
  • Tendon (a cord of fibrous tissue)
  • Aponeurosis (strong sheet of fibrous tissue)
  • Raphe (interdigitation of tendinous ends of fibers of flat muscles)

Classification of Skeletal Muscles

• Muscles are classisfied by direction of fibers:
  • Parallel: Muscle fibers are parallel to the long axis of muscle:
    • Strap like
    • Fusiform
    • Quadrilateral
  • Circular: Muscle fibers surround a circular orifice
  • Convergent: Muscle fibers converge near the point of insertion
  • Oblique: Muscle fibers run obliquely (pennate):
    • Unipennate
    • Bipennate
    • Multipennate

Anatomical Terms of Movement

• Flexion and Extension:
  • Movements that occur in the sagittal plane
  • Flexion:
    • Decreases the angle between two body parts
  • Extension:
    • Increases the angle between two body parts
• Abduction and Adduction:
  • Terms describing movements towards or away from the midline of the body
    • Abduction: Movement away from the midline
      • E.g. raising the arms out to the sides of the body
    • Adduction: Movement towards the midline
      • E.g. squeezing the legs together
• Medial and Lateral Rotation:
  • Describe movement of the limbs around their long axis
  • Medial rotation:
    • Rotating movement towards the midline
  • Lateral rotation:
    • Rotating movement away from the midline

Skeletal Muscle Action

• Prime mover:
  • The chief muscle or member of a muscle group responsible for a movement
  • E.g., Quadriceps femoris muscle in extending the knee.
• Antagonist:
  • Muscle opposing the action of the prime mover
  • Must be relaxed when the prime mover contracts
• Synergist:
  • Muscle that crosses several joints before reaching its target and contracts to stabilize other joints
  • This prevents unwanted movements

Nerve Supply of Skeletal Muscle

• A mixed nerve (nerve trunk) supplies a muscle:
  • Contains motor fibers (60%), sensory fibers (40%), and some sympathetic fibers
• Motor point of the muscle:
  • Area where the motor nerve enters the muscle and is most electrically excitable
  • Lies at the junction of the upper and middle one third of the fleshy belly of the muscle

Naming of Skeletal Muscles

• Skeletal muscles are named according to:
  • Shape, size, heads, position, depth, attachments, and actions.
• Muscle Tone:
  • A slight unconscious continuous contraction in skeletal muscle.

Smooth Muscles

• These muscles are involuntary
• Found in the GIT, blood and lymph vessels walls, in various hollow organs, and the stomach

Cardiac Muscles

• These muscles attach to the heart
• Responsible for regulating blood movement through heart chambers
• Involuntary

Learn about the skeletal system. Understand the organization, parts, and bone types. Explore the structure of bones, vertebrae, skull, girdles, extremities, joints, and muscles.