14.2 - Musculoskeletal System of Humans

Human Skeletal System Overview

• The human skeletal system consists of the axial skeleton and appendicular skeleton.

Axial Skeleton

• Forms the main axis of the body.
• Consists of the skull, vertebral column, ribs, and sternum (breastbone).

Appendicular Skeleton

• Consists of the pectoral girdle, pelvic girdle, upper limbs, and lower limbs.
• Pectoral girdle: clavicle and scapula.
• Upper limb: humerus, radius, ulna, carpus, metacarpals, and phalanges.
• Pelvic girdle: ilium, ischium, and pubis.
• Lower limb: femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges.

Human Vertebral Column

• Vertebrae on different parts of the vertebral column differ in size and shape.
• Each vertebra has a neural canal that contains the spinal cord.

Types of Vertebrae

• Thoracic vertebrae:
  • Long spinous process.
  • Spinous and transverse processes are attachment sites for muscles and ligaments.
  • Transverse processes, except those of the 11th and 12th thoracic vertebrae, have facets for articulation with the ribs.
• Lumbar vertebrae:
  • Biggest and strongest vertebrae.
  • Short spinous process.
  • Large centrum bears the weight of the lower back.
• Sacral vertebrae: five vertebrae fuse to form a triangular bone.
• Caudal vertebrae: four bones fuse to form a triangular structure pointed at one end.
• Atlas vertebra:
  • First cervical vertebra.
  • Large neural canal, small spinous process, a pair of transverse foramina, and no centrum.
  • Articulates with the skull.
• Axial vertebra:
  • Second cervical vertebra.
  • Large spinous process, small transverse process, and a pair of transverse foramina.
  • Odontoid process articulates with the facet of the atlas vertebra’s neural canal.
• Cervical vertebrae: short spinous process, small centrum, wide and short transverse process, and a pair of transverse foramina.

Comparison of Cervical, Thoracic, and Lumbar Vertebrae

Feature	Cervical vertebrae	Thoracic vertebrae	Lumbar vertebrae
Spinous process	Short	Long	Short
Transverse process	Wide and short	Long	Short
Centrum	Small	Medium-sized	Large
Transverse foramina	One pair	Absent	Absent

• Similarity: all vertebrae have spinous and transverse processes, a centrum except the atlas vertebra, and a neural canal.

Joints

• A joint is a point where two or more bones meet, or where cartilage and bone meet.
• Immovable joint: suture at the cranium of the skull.
• Slightly moveable joint: cartilage discs between vertebrae; cartilage between the first rib and sternum.
• Freely moveable joint: hinge joint and ball-and-socket joint.

• Capsule: fibrous tissue that encloses and protects the joint.
• Cartilage: cushions the joint, absorbs shocks, reduces friction between bone ends, and protects bones from wearing out.
• Synovial membrane: secretes synovial fluid into the synovial cavity.
• Synovial fluid: lubricates the joint and reduces friction between bone ends.

Types of Freely Moveable Joints

• Ball-and-socket joint:
  • Allows rotational movement in all directions.
  • Examples: shoulder joint between the humerus and pectoral girdle; hip joint between the femur and pelvic girdle.
• Hinge joint:
  • Allows movement in one plane.
  • Examples: knee, elbow, and phalanges of the fingers and toes.

Skeletal Muscle, Tendon, and Ligament

• Skeletal muscle:
  • Consists of bundles of muscle fibres with a large supply of nerves and blood vessels.
  • Muscles work in antagonistic pairs: when one contracts, the other relaxes.
  • An extensor straightens a limb when it contracts; the triceps is an extensor.
  • A flexor bends a limb when it contracts; the biceps is a flexor.
• Tendon: connects muscle to bone; consists of strong fibres that are non-elastic but flexible.
• Ligament: connects bone to bone; consists of strong, elastic, and tough connective tissue bundles; supports and strengthens joints and allows bones to bend at joints.