Anatomy of Joints

Joints

• A joint is the site where two or more bones articulate or come together.
• Joints can be classified into three types: fibrous, cartilaginous, and synovial.

Fibrous Joints

• Fibrous joints are immovable joints that have fibrous tissue between the bones.
• Examples include joints between the bones of the skull (sutures) and those between the teeth and the maxilla and mandible.

Cartilaginous Joints

• Cartilaginous joints are slightly movable joints that have a pad of fibrocartilage between the ends of the bones.
• The pad of cartilage absorbs shocks and allows for very slight movement.
• Examples include the symphysis pubis and the joints between the vertebral bodies.

Synovial Joints

• Synovial joints are freely movable joints that have a space or capsule between the bones.
• Characteristics of synovial joints include:
  • Hyaline articular cartilage covers the ends of the bones, providing a smooth joint surface and reducing friction.
  • A capsule surrounds the joint, holding the bones together.
  • A synovial membrane lines the capsule, secreting synovial fluid that nourishes the joint cavity, contains phagocytes to remove microbes and debris, acts as a lubricant, and maintains joint stability.
  • In certain joints, small sacs of synovial fluid (bursae) act as cushions to prevent friction.
• Types of synovial joints include:
  • Ball and socket joints (e.g. shoulder and hip joints)
  • Hinge joints (e.g. elbow, knee, and ankle joints)
  • Gliding joints (e.g. sternoclavicular and acromioclavicular joints)
  • Pivot joints (e.g. proximal and distal radioulnar joints)
  • Condyloid and saddle joints (e.g. wrist and temporomandibular joints)

Main Synovial Joints of the Limbs

• Shoulder joint: a ball and socket joint between the glenoid cavity of the scapula and the head of the humerus.
• Elbow joint: a hinge joint between the trochlea and capitulum of the humerus and the trochlear notch of the ulna and the head of the radius.
• Wrist joint: a condyloid joint between the distal end of the radius and the proximal ends of the scaphoid, lunate, and triquetral bones.
• Joints of the hands and fingers: synovial joints between the carpal bones, between the carpal and metacarpal bones, between the metacarpal bones and proximal phalanges, and between the phalanges.
• Hip joint: a ball and socket joint between the acetabulum of the innominate bone and the head of the femur.
• Knee joint: a hinge joint between the condyles of the femur, the condyles of the tibia, and the posterior surface of the patella.
• Ankle joint: a hinge joint between the distal end of the tibia and its malleolus, the distal end of the fibula (lateral malleolus), and the talus.
• Joints of the foot and toes: synovial joints between the tarsal bones, between the tarsal and metatarsal bones, between the metatarsals and proximal phalanges, and between the phalanges.

Muscular System

• The muscular system consists of three types of muscle tissue: skeletal, smooth, and cardiac muscle.
• Skeletal muscles are voluntary muscles that produce body movements and are attached to bones by tendons.

Skeletal Muscle Organization

• Multiple layers of connective tissue within the muscle are important for transmitting the force of contraction from individual muscle cells to the skeleton.
• The three layers of connective tissue are:
  • Epimysium: a connective tissue sheath that covers the entire muscle.
  • Perimysium: a connective tissue sheath attached to each bundle of muscle fibers.
  • Endomysium: a thin layer of connective tissue enveloping individual muscle cells.

Structure of a Skeletal Muscle Cell

• Each cell has several nuclei found just below the cell membrane (sarcolemma).
• The cytoplasm of muscle cells (sarcoplasm) is filled with:
  • Tiny filaments (actin and myosin) that run longitudinally the length of the muscle.
  • Many mitochondria.
  • Myoglobin (an oxygen reserve).
• Large intracellular stores of calcium, which is released into the sarcoplasm by nerve stimulation of muscle, are essential for contractile activity.

Contraction of Skeletal Muscle

• Skeletal muscle cells contract in response to stimulation of a nerve fiber.
• The process of contraction involves:
  • The release of calcium from intracellular stores.
  • The binding of myosin to actin filaments, forming cross-bridges.
  • The sliding of filaments past each other, shortening the sarcomere.
  • The relaxation of the muscle when stimulation stops.

Muscles of Different Parts of the Body

• There are many muscles involved in changing facial expression, movement of the lower jaw, and other functions.
• Specific muscles of the face, neck, back, and abdominal wall have distinct functions, such as:
  • Raising eyebrows (occipitofrontalis).
  • Closing the eye (orbicularis oculi).
  • Closing the lips (orbicularis oris).
  • Chewing and moving the jaw (masseter, temporalis, pterygoid).
  • Turning the head (sternocleidomastoid).
  • Pulling the head backwards (trapezius).

Muscles of the Back

• There are six pairs of large muscles in the back, including:
  • Trapezius: pulls the head backwards, squares the shoulders, and controls scapular movements.
  • Latissimus dorsi: adducts, medially rotates, and extends the arm.
  • Teres major: extends, adducts, and medially rotates the arm.
  • Quadratus lumborum: causes extension of the vertebral column and lateral flexion of the lumbar region.

Muscles of the Abdominal Wall

• There are five pairs of muscles that form the abdominal wall, including:
  • Rectus abdominis: flexes the vertebral column.
  • External oblique: compresses the abdominal organs and flexes the vertebral column.
  • Internal oblique: compresses the abdominal organs and flexes the vertebral column.
  • Transversus abdominis: compresses the abdominal organs and flexes the vertebral column.

Muscles of the Shoulder

• Specific muscles of the shoulder, including:
  • Coracobrachialis: flexes the shoulder joint.
  • Deltoid: flexes, abducts, and extends the shoulder joint.
  • Pectoralis major: flexes and adducts the arm.

Muscles of the Upper Limbs

• Specific muscles of the upper limbs, including:
  • Biceps: flexes the shoulder joint and assists with flexion and supination at the elbow joint.
  • Brachialis: flexes the elbow joint.
  • Triceps: extends the elbow joint.

Muscles of the Hip and Lower Limbs

• Specific muscles of the hip and lower limbs, including:
  • Psoas: flexes the hip joint.
  • Iliacus: flexes the hip joint.
  • Gluteal muscles: extend, abduct, and medially rotate the hip joint.
  • Sartorius: flexes the hip and knee joints.
  • Adductor group: adducts and medially rotates the thigh.
  • Quadriceps femoris: extends the knee joint.
  • Gastrocnemius and soleus: flex the foot and ankle.