Anatomy of 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 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 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 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)

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.