Articulations And Body Movements Review Sheet

The intricate dance of human movement relies on the seamless interplay between bones, muscles, and joints. Understanding articulations, or joints, and the movements they permit is fundamental to comprehending human biomechanics. This review sheet delves into the world of articulations, exploring their classification, structure, and the diverse range of movements they facilitate, providing a comprehensive guide for students and enthusiasts alike.

Classifying Articulations: A Structural and Functional Overview

Articulations are typically classified based on two primary criteria: their structure and their function.

Structural Classification

This classification focuses on the material that binds the bones together at the joint, as well as the presence or absence of a joint cavity. The three main structural classifications are:

• Fibrous Joints: These joints are characterized by the presence of fibrous connective tissue connecting the articulating bones. They lack a joint cavity and generally allow for little to no movement.

  • Sutures: Found exclusively in the skull, sutures are immovable (synarthrotic) joints where the interlocking edges of the bones are held together by short connective tissue fibers.
  • Syndesmoses: These joints involve bones connected by ligaments, cords, or interosseous membranes. The amount of movement allowed depends on the length of the connecting fibers. An example is the distal tibiofibular joint, which allows slight "give" or movement.
  • Gomphoses: These are "peg-in-socket" fibrous joints. The only example in the human body is the articulation of a tooth with its bony socket in the maxilla or mandible. The fibrous connection is the periodontal ligament.

• Cartilaginous Joints: In these joints, the articulating bones are united by cartilage. Like fibrous joints, they lack a joint cavity.

  • Synchondroses: These joints involve a bar or plate of hyaline cartilage uniting the bones. Most synchondroses are synarthrotic (immovable). A classic example is the epiphyseal plate in long bones of children. Another example is the joint between the first rib and the sternum, which is also synarthrotic.
  • Symphyses: In symphyses, hyaline cartilage covers the articulating surfaces of the bones, but the cartilage is fused to an intervening pad or plate of fibrocartilage. Symphyses are amphiarthrotic joints (slightly movable) designed for strength with flexibility. Examples include the intervertebral joints and the pubic symphysis.

• Synovial Joints: These are the most common type of joint in the body and are characterized by a fluid-filled joint cavity. Synovial joints allow for a wide range of motion and are therefore diarthrotic (freely movable). Their distinguishing features include:

  • Articular Cartilage: Hyaline cartilage covers the opposing bone surfaces. These glassy-smooth cartilages absorb compression placed on the joint and keep the bone ends from being crushed.
  • Joint (Articular) Cavity: This is a potential space that contains a small amount of synovial fluid.
  • Articular Capsule: The joint cavity is enclosed by a two-layered articular capsule. The external layer, the fibrous layer, is composed of dense irregular connective tissue that is continuous with the periostea of the articulating bones. It strengthens the joint so that the bones are not pulled apart. The inner layer is a synovial membrane composed of loose connective tissue. Besides lining the fibrous layer internally, it covers all internal joint surfaces that are not hyaline cartilage. The synovial membrane's function is to make synovial fluid.
  • Synovial Fluid: This viscous, slippery fluid fills all free spaces within the joint capsule. Essentially, synovial fluid is a filtrate of blood plasma. It also contains hyaluronic acid secreted by fibroblasts in the synovial membrane, making it viscous. Synovial fluid lubricates the articular cartilages; it also nourishes the cartilage.
  • Reinforcing Ligaments: These are bandlike ligaments that reinforce synovial joints. Most often, they are capsular ligaments, thickened parts of the fibrous layer. In other cases, they remain distinct and are found outside the capsule (extracapsular ligaments) or deep to it (intracapsular ligaments).
  • Nerves and Blood Vessels: Synovial joints are richly supplied with sensory nerve fibers that innervate the capsule. Some detect pain, but most monitor joint position and stretch. Synovial joints also have a rich blood supply. Capillary beds in the synovial membrane produce the blood filtrate that is the basis of synovial fluid.
  • Other Features:
    • Fatty Pads: Some synovial joints have cushioning fatty pads between the fibrous layer and the synovial membrane or bone.
    • Articular Discs (Menisci): Some synovial joints, such as the knee and jaw, have fibrocartilage pads called articular discs or menisci. These discs extend inward from the articular capsule and divide the synovial cavity in two. Articular discs improve the fit between articulating bone ends, making the joint more stable and minimizing wear and tear on the joint surfaces.
    • Bursae: Bursae are flattened fibrous sacs lined with synovial membrane and containing a thin film of synovial fluid. They occur where ligaments, muscle, skin, tendons, or bones rub together.
    • Tendon Sheaths: A tendon sheath is essentially an elongated bursa that wraps completely around a tendon subjected to friction. They are common where several tendons are crowded together within narrow canals (in the wrist, for example).

Functional Classification

This classification focuses on the amount of movement allowed at the joint. The three functional classifications are:

• Synarthroses: Immovable joints. These are common in the axial skeleton.
• Amphiarthroses: Slightly movable joints. These are also common in the axial skeleton.
• Diarthroses: Freely movable joints. These predominate in the limbs.

Types of Synovial Joints: A Spectrum of Movement

Synovial joints are further classified into six types, based on the shape of their articular surfaces and the movements they allow:

1. Plane Joints: Articular surfaces are essentially flat, allowing only short gliding or translational movements. Gliding does not involve rotation around any axis; therefore, gliding movements are nonaxial. Examples include intercarpal and intertarsal joints, and the joints between vertebral articular surfaces.
2. Hinge Joints: A cylindrical end of one bone articulates with a trough-shaped surface on another bone. Motion is along a single plane; hinge joints permit flexion and extension only. Hinge joints are uniaxial, meaning movement is allowed around one axis only. Examples include the elbow joint, interphalangeal joints.
3. Pivot Joints: A rounded end of one bone protrudes into a "sleeve" or ring composed of bone (and possibly ligaments) of the other. The only movement allowed is rotation of one bone around its long axis. Pivot joints are also uniaxial joints. Examples include the proximal radioulnar joint (allows rotation of the radius during pronation and supination of the forearm) and the atlantoaxial joint (allows the head to rotate on the neck).
4. Condylar Joints: An oval articular surface of one bone fits into a complementary depression in another. Condylar joints permit all angular motions (flexion, extension, abduction, and adduction) but do not allow rotation. Therefore, condylar joints are biaxial joints, meaning movement can occur around two axes. Examples include radiocarpal (wrist) joints, and metacarpophalangeal (knuckle) joints.
5. Saddle Joints: Each articular surface has both concave and convex areas, like a saddle. Saddle joints allow the same movements as condylar joints (flexion, extension, abduction, adduction), but saddle joints allow greater freedom of movement. Each saddle joint is biaxial. The best example is the carpometacarpal joint of the thumb.
6. Ball-and-Socket Joints: The spherical or hemispherical head of one bone articulates with the cuplike socket of another. These joints are multiaxial joints and allow the most freely moving synovial joints. All angular motions (flexion, extension, abduction, adduction, and circumduction) are allowed. Rotation is also permitted. Examples include the shoulder and hip joints.

Body Movements: A Comprehensive Lexicon

Understanding the specific movements possible at synovial joints is crucial for analyzing human motion. Here's a detailed overview of common body movements:

• Gliding: One flat bone surface glides or slips over another similar surface.
• Angular Movements: These movements change the angle between two bones.
  • Flexion: A bending movement, usually along the sagittal plane, that decreases the angle of the joint and brings the articulating bones closer together. Examples: bending the head to the chest; bending the knee; bending the elbow.
  • Extension: The reverse of flexion; it involves increasing the angle between the articulating bones. Examples: straightening a flexed neck, body, elbow, or knee.
  • Hyperextension: Excessive extension beyond normal range of motion. Examples: looking up at the ceiling; bending the trunk backward.
  • Abduction: Movement of a limb away from the midline or median plane of the body, along the frontal plane. Example: raising the arm or thigh laterally.
  • Adduction: Movement of a limb toward the midline of the body. Example: bringing the arm or thigh back to the midline.
  • Circumduction: Moving a limb so that it describes a cone in space. The distal end of the limb moves in a circle, while the point of the cone (the shoulder or hip joint) is more or less stationary. Circumduction involves flexion, abduction, extension, and adduction in succession.
• Rotation: Turning of a bone around its long axis. It is the only movement allowed between the first two cervical vertebrae and is common at the hip and shoulder joints.
  • Medial Rotation: Rotation toward the midline.
  • Lateral Rotation: Rotation away from the midline.
• Special Movements: These movements occur at only a few joints.
  • Pronation: Movement of the forearm so that the radius rotates over the ulna. The palm faces posteriorly or inferiorly.
  • Supination: Movement of the forearm so that the radius and ulna are parallel. The palm faces anteriorly or superiorly.
  • Dorsiflexion: Lifting the foot so that its superior surface approaches the shin.
  • Plantar Flexion: Depressing the foot (pointing the toes).
  • Inversion: The sole of the foot turns medially.
  • Eversion: The sole of the foot turns laterally.
  • Protraction: A nonangular anterior movement in the transverse plane. Example: jutting out the jaw.
  • Retraction: A nonangular posterior movement in the transverse plane. Example: pulling the jaw back.
  • Elevation: Lifting a body part superiorly. Example: shrugging the shoulders.
  • Depression: Moving the elevated part inferiorly. Example: lowering the shoulders.
  • Opposition: Movement of the thumb to touch the tips of the other fingers of the same hand. This movement makes the human hand a fine tool for grasping and manipulating objects.

Factors Influencing Joint Stability

While synovial joints allow for a wide range of motion, their stability can be compromised if the forces acting on them are excessive or if the supporting structures are weakened. Several factors contribute to joint stability:

• Shape of Articular Surfaces: The shape of the articulating surfaces plays a role, although a minor one. In some joints, such as the hip joint, the socket is deep and provides a secure fit. In other joints, such as the knee, the articular surfaces are shallow and rely more on ligaments and muscles for stability.
• Ligament Number and Location: Ligaments prevent excessive or unwanted motions. The more ligaments a joint has, the stronger it is.
• Muscle Tone: Muscle tone, which is the constant, low-level activity of muscles, is the most important stabilizing factor. Muscle tendons that cross the joint are kept taut by muscle tone.

Common Joint Injuries and Conditions

Understanding the structure and function of joints allows us to better appreciate the mechanisms behind common joint injuries and conditions.

• Sprains: Ligaments reinforcing a joint are stretched or torn. Because ligaments have a poor blood supply, sprains heal slowly and are painful.
• Cartilage Injuries: The knee is very susceptible to sports injuries because its articular cartilages (menisci) are frequently subjected to compression and shear stress at the same time. Because cartilage is avascular, it cannot repair itself.
• Dislocations (Luxations): Occur when bones are forced out of alignment. They are usually accompanied by sprains, inflammation, and difficulty in moving the joint. Dislocations are caused by serious falls and are common sports injuries. The shoulder, elbow, fingers, and hip are most commonly dislocated.
• Subluxation: A partial dislocation of a joint.
• Bursitis: Inflammation of a bursa, usually caused by a blow or friction.
• Tendonitis: Inflammation of tendon sheaths, typically caused by overuse.
• Arthritis: Describes over 100 different inflammatory or degenerative diseases that damage the joints. The most widespread crippling disease in the United States. Symptoms: pain, stiffness, and swelling of the joint. Acute forms are usually caused by bacterial invasion and are treated with antibiotics. Chronic forms include osteoarthritis, rheumatoid arthritis, and gouty arthritis.
  • Osteoarthritis (OA): The most common chronic arthritis. OA is a degenerative condition, often called "wear-and-tear arthritis." OA is most common in the aged, and probably related to the normal aging process. Cartilage is broken down more quickly than it is replaced. The exposed bone thickens and may form bony spurs (osteophytes) that enlarge the bone ends and restrict movement. Joints most affected are cervical and lumbar spine, fingers, knuckles, knees, and hips. OA is slow and irreversible, but controllable with medication (pain relievers such as aspirin and ibuprofen), moderate activity, magnetic therapy, glucosamine, and chondroitin sulfate.
  • Rheumatoid Arthritis (RA): A chronic inflammatory disorder. RA arises between the ages of 40 and 50, but may occur at any age. RA affects three times as many women as men. RA is an autoimmune disease - a disorder in which the body's immune system attacks its own tissues. RA begins with inflammation of the synovial membrane (synovitis) of the affected joints. Inflammatory blood cells migrate into the joint cavity from the bloodstream. The synovial membrane thickens and erodes the articular cartilages. Scar tissue forms and connects the bone ends. The scar tissue eventually ossifies, and the bone ends become fused (ankylosis). RA ultimately deforms the joints and disables the sufferer.
  • Gouty Arthritis: Uric acid (a normal waste product of nucleic acid metabolism) is normally excreted in the urine. In gouty arthritis, blood levels of uric acid rise excessively, and it is deposited as needle-shaped urate crystals in the soft tissues of the joints. Gouty arthritis is an inflammatory response. Gouty arthritis typically affects one joint at a time, often at the base of the great toe. In untreated gouty arthritis, the bone ends fuse and immobilize the joint.

Frequently Asked Questions (FAQ)

• What is the difference between a ligament and a tendon?
  • Ligaments connect bone to bone, providing stability to joints. Tendons connect muscle to bone, facilitating movement.
• What is the role of synovial fluid in a joint?
  • Synovial fluid lubricates the articular cartilages, reduces friction, and provides nourishment to the cartilage cells.
• Why are some joints more prone to injury than others?
  • Joints with a wider range of motion, shallower sockets, or weaker supporting ligaments are generally more susceptible to injury.
• Can arthritis be cured?
  • While there is no cure for most forms of arthritis, treatments are available to manage symptoms, reduce pain, and improve joint function.
• How can I maintain healthy joints?
  • Regular exercise, maintaining a healthy weight, and avoiding excessive stress on joints can help maintain joint health.

Conclusion

Articulations are the linchpins of human movement, enabling us to perform a wide range of activities, from delicate finger movements to powerful athletic feats. Understanding the classification, structure, and function of joints is essential for anyone interested in human anatomy, physiology, or biomechanics. This review sheet provides a solid foundation for further exploration of the fascinating world of articulations and body movements. By appreciating the intricacies of joint function, we can better understand how to prevent injuries, manage joint conditions, and optimize human performance.