Joints – Structure & Function of Joints

Introduction To Human Skeleton:

The human skeleton is the internal framework of the body. It is composed of 270 bones at birth. This total decreases to 206 bones by adulthood after some bones have fused. The bone mass in the skeleton reaches maximum density around age 30. The human skeleton can be divided into the axial skeleton and the appendicular skeleton. The axial skeleton is formed by the vertebral column, the rib cage, and the skull. The appendicular skeleton, which is attached to the axial skeleton, is formed by the pectoral girdles, the pelvic, and the bones of the upper and lower limbs.

The individual bones are attached in such a way that a large variety of co-ordinate movements are made possible in different parts of the body. These movements are made possible by skeletal muscles, the fact that the bones act as levers, cartilage which reduces friction, and ligaments that prevent dislocation, and the presence of movable joints. The site or place where two or more bones of the skeleton are attached is called a joint or place of articulation.


Structure & Function of Joints:

A joint is formed where two or more bones come in close contact with the body and are attached by ligaments or cartilage.

Types of Joints:

Joints can be classified according to the degree and type of movement they allow. The following types of joints can be recognized:

a) Fibrous (or immovable) Joints: These joints are firmly held together by a thin layer of strong connective tissue. There is no movement between the bones such as the sutures of the skull and the teeth in their sockets.

b) Cartilaginous Joints: Cartilaginous joints are joints where the articular surfaces of the bones forming the joints are attached using white fibrocartilaginous discs and ligaments which allow only a limited degree of movement. Examples are the cartilaginous joint between the vertebrae, the cartilage in the symphysis which binds the pubic bones together at the front of the pelvic girdle, and the cartilage in the joint between the sacrum and the hip bone.

c) Synovial Joints: Synovial joints are freely movable joints and show free movement including flexion (bending), extension (straightening or bending), abduction (away from the middle of the body), adduction (towards the midline), rotation, supination (turning the palm up, inversion (turning the sole inward) and eversion (turning the sole outward).

Synovial joints
Fig: Synovial joints

Synovial joints are the most evolved and therefore most mobile type of joints. They possess the following characteristic features:

  • Their articular surfaces are covered with hyaline cartilage. This articular cartilage is avascular, non-nervous, and elastic. Lubricated with synovial fluid, the cartilage forms slippery surfaces for free movements.
  • Between the articular surfaces, there is a joint cavity filled with synovial fluid. The cavity may be partially or completely subdivided by an articular disc known as the meniscus.
  • The joint is surrounded by an articular capsule which is fibrous and is lined by a synovial membrane. Because of its rich nerve supply, the fibrous capsule is sensitive to stretches imposed by movements.
  • The synovial membrane lines the entire joint except the articular surfaces covered by hyaline cartilage. It is this membrane that secretes the slimy fluid called synovial fluid which lubricates the joint and nourishes the articular cartilage. Synovial joints are the only joints that have a space between the adjoining bones. This space, referred to as the synovial (or joint) cavity, is filled with synovial fluid.

Synovial joints can be subdivided into the following groups according to the type of movement they carry out.

Functions of the Skeleton:

1. Support: The skeleton is the framework of the body, it supports the softer tissues and provides points of attachment for most skeletal muscles.

2. Protection: The skeleton provides mechanical protection for many of the body’s internal organs, reducing the risk of injury to them.

For example, cranial bones protect the brain, vertebrae protect the spinal cord, and the ribcage protects the heart and lungs.

3. Assisting in Movement: Skeletal muscles are attached to bones, therefore when the associated muscles contract they cause bones to move.

4. Storage of Minerals: Bone tissues store several minerals, including calcium (Ca) and phosphorus (P). When required, the bone releases minerals into the blood, facilitating the balance of minerals in the body.

5. Production of Blood Cells: The red bone marrow inside some larger bones produce blood cells (RBC, WBC, and Platelets)

6. Storage of Chemical Energy: With increasing age some bone marrow changes from ‘redbone marrow’ to ‘yellow bone marrow’.

Yellow bone marrow consists mainly of adipose cells, and a few blood cells. It is an important chemical energy reserve.

Bone Structure:

Each bone in the skeleton contains two forms of tissue: compact (dense) bone that is a relatively solid and spongy (cancellous) bone that forms an open network of struts and plates. Compact bone is found on the external surface of the bone. Spongy bone is located inside the bone. The proportion of compact and spongy bone varies with the shape of the bone. Compact bone is thickest where stresses arrive from a limited range of directions. Spongy bone is located where bones are not heavily stressed or where stresses arrive from many directions. Spongy bone is much lighter than compact bone, which helps to reduce the weight of the skeleton and makes it easier for muscles to move the bones.

  • Tendons: These attach muscle to bone.
  • Ligaments: These attach bone to bone.

Skeletal muscles: These muscles contract to pull on tendons and move the bones of the skeleton. In addition to producing skeletal movement, muscles also maintain posture and body position, support soft tissues, guard entrances and exits to the digestive and urinary tracts, and maintain body temperature.

Structure of the bone nerves
Fig: Structure of the bone nerves

Nerves control the contraction of skeletal muscles, interpret sensory information, and coordinate the activities of the body’s organ systems.

Cartilage: This is a type of connective tissue. It is a firm gel-like substance. The body contains three major types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage.

Hyaline cartilage: It is the most common type of cartilage. This type of cartilage provides stiff but somewhat flexible support. Examples in adults include the tips of ribs (where they meet the sternum) and part of the nasal septum. Another example is articular cartilage, which is cartilage that covers the ends of bones within a joint. The surfaces of articular cartilage are slick and smooth, which reduces friction during joint movement.

Elastic cartilage: It provides support but can tolerate distortion without damage and return to its original shape. The external flap of the ear is one place where elastic cartilage can be found.

Fibrocartilage resists: Fibrocartilage resists compression, prevents bone-to-bone contact, and limits relative movement. Fibrocartilage can be found within the knee joint, between the pubic bones of the pelvis, and between the spinal vertebrae.

The disease of Bones and Joints:

• Rheumatoid arthritis

• Osteoporosis

• Gout

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