Joints : Locomotion and Movement – Class – 11

Joints: Locomotion and Movement – Class-11: Joints are the structural arrangement of tissues in such a manner that it helps in joining the skeletal system together. They are essential for the movement of the body. They are the point of contact between bone and bone or bone and cartilage. The shape of a joint depends on its function.

What are Joints?

Joints can be defined as the point where two or more bones join together that may or may not show movement. It is an articulation where the bones, teeth, and cartilage connects together that helps in all types of movement in the body with the help of the force generated by the muscles. The types and functioning of the joint depend on its structure and the location where it is present.

Classification of Joints

Structural Classification

Based on the type of connective tissue that forms a joint it is classified into three types;

1. Fibrous Joint
2. Cartilaginous Joints
3. Synovial Joints

Fibrous Joints

The joints that are made of fibrous connective tissue are called fibrous joints. They are joined so tightly that they lack any joint cavity and thus are generally immobile or have a very slight movement. They can be of three types;

1. Suture: Two or more joints in the skull of an adult human are joined with the help of a suture. This makes the cranium immovable. E.g. sagittal suture, coronal suture, etc.
2. Syndesmosis: Bones are joined with the help of collagen fibers that keep them close together but prevent touching each other. E.g. Tibia-fibular joint, etc.
3. Gomphosis: It is a peg and socket type fibrous joint. E.g. fibrous ligament joins the root of the tooth to the socket of the gum.

Cartilaginous joints

The bones are joined by the help of hyaline cartilages which lack any joint cavity but provide a little degree of movement. This type of joint is called cartilaginous joint. This joint can be of three types;

1. Synchondrosis: It is a temporary joint seen in large bones that helps during bone growth but doesn’t help in movement. It is composed of a hyaline cartilage (epiphyseal plate) that joins the epiphysis and diaphysis. Usually, when the bone growth stops it is replaced by a bone. E.g. epiphyseal plate of the humerus.
2. Symphysis: It is also called secondary synchondrosis as it remains present even after the bone stops growing. Here, a thin layer of hyaline cartilage is present in between two bones that acts as a shock absorber and provides a point for sudden movement. E.g. pubis symphysis.

Synovial joints

The major type of joints that is present in our body provides a great degree of movement. The presence of smooth hyaline cartilage that covers the articulation point at the end of a bone along with a thick fluid called the synovial fluid that lubricates the joint makes the joint movable. This point is covered by a capsular structure to prevent the loss of the fluid.

Characteristics of a Synovial joint

1. The cavity in the joint is filled with synovial fluid.
2. The cavity is covered by a capsule made of fibrous connective tissue.
3. The inner surface of the cavity is lined by a synovial membrane.
4. The articulating ends of the bones are covered by soft hyaline cartilage to prevent frit ion between the bones.
5. The joint and the bone are held together by the help of ligaments to prevent separation.
6. Muscles are attached to the joints by the help of tendons.

Types of Synovial Joints

1. Gliding joint: Small-sized joints that are formed when flat or round surfaces articulate so that one bone can slide over the other in any possible direction. These are the most common type of synovial joints.  E.g. between the carpals of the wrist, tarsals of the foot, etc.
2. Hinge joint: Unidirectional joints which move like hinges of a door. Here the convex surface of one bone articulates with the concave surface of another bone to facilitate the movement. E.g. Knee joint, elbow joint, etc.
3. Ball and Socket joint: A joint in which the globe-like head of one bone fits into the cup-like cavity of another bone to allow free movement in all directions. This joint looks like a ball and socket. E.g. Shoulder and hip joint.
4. Pivot joint: A joint in which the rounded head of one bone is surrounded by a collar-like structure of another bone made partly of bone and part of the ligament. This type of joint only provides movement in one direction. E.g. atlas and axis joint in neck, etc.
5. Ellipsoid joint: Also called the condyloid joint. It is a joint in which the oval end of one bone fits into the depression of another bone. The joint is surrounded by ligaments and muscles in such a manner that movement in two planes is possible. E.g. joint between the metacarpals and phalanges of fingers, the joint between the radius and carpals of the wrist, etc.
6. Saddle joint: A joint in which the articulating bone is shaped like a saddle and its rider allows different types of movement straightening, bending, side to side, and rotation. In this joint concave area of one bone fits with the convex area of another bone. E.g. Joint between the trapezium and metacarpal of the thumb.

Functional Classification

In this type of classification, the bones are classified based on the degree of movement allowed by the joints. It is of three types;

Synarthrosis

This type of joint is immobile and provides a strong bond between the articulating bones. Found in places where protection of internal organs is required, and thus does not provide any mobility. E.g. sutures of the skull, syndesmosis joints, gomphosis joints, and synchondrosis joints.

Amphiarthrosis

This type of joint shows a little degree of movement and is made of cartilage. E.g. joints between the vertebrae, and symphysis.

Diarthrosis

This type of joint shows free movement to provide maximum mobility to different parts of the body like the synovial joints. It is of three types;

1. Uniaxial joints: When the movement takes place only in one plane. E.g. hinge joint and pivot joint.
2. Biaxial joints: When the movement takes place in two planes. E.g. ellipsoid joint, and saddle joint.
3. Multiaxial joints: When the movement takes place in many planes. E.g. gliding joint, and ball and socket joint.

FAQs on Joints

Q1: What is the basis of the difference between the uniaxial, biaxial, and multiaxial joints?

Answer:

These joints are differentiated based on the planes of movement they provide uniaxial joints provide movement only in one plane, biaxial joints provide movement in two planes, and multiaxial joints provide movement in multiple planes.

Q2: What are the primary functions of a joint?

Answer:

The primary functions of a joint are;

1. To connect bones.
2. To help bear the body weight.
3. To allow movement in some body structures.

Q3: Which layer of the articular capsule secretes the synovial fluid?

Answer:

The inner synovial layer made up of serous connective tissue absorbs and secretes the synovial fluid.

Q4: Where in the body sutures are found?

Answer:

Sutures are found only in the skull of the human body.