Joints: Definition, Types, Structure, Location And Examples

Biology

Topic: Meaning Of Joints And Types

Table Of Contents

  • Definition Of Joint
  • Types Of Joint
  • Structure Of A Joint
  • How Muscles Act On Bones To Cause Movement
  • Movement Of Fore Limbs Or Elbow Joint

Definition Of Joints

A joint is a region where two or more bones meet or articulate. Movement of the body or parts of the body is made possible with aid of joints and muscles. Joints are held together firmly by ligaments which are made of stiff, partially elastic fibres. Ligaments join bones to bones.

Types Of Joints

There are two main types of joints in mammals. These are immovable or fixed joints and movable joints.

1. Immovable or Fixed Joints

Immovable joints are joints or regions where two or more bones are firmly attached to one another or are firmly fixed by ligaments in such a way that movement of these bones are not possible.

Examples of places in the body where immovable joints are found are the skull and pelvic girdle. Immovable joints of the skull are called sutures.

2. Movable Joints

Movable joints are joints or regions where two or more bones meet in such a way that the bones move over each other, making movement possible. Most of the bones in the body are held by movable joints.

Types of Movable Joints

There are four main types of movable joints. These are:

i. Ball and socket joint
ii. Hinge joint
iii. Gliding or sliding joint
iv. Pivot joint

i. Ball and socket joints: Ball and socket joints allow movement in planes or directions (i.e up to 360 degree). This joint is found in the shoulder and hip joint. In the shoulder joint, the head of the humerus is a ball-like structure which fits into the glenoid cavity of the scapula (pectoral girdle) this allows movement in all directions. Similarly, in hip joint, the round head of the femur fits into the acetabulum of the pelvic girdle to form a freely articulating joint.

ii. Hinge joints: Hinge joint allows movement in one plane or direction only (i.e. up to 180 degree). Examples of hinge joints are found in the elbow and knee joints. The elbow joint is found between the humerus and ulna/radius. Similarly, the knee joint is found between the femur and tibia/fibula. As the name implies, each set of the bones making up the hinge joint functions like one half of a hinge.

iii. Gliding or sliding joints: The glinding joints allow the sliding of bones over one another. Examples of these joints are found at the wrist and ankle. They allow the hand and foot to be moved up and down or rotated slightly.

iv. Pivot joints: Pivot joints allow nodding or rotation of one part of the body on another. It is found between the atlas and axis vertebrae. The odontoid process of the axis acts as a pivot which allows the rotation of the head on the vertebral column (i.e. The atlas and the skull together rotate about the odontoid process).

Structure of a Joint

The main parts of a joint consist the following structures:

1. Ligaments: These are tough, partly elastic bands and tissue. They hold two bones together at a joint. In other words, they join one bone to another. They are able to accommodate movement at the joints because of their elastic nature.

ii. Tendons: Tendons are extension of connective tissue which surround the muscles. Unlike ligaments, they are non-elastic in nature. They connect muscles to bones.

iii. Articular cartilage: These are found at the surface of bones at joints. They play the role of cushioning the bones by protecting them from wear and tear during movement. They prevent the articulating (touching) surface from being worn out due to friction.

iv. Synovial membrane: Synovial membrane is responsible for the secretion of synovial fluid.

v. Synovial fluid: This is the fluid secreted by the synovial membrane. It lubricates the joints and thus reduces shock as well as friction between two bones.

vi. Capsule: Capsule is the space or sac which contains the synovial fluid.

How Muscles Act on Bones to Cause Movement

Muscles are bundles of long, thin cells enclosed in sheaths of connective tissue. Muscles are attached to the bones at two points. One of the points of attachment is called origin of the muscles. This is where the muscles are attached to an immovable or rigid bones, e.g. Shoulder blade. The other point of attachment is called insertion. This is where the muscles are attached to a movable bone, e.g. Radius. Muscles are attached to the bones by means of non-elastic, tough, whitish cord of fibrous materials called tendons. Muscles can only contract and relax but they cannot lengthen or expand. When a muscle contracts, it becomes shorter and thicker. Thus a pulling force is exerted on the bone in which it is attached. When a muscle relaxes, it lengthens and becomes thinner. Most muscles act in pairs called antagonistic pairs so that while one member is contracting, the other is relaxing. One member is called extensor and this tends to extend or straighten a limb by its contraction; the other member is called flexor, this bends or flexes the limb.

Movement of Fore Limbs or Elbow Joint

The muscles of the upper arm on humerus are referred to as biceps and triceps. Bicep muscles are found at the front of the humerus and are attached to the scapula by means of two tendons. The tricep muscles are found at the back of the humerus. The contraction and relaxation of these muscles bring about bending and straightening of the limb. The muscles are antagonistic muscles, that is, they work together in pairs in opposite ways. When an impulse is received from the central nervous system, the biceps (flexors) contract by becoming shorter and thicker, and at the same time, the triceps (extensors) relax. Since the tendons do not stretch, the shortening of the biceps results in a pull of the radius and as a result, the arm is bent.

On the other hand, when the triceps muscle (extensor muscles) contract, becoming shorter and thicker at the same time, the bicep muscles (flexors) relax, a force is exerted on the ulna and the arm is straightened as a result. Energy is involved in the movement of limbs. The muscular energy comes from the oxidation of glycogen which is stored within the muscles (i.e. Tissue respiration).