There are two main types of joints in mammals:
- movable joints
- immovable or fixed joints
1. Movable Joints: These are joints by which bones mover over one another. It is the commonest type of joint in man.
Types of Movable Joints
There are four main types of movable joints. These are:
- ball and socket joints
- hinge joints
- gliding or sliding joints
- pivot joints
(i) Ball and socket joints: Ball and socket joints allow movement in all directions (360 degree). Examples are the shoulder joints and hip joints. In the shoulder joint, the head of the humerus is a ball-like structure which fits into the glenoid cavity of the scapula. This allows movement in all directions. Similarly, in the hip joint, the round head of the femur fits into the acetabulum of the pelvis to form a freely articulating joints.
(ii) Hinge joints: Hinge joints allow movement in one direction only (up to 180 degree). Examples are the elbow and the knee. As the name implies, each set of bones making up hinge joints functions like one half of a hinge. The elbow joint is found between the humerus and ulna/radius, while the knee joint is found between the femur and tibia/fibula.
(iii) Gliding or sliding joints: Gliding or sliding joints allow the sliding of bones over one another. They allow the hand and foot to be moved up and down or rotated slightly. Examples are the wrist and ankle.
(iv) Pivot joints: Pivot joints allow the rotation or nodding of one part of the body on one another. They are 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
A joint is usually enclosed in a capsule made of ligaments. The articulating surfaces of the bones are covered by smooth cartilage. This prevents the surfaces of the bones from being worn out when they rub against one another. Between the surfaces of the articular cartilage is a sac lined by synovial membrane. It is filled with a thick, viscous fluid, the synovial fluid. This structure allows free movement of the bones meeting at the joint.
The features of each structure of a joint are explained below.
(i) Ligaments: These are tough, partly elastic bands of 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: These are extension of connective tissues which surround the muscles. Unlike the ligaments, they are non-elastic in nature. They connect muscles to bones.
(iii) Articular cartilage: These are found at the surfaces 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: This 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: This is the space or sac which contains the synovial fluid.
HOW MOVEMENT TAKES PLACE BY THE ACTION OF MUSCLES ON BONES
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 bone, e.g shoulder blade. The other point of attachment is called insertion. This is where muscles are attached to a movable bone, e,g radius.
Muscles are attached to bones by means of non-elastic, tough, whitish cord of fibrous materials called tendons. Muscles can only contract and relax but they cannot 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 expands 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 scapular 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, i.e, 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 stretc, the shortening of the triceps results in a pull of the radius and as a result, the arm is bent.
On the other hand, when 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).
0 comments:
Post a Comment