Biology and Health Sciences

Key unit competency 

To be able to describe types of joints and relate the structure of joints to their functions.

Learning objectives 

After studying this unit, I should be able to: 

• Differentiate between hinge joint and ball and, socket joint. 

• Outline the function of bones, ligaments, tendons, cartilage, nerves and synovial fluid in the joint. 

• Analyse the structure of immovable and movable joints. 

• Demonstrate how antagonistic muscles bring about movement at the hinge joint. 

• Appreciate the link between skeletal muscles and bones in movement. 

• Develop good habits that maintain safety of the body joints.

Introduction 

In senior one, you learnt about skeletal systems of organisms. What did we say is the importance of the skeleton in our body? Can you give examples of movable and fixed joints? 

Look at the pictures below.

What is the role played by joints in pictures A and B above? We are able to move different parts of our body because of joints that occur between different bones. 

However, there are joints that are not movable at all. In this unit, you shall learn the different types of joints, their functions and how to keep our joints healthy. 

11.1 Types of joints 

A joint is a point where a bone meets another bone or bones. Except for pelvic, sacral, skull and sternal bones, joints allow movement and provide mechanical support. Muscles pull on bones to make joints move. Therefore, without joints, movement would not be possible. Most joints only allow slight movement while other joints allow no movement at all. 

Joints develop between adjacent bones. However, movement is important in determining the type of joint that develops. Restricted movement even in a freely movable joint can transform it to an immovable joint. 

Activity 11.1: Identifying the location of the movable and immovable joints in the body 

What to do 

1.  Try to locate the position of the movable and immovable joints in your body and note them down.  

2. Write down the examples of each of the following categories of  joints; 

3. Try to demonstrate while your partner watches the 360^o movement of the shoulder joint.

Study question

Look for the meaning of the following terms: bone, ligament, muscle, cartilage and tendon.

Bone – is a hard, tough connective tissue composed of mineral salts such as calcium and phosphate.  It is abundant in all animal skeletons. 

Cartilage – this is a skeletal connective tissue which is softer than a bone. It supports the trachea, nose, oesophagus and pinna of the ear. 

Ligament-this is a fibrous tissue which join one bone to another. They are elastic to allow movement at a joint. 

Tendon –a tough connective tissue which attaches a muscle to a bone. They are inelastic. 

Muscle- this is a contractile tissue specialised for contraction and relaxation. They cover the skeleton. Muscles are responsible for locomotion and other type of movement in animals. 

Joints are classified structurally depending on how the bones articulate or functionally depending on the degree of movement between the articulating bones. 

i. Structural classification 

ii. Functional classification 

(a)  Movable joints- allow some degree of movement. There are two categories; 

(b)  Immovable joints-permit very little or no mobility.

Fixed or immovable joints 

These are joints that do not allow any movement, for example, joints in the cranium commonly known as sutures. These joints have seams between the bones of the skull. They are not smooth but rather have interlocking finger-like processes that increase stability. Between the bones are dense fibrous tissues.

Fixed joints are also found in the pelvic girdle. 

Movable joints 

These are joints that allow movement of body parts to take place. Some movable joints allow only a small degree of movement while others allow a wide range of movement. There are several movable joints which include:

(a) Gliding joints 

These joints consist of two opposing flat surfaces that allow slight amount of gliding motion. They occur between the vertebrae. They have no fluid between them but instead have a larger cartilage between them known as intervertebral disc which reduces friction during movement. They are also found in the wrist and ankle.

(b) Synovial joints 

These joints are classified depending on the shape of the adjoining articular surface.  

They include: 

(i)  Saddle joints - comprises of two saddle-shaped articulating surfaces that are oriented at right angles to each other. Example is the joint of the thumb.

(ii) Hinge joints

Activity 11.2: Determining the position of  hinge joint 

1. When you open and close a door, how does it move? 

2.  Ask a friend to open and close a door as you watch. 

     • What do you notice?  

You may have realised that the door can move only towards you  or away from you. Notice too that the door is held in place by hinges which allow this movement. 

3.  Is there any other possible way the door can move?  Try it and find out.  

The movement of the door can be described as a back and forth movement only. When something held at a point, moves in such a way, it is said to move along one plane only.

4.  Now, try and move different parts of your arm or leg. At which joints do you think there is movement in one plane only? 

5.  Name the position of these joints. 

6.  You can use an arm model with a rubber band to demonstrate movement of joints. 

Hinge joints allow movement in one plane. The joints at the elbow, knee and finger joints show this back and forth movement as the only type of movement.

(iii) Pivot joints 

It consists of a cylindrical like bony process of one bone that rotates within a ring partially composed of bone and cartilage. Examples include articulation of the axis with the atlas and the articulation between the head of radius and the proximal end of the ulna. 

(iv) Ball and socket joint 

Move your arm at the elbow. Then move your arm at the shoulder in as many ways as possible. What was the difference in the type of movements at the elbow joint and at the shoulder joint? 

These types of joints allow the greatest flexibility of movement of all joints. It consists of a ball-shaped head end of one bone that fits into the cavity (socket) of an adjacent bone. This type of joint allows wide range of movement in almost any direction. The shoulder joint is an example of a ball and socket joint. The other is the hip joint. Bones at hinge joint and ball and-socket joint are held in place by ligaments which join them together.   

Structure of a synovial joint 

The articular surfaces of bones at synovial joint are covered with articular cartilage that provides a smooth surface where the bones meet. The adjoining bones are enclosed within a synovial cavity that is surrounded by a fibrous capsule that holds the bones together. Some portions of the capsule may thicken to form ligaments. 

A synovial membrane lines the synovial cavity. The membrane produces synovial fluid that is a complex mixture of cells and nutrients such as proteins, polysaccharides and fats. The synovial fluid covers the surfaces of the joint and provides a lubricating film. Examples include: hip joint, shoulder joint and knee joint.

Self-evaluation Test 11.1 

1.  Assume that a sharp object penetrated a synovial joint. From the list of structures write the order in which they would most likely be penetrated.

         1 Muscle or tendon   

         2 Ligament  

         3 Articular cartilage  

         4 Fibrous capsule  

         5 Skin  

         6 Synovial membrane 

                    A.  5,1,2,4,3,6 

                    B.  5,1,2,4,6,3 

                    C.  5,1,2,6,4,3 

                    D.  5,2,1,4,3,6 

2.  Which two joints move in the same way? 

            A.  Knee joint and shoulder joint 

            B.  Knee joint and elbow joint 

            C.  Neck joint and knee joint 

            D.  Neck joint and ankle joint 

3.  Which of the following is used to classify joints in the body? 

             A.  The number of bones that articulate. 

             B.  The embryonic tissues formed at the joint. 

             C.  The connective tissues that bind the bones together. 

             D.  The degree of movement at the joint. 

             E.  Both C and D.

11.2  Action of antagonistic muscles in the movement of a hinge joint

Roll up your sleeves. Make a fist with your hand tightly. Bend your arm at the elbow. Observe the inner part of the upper arm. You may have noted that the inner muscles of your upper arm bulge. What do you think causes the bulge? 

The biceps and triceps are referred to as antagonistic muscles.  They are found on the upper part of the forearm. To lift the arm, the biceps contracts while the triceps relaxes. Joints provide a fulcrum point for muscles to act. The joint action in the upward movement of the arm is flexion resulting to the decreasing of the joint angle. Biceps muscles are therefore called flexor muscles. To straighten the arm, triceps muscles contract while biceps relaxes; the joint action is extension that results to increase in the joint angle. 

Antagonistic muscles occur in pairs and oppose a specific movement such that when one muscle contracts, the other relaxes. This means that they never contract or relax at the same time. When the lower arm is straightened at the elbow, we say that the arm is extended. When the arm is bent at the elbow, we say the arm is flexed. When the arm is extended, the triceps muscles are in a contracted state and so, they are tight and short in length. The bicep muscles on the other hand are relaxed and therefore stretched.

Basically the movement of the arm is purely brought about by the opposing actions of the biceps and triceps.

Self-evaluation Test 11.2 

1.  Suppose the biceps muscles are paralysed, which arm movement will not be possible? 

2. Why must muscles work in antagonistic pairs at joints?

11.3 Practices that promote healthy bones and joints 

Discussion corner

1.  What practices can cause bone fractures? 

2. To maintain healthy bones and enhance quick recovery from fractures we should take food rich in calcium, zinc ions and magnesium, ions. 

3. Sunlight enhance strong bones. 

4. Elderly people develop a bent back. 

5. Present your work to the rest of the class. 

Keeping bones and joints healthy becomes more important as we age. Serious conditions such as osteoporosis and arthritis can make it hard to move around. They may lead to even more medical problems. There are simple things that you can do to reduce your likelihood of developing these conditions or at least prevent them from becoming worse. 

Simple lifestyle changes can help to protect bones and joints. They include: 

1. Eating healthy food appropriate for bone health, for example: 

2.  Avoiding alcohol consumption. Taking alcohol interferes with the body’s ability to absorb vitamins and minerals. It also results in an increase in hormones that deplete bone density. 

3.  Weight bearing exercises which helps the body to build more bone mass and maintain the bone density. Some examples of weight bearing exercises include: 

4.  Avoiding smoking. Smoking has been shown to cause bone mass depletion. Smoking also causes many other serious health problems as well.

5.  Maintaining a healthy body weight and posture. It is important to maintain a healthy body weight to keep your bones and joints healthy. If you are underweight, you are at a higher risk of bone loss. If you are overweight, you may be causing extra stress on your joints. This may put you at risk of developing osteoarthritis. 

Self-evaluation Test 11.3 

1.  Explain why it is important to maintain a healthy body weight.

2.  Give reasons why we should avoid alcohol and smoking. 

3.  Give the benefits of putting on the right size of shoes or non-fitting clothes.

Unit summary 

• A joint is a point where two bones meet (articulate). 

• Joints can be classified according to function, type of connective tissue that binds them or presence or absence of fluid.

• Fibrous joints allow little or no movement; cartilaginous joints are slightly movable while synovial joints allow considerable movement. 

• The shoulder joint is a ball and socket joint between the head of humerus and the glenoid cavity of scapula. This joint allows a wide range of movement. 

• Synovial joints include: hinge joints, ball and socket joints and pivot. 

• The elbow joint is a compound hinge joint between the humerus, sigmoid notch of ulna and radius. Movement at this joint is limited to flexion and extension. 

• Antagonistic muscles include the biceps which is the flexor muscle while triceps is the extensor muscle. 

• Good health habits, for example, eating a balanced diet rich in calcium and vitamin D, and plenty of physical activity is important in keeping bones healthy.