Skeletal Joints and Muscles
By Joanna George-Johnson, Contributor

Where bones meet is called joints. Joints facilitate flexibility and movement. There are different ways of categorising joints:

• By the make-up of the joints.
• The degree of movement that they allow.

Fibrous: Joints that connect bones without allowing any movement. The bones of your skull and pelvis are held together by fibrous joints.

Cartilaginous: These joints are joints in which the bones are attached by cartilage. These joints allow for only a little movement, e.g., between vertebrae bones, hold and cushion bones, but allow only tiny movements.

Synovial: They allow for much more movement than other joints. Cavities between bones in synovial joints are filled with synovial fluid. This fluid helps lubricate and protect the bones. Examples of such joints include our knees and elbows.

The diagram below shows the general composition of a synovial joint.





The diagrams below show examples of (a) hinge joint and (b) ball and socket joint.

At the synovial joints, the bones are held in place by ligaments. These are very tough fibrous tissues that stretch slightly to facilitate movement. (The same thing that people usually refers to as the 'gristle' in soup bones.)

Muscles

So far, we have learnt that our skeleton is made up of strong bones held together at joints by tough ligaments. With all these features, the skeleton would not be able to move. What are needed for movement to occur are muscles.

Muscles are attached to bones at strategic points so that they can pull the bones in different positions.

Muscles attached to bones are called skeletal muscles.

Muscles are only capable of pulling; they cannot push the bones. The way in which a muscle does this is called contraction.

When a muscle contracts it gets shorter and thicker; in doing so, it produces a very strong pull on the bones.

Muscles are attached to bones by tendons. Tendons are tough inelastic fibrous tissues that provide a strong anchor of muscles to bones.

In order to move the body, the muscles must contract in a very organised manner. One set contracts first to move the limb in one direction; the other contracts to move the limb in the opposite direction. This organisation is directed by the brain.

Muscles that work in this organised fashion together are called antagonistic.

The diagrams above show how the muscles of the arm work when it is being lifted or when it is lowered.

As demonstrated, when the arm is being lifted the biceps that are at the front contract (shorten). This pulls on the ulna, where it is attached, thus lifting the arm. During this time. the triceps that are at the back relax (remain lengthen).

Conversely, to lower the arm, the triceps contract and pull on the ulna, which lowers the arm. At the same time, the biceps relax.

Points to Remember

• A muscle which bends a limb at a joint, causing limb to flex is called the flexor muscle.
  
  
• The antagonistic muscle, which straightens and extends the limb when it contracts is called the extensor.
  
  
• The biceps are an example of flexor muscles, whereas the triceps are an example of extensor muscles.

Homework:

1. Label the diagram below:

2. Give the function of structure - in the diagram above.

3. Give the differences between ligaments and tendons in structure function and properties.

Joanna George-Johnson teaches at Ardenne High School masterbio@gmail.com.