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Musculoskeletal tissues 

The tissues which provide structure to the body and enable movement are part of the musculoskeletal system. The tissues in the this system include the bones, cartilage, joint, tendons, ligaments and muscles. In this section we will examine each of these types of tissues, so that in the next section on locomotion, we can understand how these structures work together to bring about movement.

• Bone: hard mineralized tissue that determines the structure of the body and provides attachments for muscles.
• Cartilage: flexible connective tissue that is usually found in many areas of the body including the joints between bones.
• Joints: region where bones meet, a variety of joint types allow for a range of movements in a number of different planes.
• Ligament: tissue that connects bones to other bones.
• Tendons: fibrous connective tissue that connects muscle to bone and transfers the force generated by the muscles into skeletal movement.
• Muscles: made up of fibres that are capable of contraction and therefore capable of bringing about movement.

Cartilage 

Cartilage is a tough semi-transparent flexible tissue that consists of a tough matrix or jelly-like substance. The matrix is made up of collagen (a protein) and proteins with special carbohydrate chains called proteoglycans. Cartilage is enclosed by a fibrous capsule called the perichondrium. It consists of living cells called chondrocytes which secrete a rubbery protein matrix called chondrin. Chondrocytes occur in small fluid-filled spaces called lacunae which are scattered throughout the matrix. There are no blood vessels or nerves in the matrix.

Cartilage and bone

Infant and young children do not have bones like those of adults. Their bones are made mostly of cartilage – a firm, elastic, fibrous material. As the individual grows and matures, the cartilage is gradually replaced by bone cells which deposit crystals of calcium carbonate and calcium phosphate. This process called ossification greatly increases the strength of the bone.

Joints 

A joint is a point at which two bones make contact. It allows movement in many planes.

1. Fibrous joints: Joins bones where no movement is allowed. An example of this includes the bones of your cranium (the skull).
2. Cartilaginous joints: These allow slight, restricted movement. An example is the discs between the vertebrae of the spine.
3. Synovial joints: These allow free movement in one or more directions. Examples include the joints of the pelvic and pectoral girdles. They also facilitate movements like standing, sitting, walking and running.

Another way of categorising joints is movable and immovable joints. Most joints in the skeleton are movable joints. Movable joints are also known as synovial joints. Synovial joints are characterised by the existence of capsules, which contain synovial fluid. The synovial fluid helps to prevent friction during movement.

Example of a synovial joint.

There are a number of different types of synovial joints. The four main types of synovial joints include:

1. Ball and socket joint: Found in structures such as the shoulder. It allows forwards/backwards, up/down and roundabout movement.
2. Hinge joint: Found in structures such as the elbow. It allows the forearm to move up and down and acts like the hinge of a door.
3. Pivot joint: Allows turning of the head in a rotational movement from side to side.
4. Gliding joint: Found in the wrist and foot. It allows bones to slide over one another.

Movement at joints

Joints occur where two bones meet. Different types of joints allow for different types of movements. In this activity you will need to identify the different joint types, identify where they are located in the body and describe their motion.

Instructions

For each of the following joints, you need to:

• give an example of their location in the body
• describe their motion

1. Fibrous joints
2. Ball and socket joints
3. Gliding joints
4. Hinge joints
5. Pivot joints

Tendons and ligaments 

Tendon and ligaments are dense bands of dense connective tissue. Ligaments join bone to bone, and tendons join muscles to bone. An example of a ligament is the anterior cruciate ligament (ACL) of the knee, and an example of a tendon is the Achilles tendon, which attaches your calf muscle to your heel. Tendons and ligaments are similar structures, but they have some important differences, which are summarised in table below.

Comparison of ligaments and tendons

      Antagonistic muscles 

Voluntary muscles are normally connected to at least two bones. The point of attachment to the movable bone is called the point of insertion and the point of attachment of a muscle to the immovable bone is called the origin. Most muscles work in pairs and when a muscle works it needs to have an agonist and an antagonist.

An agonist is a muscle that acts to move a limb out of a particular position (contraction). An antagonist is a muscle that acts in opposition to the specific movement generated by the agonist and is responsible for returning the limb back to its original position (relaxation). Antagonistic pairs of muscles are necessary because each muscle can only exert a pulling force. A muscle cannot push itself back to its starting position. Therefore another muscle is required to pull in the opposite direction in order to return the agonist muscle back to its starting position. An example of this can be found in the contraction and relaxation of the biceps and triceps muscles when moving your forearm.

Example: Biceps and triceps

In the case of the biceps the two bones involved are the scapula (origin) and the humerus (insertion). The biceps muscle gets its name from having two tendons attached to the scapula. The tendons join to form a single muscle body, and then splits again into two tendons, one of which inserts at the radius, and the other of which inserts at the ulna. When the biceps muscle contracts, the forearm is lifted or bent, decreasing the angle between the forearm and humerus and flexing your arm. This ability of the biceps to decrease the angle between the joints results in it being called a flexor muscle.

An Illustration of the triceps (extensor) and biceps (flexor) muscles

The biceps brachii muscle gets its name from being a two-headed muscle, attaching to the scapula at two points. Although it is commonly referred to as a `bicep’, biceps is the correct form even in the singular. Similarly, the triceps brachii muscle joins at three points, and should be referred to as the triceps, whether you are talking about one or both arms.

Straightening of the forearm

When the arm is bent the biceps cannot contract since it is already in a contracted state. Muscles can only cause movement by pulling as they contract, not by pushing when they relax. Therefore, the straightening of the arm is brought about by the contraction of the triceps muscle (an extensor muscle) as it increases the angle between forearm and humerus. The triceps has three points of origin, two on the humerus and one on the scapula, and a single point of insertion on the ulna.