Locomotor System: Joints

Joints (Articulations)

Joints are structures responsible for joining different bones. They support the weight of the body and allow the movement of bones. 

According to their movement, articulations can be classified as:

• Synarthrosis: they do not allow any movement. The joints of the cranial bones are the most typical examples.
• Amphiarthrosis: they allow slight movements. The joints of the vertebrae are the most typical examples. 
• Diarthrosis or synovial joints: they allow complete movements. The bones are linked by ligaments. And the part of the bones in contact with other bones are in covered by cartilage. The space between the cartilaginous pieces that cover the bones in the diarthrosis are filled with a liquid that prevent them from friction and it is called synovial fluid. 

According to the type of movement, the synovial articulation can be classified as:

Locomotor System: Skeleton

Skeleton

Introduction

The skeleton is the main system to support the body. It forms an internal hard structure that supports other organs, protecting them. Some delicate organs are enclosed in a sort of armour made of bones. The brain, for instance, is enclosed in the cranium and the lungs and heart are protected by the rib cage.

The skeletal muscles are attached to bones and the contraction of different muscles move the bones they are joined to, providing movement to the body.

The bones are, besides, the main reservoir for calcium in human body. When the calcium is required in the blood, it is extracted from the bones.

Finally, in the bone marrow, which can be found in the interior of large bones. The haematopoiesis process is carried out to produce blood cells.

The bones have three parts:

• Periosteum: it is the outermost layer that surrounds the bone. It is made up of connective tissue and it is related to the growth the bone thickness.
• Compact bone: it is the hard part of the bone and its main structural component. It is made up of a hard extracellular matrix, rich in collagen and calcium, and cells that maintain this matrix that are called osteocytes. Another less abundant type of cells are the osteoclasts, located in the interior part of the compact bone and responsible for destroying extracellular matrix in order to release calcium into the blood. The extracellular matrix is extremely ordered, forming cylindrical structures called osteons (also known as harversian systems). In the centre of the osteons there is canal where the blood vessels and nerves can be found. Each bone is, in its compact part, made up of thousands of parallel osteons.
• Spongy bone: it is in the interior of large bones. It is a complex network of tissue that forms a trabecular system, similar to a sponge. The interior of this trabecular system is the place where the stem cells responsible for producing blood cells are nested. Due to this, it is the place where the haematopoietic process occurs.