Skeletal system or skeleton is defined as the framework of
hard, articulated structures that provide physical support, attachment for
skeletal muscles, and protection for the bodies of animals. Like other
vertebrates, the human skeleton is on the inside of body and is called endoskeleton.
In the living body, the skeleton is very much alive. Bones and cartilages are
made of living cells and also have nerves and blood vessels in them. They grow
and have the ability to repair themselves.
Role of Skeletal System
The big functions of
skeletal system are protection, support and movements. In our body, skeleton
works very closely with the muscular system to help us move. Similarly,
skeleton provides protection to many internal organs e.g. skull protects brain,
vertebral column protects spinal cord and ribs protect most of our other
internal organs. Vertebral column also provides the main support to our body
mass.
Bone and Cartilage
Overall, the human skeleton is made of bony framework but in
certain parts, this framework is supplemented by cartilage.
Cartilage
It is a dense, clear blue-white firm connective tissue (but less strong than bone). The cells of cartilage are called chondrocytes. Each chondrocyte lies in a fluid space called lacuna present in the matrix of cartilage. The matrix of cartilage contains also collagen fibers. Blood vessels do not enter cartilage.
There are three types of cartilage:
Hyaline cartilage is strong yet flexible. It is found covering the ends of the long bones, in the nose, larynx, trachea and bronchial tubes.
Elastic cartilage is similar in structure to hyaline cartilage. It is also quite strong but has elasticity due to a network of elastic fibers in addition to collagen fibers. It is found in epiglottis, pinna etc.
Fibrous cartilage is very tough and less flexible due to large number of thick
collagen fibers present in knitted form. It is found in intervertebral discs.
Bone
It is the hardest connective tissue in body. Bones not only move, support and protect the various parts of body but also produce red and white blood cells and store minerals. The hard outer layer of a bone is called compact bone while the interior of bone is soft and porous. It is called spongy bone. Spongy bone contains blood vessels and bone marrow.
Like cartilage, the
matrix of bones also contains collagen. But it also contains minerals e.g.
calcium and phosphate. Cartilage contains a single type of cell.
On the other hand, bones contain different types of cell. The mature bone cells
are called osteocytes.
Components of Human Skeleton
The 206 bones in the adult human skeleton are organized into a
longitudinal axis i.e. axial skeleton and appendicular skeleton.
Axial skeleton consists
of the 80 bones in the head and trunk of body. It is composed of five parts.
Skull contains 22 bones out of which 8 are cranial bones (enclosing the brain)
and 14 are facial bones. There are 6 middle ear ossicles (3 in each ear). There
is also a hyoid bone in neck. Vertebral column contains 26 bones (vertebrae).
The chest is made of a chest bone called sternum and 24 (12 pairs) ribs.
Appendicular Skeleton is composed of 126 bones. Pectoral (shoulder) girdle is made of 4
bones. Arms have 6 bones. Both hands have 54 bones. Pelvic girdle (hips) has 2
bones. Legs have 6 bones. Both feet have 54 bones.
Types of Joints
A joint is the location at which two or more bones make contact. They allow movement and provide mechanical support. Joints can be classified on the basis of the degree of movement they allow.
Immoveable (Fixed) joints: Such joints allow no movement e.g. the joints between the skull bones.
Slightly
moveable joints: Such joints allow slight movements e.g. joints between the
vertebrae.
Moveable joints: They
allow a variety of movements e.g. shoulder joint, hip joint, elbow joint, knee
joint etc. There are many types of moveable joints in body. The main types are
hinge joints and ball-and-socket joints. Hinge joints move back and forth like
the hinge on a door and allow movements in one plane only. The knee and elbow
are hinge joints.
Ball-and-socket: These joints
allow movement in all directions. The hip and shoulder joints are ball-and-socket
joints.
Roles of Tendons and Ligaments Tendons and ligaments are bands of connective
tissue (made of collagen). Tendons are tough bands and attach muscles to bones.
When a muscle contracts tendon exerts a pulling force on the attached bone,
which moves as a result. Ligaments are strong but flexible bands and join one
bone to another at joints. They prevent dislocation of bones at joints.
Muscles and Movement
When bones move at joints, they produce movements. The movements in
bones are brought about by the contractions of skeletal muscles, which are
attached with them by tendons.
One end of a skeletal muscle is always attached with some
immoveable bone. This end of muscle is called the origin. Other end of muscle
is attached with a moveable bone and is called the insertion. When a muscle is
stimulated by a nerve impulse, it contracts to become shorter and thicker. Due
to this contraction, it pulls the moveable bone (at insertion).
Skeletal muscles are usually in pairs of antagonists. In an
antagonistic pair, both muscles do opposite jobs. When one muscle contracts the
other relaxes and this phenomenon is known as antagonism (antagonistic action).
When a muscle contracts and bends the joint, it is known as flexor muscle and
the movement is called flexion. When a muscle contracts and straightens
the joint, it is known as extensor muscle and the movement is called extension.
Its examples are Biceps and Triceps muscles.
Biceps is a
flexor muscle on the front of the upper arm bone.
Triceps is an
extensor muscle on the back of arm.
Both Biceps and Triceps muscle have their origin at pectoral girdle
and insertion at one of the two bones of forearm. When biceps contracts, the
forearm (insertion end) is pulled upward. It is the flexion of elbow joint.
During this flexion, triceps muscle relaxes. When triceps muscle contracts,
forearm is pulled down. It is the extension at elbow joint. During it, biceps
muscle relaxes. In this way, biceps and triceps make up an antagonistic pair of
muscles. Similar pairs, working antagonistically across other joints, provide
for almost all the movements of skeleton.
Disorders of Skeletal System
Important disorders of skeletal system are:
Osteoporosis is a bone disease in adults, especially
in old people. It is more common in old women. In osteoporosis, there is a
decrease in the density of bones due to loss of calcium and phosphorus. It may
be due to malnutrition (lack of proteins and Vitamin C), lack of physical
activities or deficiency of estrogen hormone.
Arthritis means “inflammation in joints”. It is also very common in old age and in women. It is characterised by pain and stiffness in joints (particularly in the weight bearing joints e.g. hip joint, ankle joint etc.). The treatment of arthritis includes pain killer and anti-inflammatory medicines. There are many types of arthritis:
Osteo-arthritis: It is
due to degeneration in the cartilage present at joints or due to decreased
lubricant production at joints. In this arthritis, fusion of the bones at joint
may occur and joints may become totally immoveable.
Rheumatoid arthritis: It involves the inflammation of the membranes at joints. Its
symptoms include fatigue, low-grade fever, pain and stiffness in joints.
Gout: It is characterised by the accumulation of uric acid crystals in moveable joints. It generally attacks the toe joints.