Skeletal System Definition

The skeletal system in humans is the mineralized internal framework and scaffolding of the body consisting of bones, joints and associated cartilages. An adult human has 206 bones in their body and variety of different joints.

Diagram of the skeleton
Image shows a human skeleton with the major bones labeled.

The skeleton can be divided into two components, the axial skeleton and the appendicular skeleton. The axial skeleton is formed around the central axis of the body and thus includes the skull, spine, and ribcage. It protects the brain, spinal cord, heart, lungs, esophagus and major sense organs like the eyes, ears, nose and tongue. The appendicular skeleton is related to the limbs and consists of the bones of the arms and legs, as well as the shoulder and hip girdles.

Composition of the Skeletal System

Bones serve a variety of functions, primary among them being movement. Two bones or cartilages are held together at a joint through tough connective tissues called ligaments. Muscles are securely attached to bones through flexible but inelastic connective tissue called tendons. Muscles, joints, tendons and ligaments are part of the intricate machinery that allows the movement of different bones.

Functionally, joints can be divided into three classes based on the range of movement they allow in the associated bones. Immovable joints are formed when two bones are held together by fibrous connective tissue with no synovial fluid. These kinds of joints hold the bones of the cranium together. Partially movable joints are also called cartilaginous joints and are present in the spine and ribs. The third type of joints are called synovial joints and have a fluid-filled synovial cavity that allows the interfacing bones the largest range of movement. Based on the structure of the synovial joints, they can be classified into 6 types, including the hinge joints of the fingers and the ball and socket joints of the hips and shoulders.

Each bone is made of complex sets of cells, tissues and a specialized extracellular matrix. The two main cell types are called osteoblasts and osteoclasts with mostly opposing functions. While osteoblasts are involved in the formation of bone, osteoclasts are associated with a reduction in bone mass. The extracellular matrix of the bone consists of collagen and other organic fibers as well as the inorganic component containing calcium salts such as hydroxyapatite. In the interior of bones, a soft tissue called the bone marrow plays an important role in immunity and hematopoiesis. The bone is also richly supplied with nerves and blood vessels.

Functions of the Skeletal System

The first and most apparent function of the skeletal system is to provide a framework for the body. The presence of a firm bony skeleton allows the organism to have a distinctive shape adapted towards a particular lifestyle. For instance, in a fast moving animal like the cheetah, the skeleton contains long, thin limb bones and an extremely flexible spine. The structure of the skeleton also allows it to absorb the impact of running at high speeds. The bones of birds are hollow, light and create a streamlined body adapted for flight. Many animals even have sexual dimorphism in their skeletons. In humans, while this dimorphism is fairly limited, there are differences in the angle of the pelvic bones, to accommodate pregnancy. Even invertebrates like snails and prawns often have hard exoskeletons to protect themselves from predators. Soft invertebrates without a skeletal system like hermit crabs, adopt the empty hard exoskeletons of other animals like snails or bivalves. All vertebrates contain an endoskeleton at some point in their life cycle, especially a vertebral column to support the central axis of the body.

The rigid endoskeleton allows the body to rise up above the ground or stand upright, and bears the weight of the organism, and provides the scaffolding for movement. Muscles generate the force required to move bones at joints. Muscle fibers contain actin and myosin, two protein filaments that can slide past each other to change the length of the muscle. When a nerve impulse arrives at the neuromuscular junction, it signals the muscle to contract. The force generated by the contracting muscle either pulls two bones together or apart, based on the nature of the interaction between the muscle and joint.

The central part of a bone contains the bone marrow, the primary site for hematopoiesis in adult humans. There are two types of bone marrow in adults – 50% is made of red bone marrow containing hematopoietic stem cells and supportive tissue. The rest is yellow bone marrow made of fat and its proportion increases with age. The marrow is richly vascularized, and in newborns, is made completely of red bone marrow. Even in adults, the proportion of yellow bone marrow can decrease to accommodate a sudden spurt in blood cell production. This could be due to extreme blood loss after injury. The bone marrow composition also changes during pregnancy and lactation in mammals. Over the course of gestation, blood volume increases by about 1.5 liters, and even the concentration of red blood cells and white blood cells increase.

The bones of the skeletal system act as a storehouse for calcium ions, changing the quantum of mineralized deposits within bones to maintain plasma calcium ion concentration within a narrow range. Calcium ions can affect crucial sodium ion channels in the plasma membrane of every cell, thereby affecting overall homeostasis. For this reason, changes to the concentration of calcium ions have particularly adverse effects on excitable cells in the nervous system, and in cardiac, skeletal and smooth muscle. Different interacting hormones maintain the balance of calcium ions in the plasma and bones, especially the parathyroid hormone secreted from the parathyroid glands in the neck.

Osteoblasts in bone also have an endocrine function, secreting a hormone called osteocalcin. It requires vitamin K to be synthesized and is an anabolic hormone. It mediates an increase in insulin levels, and increases the sensitivity of the body to insulin. Osteocalcin contributes towards an increase in bone mass and bone mineralization.

Diseases of the Skeletal System

Diseases of the skeletal system could be confined to one section of the skeleton such as changes to the curvature of the spine, or be a generic disorder affecting all bones and joints such as arthritis or osteoporosis.

Curvature of the Spine

The bulk of the weight of the upper body is transmitted along the central axis towards the legs. When the bones or muscles of the back or not functioning optimally, it can lead initially to accommodative changes in posture and thereafter to pain, injury or permanent deformity. Since the spine surrounds the spinal cord, abnormalities in the skeletal structure of the spine can affect the nervous system, either manifesting as pain, tingling or numbing in the extremities. Additionally, the spine supports the ribcage, enclosing the heart, lungs and diaphragm. Thus, spinal deformities can also lead to shortness of breath, palpitations or even cardiac arrhythmias.

The spine in healthy individuals is S-shaped, with a convex curvature for the thoracic region and the concave tilt in the cervical and lumbar regions. This shape for the spine is ideally suited for an upright walking posture. If either the thoracic or lumbar regions have a change in curvature or there is sideways bend to the spine, it can lead to back pain, difficulty with breathing, digestion, mobility and reproduction. Kyphosis is the term for the convex curve of the thoracic region and excessive curvature in this region is called hyperkyphosis. Extreme hyperkyphosis presents as a hunchback. This could arise from genetic factors or poor posture due to obesity or osteoporosis or arthritis. The normal concave structure of the lumbar region is called lordosis, and when the region is overly arched, it is called lumbar hyperlordosis. In hyperlordosis, shoulders appear to be pushed back, while the abdominal region seems to be jutting forward.

Lordosis
Image shows a person with hyperlordosis. In a healthy spine, the midpoint of the spine (A) would be directly above the knee (B).

Hyperlordosis can arise from genetic factors, poor posture, or even deficient muscle strength. When the spine has a sideways tilt, or a lateral bend, it is called scoliosis and could be associated with both hyperkyphosis and hyperlordosis.

Osteoporosis

Osteoporosis is a condition marked by bone resorption. This reduces bone mass and density, thereby enhancing the probability of fractures from stresses as minor as sneezing. Although osteoporosis is commonly associated with ageing, even smoking, obesity, diet, some medications and alcohol consumption can contribute to the progression of the ailment. Estrogen in premenopausal women has a protective role in bone health. Weight training, exercise, and a diet containing adequate calcium, iron, phosphorous as well as Vitamin D, help in enhancing bone density and bone mass. There is some evidence that the pH of blood plays a role in the release of calcium stores from bones and the extent of bone mineralization, since calcium salts are often used as buffers in acidic environments in the body. Hormones that affect osteoclast or osteoblast function, also affect bone density. Parathyroid hormone, estrogen and even testosterone help in maintaining bone health.

Arthritis

Arthritis includes a number of joint disorders that are characterized by stiffness, inflammation and pain. While there is a range of potential causes, arthritis usually worsens with age, affecting the joints that are used most frequently – especially the joints in fingers, hips and knees. Arthritis, therefore causes disability, restricts movement and impairs fine motor skills.

Interesting Facts

  • Three bones in the inner ear, called malleus, incus and stapes, are the smallest bones in the human body. The thigh bone or femur, is the largest bone.
  • The hyoid bone situated behind the lower jaw is also called a ‘floating’ bone because it is not part of any joint, and is not directly attached to any other bone.
  • The position of the hyoid bone makes it extremely resistant to fractures. However, autopsies that reveal a broken hyoid bone indicate death from strangulation.
  • Plaster of Paris (POP) casts for immobilizing a fractured bone were invented in 1851.
  • Newborn babies have about 300 bones. Many of these bones fuse together to form the 206 bones of the adult.
  • Though teeth are considered a part of the skeletal system, they aren’t counted among the 206 bones.

Related Biology Terms

  • Bivalve – Class of molluscs containing an exoskeleton made of two hinged parts.
  • Hematopoiesis – Process of creating new blood cells from hematopoietic stem cells.
  • Hydroxyapatite – Naturally occurring calcium salt with the formula Ca5(PO4)3(OH) that makes up 70% of bone weight. Modified hydroxyapatite forms the mineral component of dentin and enamel in teeth.
  • Synovial Fluid – Viscous fluid found in the cavities of some joints, containing proteins from blood plasma as well as secreted biomolecules such as hyaluronan. Main function is to reduce friction in the joint.

Quiz

1. Which of these is an inflammatory condition?
A. Lordosis
B. Scoliosis
C. Kyphosis
D. Arthritis

Answer to Question #1

2. How would regularly consuming cola affect bone health?
A. Cola decreases blood pH, which leads to mobilization of calcium stores of the bone
B. Cola has high sugar content that leads to more energy for forming bone
C. Cola drinks change the hormone secretion of the body and signal an increase in bone mineralization
D. All of the above

Answer to Question #2

3. Why is the hyoid bone called a floating bone?
A. It is surrounded by synovial fluid
B. It does not directly interact with any other bone
C. It looks like a log
D. All of the above

Answer to Question #3
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