The musculoskeletal system functions to provide movement, support, and stability, but also has important functions such as mineral storage, hematopoiesis (blood cell development) and, glycogen storage. It’s divided into two main parts:
The muscular system
The skeletal system provides a framework for skeletal muscle to attach to, whilst the skeletal muscle moves the skeleton, acting as a stimulus to maintain bone density. Therefore, both systems work synergistically to ensure optimal movement capabilities.
The Muscular System
Muscle tissue is specialized contractile tissue that can be classified into 3 groups:
Cardiac muscle (only found in your heart)
Smooth muscle (found in the walls of blood vessels and hollow organs)
Skeletal muscle (found attached to bones by tendons)
Cardiac and smooth muscle is controlled by the autonomic nervous system, meaning their functioning is not under voluntary control. We do not consciously control our heartbeats or the constriction or dilation of our blood vessels, but we can control the movement of our limbs. Therefore, skeletal muscles are under voluntary control and have a close connection to our nervous system.
Besides providing postural support and stability, the muscular system has another important function related to heat production. Interestingly, muscle tissue is one of the most metabolically active tissues in the body and produces approximately 85 % of heat due to muscle contraction. Muscle tissue is therefore essential for maintaining normal body temperature.
The Skeletal System
The skeletal system consists of 206 bones and their associated cartilages. Bones are rigid structures composed of calcified connective tissue, and bone cells that either build-up, break down, or form bone continuously. The skeletal system has important functions, such as:
Bones connect to skeletal muscles which allow movement and maintain posture and stability.
Protection of internal organs
The rib cage protects vital organs, such as the heart, lungs, and important arteries, and the skull protects the brain.
The bone marrow found in the spongy bones is the site where new blood cells such as red and white blood cells and platelets are formed.
The bones are a storage site for calcium and phosphorous and function to maintain the levels of these minerals in the blood.
There are a variety of disorders that can affect the musculoskeletal system that ranges from diseases to minor disabilities, with several of them being underpinned by genetic abnormalities.
The role of vitamin D in bone health is essential. It facilitates the absorption of calcium into the body through the digestive system, which then allows for the mineralization (growth and maintenance) of bone 1. Research suggests that severe bone abnormalities occur when vitamin D uptake or its receptor are dysfunctional, which has a knock-on effect on calcium and phosphorous absorption, and therefore bone development.
The vitamin D receptor
The vitamin D receptor (VDR) regulates the expression of numerous genes involved in calcium uptake and bone mass formation.
The main SNPs that have been detected in the VDR gene are Fok I, Bsm I, and Apa I 2 which have been linked to calcium metabolism and bone mass formation by controlling VDR expression. It has been observed that the presence of the Fok I SNP correlated with a decrease in bone density and an increased risk of osteoporosis 2. A role for the other SNPs has also been identified in bone mineralization abnormalities.
The vitamin D binding protein
This protein is synthesized in the liver and transports vitamin D to its target tissues. It is encoded by a gene that is also prone to SNPs that alter its transport function. Several studies have found that certain SNPs in this gene lead to lower levels of vitamin D in the blood 3, and therefore bone mineralization disorders such as osteoporosis.
Vitamin D enzymes
Just as the transport of vitamin D is crucial for bone development, its optimum metabolism is also required to ensure that blood levels of vitamin D are maintained. The conversion of vitamin D to a more active form is reliant on an enzyme that functions in the liver. SNPs in the gene of this enzyme have been linked to lower circulating levels of vitamin D 4.
Several growth factors are essential for the development of bone, particularly during early development. Their receptors play important roles in ensuring that the growth factor signal is transmitted correctly to bone cells to ensure their proper functioning. Research has found that if these receptors are expressed at abnormally high or low levels, then bone cell production and differentiation are greatly affected, leading to a condition known as bent bone dysplasia 5.
Absorption of dietary calcium is dependent on the presence of vitamin D. Calcium pumps populate the intestine to facilitate its uptake in order to maintain blood calcium levels. Research has shown that variations in the gene of the calcium pump lead to lowered bone mineral density and hypocalcemia (decreased blood calcium levels) 6,7. Interestingly, the dysregulation of this pump has also been linked to increased blood pressure 7.