The Human Muscular and Skeletal Systems

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The musculoskeletal system relies on its framework of connective tissue and muscle cells to work correctly so the human body can move and exercise. It is important to have a healthy musculoskeletal system to ensure that when an individual matures, their bones are healthy and not at risk of developing diseases such as osteoarthritis or osteoporosis.

The Muscular System

Connective Tissue

Connective tissue forms a framework wherein epithelial tissue rests, and is the most abundant of the primary tissues, derived from embryonic mesenchyme with 3 main components: cells, fibres, and ground substance. Their primary function is to connect various parts of the body (LibreTexts, 2020). The fibroblast is a common type of cell that synthesizes the extracellular matrix, secretes collagen, and plays a role in wound healing. The ground substance is an amorphous gel-like substance in the extracellular space where all other connective tissue elements are imbedded (King, 2015). It is transparent, colourless, and fills the spaces between fibres and cells. Fibres are strands of protein that make the connective tissue pliable, and are divided into 3 types:

Collagenous – the dominant fibre type in most connective tissues. Their primary function is to add strength, and they are composed of microfibrils which are only visible using electron microscopy due to their thickness being around 1-10µm (Slomianka, 2009). They provide cushioning to many areas of the body including skin. A tensile force of several hundred kg/cm2 is required to tear collagen fibres as they only stretch by 15-20% (Study, 2020).
Reticular – very delicate fibres which form fine networks. They can be found around the kidney, spleen, and lymph nodes (Rogers, 2020).
Elastic – these can stretch to about 150% of its original length; they resume their original length if the tensile forces applied are relaxed. They provide elasticity to the dermis and assist in recovery from deformation (Kesson et al, 2005).

Muscle Cells

Adipocytes are specialised for synthesis and storage of fat. They are important for thermoregulation, storing calories as lipids, and organising energy sources in response to hormonal stimulation (Guertin, 2020). Under microscopy, it appears bloated with triglycerides. White adipocytes store energy as a single large lipid droplet, whereas brown adipocytes store energy in multiple small lipid droplets specifically for use as fuel to generate body heat through thermogenesis.

Leukocytes are white blood cells that are found in connective tissue (Encyclopaedia Britannica, 2020). They have small amount of slightly basophilic cytoplasm and a darkly stained nucleus due to condensed chromatin. They provide various immune defence responses by ingesting foreign materials and cellular debris with macrophages, by destroying infection agents, or by producing antibodies with plasma cells. Mast cells promote the process of inflammation essential for healing.

Muscle Tissue Types

Muscle tissue is highly cellular with a rich supply of blood vessels. There are 3 types of muscle tissue cell:

Skeletal – voluntary muscle, therefore humans actively control its function through nerve signals. They are cylindrical, attached to the bone and form a distinct organ of muscle tissue, blood vessels, and tendons which cover our bones, allowing movement.
Smooth – involuntary muscle, which has no striation. They are spindle-shaped and are thousands of times shorter than skeletal muscle at around 30 to 200µm. They are found in the walls of hollow organs such as uterus and stomach. Skin also contains smooth muscle which allows hair to rise in response to cold temperatures.
Cardiac – known as myocardium, this tissue is specialised to form the heart. It contracts involuntarily to keep the heart pumping blood around the body using nerve impulses, and as this is lifelong it has high endurance. Its fibres are arranged in an interlocking pattern to ensure that each fibre is in contact with others to form a network (Eske, 2019). They have pacemaker cells which set its own contraction rhythm known as autorhythmicity.

Sliding Filament Theory

Muscle contraction is a process whereby force is generated within muscle tissue, increasing force being exerted on the tendons. This involves a chemo-mechanical energy conversion that is carried out by the actin/myosin complex activity, which generates force through ATP hydrolysis. Actin and myosin myofilaments stack and overlap in regular arrays to form sarcomeres, and therefore appear striated under microscopy (Gillespie, 2009). Actin and myosin filaments slide against each other, creating muscle contraction known as Sliding Filament Theory.

Skeletal Muscle Fibres

Due to the body’s diverse requirements of its skeletal muscles, there are different types of skeletal muscle fibres: type I, type IIa, and type IIb. Type I contract slowly over long periods of time, such as during walking. Type IIa have a fast contraction speed and are less reliant on oxygen so are suited to weight training. Type IIb fibres are suited to speed and strength activities due to their rapid contraction speed (IvyRose Holistic, 2020).

The Skeletal System

The skeleton consists of 206 bones, with male and female skeletons varying; the female pelvis is larger to accommodate childbirth. The axial skeleton contains 80 bones and consists of the spine, ribcage and skull. The appendicular skeleton contains 126 bones including upper limbs, pelvis and lower limbs. It makes functions like walking and arm movements possible. The skeletal system performs 5 key functions: 1) protection – structure of bones protects internal organs from damage, like the skull protects the brain; 2) shape – bone structure gives the body shape, and determines height; 3) support – skeleton supports the body by keeping internal organs in place, like the spine helping an individual to stand straight; 4) movement – muscles are attached to bones, so when they contract they cause bones to move; red cell production – spongy tissue inside long bones manage red cell production.

Connective Tissue in the Skeletal System

Connective tissue connects muscles to bone to connect joints together. There are 3 main types:

Tendons – fibrous connective tissue known as collagen, attaching muscle to bone and transmitting the force which the muscle exerts. This occurs because tendons are bound together tightly so when a muscle contracts, tension is created which causes movement. They do not require much oxygen due to limited blood supply, so anaerobic exercises are beneficial for tendon growth (Ekomaru, 2019). However because exercises like walking can be taxing, a thin layer called tendon sheath is found around the ankles to protect the tendon from damage.
Ligaments – short band of tough fibrous connective tissue which connects bones to bones at a joint. They are arranged in parallel bundles to increase their strength. There are two types: extracapsular which hold bones in place, provide stability, and prevent dislocation injuries; and intracapsular which stabilise the knee joint and provide further support.
Cartilage – a supple, elastic tissue which allows facial movement. It provides supportive structure for the external ear (elastic), septum (hyaline), and knee joint (fibro), and acts as a shock absorber to prevent abrasion (Biology Dictionary, 2019).

Types of Bones

The main types of bone in the human body:

Flat – somewhat flattened, and provide protection like a shield such as the skull, sternum, and pelvis.
Long – includes the femur, and they function to support the weight of the body for skeletal mobility.
Short – as long as they are wide and located in the wrist and ankle joints, they provide stability and some movement.
Irregular – they have various complex shapes which help to protect internal organs, such as the spinal cord.
Sesamoid – embedded in tendons, and small. They protect tendons from wear and tear with a smooth surface, such as the patella.

Long bones have some main features. Their membrane covers the outside of the diaphysis (known as the bone shaft, made of compact bone tissue) or the epiphysis, and its main function is to act as a protective covering to supply blood and nourishment to the bone through tiny openings carried by blood vessels. The diaphysis and epiphysis meet at the metaphysis and epiphyseal line, and remnants of the growth plate is found here. The articular cartilage is smooth translucent tissue found at movable joints, allowing gliding actions to occur during movement. There are 2 types of bone tissue: the cancellous bone is a spongy area crammed between compact tissues of the flat bones (Henderson, 2020). The compact bone is the rigid covering which forms the coarsened shell that gives bone its durability.

Types of Joints

Structural classification divides joints into fibrous, cartilaginous and synovial joints depending on the material composing the joint. Fibrous joints are held together by fibrous connective tissue, with no cavity present between the bones. Cartilaginous joints are connected by cartilage, and have some movement. Synovial joints have space between the bones, which is filled with synovial fluid for lubrication to reduce friction, allowing for greater movement such as the ball and socket joint like the shoulder.

Lever System

A lever system is a rigid bar which moves on a fixed point called the fulcrum when force is applied. Movement is possible by lever systems that are formed by our muscles and joints working together. In a first class lever, the fulcrum is the middle component between effort and load. An example is extension of the elbow. In a second class lever, the load is the middle component. An example is plantar flexion like calf raises. Third class lever is the most common, where the effort is the middle component. An example is flexion and extension at knee joint like running and jumping (Waldlen, 2020).

Muscles must work in pairs to produce effective movement. Depending on the movement, muscles can be termed: antagonist – opposes the movement of the agonist; synergist – stabilise a joint where movement occurs; fixator – accountable for sustaining the origin of the agonist.

Ossification

Ossification begins during prenatal development into adulthood, and is the process wherein bone tissue is created from cartilage. In intramembranous ossification, bone develops directly from mesenchymal tissue, but in endochondral ossification, bone develops by replacing hyaline cartilage.

Bone remodelling is a lifelong process wherein mature bone tissue is turned over through the work of osteoclasts, osteocytes and osteoblasts. This repairs flaws in bones resulting from everyday movements (Wakim & Grewal, 2020). Additionally, remodelling helps regulate mineral homeostasis by releasing minerals into the blood or vice versa.

Musculoskeletal Disorders

Disorders of the musculoskeletal system occur for many reasons. While trauma is an obvious cause such as broken bones, there are many other pathologies that are not caused by trauma.

Rickets affects adolescent bone development along with painful bone deformation due to lack of vitamin D or calcium, or rarely genetics. It can occur in adults, known as osteomalacia, and is due to diet (NHS, 2020).

Upon maturity, bone tissue naturally decreases leading to weaker bone strength. Osteoporosis can be delayed by eating a balanced diet and exercising regularly (NHS, 2020). Women are at higher risk of developing osteoporosis. If an individual takes calcium regularly at a younger age, they will have stronger bones and will not weaken as easily when older.

Conclusion

The musculoskeletal system is a varied, developed framework in which joints, muscle cells and connective tissue work together to perform all the daily activities of an individual. A healthy diet is vital to ensure that diseases are not obtained, and to maintain a healthy system.

References

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