Human Musculoskeletal System: Structure and Functions

Each system of our body has different structures and functions that work together to keep our bodies healthy and functioning. Musculoskeletal system consists of 2 systems- muscular and skeletal systems. These systems are responsible for body movements, protection of internal organs, support, and posture.

Skeletal system is divided into two skeletons: axial and appendicular skeleton. Axial skeleton contains the skull, vertebral column, and thoracic cage. Main functions are protection of our brain, spinal cord, organs in the thorax and support of the head, neck, and trunk. Axial skeleton plays an important role in movement and posture. Appendicular skeleton consists of upper and lower limbs that are appended (attached) to the axial skeleton with pectoral girdle or pelvic girdle. Pectoral or shoulder girdle consists of the scapula and clavicle. It connects arms to axial skeleton. Pelvic girdle is formed by sacrum (part of the axial skeleton) and hip bones to connect legs to axial skeleton. Main function of the appendicular skeleton is movement.

Skeletons consist of bones, cartilages, ligaments, and joints. Bones are made of compact and spongy bone. Compact bone is smooth and dense and spongy bone is like honeycomb made of trabeculae and spaces that are filled with yellow or red bone marrow. Classification of bones is by their shape. Long bones are bigger in length, smaller in width and they have enlarged ends. Most of the bone contains compact bone and ends of long bones consist of spongy bone. Examples of long bones are limbs like bones in the forearm, fingers, legs. Short bones are cube-shaped bones that mostly contain spongy bones. Compact bones make an outer layer of the short bones. Examples of short bones are wrist and ankles. Flat bones are thin, flattened and curved and consist of spongy bone between 2 thin layers of compact bone. Most of the skull bones, ribs and breastbone are flat bones. Irregular bones are bones that cannot be put in any of the previously mentioned groups like vertebrae. Irregular bones are similar to short bones. They are mainly made of compact bone surrounded by compact bone.

Bones are made of 3 types of cells. Osteoblasts are immature bone cells. Their function is to build a new bone when it is growing, healing or remodeling. To make bones strong, osteoblasts are making collagen fibers that are mineralized. Osteocytes are mature bone cells located in lacunae. Their function is maintenance of the bone matrix. They can also trigger bone remodeling for maintenance of calcium homeostasis. Osteoclasts are cells that break down bone matrix during a process called bone resorption.

If osteoclasts are more active than osteoblasts, osteoporosis can occur. Osteoporosis is a disease which results in bone loss and decreased bone density. This can lead to higher risk of fractures.

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Joints and ligaments bind bones together and give mobility to the skeleton. Ligaments allow joint movement and make sure that movement does not take another direction. Joints are classified structurally and functionally. Structurally joints are classified as fibrous, cartilaginous, and synovial joints. Fibrous joints tightly connect bones by fibrous tissue. Fibrous joints can be found in the skull, where teeth meet facial bones and between tibia and fibula (lower leg bones). Cartilaginous joints have 2 types: synchondroses and symphyses. Synchondroses are immovable and linked by hyaline cartilage, but symphyses are slightly movable and linked by fibrocartilage discs. Synovial joints are found in the limbs where bone ends are separated by a joint cavity. These joints contain articular cartilage that covers bone ends, articular capsule that covers joint surface, joint cavity that contains synovial fluid and reinforcing ligaments that are the fibrous layer of the capsule. Functionally joints are classified as immovable, slightly movable and freely movable joints. Immovable joints are found in places where organ protection is important and freely movable joints are found in the limbs.

Skeletal system is responsible for other organ system protection, fat and mineral storage in bones, formation of blood cells in red bone marrow, support, and movement. (Mitchell, 2015, p.43) For movement to happen, each bone is attached to muscles with tendons. Muscles are responsible for movement of skeleton, movement in organ systems, for example heart contractions, maintenance of posture, heat generation.

There are 4 characteristics and 3 types of muscles. Excitability is the ability of a cell to receive and respond by changing membrane potential. Contractility is the ability to shorten (contract) when stimulated. Extensibility is the ability to stretch beyond their resting length while relaxed. Elasticity is the ability to recoil its resting length after being stretched. Muscles have 3 categories: skeletal, cardiac, and smooth muscle.

Skeletal muscles are attached to the skeleton and have a striated look. This is voluntary muscle and is consciously controlled. Sometimes skeletal muscle contractions can happen involuntarily when reflex is activated. Muscle fibers have multiple nuclei and contain myofibrils. Myofibrils are contractile elements made of myofilaments: actin and myosin. Actin is thin filament and myosin is thick filament. For muscle to contract, thin filaments slide past the thick filaments and muscle contracts. For muscles to remain healthy, they need to be active. If muscle activity is decreased, muscles get immobilized and their strength declines. This results in a disease called disuse atrophy.

Smooth muscle mostly makes up walls of hollow organs except the heart. This is involuntary muscle. Muscle fibers of smooth muscle are spindle-shaped with one nucleus. Fibers are often arranged in two layers that alternately contract and relax, changing shape and size of the organ.

Heart is made up of cardiac muscle. This type of muscle like smooth muscle is involuntary. Cardiac muscle cells are striated and have one nucleus. Cardiac muscle contractions are controlled by the heart's pacemaker and speed of contractions is controlled by the nervous system.

References

1. Marieb, E. N., & Hoehn, K. (2018). Human anatomy & physiology (11th ed.). Pearson Education Limited.
2. Marieb, E. N., & Keller, S. M. (2017). Essentials of Human Anatomy & Physiology (12th ed.). Pearson Education Limited.
3. Mitchell, D. T. (2015). Introduction to Anatomy & Physiology: The musculoskeletal system. New Leaf Publishing Group.
4. Palastanga, N., & Soames, R. W. (2011). Anatomy and Human Movement: Structure and function (6th ed.). Elsevier Health Sciences.