Inter-Relationship of The Skeletal Muscular System to The Skeleton

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Inter-Relationship of The Skeletal Muscular System to The Skeleton

The human skeletal system is broadly divided into two, namely the axial and appendicular skeletal systems. The former is the central skeletal system and serves to protect vital body organs as well as act as an attachment site for muscles which cause bodily movement. The latter comprises of the bones in the upper and lower limbs and the girdle bones that connect limbs to the axial skeleton (Taylor, 2020). The muscular system is essentially divided into three types of muscles namely, visceral muscles which are found in internal organs such as blood vessels and intestines to allow movement of substances, cardiac muscle which is found in the heart and is responsible for pumping of blood in the body, and the skeletal muscle which form when many tiny progenitor cells attach themselves together to form long multinucleated fibers and voluntarily contracts to move body parts closer to the axial skeleton they are attached to. The skeletal muscles are attached to two bones through tendons which are tough bands of dense connective tissue with strong collagen fibers that firmly attach muscles to bones. Essentially, muscle movement is characterized by contraction which pulls tendons to move bones close to each other.

In detail, for muscle contraction to occur there must first be a stimulation of the muscle in the form of an impulse from a motor neuron (a nerve that connects to muscle) (Taylor, 2020). A motor neuron only stimulates a number of muscle fibers in a muscle. The individual motor neuron and the muscle fibers it stimulates is known as a motor unit. Neuromuscular junction is the junction between the motor neuron axon and the motor fibers it stimulates. When an impulse reaches the muscle fibers of a motor unit, it stimulates a reaction in each sarcomere between the actin and myosin filaments. The actin and myosin filaments within the sarcomeres of muscle fibers bind to form cross-bridges and slide past one another, thereby creating a contraction. In other words, the reaction created from the arrival of an impulse stimulates the ends of the myosin filament to reach forward and attach to the actin filament, then it pulls actin towards the middle of the sarcomere. This process causes the shortening of the sarcomere.  Additionally, this process occurs simultaneously in all sarcomeres.

The contraction process also entails Troponin which is a complex of three proteins that is attached to the protein Tropomyosin within an actin filament such that when the muscle is relaxed, the Tropomyosin stops the attachment sites for the myosin cross bridges. This is the process that prevents muscle contraction. Additionally, the muscle stores calcium in its sarcoplasmic reticulum in a fluid known as sarcoplasm. Thus, when a muscle is stimulated to contract by a nerve impulse, calcium channels in the sarcoplasmic reticulum usually open to allow the minerals into the fluid. Some of the calcium released however attach themselves to the protein Troponin which leads to a change in the cell of the muscle that moves Tropomyosin out of the way so as to allow the cross bridges to attach and cause a muscle contraction.

In a nutshell, the muscle contraction process can be subdivided into four stages, namely, muscle activation through the motor nerve stimulation of impulses which causes sarcoplasmic reticulum to release calcium; muscle contraction which occurs upon release of calcium into the muscle cell and its subsequent binding with troponin to allow actin and myosin cross bridges to bind and contract by making use of ATP; recharging which involves re-synthetization of ATP; and relaxation which occurs when nerve stimulation stops and calcium is pumped taken back into sarcoplasmic reticulum.

References

Taylor, T. (2020). Muscular System – Muscles of the Human Body. Innerbody. Retrieved 2 October 2020, from https://www.innerbody.com/image/musfov.html.