Body muscles are responsible for various functions. They are essential in locomotion, maintaining posture, and controlling circulatory systems. Muscles are classified into voluntary/involuntary, and striated/non-striated. There are therefore three distinct muscles in the human body; cardiac, smooth, and skeletal ( Wingerd, 2013) . Muscles contain long bundles of muscle fibers which are composed of thousands of myofibrils. The myofibrils are made of sarcomeres which are composed of myofilaments. The myofilaments contain myosin and actin. Myosin and actin contract through the cross-bridge cycle and interact through the sliding filament model ( Wingerd, 2013) .
The cross-bridge cycle involves Adenosine Triphosphate (ATP), troponins, calcium, adenosine diphosphate (ADP), and inorganic phosphate (Pi). When one molecule of ATP is hydrolyzed, there occurs ADP and Pi ( Lumen, 2019) . The cross-bridge cycle involves the following procedure.
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ATP binds to myosin. This makes myosin to become “charged” and raised to a higher energy level state.
ATP is hydrolyzed (into ADP and Pi) by enzyme ATPase.
The chemical energy released during the hydrolysis of ATP is used to prepare the head of myosin for bonding with actin when it is available.
ADP and Pi still remain attached to the myosin. At this stage, the myosin is at its highest energy level.
Contraction of the muscle is triggered by the action of calcium ions binding with troponin: a protein. The binding uncovers the binding sites on actin.
The myosin head bonds with the active sites on actin. This forms the cross-bridge.
The Pi is then released from the myosin. As the myosin is expending its energy to a low energy state, it pulls the actin thus shortening the sarcomere (power stroke). Consequentially, the muscle contracts.
The ADP then releases from the myosin causing it to lower in energy. ATP binds to the myosin again and detaches it from the actin active site. The process is then repeated ( Lumen, 2019) .
References
Lumen. (2019). Muscle contraction and locomotion | Boundless biology . Lumen Learning – Simple Book Production. https://courses.lumenlearning.com/boundless-biology/chapter/muscle-contraction-and-locomotion/
Wingerd, B. (2013). The human body: Concepts of anatomy and physiology . Lippincott Williams & Wilkins.
