Muscular contraction begins with the nervous system sending a stimulus to the muscle fibers. This stimulus occurs at the neuromuscular junction. There is one neuromuscular junction for each muscle fiber. The neuromuscular junction contains structures that include the axon terminal of the neuron, the motor endplate, and the synaptic cleft, or neuromuscular cleft. From this point, the actions of the sliding filament theory begin. The actin and myosin filaments slide past each other, causing the muscle to shorten or lengthen. The filaments do not change in length during this action. Excitatory neurotransmitter acetylcholine (ACh), is released at the neuromuscular junction, after an action potential passes along the length of a neuron. ACh is released into the synaptic cleft between the axon terminal of the neuron and the muscle fiber. This occurs in direct response to the action potential (Coburn & Malek, 2012).
From that point, the Ach binds with ACh receptors, on the motor endplate of the muscle fiber. This occurs after the Ach moves across the synaptic cleft. This causes another action potential to be created. This new action potential moves along the sarcolemma of the muscle fiber. T-tubules are utilized by this action potential to travel to the interior of the muscle fiber. This movement causes the release of stored calcium from the sarcoplasmic reticulum. The calcium is released into the sarcoplasm. Once in the sarcoplasm, the calcium moves to the troponin molecules. The calcium then binds with the troponin molecules. These the troponin molecules are located along the length of the actin filaments. The shape of the troponin changes after the calcium binds to it. Tropomyosin is attached to the troponin. This change in shape causes the tropomyosin to expose the binding sites on actin, to the myosin head (Krans, 2010).
The exposed binding sites on the actin are able to attach to the myosin, forming a cross bridge. This myosin head of attachment pulls the actin filament toward the center of the sarcomere. The effort of the myosin pulling on the actin depletes the energy of the myosin. The depleted myosin must then detach from the cross bridge and reenergize itself. This requires a new adenosine triphosphate (ATP) molecule to be bound to the myosin. Once the ATP molecule is bound to the myosin, it can then detach and energize itself. The energizing of the myosin comes from the enzyme myosin adenosine triphosphatase (ATPase). The ATPase splits the ATP molecule. This energizes the myosin and allows for the cross bridge sequence to occur again. This sequence can continue as long as the muscle fiber is being stimulated to contract, by its motor neuron. The myosin pulling the actin toward the center of the sarcomere shortens the muscle. This process is the muscle contracting. The success or failure of this sequence is determined the external forces pulling against the cross bridge (Szent-Györgyi, 2004).
There are three basic types of contractions that can occur when the sliding filament theory is activated. Muscle contractions are usually pulling against an external force such as a barbell. When the myosin is pulling the actin through the cross bridge, the resistance determines the outcome. The contraction generates more force than the resistance, causing a concentric contraction. The external force is greater than the contraction force, causing a lengthening of the muscle. This is an eccentric contraction. The contraction force is equal to the resistance force, which stalls movement. This is an isometric contraction. The muscle will always attempt to shorten as the myosin pulls on the actin. The resistance level will cause one of the three types of contractions to occur. ATP is the primary fuel source for this sequence (Lefkowith, 2014).
Coburn, J.W., & Malek, M.H. (2012). NSCA’s essentials of personal training (2nd ed.). Champaign, IL: Human Kinetics.
Krans, J. (2010). The sliding filament theory of muscle contraction. Nature Education. Retrieved from http://www.nature.com/scitable/topicpage/the-sliding-filament-theory-of-muscle-contraction-14567666
Lefkowith, C. (2014). What does it all mean: Concentric, eccentric and isometric. Redefining Strength. Retrieved from https://redefiningstrength.com/mean-concentric-eccentric-isometric/
Szent-Györgyi, A. (2004). The early history of the biochemistry of muscle contraction. The Journal of General Physiology. Retrieved from http://jgp.rupress.org/content/123/6/631
MS, ISSA Master Trainer