The Sliding Filament Model of Muscle Contraction and Tetanus

A central idea in physiology is the sliding filament model of muscle contraction, which describes muscle force and movement. The process involves the interaction between thin actin and thick myosin filaments in muscle fibers. Tetanus is a disease that interrupts this natural occurrence caused by the so-called tetanus toxin and is a unique case of a complex systemic disorder. At the molecular level, muscle contractions regulate the interaction between actin and myosin filaments. According to B Brenner, “Muscle contraction occurs when the thin actin and thick myosin filaments slide past each other. It is generally assumed that this process is driven by cross-bridges which extend from the myosin filaments and cyclically interact with the actin filaments as ATP is hydrolyzed” (1987). The sliding filament model describes this mechanism and emphasizes the dynamic interaction between two filaments.

When nerves are stimulated, calcium ions are released, which starts the process. Actin filament binding sites are exposed when these calcium ions attach to the regulating protein troponin. Next, the actin binding sites that are exposed are contacted by myosin cross-bridges, which are extensions of the thick myosin filaments. As a result of this contact, ATP is hydrolyzed, giving the myosin heads the energy to turn and draw the actin filaments into the center of the sarcomere. The myosin heads revert to their initial shape and become ready to start the next cycle as they release ADP and inorganic phosphate.

The Tetanus toxin, produced by the bacterium Clostridium tetani, affect the neuromuscular system and leads to hyperactivity of voluntary muscles. As described by Hassel, “Tetanus toxin causes hyperactivity of voluntary muscles in the form of rigidity and spasms. Rigidity is the tonic, involuntary contraction of muscles, while spasms are shorter lasting muscle contractions that can be elicited by stretching of the muscles or by sensory stimulation; they are termed reflex spasms” (2013). When someone has tetanus, it is common for someone to experience prolonged muscle contractions for long periods of time. These involuntary muscle contractions, results from the sustained activation of the sliding filament model. The toxin interferes with the normal regulation of muscle contraction, leading to a prolonged and intense contraction of muscles.

In addition to rigidity, tetanus toxin induces spasms, which are shorter-lasting muscle contractions. These spasms can be triggered by stretching of the muscles or sensory stimulation and are termed reflex spasms. The underlying mechanism involves the dis-regulation of the normal inhibitory signals that control muscle contraction, causing an uncontrolled and exaggerated response to stimuli. The interference of tetanus in the sliding filament model can be understood by considering its impact on the neuromuscular junction and the subsequent muscle contraction process. Tetanus toxin primarily affects the release of neurotransmitters, such as acetylcholine, at the neuromuscular junction. The sustained release of acetylcholine leads to continuous stimulation of the muscle fibers, resulting in a state of constant contraction.

The sliding filament model of muscle contraction is a physiological process that underlies voluntary movement. When it comes to tetanus, this process plays an important role in depicting the prolonged muscle contractions and the spasms. In my claymation, I depicted this interaction on a smaller scale between the filaments. In the video, you can see how muscle contractions happen, how tetanus makes the muscles stay contracted, and lastly, I show the troponin and tropomyosin bonding sights. 

Works Cited

Brenner B, Eisenberg E. The mechanism of muscle contraction. Biochemical, mechanical, and structural approaches to elucidate cross-bridge action in muscle. Basic Res Cardiol. 1987;82 Suppl 2:3-16. doi:10.1007/978-3-662-11289-2_1

Hassel B. Tetanus: Pathophysiology, Treatment, and the Possibility of Using Botulinum Toxin against Tetanus-Induced Rigidity and Spasms. Toxins. 2013; 5(1):73-83. https://doi.org/10.3390/toxins5010073