What are common mistakes students make with the sliding filament model?
1 Answer
Skeletal muscle cells (fibers) are very different from typical cells. The long fibers develop through the fusion of mesodermal cells (myoblasts) until they become very large and contain hundreds of nuclei.
Because the whole muscle fiber must contract at the same time, the signal (action potential) is conducted through the cell by transverse tubules (T tubules) which have the same properties as the sarcolemma.
Within each muscle fiber are hundreds of lengthwise subdivisions called myofibrils. Myofibrils are made up of bundles of the protein filaments (myofilaments) that are responsible for muscle contraction.
The two types of myofilaments are: thin filaments: made of the protein actin, and thick filaments: made of the protein myosin.
Ion pumps concentrate calcium ions (Ca++) in the cisternae. The calcium ions are released into the contractile units of the muscle (sarcomeres) at the beginning of a muscle contraction.
Two transverse tubules encircle each sarcomere near the 2 zones of overlap. When calcium ions are released by the sarcoplasmic reticulum, thin and thick filaments interact.
During a contraction, myosin heads interact with actin filaments to form cross-bridges. The myosin head pivots, producing motion.
Thick filaments contain titin strands that recoil after stretching.
