How Actin and Myosin Interact With Each Other to Shorten a Sarcomere
Muscles are composed of two major protein filaments: a thick filament composed of the protein myosin and a thin filament composed of the protein actin. Muscle contraction occurs when these filaments slide over one another in a series of repetitive events. Let's see how myosin molecules play a role similar to the oars of a rower.
In this lesson, we will describe the basic components of a sarcomere and how they interact to contract our striated muscles.Now, let's take a look at an individual myofibril within the muscle cell. At this level, we can see the sarcomeres are butted up end to end, running the length of each myofibril. sarcomere is end to end A given myofibril contains approximately 10,000 sarcomeres, each of which is about 3 micrometers in length. While each sarcomere is small, several sarcomeres added together span the length of the muscle fiber. Each sarcomere consists of thick and thin bundles of proteins referred to as myofilaments. If we magnify a portion of the myofilaments, we can identify the molecules that compose them. Thick filaments contain myosin, while thin filaments contain actin. Actin and myosin collectively are referred to as the contractile proteins, which cause muscle shortening when they interact with each other. Additionally, thin filaments contain the regulatory proteins troponin and tropomyosin, which regulate interaction between the contractile proteins. The I band is that part of the sarcomere that contains thin filaments, while the A band contains an area of overlap between the thin and the thick filaments. As you can see, a single I band spans two neighboring sarcomeres. A Z line attaches those neighboring sarcomeres. The thin filaments are attached to the Z lines on each end of the sarcomere, while the thick filaments reside in the middle of the sarcomere.
In all cases, molecules which are enzymes that can utilise the energy stored in adenosine triphosphate (ATP) move along molecular tracks.
The process appears to require the development of tension between the front and rear of the cell, generating contractile force that eventually pulls the rear of the cell forward. This aspect of cell locomotion is impaired in mutants of Dictyostelium lacking myosin II, consistent with a role for myosin II in contracting the actin cortex and generating the force required for retraction of the trailing edge.
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