How Does a Muscle Contract? Discover the "Attach & Slide" Mechanism of Contraction
Recall from the previous section on muscle structure & function that part of the muscle tissue is made up of
myofibrils, which are made up of tiny myofilaments that are arranged in repeating sections
Sidenote: if this seems confusing, just remember that your muscles are mainly made up of 1)tiny
thin filaments, 2)membranes, 3)energy structures and 4)connective tissue. Just keep these 4 components in mind as
we move forward.
The myofilaments are composed of thick(myosin) and thin(actin+)
filaments that bind with one another to create a muscle contraction...
Basically, the thick filaments attach or bind to the thin filaments, and then pull or
slide them over the thick filaments to shorten the sarcomere. Think of the
sarcomere as one piece with thin and thick filaments. During a contraction, the thick attach with
the thin, and then pull the thin towards the center of the sarcomere, making the entire unit "shorter".
The places where the thick & thin filaments bind together are called
crossbridges. Keep the crossbridge cycle in mind because its essential to how a muscle actually
What happens during the actual crossbridge cycle?
Without getting too complicated, the simple explanation is that calcium[Ca++] is required to
get the process started. Then, the thick filament(myosin) binds with and pulls the thin filament a short
distance towards the center. After that the myosin attracts and binds a molecule of ATP, which
causes the thin filament to be released. The ATP molecule is then broken down and heat is released as a
ATP is basically the energy currency of the cell, intimately involved in muscle contractions.
In order for a muscle to contract and relax, ATP molecules are needed in sufficient concentrations. Likewise,
calcium ions[Ca++] are needed to kickstart the process. The thin & thick filaments use calcium
and ATP to work their magic of binding and releasing, which is what causes a contraction.
ATP, short for Adenosine Triphosphate, is a molecule that is the main energy source for the majority of
cellular functions, which includes the production of many things that the cell needs, such as
DNA, RNA and proteins. ATP is also involved in bridging the gap between metabolic reactions and actively
transporting molecules across cell membranes.
Calcium(Ca++) is all over the place, found inside the cell as well as outside of it and in our
bloodstream. It has many different functions with the most popular one being proper bone formation, but its also
intimately involved in the contraction of muscle cells. One of the places that (Ca++) ions are
stored is the mitochondria, an energy structure found inside a muscle fiber. As a mineral, calcium is also stored
inside the bone and released in a controlled manner for a variety of uses.