How the Work Is Done
Muscles have only one function, and that is to contract. For skeletal muscle, this results in pulling both ends of the muscle (and the structures to which they are attached) closer together. This is mediated by the two contractile proteins, actin and myosin, and the sliding movement that occurs between them during a contraction.
Banding Pattern of Striated Muscle
Skeletal muscle (and cardiac muscle) is often described as striated muscle because of the striped appearance of the individual muscles cells when observed through a microscope. The light and dark bands are caused by the amount of light that may pass through a particular region of the muscle. The denser areas are darker, leaving the less dense regions lighter by comparison. These darker bands, called A bands or anisotropic bands, contain myosin filaments. Myosin is a type of protein; groups of the protein together are called myosin filaments and have a threadlike appearance. These proteins, shaped much like the human arm, are wrapped around the cylinder of the myosin filament and secured by a light chain, leaving the remainder of the myosin molecule (heavy chain) free to move. While this creates a high-density area of the muscle, each end of the dark band also contains actin molecules that insert themselves between and overlap the myosin filaments, resulting in the highest density area and the darkest part of the A band.
However, these actin filaments do not extend to the center of the myosin filaments. They only insert themselves about a quarter of the length of myosin on each end, leaving the middle portion of the A band composed of only myosin. This region is therefore less dense than the ends and is seen as a light region in the center of the A band called the H band. Additionally, in the very center of the A band, and also in the center of the H band, is a dark line consisting of structural molecules that assist in holding the myosin filaments in the proper position. This dark line is the M line and also marks the center of the contractile unit of skeletal muscle called the sarcomere.
On either side of the dark band are lighter regions called the isotropic (I) bands. These areas contain only actin filaments, which are much less dense than myosin. Each actin filament is composed of two strands of filamentous (F) actin that are twisted together. Each F actin strand is formed from polymers of G-actin or globular actin molecules. This gives the F actin the appearance of a pearl necklace, in which each pearl represents a G-actin molecule. The I band is interrupted by a dark line (Z line or Z disk) that defines the center of the I band and is composed of structural molecules much like those of the M line in the dark band. These molecules also assist in maintaining the proper spacing of the actin molecules, which is critical to the sliding filament action that occurs during a muscle contraction.
Anatomy of a Word
sarcomere
A sarcomere is the basic segment of a muscle, a structural and functional unit that aids in the contraction of the muscle. It contains an entire A band and two halves of I bands on each end. During a contraction, the actin molecules on the ends of the cell are pulled toward the M line. The Z lines are also pulled closer together and the sarcomere as a whole shortens.
Accessory Proteins
While the structural molecules of the M and Z lines are important for the alignment of the thin actin and thick myosin filaments, other molecules play essential roles in helping regulate a muscle contraction and in returning the muscle to the relaxed state. One such molecule is tropomyosin. Two of these filamentous molecules run along the grooves between the two F actin molecules (which creates a thin actin filament) and function to mask sites on each G action molecule where myosin can bind. When tropomyosin is in this position, the muscle is relaxed.
Attached to tropomyosin is a multiunit molecule called troponin. One of troponin’s three subunits, troponin I, binds to a region of the actin molecule. The troponin T subunit binds to the tropomyosin molecule. The final subunit, troponin C, is capable of binding to a calcium ion. Thus, calcium unmasks the myosin bound regions of the actin molecules and leads to a muscle contraction.
Calcium and Its Role
In the relaxed state, calcium is stored in muscle cells inside organelles called sarcoplasmic reticula. This calcium reservoir is responsible for releasing calcium upon nerve stimulation, but also for pumping calcium back inside when the nerve signal ceases, signaling the muscle to relax.
Voltage-gated calcium release channels are closely associated with the T-tubules described earlier. As the action potential spreads across the surface and down into the T-tubules, it leads to the rapid release of calcium from the sarcoplasmic reticulum and its rapid spread throughout the cytoplasm of the muscle cell where it begins a muscle contraction through contact with the accessory proteins.
Sliding Filament Motion
When the head of myosin connects with a G-actin, a cross bridge is formed. This is only made possible because prior to the connection, a molecule of ATP was bound inside the myosin head and split into separate molecules of ADP and phosphate, which remain inside. Once the cross bridge is formed, however, the phosphate molecule is released, which triggers a change in the myosin molecule called a power stroke. Since the myosin head is bound to actin, the power stroke is the action that results in the sliding of actin closer to the M line. Since this is happening on either side of the M line, each Z line is moved closer together and the muscle as a whole contracts.
This single contraction cycle only provides a fraction of the shortening distance for the muscles. Several repeated cycles of contraction will need to be accomplished for the entire muscle to shorten the full distance. Thus, each myosin head will need to undergo a power stroke, release, reset, and repeat a number of times to shorten the muscle fully. Consider a tug-of-war team. Each individual pulls on the rope trying to pull the other team across a line. During the contest, it will be necessary for members to release their grip and pull on the rope from a new position. If each member released at the same time, the team would lose. So the members release and form new grips in an alternating fashion. Such is the case for cross bridges for striated muscle.
What causes rigor mortis?
Rigor mortis, or the “stiffness of death,” occurs as skeletal muscle cells contract at the time of death from an explosion of electrical signaling throughout the body. This signal doesn’t cease and the muscles remain locked in continuing cycles of contraction. However, since ATP is required for contraction and for resetting, the cycles stop when the supply of ATP is exhausted, at which time the muscles are locked in the last contracted power stroke and unable to rest.
During the release and resetting period, ATP is hydrolyzed into ADP and phosphate, and as long as the myosin binding site is still exposed on actin, a new cross bridge and power stroke can occur.
At the end of a muscle contraction, the nerve stops sending neurotransmitters and stops the action potential on the muscle cell. This halts the release of calcium and initiates the active transporting of calcium back into the sarcoplasmic reticulum. With calcium no longer available to bind to the accessory protein troponin, tropomyosin slides back into its original position. This blocks the binding sites on actin and leaves myosin in its resting position.
Energy Sources for Contraction
Energy, in the form of ATP, is required for muscle to shorten. At the onset of muscle contraction, the raw material used to produce new ATP is pulled from the plasma of the blood and becomes circulating glucose, which can be used for the immediate production of energy. As the work continues or if the work intensifies and as plasma glucose is in short supply, muscle cells draw on plasma triglyceride resources, such as those present in fat cells. For high-intensity work, the demand for energy by muscle cells exceeds what can be supplied by plasma molecules or fat cells. In this case, the body recruits glucose from storage. The liver stores glucose as glycogen, which can be tapped to return glucose into the plasma, supplying the muscles with a rich source of raw material for immediate ATP production.