Skeletal muscle is also called striated muscle, because when it is viewed under polarized light or stained with an indicator, you can see alternating stripes of light and dark.
Skeletal muscle has a complex structure that is essential to how it contracts. We will tease apart a skeletal muscle, starting with the largest structures and working our way to the smaller ones. The basic action of any muscle is contraction. For example, when you think about moving your arm using your biceps muscle, your brain sends a signal down a nerve cell telling your biceps muscle to contract. The amount of force that the muscle creates varies -- the muscle can contract a little or a lot depending on the signal that the nerve sends. All that any muscle can do is create contraction force.
A muscle is a bundle of many cells called fibers. You can think of muscle fibers as long cylinders, and compared to other cells in your body, muscle fibers are quite big. They are from about 1 to 40 microns long and 10 to 100 microns in diameter. For comparison, a strand of hair is about 100 microns in diameter, and a typical cell in your body is about 10 microns in diameter. A muscle fiber contains many myofibrils, which are cylinders of muscle proteins. These proteins allow a muscle cell to contract. Myofibrils contain two types of filaments that run along the long axis of the fiber, and these filaments are arranged in hexagonal patterns. There are thick and thin filaments. Each thick filament is surrounded by six thin filaments.
Thick and thin filaments are attached to another structure called the Z-disk or Z-line, which runs perpendicular to the long axis of the fiber (the myofibril that runs from one Z-line to another is called a sarcomere). Running vertically down the Z-line is a small tube called the transverse or T-tubule, which is actually part of the cell membrane that extends deep inside the fiber. Inside the fiber, stretching along the long axis between T-tubules, is a membrane system called the sarcoplasmic reticulum, which stores and releases the calcium ions that trigger muscle contraction.
Contracting a Muscle During contraction, the thin filaments slide past the thick filaments, shortening the sarcomere. The thick and thin filaments do the actual work of a muscle, and the way they do this is pretty cool. Thick filaments are made of a protein called myosin. At the molecular level, a thick filament is a shaft of myosin molecules arranged in a cylinder. Thin filaments are made of another protein called actin. The thin filaments look like two strands of pearls twisted around each other.
During contraction, the myosin thick filaments grab on to the actin thin filaments by forming crossbridges. The thick filaments pull the thin filaments past them, making the sarcomere shorter. In a muscle fiber, the signal for contraction is synchronized over the entire fiber so that all of the myofibrils that make up the sarcomere shorten simultaneously.
There are two structures in the grooves of each thin filament that enable the thin filaments to slide along the thick ones: a long, rod-like protein called tropomyosin and a shorter, bead-like protein complex called troponin. Troponin and tropomyosin are the molecular switches that control the interaction of actin and myosin during contraction.
While the sliding of filaments explains how the muscle shortens, it does not explain how the muscle creates the force required for shortening. To understand how this force is created, let's think about how you pull something up with a rope:
Grab the rope with both hands, arms extended.
Loosen your grip with one hand, let's say the left hand, and maintain your grip with the right.
With your right hand holding the rope, change your right arm's shape to shorten its reach and pull the rope toward you.
Grab the rope with your extended left hand and release your right hand's grip.
Change your left arm's shape to shorten it and pull the rope, returning your right arm to its original extended position so it can grab the rope.
Repeat steps 2 through 5, alternating arms, until you finish.
Muscles create force by cycling myosin crossbridges.
To understand how muscle creates force, let's apply the rope example. Myosin molecules are golf-club shaped. For our example, the myosin clubhead (along with the crossbridge it forms) is your arm, and the actin filament is the rope:
During contraction, the myosin molecule forms a chemical bond with an actin molecule on the thin filament (gripping the rope). This chemical bond is the crossbridge. For clarity, only one cross-bridge is shown in the figure above (focusing on one arm).
Initially, the crossbridge is extended (your arm extending) with adenosine diphosphate (ADP) and inorganic phosphate (Pi) attached to the myosin.
As soon as the crossbridge is formed, the myosin head bends (your arm shortening), thereby creating force and sliding the actin filament past the myosin (pulling the rope). This process is called the power stroke. During the power stroke, myosin releases the ADP and Pi.
Once ADP and Pi are released, a molecule of adenosine triphosphate (ATP) binds to the myosin. When the ATP binds, the myosin releases the actin molecule (letting go of the rope).
When the actin is released, the ATP molecule gets split into ADP and Pi by the myosin. The energy from the ATP resets the myosin head to its original position (re-extending your arm).
The process is repeated. The actions of the myosin molecules are not synchronized -- at any given moment, some myosins are attaching to the actin filament (gripping the rope), others are creating force (pulling the rope) and others are releasing the actin filament (releasing the rope).
The contractions of all muscles are triggered by electrical impulses, whether transmitted by nerve cells, created internally (as with a pacemaker) or applied externally (as with an electrical-shock stimulus).
Triggering and Reversing Contraction The coupling process leading from electrical signal (excitation) to contraction in skeletal muscle.
The trigger for a muscle contraction is an electrical impulse. The electrical signal sets off a series of events that lead to crossbridge cycling between myosin and actin, which generates force. The series of events is slightly different between skeletal, smooth and cardiac muscle. Let's take a look at what occurs within a skeletal muscle, from excitation to contraction to relaxation:
An electrical signal (action potential) travels down a nerve cell, causing it to release a chemical message (neurotransmitter) into a small gap between the nerve cell and muscle cell. This gap is called the synapse.
The neurotransmitter crosses the gap, binds to a protein (receptor) on the muscle-cell membrane and causes an action potential in the muscle cell.
The action potential rapidly spreads along the muscle cell and enters the cell through the T-tubule.
The action potential opens gates in the muscle's calcium store (sarcoplasmic reticulum).
Calcium ions flow into the cytoplasm, which is where the actin and myosin filaments are.
Calcium ions bind to troponin-tropomyosin molecules located in the grooves of the actin filaments. Normally, the rod-like tropomyosin molecule covers the sites on actin where myosin can form crossbridges.
Upon binding calcium ions, troponin changes shape and slides tropomyosin out of the groove, exposing the actin-myosin binding sites.
Myosin interacts with actin by cycling crossbridges, as described previously. The muscle thereby creates force, and shortens.
After the action potential has passed, the calcium gates close, and calcium pumps located on the sarcoplasmic reticulum remove calcium from the cytoplasm.
As the calcium gets pumped back into the sarcoplasmic reticulum, calcium ions come off the troponin.
The troponin returns to its normal shape and allows tropomyosin to cover the actin-myosin binding sites on the actin filament.
Because no binding sites are available now, no crossbridges can form, and the muscle relaxes.
As you can see, muscle contraction is regulated by the level of calcium ions in the cytoplasm. In skeletal muscle, calcium ions work at the level of actin (actin-regulated contraction). They move the troponin-tropomyosin complex off the binding sites, allowing actin and myosin to interact.
All of this activity requires energy. Muscles use energy in the form of ATP. The energy from ATP is used to reset the myosin crossbridge head and release the actin filament. To make ATP, the muscle does the following:
Breaks down creatine phosphate, adding the phosphate to ADP to create ATP
Carries out anaerobic respiration, by which glucose is broken down to lactic acid and ATP is formed
Carries out aerobic respiration, by which glucose, glycogen, fats and amino acids are broken down in the presence of oxygen to produce ATP (see How Exercise Works for details).
Muscles have a mixture of two basic types of fibers: fast twitch and slow twitch. Fast-twitch fibers are capable of developing greater forces, contracting faster and have greater anaerobic capacity. In contrast, slow-twitch fibers develop force slowly, can maintain contractions longer and have higher aerobic capacity. Training can increase muscle mass, probably by changing the size and number of muscle fibers rather than the types of fibers. Some athletes also use performance-enhancing drugs, specifically anabolic steroids, to build muscle, although this practice is dangerous and is banned in most athletic competitions.
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