Now that we've introduced the sarcomere, let's see how its structure allows skeletal muscles to contract. In order for a skeletal muscle to contract, the muscle's sarcomeres must contract. When a sarcomere contracts, the distance between the Z lines becomes shorter. The lengths of the individual filaments stay the same, but they move past each other and the region of overlap increases. The I bands, regions of only thin filaments, shorten, and the H zone, the region of only thick filaments, disappears.

The sliding of the filaments occurs because of interactions between the thin-filament actin strands and the thick- filament myosin heads. Let's look at what many scientists currently believe are the molecular events that occur during skeletal muscle contraction.

The myosin head takes two shapes: a high-energy conformation and a low-energy conformation. An ATP molecule binds to the low-energy conformation. When the ATP is hydrolyzed to form ADP and a phosphate, the head changes shape into the high-energy conformation. The high-energy myosin head binds to a specific site on an actin molecule. ADP and the phosphate are released, and the myosin head changes shape back to the low-energy conformation. The change in shape pulls the thin filament toward the center of the sarcomere.

Each thick filament has around 350 myosin heads, and each one can go through this cycle about five times per second. Therefore, the filaments can move past each other quite rapidly. A skeletal muscle cell stores enough ATP for only a few contractions.

Most of the energy for contraction is stored in phosphagens, substances that can supply a phosphate group to convert ADP to ATP. In vertebrates, the major phosphagen used is creatine phosphate. This is why some athletes take creatine phosphate supplements to enhance their muscle performance.

Since muscle cells store energy for muscle contraction, our body needs a mechanism to keep the muscles from contracting all time. Muscle contraction is inhibited by the tropomyosin of the thin filament. Tropomyosin blocks the site on actin to which the myosin head binds. The location of the tropomyosin is controlled by a set of regulatory proteins called the troponin complex.

For the muscle to contract, the tropomyosin must be removed from the myosin binding sites. When calcium ions bind to the troponin complex, the interaction between the troponin complex and tropomyosin is altered. This causes the tropomyosin to change its shape, exposing the myosin binding sites on actin.

The calcium ion concentration in a muscle cell is controlled by a specialized type of endoplasmic reticulum called the sarcoplasmic reticulum. A signal from a motor neuron, telling the muscle to contract, causes the sarcoplasmic reticulum to release calcium ions. Tropomyosin inhibition is removed and the muscle can contract.

Now that we've seen how muscle contractions occur, think back to picking up your pencil and your keyboard. Depending on what you're doing, you can use a different amount of muscle contraction. For example, you had to use greater muscle contractions to pick up the keyboard than to pick up the pencil. But at the cellular level, the contraction of a single muscle fiber is all or nothing— it either contracts or it doesn't. So how do we control a whole-muscle contraction?

The length and strength of whole-muscle contractions are regulated. The frequency of signals from the motor neuron regulates both the length and strength of a whole-muscle contraction. A single signal will cause muscle tension for only 100 milliseconds or less. If another signal arrives before the first response finishes, the signals are added together, and the tension is stronger and lasts longer. Motor nerve signals usually come in an overlapping series, which all blur into one sustained contraction, called tetanus.

Another way the strength of a whole-muscle contraction is controlled, is by the organization of muscle cells into functional units called motor units. Each muscle cell is controlled by a single motor neuron. But each motor neuron branches to control many muscle cells at the same time. A group of muscle cells controlled by a motor neuron is a motor unit. Some motor units contain more muscle cells than others, so different levels of muscle tension occur when different motor units contract. The strength of the muscle tension is controlled by which and how many motor units contract.

In addition to the frequency of nerve signals, the length of whole-muscle contraction is controlled by how long calcium ions are present. This is because the presence of calcium ions is what regulates the binding of myosin to actin. Fast muscle fibers contract only for a short period of time. They are used for short, powerful contractions, like sprinting. Slow muscle fibers contract for a longer period of time and are used, for example, to maintain good posture. Slow muscle fibers have less sarcoplasmic reticulum, so the calcium released to cause the contraction is not reabsorbed as quickly and the contraction lasts longer.

Let's see how well you understand the regulation of muscle contraction. Match the mechanism of regulation with what aspect of muscle contraction it affects. Click Submit to see if you're correct.

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The presence of calcium ions controls whether or not a muscle contraction occurs. Which and how many motor units contract control only the strength of the contraction. The amount of sarcoplasmic reticulum controls how long calcium ions are present, and therefore controls only the length of contraction. Since signals from motor neurons can add together to give a longer and stronger response, the frequency of nerve signals controls both the strength and the length of contraction.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education