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Muscular System for AP Biology

By — McGraw-Hill Professional
Updated on Oct 24, 2011

Practice problems for these concepts can be found at: Human Physiology Review Questions for AP Biology

Our tour of the muscle types of the body will include a look at the types of muscles and a quick demonstration of muscle contraction. There are three main types of muscle: skeletal, mooth, and cardiac:

  1. Skeletal muscle. Muscle type that works when you do pushups, lift a book, and do other voluntary activities. Skeletal muscle cells contain multiple nuclei. This muscle type has a striated appearance.
  2. Smooth muscle. Involuntary muscle that contracts slowly and is controlled by the ANS. Smooth muscle cells contain a single nucleus. Found in arteries, walls of digestive tract, bladder, and elsewhere. Smooth muscle is not striated in appearance.
  3. Cardiac muscle. Involuntary muscle of the heart. Cardiac muscle cells contain multiple nuclei. Cardiac muscle cells are striated in appearance.

Muscle cells are activated by the mechanism described earlier involving the action potentials and ion channels. When an action potential reaches a muscle cell, acetylcholine is released at the neuromuscular junction—the space between the motor neuron and the muscle cell. This neurotransmitter depolarizes the muscle cell and through a series of intracellular reactions causes the release of large amounts of stored calcium inside the cell, leading to muscle contraction. Muscle contraction stops when the calcium is taken back up by the sarcoplasmic reticulum of the cell.

Folks, we are going to be treated to a demonstration of skeletal muscle contraction. Skeletal muscle consists of fiber bundles, which are composed of myofibrils. What are myofibrils? Good question. They are structures that are made up of a combination of myo- filaments called thin filaments (actin) and thick filaments (myosin).

The Actin-Myosin "Tango"

It takes two to tango, and myosin and actin are up to the task. Myosin is the lead partner of this dynamic duo and powers muscle contraction. Myosin, the heart of the thick fibers, has a "head" and a "tail." The tails of the numerous myosin molecules unite to form the "thick filament" seen in Figure 15.6. The heads of the myosin molecules stick out from the thick filament and serve as the contact point with the actin. The head can exist in two forms: low and high energy. A relaxed muscle begins with the myosin heads in the low-energy form, attached to ATP. If the ATP is converted into ADP and phosphate, the myosin changes to the higher-energy form and is ready to dance. Myosin smoothly approaches its beloved partner, actin. When ready, the myosin and actin attach to each other, forming the "crossbridge." As they get ready to slide, myosin loses its ADP and phosphate, releasing its energy, and causing it to elegantly tilt its head to one side … sliding the beautiful actin toward the center of the sarcomere (Figure 15.6b). The two part ways when myosin again binds to ATP, bringing us back to where we started (Figure 15.6a).

Muscular System

Muscular System

Control mechanisms are often mentioned on the AP Biology exam and make good essay material. It would be really annoying and awkward if our muscles were contracting all the time. So, it makes sense that there must be some way to control the contraction. Myosin is only able to dance with actin if a regulatory protein, known as tropomyosin, is not blocking the attachment site on actin. The key to the removal of tropomyosin is the presence of calcium ions. Tropomyosin is also bound to another regulatory protein known as troponin. Calcium causes these two to do their own little dance and shuffle away from the actin-myosin binding site. This allows the actin–myosin dance to occur and muscle contraction to follow. When the calcium is gone, the dance is complete, and the filaments separate from each other.

Practice problems for these concepts can be found at: Human Physiology Review Questions for AP Biology

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