Cell Movement in Muscle Contraction

Much of what scientists know about cell movement has been learned from studying the contraction of muscle cells. Vertebrate animals are able to move via the contraction of skeletal muscles, which in turn can occur only through contractions initiated at the cellular level. For example, when skeletal muscle cells contract, the skeletal muscle tissue as a whole contracts. As a result of this, a bone moves. Then the organism moves. In essence, an organism's complex movement can only be initiated through movement of the tiniest cell.

Skeletal muscles move bones by contracting and shortening. Each muscle is composed of long fibers that run along the length of it. Each individual fiber is considered a single muscle cell called a myocyte. Components of muscle fibers include:

• Myofibril - a bundle of thick and thin filaments that make up muscle fibers. Within a myofibril are thin filaments called actin, and thick filaments called myosin.
• Actin - also called G-actin, is a protein found in all eukaryotic cells. Its monomeric form is the subunit of actin filaments. Alternating with the actin filaments are thick filaments of myosin.
• Myosin - a motor protein that uses ATP to drive movements along actin filaments. Each myosin fiber has a bulbous head region and a long tail. This tail attaches to the tails of other myosin fibers, creating the thickness of myosin.

These are repeating units made of actin and myosin that are found in muscle myofibrils, each called a sarcomere. Myofibrils are made up of repeating sarcomeres, which appear as light and dark bands microscopically. The arrangement of thick myosin filaments in the myofibrils causes them to refract light, thereby producing a visually dark band called an A band; in between the dark A bands are regions composed of thin actin filaments, which appear as lighter areas in contrast to the dark bands. These light bands are called I bands. Actin molecules are bound to the Z disks, which form the border of the sarcomere. Within the A band is a region called the H zone that appears even lighter than the rest of the A band under a polarization microscope. Within each muscle cell, myosin molecules convert the chemical energy of ATP into the coordinated movement of thick and thin filaments. When a muscle cell contracts, the muscle itself gets shorter, but the actin and myosin fibers remain the same length. The motion of actin and myosin filaments sliding past one another during a muscle contraction is known as the sliding filament model. The head region binds to adenosine triphosphate (ATP), the energy molecule of the cell, thereby increasing the energy level of the fiber. This gives the myosin head the energy it needs to attach itself to the actin filaments. A muscle contraction occurs in the following sequence of events:

• The start of a muscle contraction induces the release of calcium ions.
• This calcium binds to troponin, the protein associated with the actin filaments, which causes tropomyosin to move away from and expose the active myosin-binding site of the actin filament. This myosin-binding site is the site of origin of the muscle contraction.
• The head of the myosin fiber, which is bound to ATP, interacts with the active site of actin. Once the actin and ATP are bound together, a cross bridge is formed. A cross bridge is the structure formed when the myosin head is bound to the actin.
• The myosin heads pull the actin molecule, called the power stroke, causing it to slide past the myosin filament and thereby shorten the muscle. while releasing then returns to a lower energy state because of the release of the phosphate from the ATP.
• The myosin releases ADP. New ATP binds to the myosin heads, thereby breaking the actin-myosin bond at the myosin binding site.
• The actin moves toward the center of the sarcomere with each myosin cycle. As a result, the myosin head attaches at a site farther along the actin fiber, shortening the muscle each time.

The arrival of an action potential at the end of a motor neuron causes the release of the neurotransmitter acetylcholine, which stimulates the muscle to shorten. As the muscle shortens, it pulls on the tendon (a strong fibrous tissue), which pulls on the bone. This process occurs repeatedly until the muscle has shortened enough to the move the bone as required, causing the nerve signal to stop. Muscles can only pull on bone; they cannot push. In order for the bone to move back the other way, a different muscle must contract while the original muscle relaxes. A muscle contraction requires multiple binding and release events between actin and myosin filaments.

Myosin has approximately 300 active heads in each filament, which can form approximately five cross bridges per second. It is this rapid attachment and release that makes actin and myosin slide past each other. A protein called troponin attaches itself to another protein called tropomyosin, which is found in the grooves along the actin fibers. When the muscle is relaxed, the tropomyosin blocks the attachment of the myosin, thereby preventing cross bridges from forming.

When skeletal muscle cells are at rest, sufficient ATP is readily available for only a few muscle contractions. When muscle cells need to contract more than once, or for longer periods of time, creatine phosphate and glycogen are used to generate additional ATP to energize the contraction. Creatine phosphate releases a phosphate molecule to adenine diphosphate (ADP) so it can hydrolyze into ATP. Additionally, glycogen breaks down into glucose, which enters into the cellular respiration pathway to generate more ATP.

Cardiac muscle cells are found only in the tissues that make up the heart. These cells create a striated appearance in the heart tissue but are considered involuntary. They can produce their own contractions without the input of the nervous system. This is because of the specialized cells that act to initiate the contraction. Each of these pacemaker cells sends a signal throughout the heart by way of an intercalated disk, which is a structure that connects cardiac muscle cells and supports cardiac muscle contraction. The electrical signals from the pacemakers are transported through the intercalated disks to generate the action potentials needed to contract the muscle tissue.

Cardiac excitation-contraction coupling is the contraction and immediate relaxation of the heart muscle. This occurs when there is an influx of calcium and sodium ions into the cytoplasm of the cardiac muscle cells. These calcium and sodium ions trigger action potentials to travel down the sarcomere and change the structure of the myosin heads. Similar to skeletal muscle tissue, the myosin within cardiac muscle binds to the actin to form a cross bridge with the input of ATP. Myosin and actin pull on each other, shortening the length of the muscle. The muscle stays contracted as long as the calcium and sodium levels are high, but because the stimulus is short lived, the cardiac muscle relaxes quickly in order to contract again almost immediately. This keeps blood moving through the heart.