Chapter 5 test highlights

1. Latent period

Begins at stimulation and lasts about 2 msec. During this period, the action potential sweeps across the sarcolemma, and the SR releases calcium ions. The muscle fiber does not produce tension during the latent period, because the contraction cycle has yet to begin.

Motor Units and Tension Production: Part 2

A typical skeletal muscle contains thousands of muscle fibers. Some motor neurons control a few muscle fibers, but most control hundreds of them. All the muscle fibers controlled by a single motor neuron constitute a motor neuron. The size of a motor unit is an indication of how fine the control of movement can be. In the muscles of the eye where precise control is extremely important, a motor neuron may control 4-6 muscle fibers. We have much less precise control over our leg muscles, where a single neuron may control 1,000-2,000 muscle fibers. The muscle fibers of each motor unit are intermingled with those of other motor units. Because of this intermingling, the direction of pull exerted on the tendon does not change when the number of activated motor units changes.

Perimysium

Divides the skeletal muscle into a series of compartments. Each compartment contains a bundle of muscle fibers called a fascile. In addition to collagen and elastic fibers, the perimysium contains blood vessels and nerves that supply the muscle fibers within the fascicles. Each fascile receives branches of these blood vessels and nerves.

Slow Fibers: Part 1

Have only about have half the diameter of fast fibers and take three times as long to reach peak tension after stimulation. These fibers are specialized in ways that enable them to continue contracting long after a fast fiber would have become fatigued. The most important specializations support aerobic metabolism in the numerous mitochondria.

Treppe

If a skeletal muscle is stimulated a second time immediately after the relaxation phase has ended, the resulting contraction will develop a slightly higher maximum tension than did the first contraction. The increase in peak tension shown will continue over the first 30-50 stimulation. After that, the amount of tension produced will remain constant. This pattern is called a treppe, because the tension rises in stages, like the steps in a staircase. The rise is thought to result from a gradual increase int he concentration of Ca+2 in the cytosol, in part because the calcium ion pumps in the SR have too little time to recapture the ions between stimulation's. Most skeletal muscles don't undergo treppe. -Treppe is a phenomenon in cardiac muscle. It occurs if stimuli of the the same intensity are sent to the muscle fiber after a latent period.

Sliding Filament Theory

If the thin filaments are sliding toward the center of each sarcomere, alongside the thick filaments. This explanation is known as the sliding filament theory. The contraction weakens with the disappearance of the I bands, at which point the Z lines are in contract with the ends of the thick filaments. During a contraction, sliding occurs in every sarcomere along the myofibril. As a result, the myofibril gets shorter. Because myofibrils are attached to the sarcolemma at each Z line adn at either end of the muscle fiber, when myofibrils get shorter, so does the muscle fiber.

3. Relaxation phase

Lasts about 25 msec. During this period Ca+2 levels are decreasing active sites are being covered by tropomyosin, and the number of active cross bridges is declining as they detach. As a result, tension decreases to resting levels.

Fast Fibers

Most of the skeletal muscle fibers in the body are called fast fibers, because they can reach peak twitch tension in 0.01 seconds or less after stimulation. Fast fibers are large in diameter and contain densely packed myofibrils, large glycogen reserves, and relatively few mitochondria. Muscles dominated by fast fibers produce powerful contractions because the tension produced by a muscle fiber is directly proportional to the number of myofibrils. However, fast fibers fatique rapidly because their contractions use ATP in massive amounts, and they have relatively few mitochondria to generate ATP. As a result, prolonged activity is supported primarily by anaerobic metabolism

Sarcomeres

Myofibrils are bundles of thin and thick myofilaments. These myofilaments are organized into repeating functional units called sarcomeres. Sarcomeres are the smallest functional units of the muscle fiber. Interactions between the thick and thin filaments within sarcomeres are responsible for muscle contraction.

Transverse tubules or T tubules

Narrow tubes whose surfaces are continuous with the sarcolemma and extend deep into the sarcoplasm. They are filled with extracellular fluid and form passageways through the muscle fiber, like a network of tunnels through a mountain. As extensions of the sarcolemma, T tubules share the same special property of being able to conduct an electrical impulse. As a result, electrical impulses conducted by the sarcolemma also travel along the T tubules into the cell interior. The impulses, called action potentials, trigger muscle fiber contraction.

Slow Fibers: Part 2

One of the main characteristic of slow muscle fibers is that they are surrounded by a more extensive network of capillaries than is typical of fast muscle tissue . For this reason, they have drastically higher oxygen supply to support mitochondrial activity. Slow fibers also contain the red pigment myoglobin. This globular protein is similar to hemoglobin the red oxygen carrying pigment in blood. Both myoglobin and hemoglobin reversibly bind oxygen molecules. Other muscle fiber types contain small amounts of myoglobin, but it is most abundant in slow fibers. As a result, resting slow fibers hold substantial oxygen reserves that can be used during a contraction. Skeletal muscles dominated by slow fibers are dark red because slow fibers have both an extensive capillary supply and a high concentration of myoglobin

Isotonic Contractions

Tension increases and the skeletal muscle's length changes. Lifting an object off a desk, walking, and running involve isotonic contraction

2. Contraction phase

Tension increases to a peak. As the tension increases, calcium ions are binding to troponin, active sites on thin filaments are being exposed, and crossed bridge interactions are occurring. For this muscle fiber the contraction phase ends roughly 15 msec after stimulation.

Motor Units and Tension Production: Part 1

The amount of tension produced by the muscle as a whole is the sum of the tensions generated by the individual muscle fibers, since they are all pulling together. For this reason, you can control the amount of tension produced by a skeletal muscle by controlling the number of muscle fibers stimulated.

Epimysium

The epimysium muscle is a dense layer of collagen fibers that surrounds the entire muscle. It separates the muscle from nearby tissues and organs. The epimysium is connected to the deep fascia, a dense connective tissue layer.

Concentric Contraction

The muscle tension exceeds the load and the muscle shortens

Eccentric Contraction

The peak tension developed is less than the load, and the muscle elongates due to the contraction of another muscle or the pull of gravity.

Release of Calcium Ions

This action potential travels along the sarcolemma and down the T tubules to the traids. This triggers the release of calcium ions (Ca+2) from the terminal cisternae of the sarcoplasmic reticulum.

Complete Tetanus

When a higher stimulation frequency eliminates the relaxation phase, complete tetanus, occurs. Action potentials arrive so rapidly that the SR does not have time to reclaim the Ca+2. The high Ca+2 concentration in the cytosol prolongs the contraction, making it continuous.

Rigor Mortis

When death occurs, circulation ceases and the skeletal muscles are deprived of nutrients and oxygen. Within a few hours, the skeletal muscle fibers run out of ATP and the the sarcoplasmic reticulum becomes unable to pump Ca+2 out of the cytosol. Calcium ions diffusing into the cytosol extracelluar fluid or leaking out of the SR then trigger a sustained contraction. Without ATP, the cross-bridges cannot detach from the active sites, so skeletal muscles throughout the body become locked in the contracted position. Because the skeletal muscles are involved, the body becomes "stiff as a board". This physical state -rigor mortis- lasts until the lysosomal enzymes released by autolysis break down the Z lines and titin filaments. Rigor mortis begins 2-7 hours after death and ends after 1-6 days or when decomposition begins. The timing depends on environmental factors, such as temperature. Forensic pathologists can estimate the time of death on the basis of the degree of rigor mortis and environmental conditions.

Slow Fibers: Part 3

With oxygen reserves and more efficient blood supply, the mitochondria of slow fibers can contribute more ATP during contractions. In addition, demand for ATP is reduced because the cross-bridges in slow fibers cycle more slowly than those of fast fibers. Glycogen reserves of slow fibers are also smaller than those of fast fibers, because some of the mitochondrial energy production involves the breakdown of stored lipids rather than glycogen.