Chapter 12 A&Pi

In an isometric contraction

the muscle length remains the same, but the tension on the muscle increases. The sarcomeres do not shorten, and the muscle is unable to move the load on the muscle.

Describe the anatomical arrangement of a motor unit

A single motor neuron—plus all the muscle fibers it stimulates—is called a motor unit. Motor units can include many muscle fibers for less precise movement or few fibers for more delicate movement.

Describe the general structure of ATP and tell how it relates to its function

ATP consists of a molecule of adenine, a molecule of ribose sugar, and three molecules of phosphate. The last two phosphates are joined to the molecule by high-energy bonds. When the high-energy bonds are broken, the energy stored in the bonds is used for cell work.

How does ATP provide energy for muscle contraction?

ATP transfers the energy from the high-energy bonds to the myosin cross bridges. The energy is stored in the myosin until it is released when the myosin binds with the actin during muscle contraction

How are acetylcholine, Ca++, and adenosine triphosphate (ATP) involved in the excitation and contraction of skeletal muscle?

Acetylcholine is the neurotransmitter that stimulates the sarcolemma and initiates the contraction. The result of that stimulus is the release of Ca++ from the sarcoplasmic reticulum. The calcium causes the troponin to pull the tropomyosin away from the binding sites on the actin. ATP supplies the energy to the cross bridges of the myosin, so that the actin fiber can be pulled to the center of the sarcomere, causing the muscle to contract.

People who exercise seriously are sometimes told to work a muscle until they "feel the burn." In terms of how the muscle is able to release energy, explain what is going on in the muscle early in the exercise and when the muscle is "burning."

Early in the exercise, the muscle is able to keep up with the energy demands by using aerobic respiration. When the muscle cannot keep up with the energy demand because of a lack of oxygen, it must resort to anaerobic respiration. The end product of anaerobic respiration is lactic acid. An accumulation of lactic acid will cause a burning sensation in the muscle.

Describe rigor mortis

Rigor mortis is the stiffness that occurs in the muscles after death. The stiffness occurs because muscles that were contracting at the time of death need ATP to release the cross bridges. Since no more ATP is being made, the cross bridges do not release, and stiffness results.

What structures are unique to skeletal muscle fibers? Which of the structures are involved primarily in contractility and which are involved in excitability?

Myofibrils are unique to muscle cells; they are primarily involved in contraction. T tubules and sarcoplasmic reticula are also unique to muscle cells; they are primarily involved in excitability.

What are the effects of exercise on skeletal muscles?

Strength training can cause an increase in muscle size (hypertrophy). This increases the number of myofilaments in each muscle fiber. Endurance training does not usually result in muscle hypertrophy, but it does increase the muscle's ability to sustain moderate contractions for a longer period. This increases the blood supply, which allows the muscle to obtain more glucose and oxygen during exercise. Also increased is the number of mitochondria in each muscle fiber, which allows for the more efficient production of ATP.

Compare and contrast the role of Ca++ in excitation, contraction, and relaxation of skeletal muscle

The Ca++ is released into the sarcomere as a result of excitation of the sarcoplasmic reticulum. When the Ca++ binds to the troponin, it moves the tropomyosin, allowing the cross bridges to form, which causes the muscle to contract. When the calcium is transported back into the sarcoplasmic reticulum, the troponin returns the tropomyosin to its blocking position, causing relaxation.

Explain the meaning of a "unit of combined cells" as it relates to cardiac muscle. How does this structural arrangement affect its function?

The cardiac muscles do not taper at the ends like skeletal muscles but branch and form strong electrically coupled junctions with other fibers. This allows cardiac muscle fibers to form a continuous electrically coupled mass. This allows for the efficient conduction of electrical impulses necessary for coordinated pumping action.

Explain how skeletal muscles provide movement, heat, and posture. Are all of these functions unique to muscles?

The general functions of skeletal muscles are movement, heat production, and posture. Muscle contractions cause the joints to bend and the body to move. Heat is produced by the large number of contracting muscle cells. The continuous partial contraction of many skeletal muscles produces the relatively stable positions of the body when standing or sitting; this is known as posture.

Using fiber types, design a muscle for a marathon runner and a different muscle for a 100-yard-dash sprinter. Explain your choice

The marathon runner would want muscles with a high proportion of slow fibers. These muscles can maintain contractions over a long period without fatigue. A sprinter would want a muscle with a high proportion of fast fibers. Fast fibers can respond much more rapidly; however, they use so much energy that they fatigue quickly.

List and describe the different types of skeletal muscle contractions

The skeletal muscles can contract in a rapid series of contractions called incomplete tetanus. The muscle cannot fully relax before the next stimulus occurs. If the speed of the stimulus increases, the individual contractions fuse and become a single smooth contraction; this is called complete tetanus

Explain how the sliding filament theory allows for the shortening of a muscle fiber

The sliding-filament theory explains how the excitation of the muscle fiber begins with the neurotransmitter acetylcholine, causing the stimulation of the sarcolemma, T tubule, and sarcoplasmic reticulum. When the sarcoplasmic reticulum is stimulated, it releases Ca++ into the sarcomere. The Ca++ attach to the troponin, which causes the troponin to move the tropomyosin away from the actin binding site. When this happens, the myosin forms cross bridges with the actin and pulls it toward the center of the sarcomere. The muscle shortens as a result of the filaments sliding past one another. Energy for the contraction is supplied by ATP. When the calcium is transported back to the sarcoplasmic reticulum, the tropomyosin returns to the blocking position, which prevents the joining of actin and myosin.

Explain how the structure of myofilaments is related to their function

The structure of the myofilament consists of thousands of smaller subunits called myofibrils. Each myofibril consists of thick and thin filaments lying parallel to one another. Four kinds of proteins make up the myofilament: actin, troponin, and tropomyosin make up the thin filaments; myosin makes up the thick filaments. The tropomyosin twists around the actin filament and is held in place by troponin. In this position the tropomyosin is blocking the binding sites between the actin and myosin. When the muscle is stimulated to contract, Ca++ are released from the sarcoplasmic reticulum. When Ca++ attach to the troponin, they cause the troponin to move the tropomyosin away from the binding sites, allowing the myosin to pull the actin so that the muscle fiber shortens.

Which of the two smooth muscle types would be most affected by damage to the nerves that stimulate them?

Visceral smooth muscle is capable of autorhythmicity, which means it is able to contract without external stimulus. Multiunit smooth muscle is unable to do this and must rely on an external stimulus to contract. The multiunit smooth muscle would be more affected by damage to the nerve that stimulates it.

Excitability

is a characteristic shared with the nervous system.

Function of the sarcoplasmic reticulum

is the active transport and storage of Calcium ions in the canals and sacs that make up the sarcoplasmic reticulum.

Sarcoplasm

is the cytoplasm of a muscle fiber.

Sarcolemma

is the plasma membrane of a muscle fiber.

Sarcoplasmic reticulum

is the structure that is analogous to the endoplasmic reticulum of other cells.

Extensibility

refers to a muscle's ability to be pulled back to its original length. When a joint is bent, the agonist is exhibiting contractility and the antagonist is exhibiting extensibility.

Contractility

refers to a muscle's ability to get shorter.

Define the term recruited as it applies to muscles

refers to activation of motor units that lead to a muscle contraction. _____ of motor units depends on the intensity and frequency of stimulation. In general, the more intense and more frequent a stimulus, the more motor units are recruited and the stronger the contraction.

In an isotonic contraction

the tension within the muscle remains the same. As the muscle contraction begins, the sarcomeres actually shorten and move the load on the muscle.