chapter 6

Question 1

Differentiate between voluntary and involuntary muscle

Answer 1

Some muscle movements are voluntary, meaning that we have conscious control over the movements they produce. An example is deliberately picking up an object. Other muscle movements, such as the pumping action of the heart or the maintenance of muscle tone in blood vessels, are involuntary in that they are generally beyond our conscious control. You cannot will your heart to stop beating.

Question 2

What is a skeletal muscle?

Answer 2

Skeletal muscles attach to the skeleton, providing us with strength and mobility. Skeletal muscles also sculpt the body and contribute to our sense of attractiveness and wellbeing. One very important skeletal muscle is the diaphragm, without which the respiratory system would be unable to function. Some of the smallest skeletal muscles control the focus of our eyes; some of the largest are responsible for the shivering that helps keep us warm when it is cold. Nearly 40% of body weight in males and about 32% in females is skeletal muscle

Question 3

What is the difference between a tendon and a ligament?

Answer 3

ligament Dense fibrous connective tissue that connects bone to bone.

Tendons connect muscle to bone

Question 4

Differentiate between the origin and insertion of a muscle

Answer 4

The skeleton is a complex set of levers that can be pulled in many directions by contracting or relaxing skeletal muscles. One end of a skeletal muscle, called its origin, joins to a bone that remains relatively stationary. The other end of the muscle, called its insertion, attaches to another bone across a joint. When the muscle contracts, the insertion is pulled toward the origin. The origin is generally closer to the midline of the body and the insertion is farther away.

Question 5

The smallest contractile unit of a muscle myofibril. A sarcomere extends from one Z-line to the next.

Answer 5

sarcomere

Question 6

Describe what happens at a neuromuscular junction. (motor neuron, neurotransmitter)

Answer 6

Skeletal muscle cells are stimulated to contract by certain nerve cells called motor neurons. The motor neurons secrete a chemical substance called acetylcholine (ACh). Acetylcholine is a neurotransmitter, a chemical released by nerve cells that has either an excitatory or inhibitory effect on another excitable cell (another nerve cell or a muscle cell). In the case of skeletal muscle, acetylcholine excites (activates) the cells. The junction between a motor neuron and a skeletal muscle cell is called the neuromuscular junction. As indicated in Figure 6.7 step 1 , when an electrical impulse traveling in a motor neuron arrives at the neuromuscular junction acetylcholine is released from the nerve terminal. The acetylcholine diffuses across the narrow space between the neuron and the muscle cell and binds to receptor sites on the muscle cell membrane. In step 2 , the binding of acetylcholine to the receptors causes the muscle cell membrane to generate an electrical impulse of its own that travels rapidly along the cell membrane in all directions. In addition, tubelike extensions of the cell membrane called T tubules (the T stands for transverse) transmit the electrical impulse deep into the interior of the cell. The function of the T tubules is to get the electrical impulse to all parts of the cell as quickly as possible.

Question 7

Summarize the sliding filament mechanism that explains muscle contraction

Answer 7

Muscles contract when sarcomeres shorten, and sarcomeres shorten when the thick and thin filaments slide past each other, a process described previously as the sliding filament mechanism of contraction. Taking a closer look at the arrangement of thick and thin filaments in a single sarcomere (Figure 6.8a), we see that every thin filament consists of two strands of actin molecules spiraling around each other and that thick filaments are composed of many individual molecules of myosin. Myosin molecules are shaped somewhat like a golf club, with a long shaft and a rounded head. Myosin shafts form the main part of the thick filaments. The heads stick out to the side, nearly touching the thin filaments of actin. When a muscle is relaxed, the myosin heads do not quite make contact with the thin filaments, however. Muscle contraction (Figure 6.8b) occurs when the myosin heads make contact with the thin filaments, forming a cross-bridge between the two filaments. The formation of a cross-bridge causes the head to bend relative to the shaft, pulling the actin molecules toward the center of the sarcomere. The processes of cross-bridge formation and bending (the molecular events of contraction) require energy

Question 8

What is the direct source of energy for muscle contraction?

Answer 8

ATP

Question 9

Define oxygen debt

Answer 9

. After you finish exercising, note that you continue to breathe heavily for a period of time. These rapid, deep breaths help reverse your body’s oxygen debt, incurred because your muscles used more ATP early on than was provided by aerobic metabolism. The additional ATP was produced by anaerobic metabolism, with the subsequent buildup of lactic acid. After exercise, you still need oxygen to metabolize the lactic acid by aerobic pathways and to restore the muscle’s stores of ATP and creatine phosphate to their resting levels. The ability of muscle tissue to accumulate an oxygen debt and then repay it later allows muscles to perform at a near-maximal rate even before aerobic metabolism has increased.

Question 10

Define fatigue

Answer 10

Muscle fatigue is defined as a decline in muscle performance during exercise. The most common cause of fatigue is insufficient energy to meet metabolic demands, due to depletion of ATP, creatine phosphate, and glycogen stores within the muscle. However, fatigue can also be caused by psychological factors, including discomfort or the boredom of repetitive tasks.

Question 11

define motor unit

Answer 11

The motor neuron and all of the muscle cells it controls are called a motor unit (Figure 6.10). A motor unit is the smallest functional unit of muscle contraction, because as the motor neuron is activated all the muscle cells in that motor unit are activated together

Question 12

define muscle tension

Answer 12

Our strength and ability to move effectively depend on how forcefully our muscles contract. The mechanical force that muscles generate when they contract is called muscle tension. How much tension is generated by a muscle depends on three factors:

● The number of muscle cells in each motor unit (motor unit size)

● The number of motor units active at any one time

● The frequency of stimulation of individual motor units

Question 13

defineall-or-none principle

Answer 13

According to the all-or-none principle, muscle cells are completely under the control of their motor neuron. Muscle cells never contract on their own. For an individual muscle cell, there is no such thing as a half-hearted contraction, and there is no such thing as disobeying an order. Muscle cells always respond with a complete cycle of contraction and relaxation (called a twitch) every time they are stimulated by an electrical impulse, called an action potential, from their motor neuron. You will learn more about action potentials when you study the nervous system. For now, you need only understand that they are the stimuli for muscle contraction.

Question 14

define recruitment

Answer 14

Increasing tone (or force) by activating more motor units is called recruitment. The maintenance of muscle tone depends on the nervous system

Question 15

Define summation

Answer 15

. If additional stimuli arrive at the muscle cell before it has had a chance to transport calcium back into the sarcoplasmic reticulum and relax completely, the total force produced becomes greater than the force produced by one twitch alone. In effect, the force becomes greater because more calcium is present. Increasing muscle cell force by increasing the rate of stimulation of motor units is called summation.

Question 16

Differentiate between slow-twitch and fast-twitch muscle fibers.

Answer 16

Slow-twitch fibers break down ATP slowly, and so they contract slowly. They tend to make ATP as they need it by aerobic metabolism. Slow-twitch fibers contain many mitochondria and are well supplied with blood vessels, so they draw more blood and oxygen than fast-twitch fibers. They store very little glycogen because they can obtain glucose and fatty acids quickly from the blood. They store oxygen, however, in a molecule called myoglobin. The ability to maintain a temporary store of oxygen reduces the slow-twitch fiber’s need for oxygen from the bloodstream. This is especially important during the early phases of an increase in activity, before blood flow to the muscle has increased. Myoglobin and the presence of numerous blood vessels make slow-twitch fibers reddish in color, so they are sometimes called “red” muscle.

Fast-twitch fibers can contract more quickly than slow-twitch fibers because they break down ATP more quickly. They have fewer mitochondria, fewer blood vessels, and little or no myoglobin compared to slow-twitch fibers, so they’re called “white” muscle. Fast-twitch fibers store large amounts of glycogen and tend to rely heavily on creatine phosphate and anaerobic metabolism for quick bursts of high energy. Their contractions are rapid and powerful but cannot be sustained for long. Fast-twitch fibers depend on aerobic mechanisms for any activity that is sustained, but they have the capability of using anaerobic mechanisms for brief periods when bursts of power are needed. During periods of anaerobic activity, they tend to accumulate lactic acid, which causes them to become fatigued quickly.

Question 17

Which fiber type has more mitochondria?

Answer 17

Slow twitch

Question 18

What is the advantage of slow-twitch fibers?

Answer 18

More endurance

Question 19

What is the advantage of fast-twitch fibers

Answer 19

Strength and burst movements

Question 20

Differentiate between strength and aerobic training.

Answer 20

Strength training (Figure 6.12a) involves doing exercises that strengthen specific muscles, usually by providing some type of resistance that makes them work harder. Strength training is generally short, intense exercises such as weight lifting using free weights or weight machines. It builds more myofibrils, particularly in fast-twitch fibers, and causes the fast-twitch fibers to store more glycogen and creatine phosphate as quick energy sources. This increases the size of individual muscle cells and builds muscle mass and muscle strength, but it does not increase the number of muscle cells. In general, the heavier the weight used, the more visible the increase in muscle size. However, this does not mean that strength training will necessarily build bulging biceps. The extent of muscle development depends on many factors, including the amount of resistance used, the duration and frequency of exercise, and your own genetic predisposition. However, even low to moderate weights can lead to noticeable improvements in muscle strength.

Aerobic training (Figure 6.12b) involves activities in which the body increases its oxygen intake to meet the increased demands for oxygen by muscles. Whereas resistance training strengthens muscles, aerobic training builds endurance. With aerobic training, the number of blood capillaries supplying muscle increases. In addition, the number of mitochondria in muscle cells and the amount of myoglobin available to store oxygen both increase. The muscle fibers themselves do not increase much in mass nor do they increase in number. Aerobic exercise also improves the performance of the cardiovascular and respiratory systems. Less intense than strength training but carried out for prolonged periods, aerobic exercises include jogging, walking, biking, and swimming.

Question 21

define Muscular dystrophy

Answer 21

Serious diseases of muscle are relatively uncommon, but foremost among them is muscular dystrophy. The term actually applies to several different hereditary diseases of muscle (dystrophy means “abnormal growth”). In Duchenne muscular dystrophy, a single defective gene results in the lack of a particular muscle cell protein. The normal gene, when present, directs the cell to produce a protein called dystrophin that is part of the muscle cell membrane. The function of dystrophin is to limit the inflow of calcium into muscle cells through calcium leak channels. People with muscular dystrophy lack dystrophin, and as a result too much calcium leaks into the muscle cell through the leak channels. The high intracellular calcium concentration activates enzymes that damage muscle proteins and ultimately may kill the cell. The result is a loss of muscle fibers and muscle wasting. Eventually, much of the muscle mass is replaced with fibrous connective tissue. Many people with muscular dystrophy die before age 30, usually because of failure of the heart muscle or the skeletal muscles used for breathing. At the moment, there is no cure; however, it is an area of intense research interest, and progress is being made on several fronts.

Question 22

define tetanus

Answer 22

Tetanus is caused by a bacterial infection. The disorder is called tetanus because this is the technical term for a maximal (tetanic) muscle contraction (see Figure 6.10). Generally, the infection is acquired by a puncture wound to a muscle. The bacteria produce a toxin that overstimulates the nerves controlling muscle activity, resulting in tetanic contractions. The toxin affects a variety of skeletal muscles, but especially those of the jaws and neck. Jaw muscles may contract so forcefully that they seem locked shut (the origin of its common name, “lockjaw”). Untreated, tetanus may lead to death due to exhaustion or respiratory failure.

Question 23

Muscle cramps

Answer 23

Muscle cramps are painful, uncontrollable, reflex-mediated muscle contractions. They are thought to be caused by the dehydration and ion imbalances that sometimes occur with heavy exercise. The most likely culprit is a shift in potassium ions between the intracellular and extracellular fluid, combined with excessive neural stimulation of the muscle. Muscle cramps generally can be soothed by increasing the circulation to the affected muscle through gentle stretching and massage.

Question 24

define Pulled muscles

Answer 24

Pulled muscles, sometimes called torn muscles, result from stretching a muscle too far, causing some of the fibers to tear apart. Internal bleeding, swelling, and pain often accompany a pulled muscle.

Question 25

define fasciitis

Answer 25

Fasciitis involves inflammation of the connective tissue sheath, or fascia, that surrounds a muscle (see Figure 6.3). It is usually caused by straining or tearing the fascia. Most often, it affects the sole of the foot (plantar fasciitis), where it is a common cause of heel pain. Like tendons and ligaments, fascia mends slowly. Treatment includes resting the area and protecting it from pressure. Injections of corticosteroid drugs can relieve severe pain.