Unit Six. Animal Life


22. The Animal Body and How It Moves


22.8. Muscles and How They Work


Three kinds of muscle together form the vertebrate muscular system. As we have discussed, the vertebrate body is able to move because skeletal muscles pull the bones with considerable force. The heart pumps because of the contraction of cardiac muscle. Food moves through the intestines because of the rhythmic contractions of smooth muscle.


Actions of Skeletal Muscle

Skeletal muscles move the bones of the skeleton. Some of the major human muscles are labeled on the right in figure 22.12. Muscles are attached to bones by straps of dense connective tissue called tendons. Bones pivot about flexible connections called joints, pulled back and forth by the muscles attached to them. Each muscle pulls on a specific bone. One end of the muscle, the origin, is attached by a tendon to a bone that remains stationary during a contraction. This provides an object against which the muscle can pull. The other end of the muscle, the insertion, is attached to a bone that moves if the muscle contracts. For example, origin and insertion for the sartorius muscle is labeled on the left in figure 22.12. This muscle helps bend the leg at the hip, bringing the knee to the chest. The origin of the muscle is at the hip and stays stationary. The insertion is at the knee, such that when the muscle contracts (gets shorter) the knee is pulled up toward the chest.




Figure 22.12. The muscular system.

Some of the major muscles in the human body are labeled.


Muscles can only pull, not push, because myofibrils contract rather than expand. For this reason, the muscles in the movable joints of vertebrate are attached in opposing pairs, called flexors and extensors that, when contracted, move the bones in different directions. As you can see in figure 22.13, when the flexor muscle at the back of your upper leg contracts, the lower leg is moved closer to the thigh. When the extensor muscle at the front of your upper leg contracts, the lower leg is moved in the opposite direction, away from the thigh.



Figure 22.13. Flexor and extensor muscles.

Limb movement is always the result of muscle contraction, never muscle extension. Muscles that retract limbs are called flexors; those that extend limbs are called extensors.


All muscles contract, but there are two types of muscle contractions, isotonic and isometric contractions. In isotonic contractions, the muscle shortens, moving the bones as just described. In isometric contractions, a force is exerted by the muscle, but the muscle does not shorten. This occurs when you try to lift something very heavy. Eventually, if your muscles generate enough force and you are able to lift the object, the isometric contraction becomes isotonic.


Author’s Corner

The Author Works Out

No one seeing the ring of fat decorating my middle would take me for a runner. Only in my memory do I get up with the robins, lace up my running shoes, bounce out the front door, and run the streets around Washington University before going to work. Now my 5-K runs are 30-year-old memories.

Any mention I make of my running in a race only evokes screams of laughter from my daughters, and an arch look from my wife. Memory is cruelest when it is accurate.

I remember clearly the day I stopped running. It was a cool fall morning in 1978, and I was part of a mob running a 5-K (that's 5 kilometers for the uninitiated) race, winding around the hills near the university. I started to get flashes of pain in my legs below the knees—like shin splints, but much worse. Imagine fire pouring on your bones. Did I stop running? No. Like a bonehead I kept going, "working through the pain,” and finished the race. I have never run a race since.

I had pulled a muscle in my thigh, which caused part of the pain. But that wasn't all. The pain in my lower legs wasn't shin splints, and didn't go away. A trip to the doctor revealed multiple stress fractures in both legs. The X rays of my legs looked like tiny threads had been wrapped around the shaft of each bone, like the red stripe on a barber's pole. It was summer before I could walk without pain.

What went wrong? Isn't running supposed to be GOOD for you? Not if you run improperly. In my enthusiasm to be healthy, I ignored some simple rules and paid the price. The biology lesson I ignored had to do with how bones grow. The long bones of your legs are not made of stone, solid and permanent. They are dynamic structures, constantly being re-formed and strengthened in response to the stresses to which you subject them.

To understand how bone grows, we first need to recall a bit about what bone is like. Bone, as you have learned in this chapter, is made of fibers of a flexible protein called collagen stuck together to form cartilage. While an embryo, all your bones are made of cartilage. As your adult body develops, the collagen fibers become impregnated with tiny, needle-shaped crystals of calcium phosphate, turning the cartilage into bone. The crystals are brittle but rigid, giving bone great strength. Collagen is flexible but weak, but like the epoxy of fiberglass, it acts to spread any stress over many crystals, making bone resistant to fracture. As a result, bone is both strong and flexible.



When you subject a bone in your body to stress—say, by running—the bone grows so as to withstand the greater workload. How does the bone "know” just where to add more material? When stress deforms the collagen fibers of a leg bone, the interior of the collagen fibers becomes exposed, like opening your jacket and exposing your shirt. The fiber interior produces a minute electrical charge. Cells called fibroblasts are attracted to the electricity like bugs to night lights, and secrete more collagen there. As a result, new collagen fibers are laid down on a bone along the lines of stress. Slowly, over months, calcium phosphate crystals convert the new collagen to new bone. In your legs, the new bone forms along the long stress lines that curve down along the shank of the bone.

Now go back 30 years, and visualize me pounding happily down the concrete pavement each morning. I had only recently begun to run on the sidewalk, and for an hour or more at a stretch. Every stride I took those mornings was a blow to my shinbones, a stress to which my bones no doubt began to respond by forming collagen along the spiral lines of stress. Had I run on a softer surface, the daily stress would have been far less severe. Had I gradually increased my running, new bone would have had time to form properly in response to the added stress. I gave my leg bones a lot of stress, and no time to respond to it. I pushed them too hard, too fast, and they gave way.

Nor was my improper running limited to overstressed leg bones. Remember that pulled thigh muscle? In my excessive enthusiasm, I never warmed up before I ran. I was having too much fun to worry about such details. Wiser now, I am sure the pulled thigh muscle was a direct result of failing to properly stretch before running.

I was reminded of that pulled muscle recently, listening to a good friend of my wife's describe how she sets out early each morning for a long run without stretching or warming up. I can see her in my mind's eye, bundled up warmly on the cooler mornings, an enthusiastic gazelle pounding down the pavement in search of good health. Unless she uses more sense than I did, she may fail to find it.


Muscle Contraction

Recall from figure 22.6 that myofibrils are composed of bundles of myofilaments. Far too fine to see with the naked eye, the individual myofilaments of vertebrate muscles are only 8 to 12 nanometers thick. Each is a long, threadlike filament of the pro- terns actin or myosin. An actin filament consists of two strings of actin molecules wrapped around one another, like two strands of pearls loosely wound together. A myosin filament is also composed of two strings of protein wound about each other, but a myosin filament is about twice as long as an actin filament, and the myosin strings have a very unusual shape. One end of a myosin filament consists of a very long rod, while the other end consists of a double-headed globular region, or “head.” Overall, a myosin filament looks a bit like a two-headed snake. This odd structure is the key to how muscles work.


How Myofilaments Contract

The sliding filament model of muscle contraction, seen in the Key Biological Process illustration below, describes how actin and myosin cause muscles to contract. Focus on the knobshaped myosin head in panel 1. When a muscle contraction begins, the heads of the myosin filaments move first. Like flexing your hand downward at the wrist, the heads bend backward and inward as in panel 2. This moves them closer to their rodlike backbones and several nanometers in the direction of the flex. In itself, this myosin head-flex accomplishes nothing—but the myosin head is attached to the actin filament! As a result, the actin filament is pulled along with the myosin head as it flexes, causing the actin filament to slide by the myosin filament in the direction of the flex (the dotted circles in panel 2 indicate the movement of the actin filament). As one after another myosin head flexes, the myosin in effect “walks” step by step along the actin. Each step uses a molecule of ATP to recock the myosin head (in panel 3) before it attaches to the actin again (panel 4), ready for the next flex.

How does this sliding of actin past myosin lead to myofibril contraction and muscle cell movement? The actin filament is anchored at one end, at a position in striated muscle called the Z line, indicated by the lavender-colored bars toward the edges in the Key Biological Process illustration on the facing page. Two Z lines with the actin and myosin filaments in between make up a contractile unit called a sarcomere. Because it is tethered like this, the actin cannot simply move off. Instead, the actin pulls the anchor with it! As actin moves past myosin, it drags the Z line toward the myosin. The secret of muscle contraction is that each myosin is interposed between two pairs of actin filaments, which are anchored at both ends to Z lines, as shown in panel 1. One moving to the left and the other to the right, the two pairs of actin molecules drag the Z lines toward each other as they slide past the myosin core, shown progressively in panel 2 and panel 3. As the Z lines are pulled closer together, the plasma membranes to which they are attached move toward one another, and the cell contracts.



The Role of Calcium Ions

When a muscle is relaxed, its myosin heads are “cocked” and ready, but are unable to bind to actin. This is because the attachment sites for the myosin heads on the actin are physically blocked by another protein, known as tropomyosin. Myosin heads therefore cannot bind to actin in the relaxed muscle, and the filaments cannot slide.

For a muscle to contract, the tropomyosin must first be moved out of the way so that the myosin heads can bind to actin. This requires calcium ions (Ca++). When the Ca++ concentration of the muscle cell cytoplasm increases, Ca++, acting through another protein, causes the tropomyosin to move out of the way. When this repositioning has occurred, the myosin heads attach to actin and, using ATP energy, move along the actin in a stepwise fashion to shorten the myofibril.

Muscle fibers store Ca++ in a modified endoplasmic reticulum called the sarcoplasmic reticulum. When a muscle fiber is stimulated to contract, Ca++ is released from the sarcoplasmic reticulum and diffuses into the myofibrils, where it initiates contraction. When a muscle works too hard, the Ca++ channels become leaky, releasing small amounts of Ca++ that act to weaken muscle contractions and result in muscle fatigue.



Key Learning Outcome 22.8. Muscles are made of many tiny threadlike filaments of actin and myosin called myofilaments. Muscles work by using ATP to power the sliding of myosin along actin, causing the myofibrils to contract.


Inquiry & Analysis

Which Mode of Locomotion Is the Most Efficient?

Running, flying, and swimming require more energy than sitting still, but how do they compare? The greatest differences between moving on land, in the air, and in water result from the differences in support and resistance to movement provided by water and air. The weight of swimming animals is fully supported by the surrounding water, and no effort goes into supporting the body, while running and flying animals must support the full weight of their bodies. On the other hand, water presents considerable resistance to movement, air much less, so that flying and running require less energy to push the medium out of the way.

A simple way to compare the costs of moving for different animals is to determine how much energy it takes to move. The energy cost to run, fly, or swim is in each case the energy required to move one unit of body mass over one unit of distance with that mode of locomotion. (Energy is measured in the metric system as a kilocalorie [kcal] or, technically, 4.184 kilojoules [note that the Calorie measured in food diets and written with a capital C is equivalent to 1 kcal]; body mass is measured in kilograms, where 1 kilogram [kg] is 2.2 pounds; distance is measured in kilometers, where 1 kilometer [km] is 0.62 miles). The graph to the right displays three such "cost-of-motion” studies. The blue squares represent running; the red circles, flying; and the green triangles, swimming. In each study, the line is drawn as the statistical "best-fit” for the points. Some animals like humans have data in two lines, as they both run (well) and swim (poorly). Ducks have data in all three lines, as they not only fly (very well), but also run and swim (poorly).



1. Applying Concepts

a. Variables. In the graph, what is the dependent variable?

b. Comparing Continuous Variables. Do the three modes of locomotion have the same or different costs?

2. Interpreting Data

a. For any given mode of locomotion, what is the impact of body size on cost of moving?

b. Is the impact of body mass the same for all three modes of locomotion? If not, which mode's cost is least affected by body mass? Why do you think this is so?

3. Making Inferences

a. Comparing the energy costs of running versus flying for animals of the same body size, which mode of locomotion is the most expensive? Why would you expect this to be so?

b. Comparing the energy costs of swimming to flying, which uses the least energy? Why would you expect this to be so?

4. Drawing Conclusions In general, which mode of locomotion is the most efficient? The least efficient? Why do you think this is so?

5. Further Analysis

a. How would you expect the slithering of a snake to compare to the three modes of locomotion examined here? Why?

b. Do you think the costs of running by an athlete decrease with training? Why? How might you go about testing this?



Test Your Understanding

1. One of the innovations in animal body design, segmentation, allowed for

a. development of efficient internal organ systems.

b. more flexible movement as individual segments can move independently of each other.

c. locating organs in different areas of the body.

d. early determination of embryonic cells.

2. Which of the following is the correct organization sequence from smallest to largest in animals?

a. cells, tissues, organs, organ systems, organism

b. organism, organ systems, organs, tissues, cells

c. tissues, organs, cells, organ systems, organism

d. organs, tissues, cells, organism, organ systems

3. Which of the following is not a function of the epithelial tissue?

a. secrete materials

b. provide sensory surfaces

c. move the body

d. protect underlying tissue from damage and dehydration

4. An example of connective tissue is

a. nerve cells in your fingers.

b. skin cells.

c. brain cells.

d. red blood cells.

5. When a person has osteoporosis, the work of _____ falls behind the work of _____.

a. osteoclasts; osteoblasts     

b. osteoclasts; collagen 

c. osteoblasts; osteoclasts

d. osteoblasts; collagen

6. Nerve impulses pass from one nerve cell to another through the use of

a. hormones.           

b. neurotransmitters.

c. pheromones.

d. calcium ions.

7. The type of muscle used to move the leg when walking is

a. skeletal.              

b. cardiac.

c. smooth.               

d. All of the these are correct.

8. The vertebral column is part of the

a. appendicular skeleton.

b. axial skeleton.

c. hydrostatic skeleton.

d. exoskeleton.

9. Movement of a limb in two directions requires a pair of muscles because

a. a single muscle can only pull and not push.

b. a single muscle can only push and not pull.

c. moving a limb requires more force than one muscle can generate.

d. None of the above.

10. The role of calcium in the process of muscle contraction is to

a. gather ATP for the myosin to use.

b. cause the myosin head to shift position, contracting the myofibril.

c. cause the myosin head to detach from the actin, causing the muscle to relax.

d. expose myosin attachment sites on actin.