CK-12 Life Science

by CK-12 Foundation

  Stretching before and after activity helps prevent injuries. Regular stretching improves the flexibility of muscles and tendons. It also improves the range of motion of your joints. Stretching can also improve your posture, and helps prevent some aches and pains associated with tight muscles.

  Lesson Summary

  Bones, cartilage, and ligaments make up the skeletal system. The skeleton supports the body against the pull of gravity. The skeleton provides a framework that supports and protects the soft organs of the body. Bones work together with muscles as simple mechanical lever systems to move the body. Blood cells are made mostly inside the bone marrow. Bones store calcium.

  There are three types of joints in the body: fixed, partly movable, and movable. Fixed joints do not allow any bone movement. Partly movable joints allow only a little movement. Movable joints allow movement and provide mechanical support for the body. Joints are a type of lever, which is a rigid object that is used to increase the mechanical force that can be applied to another object. Joints reduce the amount of energy that is spent moving the body around. Calcium and vitamin D are two of the most important nutrients for a healthy skeletal system.

  Bones need calcium to grow properly. Vitamin D deficiency results in problems with bone growth and hardening. Osteoporosis is a disease in which bones become lighter and more porous than they should be. Light and porous bones are more likely to break than dense bones. Osteomalacia is a bone disease in which the bones do not harden properly and they can break easily. Osteoarthritis is a condition in which the cartilage at the ends of the bones breaks down. The break down of the cartilage leads to pain and stiffness in the joint. A sprain is an injury to a ligament. A fracture is a break or crack in a bone.

  Review Questions

  What are the organs of the skeletal system?

  Name one tissue of the skeletal system.

  List four functions of the skeletal system.

  Name three types of movable joints.

  “All joints in the body are movable.” Do you agree with this statement? Explain why or why not. (Intermediate

  How are the joints in your body similar to levers?

  Why is calcium important for a healthy skeletal system?

  The recommended daily amount of calcium for teenagers is 1300 mg. If a person gets only 1000 mg a day, what percentage of the recommended daily amount are they getting?

  Name two things you can do to keep your skeletal system healthy.

  What part of the skeletal system does osteoarthritis affect?

  Why might a doctor need to insert pins into a broken bone?

  Further Reading / Supplemental Links

  http://www.girlshealth.gov/bones

  [http://www.cdc.gov/nccdphp/dnpa/nutrition/nutrition_for_everyone/basics/calcium.htm http://www.cdc.gov/nccdphp/dnpa/nutrition/nutrition_for_everyone/basics/calcium.htm

  http://en.wikipedia.org/wiki

  Vocabulary

  ball and socket joint

  Joint structure in which the ball-shaped surface of one bone fits into the cuplike depression in another bone; examples include the shoulder and hip joints.

  bone marrow

  Soft connective tissue found inside many bones; site of blood cell formation.

  cartilage

  Smooth covering found at the end of bones; made of tough collagen protein fibers; creates smooth surfaces for the easy movement of bones against each other.

  compact bone

  The dense, hard outer layer of a bone.

  fracture

  Bone injury, often called a "break;" usually caused by excess bending stress on bone.

  gliding joint

  Joint structure that allows one bone to slide over the other; examples includes the joints in the wrists and ankles.

  hinge joint

  Joint structure in which the ends of bones are shaped in a way that allows motion in two directions only (forward and backward); examples include the knees and elbows.

  joint

  Point at which two or more bones meet.

  ligaments

  Fibrous tissue that connects bones to other bones; made of tough collagen fibers.

  movable joint

  Most mobile type of joint; the most common type of joint in the body.

  osteoarthritis

  A condition in which the cartilage at the ends of the bones breaks down.

  osteoporosis

  Disease in which bones become lighter and more porous than normal.

  periosteum

  Tough, shiny, white membrane that covers all surfaces of bones.

  pivot joint

  Joint structure in which the end on one bone rotates within a ring-type structure which can be made partly of bone and partly of ligament; example includes the joint between the radius and ulna.

  skeletal system

  Body system that is made up of bones, cartilage, and ligaments.

  skeleton

  Sturdy scaffolding of bones and cartilage that is found inside vertebrates.

  spongy bone

  Lighter and less dense than compact bone; found toward the center of the bone.

  sprain

  A ligament injury; usually caused by the sudden overstretching of a joint which causes tearing.

  Points to Consider

  How does your skeletal system interact with your muscular system?

  How might a broken bone affect the functioning of the muscular system?

  How do tendons differ from ligaments? How are they similar?

  Lesson 16.4: The Muscular System

  Lesson Objectives

  Identify the three muscle types in the body.

  Describe how skeletal muscles and bones work together to move the body.

  Describe how exercise affects the muscular system.

  Identify two types of injuries to the muscular system.

  Check Your Understanding

  What is muscle tissue?

  What is the function of the muscular system?

  Introduction

  The muscular system is the body system that allows us to move. You depend on many muscles to keep you alive. Your heart, which is mostly muscle, pumps blood around your body. Muscles are always moving in your body. Certain muscle movements happen without you thinking about them, while you can control other muscle movements. In this lesson you will learn about the different types of muscles in your body and how your muscular system works with the other body systems to keep you alive and healthy. You will also learn how and why regular physical activity is important for good health.

  Types of Muscles

  Each muscle in the body is made up of cells called muscle fibers. Muscle fibers are long, thin cells that can do something that other cells cannot do—they are able to get shorter. Shortening of muscle fibers is called contraction. Nearly all movement in the body is the result of muscle contraction.

  You are aware of and can control certain muscle movements. Other muscle movements you are not aware of and cannot control. Muscles that you can control are called voluntary muscles. Muscles that you cannot control are called involuntary muscles. There are three different types of muscles in the body (Figure below): skeletal, smooth, and cardiac muscle. Skeletal muscle is voluntary muscle. Smooth muscle and cardiac muscle are involuntary muscles.

  Skeletal muscle is usually attached to the skeleton. Skeletal muscles move the body. They usually contract voluntarily, but they can contract involuntarily by reflexes. For example, you can choose to move your arms, but your arm would move automatically if you were to burn your finger on a stove top.

  Smooth muscle is found within the walls of organs and structures such as the esophagus, stomach, intestines, and blood vessels. Unlike skeletal muscle, smooth muscle is involuntary muscle which means it not under your control.

  Cardiac muscle is also an involuntary muscle but is a specialized kind of muscle found only in the heart.

  Figure 16.22

  There are three types of muscles in the body: cardia
c, skeletal, and smooth. Everyone has the same three types of muscle tissue, no matter their age.

  Muscles, Bones, and Movement

  Skeletal muscles are attached to the skeleton by tendons. A tendon is a tough band of connective tissue that connects a muscle to a bone. Tendons are similar to ligaments except that ligaments join bone to each other. Muscles move the body by contracting against the skeleton. When muscles contract they get shorter, when they relax, they get longer. By contracting and relaxing, muscles pull on bones and allow the body to move. Muscles work together in pairs. Each muscle in the pair works against the other to move bones at the joints of the body. The muscle that contracts to cause a joint to bend is called the flexor. The muscle that contracts to cause the joint to straighten is called the extensor.

  For example, the biceps and triceps muscles work together to allow you to bend and straighten your elbow. Your biceps muscle, shown in Figure below, contracts, and at the same time the triceps muscle relaxes. The contracting biceps pull on the radius bone and the elbow bends. To straighten the arm, the biceps muscle relaxes and the triceps on the opposite side of the elbow joint contracts. The biceps is the flexor and the triceps is the extensor of your elbow joint. In this way the joints of your body act like levers. This lever action of your joints reduces the amount of energy you have to spend to make large body movements.

  Figure 16.23

  The biceps and triceps act against one another to bend and straighten the elbow joint. To bend the elbow, the biceps contract and the triceps relax. To straighten the elbow, the triceps contract and the biceps relax.

  Muscles and the Nervous System

  Muscles are controlled by the nervous system (see the Controlling the Body chapter). Nerves send messages to the muscular system from the brain. Nerves also send messages to the brain from the muscles. Remember that smooth and cardiac muscles are involuntary muscles. This means that you cannot control the nerve messages that get sent to and from these muscles. For example, you cannot make your heart muscle stop beating. Likewise, you cannot make food stop moving through your intestines. You can however control the movement of your skeletal muscle. When you want to move your foot, electrical messages called impulses move along nerve cells from your brain to the muscles of your foot. At the point at which the nerve cell and muscle cells meet, the electrical message is converted to a chemical message. The muscle cells receive the chemical message, which causes tiny protein fibers inside the muscle cells to get shorter. The muscles contract, pulling on the bones, and your foot moves.

  Contraction

  A muscle contraction occurs when a muscle fiber, which is a muscle cell, generates tension through the movement of actin and myosin, two of the proteins involved in this process (see below).

  Each muscle fiber contains cellular proteins and hundreds or thousands of myofibrils. Each myofibril is a long, cylindrical organelle that is made up of two types of protein filaments: actin and myosin. The actin filament is thin and threadlike, while the myosin filament is thicker. Myosin has a “head” region that uses energy from ATP to “walk” along the actin thin filament (Figure below). The overlapping arrangement of actin and myosin filaments gives skeletal muscle its striated appearance. The actin and myosin filaments are organized into repeating units called sarcomeres, which can be seen in Figure above. The sarcomeres stretch from one Z-line to the next, with thin actin filaments anchored to these Z lines. When each end of the myosin thick filament moves along the actin filament, the two actin filaments at opposite sides of the sacromere are drawn closer together and the sarcomere shortens, as shown in Figure below. When a muscle fiber contracts, all sarcomeres contract at the same time, which pulls on the fiber ends.

  Figure 16.24

  The components of muscle contraction. The is the functional unit of muscle contraction; it reaches from one Z-line to the next (labeled Z-disk in ). In a relaxed muscle, the actin (thin filament) and myosin (thick filament) overlap. In a muscle contraction, the filaments slide past each other, shortening the sacromere. This model of contraction is called the sliding filament model.

  Figure 16.25

  When each end of the myosin thick filament moves along the actin filament, the two actin filaments at opposite sides of the sacromere are drawn closer together and the sarcomere shortens.

  The Sliding Filament Theory

  The widely accepted theory of how muscles contract is called the sliding-filament model (also known as the sliding filament theory), which is shown in Figure below. The presence of calcium ions (Ca2+) allows for the interaction of actin and myosin. In the resting state, these two proteins are prevented from coming into contact. Two other proteins, troponin and tropomyosin, act as a barrier between the actin and myosin, preventing contact between them. When Ca2+ binds to the actin filament, the shape of the troponin-tropomyosin complex changes, allowing actin and myosin to contact with each other. Below is an outline of the sliding filament theory.

  Once an action potential (see the Controlling the Body chapter) reaches a muscle fiber, the action potential spreads through the muscle fiber's network, activating specialized storage sites throughout the muscle, called the sarcoplasmic reticulum, to release calcium ions (Ca++). The sarcoplasmic reticulum is a special type of smooth endoplasmic reticulum found in smooth and skeletal muscle that contains large amounts of Ca++.

  The calcium ions bind to actin filaments of the myofibrils and activate the actin for attachment by the myosin heads filaments.

  Activated myosin binds strongly to the actin filament. Upon strong binding, myosin rotates at the myosin-actin junction, which bends a region in the “neck” of the myosin “head,” as shown in the Figure below.

  Shortening of the muscle fiber occurs when the bending neck of the myosin region pulls the actin and myosin filaments across each other. Meanwhile, the myosin heads remain attached to the actin filament, as shown in Figure below.

  The binding of ATP allows the myosin heads to detach from actin. While detached, ATP breaks down to adenosine diphosphate and an inorganic phosphate (ADP + Pi). The breaking of the chemical bond in ATP gives energy to the myosin head, allowing it to bind to actin again.

  Steps 4 and 5 repeat as long as ATP is available and Ca++ is present on the actin filament. The collective bending of numerous myosin heads (all in the same direction) moves the actin filament relative to the myosin filament which causes a shortening of the sacromere. Overall, this process results in muscle contraction. The sarcoplasmic reticulum actively pumps Ca++ back into itself. Muscle contraction stops when Ca++ is removed from the immediate environment of the myofilaments.

  Figure 16.26

  The process of actin and myosin sliding past one another is called crossbridge cycling, and it occurs in all muscle types. Myosin is a molecular motor that moves along the passive actin. Each thick myosin filament has little extensions or heads, that walk along the thin actin filaments during contraction. In this way the thick filament slides over thin filament. The actin filaments transmit the force generated by myosin to the ends of the muscle, which causes the muscle to shorten.

  Muscles and Exercise

  Your muscles are important for carrying out everyday activities. The ability of your body to carry out your daily activities without getting out of breath, sore, or overly tired is called physical fitness. Physical fitness also describes the body’s ability to respond to emergencies and to avoid getting sick. A person can have a good level of physical fitness or a poor level of fitness. For example, a person who becomes breathless and tired after climbing a flight of stairs is not physically fit.

  Physical exercise is any activity that maintains or improves physical fitness and overall health. Regular physical exercise is important in preventing lifestyle diseases such as heart disease, cardiovascular disease, Type 2 diabetes, and obesity.

  Regular exercise improves the health of the muscular system. Muscles that are exercised are bigger and stronger than muscles that are not exercised. Exercise improves both muscul
ar strength and muscular endurance. Muscular strength is the ability of a muscle to exert force during a contraction. Muscular endurance is the ability of a muscle to continue to contract over a long time without getting tired. Two types of exercises help improve the fitness of muscles, anaerobic exercise and aerobic exercise.

  Exercises are grouped into three types depending on the effect they have on the body:

  Aerobic exercises such as cycling, walking, and running, increase muscular endurance.

  Anaerobic exercises such as weight training, or sprinting increase muscle strength.

  Flexibility exercises such as stretching, improve the range of motion of muscles and joints. Regular stretching helps avoid activity-related injuries.

  Anaerobic Exercise and Muscular Strength

  Anaerobic exercises cause muscles to get bigger and stronger. Anaerobic exercises use a resistance against which the muscle has to work to lift or push away. The resistance can be a weight or a person’s own body weight, as shown in Figure below. As a result of repeated muscle contractions, muscle fibers build up larger energy stores and the muscle tissue gets bigger. The larger a muscle is the greater the force it can apply to lift a weight or move a body joint. The muscles of weight lifters are large, and are therefore strong.