Chapter 8: The Muscular System

Chapter Synopsis

This chapter describes the general structure and overall function of muscle tissue. There are three major types of muscle tissue: skeletal or striated, smooth or nonstriated, and cardiac. The muscular system is often referred to as the power system because it provides all the motion needed for the body to move. Muscle contractions are designed as one of several types: isotonic, isometric, twitch, or tetanic. Muscles must be used to prevent atrophy. Scientific evidence points to the fact that proper exercise affects the entire system. This chapter covers major muscular disorders as well.

Chapter Objectives and Notes

1.      List, locate in the body, and compare the structure and function of the three major types of muscle tissue.

a.       There are three major types of muscle tissue.

                           i.      Striated muscle, also known as skeletal or voluntary muscle, when observed under the microscope, appears as bundles of fine threads with many crosswise stripes. This type of muscle tissue is always attached to bones in our body and its contractions are controlled voluntarily.

                         ii.      Another type of muscle tissue called nonstriated, smooth, or involuntary, when viewed under the microscope shows no cross stripes. It forms most of our internal body organs, and we have no control over its contractions.

                        iii.      The third type of muscle tissue, called cardiac, is made up of branching cells with very faint striations. It makes up the heart.

2.      Discuss the microscopic structure of a skeletal muscle sarcomere and motor unit.

a.                   Skeletal muscle tissue, when observed under the microscope, is found to be made up of fine, threadlike structures called myofilaments: thick, composed of a protein called myosin, and thin, composed of a protein called actin. These myofilaments are arranged in the form of dark and light stripes that are separated from each other by dark bands called Z-lines. Dark stripes are called A-bands, and they are made up of both actin and myosin. Light bands are called I-bands, and they are made up only of actin. The Z-line runs through the I-band region. The basic unit of muscle contraction, called the sarcomere, is defined as the area that runs from one Z-line to the next. Shortening of the sarcomere or moving of thin and thick molecules toward one another serves as the basis of muscle contraction.

b.                  Muscle cells are stimulated by a nerve fiber that enters the muscle fiber. This nerve is called the motor neuron. The motor neuron, together with the muscle cell it innervates, is called the motor unit.

3.      Discuss how a muscle is stimulated and compare the major types of skeletal muscle contractions.

a.       The point of contact between a neuron and a muscle fiber is called the neuromuscular junction. Chemicals must pass across this junction to initiate contraction

b.      One type of muscle contraction pattern that can be produced is called isometric. Isometric contraction does not produce movement. It causes muscle length to remain the same while tension within the muscle is increased. Pushing against an immovable object is an example of isometric contraction.

c.       Another type of contraction is called isotonic. It does produce movement at a joint. The muscle does shorten, and the insertion end moves toward the point of origin. Lifting an object is an example of isotonic contraction.

d.      A twitch contraction is a quick, jerky contraction in response to a single stimulus. A tetanic contraction is more sustained than a twitch. It is produced by a series of stimuli bombarding the muscle in rapid succession.

4.      Name and identify on a model or diagram and give the function of the major muscles of the body discussed in this chapter. Following is a list of the major muscles of the body and their function.

Muscle                                    Function

                        Pectoralis major                        Flexes upper arm; helps adduct upper arm

                        Latissimus dorsi                        Extends upper arm; helps adduct upper arm

                        Deltoid                                     Abducts upper arm

                        Biceps brachii                           Flexes lower arm

                        Triceps brachii              Extends lower arm

                        Iliopsoas                                   Flexes trunk or thigh

                        Sartorius                                   Flexes thigh; rotates lower leg

                        Gluteus maximus                       Extends thigh

                        Adductors                                Adducts thigh

                        Hamstring group                       Flexes lower leg

                        Quadriceps group                     Extends lower leg

                        Gastrocnemius              Plantar flexes foot

                        Rectus abdominis                      Flexes trunk

                        Trapezius                                  Extends head and neck

                        Sternocleidomastoid                 Rotates and extends head

                        Orbicularis oculi                        Closes eye

                        Zygomaticus                             Elevates corners of mouth and lips

                        Orbicularis oris             Draws lips together

                        Masseter                                  Closes jaw

5.      List and explain the most common types of movement produced by skeletal muscles. Types of movement produced by skeletal muscle contractions include the following.

a.       Flexion—a movement that makes the angle between two bones at their joint smaller than it was at the beginning of the movement.

b.      Extension—a movement that makes the angle between two bones at their joint larger than it was at the beginning of the movement.

c.       Abduction—a movement that takes a part away from the midline of the body.

d.      Adduction—a movement that takes a part toward the midline of the body.

e.       Rotation—a movement around a longitudinal axis.

6.      Name and describe the major disorders of skeletal muscles.

a.       Muscle injuries, caused by overexertion or trauma, are characterized by myalgia and result from over-stretching or tearing of muscle fibers. If it involves ligament damage, it may be called a sprain.

b.      Muscle infections caused by bacteria, viruses, and parasites often produce widespread myositis as seen in trichinosis and the flu.

c.       Muscular dystrophy is a group of genetic disorders characterized by atrophy of skeletal muscle tissue. Duchenne muscular dystrophy (DMD) is most common form, believed to be caused by a missing fragment in an X chromosome.

d.      Myasthenia gravis is characterized by weakness and chronic fatigue.

Chapter 8 Review


Susan, age 39, is a secretary. She spends most of her working day at a desk. Recently she has complained of feeling tired even though she receives 7 to 8 hours of sleep a night. Her doctor has recommended a walking program to counteract her sedentary lifestyle. She is reluctant to begin.

1.      What could you tell Susan about the benefits of regular exercise?






2.      Susan asks, “Is there really any danger in not exercising?” She is given the following information. (Fill in the missing words.) When muscles are inactive for a period of time, they actually shrink in size and mass, with a reduction in muscle strength making them less efficient. This is technically called

(A) __________, which is just the opposite of muscles that increase in size, called (B) __________.

3.      She asks if you are exercising, and you tell her you are running, which is a type of (A) training, such as bicycling or walking on a treadmill. What physical benefits are derived from this type of regular exercise?






4.      Susan spends much of her day at work typing. This repetitive motion may in time cause her difficulty. A common inflammatory problem often caused by such repetitive movement of the wrist and other joints is called (A)_________________ ? What are some of the symptoms present with inflammation of the tendon sheath? (B) ________________________ (C) _______________________________ (D) ________________________________________
If swelling occurs around the tendons in the area of the wrist, it is called (E) _____________________.


Define the following vocabulary words:

5.                                                                                                                                                                                          Abduction

6.                                                                                                                                                                                          Adduction

7.                                                                                                                                                                                          Atrophy

8.                                                                                                                                                                                          Flexion

9.                                                                                                                                                                                          Hypertrophy

10.                                                                                                                                                                                      Isometric

11.                                                                                                                                                                                      Motor neuron

12.                                                                                                                                                                                      Pronation

13.                                                                                                                                                                                      Supination

14.                                                                                                                                                                                      Tendon


Match the descriptions in Column A to the terms in Column B. Write the corresponding letter in the blank provided.

Column A                                                                                 Column B

15. ____ Moving parts away from the midline              A.        Extension

16. ____ Moving palm from anterior to posterior position         B.         Flexion

17. ____ Standing on your toes                                               C.        Abduction

18. ____ Makes joint angle larger                                            D.        Adduction

19. ____ Movement around longitudinal axis                E.         Rotation

20. ____ Palms turned to anterior position                               F.         Supination

21. ____ Makes joint angle smaller                                          G.        Pronation

22. ____ Moving a part toward the middle                               H.        Dorsiflexion     

23. ____ Movement to elevate the top of foot               I.          Plantar flexion  

24. ____ Cardiac muscle                                                          J.          Involuntary muscle