12 Muscles

Striated muscle sarcomeres and sarcoplasmic reticulum between the fibers

Learning Objectives


    1. 12.1 Draw and label a series of diagrams to show the different levels of organization of skeletal muscle.

    2. 12.2 Diagram the sliding filament theory of contraction.

    3. 12.3 Diagram the molecular events of excitation-contraction coupling and the contractile cycle.

    4. 12.4 Discuss the different possible causes for muscle fatigue.

    5. 12.5 Discuss the differences between slow-twitch fibers, fast-twitch oxidative-glycolytic fibers, and fast-twitch glycolytic fibers.

    6. 12.6 Explain how muscle length influences force of contraction.

    7. 12.7 Distinguish between summation and the different types of tetanus.

    8. 12.8 Define a motor unit and explain how skeletal muscles use them to create graded contractions.


    1. 12.9 Compare and contrast isometric and isotonic contractions.

    2. 12.10 Describe and give examples of how bones and muscles form fulcrums and levers.


    1. 12.11 Diagram smooth muscle anatomy.

    2. 12.12 Diagram smooth muscle contraction and relaxation.

    3. 12.13 Explain slow wave potentials, pacemaker potentials, and pharmacomechanical coupling.


    1. 12.14 Compare and contrast cardiac muscle with skeletal and smooth muscle.

Background Basics

  1. Tendons

  2. Kinases and phosphatases

  3. Isozymes

  4. Anaerobic and aerobic metabolism

  5. Glycolysis

  6. Tonic control

  7. Nitric oxide

  8. Threshold

  9. Summation

  10. Autonomic neurons

  11. Somatic motor neurons

  12. Neuromuscular junction

It was his first time to be the starting pitcher. As he ran from the bullpen onto the field, his heart was pounding and his stomach felt as if it were tied in knots. He stepped onto the mound and gathered his thoughts before throwing his first practice pitch. Gradually, as he went through the familiar routine of throwing and catching the baseball, his heart slowed and his stomach relaxed. It was going to be a good game.

The pitcher’s pounding heart, queasy stomach, and movements as he runs and throws all result from muscle contraction. Our muscles have two common functions: to generate motion and to generate force. Our skeletal muscles also generate heat and contribute significantly to the homeostasis of body temperature. When cold conditions threaten homeostasis, the brain may direct our muscles to shiver, creating additional heat.

The human body has three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle. Most skeletal muscles are attached to the bones of the skeleton, enabling these muscles to control body movement. Cardiac muscle {kardia, heart} is found only in the heart and moves blood through the circulatory system. Skeletal and cardiac muscles are classified as striated muscles {stria, groove} because of their alternating light and dark bands seen under the light microscope (Fig. 12.1a, b).

FIG. 12.1 The three types of muscles

Smooth muscle is the primary muscle of internal organs and tubes, such as the stomach, urinary bladder, and blood vessels. Its primary function is to influence the movement of material into, out of, and within the body. An example is the passage of food through the gastrointestinal tract. Viewed under the microscope, smooth muscle lacks the obvious cross-bands of striated muscles (Fig. 12.1c). Its lack of banding results from the less organized arrangement of contractile fibers within the muscle cells.

Skeletal muscles are often described as voluntary muscles, and smooth and cardiac muscle as involuntary. However, this is not a precise classification. Skeletal muscles can contract without conscious direction, and we can learn a certain degree of conscious control over some smooth and cardiac muscle.

Skeletal muscles are unique in that they contract only in response to a signal from a somatic motor neuron. They cannot initiate their own contraction, and their contraction is not influenced directly by hormones.

In contrast, cardiac and smooth muscle have multiple levels of control. Their primary extrinsic control arises through autonomic innervation, but some types of smooth and cardiac muscle can contract spontaneously, without signals from the central nervous system. In addition, the activity of cardiac and some smooth muscle is subject to modulation by the endocrine system. Despite these differences, smooth and cardiac muscle share many properties with skeletal muscle.

In this chapter, we discuss skeletal and smooth muscle anatomy and contraction, and conclude by comparing the properties of skeletal muscle, smooth muscle, and cardiac muscle. All three muscle types have certain properties in common. The signal to initiate muscle contraction is an intracellular calcium signal, and movement is created when a motor protein called myosin uses energy from adenosine triphosphate (ATP) to change its conformation. The details of these processes vary with the different muscle types.