Basics of Muscular System Summary Questions

MUSCLE TISSUE

Athletes rely on toned skeletal muscles to supply the force required for movement.

 

Introduction

Chapter Objectives

After studying this chapter, you will be able to:

• • Explain the organization of muscle tissue
  • Describe the function and structure of skeletal, cardiac muscle, and smooth muscle
  • Explain how muscles work with tendons to move the body
  • Describe how muscles contract and relax
  • Define the process of muscle metabolism
  • Explain how the nervous system controls muscle tension
  • Relate the connections between exercise and muscle performance
  • Explain the development and regeneration of muscle tissue

 

When most people think of muscles, they think of the muscles that are visible just under the skin, particularly of the limbs.

• These are skeletal muscles, so-named because most of them move the skeleton.
• But there are two other types of muscle in the body, with distinctly different jobs.
  • Cardiac muscle, found in the heart, is concerned with pumping blood through the circulatory system.
  • Smooth muscle is concerned with various involuntary movements, such as having one’s hair stand on end when cold or frightened, or moving food through the digestive system.
• This chapter will examine the structure and function of these three types of muscles.

 

The body contains three types of muscle tissue.

 

10.1 | Overview of Muscle Tissues

By the end of this section, you will be able to:

• • Describe the different types of muscle
  • Explain contractibility and extensibility

 

Muscle is one of the four primary tissue types of the body, and the body contains three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle.

• All three muscle tissues have some properties in common; they all exhibit a quality called excitability as their plasma membranes can change their electrical states (from polarized to depolarized) and send an electrical wave called an action potential along the entire length of the membrane.
  • While the nervous system can influence the excitability of cardiac and smooth muscle to some degree, skeletal muscle completely depends on signaling from the nervous system to work properly.
  • On the other hand, both cardiac muscle and smooth muscle can respond to other stimuli, such as hormones and local stimuli.
  • The muscles all begin the actual process of contracting (shortening) when a protein called actin is pulled by a protein called myosin.
    • This occurs in striated muscle (skeletal and cardiac) after specific binding sites on the actin have been exposed in response to the interaction between calcium ions (Ca++) and proteins (troponin and tropomyosin) that “shield” the actin binding sites.
    • Ca++ also is required for the contraction of smooth muscle, although its role is different: here Ca++ activates enzymes, which in turn activate myosin heads.
    • All muscles require adenosine triphosphate (ATP) to continue the process of contracting, and they all relax when the Ca++ is removed and the actin-binding sites are re-shielded.
• A muscle can return to its original length when relaxed due to a quality of muscle tissue called elasticity.
  • It can recoil back to its original length due to elastic fibers.
• Muscle tissue also has the quality of extensibility; it can stretch or extend.
• Contractility allows muscle tissue to pull on its attachment points and shorten with force.

 

Differences among the three muscle types include the microscopic organization of their contractile proteins—actin and myosin.

• The actin and myosin proteins are arranged very regularly in the cytoplasm of individual muscle cells (referred to as fibers) in both skeletal muscle and cardiac muscle, which creates a pattern, or stripes, called striations.
  • The striations are visible with a light microscope under high magnification.
  • Skeletal muscle fibers are multinucleated structures that compose the skeletal muscle.
  • Cardiac muscle fibers each have one to two nuclei and are physically and electrically connected to each other so that the entire heart contracts as one unit (called a syncytium).

 

Because the actin and myosin are not arranged in such regular fashion in smooth muscle, the cytoplasm of a smooth muscle fiber (which has only a single nucleus) has a uniform, nonstriated appearance (resulting in the name smooth muscle).

• However, the less organized appearance of smooth muscle should not be interpreted as less efficient.
• Smooth muscle in the walls of arteries is a critical component that regulates blood pressure necessary to push blood through the circulatory system; and smooth muscle in the skin, visceral organs, and internal passageways is essential for moving all materials through the body.

 

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Muscle Types

 

10.2 | Skeletal Muscle

By the end of this section, you will be able to:

• • Describe the layers of connective tissues packaging skeletal muscle
  • Explain how muscles work with tendons to move the body
  • Identify areas of the skeletal muscle fibers
  • Describe excitation-contraction coupling

 

The best-known feature of skeletal muscle is its ability to contract and cause movement.

• Skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture.
• Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position.
• Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation.
• Joints can become misaligned or dislocated entirely by pulling on the associated bones; muscles work to keep joints stable.
• Skeletal muscles are located throughout the body at the openings of internal tracts to control the movement of various substances.
  • These muscles allow functions, such as swallowing, urination, and defecation, to be under voluntary control.
• Skeletal muscles also protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs.

 

Skeletal muscles contribute to the maintenance of homeostasis in the body by generating heat.

• Muscle contraction requires energy, and when ATP is broken down, heat is produced.
• This heat is very noticeable during exercise, when sustained muscle movement causes body temperature to rise, and in cases of extreme cold, when shivering produces random skeletal muscle contractions to generate heat.

 

Each skeletal muscle is an organ that consists of various integrated tissues.

• These tissues include the skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue.
• Each skeletal muscle has three layers of connective tissue (called “mysia”) that enclose it and provide structure to the muscle as a whole, and also compartmentalize the muscle fibers within the muscle.
• Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium, which allows a muscle to contract and move powerfully while maintaining its structural integrity.
  • The epimysium also separates muscle from other tissues and organs in the area, allowing the muscle to move independently.

 

 

Bundles of muscle fibers, called fascicles, are covered by the perimysium. Muscle fibers are covered by the endomysium.

Inside each skeletal muscle, muscle fibers are organized into individual bundles, each called a fascicle, by a middle layer of connective tissue called the perimysium.

• This fascicular organization is common in muscles of the limbs; it allows the nervous system to trigger a specific movement of a muscle by activating a subset of muscle fibers within a bundle, or fascicle of the muscle.
• Inside each fascicle, each muscle fiber is encased in a thin connective tissue layer of collagen and reticular fibers called the endomysium.
  • The endomysium contains the extracellular fluid and nutrients to support the muscle fiber.
  • These nutrients are supplied via blood to the muscle tissue.

 

In skeletal muscles that work with tendons to pull on bones, the collagen in the three tissue layers (the mysia) intertwines with the collagen of a tendon.

• At the other end of the tendon, it fuses with the periosteum coating the bone.
• The tension created by contraction of the muscle fibers is then transferred though the mysia, to the tendon, and then to the periosteum to pull on the bone for movement of the skeleton.
• In other places, the mysia may fuse with a broad, tendon-like sheet called an aponeurosis, or to fascia, the connective tissue between skin and bones.
  • The broad sheet of connective tissue in the lower back that the latissimus dorsi muscles (the “lats”) fuse into is an example of an aponeurosis.

 

Every skeletal muscle is also richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal.

• In addition, every muscle fiber in a skeletal muscle is supplied by the axon branch of a somatic motor neuron, which signals the fiber to contract.
• Unlike cardiac and smooth muscle, the only way to functionally contract a skeletal muscle is through signaling from the nervous system.

 

Skeletal Muscle Fibers

Because skeletal muscle cells are long and cylindrical, they are commonly referred to as muscle fibers.

• Skeletal muscle fibers can be quite large for human cells, with diameters up to 100 μm and lengths up to 30 cm (11.8 in) in the Sartorius of the upper leg.
• During early development, embryonic myoblasts, each with its own nucleus, fuse with up to hundreds of other myoblasts to form the multinucleated skeletal muscle fibers.
• Multiple nuclei mean multiple copies of genes, permitting the production of the large amounts of proteins and enzymes needed for muscle contraction.

 

Some other terminology associated with muscle fibers is rooted in the Greek sarco, which means “flesh.”

• The plasma membrane of muscle fibers is called the sarcolemma, the cytoplasm is referred to as sarcoplasm, and the specialized smooth endoplasmic reticulum, which stores, releases, and retrieves calcium ions (Ca++) is called the sarcoplasmic reticulum (SR).
• As will soon be described, the functional unit of a skeletal muscle fiber is the sarcomere, a highly organized arrangement of the contractile myofilaments actin (thin filament) and myosin (thick filament), along with other support proteins.

 

 

A skeletal muscle fiber is surrounded by a plasma membrane called the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells. A muscle fiber is composed of many fibrils, which give the cell its striated appearance.

 

The Sarcomere

The striated appearance of skeletal muscle fibers is due to the arrangement of the myofilaments of actin and myosin in sequential order from one end of the muscle fiber to the other.

• Each packet of these microfilaments and their regulatory proteins, troponin and tropomyosin (along with other proteins) is called a sarcomere.

 

The sarcomere is the functional unit of the muscle fiber.

• The sarcomere itself is bundled within the myofibril that runs the entire length of the muscle fiber and attaches to the sarcolemma at its end.
• As myofibrils contract, the entire muscle cell contracts.
  • Because myofibrils are only approximately 1.2 μm in diameter, hundreds to thousands (each with thousands of sarcomeres) can be found inside one muscle fiber.
• Each sarcomere is approximately 2 μm in length with a three-dimensional cylinder-like arrangement and is bordered by structures called Z-discs (also called Z-lines, because pictures are two-dimensional), to which the actin myofilaments are anchored.
• Because the actin and its troponin-tropomyosin complex (projecting from the Z-discs toward the center of the sarcomere) form strands that are thinner than the myosin, it is called the thin filament of the sarcomere.
• Likewise, because the myosin strands and their multiple heads (projecting from the center of the sarcomere, toward but not all to way to, the Z-discs) have more mass and are thicker, they are called the thick filament of the sarcomere.

 

Figure 10.5 The Sarcomere The sarcomere, the region from one Z-line to the next Z-line, is the functional unit of a skeletal muscle fiber.

 

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Muscle Crash Course: part 1