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Biology LibreTexts

15.3: Types of Muscle Tissue

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  • Work Those Eye Muscles!

    He turns his gray-blue eyes in your direction—a very small movement, considering the conspicuously large and strong external eye muscles that control eyeball movements. These muscles have been called the strongest muscles in the human body relative to the work they do. However, the external eye muscles actually do a surprising amount of work. Eye movements occur almost constantly during waking hours, especially when we are scanning faces or reading. Eye muscles are also exercised nightly during the phase of sleep called rapid eye movement sleep. External eye muscles can move the eyes because they are made mainly of muscle tissue.

    Blue Grey Eyes
    Figure \(\PageIndex{1}\): Image used with permission (CC); Miansari66; via Wikimedia.org: Blue grey Eyes. jpg)

    What is Muscle Tissue?

    Muscle Tissue
    Figure \(\PageIndex{2}\): 1) Skeletal muscle cells are long tubular cells with striations (3) and multiple nuclei (4). The nuclei are embedded in the cell membrane (5) so that they are just inside the cell. This type of tissue occurs in the muscles that are attached to the skeleton. Skeletal muscles function involuntary movements of the body.2) Smooth muscle cells are spindle-shaped (6), and each cell has a single nucleus (7). Unlike skeletal muscle, there are no striations. The smooth muscle acts involuntarily and functions in the movement of substances in the lumens. They are primarily found in blood vessel walls and walls along the digestive tract. 3) Cardiac muscle cells branch off from each other, rather than remaining along with each other like the cells in the skeletal and smooth muscle tissues. Because of this, there are junctions between adjacent cells (9). The cells have striations (8), and each cell has a single nucleus (10). This type of tissue occurs in the wall of the heart and its primary function is for pumping blood. This is an involuntary action. (Mdunning13 [CC BY-SA 3.0] via wikimedia.org:Muscle Tissue (1).svg )

    Muscle tissue is a soft tissue that makes up most of the tissues in the muscles of the human muscular system. Other tissues in muscles are connective tissues, such as tendons that attach skeletal muscles to bones and sheaths of connective tissues that cover or line muscle tissues. Only muscle tissue per se, however, has cells with the ability to contract.

    There are three major types of muscle tissues in the human body: skeletal, smooth, and cardiac muscle tissues. Figure \(\PageIndex{2}\) shows how the three types of muscle tissues appear under a microscope. When you read about each type below, you will learn why the three types appear as they do.

    Skeletal Muscle Tissue

    Muscles anterior labeled

    Figure \(\PageIndex{3}\): This figure shows major skeletal muscles in the front (anterior) of the body. (public domain; Mikael Häggström, 2014; via wikimedia.org; Muscles anterior labeled.png)

    Muscle posterior labeled
    Figure \(\PageIndex{4}\): This figure shows major skeletal muscles in the back (posterior) of the body. (public domain; Mikael Häggström, 2014; via wikimedia.org; muscles posterior labeled.png)

    Skeletal muscle is muscle tissue that is attached to bones by tendons, which are bundles of collagen fibers. Whether you are moving your eyes or running a marathon, you are using skeletal muscles. Contractions of skeletal muscles are voluntary, or under the conscious control of the central nervous system via the somatic nervous system. Skeletal muscle tissue is the most common type of muscle tissue in the human body. By weight, an average adult male is about 42 percent skeletal muscles, and the average adult female is about 36 percent skeletal muscles. Some of the major skeletal muscles in the human body are labeled in Figure \(\PageIndex{3}\) and Figure \(\PageIndex{4}\).

    Skeletal Muscle Pairs

    muscle Flexion extension
    Figure \(\PageIndex{5}\): Triceps and biceps muscles in the upper arm are opposing muscles that move the arm at the elbow in opposite directions. Image used with permission (CC BY-NC 3.0; CK-12 Foundation)

    To move bones in opposite directions, skeletal muscles often consist of muscle pairs that work in opposition to one another. For example, when the biceps muscle (on the front of the upper arm) contracts, it can cause the elbow joint to flex or bend the arm, as shown in the figure below. When the triceps muscle (on the back of the upper arm) contracts, it can cause the elbow to extend or straighten the arm. The biceps and triceps muscles are an example of a muscle pair where the muscles work in opposition to each other.

    Skeletal Muscle Structure

    Each skeletal muscle consists of hundreds — or even thousands — of skeletal muscle fibers, which are long, string-like cells. As shown in the figure below, skeletal muscle fibers are individually wrapped in connective tissue called endomysium. The skeletal muscle fibers are bundled together in units called muscle fascicles, which are surrounded by sheaths of connective tissue called perimysium. Each fascicle contains between ten and 100 (or even more!) skeletal muscle fibers. Fascicles, in turn, are bundled together to form individual skeletal muscles, which are wrapped in connective tissue called epimysium. The connective tissues in skeletal muscles have a variety of functions. They support and protect muscle fibers, allowing them to withstand the forces of contraction by distributing the forces applied to the muscle. They also provide pathways for nerves and blood vessels to reach the muscles. In addition, the epimysium anchors the muscles to tendons.

    connective tissue of skeletal muscle
    Figure \(\PageIndex{6}\): Each skeletal muscle has a structure of bundles within bundles. Bundles of muscle fibers make up a muscle fascicle, and bundles of fascicles make up a skeletal muscle. At each level of bundling, a connective tissue membrane surrounds the bundle. Image used with permission (Arcadian; public domain; via wikimedia.org; muscle structure)

    The same bundles-within-bundles structure is replicated within each muscle fiber. As shown in the figure below, a muscle fiber consists of a bundle of myofibrils, which are themselves bundles of protein filaments. These protein filaments consist of thin filaments of the protein actin — which are anchored to structures called Z discs — and thick filaments of the protein myosin. The filaments are arranged together within a myofibril in repeating units called sarcomeres, which run from one Z disc to the next. The sarcomere is the basic functional unit of skeletal (and cardiac) muscles. It contracts as actin and myosin filaments slide over one another. Skeletal muscle tissue is said to be striated because it appears striped. It has this appearance because of the regular, alternating A (dark) and I (light) bands of filaments arranged in sarcomeres inside the muscle fibers. Other components of a skeletal muscle fiber include multiple nuclei and mitochondria.

     Muscle Fiber with myofibrils
    Figure \(\PageIndex{7}\): Bundles of protein filaments form a myofibril, and bundles of myofibrils make up a single muscle fiber. I and A bands refer to the positioning of myosin and actin fibers in a myofibril. Sarcoplasmic reticulum is a specialized type of endoplasmic reticulum that forms a network around each myofibril. It serves as a reservoir for calcium ions, which are needed for muscle contractions. H zones and Z discs are also involved in muscle contractions, which you can read about in the concept of Muscle Contraction. Image used with permission (CC BY-NC 3.0; OpenStax; via wikimedia.org; muscle fibers)

    Slow- and Fast-Twitch Skeletal Muscle Fibers

    Skeletal muscle fibers can be divided into two types, called slow-twitch (or type I) muscle fibers and fast-twitch (or type II) muscle fibers.

    • Slow-twitch muscle fibers are dense with capillaries and rich in mitochondria and myoglobin, which is a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibers, slow-twitch fibers can carry more oxygen and sustain aerobic (oxygen-using) activity. Slow-twitch fibers can contract for long periods of time, but not with very much force. They are relied upon primarily in endurance events, such as distance running or cycling.
    • Fast-twitch muscle fibers contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fiber can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibers can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibers, fast-twitch fibers contribute more to muscle strength and have a greater potential for increasing in mass. They are relied upon primarily in short, strenuous events, such as sprinting or weight lifting.

    Proportions of fiber types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fiber than the other. Generally, an individual who has more slow-twitch fibers is better suited for activities requiring endurance, whereas an individual who has more fast-twitch fibers is better suited for activities requiring short bursts of power.

    Smooth Muscle

    Smooth muscle is muscle tissue in the walls of internal organs and other internal structures such as blood vessels. When smooth muscles contract, they help the organs and vessels carry out their functions. When smooth muscles in the stomach wall contract, for example, they squeeze the food inside the stomach, helping to mix and churn the food and break it into smaller pieces. This is an important part of digestion. Contractions of smooth muscles are involuntary, so they are not under conscious control. Instead, they are controlled by the autonomic nervous system, hormones, neurotransmitters, and other physiological factors.

    Structure of Smooth Muscle

    Actin myosin filaments of smooth muscle
    Figure \(\PageIndex{8}\): The basic mechanism of muscle contraction in smooth muscle is the same as that in other types of muscle tissue. Image used with permission (Boumphreyfr [CC BY-SA 3.0]; via wikimedia.org; actin myosin filaments.png)

    The cells that make up smooth muscle are generally called myocytes. Unlike the muscle fibers of striated muscle tissue, the myocytes of smooth muscle tissue do not have their filaments arranged in sarcomeres. Therefore, smooth tissue is not striated. However, the myocytes of smooth muscle do contain myofibrils, which in turn contain bundles of myosin and actin filaments. The filaments cause contractions when they slide over each other, as shown in Figure \(\PageIndex{8}\).

    Functions of Smooth Muscle

    Placenta
    Figure \(\PageIndex{9}\): The muscular uterine wall stretches to a great extent to accommodate a growing fetus, yet it can still contract with great force during the labor that precedes childbirth. At that time, it can exert up to 100 pounds of force. Image used with permission (Magnus Manske; public domain; via wikimedia.org; placenta.svg)

    Unlike striated muscle, smooth muscle can sustain very long-term contractions. Smooth muscle can also stretch and still maintain its contractile function, which striated muscle cannot. The elasticity of smooth muscle is enhanced by an extracellular matrix secreted by myocytes. The matrix consists of elastin, collagen, and other stretchy fibers. The ability to stretch and still contract is an important attribute of smooth muscle in organs such as the stomach and uterus (Figure \(\PageIndex{9}\)), both of which must stretch considerably as they perform their normal functions.

    The following list indicates where many smooth muscles are found, along with some of their specific functions.

    • Walls of organs of the gastrointestinal tract (such as the esophagus, stomach, and intestines), moving food through the tract by peristalsis
    • Walls of air passages of the respiratory tract (such as the bronchi), controlling the diameter of the passages and the volume of air that can pass through them
    • Walls of organs of the male and female reproductive tracts; in the uterus, for example, pushing a baby out of the uterus and into the birth canal
    • Walls of structures of the urinary system, including the urinary bladder, allowing the bladder to expand so it can hold more urine, and then contract as urine is released
    • Walls of blood vessels, controlling the diameter of the vessels and thereby affecting blood flow and blood pressure
    • Walls of lymphatic vessels, squeezing the fluid called lymph through the vessels
    • Iris of the eyes, controlling the size of the pupils and thereby the amount of light entering the eyes
    • Arrector pili in the skin, raising hairs in hair follicles in the dermis
     

    Cardiac Muscle

    Heart Wall
    Figure \(\PageIndex{10}\): The thick wall of the heart consists mainly of cardiac muscle tissue called myocardium. (OpenStax College [CC BY 3.0 )

    Cardiac muscle is found only in the wall of the heart. It is also called myocardium. As shown in the figure below, the myocardium is enclosed within connective tissues, including the endocardium on the inside of the heart and pericardium on the outside of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Contractions of cardiac muscle are involuntary, like those of smooth muscles. They are controlled by electrical impulses from specialized cardiac muscle cells in an area of the heart muscle called the sinoatrial node.

    Like skeletal muscle, cardiac muscle is striated because its filaments are arranged in sarcomeres inside the muscle fibers. However, in cardiac muscle, the myofibrils are branched at irregular angles rather than arranged in parallel rows (as they are in skeletal muscle). This explains why cardiac and skeletal muscle tissues look different from one another.

    The cells of cardiac muscle tissue are arranged in interconnected networks. This arrangement allows rapid transmission of electrical impulses, which stimulate virtually simultaneous contractions of the cells. This enables the cells to coordinate contractions of the heart muscle.

    The heart is the muscle that performs the greatest amount of physical work in the course of a lifetime. Although the power output of the heart is much less than the maximum power output of some other muscles in the human body, the heart does its work continuously over an entire lifetime without rest. Cardiac muscle contains a great many mitochondria, which produce ATP for energy and help the heart resist fatigue.

    Feature: Human Biology in the News

    The human heart develops in a sequence of events that are controlled by communication among different types of cells, including cells that will become myocardium (the cardiac muscle that forms the wall of the heart) and cells that will become endocardium (the connective tissue that covers the inside surface of the myocardium). If communication among the cells is abnormal, it can lead to a variety of heart defects, such as cardiac hypertrophy, or abnormal enlargement of the heart muscle. Cardiac hypertrophy causes the heart to thicken and weaken over time, so it is less able to pump blood. Eventually, heart failure may develop, causing fluid to build up in the lungs and extremities.

    Abnormal cell communication is the mechanism by which a mutation called PTPN11 leads to cardiac hypertrophy in a disorder referred to as NSML (Noonan Syndrome with Multiple Lentigines). New research by scientists at Beth Israel Deaconess Medical Center in Boston has determined in which type of cells abnormalities occur that lead to NSML. In the research, the scientists engineered mouse models to express the PTPN11 mutation as they developed. The researchers manipulated the mouse models so that in some of the mice, the mutation was expressed only in cells that would develop into the myocardium, whereas in other mice, the mutation was expressed only in cells that would develop into endocardium. Unexpectedly, hypertrophy of the heart occurred only in the mice that expressed the mutation in endocardial cells, not in myocardial cells, which had long been assumed to be the cells affected. The results of the research suggest potential targets for the treatment of NSML. They may also help scientists understand the causes of other types of cardiac disorders that are much more common than NSML.

    Summary

    • Muscle tissue is a soft tissue that makes up most of the tissues in the muscles of the human muscular system. It is the only type of tissue that has cells with the ability to contract.
    • Skeletal muscle tissue is attached to bones by tendons. It allows voluntary body movements.
    • Skeletal muscle is the most common type of muscle tissue in the human body. To move bones in opposite directions, skeletal muscles often consist of pairs of muscles that work in opposition to one another to move bones in different directions at joints.
    • Skeletal muscle fibers are bundled together in units called muscle fascicles, which are bundled together to form individual skeletal muscles. Skeletal muscles also have connective tissue supporting and protecting the muscle tissue.
    • Each skeletal muscle fiber consists of a bundle of myofibrils, which are bundles of protein filaments. The filaments are arranged in repeating units called sarcomeres, which are the basic functional units of skeletal muscles. Skeletal muscle tissue is striated because of the pattern of sarcomeres in its fibers.
    • Skeletal muscle fibers can be divided into two types, called slow-twitch and fast-twitch fibers. Slow-twitch fibers are used mainly in aerobic endurance activities, such as long-distance running. Fast-twitch fibers are used mainly for non-aerobic, strenuous activities, such as sprinting. Proportions of the two types of fibers vary from muscle to muscle and person to person.
    • Smooth muscle tissue is found in the walls of internal organs and vessels. When smooth muscles contract, they help the organs and vessels carry out their functions. Contractions of smooth muscles are involuntary and controlled by the autonomic nervous system, hormones, and other substances.
    • Cells of smooth muscle tissue are not striated because they lack sarcomeres, but the cells contract in the same basic way as striated muscle cells. Unlike striated muscle, smooth muscle can sustain very long-term contractions and maintain its contractile function, even when stretched.
    • Cardiac muscle tissue is found only in the wall of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Contractions of cardiac muscle are involuntary, like those of smooth muscles. They are controlled by electrical impulses from specialized cardiac cells.
    • Like skeletal muscle, cardiac muscle is striated because its filaments are arranged in sarcomeres inside the muscle fibers. However, the myofibrils are branched instead of arranged in parallel rows, making cardiac and skeletal muscle tissues look different from one another.
    • The heart is the muscle that performs the greatest amount of physical work in the course of a lifetime. Its cells contain a great many mitochondria to produce ATP for energy and help the heart resist fatigue.

    Review

    1. What is muscle tissue?

    2. Where is skeletal muscle found, and what is its general function?

    3. Why do many skeletal muscles work in pairs?

    4. Describe the structure of a skeletal muscle.

    5. Relate muscle fiber structure to the functional units of muscles.

    6. Why is skeletal muscle tissue striated?

    7. Compare and contrast slow-twitch and fast-twitch skeletal muscle fibers.

    8. Where is smooth muscle found? What controls the contraction of smooth muscle?

    9. Compare and contrast smooth muscle and striated muscle (such as skeletal muscle).

    10. Where is cardiac muscle found? What controls its contractions?

    11. Both cardiac and skeletal muscle tissues are striated, but they look different from one another. Why?

    12. The heart muscle is smaller and less powerful than some other muscles in the body. Why is the heart the muscle that performs the greatest amount of physical work in the course of a lifetime? How does the heart resist fatigue?

    13. Arrange the following units within a skeletal muscle in order, from smallest to largest: fascicle; sarcomere; muscle fiber; myofibril

    14. Give one example of connective tissue that is found in muscles. Describe one of its functions.

    15. True or False: skeletal muscle fibers are cells with multiple nuclei.

    Explore More

    You can learn more about the three types of muscle tissues by watching this Khan Academy video:

    Check out this video to learn about pacemakers for the heart and how they work: