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3.5: Muscle Tissue

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

    • Identify the three types of muscle tissue
    • Compare and contrast the functions of each muscle tissue type
    • Explain how muscle tissue can enable motion

    Muscle tissue is characterized by properties that allow movement. Muscle cells are excitable; they respond to a stimulus. They are contractile, meaning they can shorten and generate a pulling force. When attached between two movable objects, such as bones on either side of a joint, contractions of the muscles cause the bones to move. Some muscle movement is voluntary, which means it is under conscious control. For example, a person decides to open a book and read a chapter on anatomy. Other movements are involuntary, meaning they are not under conscious control, such as the contraction of your pupil in bright light. Muscle tissue is classified into three types according to structure and function: skeletal, cardiac, and smooth (Table \(\PageIndex{1}\) and Figure \(\PageIndex{2}\) ).

    Table \(\PageIndex{1}\): Comparison of Structure and Properties of Muscle Tissue Types
    Tissue Histology Function Location
    Skeletal Long cylindrical fiber, striated, many peripherally located nuclei Voluntary movement, produces heat, protects organs Attached to bones and around entrance points to body (e.g., mouth, anus)
    Cardiac Short, branched, striated, single central nucleus Contracts to pump blood Heart
    Smooth Short, spindle-shaped, no evident striation, single nucleus in each fiber Involuntary movement, moves food, involuntary control of respiration, moves secretions, regulates flow of blood in arteries by contraction Walls of major organs and passageways

    Skeletal muscle is attached to bones and its contraction makes possible locomotion, facial expressions, posture, and other voluntary movements of the body. Forty percent of your body mass is made up of skeletal muscle. Skeletal muscles generate heat as a byproduct of their contraction and thus participate in thermal homeostasis. Shivering is an involuntary contraction of skeletal muscles in response to perceived lower than normal body temperature. The muscle cell, or myocyte, develops from myoblasts derived from the mesoderm. Myocytes and their numbers remain relatively constant throughout life. Skeletal muscle tissue is arranged in bundles surrounded by connective tissue. Under the light microscope, muscle cells appear striated with many nuclei squeezed along the membranes. The striation is due to the regular alternation of the contractile proteins actin and myosin, along with the structural proteins that couple the contractile proteins to connective tissues. The cells are multinucleated as a result of the fusion of the many myoblasts that fuse to form each long muscle fiber. Skeletal muscle is discussed in much greater detail in a later chapter.

    Cardiac muscle forms the contractile walls of the heart. The cells of cardiac muscle, known as cardiomyocytes, also appear striated under the microscope. Unlike skeletal muscle fibers, cardiomyocytes are single cells typically with a single centrally located nucleus. A principal characteristic of cardiomyocytes is that they contract on their own intrinsic rhythms without any external stimulation; they are described as autorhythmic. Cardiomyocytes attach to one another with specialized cell junctions called intercalated discs (Figure \(\PageIndex{1}\)). Intercalated discs have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle fibers that are, essentially, a mechanical and electrochemical syncytium (interconnected mass of fibers) allowing the cells to synchronize their actions. The cardiac muscle pumps blood through the body and is under involuntary control. The attachment junctions hold adjacent cells together across the dynamic pressures changes of the cardiac cycle.

    cardiac muscle fibers with intercalated discs
    Figure \(\PageIndex{1}\): Cardiac Muscle. Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes. (Image credit: "Cardiac Muscle" by OpenStax is licensed under CC BY 3.0)

    Smooth muscle tissue contraction is responsible for involuntary movements in the internal organs. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and arteries. Smooth muscle is also present in the eyes, where it functions to change the size of the iris and alter the shape of the lens; and in the skin where it causes hair to stand erect in response to cold temperature or fear. Each cell is spindle shaped with a single nucleus and no visible striations (Figure \(\PageIndex{2}\)).

    Skeletal, Smooth, and Cardiac muscle tissue as viewed under the microscope
    Figure \(\PageIndex{2}\): Muscle Tissue. (a) Skeletal muscle cells have prominent striation and nuclei on their periphery. (b) Smooth muscle cells have a single nucleus and no visible striations. (c) Cardiac muscle cells appear striated and have a single nucleus. From top, LM × 1600, LM × 1600, LM × 1600. (Image credit: "Skeletal Smooth Cardiac" by OpenStax is licensed under CC BY 3.0/ Micrographs provided by the Regents of University of Michigan Medical School © 2012)

    Concept Review

    The three types of muscle cells are skeletal, cardiac, and smooth. Their morphologies match their specific functions in the body. Skeletal muscle is voluntary and responds to conscious stimuli. The cells are striated and multinucleated appearing as long, unbranched cylinders. Cardiac muscle is involuntary and found only in the heart. Each cell is striated with a single nucleus and they attach to one another to form long fibers. Cells are attached to one another at intercalated discs. The cells are interconnected physically and electrochemically to act as a syncytium. Cardiac muscle cells contract autonomously and involuntarily. Smooth muscle is involuntary. Each cell is a spindle-shaped fiber and contains a single nucleus. No striations are evident because the actin and myosin filaments do not align in the cytoplasm.

    Review Questions

    Q. Striations, cylindrical cells, and multiple nuclei are observed in ________.

    A. skeletal muscle only

    B. cardiac muscle only

    C. smooth muscle only

    D. skeletal and cardiac muscles

    Answer

    Answer: A

    Q. The cells of muscles, myocytes, develop from ________.

    A. myoblasts

    B. endoderm

    C. fibrocytes

    D. chondrocytes

    Answer

    Answer: A

    Q. Skeletal muscle is composed of very hard working cells. Which organelles do you expect to find in abundance in skeletal muscle cell?

    A. nuclei

    B. striations

    C. golgi bodies

    D. mitochondria

    Answer

    Answer: D

    Critical Thinking Questions

    Q. You are watching cells in a dish spontaneously contract. They are all contracting at different rates; some fast, some slow. After a while, several cells link up and they begin contracting in synchrony. Discuss what is going on and what type of cells you are looking at.

    Answer

    A. The cells in the dish are cardiomyocytes, cardiac muscle cells. They have an intrinsic ability to contract. When they link up, they form intercalating discs that allow the cells to communicate with each other and begin contracting in synchrony.

    Q. Why does skeletal muscle look striated?

    Answer

    A. Under the light microscope, cells appear striated due to the arrangement of the contractile proteins actin and myosin.

    Glossary

    cardiac muscle
    heart muscle, under involuntary control, composed of striated cells that attach to form fibers, each cell contains a single nucleus, contracts autonomously
    myocyte
    muscle cells
    skeletal muscle
    usually attached to bone, under voluntary control, each cell is a fiber that is multinucleated and striated
    smooth muscle
    under involuntary control, moves internal organs, cells contain a single nucleus, are spindle-shaped, and do not appear striated; each cell is a fiber
    striation
    alignment of parallel actin and myosin filaments which form a banded pattern

    Contributors and Attributions

    This page titled 3.5: Muscle Tissue is shared under a CC BY license and was authored, remixed, and/or curated by Whitney Menefee, Julie Jenks, Chiara Mazzasette, & Kim-Leiloni Nguyen (ASCCC Open Educational Resources Initiative) .