3.5: Muscle Tissue

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 that is either electrical or chemical.
• 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.
• They are extensible, meaning they and stretch out/lengthen.
• They are elastic and will return to their resting length after stretching.

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	Functions	Locations
Skeletal	Long, cylindrical myofiber Striated Many peripherally located nuclei	Voluntary movement of skeleton and skin Produces heat Protects organs	Attached to bones Attached to skin Around entrance/exit points to body (e.g., mouth, anus)
Cardiac	Short, branched fiber Striated Single central nucleus	Involuntary, rhythmic movement of heart	Heart
Smooth	Short, spindle-shaped fiber No evident striations Single central nucleus	Involuntary contraction controls rate and volume of passage through a tube (food, air, blood, urine, etc.)	Walls of major tubular visceral organs Intraocular muscles

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 myofiber, develops from the fusion of myoblasts derived from the mesoderm. The number of myofibers you have stays relatively constant throughout life. Under the light microscope, muscle cells appear striped (striated) and have many nuclei squeezed along the membranes (Figure \(\PageIndex{1.A}\)). The striations are due to the regular alternation of the contractile proteins actin and myosin along the length of each cell. The cells are multinucleated as a result of the fusion of the many myoblasts that join to form each long muscle fiber. Skeletal muscle is discussed in much greater in a later chapter.

Figure \(\PageIndex{1}\): Muscle Tissues Overview. All muscle tissue types have elongated cells that are arranged parallel to each other, giving them a singular axis for contraction.  In skeletal muscle (A) the myofibers are long, skinny rods with nuclei on the periphery of the cell.  In cardiac muscle (B) the cells are shorted and brached with a centrally located nucleus.  Smooth muscle (C) is the only type that is not striated.  It has cells that taper at each end and a central nucleus. (Image credit: "Muscle Tissue Overview" by Virginia Commonwealth University is licensed under CC BY-NC-SA 4.0)

Cardiac muscle forms the contractile walls of the heart. The cells of cardiac muscle, known as cardiomyocytes or cardiac myofibers, also appear striated under the microscope, but their striations are not as prominant as those in skeletal muscle. Unlike skeletal muscle fibers, cardiac myofibers typically have a single, centrally located nucleus. A principal characteristic of cardiac muscle is that the cells contract without any external stimulation. They are described as autorhythmic, but their contraction rate can be altered by the nervous system. Cardiac myofibers attach to multiple neighbors with specialized cell junctions called intercalated discs (Figure \(\PageIndex{1.B}\)). 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.

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 \(\PageIndex{1.C}\)).

Figure \(\PageIndex{2}\): Histology of Muscle Tissue Types at 400x. (a) Skeletal muscle cells have prominent striations and nuclei on their periphery. (b) Smooth muscle cells have a single nucleus and no visible striations. (c) Cardiac muscle cells appear faintly striated and have a single nucleus. (Image credit: "Histology of Muscle Tissue Types" by Jennifer Lange, based on micrographs provided by the Regents of University of Michigan Medical School © 2022 under CC-BY-NC-SA.)

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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.

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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 skeletal muscles, myofibers, 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 regular alternating 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

myofibers

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

striations

light and dark alternating bands within the myofibers of skeletal and cardiac muscle tissues

Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology