5.1: The Three Types of Muscle Tissue

Muscle tissue allows movement and exists in three types: skeletal, cardiac, and smooth. 

Master this section and you'll be able to

•  Identify and compare the three types of muscle tissue (skeletal, cardiac, and smooth) based on structure, function, and location in the body.
• List the four functional properties of muscle tissue and explain how they contribute to muscle movement.
• Differentiate between voluntary and involuntary muscle control, providing examples of each from everyday body functions.

Muscle is one of the four primary tissue types of the body. There are three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue is characterized by properties that allow movement.  All muscle tissue have four functional properties in common which include excitability, contractility, extensibility, and elasticity.

• Excitability:  Muscle cells can respond to a stimulus. The plasma membrane can change electrical states and send an electrical wave called an action potential along the entire surface of each cell.
• Contractility: Muscle cells can shorten and generate a pulling force. When attached between two movable objects  — bones — contractions of the muscles cause the bones to move. 
• Extensibility: Muscle cells can stretch or elongate.
• Elasticity:  Following contraction or extension, a muscle can recoil to its original length when relaxed due to elastic proteins.

Some muscle movements are voluntary, meaning you control them consciously. Think of it like choosing to open a book and read a chapter on anatomy (😉) — your brain gives the command, and your muscles follow. Other movements are involuntary, happening automatically without you even thinking about them. A good example is your pupil shrinking in bright light. You cannot “tell” your eye to do that — it just happens.

The Three Muscle Tissue Types

Muscle tissue is divided into three types based on structure and function: skeletal, cardiac, and smoothOne of the easiest ways to tell the three muscle types apart is by looking at them under a microscope. Inside each muscle cell (or myofiber) are the contractile proteins actin and myosin, which slide past each other to make the cell contract.

• Skeletal and cardiac muscle keep things very organized. Their actin and myosin are stacked in a repeating, brick-like pattern. This arrangement produces visible stripes called striations. Under a light microscope, you can actually see these as alternating light–dark bands. (See figure below.) The stripes stand out more clearly in skeletal muscle than in cardiac muscle.

• Smooth muscle, on the other hand, is less of a perfectionist. Its actin and myosin are arranged irregularly, so the cells look uniform rather than striped. This smooth look is the reason for the name smooth muscle.

Comparison of Structure and Properties of the Three 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

 

1) Skeletal Muscle Tissue

Skeletal myofibers (muscle cells) are long, cylindrically-shaped, and striated. They can be just a few millimeters or stretch out to many centimeters — often extending beyond the sides of a microscope image.

➡️ Record holder: The sartorius, the longest muscle in the body, has myofibers that average about 42 cm (16.5 in) in length!

Skeletal muscle makes up about 40% of your body weight. It contains 50–75% of all body proteins — a reminder of how “protein-rich” muscle tissue really is.

Skeletal muscle cells have multiple nuclei — the longer the myofiber, the more nuclei. This helps efficiently manage the cell’s activities across this length, because each nucleus controls the metabolism and protein production for the area of cytoplasm near it.
How do skeletal muscle cells end up with so many nuclei?
During development, skeletal muscle fibers form through the fusion of myoblasts. Myoblasts are small precursor cells, each containing its own nucleus. When many myoblasts join together to create one long skeletal muscle fiber, all their nuclei remain inside. This is why skeletal muscle fibers are multinucleated — they are essentially a team of cells that merged into one giant cell.

Unlike in most cells, the nuclei in skeletal muscle fibers are pushed to the cell’s edges (see arrows in the image below). This makes room in the center for the thousands of contractile proteins packed inside.

Skeletal muscle is the “voluntary” muscle type. Anytime you consciously choose to contract or relax a muscle, you are using skeletal muscle. Examples include:

• Moving the skeleton: arms, legs, back, neck,  etc.
• Skin movement: facial expressions.
• Breathing: muscles of respiration, including the diaphragm.
• Specialized control of openings: external anal and external urethral sphincters.
• Speech and eating: your tongue is skeletal muscle, too!

  Figure \(\PageIndex{1}\): Skeletal muscle fibers at 200x (longitudinal section). Long, pink striated skeletal muscle fibers with dark oval nuclei along the edges, marked by arrows.

 

2) Cardiac Muscle Tissue

Cardiac muscle tissue is found only in the heart. The heart's highly coordinated contractions keep blood moving through the vessels of the circulatory system.

The structure of cardiac muscle fibers differs from the skeletal muscle fibers by size and nuclei: cardiac muscle fibers are shorter than skeletal muscle fibers, and most have a single nucleus, located in the center of the cell.

The energy demand of the heart muscle cells is huge because the heart never gets a break. The cardiac myofibers therefore contain many mitochondria and a lot ot of myoglobin, producing ATP mainly through aerobic metabolism.

Unlike skeletal muscle cells, cardiac muscle cells are branching.  This extensive branching gives the tissue a strong, interwoven structure.

Cardiac muscle cells are joined at their ends by intercalated disks (see image below). These specialized junctions are part of the sarcolemma (cell membrane) and have two special structures which serve two main purposes:

• Gap Junctions
  • Form channels that connect neighboring cells.
  • Allow electrical signals to pass rapidly from one fiber to the next.
  • This ensures the entire heart contracts in a coordinated way (not like a crowd clapping out of sync at a concert).
     
• Desmosomes
  • Act like spot welds, anchoring cells together.
  • Prevent fibers from pulling apart when the heart contracts forcefully, beat after beat.

✅ Key Takeaway: Cardiac muscle is built for endurance and teamwork. It has energy-rich cells, a branching structure, and intercalated disks that keep all fibers contracting in sync without tearing apart.

Figure \(\PageIndex{2}\):  Cardiac Muscle Fibers at 400x (longitudinal section). Indicated by arrows are the intercalated discs. The outlines show the branching fibers. (Image credits: Types of Muscle Tissue by Jennifer Lange are licensed under CC-BY-SA-NC 4.0. Micrographs A1, A2, and C1 provided by the Regents of University of Michigan Medical School, micrographs B1, B2, and C2 provided by Virginia Commonwealth University.)

 

3) Smooth Muscle Tissue

Smooth muscle is found throughout the body, lining the walls of hollow organs and passageways. It is called smooth because, unlike skeletal and cardiac muscle, its cells do not have visible striations under the microscope.

Smooth muscle fibers are spindle-shaped (wide in the middle, tapered at both ends, like a football). Each fiber has a single, centrally located nucleus (see image below). Smooth muscle cells are much shorter than skeletal muscle fibers, ranging from 30–200 μm in length.

Each fiber produces its own thin layer of connective tissue called the endomysium.

Unlike skeletal muscle, both smooth muscle and cardiac muscle are involuntary, meaning they contract automatically without conscious control. Smooth muscle is found in the:

• Digestive system: stomach and intestines
• Urinary system: urinary bladder, ureters
• Reproductive system: uterus and related structures
• Circulatory system: walls of arteries and veins
• Respiratory system: tracts that carry air to and from the lungs
• Eyes: controls the size of the iris and the shape of the lens
• Skin: attached to hair follicles; contraction causes “goosebumps” in response to cold or fear

Figure \(\PageIndex{3}\):  Smooth Muscle Fibers at 400x. Histology image of smooth muscle tissue with cross section (CS) and longitudinal section (LS). Outlined is a single cell in each section. Smooth muscle cells are spindle shaped and taper to points at the ends. They have a centrally located nucleus.  Because of this shape, when viewed in CS the myofibers will appear to have different diameters and only those cut through the center will show the nucleus. 
(Image credit: "Smooth Muscle Fibers" by Jennifer Lange is licensed under CC-BY-SA-NC 4.0. Micrographs from Virginia Commonwealth University.)

 

Interactive Element

Watch this video and learn more about muscle tissue:

 

 In looking through a microscope how could you distinguish skeletal muscle tissue from smooth muscle?