Types of Muscle Tissue

Muscle tissue is one of the basic tissue types. Histologically, the muscles of the body can be classified into 3 types: skeletal, smooth, and cardiac. The 3 types of muscle tissue are based on the morphologic and functional properties of the cells. One of the defining characteristics of muscle tissue is its contractility, which generates forces that move the musculoskeletal system as well as cause movement in the vasculature and multiple organ systems. This contractility is due to specialized proteins known as myofilaments, which create organized structures that have the ability to lengthen and contract.

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Muscle tissue is made up of muscle cells known as myocytes, which is one of the primary tissue types.


  • Develops primarily from mesoderm 
  • Intraocular smooth muscles: ectoderm
  • Myoblasts (embryonic muscle cells): from mesenchyme

Primary characteristics

  • Contractibility: 
    • Universal muscle cell property
    • Requires special protein filaments called myofilaments
    • Myofilaments include actin (thin), myosin (thick), and other proteins 
  • Excitability: responds to stimulus (including electrical, hormonal, and mechanical)
  • Extensibility: ability to extend/stretch
  • Elasticity: ability to recoil/return to normal shape when tension is released.


  • Movement (by exerting a physical force on bone)
  • Stability:
    • Support of the skeleton
    • Stabilize joints
    • Maintain posture
  • Control of body passages and openings:
    • Create the diameter of blood vessels via vasoconstriction and vasodilation
    • Move food through the GI tract via peristalsis
    • Sphincters control body openings:
      • How much light enters the eyes 
      • When food passes through certain parts of the GI tract
  • Heat production: muscle contraction generates heat.


There are 3 types of muscle tissue based on morphologic and functional differences: 

  • Skeletal muscles: 
    • Movement of skeleton and other structures (e.g., the eyes)
    • Long, multinucleated cells with striations
    • Primarily under voluntary control (though some actions are automatic)
  • Smooth muscles: 
    • Walls of vessels/hollow organs (e.g., intestines, blood vessels)
    • Fusiform cells without striations (lack banding pattern)
    • Slower, involuntary contractions
  • Cardiac muscle/myocardium: 
    • Form most of the walls of the heart
    • Striated, elongated, branched cells 
    • Under involuntary control

Striated versus nonstriated muscle

  • Related to appearance of contractile proteins on microscopy
  • Striated: 
    • Actin and myosin myofilament proteins are arranged in a regular pattern of functional units known as sarcomeres.
    • Skeletal and cardiac muscle
  • Nonstriated: 
    • Actin and myosin proteins are arranged in an irregular pattern (i.e., they lack typical sarcomere organization).
    • Smooth muscle

Skeletal Muscle

General characteristics

  • Type: striated muscle tissue
  • Contributes approximately 40% of total human body weight
  • Over 650 skeletal muscles
  • Controlled by the somatic nervous system
  • Primarily voluntary control

Gross structure of muscle and surrounding connective tissue

The muscle is arranged in a hierarchical structure:

  • Whole muscle:
    • Made up of multiple muscle fascicles
    • Surrounded by epimysium: 
      • External sheath of connective tissue surrounding the whole muscle
      • Separates whole muscles from one another
      • Covered by fascia
      • Contains collagen fibers, which become continuous with periosteum of bone
  • Muscle fascicle: 
    • Bundles of individual muscle fibers
    • Surrounded by perimysium:
      • Thin sheaths of connective tissue
      • Continuous with epimysium at their ends
  • Muscle fibers: 
    • Individual muscle cells (but typically called “fibers” because they are so long)
    • Immediately encased by sarcolemma (muscle cell–specific cell membrane)
    • Surrounded by endomysium: 
      • Thin sheaths of areolar connective tissue
      • Contain capillaries and nerve fibers to supply each cell/fiber
      • Continuous with perimysium and epimysium at their ends
    • Have several hundred to several thousand myofibrils in each muscle fiber
  • Myofibrils: 
    • Long functional subunits made up of myofilaments within a muscle cell
    • Surrounded by sarcoplasmic reticulum
    • Take up a majority of the sarcoplasm 
  • Myofilaments: individual contractile proteins

Microscopic structures

  • General skeletal muscle fiber characteristics:
    • Multinucleated cells
    • Nuclei sit on the periphery of the cell.
    • Diameter: 10–100 µm  
  • Sarcolemma: 
    • Muscle cell membrane
    • Contain transverse (T) tubules: 
      • Channel-like openings that penetrate through the fiber, carrying electrical signals to all the myofibrils within it.
      • In close contact with the sarcoplasmic reticulum
  • Sarcoplasm: 
    • Muscle cell cytoplasm
    • Primarily filled with protein bundles called myofibrils
    • Other organelles exist between the myofibrils.
    • Contains high amounts of:
      • Myoglobin: binds/stores O2 until it is needed
      • Glycogen: used for energy
  • Sarcoplasmic reticulum: 
    • Specialized smooth endoplasmic reticulum
    • Forms a network around each myofibril
    • Ends dilate into structures called the terminal cisternae, which line the T tubules
    • Stores calcium, which can be released via gated channels (important during muscle contraction)
  • Myofilaments:
    • Refers to individual proteins that together cause muscle contraction.
    • Contractile proteins: 
      • Actin: thin myofilaments made of 2 long-coiling protein strands
      • Myosin: thick myofilament proteins with a main shaft and a globular head on each end
    • Regulatory proteins: regulate binding of actin to myosin
      • Tropomyosin: blocks the binding sites on actin when muscle is relaxed
      • Troponin: calcium-binding proteins that control contractions
    • Elastic filament titin: runs through the core of the myosin, emerges from the end of it, and connects to the Z line
    • Myofilaments are organized into sarcomeres:
      • Functional units of striated muscles
      • Made up of regular repeating units of interlocking actin and myosin chains
      • Sarcomeres interlock end to end with each other, forming the long myofibrils.
    • Many myofilaments arranged end to end and in parallel make up a myofibril

Microscopic organization: bands seen in striated muscle

The myofibrils are organized in a pattern that creates different bands and zones when viewed under microscopy. These bands are created by overlapping actin and myosin strands.

  • Myosin: thick straight filaments arranged in parallel
  • Actin: 
    • Thin filaments
    • Connected to each other at the Z line
    • Located between each myosin filament
  • Z band (also called the Z line or Z disc): 
    • Anchors and separates one sarcomere from another
    • A sarcomere is defined as the region between 2 Z bands.
  • A (anisotropic) bands:
    • Dark bands on microscopy → memory trick: dark has an A in it
    • Formed by entire length of thick myosin filaments, which include overlapping actin filaments at the ends
  • I (isotropic) bands:
    • Light bands on microscopy → memory trick: light has an I in it
    • Consist of only thin actin filaments
    • I bands are between the A bands.
    • I bands include the Z band.
  • H zone:
    • Lighter zone in the middle of the A band
    • Consists of only myosin filaments → excludes the ends of the myosin that overlap with actin
  • M bands:
    • Fine, dark line in the center of the H zone
    • Myosin-binding proteins attach here.
  • Striated muscle: gets its name from the ordered appearance of these bands on microscopy, which look like stripes

Types of skeletal muscle fibers

There are 3 primary types of skeletal muscle fibers, found in different muscles throughout the body based on their function.

  • Type I fibers: slow-twitch muscles:
    • Slow oxidative fibers
    • Fatigue-resistant motor units
    • Small red fibers
    • Example: back muscles used to maintain posture
  • Type II fibers: fast-twitch muscles:
    • Type IIA:
      • Fast oxidative, glycolytic fibers
      • Fatigue-resistant
      • Intermediate/medium size
      • Used in movement that requires high sustained power
    • Type IIB:
      • Fast glycolytic fibers
      • Store large amounts of glycogen
      • Fatigue-prone due to buildup of lactic acid during use
      • Large pink fibers

Motor innervation of skeletal muscle fiber: the neuromuscular junction

Skeletal muscle cell contraction requires stimulation by an action potential from motor neurons. 

  • Neuromuscular junction (also called an end plate): 
    • Synapse between skeletal muscle cell and motor neuron
    • Each skeletal muscle cell (fiber) has 1 neuromuscular junction around the midpoint of cell.
    • Synaptic knob: a swelling at the end of the motor neuron
    • Motor end plate: depressions in the sarcolemma in close association with the synaptic knob
    • Synaptic cleft: the space between the synaptic knob and the motor end plate
  • Acetylcholine is released from synaptic vesicles in the synaptic knob → activates receptors on the motor end plate
  • Motor unit: 
    • A group of muscle fibers working together that are controlled by a single motor neuron
    • Small motor units:
      • Only a few muscle fibers per neuron
      • Allow for fine muscle control
    • Large motor units:
      • Up to several hundred muscle fibers innervated by a single neuron
      • Used in large postural muscles

Muscle insertion into bone

  • Muscles attach to bone via tendons.
  • Tendons are formed from the 3 connective tissue layers surrounding the muscles:
    • Epimysium
    • Perimysium
    • Endomysium
    • Additional connective tissue for added strength
  • Tendons become continuous with the periosteum of bone.
  • Force generated by the muscle cells is transferred to the surrounding connective tissue → tendon → bone (generating movement)

Gross organizational patterns of skeletal muscles

There are several different types of organizational patterns based on the arrangement of bundles the muscle fascicles:

  • Fusiform: thick in the middle and tapered at each end (e.g., biceps brachii)
  • Parallel: uniform width of parallel fascicles running along the long axis of a muscle (e.g., rectus abdominis)
  • Convergent: fan-shaped, having a broad origin inserting with a single tendon (e.g., pectoralis major)
  • Pennate: feather-shaped, having shorter fascicles attaching to a central tendon at an oblique angle
    • Unipennate: fascicles all approach the tendon from the same side (e.g., extensor digitorum)
    • Bipennate: fascicles approach the tendon from both sides (e.g., rectus femoris)
    • Multipennate: shaped like a bunch of feathers approaching a single tendon (e.g., deltoid)
  • Circular: sphincter or orbicular muscles (e.g., pyloric sphincter in the stomach, orbicularis oculi of the eyelids)
Types of organization within muscles

Types of organization within muscles

Image: “The skeletal muscles of the body typically come in seven different general shapes” by OpenStax College. License: CC BY 4.0

Smooth Muscle

General characteristics

  • Type: nonstriated muscle
  • Called smooth because of the lack of striations on microscopy
  • Involuntary muscles that generally control internal organs and vessels.


Smooth muscle is primarily found in the walls of hollow structures, including:

  • Vasculature 
  • GI tract: 
    • Esophagus
    • Stomach
    • Small and large intestines
    • Rectum
  • Respiratory tract: 
    • Trachea
    • Bronchi and bronchioles
  • Female reproductive tract: 
    • Uterus
    • Fallopian tubes
    • Vagina
  • Urinary tract: 
    • Ureters
    • Urinary bladder
    • Urethra
  • Iris of the eye
  • Piloerector muscles in hair follicles

Regulatory control

  • Under involuntary control
  • Innervation: via the ANS
  • Also influenced by hormones 

Microscopic structure

  • 1 central nucleus per smooth muscle cell
  • Fusiform shape (tapered at the ends) arranged in parallel to one another
  • Impulses transmitted through gap junctions, allowing for peristalsis
  • Consists of myosin and actin filaments:
    • Not arranged in well-ordered sarcomeres
    • Filaments are arranged in a more irregular pattern.
  • Dense bodies:
    • Small masses of proteins scattered throughout the sarcoplasma and on the inner face of the sarcolemma
    • Thin and thick filaments (actin and myosin): connect to dense bodies, which are functionally like Z discs of striated muscle.
    • Intermediate filaments: connect dense bodies to one another
  • Less sarcoplasmic reticulum
  • No T tubules
  • Structure allows for sustained contraction.

Types of smooth muscle

There are 2 primary types of smooth muscle tissue:

  • Single-unit type:
    • Found in blood vessels and most visceral organs, including those in the digestive, respiratory, urinary, and reproductive tracts
    • More common than the multiple-unit type
    • Often forms multiple layers (e.g., circular and longitudinal layers in the GI tract)
    • Myocytes are electrically coupled via gap junctions: 
      • Transmit impulses to adjacent myocytes → produce a functional syncytium (a large number of cells contracting as a single unit)
      • Allow slow, wave-like contraction
  • Multiple-unit type:
    • Individual cells are separated by basement membrane.
    • Lack gap junctions
    • Each cell contracts independently from one another.
    • Found in: 
      • Largest arteries and pulmonary passages
      • Piloerector muscles of hair follicles
      • Iris of the eye

Cardiac Muscle

General characteristics

  • Type: striated muscle
  • Works autonomously → has its own pacemaker cells
  • Found almost exclusively in the heart (a few cells in aorta and superior vena cava)
  • Central nucleus
  • Cells have multiple branches → 1 cell connects to many others via intercalated discs
  • Cells are arranged in a woven pattern in spiraling layers:
    • Produces a wave of contraction, wringing out the heart chambers when they contract
    • Muscle of ventricles much thicker than atria
  • Contain many mitochondria to produce ATP to meet energy demands of cells
  • Actin and myosin filament arrangement similar to that seen in skeletal muscle → sarcomeres form the striation pattern

Intercalated discs

  • Unique to cardiac cells
  • Connect neighboring cardiomyocytes with each other, end to end
  • Irregular, transverse, thick parts of the sarcolemma at the terminal ends of the cell branches 
  • Composed of:
    • Desmosomes: proteins anchoring 1 cell to another
    • Gap junctions: 
      • Electrical synapses between cells
      • Allow rapid transmission of electrical impulses throughout the cardiac muscle 
      • Produces synchronized contraction of cardiomyocytes
Structure of the intercalated discs within cardiac muscle

Structure of the intercalated discs within cardiac muscle

Image: “Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes” by OpenStax College. License: CC BY 4.0

Control of contractions

  • Can contract without nervous stimulation (unlike skeletal muscle)
  • Contains intrinsic pacemaker cells within the sinoatrial (SA) node 
  • Receives fibers from the ANS, which can affect:
    • Heart rate
    • Contraction strength

Related videos

Comparison of Skeletal, Smooth, and Cardiac Muscle Tissue

Table: Characteristics of muscle types
TypeLocationStriated versus nonstriatedMotor end platesCharacteristics of cellsControl
SkeletalSkeletal musclesStriatedPresent
  • Long, cylindrical
  • Multinucleated
  • Walls of hollow organs
  • Blood vessels
  • Shorter, tapered cells
  • Single central nucleus
CardiacWall of heartStriatedAbsent (connected via intercalated discs)
  • Branching networks
  • Single central nucleus

Clinical Relevance

  • Myositis: inflammation of muscle tissue. Myositis is generally secondary to infections or inflammatory disorders.
  • Polymyositis: autoimmune inflammatory myopathy caused by T-cell–mediated muscle injury. The etiology is unclear, but there are several genetic and environmental associations. Polymyositis is most commonly seen in middle-aged women and rarely affects children. Presentation is with progressive, symmetric, proximal muscle weakness and constitutional symptoms. Complications may arise from respiratory, cardiac, or GI involvement. 
  • Rhabdomyolysis: characterized by muscle necrosis and the release of toxic intracellular contents, especially myoglobin, into the circulation. Rhabdomyolysis can result from trauma or direct muscle injuries; however, nonexertional and nontraumatic etiologies (e.g., heatstroke, immobilization, medication side effects) can also lead to muscle breakdown. The classic triad of symptoms includes myalgia, weakness, and tea-colored urine, but the presentation can be nonspecific. 
  • Compartment syndrome: condition that occurs when increased pressure in a closed muscle compartment exceeds the pressure required to perfuse the compartment, resulting in muscle and nerve ischemia. Compartment syndrome is often caused by trauma, such as long-bone fractures, crush injuries, and burns, but it can also be caused by nontraumatic etiologies, such as intense muscle activity, group A streptococcal infections, and too-tight casts.
  • Myasthenia gravis: autoimmune disorder caused by antibodies against postsynaptic acetylcholine receptors at the neuromuscular junction. The disorder can affect ocular, bulbar, extremity and respiratory muscles, causing weakness and fatigue that fluctuates throughout the day. 
  • Duchenne muscular dystrophy (DMD): X-linked recessive genetic disorder that is caused by a mutation in the DMD gene. This mutation leads to production of abnormal dystrophin, resulting in muscle fiber destruction and replacement with fatty or fibrous tissue. Affected boys present with progressive proximal muscle weakness that leads to the eventual loss of ambulation, contractures, scoliosis, cardiomyopathy, and respiratory failure. 
  • Myocardial infarction: ischemia and death of an area of myocardial tissue due to insufficient blood flow and oxygenation, usually from thrombus formation on a ruptured atherosclerotic plaque in the epicardial arteries. Clinical presentation is most commonly with chest pain, but women and individuals with diabetes may have atypical symptoms. 
  • Cardiomyopathies: group of myocardial diseases associated with structural changes of the heart muscles (myocardium) and impaired systolic and/or diastolic function, in the absence of other heart disorders (such as coronary artery disease or hypertension). The list of causes is extensive, ranging from familial disorders to underlying diseases and infections. Symptoms include chest pain, dyspnea, palpitations, and syncope. Some individuals may be asymptomatic and/or present with sudden cardiac death. 
  • Uterine leiomyoma and leiomyosarcomas: Uterine leiomyomas (or uterine fibroids) are benign tumors arising from smooth muscle cells in the uterine myometrium. Leiomyosarcomas are malignant tumors, arising de novo (not from fibroids). Both conditions present with abnormal bleeding, pelvic pain, and/or bulk symptoms. Fibroids are identified as a hypoechoic, well-circumscribed, round masses on pelvic ultrasound. Leiomyosarcomas are usually only diagnosed on a postoperative specimen.


  1. Saladin, K.S., Miller, L. (2004). Anatomy and physiology, 3rd ed. McGraw-Hill Education, pp. 408–417, 432–434.
  2. Mescher, A.L. (Ed). (2021). Junqueira’s Basic Histology Text and Atlas, 16th ed. McGraw-Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=3047&sectionid=255121153
  3. Paulsen, D.F. (Ed.). (2010). Histology & Cell Biology: Examination & Board Review, 5th ed. McGraw-Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=563&sectionid=42045304
  4. Darras, B. (2021). Duchenne and Becker muscular dystrophy: clinical features and diagnosis. UpToDate. Retrieved July 21, 2021, from https://www.uptodate.com/contents/duchenne-and-becker-muscular-dystrophy-clinical-features-and-diagnosis
  5. Mohrman, D.E., Heller, L. (Eds.). (2018). Cardiovascular Physiology, 9th ed. McGraw-Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=2432&sectionid=190800450

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