Types of Muscle Tissue

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Introduction

Muscles represent one of four primary tissue types found in the human body, the other three being epithelial tissue, connective tissue, and nervous tissue.1 

Muscles account for approximately 40% of a person’s weight and there are more than 650 muscles in the human body.2

The functional units of a muscle are termed myocytes, multi-nucleated cells that make up the muscle tissue.4

There are three main categories of muscle tissue found within the human body: skeletal muscle, cardiac muscle, and smooth muscle. Each type has unique histological features, enabling the muscle to carry out its specific functions.5


Skeletal muscle

Composition

Skeletal muscle tissue is highly organised and arranged in long bundles of cylindrical muscle fibres (Figure 1).6 

The muscle fibres vary in size, shape, and arrangement, from being small, broad, and parallel to being large, narrow, and oblique.7 

Each skeletal muscle is organised into hundreds to thousands of muscle fibres working together as a unit.

There are different layers of connective tissue surrounding each layer of muscle. The outermost part of the muscle is surrounded by a connective tissue sheath termed epimysium. Around each bundle of muscle fibres lies the perimysium (a.k.a. fascia) and surrounding each muscle fibre lies the endomysium.6

Skeletal fibres are striated in appearance corresponding to the repeated units of actin and myosin filaments organised into sarcomeres seen as distinct Z lines under a microscope.8

There is an abundance of mitochondria, myoglobin, and glycogen storage located in the cytoplasm of skeletal myocytes.8 This is due to the high energy requirement necessary to generate the force needed for movement, the main function of skeletal muscle.

There are two types of skeletal muscle found within the human body: type 1 (slow-twitch fibres) and type 2 (fast-twitch fibres).9

Slow-twitch and fast-twitch fibres differ in their colour, diameter, myoglobin content, speed of contraction, degree of force produced, degree of fatiguability, primary energy reserve, and primary storage fuel used.9

Type 1 (slow-twitch)

Slow-twitch fibres are red, small diameter fibres with a high myoglobin content. They have a slow contraction time, low force production, and high resistance to fatigue. 

These fibres receive ATP primarily through oxidative phosphorylation with triglycerides being the primary energy source. This muscle fibre type is typically utilised for longer duration aerobic activity.9

Type 2 (fast-twitch)

Fast-twitch fibres are white, large-diameter fibres with a low myoglobin content. They have a fast contraction time, high force production and low resistance to fatigue.

These fibres receive ATP primarily through glycolysis with creatinine phosphate and glycogen being the primary energy source. This muscle fibre type is typically utilised for the shorter duration anaerobic activity.9

 cylindrical skeletal muscle fibres
Figure 1. Bundles of cylindrical skeletal muscle fibres.10

Contraction

Contraction of skeletal muscle is regulated through a protein called troponin.11 

As a nerve impulse travels along the muscle fibre membrane, it travels deeper into the interior through a transverse tubular system (T-tubules), where it reaches the calcium storing sarcoplasmic reticulum (Figure 2).11

Calcium is released into the sarcoplasm of the cell and subsequently binds to troponin on the actin filament. This causes the displacement of the tropomyosin head, exposing the actin-binding site for myosin and allows cross bridging to occur and the subsequent contraction of the muscle fibre (Figure 3).11 

The contraction of skeletal myocytes follows the ‘all or none law‘ which states that for any given nerve impulse that reaches the threshold of activation of a muscle fibre, the response will always be the same irrespective of the strength of impulse that reaches the fibre.

Increasing the number of action potentials running through the fibre and increasing the number of muscle fibres activated can increase the force of contraction of skeletal muscle.

Function

Skeletal muscle is found throughout the human body supporting the underlying skeleton through its attachment to tendons.

The primary functions of skeletal muscle include:8

  • Production of voluntary movement of the skeleton controlled through the somatic nervous system (i.e. under conscious control)
  • Maintenance of body position and posture
  • Stabilisation of joints
  • Supporting the underlying organs and soft tissue
  • Guarding against the entry and exits of the body
  • Storing nutrient reserves (e.g. calcium, magnesium, and phosphate)
  • Helping to maintain the correct body temperature by generating heat
Clinical relevance: Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a rare genetic X-linked recessive disorder resulting in a defective dystrophin gene. The dystrophin gene encodes for the dystrophin protein, a rod-shaped cytoplasmic protein responsible for anchoring the muscle fibre cytoskeleton of both skeletal cells and cardiac cells to the surrounding extracellular matrix.14

The absence of this protein leads to a progressive weakening and degeneration of the muscle fibres with progressive fibrosis as the cells start to die.15

Clinically, DMD presents more commonly in males due to the X-linked inheritance pattern and is diagnosed in early childhood when children are learning to sit, stand, and walk.15,28

Children may first present with delayed motor milestones, such as the inability to walk. Once children with DMD start to walk, parents often describe them as ‘clumsy’ or ‘falling all the time’. A slow and ungainly run is a common presentation.

The treatment of DMD is based on symptomatic control and reducing the risk of complications.14


Cardiac muscle

Composition

Cardiac muscle cells (cardiomyocytes) are specialised cells only found in the myocardium.

Cardiac myocytes are rectangular cells with one to two central nuclei arranged in an extensive branching organisation among its fibres (Figure 4).16

The cells are connected through specialised connection points called intercalated discs that have extensive gap junctions in between the discs, allowing for cell-to-cell communication (Figure 5).17

This enables the heart to act as a pump since the contraction of one cell leads to a wave of contraction through neighbouring cells spreading throughout the entire heart. This type of contraction is termed syncytium.18

Like skeletal muscle tissue, cardiac muscle tissue is striated in appearance and organised into repeating rows of sarcomeres. However, the actin and myosin filaments within cardiac cells are much less organised than what is found in skeletal muscle, as well as having shorter fibres.16

The cytoplasm of cardiomyocytes is rich in mitochondria and myoglobin due to the immediate usage of ATP necessary for the highly coordinated involuntary contractions of the heart.19

Contraction

The heart contracts in a wave-like pattern, syncytium, initiated by specialised autorhythmic cells within the sinoatrial node (SA) of the heart called pacemaker cells.17

They can spontaneously generate an electrical impulse causing the heart to contract and relax respectfully, pumping blood to the rest of the body.18

Like skeletal muscle, the contraction of cardiac myocytes is under troponin-mediated regulation.

Function

Cardiac muscle is only found within the heart and its sole function is to pump blood to the body.

Clinical relevance: Cardiomyopathy

Cardiomyopathy is a group of medical conditions that affect the underlying cardiac muscle impairing the heart’s ability to pump blood to the rest of the body.22

Cardiomyopathy can be either congenital or acquired through other diseases including hypertension, ischemic heart disease, long term alcohol/cocaine abuse and connective tissue disorders.23

There are two general types of cardiomyopathy: dilated cardiomyopathy, which results in enlarged and weakened ventricles, and restrictive cardiomyopathy, which results in ventricular stiffness and rigidity.22

In these two cases, the heart is not able to pump effectively, which leads to the heart working overtime to compensate. With time, however, the cardiomyocytes deteriorate and can eventually progress to valvular disorders, arrhythmias, and heart failure.

Clinical features of cardiomyopathy include:23

  • Shortness of breath (dyspnoea)
  • Fatigue
  • Pitting oedema and sacral oedema
  • Syncope
  • Arrhythmias
  • Cardiac murmurs

The main goal of treatment of cardiomyopathy is to slow down the progression of the disease, control symptoms and reduce complications.23


Smooth muscle

Composition

Smooth muscle tissue is organised by overlapping sheets of spindle-shaped cells characterised by a round centre with a single large central nucleus and tapered ends (Figure 6).24

Microscopically, the cells appear homogenous and are connected through end-to-end junctions called gap junctions, which form a watertight seal between the cells.25

Unlike skeletal and cardiac muscle tissue, striations cannot be visualised under the microscope or histology, hence the name smooth muscle.

An abundance of both actin and myosin filaments are still present and can be found running along the entire length of smooth muscle cells anchored through dense bodies (Figure 7). They are more loosely arranged in a stacked staircase pattern, and not in the highly organised repeating pattern of sarcomeres that give both the skeletal and cardiac muscles the appearance of striations.24

The cytoplasm of smooth muscle cells contains an abundance of Golgi apparatus, endoplasmic reticulum, desmin, vimentin, elastin and collagen, with fewer mitochondria than what is found within both skeletal and cardiac myocytes.26

Figure 6. Spindle-shaped appearance of smooth muscle cells.27

Contraction

Like all muscles, the primary function of smooth muscle is to contract.

However, unlike both skeletal muscle and cardiac muscle, their contractions are slow, powerful, persistent, long-lasting, involuntary contractions that are controlled by the autonomic nervous system (i.e. not under conscious control).24

This is sometimes referred to as a graded contraction, instead of following the all-or-none principle of skeletal muscle.24

Instead of the troponin mediated contraction, smooth muscle regulates its contraction by utilising another calcium-binding protein called calmodulin (CaM) that binds to specific L-type voltage-gated channels on smooth muscle cells.26

Figure 7. Mechanism of contraction of smooth muscle cells.28

Function

Smooth muscle lines the inner walls of the vasculature, hollow visceral organs (i.e. the urinary bladder, uterus, stomach, intestines), and the major tracts of the body (i.e. the respiratory tract, digestive tract, urogenital tract).26

The two functions of smooth muscle involve changing the diameter of organs as needed by the body and withstanding internal pressure that is exerted on the organ it lines.24

Smooth muscle plays a vital role in regulating blood pressure by altering systemic vascular resistance. 

In addition, smooth muscle is responsible for peristalsis to move food through the digestive system. 

Smooth muscle also plays a role in regulating bodily secretions. The secretion of salivary enzymes in the mouth during mastication, gastric acid and digestive enzymes in the stomach during digestion and mucous from the intestines to stabilise the acidic chyme from the stomach.

Smooth muscle lines the respiratory tract to assist in breathing and is also found in the iris of the eye controlling the amount of light entering and, in the skin, elevating the hair follicles when temperature decreases.

Clinical relevance: Asthma

Asthma is a chronic inflammatory airway disease characterised by intermittent airway obstruction.29

Asthma is an IgE mediated type 1 hypersensitivity reaction that occurs when the body encounters an allergen (i.e. dust, dander, pollen). The immune system overreacts by creating an abundance of IgE antibodies against these allergens that crosslinks with histamine containing mast cells and thus subsequently release of histamine.29

This creates an inflammatory reaction causing swelling, narrowing of the airways, overproduction of mucus, and constriction of the smooth muscle lining the airways.29

The airway smooth muscle plays an integral part in the pathogenesis of asthma responsible for the acute bronchoconstriction precipitated by hyperresponsiveness and inflammation.

There is currently no cure for asthma. However, the smooth muscle is an important pharmaceutical target point for symptom relief with the use of both bronchodilators and anti-inflammatory medication.29


Key points

The following table lists the types of muscle and characteristic features of each.

Table 1. An overview of the different types of muscle.

 

Skeletal muscle Cardiac muscle Smooth muscle

Location

Attached to the bone via tendons

The myocardium (heart)

Lining the walls of visceral organs, the circulation system, and the different tracts of the body

Number of nuclei

Multinucleated in the periphery

1-2 central nuclei

Single central nucleus

Cell shape

Long, cylindrical

Short, fusiform, branched

Short, spindle-shaped

Striations

Yes

Yes

No

Autorhythmic

No

Yes

No

Control

Voluntary

Involuntary

Involuntary

Functions

  • Bodily movement
  • Maintain body position and posture
  • Support underlying organs

Pump blood to the rest of the body

  • Peristalsis of food through the digestive tract
  • Regulation of blood pressure
  • Excretion of urine
  • Contractions during labour and propulsion of sperm
  • Regulation of bronchiole diameter
  • Raises hair on the skin (arrector pili muscles)
  • Dilation and constriction of the pupil

Histology overview

 
Figure 8. Histological appearance of the three muscle types. A: skeletal muscle. B: smooth muscle. C: cardiac muscle. 30

Reviewer

Olof D. Olafardottir

PhD in Physical Anthropology

Teaching Assistant in Anatomy & Physiology at Indiana University


Editor

Dr Chris Jefferies


References

  1. Julie Doll. Tissue types. Published in 2020. Available from: [LINK]
  2. Jill Seladi-Schulman. How many muscles are in the human body?. Published in 2020. Available from: [LINK]
  3. Lana Burgess. What are the main functions of the muscular system?. Published in 2018. Available from: [LINK]
  4. Rachel Baxter. Types of muscle cells. Published in 2020. Available from: [LINK]
  5. Gordon A. Starkebaum. Types of muscle tissue. Published in 2019. Available from: [LINK]
  6. National Institute of Health. Structure of skeletal muscle. Published in 2020. Available from: [LINK]
  7. Mark Hill. Musculoskeletal muscle: Muscle development. Published in 2020. Available from: [LINK]
  8. ClaraFranzini-Armstrong and Andrew G.Engel. Skeletal muscle: Architecture of membrane systems. Published in 2012. Available from: [LINK]
  9. Walter R. Frontera and Julien Ochala. Skeletal muscle: A brief review of structure and function. Published in 2015. Available from: [doi10.1007/s00223-014-9915-y]
  10. Figure 1. Wikimedia Commons. Histology: Bundles of cylindrical skeletal muscle fibers. Licence: [Public domain]. Available from: [LINK]
  11. Hakan Ömeroğlu and Suna Ömeroğlu. Skeletal muscle: Structure, function, and repair. Published in 2016. Available from: [LINK]
  12. Figure 3. Wikimedia Commons. Microscopic arrangement of skeletal muscle fibers. Licence: [CC BY-SA]. Available from: [LINK]
  13. Figure 2. Wikimedia Commons. Arrangement of the transverse tubular system. Licence: [CC-BY]. Available from: [LINK]
  14. National Organization for Rare Disorders. Duchenne muscular dystrophy. Published in 2016. Available from: [LINK]
  15. National Institute of Health. Duchenne muscular dystrophy. Published in 2020. Available from: [LINK]
  16. Tim Barclay. Cardiac muscle tissue. Published in 2017. Available from: [LINK]
  17. Gloria Lotha. Cardiac muscle. Published in 2019. Available from: [LINK]
  18. Jamie Eske. What to know about cardiac muscle tissue. Published in 2019. Available from: [LINK]
  19. Pinnell Jeremy, Turner Simon, and Howell Simon. Cardiac muscle physiology. Published in 2007. Available from: [LINK]
  20. Figure 4. OpenStax College – Anatomy & Physiology. Histology: branched pattern of cardiac myofibers. Licence: [CC-BY]. Available from: [LINK]
  21. Figure 5. OpenStax College – Anatomy & Physiology. Intercalated disc arrangement between cardiomyocytes. Licence: [CC-BY]. Available from: [LINK]
  22. National Institute of Health. Cardiomyopathy. Published in 2020. Available from: [LINK]
  23. Centers for Disease Control and Prevention. Cardiomyopathy. Published in 2019. Available from: [LINK]
  24. David P. Wilson. Vascular smooth muscle structure and function. Published in 2011. Available from: [LINK]
  25. Angel Vodenicharov. Structure and function of smooth muscle with special reference to mast cells. Published in 2012. Available from: [doi:10.5772/48566]
  26. Brant B. Hafen and Bracken Burns. Physiology, smooth muscle. Published in 2018. Available from: [LINK]
  27. Figure 6. Wikimedia Commons. Histology: Spindle shaped appearance of smooth muscle cells. Licence: [Public domain]. Available from: [LINK]
  28. Figure 7. OpenStax College – Anatomy & Physiology. Mechanism of contraction of smooth muscle cells. Licence: [CC-BY]. Available from: [LINK]
  29. James Conlan. Asthma. Published in 2019. Available from: [LINK]
  30. Figure 8. Wikimedia Commons. Histological appearance of the 3 muscle types (a)Skeletal (b)Smooth (c)Cardiac. Licence: [CC-BY]. Available from: [LINK]

 

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