The body contains three types of muscle tissue: (a) skeletal muscle, (b) smooth muscle, and (c) cardiac muscle. (Same magnification)
A schematic diagram of the different types of muscle cells (same order as above).
Muscle tissue is a soft tissue that composes muscles in animal bodies, and gives rise to muscles' ability to contract. This is opposed to other components or tissues in muscle such as tendons or perimysium.[clarification needed] It is formed during embryonic development through a process known as myogenesis.
Muscle tissue varies with function and location in the body. In mammals the three types are: skeletal or striated muscle; smooth or non-striated muscle; and cardiac muscle, which is sometimes known as semi-striated. Smooth and cardiac muscle contracts involuntarily, without conscious intervention. These muscle types may be activated both through interaction of the central nervous system (CNS) as well as by receiving innervation from peripheral plexa or endocrine (hormonal) activation. Striated or skeletal muscle only contracts voluntarily, upon influence of the central nervous system. Reflexes are a form of non-conscious activation of skeletal muscles, but nonetheless arise through activation of the CNS, albeit not engaging cortical structures until after the contraction has occurred.
The different muscle types vary in their response to neurotransmitters and endocrine substances such as acetyl-choline, noradrenalin, adrenalin, nitric oxide and others. This depends on muscle type and where the muscle is located.
Muscle cells (myocytes) are elongated and classified and or compatible as either striated muscle cells or smooth muscle cells depending on the presence or absence, respectively, of organized, regularly repeated arrangements of myofibrillar contractile proteins called myofilaments. Striated muscle is further classified as either skeletal or cardiac muscle. Thus, muscle tissue can be described as being one of three different types:
- Skeletal muscle or "voluntary muscle" or "striated muscle" is anchored by tendons (or by aponeuroses at a few places) to bone and is used to effect skeletal movement such as locomotion and in maintaining posture. Though this postural control is generally maintained as an unconscious reflex (see proprioception), the muscles responsible react to conscious control like non-postural muscles. An average adult male is made up of 42% of skeletal muscle and an average adult female is made up of 36% (as a percentage of body mass). It also has striations unlike smooth muscle.
- Smooth muscle or "involuntary muscle" or "non-striated muscle" is found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin (in which it controls erection of body hair). Unlike skeletal muscle, smooth muscle is not under conscious control.
In vertebrates, there is a third muscle tissue recognized:
- Cardiac muscle (myocardium), is also an "involuntary muscle" but is more similar in structure to skeletal muscle, and is found only in the heart.
Cardiac and skeletal muscles are "striated" in that they contain sarcomeres and are packed into highly regular arrangements of bundles; smooth muscle has neither. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles (called intercalated discs). Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.
Comparison of types
|smooth muscle||cardiac muscle||skeletal muscle|
|Fibers||fusiform, short (<0.4 mm)||branching||cylindrical, long (<15 cm)|
|Mitochondria||few||numerous||many to few (by type)|
|Sarcomeres||none||present, max. length 2.6 µm||present, max. length 3.7 µm|
|Syncytium||none (independent cells)||none (but functional as such)||present|
|Sarcoplasmic reticulum||little elaborated||moderately elaborated||highly elaborated|
|Self-regulation||spontaneous action (slow)||yes (rapid)||none (requires nerve stimulus)|
|Response to stimulus||unresponsive||"all-or-nothing"||"all-or-nothing"|
|Workspace||Force/length curve is variable||the increase in the force/length curve||at the peak of the force/length curve|
|Response to stimulus|
Skeletal muscle is further divided into several subtypes:
- Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity.
- Type I muscle fiber are sometimes broken down into Type I and Type Ic categories, as a result of recent research.
- Type II, fast twitch muscle, has three major kinds that are, in order of increasing contractile speed:
- Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red when deoxygenated.
- Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up of lactic acid). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.
- Type IIb, which is anaerobic, glycolytic, "white" muscle that is even less dense in mitochondria and myoglobin. In small animals like rodents this is the major fast muscle type, explaining the pale color of their flesh.
The smooth muscle fibres taper at both ends and do not show striation. Cell junctions hold them together and they are bundled together in a connective tissue sheath. The wall of internal organs such as the blood vessels,stomach and intestine contains this type of muscle tisse. Smooth muscles are involuntary
Cardiac muscle tissue occurs only in the heart. Its cells are joined end to end. The resulting fibers are branched and interconnected in complex networks. Each cell has a single nucleus. At its end, where it touches another cell, there is a specialized intercellular junction called an intercalated disc, which occurs only in cardiac tissue. Cardiac muscle is controlled involuntarily and, in fact, can continue to function without being stimulated by nerve impulses. This tissue makes up the bulk of the heart and is responsible for pumping blood through the heart chambers into the blood vessels.
|This section requires expansion. (March 2014)|
- Pratt, Rebecca. "Muscle Tissue". AnatomyOne. Amirsys, Inc. Retrieved 26 October 2012.
- Marieb, Elaine; Hoehn, Katja (2007). Human Anatomy & Physiology (7th ed.). Pearson Benjamin Cummings. p. 317. ISBN 0-8053-5387-9.
- McCloud, Aaron (30 November 2011). "Build Fast Twitch Muscle Fibers". Complete Strength Training. Retrieved 30 November 2011.
- Larsson, L; Edström, L; Lindegren, B; Gorza, L; Schiaffino, S (July 1991). "MHC composition and enzyme-histochemical and physiological properties of a novel fast-twitch motor unit type". The American Journal of Physiology 261 (1 pt 1): C93–101. PMID 1858863. Retrieved 2006-06-11.
- Smerdu, V; Karsch-Mizrachi, I; Campione, M; Leinwand, L; Schiaffino, S (December 1994). "Type IIx myosin heavy chain transcripts are expressed in type IIb fibers of human skeletal muscle". The American Journal of Physiology 267 (6 pt 1): C1723–1728. PMID 7545970. Retrieved 2006-06-11. Note: Access to full text requires subscription; abstract freely available