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Muscle tissue is a specialized tissue found in animals which functions by contracting, thereby applying forces to different parts of the body. Muscle tissue consists of fibers of muscle cells connected together in sheets and fibers. Together these sheets and fibers and known as muscles, and control the movements of organisms as well as many other contractile functions. There are three different types of muscle found in animals, depending on their use. While these muscles differ slightly, they function in a similar way.
The three types of muscle tissue are skeletal, smooth, and cardiac.
The 3 types of muscle tissue are cardiac, smooth, and skeletal. Cardiac muscle cells are located in the walls of the heart, appear striated, and are under involuntary control. Smooth muscle fibers are located in walls of hollow visceral organs, except the heart, appear spindle-shaped, and are also under involuntary control. Skeletal muscle fibers occur in muscles that are attached to the skeleton. They are striated in appearance and are under voluntary control.
Key Points
Muscle tissue can be divided functionally, is it under voluntary or involuntary control; and morphologically, striated or non-striated.
By applying these classifications three muscle types can be described; skeletal, cardiac, and smooth.
Skeletal muscle is voluntary and striated, cardiac muscle is involuntary and striated and smooth muscle is involuntary and non-striated.
Key Terms
skeletal muscle: The voluntary muscle of vertebrates, which is striated and anchored by tendons to bone, is used to effect skeletal movement such as locomotion.
involuntary: A muscle movement not under conscious control e.g. the beating of the heart.
striated: The striped appearance of certain muscle types in which myofibrils are aligned to produce a constant directional tension.
voluntary: A muscle movement under conscious control e.g. deciding to move the forearm.
smooth muscle: Involuntary muscle that is found within the intestines, throat, uterus, and blood vessel walls.
cardiac muscle: The striated and involuntary muscle of the vertebrate heart.
Muscle Tissue
Muscle is a soft tissue that is highly specialized for the production of tension which results in the generation of force. Muscle cells, or myocytes, contain myofibrils comprised of actin and myosin myofilaments which slide past each other producing tension that changes the shape of the myocyte. Numerous myocytes make up muscle tissue and the controlled production of tension in these cells can generate significant force.
Types of Muscle Tissue
Muscle tissue can be classified functionally, voluntary or involuntary, and morphologically striated or non-striated. Voluntary refers to whether the muscle is under conscious control, striation refers to the presence of visible banding within myocytes which occurs due to the organization of myofibrils to produce a constant direction of tension.
By applying the above classifications it is possible to describe three forms of muscle tissue that perform the wide range of functions described.
Skeletal Muscle
Skeletal muscle mainly attaches to the skeletal system via tendons to maintain posture and control movement for example contraction of the biceps muscle, attached to the scapula and radius, will raise the forearm. Some skeletal muscle can attach directly to other muscles or the skin, as seen in the face where numerous muscles control facial expression.
Skeletal muscle is under voluntary control, although this can be subconscious for example when maintaining posture or balance. Morphologically skeletal myocytes are elongated and tubular and appear striated with multiple peripheral nuclei.
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 is 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:[rx]
- Type IIa, which, like a 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.[rx]
- Type IIb, which is an 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.
Cardiac Muscle Tissue
Cardiac muscle tissue is found only in the heart where cardiac contractions pump blood throughout the body and maintain blood pressure.
As with skeletal muscle cardiac muscle is striated, however, it is not consciously controlled and so is involuntary. Cardiac muscle can be further differentiated from skeletal muscle by the presence of intercalated discs which control the synchronized contraction of cardiac tissues. Cardiac myocytes are shorter than skeletal equivalents and contain only one or two centrally located nuclei.
Smooth Muscle Tissue
Smooth muscle tissue is found associated with numerous other organs and tissue systems such as the digestive system or respiratory system. It plays an important role in the regulation of flow in such tissues for example aiding the movement of food through the digestive system via peristalsis.
Smooth muscle is non-striated, although it contains the same myofilaments they are just organized differently, and involuntary. Smooth muscle myocytes are spindle-shaped with a single centrally located nucleus.
The function of Muscle Tissue
Muscle tissue functions as a single unit and is often connected to the same nerve bundles. A nerve impulse traveling from the brain or another outside signal tells the muscle to contract. The nerve impulse is transferred almost instantaneously to all the nerve cells in the muscle tissue, and the entire muscle contracts.
At the cellular level, each muscle cell has a complex of proteins containing actin and myosin. These proteins slide past one another when the signal to contract is received. The filaments are connected to the ends of the cells, and as they slide past one another, the cell contracts in length. A single cell can contract up to 70% in length, which shortens the entire muscle when contraction happens. Muscle tissue can be used to move bones, compress chambers, or squeeze various organs. These different types of muscle tissue are discussed below.
Comparison of types
| smooth muscle | cardiac muscle | skeletal muscle | |
| Anatomy | |||
| Neuromuscular junction | none | none | present |
| Fibers | fusiform, short (<0.4 mm) | branching | cylindrical, long (<15 cm) |
| Mitochondria | few | numerous | many to few (by type) |
| Nuclei | 1 | 1 | >1 |
| 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 |
| ATPase | little | moderate | abundant |
| Physiology | |||
| Self-regulation | spontaneous action (slow) | yes (rapid) | none (requires nerve stimulus) |
| Response to stimulus | unresponsive | “all-or-nothing” | “all-or-nothing” |
| Action potential | yes | yes | yes |
| Workspace | Force/length curve is variable | the increase in the force/length curve | at the peak of the force/length curve |
