Types of Muscle Tissue: Skeletal, Cardiac & Smooth

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  1. 0:58 Muscle histology
  2. 3:12 Muscular contraction
  3. 5:10 Regulation of contraction
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Taught by

John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Have you ever wondered why muscle has different names such as striated, smooth, voluntary, or involuntary? This lesson describes the different types of muscle tissue based on their histology, mechanism of contraction, and regulation.

Muscle Tissue

The three types of muscle tissue
Muscle Tissue Types

Muscle is one of four different tissues found in our bodies. The other general tissue types include epithelial tissue, connective tissue, and nervous tissue. Specifically, we have three different types of muscle tissue, including skeletal, cardiac, and smooth. Each type of muscle is unique in terms of its structure and its function. Regardless of the specific type, all muscle serves the same general function, and that is support and movement. In other words, muscles contract, thus causing the body and body parts to move. In this lesson, we will compare and contrast the three different muscle types in terms of their structure and function.

Muscle Histology

Tissues are made of cells, and a muscle cell is referred to as a myocyte or sometimes simply a muscle fiber because some are long and they look like a fiber. Let's first compare muscle tissue based on what you would see under a microscope; that is, its histology.

Both cardiac and skeletal muscle are termed striated muscle as they have striations that run across their muscle fibers. In contrast, smooth muscle is not striated, and it's, well, smooth. The striations are end-to-end junctions of repeating units that are referred to as sarcomeres. A sarcomere is a functional unit of striated muscle as it contains all the tools necessary for contraction. In this sense, you can think of a smooth muscle as a giant sarcomere. Skeletal and cardiac muscle, while both striated, can be distinguished based on arrangement of the fibers. While skeletal muscle fibers are long and linear, cardiac muscle fibers are short and branched.

Cardiac muscle myocytes run in different directions
Cardiac Muscle Structure
Looking at skeletal muscle with a microscope, we can see the fibers stacked neatly together in a parallel arrangement. Additionally, these fibers are so long, and they run the entire length of the muscle organ. I guess this is what my mother expected my room to look like - nice and orderly. Cardiac muscle, on the other hand, looks more like my kid's room - a complete mess. As the myocytes are branched, they run in different directions as they interconnect with one another. Additionally, cardiac fibers are connected end-to-end with each other with special structures called intercalated discs. The intercalated discs, like the striations separating the sarcomeres, run across the fibers. However, they appear darker on the slide, and they allow for direct communication between the adjacent myocytes. Smooth muscles have a characteristic spindle shape.

Muscular Contraction

Skeletal muscle tissue is found in our skeletal muscles; for example, the biceps. Cardiac muscle is found in our heart, and smooth muscle is found in our visceral, or hollow, organs - for example, blood vessels and intestines. All muscles contract as a result of interaction between special proteins within the myocytes. Skeletal, cardiac, and smooth muscle cells synthesize the contractile proteins actin and myosin, which are needed for muscular contraction.

When the muscle cell is stimulated, actin and myosin interact in such a way to cause the cell to shorten and, thus, the muscle to contract. Furthermore, all muscles contract in response to intracellular calcium. In striated muscle, calcium binds to a regulatory protein called troponin, which allows actin and myosin to interact with each other. In smooth muscle, calcium binds to the regulatory protein calmodulin, which allows the contractile proteins to interact and, thus, the muscle to contract.

Actin and myosin interact during muscle cell stimulation, causing muscle contractions
Muscular Contraction

Where does the calcium come from? In skeletal muscle, the calcium is released from internal stores that we call sarcoplasmic reticulum. Cardiac and smooth muscle contraction is dependent upon extracellular calcium in addition to the intracellular calcium from the sarcoplasmic reticulum.

Regulation of Contraction

The muscle tissues can be differentiated based on how contraction is regulated by the nervous system. Skeletal muscle is sometimes referred to as voluntary muscle as contraction requires stimulation by a motor neuron. Specifically, neurons innervating skeletal muscles are referred to as somatic motor neurons, and they can be activated with conscious thought or even spinal reflexes. For example, we can choose to contract our leg muscles in order to walk. Additionally, these same muscles will contract in response to tapping on your knee. Either way, somatic motor neurons are needed to stimulate skeletal muscle contraction.

Cardiac and smooth muscle are referred to as involuntary muscle as contraction can occur without neural stimulation. The ability of muscle to contract without neural stimulation is referred to as automaticity. An experiment is frequently done in the academic laboratory to demonstrate automaticity. A frog's heart will continue to beat even after it's removed from the body. It is important to note that the nervous system regulates cardiac and smooth muscle contraction.

Cardiac muscles contract without neural stimulation
Involuntary Muscle Contraction
Autonomic motor neurons of the involuntary nervous system innervate these muscles and can change the rate or the strength of contraction. For example, our heart beats faster when we're nervous. We don't have control over this because the autonomic nervous system changes our heart rate for us.

Lesson Summary

In summary, the three types of muscle tissue are skeletal, cardiac, and smooth. Cardiac and skeletal are both striated muscle as repeating units called sarcomeres give the cells a striated appearance, while smooth muscle lacks such striations. Skeletal fibers are long and linear, while cardiac fibers are short and branched. Smooth muscles lack striations and have a characteristic spindle-shape appearance. Skeletal muscle fibers are arranged side-by-side and run the entire length of the muscle, while cardiac muscle fibers connect end-to-end at intercalated discs.

The contractile proteins actin and myosin interact with one another, causing contraction. Calcium binds troponin in striated muscle and calmodulin in smooth muscle, thus allowing for contraction. Skeletal muscle is termed voluntary as it is stimulated by the somatic motor neurons, while cardiac and smooth muscle are involuntary as neural stimulation is not required in order for them to contract. The ability of muscle to contract on its own is termed automaticity.

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