9.3: Types of Muscle Tissue (2023)

Work those eye muscles!

Roll your eyes - a small movement, considering the visibly large and strong external eye muscles that control eyeball movements. These muscles are called the strongest muscles in the human body in terms of the work they do. However, the outer eye muscles perform a surprising amount of work. Eye movements occur almost constantly during waking hours, especially when examining faces or reading. The eye muscles are also exercised every night during the phase of sleep called rapid eye movement sleep. The outer eye muscles can move the eyes because they are mostly made of muscle tissue.

Skeletal muscle pairs

To move bones in opposite directions, skeletal muscles usually consist of pairs of muscles that work in opposition to one another. For example, when the biceps muscle (on the front of the arm) contracts, it can cause the elbow joint to flex or bend the arm, as shown in Figure \(\PageIndex{5}\). When the triceps muscle (on the back of the arm) contracts, it can cause the elbow to extend or straighten the arm. The biceps and triceps muscles are examples of muscle pairs where the muscles work in opposition to each other.

Skeletal muscle structure

Each skeletal muscle consists of hundreds – or eventhousands– from skeletal muscle fibers, which are long thread-like cells. As shown in Figure \(\PageIndex{6}\), skeletal muscle fibers are individually wrapped in connective tissue calledendomysium. Skeletal muscle fibers are grouped into units calledmuscle fascicles, surrounded by connective tissue sheaths calledperimysium. Each fascicle contains between ten and 100 (or even more!) skeletal muscle fibers. The fascicles, in turn, are grouped together to form individual skeletal muscles, which are wrapped in connective tissue calledepimysium. The connective tissues of skeletal muscles have a variety of functions. They support and protect the muscle fibres, allowing them to resist the forces of contraction by distributing the forces applied to the muscle. They also provide pathways for nerves and blood vessels to reach the muscles. In addition, the epimysium anchors muscles to tendons.

The same structure of bundles within bundles is replicated within each muscle fiber. As shown in Figure \(\PageIndex{7}\), a muscle fiber consists of a bundle of myofibrils, which are bundles of protein filaments. These protein filaments consist of thin filaments of the protein actin, anchored in structures called Z disks – and thick filaments of the protein myosin. Filaments are arranged together within a myofibril in repeating units calledsarcomeres,that go from one Z disk to the next. The sarcomere is the basic functional unit of skeletal (and cardiac) muscle. It contracts as the actin and myosin filaments slide past each other. Skeletal muscle tissue is said to be striated because it looks striped. It has this appearance because of regular, alternating bands of A (dark) and I (light) filaments arranged in sarcomeres within muscle fibers. Other components of a skeletal muscle fiber include multiple nuclei and mitochondria.

Slow and fast twitch skeletal muscle fibers

Skeletal muscle fibers can be divided into two types, called slow-twitch (or type I) muscle fibers and fast-twitch (or type II) muscle fibers.

• Slow twitch muscle fibersthey are dense in capillaries and rich in mitochondria and myoglobin, a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibers, slow-twitch fibers can carry more oxygen and support aerobic (oxygen-using) activity. Slow-twitch fibers can contract for long periods of time, but not very forcefully. They are primarily used in endurance events such as long-distance running or cycling.
• Fast twitch muscle fiberscontain fewer capillaries and mitochondria and less myoglobin. This type of muscle fiber can contract quickly and powerfully, but it tires very quickly. Fast-twitch fibers can only sustain short bursts of anaerobic activity (one that doesn't use oxygen). Relative to slow-twitch fibers, fast-twitch fibers contribute more to muscle strength and have greater potential to increase mass. They are mainly used in short, strenuous events such as running or weight lifting.

The proportions of fiber types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a higher percentage of one muscle fiber type than another. Generally, an individual who has more slow-twitch fibers is better suited for activities that require endurance. In contrast, an individual who has more fast-twitch fibers is better suited for activities that require small bursts of energy.

smooth muscle structure

The cells that make up smooth muscle are often calledmyocytes.Unlike the muscle fibers of striated muscle tissue, smooth muscle tissue myocytes do not have their filaments arranged in sarcomeres. Therefore, smooth fabric is not striated. However, smooth muscle myocytes contain myofibrils, which contain bundles of myosin and actin filaments. The filaments cause contractions when they slide over each other, as shown in Figure \(\PageIndex{8}\).

smooth muscle functions

Unlike striated muscle, smooth muscle can sustain long-term contractions. Smooth muscle can also elongate and still maintain its contractile function, which striated muscle cannot. An extracellular matrix secreted by myocytes increases smooth muscle elasticity. The matrix consists of elastin, collagen and other elastic fibers. The ability to stretch and yet contract is an important attribute of smooth muscle in organs such as the stomach and uterus (Figure \(\PageIndex{9}\)), both of which must stretch considerably as they perform their normal functions.

The following list indicates where many smooth muscles are located, along with some of their specific functions.

• Walls of the gastrointestinal tract (such as the esophagus, stomach, and intestines), moving food through the tract by peristalsis.
• Walls of the air passages of the respiratory tract (such as the bronchi), controlling the diameter of the passages and the volume of air that can pass through them
• Walls of male and female reproductive organs; in the womb, for example, pushing a baby out of the womb and into the birth canal
• The walls of structures in the urinary system, including the urinary bladder, allow the bladder to expand so it can hold more urine and then contract as urine is released.
• Walls of blood vessels, controlling the diameter of the vessels and thereby affecting blood flow and blood pressure
• Lymphatic vessel walls, squeezing fluid called lymph through the vessels.
• Irises of the eyes, controlling the size of the pupils and therefore the amount of light that enters the eyes
• Arrector pili in the skin, raising hairs in hair follicles in the dermis.

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You can learn more about the three types of muscle tissue by watching this Khan Academy video: