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Concise Notes On Muscle Tissue, Nothing Missing.
Figure 15.3.215.3.2: 1) Skeletal muscle cells are long tubular cells with striations (3) and multiple nuclei (4). The nuclei are embedded in the cell membrane (5) so that they are just inside the cell. This type of tissue occurs in the muscles that are attached to the skeleton. Skeletal muscles function involuntary movements of the body.2) Smooth muscle cells are spindle-shaped (6), and each cell has a single nucleus (7). Unlike skeletal muscle, there are no striations. The smooth muscle acts involuntarily and functions in the movement of substances in the lumens. They are primarily found in blood vessel walls and walls along the digestive tract. 3) Cardiac muscle cells branch off from each other, rather than remaining along with each other like the
cells in the skeletal and smooth muscle tissues. Because of this, there are junctions between adjacent cells (9). The cells have striations (8), and each cell has a single nucleus (10). This type of tissue occurs in the wall of the heart and its primary function is for pumping blood. This is an involuntary action. (Mdunning [CC BY-SA 3.0] via wikimedia.org:Muscle Tissue (1).svg )
Muscle tissue is a soft tissue that makes up most of the tissues in the muscles of the human muscular system. Other tissues in muscles are connective tissues, such as tendons that attach skeletal muscles to bones and sheaths of connective tissues that cover or line muscle tissues. Only muscle tissue per se, however, has cells with the ability to contract.
There are three major types of muscle tissues in the human body: skeletal, smooth, and cardiac muscle tissues. Figure 15.3.215.3.2 shows how the three types of muscle tissues appear under a microscope. When you read about each type below, you will learn why the three types appear as they do.
Skeletal Muscle Tissue
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Figure 15.3.415.3.4: This figure shows major skeletal muscles
in the back (posterior) of the body. (public domain; Mikael Häggström, 2014; via wikimedia.org; muscles posterior labeled.png)
Skeletal muscle is muscle tissue that is attached to bones by tendons, which are bundles of collagen fibers. Whether you are moving your eyes or running a marathon, you are using skeletal muscles. Contractions of skeletal muscles are voluntary, or under the conscious control of the central nervous system via the somatic nervous system. Skeletal muscle tissue is the most common type of muscle tissue in the human body. By weight, an average adult male is about 42 percent skeletal muscles, and the average adult female is about 36 percent skeletal muscles. Some of the major skeletal muscles in the human body are labeled in Figure 15.3.315.3.3 and Figure 15.3.415.3.4.
Skeletal Muscle Pairs
are an example of a muscle pair where the muscles work in opposition to each other.
Skeletal Muscle Structure
Each skeletal muscle consists of hundreds — or even thousands — of skeletal muscle fibers, which are long, string-like cells. As shown in the figure below, skeletal muscle fibers are individually wrapped in connective tissue called endomysium. The skeletal muscle fibers are bundled together in units called muscle fascicles, which are surrounded by sheaths of connective tissue called perimysium. Each fascicle contains between ten and 100 (or even more!) skeletal muscle fibers. Fascicles, in turn, are bundled together to form individual skeletal muscles, which are wrapped in connective tissue called epimysium. The connective tissues in skeletal muscles have a variety of functions. They support and protect muscle fibers, allowing them to withstand 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 the muscles to tendons.
Figure 15.3.615.3.6: Each skeletal muscle has a structure of bundles within bundles. Bundles of muscle fibers make up a muscle fascicle, and bundles of fascicles make up a skeletal muscle. At each level of bundling, a connective tissue membrane surrounds the bundle. (Arcadian; public domain; via wikimedia.org; muscle structure)
The same bundles-within-bundles structure is replicated within each muscle fiber. As shown in the figure below, a muscle fiber consists of a bundle of myofibrils, which are themselves bundles of protein filaments. These protein filaments consist of thin filaments of the protein actin — which are anchored to structures called Z discs — and thick filaments of the protein myosin. The filaments are arranged together within a myofibril in repeating units called sarcomeres, which run from one Z disc to the next. The sarcomere is the basic functional unit of skeletal (and cardiac) muscles. It contracts as actin and myosin filaments slide over one another. Skeletal muscle tissue is said to be striated because it appears striped. It has this appearance
wikimedia.org; muscle fibers)
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 fibers are dense with capillaries and rich in mitochondria and myoglobin, which is a protein that stores oxygen until needed for muscle activity. Relative to fast-twitch fibers, slow-twitch fibers can carry more oxygen and sustain aerobic (oxygen-using) activity. Slow-twitch fibers can contract for long periods of time, but not with very much force. They are relied upon primarily in endurance events, such as distance running or cycling.
Fast-twitch muscle fibers contain fewer capillaries and mitochondria and less myoglobin. This type of muscle fiber can contract rapidly and powerfully, but it fatigues very quickly. Fast-twitch fibers can sustain only short, anaerobic (non-oxygen-using) bursts of activity. Relative to slow-twitch fibers, fast-twitch fibers contribute more to muscle strength and have a greater potential for increasing in mass. They are relied upon primarily in short, strenuous events, such as sprinting or weight lifting.
Proportions of fiber types vary considerably from muscle to muscle and from person to person. Individuals may be genetically predisposed to have a larger percentage of one type of muscle fiber than the other. Generally, an individual who has more slow-twitch fibers is better suited for activities requiring endurance, whereas an individual who has more fast-twitch
fibers is better suited for activities requiring short bursts of power.
Smooth Muscle
Smooth muscle is muscle tissue in the walls of internal organs and other internal structures such as blood vessels. When smooth muscles contract, they help the organs and vessels carry out their functions. When smooth muscles in the stomach wall contract, for example, they squeeze the food inside the stomach, helping to mix and churn the food and break it into smaller pieces. This is an important part of digestion. Contractions of smooth muscles are involuntary, so they are not under conscious control. Instead, they are controlled by the autonomic nervous system, hormones, neurotransmitters, and other physiological factors.
Structure of Smooth Muscle
Figure 15.3.915.3.9: The muscular uterine wall stretches to a great extent to accommodate a growing fetus, yet it can still contract with great force during the labor that precedes childbirth. At that time, it can exert up to 100 pounds of force. (Magnus Manske; public domain; via wikimedia.org; placenta.svg)
Unlike striated muscle, smooth muscle can sustain very long-term contractions. Smooth muscle can also stretch and still maintain its contractile function, which striated muscle cannot. The elasticity of smooth muscle is enhanced by an extracellular matrix secreted by myocytes. The matrix consists of elastin, collagen, and other stretchy fibers. The ability to stretch and still contract is an important attribute of smooth muscle in organs
such as the stomach and uterus (Figure 15.3.915.3.9), both of which must stretch considerably as they perform their normal functions.
The following list indicates where many smooth muscles are found, along with some of their specific functions.
Walls of organs of the gastrointestinal tract (such as the esophagus, stomach, and intestines), moving food through the tract by peristalsis
Walls of 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 organs of the male and female reproductive tracts; in the uterus, for example, pushing a baby out of the uterus and into the birth canal
Walls of structures of the urinary system, including the urinary bladder, allowing 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
Walls of lymphatic vessels, squeezing the fluid called lymph through the vessels
Iris of the eyes, controlling the size of the pupils and thereby the amount of light entering the eyes
beats and pumps blood. Contractions of cardiac muscle are involuntary, like those of smooth muscles. They are controlled by electrical impulses from specialized cardiac muscle cells in an area of the heart muscle called the sinoatrial node.
Like skeletal muscle, cardiac muscle is striated because its filaments are arranged in sarcomeres inside the muscle fibers. However, in cardiac muscle, the myofibrils are branched at irregular angles rather than arranged in parallel rows (as they are in skeletal muscle). This explains why cardiac and skeletal muscle tissues look different from one another.
The cells of cardiac muscle tissue are arranged in interconnected networks. This arrangement allows rapid transmission of electrical impulses, which stimulate virtually simultaneous contractions of the cells. This enables the cells to coordinate contractions of the heart muscle.
The heart is the muscle that performs the greatest amount of physical work in the course of a lifetime. Although the power output of the heart is much less than the maximum power output of some other muscles in the human body, the heart does its work continuously over an entire lifetime without rest. Cardiac muscle contains a great many mitochondria, which produce ATP for energy and help the heart resist fatigue.
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