Chapter 10 - Outline - OpenStax 2021 PDF

**[CHAPTER TEN]** **SKELETAL MUSCLE TISSUE** 1. A. All muscle tissue is derived from mesoderm. Muscle tissue, one of the four principle tissue types, consists chiefly of muscles cells that are highly specialized for contractions. Without the three types of muscle tissue -- skeletal...

**[CHAPTER TEN]** **SKELETAL MUSCLE TISSUE** 1. A. All muscle tissue is derived from mesoderm. Muscle tissue, one of the four principle tissue types, consists chiefly of muscles cells that are highly specialized for contractions. Without the three types of muscle tissue -- skeletal muscle, smooth muscle, and cardiac muscle -- nothing in the body would move, and no body movement could occur. B. Characteristics of Muscle Tissue: 1. **[Excitability]** -- their plasma membranes can change their electrical states (from polarized to depolarized) and send an electrical wave called an action potential along the entire length of the membrane when stimulated by a motor neuron or hormone. 2. **[Contractility]** -- muscles pull on their attachment points and shorten with force. 3. **[Extensibility]** -- it can stretch or extend. 4. **[Elasticity]** -- after contracting or extending, muscle can return to their original length. C. Types of Muscle Tissue 1. **[Skeletal Muscle Tissue]** = found attached to bones; accounts for 40% of the body mass; long, cylindrical-shaped cells; multi-nucleated cells with the nuclei peripherally located, voluntary, possesses striation; quick twitch with short contractions. Skeletal muscle has many functions (see list below). 2. **[Cardiac Muscle Tissue]** = found only in the heart; cells are short and branched; usually uni-nucleated but occasionally can be bi-nucleated; involuntary; possesses striations and intercalated discs; intermediate twitch with intermediate contractions. Cardiac muscle moves blood and maintains blood pressure. 3. **[Smooth Muscle Tissue]** = found throughout the body in the walls of hollow organs of the digestive, respiratory, urinary, and reproductive tracts; the walls of blood vessels and in the arrector pili muscle of skin. The cells are short and spindle-shaped, lack striations and intercalated discs, and have only a single nucleus per cell. They demonstrate slow twitch and long contractions. Smooth muscle moves food, urine, and reproductive secretions; controls the diameter of respiratory passageways and regulates the diameter of blood vessels. **10.2 Skeletal Muscle** A. 1. 2. 3. 4. 5. 6. B. 1. a. b. c. d. e. f. 2. a. b. c. d. 3. a. b. c. d. e. f. g. C. 1. Each myofibril consists of approximately 10,000 sarcomeres, each with a resting length of about 2 mm. Each sarcomere is composed of a very specific arrangement of the myofilaments of actin and myosin. a. The **[A band]** is the dense region of the sarcomere that contains overlapping thick and thin filaments (both myosin and actin). b. On either side of the A band is an area that contains only thin filaments (actin) called the **[I band]**. c. In the middle of each A band, is an area that contains only thick filaments (myosin) called the **[H band]**. d. The **[M Line]**, located in the middle of each H band, anchors the central portion of each thick filament. e. **[Z Lines]** mark the boundary between adjacent sarcomeres. Z lines consist of proteins called **actinins**, which anchor the thin filaments of adjacent sarcomeres. 2. Anatomy of the myofilaments (actin and myosin) a. i. ii. iii. iv. b. i. ii. iii. iv. v. **10.3 Skeletal Muscle Contraction and Relaxation** A. Neuromuscular junction and nerve stimulation 1. Skeletal muscle cells are stimulated by a **motor neuron**. 2. The axon of each motor neuron branches extensively to form numerous cellular extensions called **[synaptic terminals]**. 3. The synaptic terminals then interact with the sarcolemma of the muscle fiber at a specialized site called the **[neuromuscular junction]**. 4. When the electrical impulse (action potential) reaches the synaptic terminals, **calcium channels** with the synaptic terminal begin to open, causing calcium to rush into the axon terminal. 5. As a result of the influx of calcium, **[synaptic vesicles]** containing the neurotransmitter **[acetylcholine]** (Ach) fuse with the axon membrane releasing the neurotransmitter into the **synaptic cleft** by exocytosis. 6. Acetylcholine then diffuses across the synaptic cleft and attaches to receptors (also known as **ion** **channels**) on a highly folded region of the sarcolemma called the **motor end plate**. 7. The binding of acetylcholine to the channels causes the channel to open and influx of sodium ions into the muscle cell resulting in **depolarization** of the sarcolemma and eventually leading to **an action potential** and contraction. 8. As the action potential spreads away from the motor end plate and across the sarcolemma, acetylcholine is swiftly broken down by **[acetylcholinesterase]**. The destruction of Ach prevents continued muscle contraction in the absence of nervous stimulation. B. 1. 2. 3. 4. 5. 6. 7. Action potentials are considered **[all or none responses]** because once initiated, they are unstoppable. C. 1. 2. 3. D. 1. 2. 3. 4. 5. 6. 7. E. 1. 2. a. b. c. d. 3. a. b. c. 4. **Nervous System Control of Muscle Tension** A. Classification of Muscle Contractions 1. **[Isotonic contractions]** = tension rises until it exceeds the load so that the load is "moved". Once the load moves, tension in the muscle now remains constant. 2. **[Concentric contraction]** = contractions that permit the muscle to shorten 3. **[Eccentric contraction]** = contractions that cause the muscle to lengthen 4. **[Isometric contractions]** = tension increases to peak but the muscle neither shortens nor lengthens. Instead, the tendons stretch. 5. Tension production is greatest when a muscle is stimulated at its optimal length. There is no mechanism to regulate the amount of tension produced in a contraction by changing the number of contracting sarcomeres. When calcium ions are released, they are released from all triads in the muscle fiber, thus, the muscle fiber is either contracting or relaxed. B. **[Motor units]** = all the muscle fibers controlled by a single motor neuron. 1. 2. 3. During a sustained contraction, motor units are activated on a rotating basis called **[asynchronous motor unit summation]**. This ensures that each motor unit has an opportunity to recover before it is stimulated again. C. **[Myograms]** = a record of muscle contractions (force of contraction in relationship to time). 1. 2. a. b. c. 3. Two factors determine the amount of tension produced by a skeletal muscle: 1) the amount of tension produced by each stimulated muscle fiber; and 2) the total number of muscle fibers stimulated at any given moment. a. b. c. d. D. A variable number of motor units is always active, even when the entire muscle is not contracting. This creates a resting tension called **[muscle tone]**. a. **[Hypotonia]** is the absence of the low-level contractions that lead to muscle tone and can result from damage to parts of the central nervous system (CNS), such as the cerebellum, or from loss of innervations to a skeletal muscle, as in poliomyelitis. Hypotonic muscles have a flaccid appearance and display functional impairments, such as weak reflexes. b. **[Hypertonia]** is excessive muscle tone is referred to as **hypertonia**, accompanied by hyperreflexia (excessive reflex responses), often the result of damage to upper motor neurons in the CNS. Hypertonia can present with muscle rigidity (as seen in Parkinson's disease) or spasticity, a phasic change in muscle tone, where a limb will "snap" back from passive stretching (as seen in some strokes). **10.5 Types of Skeletal Muscle Fibers** A. B. C. **10.6 Exercise and Muscle Performance** A. 1. **[Hypertrophy]** = an enlargement of stimulated muscles due to physical training. The number of fibers within the muscle does not change but instead each muscle fiber increases in diameter. Example: body builders from exhaustive training and/or the use of steroid hormones. 2. **[Atrophy]** = lack of use of a muscle causes it to become flaccid, loses tone and power, and decreases in diameter. Example: a person wearing a cast for several months. Age-related muscle atrophy is called **[sarcopenia]**. 3. **[Paralysis]** = loss of voluntary control of a muscle. Can result from many conditions: a. b. c. d. e. B. Endurance Exercise 4. Slow fibers are predominantly used in endurance exercises that require little force but involve numerous repetitions. The aerobic metabolism used by slow-twitch fibers allows them to maintain contractions over long periods. 5. Endurance training modifies these slow fibers to make them even more efficient by producing more mitochondria to enable more aerobic metabolism and more ATP production. 6. Endurance exercise can also increase the amount of myoglobin in a cell, as increased aerobic respiration increases the need for oxygen. Myoglobin is found in the sarcoplasm and acts as an oxygen storage supply for the mitochondria. 7. The training can trigger the formation of more extensive capillary networks around the fiber, a process called **[angiogenesis]**, to supply oxygen and remove metabolic waste. C. Resistance Exercise 8. Resistance exercises, as opposed to endurance exercise, require large amounts of FG fibers to produce short, powerful movements that are not repeated over long periods. 9. Resistance exercise affects muscles by increasing the formation of myofibrils, thereby increasing the thickness of muscle fibers. This added structure causes hypertrophy, or the enlargement of muscles, exemplified by the large skeletal muscles seen in body builders and other athletes. 10. Except for the hypertrophy that follows an increase in the number of sarcomeres and myofibrils in a skeletal muscle, the cellular changes observed during endurance training do not usually occur with resistance training. There is usually no significant increase in mitochondria or capillary density. However, resistance training does increase the development of connective tissue, which adds to the overall mass of the muscle and helps to contain muscles as they produce increasingly powerful contractions. D. Performance-Enhancing Substances 11. Some athletes attempt to boost their performance by using various agents that may enhance muscle performance. **Anabolic steroids** are one of the more widely known agents used to boost muscle mass and increase power output. Anabolic steroids are a form of testosterone, a male sex hormone that stimulates muscle formation, leading to increased muscle mass. 12. Endurance athletes may also try to boost the availability of oxygen to muscles to increase aerobic respiration by using substances such as **erythropoietin** (EPO), a hormone normally produced in the kidneys, which triggers the production of red blood cells. The extra oxygen carried by these blood cells can then be used by muscles for aerobic respiration. 13. **Human growth hormone** (hGH) is another supplement, and although it can facilitate building muscle mass, its main role is to promote the healing of muscle and other tissues after strenuous exercise. Increased hGH may allow for faster recovery after muscle damage, reducing the rest required after exercise, and allowing for more sustained high-level performance. **10.7 Cardiac Muscle Tissue** A. Cardiac muscle cells possess a single, centrally located nucleus; they are uni-nucleated. B. Cardiac muscle cells are relatively small and are columnar-shaped. Unlike skeletal muscle cells, cardiac muscle cells branch. C. Cardiac muscle cells are metabolically very active and therefore possess large numbers of mitochondria and are rich supplied with capillaries. D. Cardiac muscle cells possess **[striations]** because of the highly organized arrangement of the myofibrils into repeated sarcomeres. E. Approximately, 1% of cardiac muscle cells are capable of generating their own electrical impulse and are therefore called **[autorhythmic]**. F. Cardiac muscle tissue possesses **[intercalated discs]** where the plasma membranes of two adjacent cardiac muscle cells are extensively intertwined and bound together by **gap junctions and desmosomes**. These connections help stabilize the relative positions of the adjacent cells. It also allows a direct electrical, chemical, and mechanical connection between the two muscle cells so that the cardiac muscle cells act as an enormous single cell. This ability to behave as a single coordinated unit is called **[functional syncytium]**. G. Cardiac muscle cell contraction last longer than skeletal muscle fiber contraction primarily due to differences in membrane permeability. Calcium channels remain open in cardiac muscle cells for an extended time resulting in a prolonged **refractory period**. **10.8 Smooth Muscle** A. Smooth muscle cells are relatively long and slender, ranging from 5 to 10 um in diameter and 30 to 200 um in length. B. Although actin and myosin filaments are utilized in the contraction of smooth muscle, they arranged differently from that of skeletal and cardiac muscle. There are no sarcomeres or myofibrils. As a result, there are no striations in smooth muscle and is called un-striated muscle. C. Thin fibers (actin) are attached to **dense bodies** rather than Z lines and thick filaments (myosin) have more heads per thick filament and are scattered throughout the sarcoplasm. D. Furthermore, there are no T-tubules, troponin, or tropomyosin, and the sarcoplasmic reticulum (SR) forms a loose network throughout the sarcoplasm. Because the troponin-tropomyosin interaction is not used, smooth muscle relies on the protein **calmodulin** to form a calcium-calmodulin complex to regulate cross-bridge formation. E. Visceral smooth muscle cells have no direct contact with motor neurons but are connected to each other by gap junctions so whenever a contraction is stimulated, its electrical signal can spread from cell to cell. **Pacesetter cells** are present in areas where peristalsis, or rhythmic contraction, is necessary. F. Unlike skeletal and cardiac muscle, smooth muscle tissue can produce more muscle cells called **hyperplasia**. **10.9 Development & Regeneration of Muscle Tissue** A. B. C. D. Smooth muscle tissue can regenerate from a type of stem cell called a **pericyte**, which is found in some small blood vessels. Pericytes allow smooth muscle cells to regenerate and repair much more readily than skeletal and cardiac muscle tissue.