Bio 17.2 Muscular System PDF

Enter word / phrase to search UBook text Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 17: Muscular System 570 Lesson 17.2 **Characteristics of Specific Muscle Types** Introduction The force produced by muscle contraction can cause bodily movement or stabilizat...

Enter word / phrase to search UBook text Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 17: Muscular System 570 Lesson 17.2 **Characteristics of Specific Muscle Types** Introduction The force produced by muscle contraction can cause bodily movement or stabilization of position. In addition, this force can compress or expand tubular cavities to cause, modulate, or prevent physical transport of a substance (eg, air, blood). There are three broad classifications of muscle, **skeletal**, **cardiac**, and **smooth**, with each class specialized for specific functions (Figure 17.14). **Figure 17.14** Examples of the function of the three muscle types. 17.2.01 Skeletal Muscle **Skeletal muscle** is found throughout the body, typically in muscles attached at each end to bone. When thin sections of skeletal muscle samples are viewed under a microscope, regular light and dark regions, known as **striations**, are apparent. Skeletal muscle cells are long and cylindrical [fibers](javascript:void(0)) and are multinucleated (ie, contain multiple nuclei per cell). The microscopic organization of skeletal muscle fibers is discussed in further detail in Lesson 17.1. Skeletal muscle fibers have specific mitochondrial enzymes and myosin ATPases that enable these fibers to contract with varying degrees of force, velocity, and endurance. Although skeletal muscles are **voluntary muscles**, the control of some of these muscles allows for conscious interruption or activation of contraction (eg, control of the diaphragm muscle during breathing or of certain sphincters composed of skeletal muscle). Skeletal muscle is the only type of muscle innervated by the [somatic nervous system](javascript:void(0)). Generation of an [action potential](javascript:void(0)) at the motor end plate of a [neuromuscular junction](javascript:void(0)) (ie, the interface between a somatic motor neuron and a skeletal muscle fiber) causes depolarization to spread across the sarcolemma. Action potentials propagate along transverse tubules (t-tubules) just as they propagate along the sarcolemma, carrying the wave of depolarization deep into muscle fibers and resulting in the rapid and complete depolarization of the muscle fiber (Figure 17.15). A diagram of a baby in uterus Description automatically generated Chapter 17: Muscular System 571 **Figure 17.15** Depolarization in a skeletal muscle fiber. Voltage-sensing calcium channels in t-tubules and neighboring calcium channels in the sarcoplasmic reticulum largely span the small space between the external (t-tubular) and internal (sarcoplasmic reticulum) membranes of striated muscle fibers (Figure 17.16). In skeletal muscle, voltage-sensing calcium channels detect depolarization and cause the calcium channels in the sarcoplasmic reticulum to open, thereby causing contraction to occur. The Ca2+ released during contraction is rapidly taken back up into the sarcoplasmic reticulum by the sarcoplasmic reticulum Ca2+-ATPase (SERCA). **Figure 17.16** Depolarization-induced release and subsequent reuptake of Ca2+ in skeletal muscle.  A diagram of a structure Description automatically generated Chapter 17: Muscular System 572 The force produced when skeletal muscle contracts is dependent upon several factors. These factors include the number of muscle fibers activated, the frequency at which muscle fibers are activated, and the length of activated muscle fibers. Groups of muscle fibers distributed throughout a muscle but controlled by the same motor neuron are called **motor units**. Certain motor units are more readily stimulated than others, resulting in a progressive accumulation of active motor units as the intensity of nervous stimulation increases. This process, called **motor unit recruitment**, increases the proportion of active muscle fibers in a muscle during a contraction, thereby increasing the force produced (Figure 17.17). **Figure 17.17** Skeletal muscle motor units and motor unit recruitment. A single motor unit is composed of muscle fibers with identical activity levels; therefore, all the fibers in a motor unit are of a single fiber type. As muscle force production begins, motor units with low activation thresholds are activated first, when the level of neural stimulation is also low. These lowest threshold motor units activate *slow* (ie, slower-contracting), o*xidative* (ie, showing a preference for ATP generation via oxygen-requiring mitochondrial metabolism) muscle fibers. As force production increases further, motor units with higher activation thresholds that innervate *fast* (ie, faster-contracting), oxidative fibers are recruited. As force production approaches the maximum, the final  Chapter 17: Muscular System 573 motor units recruited are those with the highest thresholds for activation, or the fast, *glycolytic* (ie, showing a preference for ATP generation via glycolysis) fibers. As motor unit activation frequency increases, the interval during which Ca2+ can be pumped back into the sarcoplasmic reticulum and the muscle fiber can relax decreases. When a new burst of excitation activates Ca2+ release before relaxation from the previous stimulus has occurred, active force production occurs in a muscle fiber in which tension is already above the baseline. As a result, isolated **twitch** muscle contractions in response to individual action potentials fuse with one another, producing a \"staircase\" of twitches and greater total force than when each burst of stimulation occurs in isolation (Figure 17.18). **Figure 17.18** Individual muscle twitches can sum to produce a tetanus. A contraction in which individual twitches fuse is called a **tetanic contraction** (or simply a **tetanus**). At high enough frequencies of stimulation, individual twitches are no longer discernible, and force production smoothly approaches a maximum plateau level with each burst of high-frequency stimulation. When individual stimulation peaks are discernible within a tetanus, the tetanus is described as **unfused**; a tetanus in which individual peaks are not discernible is called a **fused tetanus**, as shown in Figure 17.18. The physical length of a muscle fiber as it contracts also affects the tension produced in the muscle. Force production is maximal when sarcomere lengths allow maximal actin-myosin overlap, and stretching a shortened muscle will increase force production up to the point at which overlap begins diminishing (ie, excessive stretching) or the fiber is damaged (Figure 17.19). Below the ideal sarcomere length, actin and myosin filaments from one side of the sarcomere crowd structures from the other side of the sarcomere (eg, Z line), resulting in hindered force production. Chapter 17: Muscular System 574 **Figure 17.19** Relationship between force of contraction and sarcomere length. Due to its large mass, contraction of skeletal muscle can exert a large influence on whole-body energy expenditure and [thermoregulation](javascript:void(0)). During exercise, increased skeletal muscle energy utilization can greatly increase the rate of whole-body energy expenditure, and the associated increase in heat production can challenge the body\'s thermoregulatory mechanisms. In addition, cold exposure can induce rapid, involuntary muscle contractions (ie, shivering) that can also markedly increase whole-body energy expenditure and heat production (**thermogenesis**, Figure 17.20).  Chapter 17: Muscular System 575 **Figure 17.20** Skeletal muscle thermogenesis resulting from shivering. Similar to the inevitable increases in energy expenditure and heat production, squeezing of blood vessels during skeletal muscle contractions is unavoidable. Veins are more affected by this squeezing than arteries because veins are more easily compressed. This contraction-induced compression of veins, known as the **skeletal muscle pump**, can serve an important function: Squeezing blood from skeletal muscle toward the heart increases venous return (ie, the amount of blood returning to the heart). Because gravity causes pooling of blood in the lower extremities, the effect of the skeletal muscle pump is most pronounced during contractions of the leg muscles, as shown in Figure 17.21. A diagram of the muscles of the leg Description automatically generated Chapter 17: Muscular System 576 **Figure 17.21** The skeletal muscle pump. Skeletal muscle fibers can be grouped into three broad categories according to their characteristics (eg, speed of contraction, myoglobin content, enzyme activity). These categories and their characteristics are summarized in Table 17.1.  Chapter 17: Muscular System 577 **Table 17.1** General characteristics of typical skeletal muscle fiber types. **Fiber type** **Slow oxidative(type 1)** **Fast oxidative(type 2A)** **Fast glycolytic(type 2X)** **Speed of contraction** Slow Medium Fast **Best use** Endurance activity (eg, long-distance running, posture) Moderate endurance activity (eg, medium-distance running) Explosive movements (eg, weight lifting, jumping) **Resistance to fatigue** Fatigue resistant Intermediate susceptibility to fatigue Easily fatigable **Primary source of ATP** Aerobic respiration Anaerobic glycolysis and aerobic respiration Anaerobic glycolysis **Mitochondria** Plentiful Plentiful to moderately plentiful Few **Capillaries** Plentiful Plentiful to moderately plentiful Few **Myoglobin** Plentiful Plentiful to moderately plentiful Few **Appearance** Red Intermediate White **Concept Check 17.2** Fill in the blanks in the sentences below with the words \"fast,\" \"glycolytic,\" \"motor unit,\" \"muscle fiber type,\" \"oxidative,\" and \"slow.\" A \_\_\_\_\_\_\_\_\_\_\_\_\_ is a collection of skeletal myocytes voluntarily activated together and composed of a single \_\_\_\_\_\_\_\_\_\_\_\_\_\_. As the force produced by a voluntary muscle contraction gradually increases, any motor units with \_\_\_\_\_\_\_\_\_\_, \_\_\_\_\_\_\_\_\_\_ muscle fibers are activated first, followed by any motor units with fast, oxidative fibers. \_\_\_\_\_\_\_\_\_\_, \_\_\_\_\_\_\_\_\_\_ muscle fibers are recruited last, as the force produced approaches the maximum. [**Solution**](javascript:void(0)) Chapter 17: Muscular System 578 17.2.02 Cardiac Muscle **Cardiac muscle** is found in the walls of the [heart](javascript:void(0)) and is specialized to generate the powerful, coordinated contractions responsible for the continuous pumping of blood through the vessels of the circulation (Figure 17.22). Microscopically, cardiac muscle appears striated like skeletal muscle, but cardiac muscle fibers are much shorter and can branch, and each cardiac muscle cell contains only one or two nuclei. In addition, cardiac muscle fibers are generally similar to each other, with abundant mitochondria and capillaries, reflecting the single function (ie, blood-pumping) and high metabolic demand of the heart. **Figure 17.22** Cardiac muscle is found in the atria and ventricles of the heart. Cardiac muscle cells are connected to adjacent cells via **intercalated discs**. These discs are regions of cell contact that contain both desmosomes (to prevent cells from separating during contraction) and gap junctions (to facilitate direct ion exchange for synchronized contraction), as shown in Figure 17.23. Cardiac muscle is under involuntary control and is **myogenic**, meaning that it *does not require* nervous system input to contract. Instead, specialized cells spontaneously depolarize to produce electrical impulses that spread through the cardiac muscle via intercalated discs. However, cardiac muscle contraction can be *regulated* by neural and hormonal input. For example, [parasympathetic](javascript:void(0)) signaling slows the rate of contraction, and sympathetic and hormonal signaling can increase heart rate (see Concept 13.1.13).  Chapter 17: Muscular System 579 **Figure 17.23** Intercalated discs in cardiac muscle. As in skeletal muscle, t-tubules in cardiac muscle contain voltage-sensing Ca2+ channels near the Ca2+ channels in the sarcoplasmic reticulum. In cardiac muscle, however, depolarization-induced Ca2+ entry through the voltage-sensing Ca2+ channels is essential in triggering [contraction](javascript:void(0)). In addition, although some of the Ca2+ released during contraction is pumped back into the sarcoplasmic reticulum by sarcoplasmic reticulum Ca2+-ATPases, release of Ca2+ back into the extracellular fluid is also an important contributor to the restoration of the baseline Ca2+ concentration after a cardiac contraction. Because each heart chamber is a functional syncytium (ie, a collection of connected cells effectively sharing cytoplasm via the pores formed by gap junctions), all the muscle cells in each chamber are activated to contract with each heartbeat. As a result, recruitment of additional muscle fibers is not possible, unlike in skeletal muscle contraction. In addition, action potentials in cardiac muscle cells last much longer than in skeletal muscle cells. Consequently, the refractory period (see Concept 12.2.02) for each action potential extends until muscle relaxation has occurred, and frequency summation cannot occur. However, cardiac muscle cells can greatly increase their force production when stretched, and contractions initiated at greater sarcomere lengths generate more force (up to a point). As in skeletal muscle cells, this general phenomenon is referred to as the length-tension relationship in the context of muscle fiber and sarcomere lengths. At the whole-heart level, greater atrial and ventricular blood volumes stretch cardiac muscle fibers, producing more forceful contractions and increased pumping of blood, a phenomenon called the Frank-Starling mechanism. 17.2.03 Smooth Muscle **Smooth muscle**, so named because it lacks the microscopic striations found in skeletal and cardiac muscle, is found in internal organs and tissues (Figure 17.24). This muscle type exhibits a wide range of specialization to allow periodic or sustained contractions, typically to squeeze or compress a tubular structure (eg, [gastrointestinal tract](javascript:void(0)), blood vessel). Such contractions propel or prevent movement of materials (eg, chyme); smooth muscle contractions can also fine-tune flow (eg, blood in vessels). Chapter 17: Muscular System 580 **Figure 17.24** Smooth muscle in blood vessels. Smooth muscle shares some characteristics with striated muscle, such as the essential role of cytoplasmic Ca2+ entry in causing contraction and force production via actin-myosin interaction, but important differences exist. For example, unlike striated muscles, the small myocytes in smooth muscle do not possess t-tubules to carry waves of depolarization deeper into the cells. Figure 17.25 depicts smooth muscle cells in relaxed and contracted states in blood vessels.  Like the heart, smooth muscle is innervated by the autonomic nervous system and can be myogenic. Also similar to cardiac muscle, relaxation is facilitated by both Ca2+ reuptake into the sarcoplasmic reticulum as well as transport of Ca2+ back to the extracellular fluid. Some smooth muscles also resemble the heart in that gap junctions allow the cells in such muscles to act as a functional syncytium. In these cells, sharing of cytoplasm allows activation of all cells simultaneously