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Muscle Tissue Chapter 11 387   Chapter Introduction In this chapter we wi...

Muscle Tissue Chapter 11 387   Chapter Introduction In this chapter we will learn… The word “muscle” in A&P can be used in several ways. We may be talking about an organ (i.e., the biceps brachii muscle in your arm) or a tissue (muscle tissue). Of the tissues, we could be talking about Nucleus the tissue that comprises a muscle under your skin such as the biceps brachii, or the muscle that lines your stomach or blood vessels, or Nucleus the tissue that makes up the bulk of the heart. Whether it functions to move food, pump blood, or help you to walk, we will discover all of the muscle tissues of the human body in this chapter. The differences among How skeletal muscle the muscle tissue types contraction is achieved 11.1 Overview of Muscle Tissues Learning Objectives: By the end of this section, you will be able to: 11.1.1 Describe the major functions of muscle 11.1.3 Compare and contrast the general tissue. microscopic characteristics of skeletal, cardiac, and smooth muscle. 11.1.2 Describe the structure, location in the body, and function of skeletal, cardiac, and smooth muscle. Remember from Chapter 5 that there are four tissue types in the human body. Every organ and organ system is made up of a combination of these four types. One of these—muscle tissue—is the subject of this chapter. One of the challenges learners can sometimes struggle with when discussing muscle tissue is keeping in mind when we are discussing a muscle as an organ (a named muscle such as the abdominals or your deltoid) and when we are considering muscle as a tissue (a group of cells work- ing together to perform one function). In this chapter we discuss the three muscle tissues, while in Chapter 12 we will examine the individual skeletal muscle organs of the body. The three muscle tissues—skeletal, smooth, and cardiac—are very different The Human Anatomy and Physiology Society includes more than 1,700 educators who work together to promote excellence in the teaching of this subject area. The HAPS A&P Learning Outcomes measure student mastery of the content typically covered in a two-semester Human A&P curriculum at the undergraduate level. The full Learning Outcomes are available at https://www.hapsweb.org. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 388 Unit 2 Support and Movement in structure and appearance. They share some functions and differ in others. LO 11.1.1 Obviously, the best-known feature of all muscle tissue types is their ability to con- tract and cause movement. When we think of the movement caused by smooth and cardiac muscle contraction, we can think of the movement of internal mate- rial. Cardiac muscle makes up the walls of the heart and therefore surrounds the blood-filled chambers. When it contracts it squeezes on that internal material—the blood—and propels it through the circulatory system. Similarly, smooth muscle makes up the walls of our hollow organs such as blood vessels, intestines, the uterus, and so on. When smooth muscle contracts, it propels the internal material, such as food or menstrual blood, in a certain direction. Skeletal muscle contraction, on the other hand, causes external movement. Our limbs move in space, the skin of our face contorts into a smile, and so forth. Beyond this function of contraction in various forms, skeletal muscle functions to maintain posture. Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position. Muscles also stabilize the joints. Skel- etal muscles can function to protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs. Think about how you would react, for example, if someone swung a punch toward your belly during a boxing class; you would likely tense your abdominal skeletal muscles to act as a protective shield. Skeletal muscles contribute to the maintenance of homeostasis in the body by generating heat. Muscle contraction requires energy, and when ATP is broken down, heat is produced. This heat is very noticeable during exercise, when sustained muscle movement causes body temperature to rise, and in cases of extreme cold, when shiver- ing produces random skeletal muscle contractions to generate heat. LO 11.1.2 You will find that when talking about muscle tissue, skeletal muscle is discussed the most. Due to several physiological and anatomical factors, skeletal muscle is the most well-studied and well-understood. Because skeletal muscle is featured so heav- ily in our understanding of muscle tissue, let’s begin by comparing and contrasting the three tissue types. Skeletal muscle, cardiac muscle, and smooth muscle all exhibit a quality called excitability; they can change from relaxed to contracted based on electrical properties at their plasma membranes. The trigger or control over this change is different for the different forms of muscle, however. Smooth and cardiac muscle tissues are considered involuntary because the conscious brain cannot con- trol their contraction. Unconscious aspects of the nervous system can influence the excitability of cardiac and smooth muscle to some degree, but you can’t stop your heart from beating just by using your willpower. Skeletal muscle, on the other hand, completely depends on signaling from the nervous system to work properly, so we refer to skeletal muscle as voluntary. Hormones can also influence muscle contrac- tion, acting primarily on cardiac and smooth muscle. The hormone epinephrine, which is also referred to as adrenaline, can increase how hard your heart contracts, leading to the heart-thumping feeling you might experience when startled or on a roller coaster ride. All muscle tissue can return to its original length after contraction due to a quality of muscle tissue called elasticity. As with other tissues, such as the skin, the ability to recoil back to original length is due to elastic fibers. Muscle tissue also has the quality of extensibility; it can stretch or extend. Contractility allows muscle tissue to pull on its attachment points and shorten with force. LO 11.1.3 All three types of muscle cells have the same internal components, includ- ing contractile proteins, mitochondria, nuclei, and a plasma membrane. However, significant differences exist among the three muscle types in terms of how these Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Muscle Tissue Chapter 11 389   components are organized. Skeletal muscle cells are long multinucleated struc- tures that compose the skeletal muscle. Skeletal muscle cells are typically as long or almost as long as the muscle organ they are arranged in; for this reason, they are often referred to as muscle fibers, which is a synonym for muscle cell. If you glance at your bicep, you can estimate its length to be probably somewhere around nine inches. Within that bicep muscle, many, many skeletal muscle cells are also about the length of that muscle. These skeletal muscle cells result from the fusion of hundreds or thousands of individual cells. Each original cell was much longer and had its own nucleus, and when they fused, they resulted in a longer cell with many nuclei. In con- trast, cardiac muscle cells each have one to two nuclei and are physically and electri- cally connected to each other with gap junctions so that a change that occurs in one cell is spread to its neighbors. This structural togetherness allows the entire heart to contract as one unit. Smooth muscle cells are small and shaped like an American football, and each has one single nucleus. In addition to the differences in cell size, shape, and number of nuclei, there is an additional key microscopic difference among these tissue types. The arrangement of the contractile proteins in skeletal and cardiac muscle follows a precise pattern—so precise, in fact, that these muscle cells look striped under the microscope. This char- acteristic of being striped or striated is a defining characteristic of skeletal and cardiac muscle cells and tissue. Because the contractile proteins are not arranged in such regu- lar fashion in smooth muscle, the cell has a uniform, nonstriated appearance, which early anatomists referred to as “smooth” (Figure 11.1). Figure 11.1 The Three Types of Muscle Tissue Three types of muscle tissue are found in the body: (A) Skeletal muscle (associated with the bones), (B) Smooth muscle (found in the walls of tubular organs), and (C) Cardiac muscle (found only in the heart). Nucleus Striations Mediscan/Alamy Stock Photo A Skeletal muscle Nucleus Smooth muscle ÿbers Steve Gschmeissner/Science Photo Library/Getty Images B Smooth muscle Intercalated disc (junction Nucleus that contains gap junctions) Choksawatdikorn/Science Photo Library/Getty Images Striations C Cardiac muscle Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 390 Unit 2 Support and Movement Learning Check 1. Which of the following is not a muscle tissue type? a. Cardiac c. Skeletal b. Smooth d. Lung 2. Which of the following do you expect to occur when smooth muscle tissues contract? a. Blood will be ejected out of the heart. c. Food will pass through the esophagus. b. The rib cage will expand to allow air in the lungs. d. Flexion will occur at the elbow. 3. Define excitability: a. A muscle’s ability to change from contracted to relaxed based on electrical properties. b. A muscle’s ability to return to its original length after contraction. c. A muscle’s ability to stretch, extend, or recoil back. d. A muscle’s ability to pull on its attachment point and shorten with force. 4. Describe the microscopic features of smooth muscle tissues. Please select all that apply. a. Has striations c. Has one nucleus b. Cells (fibers) run the length of the entire muscle d. American football-shaped 11.2 Skeletal Muscle Learning Objectives: By the end of this section, you will be able to: 11.2.1 Describe the organization of skeletal muscle, thick [myosin] myofilaments, thin [actin] from cell (skeletal muscle fiber) to whole myofilaments, troponin, tropomyosin). muscle. 11.2.4 Define sarcomere. 11.2.2 Name the connective tissue layers that surround each skeletal muscle fiber, fascicle, 11.2.5 Describe the arrangement and composition entire muscle, and group of muscles, and of the following components of a sarcomere: indicate the specific type of connective tissue A-band, I-band, H-zone, Z-disc (line), and that composes each of these layers. M-line. 11.2.3 Describe the components within a skeletal 11.2.6 Describe the structure of the neuromuscular muscle fiber (e.g., sarcolemma, transverse [T] junction. tubules, sarcoplasmic reticulum, myofibrils, Because skeletal muscle makes up an enormous proportion of the mass of the human body and skeletal muscle cells are the most well studied, we will examine the structure and function of these cells in more detail. Each skeletal muscle is an organ that, like all LO 11.2.1 organs, consists of multiple integrated tissues. These tissues include the skeletal muscle cells, blood vessels, nerve fibers, and wrappings of connective tissue. Each skeletal mus- cle is wrapped by a sheath of dense, irregular connective tissue called the epimysium LO 11.2.2 (see the “Anatomy of a Muscle [Organ]” feature). The epimysium separates muscle from other tissues and organs in the area, allowing the muscle to move independently. An additional layer of dense irregular connective tissue, fascia, may be external to the epimysium. Fascia is visible during dissection and also during cooking, if you have ever noticed the thin, iridescent layer of tissue that separates chicken breast from the chicken tender, for example, you’re examining the fascia! The fibers of the epimysium are continuous with the fibers of the tendon, which are continuous with the fibers of the periosteum, uniting muscle to bone for effective movements. The tension created Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Muscle Tissue Chapter 11 391   by contraction of the muscle fibers is transferred though the epimysium to the tendon, and then to the periosteum to pull on the bone for movement of the skeleton. In other places, the epimysium may fuse with a broad, tendon-like sheet called an aponeurosis, or to fascia, the connective tissue between skin and bones. The inside of each skeletal muscle organ is organized and subdivided. Muscle cells are organized into bundles, each called a fascicle. Fascicles are wrapped with a layer of connective tissue called the perimysium. The position of the epimysium allows for passage of blood vessels and nerves in the protected spaces between the fascicles. The fascicular organization allows the nervous system to fine-tune the activation of the muscle by triggering only a subset of muscle fibers within a fascicle, or a subset of fascicles within the muscle. Inside each fascicle, the long muscle cells are each encased in a thin connective tissue layer of collagen and reticular fibers called the endomysium. The endomysium is external to the plasma membrane and contains the extracellular fluid and nutrients to support the muscle fiber. The blood and nervous supply to the muscle is woven throughout these layers of connective tissue. Skeletal muscle requires a tremendous amount of the body’s oxygen and calories, which are supplied by blood vessels. Waste generated by muscle cell metabo- lism is removed through the blood as well. In times of strenuous activity, the blood supply may not be able to provide sufficient nutrients or remove wastes quickly enough. We will discuss adaptations and consequences of this scenario later on in this chapter. Remember that skeletal muscle, unlike cardiac and smooth muscle, only contracts in response to signaling from the nervous system, so muscle organs are highly innervated as well. Anatomy of... A Muscle (Organ) The entire muscle is covered by a layer called the epimysium. The ÿbers of this connective tissue (CT) covering are continuous with the ÿbers of the tendons that connect the muscle to bone. Each bundle is a fascicle. Epimysium Bundles of muscle Perimysium cells/ÿbers are wrapped in a covering of CT called the perimysium. Each muscle cell/ÿber is covered by Endomysium a layer called the endomysium. The ÿbers of the endomysium are continuous within the perimysium that envelops the fascicle. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 392 Unit 2 Support and Movement Table 11.1 Muscle Cell Components 11.2a Skeletal Muscle Cells Cell Components Name in a Typical Cell Much of the terminology associated with muscle cells is rooted in the Greek sarco, which means “flesh.” The muscle cell has specialized terms for some of the cell compo- Sarcolemma Plasma membrane nents we learned about in Chapter 4, summarized in Table 11.1. The plasma membrane Sarcoplasm Cytoplasm is called the sarcolemma, the cytoplasm is referred to as sarcoplasm, and the spe- Sarcoplasmic Endoplasmic cialized smooth endoplasmic reticulum, is called the sarcoplasmic reticulum (SR). reticulum (SR) reticulum (ER) These features have counterparts in other cells, but they have a specialized structure to LO 11.2.3 match their function in skeletal muscle cells. The sarcolemma is made of a phospho- lipid bilayer, just like it is in other cells; however, the sarcolemma of a muscle cell is a bit different than other cells. While the plasma membrane of a typical cell is typically a smooth, rounded surface, the sarcolemma is punctuated by deep invaginations called T-tubules (“T” stands for “transverse”) (Figure 11.2). These invaginations serve a criti- cal function for muscle cells. As we will learn a bit later in this chapter, skeletal muscle cells contract in response to electrical changes on their membrane surfaces. These T-tubules bring the electrical signal deep into the cell so that every myofibril through- out the entire cell can respond to it. The sarcoplasmic reticulum is also a bit different in skeletal muscle cells. Unlike the endoplasmic reticulum, which functions mostly in protein and other molecular manufacturing, the sarcoplasmic reticulum functions primarily in the storage and release of calcium ions. Calcium ions play a critical role in muscle cell contraction (as we will learn a bit later in this chapter) and so the sarcoplasmic reticulum must have a physically intimate relationship with the plasma membrane, where the electrical signal that causes contraction travels. Thus, in Figure 11.2 you can see that the T-tubules dive into the depths of the cell and are sur- rounded by the sarcoplasmic reticulum. This physical connection allows the transla- tion of the membrane electrical signal to the calcium-storing sarcoplasmic reticulum. Skeletal muscle cells are multinucleated, as described previously; however, as we examine a muscle cell cross section in the “Anatomy of a Skeletal Muscle Cell” feature, we can see that all of these nuclei are pushed out to the periphery of the cell, almost shoved against the sarcolemma. This is quite different from the typical cell, in which the nucleus is most often seen residing in the middle of Figure 11.2 The Sarcolemma, the T-Tubules, and the Sarcoplasmic Reticulum Invaginations of the plasma membrane called T-tubules bring the electrical impulses that travel along the membrane into the depths of the cell. The SR, which stores calcium, abuts the T-tubules. When an electrical impulse travels along a T-tubule, the SR membrane is affected and releases calcium into the sarcoplasm. Myoÿbrils Mitochondrion Sarcolemma Triad T-tubule Nucleus Sarcoplasmic reticulum Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Muscle Tissue Chapter 11 393   Anatomy of... A Skeletal Muscle Cell The nuclei of muscle cells are Sarcolemma (plasma membrane) situated in the periphery, just under the sarcolemma. Mitochondria are plentiful in muscle, Endomysium generating the ATP (connective necessary for contraction. tissue covering) Myofibrils are long cylinders of contractile proteins that shorten during contraction. the cytoplasm; the internal volume of a muscle cell, on the other hand, is almost entirely taken up by myofibrils, which are the contractile machinery. A myofibril is a long cylinder of carefully arranged contractile proteins that shorten during con- traction. As the myofibrils shorten, they pull on the epimysium at the ends of the muscle, thereby pulling on the tendons of the muscle and moving the bones closer together. Scattered among the myofibrils are mitochondria, organelles that contrib- ute to the conversion of glucose to ATP. 11.2b The Sarcomere The myofibrils are long cylinders of contractile proteins. The precise arrangement Learning Connection of these proteins within the myofibril leads to the striated appearance of skeletal and cardiac muscle cells. There are several proteins found in sarcomeres, the two main Macro to Micro proteins are myosin and actin. Both myosin and actin are small protein subunits that What are the different relationships among string together into long fibers called myofilaments (Figure 11.3A). Along the string of the words myofibril, muscle cell, fascicle, muscle, fascia, epimysium, perimysium, actin subunits, two additional proteins, troponin and tropomyosin (along with other and endomysium? Can you group these proteins), are found. Together, the actin, troponin, and tropomyosin filament is called terms into categories based on whether a thin filament while the myosin proteins are arranged in a thicker bundled called the you would need a microscope to see them thick filament. The thin filaments stretch horizontally from a disc (the Z disc) that or they could be seen with the naked eye? forms the borders of a frame that surrounds the thick filaments; this organized protein unit (called a sarcomere) is the functional unit of the muscle cell (Figure 11.3B). The LO 11.2.4 sarcomere is the smallest unit in which contraction occurs; each sarcomere pulls its ends together during contraction. Sarcomeres are incredibly small; you would need to Z disc (Z line) stack 45 sarcomeres end to end to build the thickness of a sheet of printer paper. Imag- ine how many sarcomeres, laid end to end, it would take to build a muscle cell, which can be inches in length! Learning Connection Myofibrils, which run the entire length of the muscle fiber, are the thickness of one Try Drawing It! sarcomere, but are many, many sarcomeres long. The end of the myofibril attaches to Draw a sarcomere for yourself. Use differ- the sarcolemma. As sarcomeres contract, the myofibril shortens, pulling on the sarco- ent colors for each of the proteins. mere at its ends, thus causing the entire muscle cell to contract. Examining the sarcomere as a whole, anatomists often use terms to describe the LO 11.2.5 patterns they see under the microscope. The region where the thick filaments are lined Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 394 Unit 2 Support and Movement Figure 11.3 Microscopic Structure of Skeletal Muscle Cells (A) The sarcomere is the functional and contractile unit of skeletal and cardiac muscle cells. Sarcomeres are bordered by Z lines. Lined up Z line to Z line, long cylinders of sarcomeres called myofibrils (B) comprise the majority of the inside of a muscle cell. Each myofibril is surrounded by a network of sarcoplasmic reticulum. Sarcomere Z line Z line Actin (thin) filament Myosin (thick) filament Actin Troponin Tropomyosin Heads subunit Actin-binding sites ATP-binding sites Tail Myosin subunits Binding site for myosin A Sarcomere Actin (thin) Sarcoplasmic ÿlament Z line H zone Z line reticulum M line Myosin (thick) I band A band I band ÿlament B up is quite dark in a microscopic image and is referred to as the A band. To the sides of the A band, regions where only thin filaments are found, are the I bands. The M line is a horizontal line at the exact center of the sarcomere, and the H zone extends laterally from the M line; it is the space between the ends of the thin filaments where only thick filaments can be found (see Figure 11.3B). 11.2c The Neuromuscular Junction LO 11.2.6 Every skeletal muscle cell has one point of contact with the neuron that controls it. Neurons that control skeletal muscle cells are called motor neurons. At this site, the neuromuscular junction (NMJ) the neuron is able to stimulate an electrical signal that travels along the length of the muscle cell and along its T-tubules (Figure 11.4). The region of the sarcolemma at the NMJ is called the motor end plate. Excitation signals from the neuron are the only way to cause contraction in a skeletal muscle cell. In contrast, cardiac and smooth muscle cells do not each have an NMJ but have a variety of stimuli that they contract and relax in response to. Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Muscle Tissue Chapter 11 395   Figure 11.4 The Neuromuscular Junction (NMJ) The neuromuscular junction is a synapse between a motor neuron and a skeletal muscle cell. The surface of the skeletal muscle cell at the synapse is the motor end plate. Motor neuron Muscle cell Sarcolemma Neuromuscular junction (NMJ) Motor end plate Myoÿbril Learning Check 1. Which of the following wraps around a fascicle? a. Endomysium c. Epimysium b. Perimysium d. Aponeurosis 2. A calcium ion is inside the T-tubule. When the T-tubule is stimulated, where would the calcium ion go next? a. Sarcolemma c. Sarcoplasmic reticulum b. Sarcoplasm d. Myofibrils 3. Which of the following make up a thick filament? a. Actin c. Troponin b. Myosin d. Tropomyosin 4. Which of the following would you expect to happen if the motor end plate was desensitized? Please select all that apply. a. Decreased calcium release c. Decreased stimulation of motor neuron b. Weaker muscle contraction d. Increased stimulation of sarcolemma 11.3 Skeletal Muscle Cell Contraction and Relaxation Learning Objectives: By the end of this section, you will be able to: 11.3.1 Define the sliding filament model of skeletal junction, excitation-contraction coupling, muscle contraction. and cross-bridge cycling. 11.3.2 Describe the sequence of events involved in 11.3.3 Describe the sequence of events involved in the contraction of a skeletal muscle fiber, skeletal muscle relaxation. including events at the neuromuscular Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 396 Unit 2 Support and Movement Reach down and pick up a pen with your fingers. Seamlessly, in just milliseconds, you were able to coordinate the contraction and relaxation of many muscles to enable this fairly simple movement. You contract and relax your muscles prob- ably hundreds of thousands of times a day without effort or thought, but this series of events, when examined molecularly, is actually quite complex! We discussed earlier that the sarcomere is the functional unit of the muscle cell, that is to say, the smallest single unit of contraction. Let’s first examine how the sarcomere contracts and then we will zoom out and look at the series of cellular events that lead to that contraction. 11.3a The Sliding Filament Model of Contraction LO 11.3.1 For a long time, scientists didn’t know exactly how the sarcomere shortened. Did the filaments fold like an accordion? Did the shape of the sarcomere change? After years of careful experimentation, scientists have accepted an explanation of contrac- tion called the sliding filament model. The contraction is accomplished with the thin filament framework slides past the static thick filaments, bringing the Z discs closer together (Figure 11.5). Since sarcomeres are lined up Z disc to Z disc down the length of the myofibril and cell, the tiny shortening of each sarcomere multiplies and signifi- cant muscle shortening occurs. So, how do the filaments slide? The answer lies in the interaction between the myosin protein in the thick filaments and the actin subunits of the thin filaments. Myosin protein filaments have a head that is held slightly away from the tail portion of the protein (see Figure 11.3A). These heads have a binding site where the myosin can bind to a specific site on the actin subunits. When that site is exposed and there is ATP available in the sarcoplasm, myosin heads will bind to actin; this binding event is called a cross-bridge. Like all proteins, when myosin binds to another LO 11.3.2 molecule, it will undergo a shape change. The myosin shape change induced by bind- ing to actin looks almost like a small sit-up of the myosin molecule. The actin thin Figure 11.5 The Sliding Filament Model of Muscle Contraction Sarcomeres contract when the myosin heads of the thick filaments pull the actin thin filament framework closer together. The Z lines move closer together and the H zone disappears. M line Z H zone Z Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Muscle Tissue Chapter 11 397   filaments are then pulled by the myosin heads toward the center of the sarcomere, bringing the Z discs closer together. The myosin shape change is small and can only pull a very short distance before it has reached its limit and must be “re-cocked” before it can pull again, a step that requires ATP. If the actin sites for binding myosin are still available and there is still ATP in the sarcoplasm, the myosin heads will reset, bind again, and pull another time, moving the Z discs even closer. This cycle will continue as long as ATP and binding sites remain available, and the Z discs get closer and closer together until maximum contraction is reached. This cycling and progress might be likened to the hand-over-hand motion of pulling an anchor up one hand- grasp and pull at a time. If cycling and contraction continues whenever ATP and binding sites are avail- able, what keeps us from having fully contracted muscles all the time? The control over muscle contraction comes from the other proteins in the thin filament, the troponin and tropomyosin. Tropomyosin protein winds around the chains of the actin filament like the ribbon on a package and covers the myosin-binding sites, thus preventing myosin from being able to bind actin. Tropomyosin is in this position during muscle relaxation. To enable contraction, something must pull tropomyosin out of the way, allowing cross-bridges to form. Tropomyosin does not sit alone on the actin filament, but rather is part of a structural complex with another protein, troponin. The troponin- tropomyosin complex is responsible for preventing the myosin “heads” from binding to actin, and consequently preventing contraction from occurring. Troponin has a bind- ing site for Ca2+ ions. At the initiation of muscle contraction, Ca2+ ions in the sarcoplasm bind to troponin. Like all proteins, when troponin binds to another molecule, it under- goes a shape change. This shape change has the effect of pulling on its partner— tropomyosin—off of its resting position, exposing the binding sites on actin and Learning Connection thus allowing contraction to proceed. In this way, we can consider the troponin- tropomyosin complex to be the brakes on the contraction machinery. So, if con- Broken Process traction is dependent on cross-bridge formation, and cross-bridges can only form What would occur if Ca2+ ions were absent when Ca2+ ions are present in the sarcoplasm, what regulates the sarcoplasmic in the muscle cell? What would occur if concentration of Ca2+? Ca2+ ions were always present? 11.3b Excitation-Contraction Coupling Skeletal muscle contraction is always controlled by the nervous system, so to exam- ine the initiation of a muscle contraction, we must look at the series of events at the NMJ, along the muscle cell sarcolemma, and within the muscle cell. The signals that travel along the motor neuron and the sarcolemma are electrical signals, so let’s start by understanding those. All living cells have an electrical gradient across their mem- branes, meaning that one side of the membrane has more positive and/or fewer nega- tive charges than the other. The difference in electrical charge is called the membrane potential and is measured by using a voltmeter that can compare one side to the other (Figure 11.6). Recall that neurons and muscle cells are both described as excitable; all excitable cells have a significant membrane potential. The inside of these membranes is usually around 260 to 290 mV, relative to the outside. This means that the inside of the membrane has fewer positively charged molecules, and/or more negatively charged molecules than the outside; the difference between these two sides is between 60 and 90 mV. This difference in charge is established due to the action of the Na+/K+ pump. Student Study Tip This pump is at work constantly in neurons and muscle cells, and each time it cycles To remember which ion enters the cell and it moves three Na+ ions to the outside of the cell and two K+ ions to the inside of the which leaves via the Na+/K+ pump, the cell cell. The action of the Na+/K+ pump effectively establishes the gradient because more says “nah” to Na+ and “K” to accepting K+ positive charges are found outside than inside. Neurons and muscle cells use their into the cell! Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 398 Unit 2 Support and Movement Figure 11.6  easurement of the M membrane potentials to generate electrical signals. They do this by controlling the Membrane Potential movement of charged particles, called ions, across their membranes to create electrical currents. The movement of ions is prevented by the membrane, because only small, Voltmeters are used to measure the mem- brane potential, the difference in charge nonpolar, and uncharged molecules can freely diffuse across the membrane. Therefore, between the inside and outside of the cell. the electrical signal, which occurs due to the movement of these ions, is achieved by Voltmeter opening and closing specialized protein channels in the membrane that allow the ions to move. Although the currents generated by ions moving through these channel pro- 0 teins are very small, they form the basis of both neural signaling and muscle contrac- –70 +70 tion. When a cell is in a resting state, not contracting or sending a signal, the difference mV across its membrane is its resting membrane potential. When a cell is in an active state, it is contracting or sending a signal, the event, and the changes in charge across Extracellular the membrane is called an action potential. ˜uid + + + + + + To examine the process of contraction and relaxation, we will break the events into three parts: the NMJ, the sarcolemma, and the sarcomere. 11.3c Events at the Neuromuscular Junction The process of contraction begins when a neuronal action potential travels along the – – – – – – motor neuron and reaches the NMJ. At the end of the axon the arrival of the action Cytosol potential causes the release of a chemical messenger, or neurotransmitter, called acetylcholine (ACh). The ACh molecules diffuse across the small space, the synaptic cleft, that spans the distance between the motor neuron and the muscle cell. ACh trav- els by diffusion and will bind to ACh receptors located within the motor end plate of the sarcolemma on the other side of the synapse. ACh receptors are proteins, and like all proteins, once ACh binds the receptor will change shape. In this case, the binding of ACh opens a channel in the ACh receptor and positively charged ions can pass through the channel. Positively charged ions will be drawn into the muscle fiber because they are attracted to the negative charges there. The resting muscle cell membrane had a significant charge difference across its membrane; it was about 90 mV less positive or more negative on the inside of the membrane compared to the outside (Figure 11.7). We can describe the membrane in this resting state as being polarized (different on Figure 11.7 The Electrical Sequence of an Action Potential The voltmeter measures the electrical charge difference between the inside and outside of the membrane of a muscle cell. The resting membrane potential is the difference, typically 290 mV between these two locations when the cell is not contracting. When Na+ channels open and Na+ can enter the cell, the inside becomes more positive than when it was at rest; it depolarizes. When the K+ ion channels open, and K+ ions can leave the cell, the inside of the cell becomes less positive again; the cell repolarizes. Depolarizing Repolarizing +30 Membrane potential (mV) 0 –90 Resting Muscle Time membrane ÿber action potential potential Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Muscle Tissue Chapter 11 399   DIGGING DEEPER: Interference at the Neuromuscular Junction A number of toxins (which are naturally occurring) and drugs (made in a laboratory) can affect the neuromuscular junction. Crotoxins, a group of poisons isolated from vipers and other snakes, affect motor neurons and their release of acetylcholine. Botulinum toxin, also called botox, similarly prevents the release of acetylcholine at the NMJ, causing paralysis. Curare is a poison isolated from the bark of some South American plants. Curare is a competitive blocker to the acetylcholine receptor, so the acetylcholine released cannot bind to the receptor. The effects of curare look identical to the effects of botox and some of the crotoxins. If acetylcholine cannot bind to its receptor, it will not have an effect on the muscle cell and no muscle contraction will take place. This lack of muscle contraction is called flaccid paralysis. Sarin gas also affects the NMJ but produces the opposite effect, resulting in excessive and uncontrolled contraction, called spastic paralysis. Sarin gas, a toxin that was origi- nally produced as a pesticide but has been used against humans in warfare and terrorist attacks, prevents acetylcholinesterase from breaking down acetylcholine. This allows acetylcholine to persist in the synapse, causing an extended period of contraction long after the motor neuron has stopped firing signals. one side compared to the other) and having a membrane potential of 290 mV. Once ACh binds and positive ions rush into the cell, the membrane of the motor end plate will depolarize, meaning that the membrane potential of the muscle fiber becomes less negative (closer to zero). Learning Check 1. Describe the appearance of a contracted muscle cell. Please select all that apply. a. Z discs are closer together. c. I bands are thicker. b. M line is closer together. d. A bands are thicker. 2. Which of the following microfilaments is responsible for pulling the other microfilament during a contraction? a. Myosin b. Actin 3. Where can you find troponin? a. Surrounding actin c. Embedded within the sarcolemma b. Surrounding myosin d. Embedded within the sarcoplasmic reticulum 4. The extracellular matrix has a net charge of 250 mV. The cytoplasm in the cell has a net charge of 170 mV. What is the membrane potential? a. 250 mV c. 420 mV b. 170 mV d. 80 mV 11.3d Events Along the Sarcolemma Along the length of the muscle cell the sarcolemma is studded with ion channels. The majority of these channels are described as voltage-gated channels. We have described many protein shape changes that occur when a molecule binds to the protein; these voltage-gated channels change shape when there is a change in the membrane potential. The two populations of voltage-gated channels on the muscle cell membrane are both closed (no ions can pass through) when the cell is polarized, and the inside is more negative than the outside. These channels change shape and open (allowing ions to pass through) when the membrane depolarizes. The first to open are voltage-gated sodium channels; these channels allow sodium ions (Na+) to enter the muscle fiber, leading to further depolarization of the cell. The action potential rapidly spreads (or Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 400 Unit 2 Support and Movement “fires”) along the entire membrane, including down each T-tubule as each segment of the membrane depolarizes (Figure 11.8). A second population of voltage gated chan- nels opens shortly after the voltage-gated sodium channels. Interspersed along the membrane are voltage-gated potassium channels. Once these channels open (they Student Study Tip are slower to open than voltage-gated sodium channels potassium ions (K+) can now Na+ channels open right away, analogous traverse the membrane. These ions will flow out of the cell because they are in a greater to a young fit person, but K+ channels are concentration inside than outside, so they will diffuse down their concentration gradi- like creaky grandmas: slow to open and close. ent. The egress of positive charges from the cell repolarizes the membrane (brings it back to its polarized state of being more positive outside and more negative inside). Things happen very quickly in the world of excitable membranes (just think about how quickly you can snap your fingers as soon as you decide to do it). Immediately following depolarization of the membrane, it repolarizes, reestablishing the negative membrane potential. Meanwhile, the ACh in the synaptic cleft is being quickly broken Figure 11.8 End Plate Potential The sarcolemma at the NMJ is known as the end plate. The action potential begins at the end plate as acetylcholine receptor channels open and allow positive ions into the cell. This influx of positivity opens voltage-gated channels on the sarcolemma. The action potential proceeds down the membrane. Acetylcholine Acetylcholine Action receptor potential Na+ + + + + + + + + – – – – – – Na+ Muscle cell End plate + + – – – – – – + + + + + + Sarcolemma Na+ Action potential K+ Na+ + + + + + + – – + + – – – – K+ Na+ – – + + – – – – + + + + + + K+ Na+ Action potential K+ Na+ + + + + + + – – – – + + – – K+ Na+ – – – – – – + + + + + + K+ Na+ Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  Muscle Tissue Chapter 11 401   down by an enzyme, acetylcholinesterase (AChE), that is found there. As less and less ACh is in the synapse to bind receptors and the sarcolemma is repolarizing, the action Learning Connection potential ends and the cell goes back to its resting membrane potential. Explain a Process Physiological processes with many steps 11.3e Events at the Sarcomere can be quite difficult to master. Try writing Now that we have learned the sequence of electrical, or excitation events in the NMJ each step of the events at a neuromuscu- and along the sarcomere, we can relate them to the mechanical contraction that occurs lar junction on a separate piece of paper, mixing them up, and then putting them at the sarcomere. The union of these physiological events is often called excitation- back in order. This activity will not only contraction coupling. The muscle cell action potential, which sweeps along the sarco- challenge your memory of the steps, but lemma as a wave, is “coupled” to the actual contraction through the release of calcium will help you to understand each function. ions (Ca2+) from its storage in the cell’s SR. For the action potential to reach the mem- brane of the SR, there are periodic invaginations in the sarcolemma, called T-tubules. These T-tubules ensure that the membrane can get close to the SR in the sarcoplasm. Student Study Tip The arrangement of a T-tubule with the membranes of SR on either side is illustrated T-tubules look like a capital “T” and in Figure 11.9. Once released, the Ca2+ interacts with the troponin-tropomyosin transmit signals deep into the cell. protein complex, moving the tropomyosin aside so that the actin-binding sites are exposed for attachment by myosin heads. Cross-bridge cycling can then occur; each sarcomere shortens, and the muscle itself contracts. Cross-bridge cycling is sustained by ATP, so as long as both ATP and Ca2+ are available in the cytoplasm, contraction will continue. The full sequence of events, from motor neuron to sarcomere, is illustrated in Figure 11.9. Figure 11.9 The Steps in Muscle Cell Contraction Muscle cell contraction begins when acetylcholine, released by the motor neuron, depolarizes the motor end plate. As the sarcolemma and T-tubules depolarize, calcium is released into the cell, triggering the movement of troponin and tropomyosin on the thin filaments away from the myosin bind- ing sites. Myosin heads, now able to form cross-bridges with actin, contract the sarcomere, the entire muscle cell and the entire muscle. 1 Depolarization of the axon terminal leads to the release of ACh. This neurotransmitter diffuses throughout the synaptic cleft, binding to receptors at the motor end The action potential travels plate and triggering the depolarization of the sarcolemma. along the axon of the motor neuron, arriving at the axon 2 The wave of depolarization travels along terminal. the sarcolemma and along the T-tubules. The depolarization of the T-tubules causes the release of Ca2+ from the sarcoplasmic reticulum. 4 Calcium Sarcoplamic reticulum Troponin As contraction occurs within Intracellular calcium allows for the formation each sarcomere, the entire of cross-bridges and contraction. muscle cell and organ shortens, pulling on the attachment points and producing tension. 3 Copyright 2023 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 402 Unit 2 Support and Movement 11.3f ATP and Muscle Contraction Our muscle cells use a majority of the calories we need each day. It is clear to see why when we examine the few different roles ATP plays in the events of contraction and relaxation and multiply that out by the number of sarcomeres in one myofibril, the number of myofibrils in one muscle cell, the number of muscle cells in a muscle, and the number of muscles in the human body (truly an impossible calculation, but you can understand that the number would be mind-boggling!). For thin filaments to continue to slide past thick filaments during muscle contraction, myosin heads must pull the actin at the binding sites, detach, return to their original shape, attach to more binding sites, pull, detach, return to their original shape, and so on. This motion of the myosin heads is similar to that of the oars when an individual rows a boat: The blades of the oars (the myosin heads) pull, are lifted from the water (detach), repositioned (re-cocked), and then immersed again to pull (Figure 11.10). Each cycle requires energy, and the action of the myosin heads in the sarcomeres repetitively pulling on the thin filaments also requires energy, which is provided by ATP. Cross-bridge formation occurs when the myosin head attaches to the actin. At this point in the cycle, adenosine diphosphate (ADP) and inorganic phosphate (Pi) are still bound to myosin (see “Anatomy of Cross-Bridge Cycling,” Step 1) from the previous