Understanding Muscle Contraction

Chapter 2 Understanding Muscle Contraction*





MUSCULAR ASPECTS OF MOVEMENT


Muscle contractions provide the basis for all human movement. To understand how movement occurs requires an appreciation of the interactions among the various systems of the body. For instance, the muscle cells (fibers) must be able to produce and utilize adenosine triphosphate (ATP) to provide the energy for contraction and force production. This process requires that the digestive, respiratory, endocrine, and cardiovascular systems operate effectively to provide muscle cells with the oxygen and nutrients they require to produce energy. For the purposes of this chapter, the assumption is that these other systems of the body are functioning properly.



Overview of Muscle Tissue


Muscle tissue produces force because of the interaction of its basic contractile elements (myofilaments), which are composed primarily of protein. The function of the muscle tissue ultimately depends on the type of muscle tissue involved: skeletal, smooth, or cardiac muscle. The force of contraction may be used for locomotion (skeletal muscle); the movement of materials through hollow tubes, such as the digestive tract or blood vessels (smooth muscle); or the pumping action of the heart (cardiac muscle). However, all muscle tissue has the ability to produce force because of certain basic characteristics common to all types.


Because skeletal muscles have various characteristics, physiologists refer to them by different names. On the one hand, skeletal muscles are under conscious control, so they are often referred to as voluntary muscles. On the other hand, skeletal muscles are sometimes referred to as striated muscles because of the repeating pattern of light and dark bands seen in the microscopic structure of the muscle. Additionally, in order to differentiate skeletal muscle fibers from intrafusal fibers found in sensory organs of the muscle, physiologists sometimes refer to skeletal muscle fibers as extrafusal muscle fibers.





Macroscopic Structure of Skeletal Muscles


The human body contains more than 400 skeletal muscles, and they account for 40% to 45% of the adult male body weight and 23% to 25% of the adult female body weight.1,2 These muscles function together in a remarkable way to provide smooth, integrated movement for a wide variety of activities, many of which require little conscious thought. Muscle action also provides the basis for sport and fitness activities, and muscle definition itself, as seen in Figure 2-1, has become the goal of the sport of bodybuilding. To understand how muscles function in bodybuilding poses, or in any other activity, it is necessary to look beneath the skin.




Organization and Connective Tissue


Skeletal muscles are organized in a systematic fashion, as depicted in Figure 2-2. Some of this organization is apparent to the naked eye, but other aspects are apparent only when muscle fibers are viewed through a simple or an electron microscope.



Skeletal muscles are attached to bones by tendons, an arrangement that allows the contraction of a muscle to move a bone. Each muscle is bound together by a thick layer (sheath) of connective tissue called fascia. Just beneath the fascia is a more delicate layer of connective tissue called epimysium that directly covers the muscle.


The interior of the muscle is subdivided into bundles of muscle fibers called fasciculi (the singular is fasciculus or fascicle), which are also surrounded by connective tissue. The sheath of connective tissue that separates the fasciculi within a skeletal muscle is called perimysium. The fasciculi are composed of many individual muscle fibers (cells), each of which is surrounded by its own sheath of connective tissue called endomysium. The three layers of connective tissue (epimysium, perimysium, and endomysium) provide the framework that holds the muscle together. These three layers of connective tissue come together at each end of the muscle to form the tendons that attach the muscle to bone. As a muscle contracts, it pulls on the connective tissue in which it is wrapped, causing the tendon to move the bone to which it is attached.




Microscopic Structure of a Muscle Fiber


Individual muscle fibers are composed primarily of smaller units called myofibrils, which are in turn made up of myofilaments. This organization of skeletal muscle is shown in Figure 2-4.



Muscle fibers, also called muscle cells, are long, cylinder-shaped cells ranging from 10 to 100 μm in diameter and 1 to 400 mm in length.25 The major structures of a muscle fiber and their functions are summarized in Table 2-1.


Table 2-1 Summary of Major Components of a Skeletal Muscle Cell





































Cell Part Description Function
Nucleus Multinucleated Is the control center for the cell
Sarcolemma Polarized cell membrane Is capable of receiving stimuli from the nervous system
Sarcoplasm Intracellular material Holds organelles and nutrients; provides the medium for glycolytic enzymatic reactions
Organelles
Myofibrils Rodlike structures composed of smaller units called myofilaments: account for 80% of muscle volume Contain contractile proteins (myofilaments), which are responsible for muscle contraction
T tubules Series of tubules that run perpendicular (transverse) to the cell and are open to the external part of the cell Spread polarization from the cell membrane into the interior of cell, which triggers the sarcoplasmic reticulum to release calcium
Sarcoplasmic reticulum Interconnecting network of tubules running parallel with and wrapped around the myofibrils Stores and releases calcium
Mitochondria Sausage or spherical-shaped organelles; numerous in a muscle cell Are the major site of energy production

From Plowman SA, Smith DL: Exercise physiology for health, fitness, and performance, ed 2, San Francisco, 2003, Benjamin Cummings.


A skeletal muscle fiber contains many nuclei, which are located just below the cell membrane. The polarized plasma membrane of a muscle cell is referred to as the sarcolemma, and it is the properties of this membrane that account for the irritability of muscle. The sarcoplasm of a muscle cell is similar to the cytoplasm of other cells, but it has specific adaptations to serve the functional needs of muscle cells, namely, increased amounts of glycogen and the oxygen binding protein myoglobin.


The muscle fiber contains the organelles found in other cells (including a large number of mitochondria) along with some specialized organelles. The organelles of specific interest are the transverse tubules, sarcoplasmic reticulum, and myofibrils. The myofibrils are composed of the protein myofilaments and are responsible for the contractile properties of muscles.





Sarcomeres


A sarcomere is the functional unit (contractile unit) of a muscle fiber. As illustrated in Figure 2-5, each sarcomere contains two types of myofilaments: thick filaments, composed primarily of the contractile protein myosin, and thin filaments, composed primarily of the contractile protein actin. Thin filaments also contain the regulatory proteins, troponin and tropomyosin. When myofilaments are viewed under an electron microscope, their arrangement gives the appearance of alternating bands of light and dark striations. The light bands are called I bands and contain only thin filaments. The dark bands are called A bands and contain thick and thin filaments, with the thick filaments running the entire length of the A band. Thus the length of the thick filament determines the length of the A band.



The names for the various regions of the sarcomere are not arbitrary; they are derived from the first letter of the German word that describes their appearance. The names for the bands describe the refraction of light through the respective bands. The I band is abbreviated from the word isotropic, which means that this area appears lighter because more light can pass through it. The A band is so named because of its anisotropic properties, meaning that it appears darker because it does not allow as much light to pass through. These properties are directly related to the type of filament present.


Each A band is interrupted in the midsection by an H zone (from the German Hellerscheibe, for “clear disc”), where there is no overlap of thick and thin filaments. Running through the center of the H zone is a dense line called the M line (from the German Mittelsclzeibe, for “middle disc”). The I bands are also interrupted at the midline by a darker area called the Z disc (from the German Zwischenscheibe, for “between disc”). A sarcomere extends from one Z disc to the successive Z disc. The Z disc serves to anchor the thin filaments to adjacent sarcomeres.


Myofilaments occupy three-dimensional space. The arrangement of the myofilaments at different points in the sarcomere is shown in Figure 2-5, D and F. Notice that in regions where the thick and thin filaments overlap, each thick filament is surrounded by six thin filaments and each thin filament is surrounded by three thick filaments.


A sarcomere consists of more than just contractile and regulatory proteins. Proteins of the cytoskeleton provide much of the internal structure of the muscle cell. Figure 2-6 diagrams the cytoskeleton of the sarcomere and its relationship to the contractile proteins.6 The M line and the Z disc hold the thick and the thin filaments in place, respectively. The elastic filament helps keep the thick filament in the middle between the two Z discs during contraction.




Molecular Structure of the Myofilaments


The sliding-filament theory of muscle contraction, which explains how muscles contract, is based on the sliding of the contractile proteins of the myofilaments over one another during muscular contraction. Therefore paying careful attention to the structure of the myofilaments is essential.



Thick Filaments


Thick filaments are composed primarily of myosin molecules (Figure 2-7). Each molecule of myosin has a rodlike tail and two globular heads (Figure 2-7, A). A typical thick filament contains approximately 200 myosin molecules.4 These molecules are oriented so that the tails form the central rodlike structure of the filament (Figure 2-7, B). The globular myosin heads extend outward and form cross-bridges when they interact with thin filaments. The myosin heads have two reactive sites: One allows it to bind with the actin filament, and one binds to ATP. Only when the myosin heads bind to the active sites on actin, forming a cross-bridge, does contraction occur.



The myosin subunits are oriented in opposite directions along the filament, forming a central section that lacks projecting heads (Figure 2-7, C). The result is a bare zone in the middle of the filament, which accounts for the H zone seen in the middle of the A band (Figure 2-7, D).



Thin Filaments


Thin filaments are composed primarily of the contractile protein actin. As illustrated in Figures 2-8, A and B, actin is composed of small globular subunits (G actin) that form long strands called fibrous actin (F actin). A filament of actin is formed by two strands of F actin coiled about each other to form a double helical structure; it resembles two strands of pearls wound around each other and may be referred to as a coiled coil (Figure 2-8, C). The actin molecules contain active sites to which myosin heads will bind during contraction.



The thin filaments also contain the regulatory proteins called tropomyosin and troponin, which regulate the interaction of actin and myosin. Tropomyosin is a long, double-stranded, helical protein that is wrapped about the long axis of the actin backbone (Figure 2-8, D). Tropomyosin serves to block the active site on actin, thereby inhibiting actin and myosin from binding under resting conditions.


Troponin is a small, globular protein complex composed of three subunits that control the position of the tropomyosin (Figure 2-9). The three units of troponin are troponin C (Tn-C), troponin I (Tn-I), and troponin T (Tn-T). Tn-C contains the calcium-binding sites, Tn-T binds troponin to tropomyosin, and Tn-I inhibits the binding of actin and myosin in the resting state (Figure 2-9, B). When calcium binds to the Tn-C subunit, the troponin complex undergoes a configurational change. Because troponin is attached to tropomyosin, the change in the shape of troponin causes tropomyosin to be removed from its blocking position, thus exposing the active sites on actin.3,4 Once the active sites are exposed, the myosin heads can bind to the actin, forming the cross-bridges (Figure 2-9, C). Thus calcium is the key to controlling the interaction of the filaments and therefore muscle contraction.




Contraction of a Muscle Fiber


In order for a muscle to contract, an AP must be generated in the motor neuron that innervates the muscle fibers that will contract. The message from the motor neuron must then be passed to the muscle fiber through the neuromuscular junction. Finally, the AP must be conducted along the sarcolemma and into the interior of the muscle fiber to initiate movement of the myofilaments. The process whereby electrical events in the sarcolemma of the muscle fiber are linked to the movement of the myofilaments is called excitation-contraction coupling.


The description of the specific changes that occur during contraction requires careful attention to three factors: the position of the myofilaments, the location of calcium ions, and the role of ATP.




Excitation-Contraction Coupling


Excitation-contraction coupling refers to the sequence of events by which an AP (an electrical event) in the sarcolemma of the muscle cell initiates the sliding of the myofilaments, resulting in contraction (a mechanical event). Excitationcontraction coupling can be categorized into three phases:





Figure 2-10 summarizes the events that occur during each phase of excitation-contraction coupling. Excitation-contraction coupling begins with depolarization and spread of an AP along the sarcolemma (point 2 in Figure 2-10) and continues with the propagation of the AP into the T tubules (point 2 in Figure 2-10). An AP in the T tubules causes the release of calcium from the lateral sacs of the sarcoplasmic reticulum (point 3 in Figure 2-10).



When calcium is released from the sarcoplasmic reticulum (the second phase), it binds to the troponin molecules on the thin filament. The binding of calcium to troponin causes troponin to undergo a configurational change, thereby removing tropomyosin from its blocking position on the actin filament (point 4 in Figure 2-10). The third phase of excitation-contraction coupling is the cross-bridging cycle (point 5 in Figure 2-10). The cross-bridging cycle describes the cyclic events that are necessary for the generation of force or tension within the myosin heads during muscle contraction. The generation of tension within the contractile elements results from the binding of the myosin heads to actin and the subsequent release of stored energy in the myosin heads. As shown in Figure 2-11, four individual steps are necessary for the cross-bridging cycle3,4,7:







The first step in the cross-bridge cycle is the binding of activated myosin heads (*M) with the active sites on actin, forming cross-bridges. In Figure 2-11 a centered dot (·) is used to indicate binding, and an asterisk (*) is used to indicate activated myosin heads. Thus A·*M indicates that the activated myosin heads are bound to actin (A), whereas A + M indicates that actin and myosin are unbound.


The second step in the cross-bridging cycle is the power stroke. During this step, activated myosin heads swivel from their high-energy, activated position to a low-energy configuration (M with no *). This movement of the myosin cross-bridges results in a slight displacement (sliding) of the thin filament over the thick filament toward the center of the sarcomere. As shown in Figure 2-11, during the second step ADP and Pi are released from the myosin heads, resulting in myosin bound only to actin (A·M).


The third step involves the binding of ATP to the myosin heads and subsequent dissociation (detachment) of the myosin cross-bridges from actin, which produces A + M·ATP. The binding of ATP molecules to the myosin heads allow the myosin heads to detach from actin. In the fourth step the breakdown of ATP provides the energy to activate the myosin heads (*M). Activation of the myosin heads is extremely important because it provides the cross-bridges with stored energy to move the actin during the power stroke. The breakdown of ATP at this step depends on the presence of myosin ATPase (also known as myofibrillar ATPase), as depicted in the following reaction:




image



Notice that the products of ATP hydrolysis, ADP + Pi, remain bound to the myosin heads and that the myosin is now in its high-energy or activated state.


The cross-bridging cycle continues as long as ATP is available and calcium is bound to troponin (Tn-C), causing the active sites on actin to be exposed. On the other hand, activated myosin remains in the resting state awaiting the next stimulus if calcium is not available in sufficient concentration to remove tropomyosin from its blocking position on actin (see step 4b in Figure 2-11). Because each cycle of the myosin cross-bridges barely displaces the actin, the myosin heads must bind to the actin and be displaced many times for a single contraction to occur. Thus myosin makes and breaks its bond with actin hundreds or even thousands of times during a single muscle twitch. In order for this make-and-break cycle to occur, myosin heads must detach from actin and then be reactivated. This detaching and reactivating process requires the cycle to be repeated and requires the presence of ATP (see step 3).4 The analogy of a spring-loaded mousetrap may be helpful for understanding the role of ATP in providing energy to activate the myosin head. It takes energy to set the trap, just as it takes the splitting of ATP to set or activate the myosin head. Once set, however, the trap releases energy when it is sprung. In a similar manner, the myosin head possesses stored energy, which is released when the myosin heads bind to actin and swivel.


A review of Figure 2-11 reveals that ATP plays several important roles in muscle contraction.


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Jul 22, 2016 | Posted by in PHYSICAL MEDICINE & REHABILITATION | Comments Off on Understanding Muscle Contraction
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