Chapter 10: Muscle Tissue • Muscle is one of the 4 primary types of tissue. It is subdivided into skeletal, cardiac and smooth muscle. I. Skeletal Muscle Tissue and the Muscular System, p. 284 Objective 1. Specify the functions of skeletal muscle tissue. • Skeletal muscles are the muscles attached to the skeletal system, which allow us to move. The muscular system includes only skeletal muscles. • Skeletal muscles are made up of muscle tissue (composed of muscle cells or fibers), connective tissues, nerves and blood vessels. • The 5 functions of skeletal muscles are: 1. To produce skeletal movement. 2. To maintain posture and body position. 3. To support soft tissues. 4. To guard the entrances and exits of the body. 5. To maintain body temperature. II. Functional Anatomy of Skeletal Muscle, p. 284 Objectives 1. Describe the organization of muscle at the tissue level 2. Explain the unique characteristics of skeletal muscle fibers. 3. Identify the structural components of a sarcomere. Organization of Connective Tissues, p. 284 Figure 10-1 • Muscles have 3 layers of connective tissues: 1. the epimysium: an exterior collagen layer connected to the deep fascia which separates the muscle from surrounding tissues. 2. the perimysium: surrounds bundles of muscles fibers called fascicles. Perimysium holds the blood vessels and nerves that supply the fascicles. 3. the endomysium: surrounds individual muscle cells (the muscle fibers), and contains the capillaries and nerve fibers that directly contact the muscle cells. Endomysium also contains satellite cells (stem cells) that repair damaged muscles. • At each end of the muscle, the endomysium, perimysium and epimysium come together to form a connective tissue attachment to the bone matrix, either a tendon (a bundle) or an aponeurosis (a sheet). Blood Vessels and Nerves, p. 285 • Skeletal muscles are voluntary muscles, controlled by nerves from the central nervous system. • An extensive vascular system supplies large amounts of oxygen to muscles, and carries away wastes. Skeletal Muscle Fibers, p. 286 Figure 10-2 • Skeletal muscle cells (fibers) are very different from typical cells. The long fibers develop through the fusion of mesodermal cells (myoblasts) until they become very large and contain hundreds of nuclei. Figure 10-3 • The cell membrane of a muscle cell is called the sarcolemma, which surrounds the sarcoplasm or cytoplasm of the muscle fiber. Muscle contractions begin with a change in the transmembrane potential. • Because the whole muscle fiber must contract at the same time, the signal (action potential) is conducted through the cell by transverse tubules (T tubules) which have the same properties as the sarcolemma. • Within each muscle fiber are hundreds of lengthwise subdivisions called myofibrils. Myofibrils are made up of bundles of the protein filaments (myofilaments) that are responsible for muscle contraction. • The 2 types of myofilaments are: 1. thin filaments: made of the protein actin, and 2. thick filaments: made of the protein myosin. • Sarcoplasmic Reticulum: Surrounding each myofibril is a membranous structure called the sarcoplasmic reticulum, which is involved in transmitting the action potential to the myofibril. The sarcoplasmic reticulum is similar in structure to the smooth endoplasmic reticulum, forming chambers called terminal cisternae which attach to T tubules. One T tubule and a pair of terminal cisternae are called a triad. • Ion pumps concentrate calcium ions (Ca++) in the cisternae. The calcium ions are released into the contractile units of the muscle (sarcomeres) at the beginning of a muscle contraction. Figure 10-4 • Sarcomeres (the contractile units of muscle) are structural units of myofibrils resulting from the organization or pattern of thick and thin filaments within the myofibril. • Skeletal muscles appear striped or striated because of the arrangement of alternating dark, thick filaments (A bands) and light, thin filaments (I bands) within their myofibrils. • The center of the A band is the midline or M line of the sarcomere. The centers of the I bands are Z lines. One sarcomere is measured from one Z line to another. • Thick filaments and thin filaments overlap in the zone of overlap, which is the densest, darkest area on a light micrograph. • The area around the M line, which has thick filaments but no thin filaments, is called the H zone. • Strands of protein (titin) reach from the tips of the thick filaments to the Z line and stabilize the filaments. Figure 10-5 • Two transverse tubules encircle each sarcomere near the 2 zones of overlap. When calcium ions are released by the sarcoplasmic reticulum, thin and thick filaments interact. Figure 10-6 (Review the functional organization of a skeletal muscle fiber.) Figure 10-7 • The complex interactions of thick and thin filaments which cause muscle contraction are determined by the structures of their protein molecules. • Thin filaments contain 4 proteins: 1. F actin (2 twisted rows of globular G actin. Active sites on G actin strands bind to myosin.) 2. nebulin (holds F actin strands together) 3. tropomyosin (a double strand, prevents actin-myosin interaction) 4. troponin (a globular protein, binds tropomyosin to G actin, controlled by Ca++) ♣ When a Ca++ ion binds to the receptor on a troponin molecule, the troponin- tropomyosin complex changes, exposing the active site of the F actin and initiating contraction. ♣ Thick Filaments contain twisted myosin subunits. The tail binds to other myosin molecules. The free head, made of 2 globular protein subunits, reaches out to the nearest thin filament. ♣ During a contraction, myosin heads interact with actin filaments to form cross- bridges. The myosin head pivots, producing motion. ♣ Thick filaments contain titin strands that recoil after stretching. Sliding Filaments and Muscle Contraction, p. 291 Figure 10-8 ♣ In skeletal muscle contraction, the thin filaments of the sarcomere slide toward the M line, in between the thick filaments. This is called the sliding filament theory. The width of the A zone stays the same, but the Z lines move closer together. III. The Contraction of Skeletal Muscle, p. 292 Objectives 1. Identify the components of the neuromuscular junction, and summarize the events involved in the neural control of skeletal muscles. 2. Explain the key steps involved in the contraction of a skeletal muscle fiber. Figure 10-9 ♣ Muscle fiber contraction is initiated by neural stimulation of a sarcolemma, causing excitation-contraction coupling. The cisternae of the sarcoplasmic reticulum release calcium ions, which trigger the interaction of thick and thin filaments, consuming ATP and producing a pulling force called tension. ♣ We will now look at each stage of skeletal muscle contraction in detail. The Control of Skeletal Muscle Activity, p. 293 Figure 10-10 ♣ Neural stimulation occurs at the neuromuscular junction (NMJ). The electrical signal or action potential travels along the nerve axon and ends at a synaptic terminal which releases a chemical neurotransmitter called acetylcholine (ACh). ♣ ACh travels across a short gap called the synaptic cleft and binds to membrane receptors on the sarcolemma called the motor end plate, causing sodium ions to rush into the sarcoplasm. An enzyme in the sarcolemma (acetylcholinesterase or AChE) then breaks down the ACh. ♣ The increase in sodium ions generates an action potential in the sarcolemma which travels along the T tubules, leading to the excitation-contraction coupling. Excitation - Contraction Coupling, p. 295 Figure 10-11 ♣ When the action potential reaches a triad, calcium ions are released, triggering contraction. ♣ This step requires the myosin heads to have previously broken down ATP and stored the potential energy in the “cocked” position. Figure 10-12 ♣ The Contraction Cycle has 5 steps: 1. Exposure of active sites 2. Formation of cross-bridges 3. Pivoting of myosin heads 4. Detachment of cross-bridges 5. Reactivation of myosin Figure 10-13 ♣ As the sarcomeres shorten, the muscle pulls together, producing tension that moves whatever it is attached to. Relaxation, p. 298 ♣ Since AChE quickly breaks down ACh, the duration of a contraction depends on: 1. the duration of the neural stimulus 2. the number of free calcium ions in the sarcoplasm 3. the availability of ATP ♣ As calcium ion concentrations in the sarcoplasm fall, calcium ions detach from troponin, and the active sites are recovered by tropomyosin. The sarcomeres will remain in the contracted state unless an outside force returns them to their stretched position. ♣ Upon death, ion pumps cease to function and calcium builds up in the sarcoplasm, causing a fixed muscular contraction called rigor mortis. Table 10-1: A review of muscle contraction from ACh release to the end of contraction. Key ♣ Skeletal muscle fibers shorten as thin filaments interact with thick filaments and sliding occurs. ♣ The trigger for contraction is the appearance of free calcium ions in the sarcoplasm; the calcium ions are released by the sarcoplasmic reticulum when the muscle fiber is stimulated by the associated motor neuron. ♣ Contraction is an active process; relaxation and return to resting length is entirely passive. IV. Tension Production, p. 300 Objectives 1. Describe the mechanism responsible for tension production in a muscle fiber, and discuss the factors that determine the peak tension developed during a contraction. 2. Discuss the factors that affect peak tension production during the contraction of an entire skeletal muscle, and explain the significance of the motor unit in this process. 3. Compare the different types of muscle contractions. Tension Production by Muscle Fibers, p. 300 ♣ As a whole, a muscle fiber is either contracted or relaxed (the all-or-none principal). ♣ The tension produced by the contraction of an individual muscle fiber can vary, depending on the number of pivoting cross-bridges; the fiber’s resting length at the time of stimulation, and the frequency of stimulation. Figure 10-14 ♣ Length-Tension Relationships: The number of pivoting cross bridges depends on the amount of overlap between thick and thin fibers.