CHAPTER 10: MUSCULAR TISSUE



OVERVIEW OF MUSCLE TISSUETypes of muscle tissue:Skeletal muscle tissue: Skeletal muscle tissue is attached to the bones of the skeleton and allows the skeleton to move. It’s striated (alternating light and dark bands of muscle proteins) and voluntary (consciously controlled by motor neurons of the somatic nervous system).Cardiac muscle tissue: Cardiac muscle tissue forms the heart wall, and it’s composed of cardiac muscle fibers (contractile cells of the heart). It’s striated and involuntary (not consciously controlled; controlled and regulated by motor neurons of the autonomic nervous system). It’s composed of pacemaker cells that function in autorhythmicity, and it remains contracted 10-15 times longer than skeletal muscle tissue. Cardiac muscle fibers have larger and more numerous mitochondria than skeletal muscle fibers, and they are autorhythmic (cells can spontaneously contract on their own without stimulation from motor neurons). Cardiac muscle fibers depend on aerobic cellular respiration to synthesize ATP, so they require a constant supply of oxygen, and they can use lactic acid produced by skeletal muscle fibers to synthesize ATP, which is important during exercise.Smooth muscle tissue: Smooth muscle tissue forms the walls of hollow internal organs (EX: blood vessels, organs in the abdominopelvic cavity – stomach, small intestine, large intestine, urinary bladder, reproductive organs, organs in the respiratory tract – bronchioles). It’s non-striated and involuntary (not consciously controlled; controlled and regulated by motor neurons of the autonomic nervous system).Functions of muscle tissue:1. Movement of body: Skeletal muscle tissue moves body structures with the coordination and integration of bones and joints.2. Stabilizes body position: Contraction of skeletal muscles aids in stabilizing joints and helps maintain body position (EX: sustained contractions of neck muscles hold the head upright).3. Stores and moves substances within the body: Muscle tissue is capable of storing and moving substances via contraction and relaxation of the tissue (EX: contraction and relaxation of smooth muscle tissue in the walls of blood vessels adjusts blood-vessel diameter in order to aid in blood circulation and regulate blood flow).4. Generates heat: Muscle tissue produces heat when it contracts, which is used to maintain body temperature (EX: shivering, which is involuntary contractions of skeletal muscle tissue, increases the rate of heat production in the body).Properties of muscle tissue:1. Electrical excitability/Irritability: This is the ability of the muscle tissue to respond to specific stimuli by producing electrical signals/messages, which causes muscle tissue to contract or relax depending on the stimuli that is detected.2. Contractility: This is the ability of muscle tissue to contract forcefully when stimulated by an electrical signal/message.3. Extensibility: This is the ability of muscle tissue to stretch without being damaged and to allow muscle tissue to contract forcefully even it is already stretched. Smooth and cardiac muscle tissues are subject to the most frequent stretching.4. Elasticity: This is the ability of muscle tissue to return to its original length and shape following contraction or extension.SKELETAL MUSCLE TISSUESkeletal muscle tissue: Each skeletal muscle is a separate organ composed of thousands of muscle fibers. Connective tissue surrounds muscle fibers, whole muscles, blood vessels, and nerves.Muscle fibers: Muscle fibers are muscle cells that compose muscle tissue and are referred to as fibers due to their elongated shape.Connective tissue components:Fascia: Sheet or band of connective tissue that supports and surrounds skeletal muscles.Superficial fascia: This type of fascia separates skeletal muscle tissue from the skin. It is, actually, the hypodermis (subcutaneous layer) of the skin and is composed of adipose tissues and areolar connective tissue. It serves as a pathway for nerves, blood vessels, and lymphatic vessels to enter and exit the muscle tissue. The adipose tissue of the superficial fascia functions to reduce heat loss from the body and protects skeletal muscles from physical trauma.Deep fascia: This type of fascia is composed of dense irregular connective tissue, and it lines the body wall and limbs, and holds muscles with similar functions together. Deep fascia functions to allow muscles to move freely.Epimysium: Outermost layer of the deep fascia composed of dense irregular connective tissue, and it surrounds the entire muscle.Perimysium: Connective tissue layer that surrounds 10-100 or more skeletal muscle fibers. It separates the muscle fibers into bundles called fascicles.Fascicles: Bundles of skeletal muscle fibers that are covered by perimysium.Endomysium: Thin sheath of areolar connective tissue that covers each individual muscle fiber within the fascicles.Tendon: Strong cord of dense regular connective tissue composed of parallel bundles of collagen fibers that attach skeletal muscle to the periosteum of the bone. Nerve and blood supply: Skeletal muscles have an extensive nerve and blood supply. Neurons stimulate the muscle tissue to contract and detect changes within muscle fibers. Blood vessels supply the muscle fibers with oxygen- and nutrient-rich blood, and the blood removes heat and waste products from the muscle fibers.Types of skeletal muscle fibers:Slow oxidative (SO) fibers: SO fibers have a small diameter and are the least powerful. They provide slow, sustained contractions for prolonged periods of time and are resistant to fatigue. They contain a high density of mitochondria and myoglobin (protein that binds to and stores oxygen) and store low concentrations of glycogen. Red muscles are composed of SO fibers, and these muscles have an extensive blood supply (EX: postural muscles of the neck to help maintain posture). SO fibers synthesize ATP via aerobic cellular respiration.Fast oxidative-glycolytic (FOG) fibers: FOG fibers have an intermediate diameter, provide fast, powerful contractions, and are moderately resistant to fatigue. They contain a high density of mitochondria and myoglobin and store high concentrations of glycogen. Skeletal muscles composed of FOG fibers appear pink-red in color, and these muscles have an extensive blood supply (EX: muscles of the lower limbs used for walking and sprinting). FOG fibers synthesize ATP via aerobic and anaerobic cellular respiration.Fast glycolytic (FG) fibers: FG fibers have a large diameter and are the most powerful. They provide fast powerful contractions but fatigue rapidly. They contain a low density of mitochondria and myoglobin and store high concentrations of glycogen. White muscles are composed of FG fibers, and these muscles lack an extensive blood supply (EX: muscles of the upper limbs used for quick, intense movements). FG fibers synthesize ATP via anaerobic cellular respiration.Microscopic anatomy of skeletal muscle fibers:Sarcolemma: The sarcolemma is the cell membrane of the skeletal muscle fiber, and it’s surrounded by endomysium. The sarcolemma functions in the transport of molecules across the membrane surface, and it separates the intracellular and extracellular environments of the cell.Sarcoplasm: The sarcoplasm is the cytoplasm of the skeletal muscle fiber, and it contains the glycosomes and myoglobin. The sarcoplasm functions as the site of metabolism within the cell.Glycosomes: Glycosomes are vesicles of stored glycogen molecules, which will be catabolized to glucose during muscle contraction.Myoglobin: Myoglobin is the red pigment (protein) that functions to store oxygen.Myofibrils: Myofibrils are rod-like organelles that comprise about 80% of the cellular volume. They are densely packed in the sarcoplasm, and hundreds to thousands of myofibrils can be located within one muscle fiber. Myofibrils are composed of sarcomeres and function in contraction.Striations: Striations are proteins that form repeated series of dark and light bands along the length of myofibrils, and they compose sarcomeres.A bands: Dark, dense middle area of the sarcomere, which extends the entire length of the thick filament, and a zone of overlap occurs at each end of the A band.I bands: Lighter, less dense area of the sarcomere that contains only thin filaments.Sarcomeres: Sarcomeres are the smallest contractile units, or functional units, of muscle fibers. They are aligned side-by-side within myofibrils, and their structure includes several proteins that allow contraction to occur.M line: The M line is composed of myomesin (protein), which holds the thick filaments together in the center of the H zone.H zone: The H zone is an area located in the center of each A band that contains only thick filaments.Z discs: The Z discs are composed of alpha-actinin (protein) and are located at the end of each sarcomere, which separate the sarcomeres in the muscle fiber. They function to anchor the thin filaments in the myofibrils.Myofilaments: Myofilaments are protein structures that compose sarcomeres. They include thick and thin filaments.Thick filaments: Thick filaments are myofilaments that are composed of myosin (protein), and they extend the entire length of the A band.Thin filaments: Thin filaments are myofilaments that are composed of actin (protein) troponin (protein), and tropomyosin (protein), and they extend across the I band and partway into the A band.Muscle proteins: Muscle proteins are found within the myofibrils.Contractile proteins: Contractile proteins function to generate force during muscle contraction in order to move (contract) the muscle fibers.Myosin: Myosin is a motor protein that is found in all three types of muscle tissue. It functions to push and pull cellular structures causing movement by converting chemical energy in ATP into mechanical energy of motion. Each myosin molecule is shaped like 2 golf clubs that are twisted together, and 300 molecules of myosin form one thick filament in a sarcomere (compartments of thick and thin filaments within muscle fibers).Actin: Actin is one protein that forms the thin filaments in a sarcomere. It functions as a myosin-binding site, where the myosin heads attach to actin in the thin filaments to allow movement (contraction) of the muscle fiber.Regulatory proteins: Regulatory proteins function to control the process of muscle contraction. They act as “on-and-off” switches for contraction.Troponin and Tropomyosin: These are regulatory proteins that compose part of the thin filaments. When the muscle is relaxed, myosin is blocked from attaching to actin because tropomyosin covers the binding site. Troponin functions to hold tropomyosin in place. In order for the muscle fiber to contract, tropomyosin must move and uncover the binding site on the thin filament to allow the myosin heads to attach to actin.Structural proteins: Structural proteins function to help maintain the alignment of the thick and thin filaments in the sarcomere. These proteins give muscle fibers elasticity, extensibility, and stability.Titin: Titin is an elastic filament that extends from the Z disc through the thick filaments to attach the filaments to the M line. It functions to hold the thick filaments place to maintain A-band organization and to help the muscle fiber return to its original shape after being stretched.Dystrophin: Dystrophin functions to link the thin filaments to the integral proteins of the sarcolemma.Sarcoplasmic reticulum (SR): Sarcoplasmic reticulum is a specialized type of smooth ER that surrounds each myofibril like a sleeve. They are tubular-shaped structures that function to regulate intracellular calcium ions by storing and releasing them into the sarcoplasm when contraction occurs.Transverse (T) tubules: The T tubules are protrusions of the sarcolemma into the muscle fiber, and they form tubular-shaped structures that run horizontally within the muscle fiber and surround the myofibrils. They are located at the A band – I band junction, and the lumen (opening) of the tubules is continuous with the extracellular space. This is important to allow ions into the tubules, which facilitates propagation of the action potential during excitation-contraction coupling. T tubules function to increase the surface area of the sarcolemma in order to facilitate conduction of the action potential within the muscle fiber.Triads: Triads are a 3-layered structure composed of two terminal cisternae of the sarcoplasmic reticulum with a T tubule between the terminal cisternae. The triads function to facilitate the conduction of the action potential into the muscle fiber, which signals the release of calcium ions from the terminal cisternae to ensure that each myofibril contracts simultaneously.Sliding filament model of contraction:This model explains how skeletal muscle fibers contract, which involves the thick and thin filaments sliding past each other in the sarcomere. The myosin heads from the thick filaments attach to actin forming cross bridges, which allow the thick filaments to move along the length of the thin filaments causing the thin filaments to move closer to the M line. As the thin filaments slide inward, the Z discs move closer together, and the sarcomere shortens, which results in contraction of the muscle fiber and, eventually, the entire muscle. Although the sarcomere shortens, the thick and thin filaments remain the same length.Physiology of skeletal muscle fibers: The physiology of muscle fibers occurs at the neuromuscular junction, and it involves stimulation of the muscle fiber via a motor neuron and excitation-contraction coupling at the level of the sarcolemma. For a brief animated explanation of skeletal muscle contraction, please view: Overview of Skeletal Muscle Contraction.Neuromuscular junction (NMJ): The neuromuscular junction is the point of contact between an axon terminal of a motor neuron and the sarcolemma of a muscle fiber. Each muscle fiber has only one NMJ located about halfway along its length.Motor neuron: A motor neuron is a nerve cell that functions to control contraction.Synaptic cleft: The synaptic cleft is a space between the axon terminal of the motor neuron and the sarcolemma of the muscle fiber. It’s approximately 1-2 micrometers wide.Acetylcholine (ACh): ACh is a neurotransmitter released from the axon terminals of the motor neuron. It’s located within synaptic vesicles inside the axon terminals, and ACh is released when an action potential stimulates the axon terminal. This causes the synaptic vesicles to move and fuse with the neurolemma (cell membrane of neuron), which causes the release of ACh via exocytosis into the synaptic cleft.Motor end plate (junctional folds): The motor end plate is the area where the axon terminal of the motor neuron makes contact with the folds of the sarcolemma. This area contains a high density of ACh receptors.ACh receptors: ACh receptors are ligand-gated ion channels that bind to ACh. When ACh binds, the channel opens to allow the movement of sodium and potassium ions into and out of the muscle fiber.Acetylcholinesterase: This is an enzyme located in the synaptic cleft that catabolizes ACh to acetic acid and choline. It removes ACh from the synaptic cleft, which stops contraction.Phase 1: Stimulation of the muscle fiber via the motor neuron, which occurs at the motor end plateStep 1: An action potential (AP) arrives at the axon terminal of a motor neuron.Step 2: Voltage-gated calcium-ion channels open and calcium ions enter the axon terminal.Step 3: Calcium-ion entry causes synaptic vesicles within the axon terminal to release ACh via exocytosis.Step 4: ACh diffuses across the synaptic cleft and binds to ACh receptors in the sarcolemma.Step 5: ACh binding causes ligand-gated ion channels to open, which simultaneously allows sodium ions into the muscle fiber and potassium ions out of the muscle fiber. A rapid increase in sodium ions entering the muscle fiber causes the sarcolemma to depolarize (positive increase in membrane potential) generating an action potential (AP).Step 6: ACh effects stop when acetylcholinesterase catabolizes ACh in the synaptic cleft.-9461517780000Phase 2: Excitation-contraction coupling, which occurs at the sarcolemmaStep 7: Following depolarization, the AP travels across the entire sarcolemma as voltage-gated sodium-ion channels open propagating the AP.Step 8: AP travels along the transverse (T) tubules.Step 9: Sarcoplasmic reticulum (SR) releases calcium ions, which bind to troponin allowing the myosin-binding sites on actin to be exposed.Step 10: Cross bridges form as myosin heads on the thick filaments bind to actin on the thin filaments and contraction begins via the sliding-filament mechanism.MUSCLE METABOLISMProduction of ATP in muscle fibers: Muscle fibers need large amounts of ATP to power a contraction, and muscle fibers synthesize the ATP that they need.Creatine phosphate: Relaxed muscle fibers produce more ATP as compared to when they are contracting. Excess adenosine triphosphate (ATP) is stored as creatine phosphate (high-energy molecule located within muscle fibers) when this molecule phosphorylates adenosine diphosphate (ADP). Storing ATP as creatine phosphate allows enough energy for a muscle to contract for a maximum of 15 seconds (short bursts of activity).Anaerobic cellular respiration: This is a series of ATP-producing reactions that does not require oxygen and can synthesize ATP when oxygen is present or absent. It produces 2 molecules of ATP from the catabolism of glucose from glycogen in muscle fibers (glycolysis). When ATP is synthesized via this metabolism, lactic acid is, also, produced and accumulates in the cytosol of the muscle fibers. Lactic acid diffuses out of the muscle fibers into the blood and passes through the liver, and the hepatocytes (liver cells) will convert the lactic acid into glucose in order to continue the production of ATP. This is a fast-acting process, which can occur in 60 seconds.Aerobic cellular respiration: This is a series of ATP-producing reactions that requires oxygen to synthesize ATP in the mitochondria. This is a slow-acting process, which can take hours to complete, but it produces more ATP than anaerobic cellular respiration (glycolysis). If glucose is catabolized using oxygen, then over 30 molecules of ATP will be synthesized per glucose. If fatty acids are catabolized using oxygen, then more than 100 molecules of ATP will be synthesized per fatty-acid molecule.Muscle fatigue: Muscle fatigue occurs when the muscle tissue is unable to maintain contraction after prolonged activity. It results from changes in the muscle fibers and may be a protective mechanism of the body to cease exercising before muscles are damaged. Possible factors that cause muscle fatigue include depletion of creatine phosphate and glycogen within muscle fibers, insufficient oxygen to muscle fibers, and an accumulation of lactic acid within muscle tissue.SMOOTH MUSCLE TISSUESmooth muscle tissue: Smooth muscle fibers are elongated cells with one centrally-located nucleus and are much thinner and shorter than skeletal muscle fibers. Typically, the two layers of smooth muscle tissue will compose the walls of viscera, and these layers are arranged perpendicular to each other, which facilitate movement of substances within the lumen of the organ. Smooth muscle tissue does not have neuromuscular junctions, but they are innervated by the motor neurons from the autonomic nervous system, which controls their contractions. Smooth muscle tissue provides slow, sustained contractions, which require low concentrations of ATP, and it can take 30 times longer to contract and relax as compared to skeletal muscle tissue.Types of smooth muscle tissue:Single-unit smooth muscle (visceral muscle): The most common type is visceral smooth muscle tissue, which forms the walls of hollow internal organs (EX: blood vessels. stomach, small intestine, large intestine, urinary bladder, uterus). Visceral smooth muscle tissue is, also, known as single-unit smooth muscle tissue because contraction of one muscle fiber causes adjacent fibers to contract in unison, and it’s autorhythmic (exhibit Spontaneous rhythmic action potentials without nervous system innervation). These smooth muscle fibers are arranged perpendicular to each other in longitudinal and circular sheets, and gap junctions allow these cells to contract as a unit. This tissue is innervated by the autonomic nervous system, and it can respond to various chemical stimuli.Multiunit smooth muscle: Multiunit smooth muscle tissue is characterized by the stimulation of one muscle fiber, which causes all other fibers to contract simultaneously regardless of where they are located in the body (EX: stimulation of one arrector pili mucscle causes all arrector pili muscles to contract and stimulation of the muscles of the iris adjusts the diameter of the pupil in both eyes simultaneously). These smooth muscle fibers are structurally independent of each other with few, if any, gap junctions, and autorhythmicity is absent. This tissue is, also, innervated by the autonomic nervous system, and it can respond to hormones.AGING AND MUSCULAR TISSUEAs humans age, we experience a slow, progressive loss of skeletal muscle tissue, which is replaced by fibrous connective tissue and adipose tissue. Loss of skeletal muscle tissue occurs due to a decrease in physical activity, decrease in muscle strength, slowing of muscular reflexes, and loss of flexibility. Skeletal muscle tissue can become stronger and muscular performance can improve with strength training (EX: lifting weights) and aerobic activities (EX: running, jogging, biking, swimming, brisk walking). ................
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