Muscular Tissue - Arab American University PDF

Muscular Tissue Spring Semester - 2024 Functions of Muscles 1. Body movement 2. Stabilizing body position 3. Storing and moving substances within the body 4. Generating heat →Thermogenesis Heat produced by the muscles → maintain body temperature, Shivering (involuntary contraction).  Pr...

Muscular Tissue Spring Semester - 2024 Functions of Muscles 1. Body movement 2. Stabilizing body position 3. Storing and moving substances within the body 4. Generating heat →Thermogenesis Heat produced by the muscles → maintain body temperature, Shivering (involuntary contraction).  Properties of Muscles 1. Electrical Excitability: ability to respond to certain stimuli by producing electrical signal. 2. Contractility: ability of muscular tissue to contract forcefully when stimulated by an action potential. 3. Extensibility: ability of muscle tissue to stretch without being damaged. 4. Elasticity: ability of muscular tissue to return to its original length & shape after contraction or extension. Types of Muscles SKELETAL MUSCLE TISSUE Components 1. Epimysium It surrounds the entire muscle. 2. Perimysium It surrounds each fascicle. 3. Endomysium It surrounds each fiber. 4. Tendon Attach a muscle to a bone. 5. Aponeurosis Broad, flattened tendon.  Sarcolemma, Transverse Tubules, and Sarcoplasm ▪ Sarcolemma: The plasma membrane of a muscle fiber (muscle cell). ▪ Transverse (T) tubules: invaginations of the sarcolemma toward the center of each muscle fiber. ▪ Within the sarcolemma is the sarcoplasm, the cytoplasm of a muscle Fiber that includes: 1. Glycogen used for synthesis of ATP. 2. Myoglobin that binds oxygen molecules in muscle fibers. Myofibril & Sarcoplasmic Reticulum ▪ Myofibrils: little threads, that represent the contractile organelles of skeletal muscle. ▪ Sarcoplasmic reticulum (SR) : A fluid-filled system of membranous sacs → encircles each myofibril. ▪ In a relaxed muscle fiber, the sarcoplasmic reticulum stores calcium ions (Ca2+). Release of Ca2+ triggers muscle contraction. (muscle cell)  The sarcomere ▪ Myofibril → Thin & Thick filaments. ▪ Filaments are arranged into Sarcomeres → basic functional unit of myofibril. ▪ The extent of overlap of the thick and thin filaments depends on whether the muscle is contracted, relaxed, or stretched. The sarcomere ‫‪The sarcomere‬‬ ‫* األجزاء مطلوبة تعيين *‬  The sarcomere ▪ A-band: The darker middle part of the sarcomere. ▪I band: is a lighter, less dense area that contains the rest of the thin filaments but no thick filaments. ▪ Zone of overlap: area where the thick and thin filaments lie side by side. Z disc: separate one sarcomere from the next. ▪ H zone: narrow area in the center that contains thick but not thin filaments. ▪ M line: Supporting proteins that hold the thick filaments in the Middle of the sarcomere. Muscle Proteins 1. Contractile 1. Actin 2. Myosin 2. Regulatory 1. Troponin 2. Tropomyosin 3. Structural 1. Titin 2. Αlfa-actinin 3. Myomesin 4. Nebulin 5. Dystrophin Muscles proteins Contractile proteins: (Myosin) ❑ Myosin is the main component of thick filaments and functions as a motor protein. ❑ Motor proteins converts the chemical energy in ATP to the mechanical energy of motion. ❑ The two projections of each myosin molecule (golf club heads) are called myosin heads. Muscles proteins Contractile proteins: (Actin) ❑ Individual actin molecules join to form an actin filament that is twisted into a Helix. ❑ On each actin molecule is a myosin-binding site, where a myosin head can attach.  Regulatory proteins Muscles proteins (Troponin and Tropomyosin) ❑ In relaxed muscle, myosin is blocked from binding to actin because tropomyosin cover the myosin binding sites on actin. ❑ The tropomyosin strands are held in place by troponin. ❑ calcium (Ca2+) bind to troponin → troponin change in shape → moves tropomyosin moved away from myosin- binding sites on actin → myosin binds to actin → muscle contraction. Contraction and Relaxation of Skeletal Muscle Fibers H Zone decreases, and A band stay the same.  Mechanism of muscle contraction: Sliding filament theory The contraction cycle consists of four steps: 1. ATP hydrolysis: The myosin head includes an ATP-binding site. This hydrolysis reaction of the ATP provides energy to the myosin head. 2. Attachment of myosin to actin to form cross-bridges. 3. Power stroke: During the power stroke, cross-bridges rotate and move the thin filaments past the thick filaments toward the center of the sarcomere (M-line). 4. Detachment of myosin from actin. The contraction cycle  Excitation-Contraction Coupling An increase in Ca2+ concentration in the sarcoplasm starts muscle contraction, and a decrease stops it. When a muscle fiber is relaxed, the concentration of Ca2+ in its sarcoplasm is very low. However, a huge amount of Ca2+ is stored inside the sarcoplasmic reticulum. As a muscle action potential propagates along the sarcolemma and into the T tubules, it causes Ca2+ release channels in the SR membrane to open. When these channels open, Ca2+ flows out of the SR into the sarcoplasm around the thick and thin filaments. As a result, the Ca2+ concentration in the sarcoplasm rises tenfold or more. The released calcium ions combine with troponin, causing it to change shape. This conformational change moves tropomyosin away from the myosin-binding sites on actin. Once these binding sites are free, myosin heads bind to them to form cross-bridges, and the contraction cycle begins.  The Neuromuscular Junction (NMJ) Somatic motor neurons → release acetylcholine (Ach). Neuro Muscular Junction (NMJ) → the synapse between a somatic motor neuron and a skeletal muscle fiber. Motor end plate → contain Ach receptors. Nerve impulse: 1. Release of Ach 2. Activation of Ach receptors → channel open → sodium flow across membrane. 3. Production of muscle action potential. 4. Termination of Ach activity : acetylcholinesterase breaks down Ach and end the effect of it.  Muscle Metabolism ATP is needed for : ◦ Contraction and relaxation ◦ Pump Ca2+ into the sarcoplasmic reticulum ◦ Metabolism Muscle fibers have three ways to produce ATP: (1) creatine phosphate (2) anaerobic glycolysis (3) aerobic respiration  1. Creatine Phosphate ‫*الرسم غير مطلوب‬ ▪ When muscle relaxed → produce more ATP→ Excess ATP → Excess synthesize creatine phosphate C.K (found only in muscle cells). ▪ Creatine kinase → catalyze the reaction  2. Anaerobic Cellular Respiration (A C R ) ‫*الرسم غير مطلوب‬ ▪ ATP producing without need for oxygen. ▪ Glucose from blood or from muscle glycogen → pyruvic acid + ATP ▪ For each glucose molecule → 2 ATP molecules  3. Aerobic Cellular Respiration ‫*الرسم غير مطلوب‬ ▪ Production of ATP in mitochondria. ▪ Each glucose molecule → 32 ATP Molecules. ▪ In prolonged activity.  Control of Muscle Tension Total force or tension that a single fiber can produce depend on the rate at which nerve impulse arrive at NMJ. Frequency of stimulation: number of impulses per second. Maximum tension is also affected by: ◦ Degree of stretch before contraction. ◦ Nutrient & oxygen availability. ◦ Number of fibers contracting in unison.  Motor Units Motor unit = Somatic motor neuron + all the skeletal muscle fibers it stimulates. SMN stimulate about 150 muscle fibers. Total strength of a contraction depend on size of motor unit and number of Muscle Units activated at the same time.  Motor Unit Recruitment The process in which the number of active motor units are increasing. Typically, Some motor units are contracting → others are relaxing. This pattern delay muscle fatigue, and allow contraction of the whole muscle to be sustained for long periods. Producing smooth movements rather than a series of jerks.  Muscle Tone ▪ Tonos = tension. ▪ Even at rest, a skeletal muscle exhibits muscle tone. Small amount of tension in the muscle due to weak, involuntary contractions of its motor units. ▪Muscle tone keeps muscle firm, but not strong enough to cause movement. ▪ Muscle tone is established by the motor neurons. ▪To sustain muscle tone → small groups of muscle units are alternatively active & inactive in shifting pattern. ▪ If motor neurons are cut or damaged →flaccid muscle.  1. Isotonic Contraction Iso= the same, tonic = tension (IT) The force of contraction developed by the muscle remains almost the same while the muscle changes in length. Used for body movements & moving objects Types : ◦ Concentric ITC: Muscle shortens ◦ Eccentric ITC: Length of the muscle increases  2. Isometric Contraction Muscle doesn't change in length. Important for maintaining posture and for supporting objects in a fixed position. Although do not result in body movement → still energy is expended. Most activities include both isotonic & isometric contraction Muscle Contraction  Skeletal Muscle Fibers ❑ Red Muscle fibers ◦ High myoglobin content ◦ Appear darker ❑ White muscle fibers ◦ Low myoglobin content ◦ Appear lighter  Skeletal Muscle Cardiac Muscle Smooth Muscle Fiber shape & Long cylindrical Branched cylindrical Fiber is thickest in middle, striation fibers, fiber, not striated striated striated Location Attached to bones Heart Walls of viscera, airways, by tendons blood vessels, arrector pili muscle of hair follicles Connective Endomysium, Endomysium, Endomysium tissue perimysium, perimysium, component epimysium Contractile Proteins yes yes No organized into sarcomeres Sarcoplasmic Abundant Some Very little reticulum Traverse tubules Yes Yes No present  Skeletal Muscle Cardiac Muscle Smooth Muscle Auto-rhythmicity No Yes Yes Nervous control Voluntary Involuntary Involuntary Contraction Acetylcholine Acetylcholine and regulated by Norepinephrine --- Source of Sarcoplasmic Reticulum SR, interstitial fluid SR , interstitial fluid calcium for (SR) contraction Regulator Troponin & Troponin & Calmodulin and proteins for tropomyosin tropomyosin others contraction Speed of Fast Moderate Slow contraction The End