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Muscle tissue Characteristics Excitability ·When a muscle is stimulated by a nerve impulse or electrical stimulus it contracts: property of excitability. Contractibility ·Stimulated muscle responds by contracting. Extensibility ·Pull on the ends of muscle it stretches; extensibility Elasticity ·Released from stretching the muscle returns to its original shape. Functions Motion ·Muscles provide the force for motion; skeletal, cardiac and smooth. Posture ·Generates a force to maintain posture Heat ·Every muscle action requires mechanical and chemical processes; generates heat; used to maintain body temp. Type Skeletal ·Muscle attached to a bone to provide for motion and posture. Cardiac ·Associated with the heart, to generate the force used to drive blood through the vessels. Smooth ·Associated with the many organs of the body; ·E.g., generates a squeezing force that pushes material digestive tract. ·Also, found in the bronchial tubes and blood vessels. down the Connective tissue Fascia ·Sheets of CT fibers (collagenous) found under the skin and between muscles and organs. ·Helps to separate one organ or structure from another. ·Consists of superficial, deep and subserous fascia deep fascia ·Dense CT, arranged in layers. ·In each layer, the fibers are parallel ·each layer is oriented in a different direction. ·Deep fascia around each muscle blends into the periosteum bone. Epimysium ·Dense CT layer which surrounds the entire muscle; continuous with the surrounding deep fascia. ·Separates the muscles from surrounding structures. Perimysium ·An extension of the epimysium within the muscle; divides the muscle into bundles of muscle cells; fascicles. Endomysium ·An extension of the perimysium within each fascicle; separates each muscle cell. of the tendons ·Dense regular CT bundles ; bind muscles to bones. ·Tendons are continuous with the epimysium of the muscle ·by implication, the peri- and endomysium as well. ·Forces generated within each muscle cell are transmitted through the various CT layers; transferred through the tendon to the bone. Skeletal muscle striated voluntary ·Under the microscope; each muscle has minute cross-banding patterns; a “striated” appearance. ·The muscle is under conscious control; voluntary. Histology Myofibers or muscle fibers or muscle cells ·All are equivalent terms. Sarcolemma ·Sarco means muscle. Lemma= plasma membrane (PM). ·Muscle cell membrane. Sarcoplasm ·The cytoplasm within the muscle cell. Multinucleate ·Muscle cells have many nuclei. ·Started out as multiple uninucleate cells; fuse into the much larger muscle fiber. ·Original nuclei are retained. transverse tubules ·Tube-like extensions of the PM into the sarcoplasm. ·They carry the stimulus ( in this case, a type of “electrical” signal) from the sarcolemma deep into the cell. sarcoplasmic reticulum ·Similar to the ER, endoplasmic reticulum. ·Stores large amounts of calcium within the ·Is closely associated with the T-Tubules. SR lumen. Calcium ·Calcium is released into the sarcoplasm in response to a stimulus. Calsequestrine ·An ion pump used to move calcium into the SR; uses ATP calcium-ATPase ·A ion pump used to move calcium into the SR; uses ATP Triad ·One T-tubule and the adjacent SR on either side. myofibrils ·Are bundles of protein filaments, called myofilaments, within the muscle fiber (cell.) ·Made of two types of myofilaments. myofilaments myofibril is made of alternating and interdigitating thin and thick myofilaments aligned with the long axis of the cell. ·Each pattern seen within each myofiber (muscle cell) is caused by this arrangement of myofilaments. ·Striated Thin ·Appears ·Made lighter under the microscope. of several proteins; actin, troponin and tropomyosin. Thick ·Appears darker under the microscope. ·Made of a protein called myosin. sarcomeres ·Myofilaments sarcomeres. are organized into some 10,000 repeating units called Z-lines end of the sacromere. Thin filaments of adjacent sarcomeres are linked together here. ·Each A-band ·Consists of overlapping thin and thick filaments. I-band ·Only thin filaments. ·The I-band shortens as the sarcomere contracts. H-zone ·Only thick filaments. This also shortens during contraction. 16 thin Actin ·Each thin filament is made of three different proteins. filaments are made of subunits called G-actin; these are globular proteins which are linked together to form a filament. ·Actin myosin binding site ·Each G-actin contains a binding site for myosin head groups (part of the thick filament) Troponin ·This protein is associated with actin and it binds calcium. Tropomyosin ·The third protein of the group ·when muscle fiber is relaxed tropomyosin covers the myosin binding site, blocks the myosin head groups from binding to actin. thick ·Consists of a bundle of proteins called myosin Myosin Tails heads=cross bridges ·Each tail has two head groups at the same end and each has two binding sites. actin binding site ·This binds to the myosin binding site on actin, when it is exposed as tropomyosin moves. ATP-binding site ·Binds to ATP ·splits ATP · released energy drives the movement of the myosin head groups. Motor end plate (neuromuscular junction) ·Signals passed down through nerves to stimulate muscle contraction ·need to be converted from an action potential (AP, nerve impulse) into a chemical signal at the neuromuscular junction. synaptic end bulbs ·Signals reach the end of the nerve; enter the synaptic end bulb. synaptic vesicles ·Signal triggers synaptic vesicles to release signaling chemicals into the synaptic cleft. acetlycholine- ACh (neurotransmitter) ·In muscles, this chemical is called acetlycholine, a neurotransmitter. acetylcholinesterase ·Ach needs to be broken down as soon as it is used by the enzyme acetlycholinesterase. Q8 20 Molecular events in muscle contraction ·Best reviewed by studying the animation. 1) Signals pass down the nerve and cause the release of NT at the neuromuscular junction. 2) A signal is generated in the sarcolemma within the junction. 3) Signal passes down the T-tubules. 4) Triggers the release of calcium from the SR. 5) The calcium diffuses into the sarcoplasm and binds to troponin on the thin filament. 6) Troponin causes tropomyosin to move and exposes the myosin binding site. 7) Myosin head groups bind to the active site on actin and then undergo a power stroke; this pulls the actin filament along 8) Head groups detach and are free to bind again only if ATP is present. 9) As nerve signals stop the calcium levels fall and tropomyosin moves back to cover the myosin binding sites. Contraction stops. Q9 23 Rigor mortis ·During ·ATP death ATP levels fall rapidly, as cellular respiration stops. is required to detach myosin head groups from the actin filaments. ·Myosin and actin filaments · muscles become rigid and are bound together locked into the position they were in at death. ·Eventually the myosin and actin filaments ·enzymatic and chemical degradation ·muscle becomes flaccid. detach Energy for muscle contraction ATP is needed for… 1) contraction ·Cocking and detachment of the myosin head. 2) calsequestrin and Ca2+-ATPase ·Pumping calcium into the SR of the sarcoplasm. 3) Na+/K+-ATPase ·Needed for impulse conduction. ATP lasts ·...only 5-6 seconds during active muscle contraction as ATP stores are used up. ATP is quickly reconstituted ·Several mechanisms that replenish the ATP stores. Sources of energy for ATP production 1) phosphocreatine (creatine phosphate, CP) ·ATP is produced by the ·another high energy phosphogen system from phosphocreatine molecule. creatine kinase ·Breaks down phosphocreatine, releasing a phosphate and energy. ·Energy is used to make new ATP. ·The first system that gets activated during contraction; it is simple produce ATP quickly. ·This store of energy lasts about 8-15 seconds. and can 2) anaerobic respiration, glycolysis ·ATP is now ·None of the made by anaerobic mechanisms. previously listed processed are turned off; rather NEW mechanisms are added. Glycogen ·A polymer of glucose. ·Primary energy source used in this process; lasts about 30-40 secs. lactic acid/pyruvic acid ·Product of anaerobic respiration is pyruvate; as no oxygen is available the pyruvate is converted to lactic acid. ·Lactic acid builds up in the muscles; changes the pH of the muscle ·Causes a decrease in the efficiency of enzymes ·Leads to soreness and fatigue. ·Muscle recovery; lactic acid must be removed quickly by a well vascularized muscle. no oxygen ·Anaerobic respiration; no oxygen is used · most of the energy in glucose is not converted ·Only 2 ATPs per glucose are produced. ·Remaining energy is trapped in lactic acid. to ATP. fermentation ·Aka, anaerobic respiration; usually not applied to humans. oxygen debt ·If oxygen is available; lactic acid can be converted back into pyruvate ·used by mitochondria to generate ATP aerobically. ·During anaerobic respiration a need for oxygen is built up; oxygen debt. 3) Oxidative metabolism Aka, aerobic respiration Oxygen ·Oxygen used by mitochondria to produce 36 net ATPs ·2 from glycolysis and 34 from Krebs and ETC. ·Can produce ATP indefinitely as long as you have oxygen and energy stores (fat, proteins or glucose.) Muscle heat ·All mechanical and chemical reactions ·Used to maintain body temperature. produce heat. Initial ·Heat generated during muscle contraction. Recovery ·Heat produced to recover from contraction; oxygen debt, replenishing ATP stores. 1. Phosphogen system 2. Glycogen-lactic acid system 3. Aerobic respiration system anaerobic respiration Creatine phosphate Glycogen Lactic acid Glucose fatty acids amino acids + 02 Glucose Creatine + PO3 Pyruvic acid ADP CO2 + H2O ATP Lasts for 10-15 secs Lasts for 30-40 secs 100 m dash, weight lifting 200-400 m dash Lasts indefinitely 30 All or none principal ·All the muscle fibers of a motor unit respond to a threshold ·contracting as fully as they are physiologically able to. stimulus Threshold stimuli ·A stimulus that is sufficient to cause a response; the response is to contract fully. Sub-threshold stimulus that is not sufficient to cause a response; none of the motor units respond. ·A Contractions Twitch ·A contraction-relaxation event in response to a single threshold stimulus. latent phase ·Period immediately after a stimulus; muscle is preparing to contract; the stimulus is moving over the sarcolemma, down the t-tubules, etc. ·No contraction yet. contraction phase ·Filaments are sliding past each other and generating the contractile force. relaxation phase ·Ca2+ levels falls and the filaments can’t interact; forces decline. refractory period ·Period of time after the initial stimulus in which another stimulus has no effect; during the time of action potential and Ca2+ diffusion. tetanus ·Sustained muscle contractions in normal muscle contractions. Incomplete, unfused or wave summation ·If a second stimulus arrives before the completion of the relaxation phase, the second contraction is added to the first, causing a stronger second contraction. Complete ·Stimulation rate is increased, the stimuli arrive during the contraction phase and the increase in tension is continuous until a maximum level is reached. treppe ·A step-wise increase in muscle contraction when the stimuli are applied immediately after the relaxation phase. Types of contractions Isotonic ·Iso= same, tonic=tension. Same tension. ·Muscle tension stays the same while the muscle length shortens. ·Activities such as walking, lifting objects, etc. involve isotonic contractions. Isometric ·Iso= same, metric=length. Same length. ·Muscle length stays the same while the muscle tension increases. Fibers types Slow-twitch red Fast twitch red Fast twitch white Myoglobin lots lots low Mitochondria lots lots low Capillaries lots lots low ATP production aerobic Aerobic Contraction slow Fast Fatigue Very resistant Moderately resistant Anaerobic Fast Low resistant to fatigue Myoglobin A protein similar to hemoglobin that binds oxygen and found in muscles. Cardiac muscle ·Heart muscle. Striated involuntary ·Striations present; unconscious control. Intercalated discs ·Characteristic of cardiac muscle; region of gap junctions between cardiocytes. Long refractory period ·Reduces the chance for an uncontrolled contractions of the atria and ventricles. Smooth muscle Non-striated involuntary ·Cross banding pattern is not present and control is not conscious. Arteries, airways & others ·Cross banding pattern is not present; control is not conscious. Slower and longer lasting contractions