Chapter 6 The Muscular System PDF

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this is a summary of the muscular system that contains information from my textbook...

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Chapter 6: The Muscular System -The essential function of muscles is to contract -Responsible for all body movements, viewed as the machines of the body

Muscle Types -3 types (Skeletal, Smooth, and Cardiac) -Similarities include: 1. Skeletal and smooth muscle cells are elongated, and thus called muscle fibers 2. The ability of muscle to shorten or contract, depends on 2 types of myofilaments 3. Terminology, when you see the prefixes myo- or mys- (muscle) or Sarco- (flesh), you will know muscle is being talked about

Skeletal Muscle -Packaged into organs called skeletal muscles that attach to the skeleton -Large, cigar shaped, multinucleate cells -Largest muscle fibers -Also known as striated muscle and voluntary muscle -Can be activated by reflex as well -Contracts rapidly with great force, but tires easily and requires rest after short periods of activity -Soft and fragile, yet can exert great power, they are not ripped apart as they exert this great force, due to connective tissue which bundles thousands of their fibers together, strengthening and supporting the muscle as a whole -Each muscle fiber is enclosed in a delicate connective tissue sheath called endomysium Several sheathed muscle fibers are then wrapped by a coarser fibrous membrane called Perimysium, forming a bundle of fibers called a fascicle

-Many fascicles bound together by a tougher overcoat of connective tissue called an epimysium, covering the entire muscle -End of epimysium that extend beyond the muscle, blend either into a strong cordlike tendon or a sheetlike aponeurosis which indirectly attaches the muscle to bone, cartilage, or connective tissue covering -Tendons also provide durability and conserve space - They are mostly tough collagen fibers, thus can cross rough bony projections -Small size means more tendons than fleshy muscles can pass over a joint

Smooth Muscle -No striations, involuntary movement, visceral -Found mainly in walls of hollow (tubelike) visceral organs such as the stomach, they propel substances along a pathway -Spindle shaped, uninucleate, surrounded by very little endomysium -Arranged in layers, 2 layers (One running circularly and the other longitudinally) -as they contact and relax alternately, they change the size and shape of the organ e.g., in moving food through the digestive tract -Muscle contraction slow and sustained

Cardiac Muscle -only found in the heart, forms bulk of heart walls -Striated, uninucleate and involuntary -Cardiac cells cushioned by small amounts of endomysium and are arranged in spiral or figure 8 shaped bundles

-As the heart contracts, its internal chambers become smaller, forcing blood into the large arteries leaving the heart -Muscle fibers are branching cells joined by special gap junctions called intercalated disks -Cardiac muscle usually contracts at a steady rate set by the heart’s pacemaker -Nervous system can stimulate the heart into high gear for short periods

Muscle Functions: Skeletal Muscles 1. Produce Movement -Responsible for the body’s mobility -Enable us to react quickly to changes in the external environment -Allow expression of emotion (silent smile/frown) 2. Maintain Posture & Body Position -Function continuously, making tiny adjustments so we remain erect/seated etc. 3. Stabilize Joints -As skeletal muscles pull on bones to cause movements, they also stabilize the joints of the skeleton -Muscle & tendons are important in reinforcing and stabilizing joints that have poorly articulating surfaces e.g., Shoulder & Knee joint 4. Generate Heat -Muscle activity generates body heat as a by-product -ATP used to power muscle contraction, nearly ¾ of Its energy escapes as heat, which is vital is maintaining normal body temperature -Skeletal muscle makes up 40% of body mass, thus it’s the muscle type most responsible for generating heat. Microscopic Anatomy of Skeletal Muscle

-Many oval nuclei can be seen beneath the plasma membrane which is called the sarcolemma in muscle fibers -The nuclei are pushed aside by long ribbonlike organelles, the myofibrils, which nearly fill the cytoplasm -Alternating light & dark bands along the length of the perfectly aligned myofibrils give the muscle fiber it’s striated appearance (light = I bands, Dark = A bands) -Light I band has a midline interruption, a darker area called the Z disc, and the dark A band has a lighter central area called the H zone -The M line in the center of the H zone contains tiny protein rods that hold adjacent thick filaments together -Myofibrils are chains of tiny contractile units called sarcomeres, which are the structural and functional units of skeletal muscle -Sarcomeres are aligned end to end in the length of the myofibrils -It is the precise arrangement of even smaller structures (myofilaments) within sacromeres that produces the striations in skeletal fibers -2 types of threadlike protein myofilaments within each sarcomere - First there’s thick filaments made of bundled molecules of the protein myosin, they extend the entire length of the dark A band, their mid parts are smooth, and their ends are studded with projections. -The projections are called myosin heads form cross bridges when they link the thick and thin filaments together during contraction -Myosin filaments are attached to the Z discs by titin (elastic filaments that run through the core of the thick filament -secondly there’s thin filaments made of the contractile protein actin, and regulatory proteins that play a role in allowing/preventing binding of myosin heads to actin -they are anchored to the Z disc - Although they overlap the ends of the thick filaments, the thin filaments do not extend into the middle of a relaxed sarcomere, thus the H zone looks a bit lighter -When the actin containing thin filaments slide towards each other during contraction the H zones disappear because the actin and myosin filaments completely overlap -Sacroplasmic reticulum (SR), is a specialized smooth endoplasmic reticulum -The interconnecting tubles and sacs of the SR surround every myofibril like a loose sweater over an arm -The major role of this system is to store calcium and to release it on demand when the muscle fiber is stimulated to contract -Calcium provides the final go signal for contraction

Skeletal Muscle Activity -Irritability (AKA responsiveness)- the ability to receive and respond to stimulus -Contractility- the ability to forcibly shorten when adequately stimulated -Extensibility -is the ability of muscle fibers to stretch -Elasticity- is their ability to recoil and resume their resting length after being stretched Stimulation of muscle contraction

-To contract, skeletal muscle fibers must be stimulated by nerve impulses -A motor unit consists of one neuron and all the skeletal muscle fibers it stimulates -When the axon reaches the muscle it branches into a number of axon terminals, each of which forms junctions with the sarcolemma of a different muscle cell -The junctions called neuromuscular junctions contain synaptic vesicles filled with a chemical referred to as a neurotransmitter -The neurotransmitter that stimulates skeletal muscle fibers is acetylcholine (ACH) -Although the nerve endings and the muscle fiber membranes are very close, they never touch, the gap between them the synaptic cleft is filled with interstitial fluid -some cases where motor nerve impulses are unable to reach the muscle, such as ALS or (amyotrophic lateral sclerosis) where motor neurons degenerate over time, resulting in paralysis that gradually worsens -the cause of ALS is unknown but characteristics include malfunctioning mitochondria, inflammation, and the generation of free radicals that damage DNA and tissue much like intense UV light. -Prognosis for patients with ALS is generally death within 3-5 years as breathing muscles will eventually be affected (suffocation) The Neuromuscular Junction

Synapse at the neuromuscular junction

1. Calcium channels open, and calcium enters the terminal

2. Calcium entry causes some of the synaptic vesicles in the axon terminal to fuse with the cell membrane and release acetylcholine 3. Which then diffuses across the synaptic cleft and attaches to membrane receptors in highly folded regions of the sarcolemma 4. If enough ACh is released, the sarcolemma at that point becomes temporarily even more permeable to sodium ions which rush into the muscle fiber and to potassium ions which diffuse out of the muscle fiber. More sodium enter than potassium leaves. This inbalance gives the cell interior an excess of positive ions which reverses the resting electrical conditions of the sarcolemma. This event, called depolarization, opens more channels that only allow sodium entry. 5. This generates an action potential, once begun it is unstoppable, it travels over the entire surface of the sarcolemma, conducting the electrical impulse from one end of the cell to the other. Result is contraction of the muscle fiber 6. A single nerve impulse produces only one contraction, because while the action potential is occurring, the enzyme acetylcholinesterase present on the sarcolemma and in the synaptic cleft, breaks down acetylcholine to acetic acid and choline

Conduction of action potentials deeper into the muscle (T-tubules & Sarcoplasmic reticulum) -Generated action potentials must be transmitted deeper into the interior of the muscle fiber -important structures that play a role in this are the transverse tubules (T-tubules) and the Sarcoplasmic reticulum

-T-tubules surround myofibrils deeper into the muscle fiber -The action potential generated at the sarcolemma would spread down the T-tubules allowing transmission of the action potential within the interior of the fiber -The sarcoplasmic reticulum houses calcium and surrounds the myofibrils -The action potential in the T-tubules leads to permeability changes in the sarcoplasmic reticulum allowing for the release of calcium

Mechanism of muscle contraction – the sliding filament theory -When the nervous system activates muscle fibers, the myosin heads attach to the binding sites on the thin filament , and sliding begins - Each cross bridge attaches and detaches several times during a contraction, generating tension that helps pull the thin filaments toward the center of the sarcomere (Walking of the myosin cross bridges, or heads along the thin filaments during muscle shortening ) -Some myosin heads are always in contact with actin so that the thin filaments cannot slide backward, the cycle continues to repeat itself -Myofilaments themselves do not shorten they just slide past each other -Formation of cross bridges requires calcium and ATP -The action potential along the T-Tubules initiates the release of calcium from the sarcoplasmic reticulum -The calcium ions trigger the binding of myosin to actin, initiating filament sliding -When the action potential ends, calcium ions are immediately returned to the SR storage areas, the regulatory proteins return to their resting shape and block myosin binding sites, the muscle fiber relaxes and settles back to it’s original length

-In a relaxed muscle fiber, the regulatory proteins (Troponin + Tropomyosin) forming part of the actin myofilaments prevent myosin binding

-When an action potential sweeps along its sarcolemma and a muscle fiber is excited, calcium ions are released from intracellular storage areas (sacs of sarcoplasmic reticulum) -The flood of calcium acts as the final trigger for contraction, because as calcium binds to the regulatory proteins on the actin filaments, the proteins change in both shape and position on the thin filaments -This action exposes myosin-binding sites on the actin, to which the myosin heads can attach, and the myosin heads immediately begin seeking out binding sites

-The free myosin heads are “cocked” much like an oar ready to be pulled on for rowing. -Myosin attachment to actin causes the myosin heads to snap toward the center of the sarcomere in a rowing motion -The filaments are slightly pulled toward the center of the sarcomere -ATP provides the energy needed to release and recock each myosin head so that it is ready to attach to a binding site farther along the thin filament Graded Responses -Muscle fiber will contract fully when stimulated adequately, it never partially contracts -The whole muscle however reacts to stimuli with graded responses or different degrees of shortening which generate different amounts of force -They can be produced in 2 ways 1. By changing the frequency of muscle stimulation 2. By changing the number of muscle fibers being stimulated at one time

Muscle response to increasingly Rapid Stimulation: -Muscle twitches (single, brief, jerky, contractions) -In most types of muscle activity nerve impulses are delivered to the muscle at a rate so rapid that the muscle does not get a chance to relax completely between stimuli -The effects of the successive contractions are “summed” together and the muscle contractions get stronger and smoother -The muscle exhibits unfused or incomplete tetanus -When the muscle is stimulated so rapidly that no evidence of relaxation is seen and the contractions are completely smooth and sustained, the muscle is in fused tetanus or complete tetanus

Muscle response to stronger stimuli -How forceful a muscle contracts depends to a large extent on how many of its cells (motor units) are stimulated -If only a few cells(motor units) are stimulated the muscle as a whole contracts only slightly -When all the motor units are active and all the muscle fibers are stimulated the muscle contraction is as strong as it can get -Muscle contractions can range from slight to vigorous depending on the work to be done

Length of the muscle fiber before contraction- influence on contraction- the length-tension relationship -What is the optimal muscle length at which maximal tension can be developed -How can this be used for maximal explosive force -every muscle has an optimal resting length at which maximal force can be achieved on a subsequent tetanic contraction -More tension can be achieved during tetanus when beginning at the optimal length compared to muscle contractions that begin with the muscle less/greater than the optimal length -Can be explained by the sliding filament mechanism

A. Thin & Thick filaments of the sarcomere overlapping regions where a maximal number of cross bridges can form for cycles of binding and bending (optimal resting muscle length) B. When the fibers are stretch to greater length at resting point, thin filaments are pulled out, decreasing the number of actin sites available for cross bridge binding, they are closer to the Z disc and go unused. Less cross bridge activity leading to less tension developed C. Extreme end of maximal stretch (70% longer than optimal), none of the thin filaments line up with the thick filaments, no cross bridges can be formed resulting in no contraction D. In compression, actin and myosin overlaps to a great extent, tension will be lowered compared to resting length. A large level of compression will lead to no contraction Type of muscle fiber- influence on muscle contraction -3 types of muscle fibers: 1. slow twitch/ oxidative muscle fibers (type 1 or red fibers) 2. Fast twitch oxidative muscle fibers (type 2A or intermediate fibers) 3. Fast twitch glycolytic muscle fibers (type 2X white fibers) -The colour designation is due to the capillary supply -Red fibers due to higher percentage of capillaries, and higher degree of myoglobin -white fibers, pale colour due to fewer capillaries and less myoglobin -Fast twitch fibers have higher myosin, ATPase activity compared to slow twitch fibers hence ATP split at faster rate in these fibers and is more readily available for cross bridge formation -The time for peak tension in fast twitch muscles is much quicker than slow twitch -Fast twitch fibers contain more actin and myosin filaments resulting in larger fiber diameter -Fast twitch fibers contain more glycogen as a metabolic energy source

-Slow twitch fibers are more resistant to fatigue owing to a greater capacity to produce ATP -Most muscles contain a mixture of the 3 muscle fibers types, and the percentage of each type within a given muscle is mainly determined by the type of activity for which that muscle is specialized -E.G back muscles used for keeping the body upright for long periods of time without fatigue contain a greater percentage of slow twitch muscles -Individuals genetically contain varying percentages of fast twitch and slow twitch muscles

Types of muscle contraction -Muscles do not always shorten when they contract -Common to all muscle contractions is that tension (force) develops in the muscle as the actin and myosin myofilaments interact and the myosin cross bridges attempt to slide the thin actin containing filaments past the thick myosin filaments -Isotonic Contractions the myofilaments are successful in their sliding movements, the muscle shortens, and movement occurs (bicep curl) -Isometric Contractions the muscles do not shorten, the myosin filaments are “spinning their wheels”, the tension in the muscle keeps increasing, they try to slide but the muscle is pitted against some more or less immovable object -Isokinetic Contractions the velocity of shortening remains constant as the muscle changes length

2 Types of Isotonic contraction -The muscle changes length at constant tension in both types -concentric the muscle shortens -eccentric the muscle lengthens as it is being stretched by an external force while contracting

Skeletal Muscle Metabolism- ATP regeneration -Muscles store very limited supplies of ATP (only a few seconds worth)

-As ATP is the only energy source that can be used directly to power muscle activity, ATP must be regenerated continuously if contraction is to continue -Working muscles use 3 pathways to regenerate ATP: 1. Direct phosphorylation of ADP by creatine phosphate -The unique high energy molecule creatine phosphate is found in muscle fibers but not other cell types -As ATP is depleted, interactions between CP and ADP result in transfers of a high energy phosphate group from CP to ADP, thus regenerating more ATP in a fraction of a second -Although muscle fibers store perhaps five times as much CP as ATP, the CP supplies are soon exhausted 2. Aerobic pathway -At rest and during light to moderate exercise, some 95% of the ATP used for muscle activity comes from aerobic respiration -Occurs in the mitochondria and involves a series of metabolic pathways that use oxygen referred to as oxidative phosphorylation -During aerobic respiration glucose is broken down completely to carbon dioxide and water, and some of the energy released as the bonds are broken is captured in the bonds of ATP molecules -Although aerobic respiration provides a rich ATP harvest (32 ATP per 1 glucose), it is fairly slow and requires continuous delivery of oxygen and nutrient fuels to the muscle to keep it going. 3. Anaerobic glycolysis and lactic acid formation -The initial steps of glucose breakdown occur via a pathway called glycolysis, which does not use oxygen and hence is anaerobic -During glycolysis, which occurs in the cytosol, glucose is broken down to pyruvic acid, and small amounts of energy are captured in ATP bonds (2 ATP per 1 glucose molecule) -As long as enough oxygen is present, the pyruvic acid then enters the oxygen- requiring aerobic pathways that occur within the mitochondria to produce more ATP -When muscle activity is intense, or oxygen and glucose delivery is temporarily inadequate to meet the needs of working muscles, the sluggish aerobic pathways cannot keep up with the demands for ATP -The pyruvic acid generated during glycolysis is converted to lactic acid -Produces only about 5% as much ATP from each glucose molecule as aerobic respiration, it is 2 ½ times faster and provides most of the ATP needed for 30 to 40 seconds of strenuous muscle activity -cons: 1) It uses huge amount of glucose for a small ATP harvest 2) Accumulating lactic acid promotes muscle soreness

Muscle Fatig...