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3.16C). The extentof overlap of the thin and thick filaments of the sarcomeresaffects the number of crossbridges formed during contraction.With greater stretch before stimulation, more force is generated, up to an optimal sarcomere length.Muscle contractions can also be characterized as isometric orisotonic.
In isometric contraction, muscle length is constantbut a change in tone is observed. An example is the contraction that occurs when we exert force against an immovableobject. Force is generated, but length remains unchanged.Sarcomere shortening is accompanied by stretch of connective tissue and cytoskeletal components of the muscle (serieselastic elements). During isotonic contraction, muscle toneremains constant while the muscle shortens.
An example isthe contraction associated with lifting a book. In reality, allmuscle movements are a combination of isometric and iso-tonic contractions. Pressing of heavy free weights involves alarge isometric component, but it also involves an isotoniccomponent. In general, isometric exercises like weightliftingare useful for body-building. With repeated isometric exercise, hypertrophy of muscle occurs, in which the number ofsarcomeres is increased (hyperplasia, or increased number ofmuscle cells, does not normally occur).
Muscle atrophy orwasting (decreased number of sarcomeres) occurs with disuse,for example during prolonged bed rest.SMOOTH MUSCLESmooth muscle is a type of nonstriated muscle found withinorgans. Contractile proteins are not organized as sarcomeresin smooth muscle; rather, actin is anchored to the cell membrane and to dense bodies within the cell. As in other types ofNerve and Muscle Physiology43SarcomereZ bandZ bandMuscle relaxedI bandA bandI bandH zoneSarcomereZ bandZ bandMuscle contractedA bandH zoneI bandA bandH zoneI bandA bandH zoneDuring muscle contraction, thin filaments of each myofibril slide deeply between thick filaments, bringing Z bands closer togetherand shortening sarcomeres. A bands remain the same width, but I bands narrow. H zones also narrow or disappear as thin filamentsencroach upon them.
Myofibrils and, consequently, muscle fibers (muscle cells), fascicles, and muscle as a whole grow thicker. Duringrelaxation, the reverse occurs.Figure 3.14 Muscle Contraction and Relaxation During skeletal muscle contraction, cyclicalcrossbridge formation between interdigitated actin and myosin filaments produces shortening of thesarcomere.muscle, actin–myosin interactions are the basis of contraction(Fig. 3.17).
A comparison of skeletal, smooth, and cardiacmuscle is shown in Table 3.3.Types of Smooth MuscleSmooth muscle is further classified as unitary or multiunittypes. Of these types, unitary smooth muscle is far moreabundant and is found in the walls of blood vessels, thebladder and the gut, among other organs. This type ofsmooth muscle is capable of sustained and often powerfulcontractions. The cells have gap junctions between them,allowing rapid and direct spread of action potentials, aselectrical potentials are directly conducted between cellsthrough ion fluxes.
Because of this, many smooth musclecells act as a single unit, producing a synchronous contraction. In contrast, multiunit smooth muscle is organized intomotor units similar to those in skeletal muscle. Cells areelectrically isolated from each other (there are no gap junctions), allowing for fine motor control. This type of muscleis found in a few specific regions such as the ciliary body ofthe eye, the vas deferens, and the piloerector muscles in theskin.Contraction of Smooth MuscleSmooth muscle contraction is controlled by multiple neurotransmitters and other chemical ligands that affect cytosolic44The Nervous System and MuscleActinTroponinTropomyosinZ bandThin filamentMyosin headgroupADP ⬃ PiThick filament(myosin)A new molecule of ATP binds to themyosin head, causing it to releasefrom the actin molecule.
Partialhydrolysis of this ATP (ADP ⬃ Pi)will “recock” the myosin head andproduce a high-affinity binding sitefor actin. If Ca2 levels are stillelevated, the crossbridge willquickly reform, causing furthersliding of the actin and myosinfilaments past each other. If Ca2 isno longer elevated, the musclerelaxes.Ca2ADP ⬃ PiCa2At rest, ATP binds to myosin headgroups and is partially hydrolyzed toproduce a high-affinity binding site foractin on the myosin head group.However, the head group cannot bindbecause of the blocking of the actinbinding sites by tropomyosin.Note: Reactions are shown occurring atonly one crossbridge, but the sameprocess takes place at all or mostcrossbridges.Ca2 released from the sarcoplasmicreticulum in response to actionpotential binds to troponin, causingtropomyosin to move and expose themyosin binding site on the actinmolecule.
The crossbridge is formed.ATPADP ⬃ PiADPⴙPiADP and Pi are released, the myosinhead flexes, and the myosin and actinfilaments slide past each other.ATPaseFigure 3.15 Biochemical Mechanics of Muscle Contraction Muscle contraction is producedwhen actin and myosin form and recycle crossbridges. The process is dependent on the presence of freeintracellular Ca2+ and the availability of ATP.Ca2+ concentration. Some of these substances produce depolarization of the cell membrane, resulting in opening ofvoltage-gated membrane Ca2+ channels and release of Ca2+from intracellular stores, in a process similar to that in skeletalmuscle.
In pharmacomechanical coupling, binding of a ligandto a membrane receptor produces an increase in intracellularCa2+ concentration, and thus smooth muscle contraction,without altering membrane potential. An example is contraction of vascular smooth muscle by binding of norepinephrineor epinephrine to α-adrenergic receptors.In phasic smooth muscle, contractions are short induration, as in skeletal muscle. In tonic smooth muscle,tension may be maintained for long periods of time with littleATP utilization. In this case, myosin is dephosphorylated whileattached to actin, forming the “latch state” (see Fig. 3.18).
Inthis state, crossbridge recycling is very slow and ATP utilizationis reduced. This latch state is important in maintaining tensionin blood vessels and sphincters.Depolarization or pharmacomechanical coupling leads toelevation of intracellular Ca2+, the common signal in smoothmuscle contraction (Fig. 3.18).
In the latter case, ligandbinding activates membrane phospholipase C, which cleavesinositol bisphosphate, thereby producing IP3, which releasesCa2+ from intracellular stores. Whether elevated by membranedepolarization or activation of the inositol pathway, Ca2+binds to the protein calmodulin, and the Ca2+-calmodulinNerve and Muscle PhysiologyA. Variation in size of motor unitSmall motor units:Muscles that perform fine movements(e.g., fingers and eyes)Motorneuron45MotorneuronLarge motor units:Muscles that perform coarse movements(e.g., musclesof posture)B.
Summation of muscle response with progressive frequency of stimulationMuscle responseStimuliC. Muscle length–muscle tension relationshipZ band4Z bandMuscle greatly contracted. Thick filamentcompressed between Z bands. Thinfilaments interfere with one another. Verylittle or no tension develops on stimulation.Tension(N/m2 x 105)3SarcomereMuscle contracted, but less than above.Thin filaments partially overlap. Less thanmaximal tension develops on stimulation.2Muscle at normal resting length. All or mostcrossbridges effective. Maximal tensiondevelops on stimulation.1Muscle stretched to some extent.Fewer crossbridges effective.
Lesstension develops on stimulation.0Muscle greatly stretched. Few or nocrossbridges effective. Minimal orno tension develops on stimulation.012Sarcomere length (m)3Figure 3.16 Grading of Muscle Tension and Length–Tension Relationship The force generated when skeletal muscle is stimulated is related to the size of the motor units stimulated (A), the numberof motor units activated and the frequency of stimulation of the muscle fibers (B), and the resting length ofmuscle fibers (C).
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