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Small motor units are involved in fine motor control, for example in fingers and eyes, whilelarger motor units are involved in coarser movements (A). Skeletal muscles also produce more force whenmore motor units are recruited. With repeated stimulation of a muscle fiber, summation occurs, in whichindividual twitches occur without complete relaxation between twitches (B). There is also a length–tensionrelationship, whereby greater resting sarcomere length (stretch of the muscle before contraction) is associated with greater force of contraction, up to an optimal resting length (C).446The Nervous System and MuscleTable 3.3Comparison of Muscle Structure and FunctionSkeletal MuscleCardiac MuscleSmooth MuscleMorphologyLong; cylindricalBranchedSpindle or fusiformNucleiMultiple; located peripherallyOne (sometimes two);located centrallyOne; located centrallySarcomereYes; striated patternYes; striated patternNoT tubulesYes; forms triad withsarcoplasmic reticulumYes; forms dyad withsarcoplasmic reticulumNo; caveolaeElectrical coupling of cellsNoYes; intercalated discscontain gap junctionsYes; gap junctionsRegenerationYes; via satellite cellsNoYesMitosisNoNoYesExtracellular Ca2+ required forcontractionNoYesYesRegulation of crossbridge formationCa2+ binding to troponinCa2+ binding to troponinCa2+-calmodulin activation ofmyosin kinase andphosphorylation of myosinControl of contractionMotor neuronsAutonomic nerves; βadrenergic agonistsAutonomic nerves; hormonesSummation of twitches by increasedstimulus frequencyYesNo*YesTension varies with filament overlapYesYesYesSTRUCTUREPHYSIOLOGYMajor differences in structure and function of skeletal, cardiac, and smooth muscle are indicated.*Cardiac muscle cannot be tetanized, but the force of contraction will increase at high stimulus frequency because of an increase in intracellular [Ca2+],a phenomenon termed “Treppe.”(Reprinted with permission from Hansen, J: Netter’s Atlas of Human Physiology, Philadelphia, Elsevier, 2002.)complex activates the enzyme myosin kinase, allowing interaction between myosin and actin, producing contraction asthese proteins slide past each other.
The contraction cyclecontinues as long as intracellular Ca2+ is elevated.Relaxation of Smooth MuscleSmooth muscle relaxation may also be induced by pharmacomechanical coupling, in which case various substancesstimulate a rise in intracellular cAMP or cGMP.
Ultimately,intracellular Ca2+ is reduced, causing relaxation. Anexample is relaxation produced by binding of epinephrine ornorepinephrine to β-adrenergic receptors on vascular smoothmuscle.CARDIAC MUSCLECardiac muscle is similar to skeletal muscle or smooth musclein some respects, and dissimilar in others (Fig. 3.19 and Table3.3). Contraction of skeletal muscle is under the voluntarycontrol of the central nervous system, whereas contractionof cardiac and smooth muscle is involuntary. Both unitarysmooth muscle and cardiac muscle have the capability forspontaneous electrical activity; cardiac contraction is normally under control of cardiac pacemakers cells in the sinoatrial (SA) node.
Gap junctions in cardiac muscle, as inunitary smooth muscle, allow synchronous contraction.These gap junctions in cardiac muscle are found in the intercalated disks between cells. Cardiac and skeletal muscles havehighly organized sarcomeres, leading to a striated appearance.CaveolaeNucleusMyosinContractedsmoothmuscle cellActinMitochondrionDense bodyCytoskeletonFigure 3.17 Smooth Muscle Structure Actin and myosin filaments of spindle-shaped smooth muscle cells are arranged quite differently thancontractile proteins in skeletal muscle. Actin is anchored to dense bodies within the muscle cell and to the plasma membrane, and the sliding ofmyosin and actin filaments produces contraction when intracellular Ca2+ is elevated, either by release from intracellular stores or flux through Ca2+channels. Caveolae are invaginations in the cell membrane and are a site of Ca2+ flux.Calcium influx and phosphorylationCa2+Contraction cycleCalmodulinCa2+PiChannelADPPiPiBindsactinCa-calmodulinSRADPMyosinkinaseIP3Phospholipase CReceptorLatch stateADPPiATPMyosinPiADPPiMyosinphosphataseADPPiPiPiMyosinphosphatase(dephosphorylation)ATPATPPi“Latch state”Figure 3.18 Excitation-Contraction Coupling of Smooth Muscle Binding of a ligand to the sarcolemma results in elevation of freeintracellular Ca2+, through either depolarization of the cell membrane and opening of Ca2+ channels, or activation of the enzyme phospholipase C.
Inthe latter case, cleavage of inositol trisphosphate by phospholipase C produces IP3, which binds to sites on the sarcoplasmic reticulum, causingrelease of stored Ca2+. In either case, Ca2+ binds to the protein calmodulin, which activates myosin kinase, initiating actin–myosin interaction. Thecontraction cycle continues as long as Ca2+ is elevated. The latch state occurs when myosin is dephosphorylated by myosin phosphatase.
In thisstate, contraction can be maintained without further ATP hydrolysis.48The Nervous System and MuscleFibroblastCollagenCapillaryMyofibrilsFiberNucleusBasement membraneZSarcoI BanmeredA BandHZTPlasma membrane (sarcolemma)Sarcoplasmic reticulumDesmosomesTLLMitochondrionIntercalated discTight junctionIntercellular spaceSarcoplasmPinocytotic vesiclesGlycogenLipidMyofibrilThin filamentThick filamentFigure 3.19 Schema of Structure of Cardiac Muscle The striated appearance of cardiac muscleis associated with the arrangement of the contractile proteins actin and myosin into sarcomeres, as in skeletal muscle.
A notable difference between these types of muscles is that cardiac muscle utilizes extracellularCa2+ as well as intracellular stores to initiate contraction. Another difference is that transverse tubules ofcardiac muscle form dyads with the terminal cisternae of the sarcoplasmic reticulum, as opposed to thetriads found in skeletal muscle. In addition, unlike skeletal muscle, cardiac muscle fibers are connected bygap junctions at intercalated disks between the cells, allowing the spread of depolarization from cell to cell,producing synchronous contraction of muscle.Calcium channel blockers are drugs that block voltagedependent Ca2+ channels (L-type Ca2+ channels). Thesedrugs are often used in antihypertensive therapy, based on theireffects on vascular smooth muscle. However, because they alsoblock voltage-dependent Ca2+ channels of the heart, they slowconduction in the heart and reduce myocardial contractility.Calcium channel blockers may belong to several classes ofchemicals, including dihydropyridines.
Nifedipine and amlodipine are examples of the dihydropyridine class. Voltage-gatedCa2+ channels of T-tubules in cardiac and skeletal muscle arealso referred to as dihydropyridine receptors, based on theirability to bind these drugs. Note, however, that while cardiacmuscle contractility is reduced by calcium blockers, skeletalmuscle contractility is unaffected.
Skeletal muscle is not dependent on extracellular Ca2+ for contraction.The velocity of contraction of skeletal muscle is dependenton muscle fiber type (fast twitch vs. slow twitch fibers);cardiac muscle contraction is slower than that of skeletalmuscle, but more rapid than contraction in smooth muscle.Cardiac and smooth muscle utilize both intracellular andextracellular sources of Ca2+, whereas the only Ca2+ sourcefor contraction of skeletal muscle is intracellular (from thesarcoplasmic reticulum). As in skeletal muscle, Ca2+ in cardiacmuscle binds to troponin to initiate crossbridge formation.Other aspects of cardiac muscle function, including mechanical function and regulation of pacemaker activity, are considered in Section 3.49CHAPTER4Organization andGeneral Functions of theNervous SystemThe nervous system consists of the central nervous system(CNS) and the peripheral nervous system (PNS).
The CNSincludes the brain and spinal cord. The peripheral nervoussystem includes nerves, ganglia, and sensory receptors outsidethe CNS. The peripheral nervous system can also be subdivided into sensory and motor divisions. Sensory nerves transmit information from various sensory receptors to the CNS;motor nerves transmit information from the CNS to musclesand glands, thereby controlling their activity.ing in lower pH.
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