Chapter 10



Chapter 10 Muscle Tissue10-4 Components of the Neuromuscular JunctionThe Control of Skeletal Muscle ActivityThe neuromuscular junction (NMJ)Special intercellular connection between the nervous system and skeletal muscle fiberControls calcium ion release into the sarcoplasm10-4 Components of the Neuromuscular JunctionExcitation–Contraction CouplingAction potential reaches a triadReleasing Ca2+Triggering contraction Requires myosin heads to be in “cocked” positionLoaded by ATP energy10-4 Skeletal Muscle Contraction The Contraction CycleContraction Cycle BeginsActive-Site Exposure Cross-Bridge Formation Myosin Head Pivoting Cross-Bridge Detachment Myosin Reactivation 10-4 Skeletal Muscle ContractionFiber ShorteningAs sarcomeres shorten, muscle pulls together, producing tensionMuscle shortening can occur at both ends of the muscle, or at only one end of the muscleThis depends on the way the muscle is attached at the ends10-4 Skeletal Muscle RelaxationRelaxationContraction durationDepends on:Duration of neural stimulusNumber of free calcium ions in sarcoplasmAvailability of ATP10-4 Skeletal Muscle RelaxationRelaxationCa2+ concentrations fallCa2+ detaches from troponinActive sites are re-covered by tropomyosinRigor MortisA fixed muscular contraction after deathCaused when:Ion pumps cease to function; ran out of ATPCalcium builds up in the sarcoplasm10-4 Skeletal Muscle Contraction and RelaxationSummarySkeletal muscle fibers shorten as thin filaments slide between thick filamentsFree Ca2+ in the sarcoplasm triggers contractionSR releases Ca2+ when a motor neuron stimulates the muscle fiber Contraction is an active processRelaxation and return to resting length are passive10-5 Tension Production and Contraction TypesTension Production by Muscles FibersAs a whole, a muscle fiber is either contracted or relaxed Depends on:The number of pivoting cross-bridgesThe fiber’s resting length at the time of stimulationThe frequency of stimulation10-5 Tension Production and Contraction TypesTension Production by Muscles FibersLength–Tension RelationshipsNumber of pivoting cross-bridges depends on:Amount of overlap between thick and thin fibersOptimum overlap produces greatest amount of tensionToo much or too little reduces efficiencyNormal resting sarcomere lengthIs 75 to 130 percent of optimal length10-5 Tension Production and Contraction TypesTension Production by Muscle FibersThe Frequency of StimulationA single neural stimulation produces:A single contraction or twitch Which lasts about 7–100 msec.Sustained muscular contractionsRequire many repeated stimuli10-5 Tension Production and Contraction TypesTension Production by Muscle FibersTwitches Latent periodThe action potential moves through sarcolemmaCausing Ca2+ release Contraction phase Calcium ions bindTension builds to peak Relaxation phase Ca2+ levels fallActive sites are covered and tension falls to resting levels10-5 Tension Production and Contraction TypesTension Production by Muscle FibersTreppeA stair-step increase in twitch tensionRepeated stimulations immediately after relaxation phaseStimulus frequency 50/secondCauses a series of contractions with increasing tension10-5 Tension Production and Contraction TypesTension Production by Muscle FibersWave summationIncreasing tension or summation of twitches Repeated stimulations before the end of relaxation phaseStimulus frequency 50/secondCauses increasing tension or summation of twitches10-5 Tension Production and Contraction TypesTension Production by Muscle FibersIncomplete tetanusTwitches reach maximum tensionIf rapid stimulation continues and muscle is not allowed to relax, twitches reach maximum level of tensionComplete tetanusIf stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction10-5 Tension Production and Contraction TypesTension Production by Skeletal MusclesDepends on:Internal tension produced by muscle fibersExternal tension exerted by muscle fibers on elastic extracellular fibersTotal number of muscle fibers stimulated10-5 Tension Production and Contraction TypesMotor Units and Tension ProductionMotor units in a skeletal muscle:Contain hundreds of muscle fibers That contract at the same timeControlled by a single motor neuron10-5 Tension Production and Contraction TypesMotor Units and Tension ProductionRecruitment (multiple motor unit summation)In a whole muscle or group of muscles, smooth motion and increasing tension are produced by slowly increasing the size or number of motor units stimulatedMaximum tensionAchieved when all motor units reach tetanusCan be sustained only a very short time10-5 Tension Production and Contraction TypesMotor Units and Tension ProductionSustained tensionLess than maximum tensionAllows motor units to rest in rotation Muscle toneThe normal tension and firmness of a muscle at restMuscle units actively maintain body position, without motion Increasing muscle tone increases metabolic energy used, even at rest10-5 Tension Production and Contraction TypesMotor Units and Tension ProductionContractions are classified based on pattern of tension productionIsotonic contraction Isometric contraction10-5 Tension Production and Contraction TypesIsotonic ContractionSkeletal muscle changes lengthResulting in motionIf muscle tension load (resistance):Muscle shortens (concentric contraction)If muscle tension load (resistance):Muscle lengthens (eccentric contraction)10-5 Tension Production and Contraction TypesIsometric ContractionSkeletal muscle develops tension, but is prevented from changing lengthiso- same, metric measure10-5 Tension Production and Contraction TypesLoad and Speed of ContractionAre inversely relatedThe heavier the load (resistance) on a muscle:The longer it takes for shortening to beginAnd the less the muscle will shorten10-5 Tension Production and Contraction TypesMuscle Relaxation and the Return to Resting LengthElastic forcesThe pull of elastic elements (tendons and ligaments)Expands the sarcomeres to resting lengthOpposing muscle contractionsReverse the direction of the original motionAre the work of opposing skeletal muscle pairs10-5 Tension Production and Contraction TypesMuscle Relaxation and the Return to Resting LengthGravityCan take the place of opposing muscle contraction to return a muscle to its resting state10-6 Energy to Power ContractionsATP Provides Energy for Muscle ContractionSustained muscle contraction uses a lot of ATP energyMuscles store enough energy to start contractionMuscle fibers must manufacture more ATP as needed10-6 Energy to Power ContractionsATP and CP ReservesAdenosine triphosphate (ATP) The active energy moleculeCreatine phosphate (CP)The storage molecule for excess ATP energy in resting muscleEnergy recharges ADP to ATPUsing the enzyme creatine kinase (CK) When CP is used up, other mechanisms generate ATP10-6 Energy to Power ContractionsATP GenerationCells produce ATP in two ways Aerobic metabolism of fatty acids in the mitochondria Anaerobic glycolysis in the cytoplasm10-6 Energy to Power ContractionsAerobic MetabolismIs the primary energy source of resting musclesBreaks down fatty acids Produces 34 ATP molecules per glucose moleculeGlycolysisIs the primary energy source for peak muscular activityProduces two ATP molecules per molecule of glucoseBreaks down glucose from glycogen stored in skeletal muscles10-6 Energy to Power ContractionsEnergy Use and the Level of Muscular ActivitySkeletal muscles at rest metabolize fatty acids and store glycogen During light activity, muscles generate ATP through anaerobic breakdown of carbohydrates, lipids, or amino acids At peak activity, energy is provided by anaerobic reactions that generate lactic acid as a by-product10-6 Energy to Power ContractionsMuscle FatigueWhen muscles can no longer perform a required activity, they are fatiguedResults of Muscle FatigueDepletion of metabolic reservesDamage to sarcolemma and sarcoplasmic reticulumLow pH (lactic acid)Muscle exhaustion and pain10-6 Energy to Power ContractionsThe Recovery PeriodThe time required after exertion for muscles to return to normal Oxygen becomes availableMitochondrial activity resumes10-6 Energy to Power ContractionsLactic Acid Removal and RecyclingThe Cori CycleThe removal and recycling of lactic acid by the liver Liver converts lactate to pyruvateGlucose is released to recharge muscle glycogen reserves10-6 Energy to Power ContractionsThe Oxygen DebtAfter exercise or other exertion:The body needs more oxygen than usual to normalize metabolic activities Resulting in heavy breathingAlso called excess postexercise oxygen consumption (EPOC)10-6 Energy to Power ContractionsHeat Production and LossActive muscles produce heatUp to 70 percent of muscle energy can be lost as heat, raising body temperatureChapter 12 Neural TissueAn Introduction to the Nervous SystemLearning Outcomes12-1Describe the anatomical and functional divisions of the nervous system.12-2Sketch and label the structure of a typical neuron, describe the functions of each component, and classify neurons on the basis of their structure and function.12-3Describe the locations and functions of the various types of neuroglia.An Introduction to the Nervous SystemLearning Outcomes12-4Explain how the resting membrane potential is created and maintained.12-5Describe the events involved in the generation and propagation of an action potential.12-6Discuss the factors that affect the speed with which action potentials are propagated. An Introduction to the Nervous SystemLearning Outcomes12-7Describe the structure of a synapse, and explain the mechanism involved in synaptic activity.12-8Describe the major types of neurotransmitters and neuromodulators, and discuss their effects on postsynaptic membranes.12-9Discuss the interactions that enable information processing to occur in neural tissue. An Introduction to the Nervous SystemThe Nervous SystemIncludes all neural tissue in the bodyNeural tissue contains two kinds of cells Neurons Cells that send and receive signals Neuroglia (glial cells) Cells that support and protect neurons An Introduction to the Nervous SystemOrgans of the Nervous SystemBrain and spinal cordSensory receptors of sense organs (eyes, ears, etc.)Nerves connect nervous system with other systems12-1 Divisions of the Nervous SystemAnatomical Divisions of the Nervous SystemCentral nervous system (CNS)Peripheral nervous system (PNS)12-1 Divisions of the Nervous SystemThe Central Nervous System (CNS)Consists of the spinal cord and brain Contains neural tissue, connective tissues, and blood vesselsFunctions of the CNS are to process and coordinate:Sensory data from inside and outside bodyMotor commands control activities of peripheral organs (e.g., skeletal muscles)Higher functions of brain: intelligence, memory, learning, emotion12-1 Divisions of the Nervous SystemThe Peripheral Nervous System (PNS)Includes all neural tissue outside the CNSFunctions of the PNSDeliver sensory information to the CNS Carry motor commands to peripheral tissues and systems12-1 Divisions of the Nervous SystemThe Peripheral Nervous System (PNS)Nerves (also called peripheral nerves)Bundles of axons with connective tissues and blood vessels Carry sensory information and motor commands in PNS Cranial nerves – connect to brainSpinal nerves – attach to spinal cord12-1 Divisions of the Nervous SystemFunctional Divisions of the PNSAfferent divisionCarries sensory information From PNS sensory receptors to CNSEfferent divisionCarries motor commandsFrom CNS to PNS muscles and glands12-1 Divisions of the Nervous SystemFunctional Divisions of the PNS Receptors and effectors of afferent divisionReceptors Detect changes or respond to stimuliNeurons and specialized cells Complex sensory organs (e.g., eyes, ears)EffectorsRespond to efferent signals Cells and organs12-1 Divisions of the Nervous SystemFunctional Divisions of the PNSThe efferent division Somatic nervous system (SNS)Controls voluntary and involuntary (reflexes) skeletal muscle contractions12-1 Divisions of the Nervous SystemFunctional Divisions of the PNSThe efferent division Autonomic nervous system (ANS) Controls subconscious actions, contractions of smooth muscle and cardiac muscle, and glandular secretionsSympathetic division has a stimulating effectParasympathetic division has a relaxing effect12-2 NeuronsNeuronsThe basic functional units of the nervous systemThe structure of neuronsThe multipolar neuronCommon in the CNS Cell body (soma)Short, branched dendritesLong, single axon12-2 NeuronsThe Cell BodyLarge nucleus and nucleolus Perikaryon (cytoplasm)Mitochondria (produce energy)RER and ribosomes (produce neurotransmitters)12-2 NeuronsThe Cell BodyCytoskeleton Neurofilaments and neurotubules in place of microfilaments and microtubulesNeurofibrils: bundles of neurofilaments that provide support for dendrites and axonNissl bodiesDense areas of RER and ribosomesMake neural tissue appear gray (gray matter)12-2 NeuronsDendritesHighly branched Dendritic spines Many fine processesReceive information from other neurons80–90 percent of neuron surface area 12-2 NeuronsThe axonIs longCarries electrical signal (action potential) to targetAxon structure is critical to function12-2 NeuronsStructures of the AxonAxoplasm Cytoplasm of axonContains neurofibrils, neurotubules, enzymes, organelles Axolemma Specialized cell membraneCovers the axoplasm12-2 NeuronsStructures of the AxonAxon hillockThick section of cell bodyAttaches to initial segment Initial segmentAttaches to axon hillock12-2 NeuronsStructures of the AxonCollateralsBranches of a single axonTelodendriaFine extensions of distal axonAxon terminalsTips of telodendria12-2 NeuronsThe Structure of NeuronsThe synapseArea where a neuron communicates with another cell12-2 NeuronsThe Structure of NeuronsThe synapsePresynaptic cellNeuron that sends messagePostsynaptic cellCell that receives messageThe synaptic cleftThe small gap that separates the presynaptic membrane and the postsynaptic membrane12-2 NeuronsThe SynapseThe synaptic terminalIs expanded area of axon of presynaptic neuronContains synaptic vesicles of neurotransmitters12-2 NeuronsNeurotransmittersAre chemical messengersAre released at presynaptic membraneAffect receptors of postsynaptic membrane Are broken down by enzymesAre reassembled at axon terminal12-2 NeuronsRecycling NeurotransmittersAxoplasmic transport Neurotubules within the axon Transport raw materialsBetween cell body and axon terminalPowered by mitochondria, kinesin, and dynein12-2 NeuronsTypes of SynapsesNeuromuscular junctionSynapse between neuron and muscleNeuroglandular junctionSynapse between neuron and gland12-2 NeuronsStructural Classification of NeuronsAnaxonic neuronsFound in brain and sense organs Bipolar neuronsFound in special sensory organs (sight, smell, hearing)Unipolar neuronsFound in sensory neurons of PNSMultipolar neuronsCommon in the CNSInclude all skeletal muscle motor neurons12-2 NeuronsAnaxonic NeuronsSmallAll cell processes look alikeBipolar NeuronsAre smallOne dendrite, one axon12-2 NeuronsUnipolar NeuronsAlso called pseudounipolar neuronsHave very long axonsFused dendrites and axon Cell body to one sideMultipolar NeuronsHave very long axonsMultiple dendrites, one axon12-2 NeuronsThree Functional Classifications of Neurons Sensory neuronsAfferent neurons of PNS Motor neuronsEfferent neurons of PNS InterneuronsAssociation neurons12-2 NeuronsFunctions of Sensory NeuronsMonitor internal environment (visceral sensory neurons)Monitor effects of external environment (somatic sensory neurons)Structures of Sensory NeuronsUnipolarCell bodies grouped in sensory gangliaProcesses (afferent fibers) extend from sensory receptors to CNS12-2 NeuronsThree Types of Sensory Receptors InteroceptorsMonitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive) Internal senses (taste, deep pressure, pain) ExteroceptorsExternal senses (touch, temperature, pressure)Distance senses (sight, smell, hearing) ProprioceptorsMonitor position and movement (skeletal muscles and joints)12-2 NeuronsMotor NeuronsCarry instructions from CNS to peripheral effectors Via efferent fibers (axons)12-2 NeuronsMotor NeuronsTwo major efferent systemsSomatic nervous system (SNS)Includes all somatic motor neurons that innervate skeletal musclesAutonomic (visceral) nervous system (ANS)Visceral motor neurons innervate all other peripheral effectors Smooth muscle, cardiac muscle, glands, adipose tissue 12-2 NeuronsMotor NeuronsTwo groups of efferent axonsSignals from CNS motor neurons to visceral effectors pass synapses at autonomic ganglia dividing axons into:Preganglionic fibers Postganglionic fibers12-2 NeuronsInterneuronsMost are located in brain, spinal cord, and autonomic gangliaBetween sensory and motor neuronsAre responsible for:Distribution of sensory informationCoordination of motor activityAre involved in higher functionsMemory, planning, learning12-3 NeurogliaNeurogliaHalf the volume of the nervous systemMany types of neuroglia in CNS and PNS12-3 NeurogliaFour Types of Neuroglia in the CNS Ependymal cells Cells with highly branched processes; contact neuroglia directly Astrocytes Large cell bodies with many processes Oligodendrocytes Smaller cell bodies with fewer processes Microglia Smallest and least numerous neuroglia with many fine-branched processes12-3 NeurogliaEpendymal CellsForm epithelium called ependymaLine central canal of spinal cord and ventricles of brainSecrete cerebrospinal fluid (CSF)Have cilia or microvilli that circulate CSFMonitor CSFContain stem cells for repair12-3 NeurogliaAstrocytesMaintain blood–brain barrier (isolates CNS)Create three-dimensional framework for CNSRepair damaged neural tissueGuide neuron developmentControl interstitial environment12-3 NeurogliaOligodendrocytesMyelinationIncreases speed of action potentialsMyelin insulates myelinated axons Makes nerves appear white 12-3 NeurogliaOligodendrocytes Nodes and internodesInternodes – myelinated segments of axon Nodes (also called nodes of Ranvier)Gaps between internodesWhere axons may branch12-3 NeurogliaMyelinationWhite matterRegions of CNS with many myelinated nerves Gray matterUnmyelinated areas of CNS12-3 NeurogliaMicrogliaMigrate through neural tissueClean up cellular debris, waste products, and pathogens12-3 NeurogliaNeuroglia of the Peripheral Nervous SystemGangliaMasses of neuron cell bodies Surrounded by neurogliaFound in the PNS12-3 NeurogliaNeuroglia of the Peripheral Nervous SystemSatellite cells Also called amphicytesSurround gangliaRegulate environment around neuron12-3 NeurogliaNeuroglia of the Peripheral Nervous SystemSchwann cells Also called neurilemma cellsForm myelin sheath (neurilemma) around peripheral axonsOne Schwann cell sheaths one segment of axonMany Schwann cells sheath entire axon12-3 NeurogliaNeurons and NeurogliaNeurons perform:All communication, information processing, and control functions of the nervous system Neuroglia preserve:Physical and biochemical structure of neural tissueNeuroglia are essential to: Survival and function of neurons12-3 NeurogliaNeural Responses to InjuriesWallerian degeneration Axon distal to injury degeneratesSchwann cellsForm path for new growthWrap new axon in myelin12-3 NeurogliaNerve Regeneration in CNSLimited by chemicals released by astrocytes that:Block growth Produce scar tissue ................
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