The Muscular SystemThe Muscular System

Chapter 8Chapter 8

Structure Structure of a Skeletal Muscleof a Skeletal Muscle

Layers of fibrous connective tissue Layers of fibrous connective tissue called called fasciafascia separate an individual separate an individual skeletal muscle from adjacent skeletal muscle from adjacent muscles and hold it in positionmuscles and hold it in positionFascia may project beyond the end of the muscle forming a tendon

Cont.Cont.Epimysium – layer of con. tissue that surrounds a skeletal musclePerimysium – extends inward from the epimysium and separates the muscle tissue into small compartmentsFascicles – bundles of skeletal fibers found in compartmentsEndomysium – con. tissue that surrounds individual skeletal fibers within fascicles

Cont.Cont.Skeletal muscle fiberSkeletal muscle fiber– Thin, elongated cylinder with rounded

ends– The cytoplasm (sarcoplasm) contains

many small, oval nuclei, mitochondria, and myofibrils

– Myofibrils contain two types of protein filaments

Myosin (thick)Actin (thin)

Striation patternStriation pattern– I bands (the light bands) are composed

of thin actin filaments directly attached to Z lines

– A bands (the dark bands) are composed of thick myosin filaments overlapping thin actin filaments

– Sarcomere – segment of a myofibril that extends from one Z line to the next Z line

– Sarcoplasmic reticulum – membranous channels that surrounds each myofibrl and runs parallel to it

– Transverse tubules – extend inward as invaginations from the fiber’s membrane and passes all the way through the fiber; contains extracellular fluid

– Both activate the muscle contraction mechanism when the fiber is stimulated

Neuromuscular JunctionNeuromuscular JunctionEach skeletal muscle fiber connects to an axon from a nerve cell, called a motor neuronThe axon extends outward from the brain or spinal cordThe muscle fiber contracts only when the motor neuron stimulates itThe connection between the motor neuron and the muscle fiber is called a neuromuscular junctionMuscle fiber membrane forms a motor end plate

The end of the motor neuron branches and projects into recesses of the muscle fiber membrane

Cytoplasm at the distal ends of these Cytoplasm at the distal ends of these motor neuron axons contains motor neuron axons contains synaptic vesiclessynaptic vesicles that store that store chemicals called chemicals called neurotransmittersneurotransmitters

Summary of Muscle ContractionSummary of Muscle Contraction

Nerve impulse travels from brain to Nerve impulse travels from brain to the end of a motor neuron axon; the end of a motor neuron axon; synaptic vesicles release a synaptic vesicles release a neurotransmitter (neurotransmitter (acetylcholineacetylcholine) ) into the into the synaptic cleftsynaptic cleft between the between the neuron and the motor end plate of neuron and the motor end plate of the muscle fiber stimulating the the muscle fiber stimulating the muscle fiber to contract.muscle fiber to contract.

Role of Myosin and ActinRole of Myosin and ActinMyosinMyosin – composed of two twisted – composed of two twisted protein strands with globular parts protein strands with globular parts called cross-bridges projecting called cross-bridges projecting outward along their lengthsoutward along their lengthsActin – a globular structure with a binding site to which the myosin cross-bridges can attach– Composed of troponin and

tropomyosin

Sliding filament modelSliding filament model– Sarcomeres shorten as myosin cross-

bridge attaches to an actin binding site that is exposed by the release of calcium ions and bends slightly, pulling the actin filament

– Then the head can release, straighten, Then the head can release, straighten, combine with another binding site combine with another binding site further down the actin filament, and pull further down the actin filament, and pull againagain

Globular portions of the myosin filaments contain Globular portions of the myosin filaments contain an enzyme, an enzyme, ATPaseATPaseATPase catalyzes the breakdown of ATP to ADP and phosphate, releasing energy This energy puts the myosin cross-bridge in a This energy puts the myosin cross-bridge in a “cocked” position“cocked” positionWhen a cocked cross-bridge binds to actin, it pulls on the thin filamentFilament moves toward the center of the Filament moves toward the center of the sarcomere, and the sarcomere shortenssarcomere, and the sarcomere shortensCycle repeats as long as ATP is available and muscle fiber is stimulated to contract

VocabularyVocabularyActinActin

MyosinMyosin

Cross bridgesCross bridges

Binding sitesBinding sites

SarcomereSarcomere

TropomysiumTropomysium

TroponinTroponin

CalciumCalcium

Sarcoplasmic Sarcoplasmic reticulumreticulum

PropagationPropagation

Action potentialAction potential

PermeabilityPermeability

ChannelsChannels

Use the above vocabulary and pictures to explain Use the above vocabulary and pictures to explain the sliding filament model.the sliding filament model.

Stimulus for ContractionStimulus for Contraction

AcetylcholineAcetylcholine – neurotransmitter – neurotransmitter that stimulates skeletal muscle that stimulates skeletal muscle fibersfibers

When nerve impulses cease, two When nerve impulses cease, two events lead to muscle relaxationevents lead to muscle relaxation

1.1. Acetylcholine is decomposed by Acetylcholine is decomposed by acetylcholinesteraseacetylcholinesterase

2.2. Calcium ions transported back into the Calcium ions transported back into the sarcoplasmic reticulumsarcoplasmic reticulum

Energy Sources for ContractionEnergy Sources for ContractionExisting ATP moleculesExisting ATP molecules

Once used up cells must regenerate ATP Once used up cells must regenerate ATP from ADP and phosphatefrom ADP and phosphate

Creatine phosphateCreatine phosphate contains high contains high energy phosphate bondsenergy phosphate bonds– An enzyme in the mitochondria catalyzes the An enzyme in the mitochondria catalyzes the

synthesis of creatine phosphate synthesis of creatine phosphate – Stores excess energy released from the Stores excess energy released from the

mitochondriamitochondria

Cellular respiration of glucoseCellular respiration of glucose

Oxygen Supply and Cellular Oxygen Supply and Cellular RespirationRespiration

Glycolysis can take place in the absence of Glycolysis can take place in the absence of oxygenoxygenMore complete breakdown of glucose More complete breakdown of glucose (cellular respiration) requires oxygen(cellular respiration) requires oxygenOxygen comes from lungs and is carried Oxygen comes from lungs and is carried by rbcs and are attached to hemoglobinby rbcs and are attached to hemoglobinMyoglobinMyoglobin is synthesized in muscle cells is synthesized in muscle cells and imparts the reddish-brown color of and imparts the reddish-brown color of skeletal muscle tissue; temporarily stores skeletal muscle tissue; temporarily stores oxygenoxygen

Oxygen DebtOxygen DebtWhen skeletal muscles are used even for a When skeletal muscles are used even for a minute or two, the muscle fibers must minute or two, the muscle fibers must increasingly use anaerobic respiration to increasingly use anaerobic respiration to obtain energyobtain energyPyruvic acid used to regenerate glycolysis, Pyruvic acid used to regenerate glycolysis, but produces lactic acidbut produces lactic acidLactic acid accumulates in the muscles, Lactic acid accumulates in the muscles, diffuses into the bloodstream, and reaches diffuses into the bloodstream, and reaches the liverthe liverIn the liver, reactions occur requiring ATP, In the liver, reactions occur requiring ATP, synthesize glucosesynthesize glucose

Oxygen debt has to be repaidOxygen debt has to be repaid

May take several hoursMay take several hours

Metabolic capacity may change with Metabolic capacity may change with training (Table 8.2)training (Table 8.2)

Muscle FatigueMuscle FatigueA muscle exercised strenuously for a A muscle exercised strenuously for a prolonged period may lose its ability to prolonged period may lose its ability to contract, a condition called contract, a condition called fatiguefatigueInterruption in the muscle’s blood supply Interruption in the muscle’s blood supply or lack of acetylcholine in motor neuron or lack of acetylcholine in motor neuron axons may also cause fatigueaxons may also cause fatigueMost often caused from the accumulation Most often caused from the accumulation of lactic acid which lowers pH and fibers of lactic acid which lowers pH and fibers no longer respond to stimulationno longer respond to stimulationCramps occur when a muscle undergoes a Cramps occur when a muscle undergoes a sustained involuntary contractionsustained involuntary contraction

Heat ProductionHeat Production

More than half of the energy More than half of the energy released in cellular respiration is heatreleased in cellular respiration is heat

Blood transports heat to other Blood transports heat to other tissues, which helps maintain body tissues, which helps maintain body temperaturetemperature

Muscular ResponsesMuscular ResponsesWhen an isolated muscle fiber is exposed to a series of stimuli of increasing strength, the fiber remains unresponsive until a certain strength of stimulation is appliedMinimal strength is called the Minimal strength is called the threshold threshold stimulusstimulusSkeletal muscle fiber does not respond partially – all-or-none responseExtent of shortening depends on Extent of shortening depends on resistanceresistance

Skeletal muscle removed from an Skeletal muscle removed from an animal can be used to show how a animal can be used to show how a whole muscle responds to whole muscle responds to stimulationstimulationMuscle is mounted in a device and then stimulated electricallyWhen the muscle contracts, it pulls When the muscle contracts, it pulls on a lever, and its movement is on a lever, and its movement is recordedrecordedResulting pattern is a myogram

TwitchTwitch - single contraction that lasts - single contraction that lasts only a fraction of a secondonly a fraction of a secondLatent period – delay between the time the stimulus was applied and the time the muscle fiber responded– Shorter than 0.01 sec. in humansShorter than 0.01 sec. in humans

Period of contraction – muscle pulls at attachmentPeriod of relaxationPeriod of relaxation – muscle – muscle returns to its former lengthreturns to its former length

SummationSummation – process that combines – process that combines the force of individual twitchesthe force of individual twitchesWhen the resulting forceful, sustained contraction lacks even partial relaxation, it is called a tetanic contraction, or tetanus

ElectromyographyElectromyography

EMG measurements record the EMG measurements record the electrical activity associated with electrical activity associated with muscle contraction, so it is not a muscle contraction, so it is not a direct measure of force direct measure of force

Muscle ToneMuscle Tone

Def. – muscle undergoes some Def. – muscle undergoes some sustained contraction when it sustained contraction when it appears to be at restappears to be at restResponse to nerve impulses that originate repeatedly from the spinal cord and stimulate a few muscle fibers

Important in maintaining postureImportant in maintaining posture

Smooth MuscleSmooth MuscleLack striationsLack striationsCan change length without changing Can change length without changing tautnesstautnessCan maintain contraction longer with a Can maintain contraction longer with a given amt. of ATPgiven amt. of ATPMultiunit smooth muscle– Muscle fibers are separate– Found in the irises of the eyes and in walls of

blood vessels– Contracts only in response to stimulation by

motor nerve impulses or certain hormones

Visceral smooth muscleVisceral smooth muscle– Composed of sheets of spindle-shaped

cells in close contact with one another– Found in the walls of hollow organsFound in the walls of hollow organs– Fibers can stimulate each other– Display rhythmic pattern of repeated Display rhythmic pattern of repeated

contractionscontractions– Peristalsis – wavelike motion that

occurs in tubular organs and helps force the contents of these organs along their lengths

Cardiac MuscleCardiac MuscleFound only in the heartFound only in the heartComposed of branching, striated cells interconnectedTwitches are longer than skeletal Twitches are longer than skeletal twitchestwitchesIntercalated discs allow muscle impulses to pass freely from cell to cellSelf-exciting and rhythmicSelf-exciting and rhythmic

Skeletal Muscle ActionsSkeletal Muscle Actions

OriginOrigin – immovable end of a muscle – immovable end of a muscleInsertion – movable end of a muscle

When a muscle contracts, its When a muscle contracts, its insertion is moved toward its origininsertion is moved toward its origin

Interaction of Skeletal MusclesInteraction of Skeletal MusclesFunction in groupsFunction in groupsPrime mover (agonist) – muscle responsible for a particular body movementSynergistsSynergists – muscles that contract – muscles that contract and assist the prime moverand assist the prime moverAntagonists – muscles that resist a prime mover’s action and cause movement in the opposite direction

Muscle disordersMuscle disordersMuscular dystrophiesMuscular dystrophies – inherited diseases; – inherited diseases; progressive weakness and deterioration; caused progressive weakness and deterioration; caused by mutation of gene which codes for dystrophinby mutation of gene which codes for dystrophinCharcot-Marie-Tooth diseaseCharcot-Marie-Tooth disease - comprises a group - comprises a group of disorders that affect peripheral nerves which of disorders that affect peripheral nerves which lie outside the brain and spinal cord and supply lie outside the brain and spinal cord and supply the muscles in the limbsthe muscles in the limbsMyotonic dystrophyMyotonic dystrophy – form of muscular dystrophy; – form of muscular dystrophy; gradual reduction in strength and controlgradual reduction in strength and controlHereditary idiopathic dilated cardiomyopathyHereditary idiopathic dilated cardiomyopathy - - the the heart becomes weakened and enlarged, and becomes weakened and enlarged, and cannot pump blood efficiently cannot pump blood efficiently

Major Skeletal MusclesMajor Skeletal Muscles

Doc, what’s wrong with me?Doc, what’s wrong with me?

Create a doctor/patient interview Create a doctor/patient interview about a given muscular disorderabout a given muscular disorder

Include symptoms, description, Include symptoms, description, cause, how diagnosis is made, risk cause, how diagnosis is made, risk factors, treatment, and alternate factors, treatment, and alternate namesnames

Must have two copies of interview Must have two copies of interview and one propand one prop