BIO 238 Copyright 2010, John Wiley & Sons, Inc. Muscular System.

BIO 238

Copyright 2010, John Wiley & Sons, Inc.

Muscular System

Overview of Muscular TissueTypes of Muscular Tissue

SkeletalCardiacSmooth

Functions of Muscular TissueProducing Body Movements

Walking and runningStabilizing Body Positions

PostureMoving Substances Within the Body

Heart muscle pumping bloodMoving substances in the digestive tract

Generating heatContracting muscle produces heatShivering increases heat production

Skeletal Muscle Tissue


Skeletal Muscle TissueSo named because most skeletal muscles move bonesSkeletal muscle tissue is striated:

Alternating light and dark bands (striations) as seen when examined with a microscope

Skeletal muscle tissue works mainly in a voluntary manner Its activity can be consciously controlled

Most skeletal muscles also are controlled subconsciously to some extentEx: the diaphragm alternately contracts and relaxes

without conscious control

Skeletal Muscle Tissue

Skeletal Muscle TissueConnective Tissue Components

FasciaDense sheet or broad band of irregular connective

tissue that surrounds musclesEpimysium

The outermost layerSeparates 10–100 muscle fibers into bundles called

fasciclesPerimysium

Surrounds numerous bundles of fasciclesEndomysium

Separates individual muscle fibers from one anotherTendon

Cord that attach a muscle to a boneAponeurosis

Broad, flattened tendon

Skeletal Muscle TissueSarcolemma

The plasma membrane of a muscle cellTransverse (T) tubules

Connect the plasma membrane to the interior of the cellMuscle action potentials travel through the T tubulesEnsure that action potential excites all parts of the muscle

fiber at the same timeSarcoplasm, the cytoplasm of a muscle fiber

Sarcoplasm includes glycogen used for synthesis of ATP and a red-colored protein called myoglobin which binds oxygen molecules

Myoglobin releases oxygen when it is needed for ATP production

Skeletal Muscle TissueNerve and Blood Supply

Neurons that stimulate skeletal muscle to contract are somatic motor neurons

The axon of a somatic motor neuron typically branches many timesEach branch extending to a different skeletal

muscle fiberEach muscle fiber is in close contact with

one or more capillaries

Skeletal Muscle TissueMicroscopic Anatomy

The number of skeletal muscle fibers is set before you are bornMost of these cells last a lifetime

Muscle growth occurs by hypertrophyAn enlargement of existing muscle fibers

Testosterone and human growth hormone stimulate hypertrophy

Satellite cells retain the capacity to regenerate damaged muscle fibers

Skeletal Muscle TissueMyofibrils

Thread-like structures which have a contractile function

Sarcoplasmic reticulum (SR)Membranous sacs which encircles each myofibrilStores calcium ions (Ca2+)Release of Ca2+ triggers muscle contraction

SarcomeresCompartments of arranged filamentsBasic contractile unit of a myofibril

FilamentsFunction in the contractile processTwo types of filaments (Thick Myosin and Thin Actin)There are two thin filaments for every thick filament

Organization of Skeletal Muscle

Myofibril and Filaments

Contraction and Relaxation of Skeletal Muscle

The Sliding Filament MechanismMyosin heads attach to and “walk” along

the thin filaments at both ends of a sarcomere

Progressively pulls the thin filaments toward the center of the sarcomere

The sarcomere shortensLeads to shortening of the entire muscle

Contraction and Relaxation of Skeletal Muscle

Contraction and Relaxation of Skeletal Muscle The Neuromuscular Junction

Motor neurons have a threadlike axon that extends from the brain or spinal cord to a group of muscle fibers

Action potentials arise at the interface of the motor neuron and muscle fiber

SynapseWhere communication occurs between a somatic motor neuron

and a muscle fiber Synaptic cleft

Gap that separates the two cells Neurotransmitter

Chemical released by the initial cell communicating with the second cell

Synaptic vesiclesSacs suspended within the synaptic end bulb containing

molecules of the neurotransmitter acetylcholine (Ach) Motor end plate

The region of the muscle cell membrane opposite the synaptic end bulbs

Contains acetylcholine receptors

Contraction and Relaxation of Skeletal MuscleNerve impulses elicit a muscle action

potential in the following way(1) Release of acetylcholine

Nerve impulse arriving at the synaptic end bulbs causes many synaptic vesicles to release ACh into the synaptic cleft

(2) Activation of ACh receptorsBinding of ACh to the receptor on the motor end plate opens

an ion channelAllows flow of Na+ to the inside of the muscle cell

(3) Production of muscle action potentialThe inflow of Na+ makes the inside of the muscle fiber more

positively charged, triggering a muscle action potentialThe muscle action potential then propagates to the SR to

release its stored Ca2+

(4) Termination of ACh activityACh effects last only briefly because it is rapidly broken down

by acetylcholinesterase (AChE)

Neuromuscular JunctionsInteractions AnimationsNeuromuscular Junctions

You must be connected to the internet to run this animation.

Contraction and Relaxation of Skeletal MuscleThe contraction cycle consists of 4 steps

ATP hydrolysisReorients and energizes the myosin head

Formation of cross-bridgesOnce myosin binds to the actin, active sites become

available. Myosin head attaches to active site on actinPower stroke

The myosin head pivots (power stroke) and causes the cross-bridge rotates, sliding the filaments towards the center of the sarcomere.

Detachment of myosin from actinAs the next ATP binds to the myosin head, the myosin

head detaches from actinThe contraction cycle repeats as long as ATP is available

and the Ca2+ level is sufficiently highContinuing cycles apply the force that shortens the

sarcomere


Nerve impulse arrives ataxon terminal of motorneuron and triggers releaseof acetylcholine (ACh).

Synaptic vesiclefilled with ACh

ACh receptor

Muscle action potential

Nerveimpulse

1

ACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).



ACh receptor


Nerveimpulse

1

2 ACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).



ACh receptorAcetylcholinesterase insynaptic cleft destroysACh so another muscleaction potential does notarise unless more ACh isreleased from motor neuron.


Nerveimpulse

1

2

3

ACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).



ACh receptorAcetylcholinesterase insynaptic cleft destroysACh so another muscleaction potential does notarise unless more ACh isreleased from motor neuron. Ca2+


Nerveimpulse

SR

Muscle AP travelling alongtransverse tubule opens Ca2+

release channels in thesarcoplasmic reticulum (SR)membrane, which allowscalcium ions to flood into the sarcoplasm.

1

2

3

4

Transverse tubuleACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).





Nerveimpulse

SR

Ca2+ binds to troponin onthe thin filament, exposingthe binding sites for myosin.



1

2

3

4

5






Nerveimpulse

SR

Contraction: power strokesuse ATP; myosin heads bindto actin, swivel, and release;thin filaments are pulled towardcenter of sarcomere.




Elevated Ca2+

1

2

3

4

5

6






Nerveimpulse

SR


Ca2+ activetransport pumps

Ca2+ release channels inSR close and Ca2+ activetransport pumps use ATPto restore low level of Ca2+ in sarcoplasm.




Elevated Ca2+

1

2

3

4

5

67






Nerveimpulse

SR


Troponin–tropomyosincomplex slides back into position where it blocks the myosinbinding sites on actin.






Elevated Ca2+

1

2

3

4

5

67

8






Nerveimpulse

SR


Troponin–tropomyosincomplex slides back into position where it blocks the myosinbinding sites on actin.

Muscle relaxes.






Elevated Ca2+

1

2

3

4

9

5

67

8

Transverse tubule

Muscle MetabolismMuscle Fatigue

Inability of muscle to maintain force of contraction after prolonged activity

Factors that contribute to muscle fatigueInadequate release of calcium ions from the SRDepletion of creatine phosphate (storage form of ATP)Insufficient oxygenDepletion of glycogen and other nutrientsBuildup of lactic acid and ADPFailure of the motor neuron to release enough

acetylcholine

Muscle MetabolismOxygen Consumption After ExerciseAfter exercise, heavy breathing

continues and oxygen consumption remains above the resting level

Oxygen debtThe added oxygen that is taken into the body

after exerciseThis added oxygen is used to restore muscle

cells to the resting level in three waysConverts lactic acid into glycogenSynthesizes creatine phosphate and ATPReplaces the oxygen removed from myoglobin

Motor UnitsConsist of a motor neuron and the muscle

fibers it stimulatesThe axon of a motor neuron branches out

forming neuromuscular junctions with different muscle fibers

A motor neuron makes contact with about 150 muscle fibers

Control of precise movements consists of many small motor units

The total strength of a contraction depends on the size of the motor units and the number that are activated

Types of Skeletal Muscle Fibers

Muscle fibers contract at different speeds, and vary in how quickly they fatigue

Muscle fibers are classified into three main typesSlow oxidative fibers (Type I)Fast oxidative-glycolytic fibers (Type IIA)Fast glycolytic fibers (Type IIB)

Slow Oxidative Fibers (SO fibers)

Least powerful type of muscle fibersAppear dark red (more myoglobin)Generate ATP mainly by aerobic cellular

respirationRich blood supply and many mitochondria

Have a slow speed of contractionVery resistant to fatigueCapable of prolonged, sustained contractions for

many hoursAdapted for maintaining posture and for

aerobic, endurance-type activities such as running a marathon

Fast Oxidative–Glycolytic Fibers (FOG fibers)

Generate considerable ATP by aerobic cellular respiration (still has decent blood supply, large amount of myglobin, and many mitochondria)

Resistance to fatigue is intermediateGenerate some ATP by anaerobic glycolysisSpeed of contraction fasterContribute to activities such as walking and

sprinting

Fast Glycolytic Fibers (FG fibers)

Generate the most powerful contractionsHave low myoglobin contentRelatively few blood capillariesFew mitochondriaAppear white in colorGenerate ATP mainly by glycolysisFibers contract strongly and quicklyFatigue quicklyAdapted for intense anaerobic movements of

short durationWeight lifting or throwing a ball

Types of Skeletal Muscle FibersDistribution and Recruitment of

Different Types of FibersMost muscles are a mixture of all three types of

muscle fibersProportions vary, depending on the action of the

muscle, the person’s training regimen, and genetic factorsPostural muscles of the neck, back, and legs have a

high proportion of SO fibersMuscles of the shoulders and arms have a high

proportion of FG fibersLeg muscles have large numbers of both SO and FOG

fibers

Types of Contractions

Isotonic contractionThe tension developed remains constant while

the muscle changes its lengthUsed for body movements and for moving

objectslike picking a book up off a table

Isometric contractionThe tension generated is not enough for the

object to be moved and the muscle does not change its length

Holding a book steady using an outstretched arm

Isotonic and Isometric Contractions

Aging and Muscular TissueAging

Brings a progressive loss of skeletal muscle mass

A decrease in maximal strengthA slowing of muscle reflexesA loss of flexibility

With aging, the relative number of slow oxidative fibers appears to increase

Aerobic activities and strength training can slow the decline in muscular performance

Muscle DisordersCramps

Involuntary, painful, sustained tetanic contractions

Possible causesChemical changes in the musclePhysical blow to the muscle

FibrosisAbnormal increase of fibrous connective

tissue in muscleOccurs from connective tissue replacing dead

muscle fibers after injury

FibrositisInflammation of connective tissue in a

muscleProduces muscular rheumatism

Muscular dystrophyInherited muscular disorders characterized

by progressive degeneration of musclesMuscles weaken and atrophy, producing

progressive crippling

MyositisInflammation of muscle tissueSymptoms similar to fibrositis

Strains or “pulled muscles”Due to excessive muscle stretching

Mild strains damage only a few muscle fibers

Severe strains tear both connective and muscle tissuesSevere impairment of muscle function


HypertrophyIncrease in muscle size due repeated activity

or hormonesAtrophy

Decrease in muscle size due to lack of useContractures

Shortened, contracted muscles or muscle groups in with the muscle atrophies and shrinks while the connective tissue thickens

Seen in people that have been immobile for while


TendonitisInflammation of the tendons

FibromyalgiaCharacterized by pain, fatigue, and stiffness

in the connective tissues of the muscles, tendons, and ligaments.

Neurological Disorders Affecting MusclesBotulism

Poisoning caused by neurotoxin from bacterium Clostridium botulinum

Prevents release of acetylcholineDeath can result due to paralysis of

breathing musclesCaused by eating contaminated, improperly

canned vegetables and meat

Myasthenia gravisExtreme muscular weakness

Improper functioning of the neuromuscular junctionAutoimmune disease

Blocks the stimulatory effect of acetylcholine by reducing or blocking ACh receptors

PoliomyelitisViral disease of motor neurons in spinal cordDestruction of motor neurons leads to

paralysisPolio vaccination is available

SpasmsSudden, involuntary contractions of a muscle

or group of musclesCauses

Irritation of motor neuronsEmotional stressNeurological disorders

Muscle Spasms Effect Blood Flow

TetanusCaused by neurotoxin of anaerobic bacterium

Clostridium tetani Usually caused by puncture woundsNeurotoxin affects motor neurons

Causes continuous stimulation and tetanic contractions of certain muscles

Mortality is high without treatmentVaccination is available

BIO 238 Copyright 2010, John Wiley & Sons, Inc. Muscular System.

Documents

Transcript of BIO 238 Copyright 2010, John Wiley & Sons, Inc. Muscular System.