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Smooth Muscles - Physiology - Lecture Notes, Study notes of Physiology

Dhirubhai Ambani Institute of Information and Communication TechnologyPhysiology

Very well accomplished lecture for Physiology students. This lecture handout includes: Smooth Muscles, Multi-Unit Smooth Muscle, Single Unit Muscle Fibers, Phosphorylation, Enzyme, Fibers, Caveoli

Typology: Study notes

2013/2014

Uploaded on 01/31/2014

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SmoothMuscles:
Theymadeupofmusclesfiberswith2to10micrometersofdiameter,andthelengthbetween50to
200micrometers.Theyaresmallerthanstriatedmuscles,theyhaveonlyonenucleus,theycontain
thesamecontractilefibers,actineandtropo‐myosin,buttheydon’thavetroponine,andtheamount
ofcontractilefibersismuchlowerbycomparisationwithstriatedmuscles.1gofsmoothmuscle
containsonly10%oftheamountofmyosininstriatedmuscles.MyosinhasalowerATPaseactivity,
until1000timelowerthanstriatedmuscles;thisisalsoareasonwhytheycancontractlonger.
Contractilefibersarenotorganizedinsarcomers,that’swhytheappearanceisnotstriatedunderthe
opticalmicroscope.Actinfilamentsaregatheredbymeansofsomestructurenameddensebodies,
around50filamentsofactinaregatheredandamongtheseactinfilamentstherearethickfilaments
ofmyosin.Densebodiesalsoattachactinfilamentstosarcolemma.Sarcoplasmicreticulumisnotso
welldevelopedasincaseofstriatedmuscles,andinthiscasetherearedepressionsatthelevelof
cellmembranecalledcaveoli.
Smoothmusclesareclassifiedinmulti‐unitandsingle‐unitsmoothmuscles.
Multi‐unitsmoothmuscle:Thiskinfofmusclefibersarefoundinthewallofthevesselsinthelevel
oftheexternalpartofthe
Inthecilliarymuscleofeyeandpiroerectormusclesofear.Thistypeofsmoothmuscledonot
contractspontinuouslybycomparisaonwithsingleunitsmoothmuscle.Theyhavearichsympathic
andparasympathicinnervation,andthistypeoffiberscontractindividualy.Thenerveending
innervatingthistypeoffibersareoftwotypesnamely:
‐ Nerveendinghavingontheirlengthsomedilatedportionscalledvaricositieswhocontain
vesiculesofneuromediatorandtheycomeincontactwithmusclesfibers.Thereisatight
contactbetweenthevaricositiesandthemusclesfibers,theneurotransmittercanbe
noradrenalineoracetylcholine.Whentheimpulsereachesthevaricositiesneurostransmiter
isreleasedandcontractionofthemusclefibersisproduced.Butbecausemediatoris
releasedfromseveralvaricosities,inthiswayseveralmusclefiberswhobelongstodifferent
motorunitcancontractatthesametime.Speciallyfoundatthelevelofvesselsanddifferent
ducts,whereaveryrapidresponseisnecessary.
‐ Thesecondtypeofnervousendings:Donotcomedirectlyincontactwiththesmooth
muscle,themediatorhastocrossacertaindistanceuntiltheyreachthesmoothfibers,that’s
whycontractiondonotoccurimmediatelyafterthereleaseofthemediator.Thelatencyisof
150milliseconds.
Singleunitmusclefibers:Betweenthecellsthereisaveryclosecontact,andthatswhythey
reacttogetherwhentheyarestimulatedasiftheyareasinglecell.Betweencellstherearesome
gapjunctionsbywhichionscanpasseasilyfromamuscletoanother,tha’twhymultiplecell
contractatthesametimeasiftheywereasinglecelle,thistypeofstructureiscalledsyncytium.
Theycarefoundincavitaryorgans,repiratorypathways,indigestivetract,uterus,ureters,atthe
levelofbloodvessels(intheinnerpart).Thesekindoffiberscancontractspontaneously,or
undertheactionornervousimpulsesorendocrinestimuli,orundertheactionofstretching.The
valueofrestngmemebranepotentialisaround‐60,‐50mV,butthelevelofrestingmembrane
potentialisnotstable,thereisafluctuation,it’slikeanondulatedline.At‐35mV,thenanaction
potentialisinitiated.
Incaseofsmoothmusclefibers,depolarizationandcontractionisproducedbycalcium.When
thedepolarizationwavereachescaveoli,calciumchannelsareopenedandcalciumpassesinside
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Smooth Muscles : They made up of muscles fibers with 2 to 10 micrometers of diameter, and the length between 50 to 200 micrometers. They are smaller than striated muscles, they have only one nucleus, they contain the same contractile fibers, actine and tropo‐myosin, but they don’t have troponine, and the amount of contractile fibers is much lower by comparisation with striated muscles. 1g of smooth muscle contains only 10% of the amount of myosin in striated muscles. Myosin has a lower ATPase activity, until 1000 time lower than striated muscles; this is also a reason why they can contract longer. Contractile fibers are not organized in sarcomers, that’s why the appearance is not striated under the optical microscope. Actin filaments are gathered by means of some structure named dense bodies, around 50 filaments of actin are gathered and among these actin filaments there are thick filaments of myosin. Dense bodies also attach actin filaments to sarcolemma. Sarcoplasmic reticulum is not so well developed as in case of striated muscles, and in this case there are depressions at the level of cell membrane called caveoli. Smooth muscles are classified in multi‐unit and single‐unit smooth muscles. Multi‐unit smooth muscle : This kinf of muscle fibers are found in the wall of the vessels in the level of the external part of the In the cilliary muscle of eye and piro erector muscles of ear. This type of smooth muscle do not contract spontinuously by comparisaon with single unit smooth muscle. They have a rich sympathic and parasympathic innervation, and this type of fibers contract individualy. The nerve ending innervating this type of fibers are of two types namely : ‐ Nerve ending having on their length some dilated portions called varicosities who contain vesicules of neuro mediator and they come in contact with muscles fibers. There is a tight contact between the varicosities and the muscles fibers, the neuro transmitter can be noradrenaline or acetyl choline. When the impulse reaches the varicosities neuro stransmiter is released and contraction of the muscle fibers is produced. But because mediator is released from several varicosities, in this way several muscle fibers who belongs to different motor unit can contract at the same time. Specially found at the level of vessels and different ducts, where a very rapid response is necessary. ‐ The second type of nervous endings: Do not come directly in contact with the smooth muscle, the mediator has to cross a certain distance until they reach the smooth fibers, that’s why contraction do not occur immediately after the release of the mediator. The latency is of 150 milliseconds. Single unit muscle fibers: Between the cells there is a very close contact, and that s why they react together when they are stimulated as if they are a single cell. Between cells there are some gap junctions by which ions can pass easily from a muscle to another, tha’t why multiple cell contract at the same time as if they were a single celle, this type of structure is called syncytium. They care found in cavitary organs, repiratory pathways, in digestive tract, uterus, ureters, at the level of blood vessels (in the inner part). These kind of fibers can contract spontaneously, or under the action or nervous impulses or endocrine stimuli, or under the action of stretching. The value of restng memebrane potential is around ‐60, ‐50mV, but the level of resting membrane potential is not stable, there is a fluctuation, it’s like an ondulated line. At ‐35mV, then an action potential is initiated. In case of smooth muscle fibers, depolarization and contraction is produced by calcium. When the depolarization wave reaches caveoli, calcium channels are opened and calcium passes inside

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according to it’s gradient concentration. Outside it’s fond 10^‐ 3 mmol/liter. Inside of cells calcium is found at 10^‐ 7 mmol/liter. Calmoduline, a polypeptide, having 48 amino‐acids, calcium that’s penetrates in smooth muscle is bind by calmoduline. Each molecule can bind 4 molecules of calcium. This complex activates an enzyme : miokinase, which belong to the light chains of globular heads of myosin. This enzyme has the role to phosphorylate the globular head of myosin, and when it’s phosphorylated, only then myosin can interact with actin. The phosphorylation of globular heads occurs under the action of another enzyme named myosin phosphatase. Only after globular heads are dephosphorylated, only then myosin globular head can detach from actin filament. When the muscle fibers are stretched, they’ll react by contraction. Example the stomach in the digestive tract Under the action of estrogen, smooth muscle fibers of uterus contact, under the action of progesterone uterus is relaxed.

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