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CONTRACTION/EXCITATION OF SKELETAL AND

SMOOTH MUSCLES

PRESENTED BY:NIYAMAT M.A CHIMTHANAWALA

M.PH I (PHARMACOLOGY)DEPT. OF PHARMACEUTICAL SCIENCES,

NAGPUR UNIVERSITY25-Aug-1525/08/15

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CONTENTS SKELETAL MUSCLESA. Physiological anatomyB. General Mechanism C. Molecular Mechanism D. Interaction of filaments and Calcium ionsE. Energetics F. Characteristics of Whole Muscle ContractionG. Specialties of contractionH. The Neuromuscular junctionI. Molecular Biology of Ach Formation & ReleaseJ. Muscle Action PotentialK. Contraction-Excitation Coupling SMOOTH MUSCLESA. TypesB. Contractile mechanism of Smooth muscleC. Nervous and Hormonal Control of Smooth Muscle ContractionD. Effect of Local Tissue Factors and Hormones on Contraction

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PHYSIOLOGICAL ANATOMY OF SKELETAL MUSCLES

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a) Connective tissue sheaths

b) Sarcolemmac) Myofibrils-

Actin & Myosin filaments

d) Sarcoplasme) Striations,

Sarcomeresf) SR, T tubules

a) Connective Tissue sheaths■ Epimysium- an “overcoat” of dense irregular connective tissue, surrounding the whole muscle. Sometimes it blends with the deep fascia that lies between neighboring muscles or the superficial fascia deep to the skin.■ Perimysium and fascicles- (bundles of sticks), the grouped muscle fibers within each skeletal muscle. Surrounding each fascicle is a layer of fibrous connective tissue called perimysium.■ Endomysium (wispy sheath of areolar connective tissue) surrounding each individual muscle fiber.

Bright Myomesin

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PHYSIOLOGICAL ANATOMY OF SKELETAL MUSCLES

STRIATED APPEARANCE

Sarcomere-the functional unit of skeletal muscle.

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b) Sarcolemma (cell membrane of the muscle fiber)Contains outer coat of numerous thin collagen fibrils. At each end of the muscle fiber, sarcolemma fuses with tendon fiber, tendon fibers bundles form the muscle tendons that then insert into the bones.

c) Myofibrils (1–2 μm in diameter),Thick filaments 16nm diameter .Each thick filament contains about 300 myosin moleculesbundled together. Each myosin molecule 4,80,000 MW ,consists of 2 heavy 200,000 MW each & 4 light polypeptide chains 20,000 MW each, and has a rod like tail attached by a flexible hinge to two globular heads .The tail consists of two intertwined helical polypeptide heavy chains. Form cross bridges and swivel at point of attachment. Thin filaments (7–8 nm thick) of actin mostly. Kidney shapedpolypeptide subunits-globular actin or G actin, (the active sites to attachto during contraction; 42,000 MW each). G actin subunits polymerized into long actin filaments called filamentous, or F,actin. Two intertwined actin filaments, resembling a twisted double strand of pearls, form the backbone of each thin filament.Thin filaments also contain several regulatory proteins.■ Tropomyosin a rod-shaped protein, spiral about actin core helpstiffen and stabilize it. Molecules arranged end to end along the actin filaments,In relaxed muscle fiber, block myosin-binding sites on actin. ■ Troponin a globular three-polypeptide complex.(TnI) inhibitory subunit binds to actin.(TnT) binds to tropomyosin and helps position it on actin.(TnC) binds calcium ions. ■ Titin elastic element;very springy filamentous molecules 30,00,000 MW hold the filaments in place. Extends from Z attaches to M line.  d) Striations, Sarcomeres, and Myofilaments Striations, a repeating series of dark A bands and light I bands.■ A band has the H zone and M line.■ I band has the Z disc (or Z line).

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e) Sarcoplasmic Reticulum is an elaborate ER.Most SR tubules run longitudinally along the myofibril, communicating with each other at the H zone. Others called terminal cisterns (“end sacs”) form larger, perpendicular cross channels at the A band–I band junctions and they always occur in pairs.CALSEQUESTRIN-a protein in SR which binds Ca2+ 40 times more.

f) T Tubules At each A band–I band junction, the sarcolemma of the muscle cell protrudes deep into the cell interior, forming an elongated tube; increase the muscle fiber’s surface area. Fusing tubelike caveolae,the lumen (cavity) is continuous with the extracellular space. Along its length, each T tubule runs between the paired terminal cisterns of the SR, forming triads, successive groupings of the three membranous structures (terminal cistern, T tubule, and terminal cistern). As they pass from one myofibril to the next, the T tubules carry nerve impulses to deepest regions of cell.

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DIFFERENCES BETWEEN TYPES OF MUSCLES

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1 tubule/sarcomere2 tubules/sarcomere

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GENERAL MECHANISM OF MUSCLE CONTRACTION

1. ACTION POTENTIAL TRAVELS FROM NERVE ENDINGS TO MUSCLE FIBERS

2. RELEASE OF NEUROTRANSMITTER ACH3. DENSE BARS (Ca2+ RELEASE) ACH ACTS ON

RECEPTOR (Ach gated cation channel)4. ACH BINDS; LARGE INFLUX OF Na+ IONS5. A.P TRAVELS ALONG MUSCLE FIBER SIMILAR TO NERVE6. A.P TRAVELS DEEP TO S.R; Ca2+ RELEASE7. Ca2+ INITIATES CONTRACTILE PROCESS (actin-myosin

slide along each other) 8. Ca2+ PUMPS BACK INTO S.R; CONTRACTION CEASES25-Aug-15

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AFTER ACTION POTENTIAL,FORMATION

OF ACH AND ITS RELEASE

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MOLECULAR MECHANISM Sliding filament mechanism of muscle contraction Molecular Characteristics of the Contractile FilamentsA. Myosin filament 1.6 micrometres, crossbridges

(axially displaced at 120 degrees) -2 pts of attachment (hinges)

B. ATPase Activity of the Myosin Head-head acts as an ATPase enzyme

C. Actin filament 1 micrometre; 2.7nm dist. Between 2 active sites.

D. Tropomyosin molecules MW 70,000; 40nm lengthE. Troponin: role in muscle contraction; 3 subunits

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Interaction of One Myosin Filament,Two Actin Filaments, and Calcium Ions

to Cause Contraction Presence of Mg2+ and ATP –binding occurs instantly. Presence of Troponin-Tropomyosin complex on Active

sites of actin inhibits binding. Ca2+ inhibits the complex; facilitates binding and

contraction. Walk-along “ratchet theory” of contraction-tilt of the head

(power stroke) -- two ends of actin filament towards centre of myosin filament.

No. of cross bridges proportional to force of contraction. Fenn effect- amt of ATP cleaved proportional to the work

done.

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ATP as the Source of Energy for Contraction—Chemical Events in the

Motion of the Myosin Heads.

ATP cleavage- ADP + iP; the head is perpendicular but not attached to actin

Ca2+ complex binding reveals active sites; myosin binds

Head conformational change; tilt called power stroke

Tilt releases ADP; new ATP replaces it Binding of ATP leads to detachment of head

from actin

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Work Output During Muscle Contraction

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W = L x D

Efficiency of contraction is best when velocity is moderate about 30%

Sources of Energy for Muscle Contractiona. ATP 4mM –duration of full muscle contraction 1-2 secb. Phosphocreatine approx 20mM –duration of 5-8 secc. Glycogen breakdown, even in absence of O2 –few sec to a

min formation of ATP 2.5times fasterd. Oxidative metabolism of fats,proteins, carbohydrates –

duration 2-4 hrs

Relation of load to velocity of contraction in skeletal muscle with a cross section of 1 sq cm and a length of 8 cm.

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Characteristics of Whole Muscle Contraction

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1.Isometric Versus Isotonic Contraction.2.Characteristics of Isometric Twitches Recorded from Different Muscles.3.Fast Versus Slow Muscle Fibers.4.Mechanics of Skeletal Muscle Contraction5.Muscle Contractions of Different Force—Force Summation. Multiple fiber summation vs Frequency summation.6.Maximum Strength of Contraction.7.Changes in Muscle Strength at the Onset of Contraction—The8.Staircase Effect (Treppe).9.Skeletal muscle tone.

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Duration of isometric contractions for different types of mammalianskeletal muscles, showing a latent period between the actionpotential (depolarization) and muscle contraction.

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MUSCLE SPECIALTIES

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HYPERTROPHY –Total mass enlargesEither due to exertion or muscle stretched greater than normalRemodeling of muscles can occur within 2 weeks.ATROPHY-Muscle mass decreases, rate of decay of proteins high, unused for

longDENERVATION ATROPHY-Complete loss of function within 2yrsHYPERPLASIA- Extreme muscle force generationPOLIOMYELITIS- Some nerve fibers destroyedRIGOR MORTIS-Contracture; loss of ATPMYASTHENIA GRAVIS-autoimmune disease; patients develop anitbodies against

Ach activated ion channels. Death might occur due to paralysis of respiratory muscles.

CONDITION NERVE FIBERS SKELETAL MUSCLE

RESTING MEMBRANE POTENTIAL

-80 TO -90 mv -80 TO -90 mv

DURATION OF A.P 5 TIMES FASTER 1-5 msecVELOCITY OF CONDUCTION

MUCH FASTER 1/13TH THAT OF MYELINATED NERVE FIBERS

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THE NEUROMUSCULAR TRANSMISSION

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IN RELAXED STATE Ca2+ CONC 10^-7 M; CONTRACTED STATE 10^ -4 M; 500 FOLD MORECa2+ PULSE LASTS 1/20TH OF A SEC IN SKELETAL MUSCLES

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NM JUNCTION• NERVE FIBER INNERVATION TO MUSCLE FIBER;

MOTOR END PLATE• SYNAPTIC GUTTER SYNAPTIC CLEFT (30nm)

NM JUNCION

• SMALLER FOLDS AROUND MUSCLE MEMBRANE (SUB NEURAL CLEFTS)

• 3,00,000 VESICLES (40 nm)AT A TIME AT SINGLE TERMINAL END PLATE

• 10,000 MOLECULES OF ACH IN EACH VESICLE

ACH SECRETION

• 125 VESICLES RELEASE UPON STIMULUS• DENSE BARS ON INSIDE OF NERVE MEMBRANE

CONTAIN VOLTAGE GATED Ca2+ CHANNELS• Ca2+ ENTRY 100 FOLD; ACH 10,000 FOLD ENTRY• Ca2+ ENTRY PROMOTES EXOCYTOSIS

ACH GATED ION CHANNELS

• 2,75,000 MW PROTEIN COMPLEX, 5 SUBUNITS 2a,B,g,d• 2 ACH MOLECULES BIND; CONF CHANGE• 0.65 nm DIAMETER OF OPEN ACH CHANNEL• LARGE QUTY Na+ FUSE INSIDE DUE TO -VEITY

MEMBRANE

• ACH FEW MSEC IN SYNAPSE BEFORE DEGRADATION BY ACHE

• ACTION POTENTIAL GENERATED AT LOCAL AREA OF END PLATE +50 TO +75 mV

MUSCLE

• HIGH SAFETY FACTOR OF MUSCLE-3 TIMES THE AMT OF END PLATE POTENTIAL REQD

• FATIGUE DUE TO STIMUALTION GREATER THAN 100 TIMES/SEC

NM JUNCTION

• NO. OF VESICLES SUFFICIENT FOR THOUSANDS OF IMPULSES

• CLATHRIN COATED PITS FORM NEW VESICLES• ENTIRE PROCESS OCCURS IN 5-10 MSEC.

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Contraction and Excitationof Smooth Muscle

1. 1-5 micrometres diameter and 20-500 micrometres length2. Diffusion occurs at 200-300 msec rate of Ca2+ inside along the conc

gradient.3. Types- a) Multi unit smooth muscle discrete (separate fibers covered by collagen like glycoprotein)

ex:ciliiary muscles, iris muscles, piloerector muscles. Nervous stimuli b) Single unit smooth muscle (hundred thousands of muscle fibers contract together) . Non-

nervous stimuli Also called syncytial smooth muscle because of its syncytial interconnections among fibers. Also called visceral smooth muscle because it is found in the walls of most viscera of the

body, including the gut, bile ducts, ureters, uterus, and many blood vessels.4. The dense bodies of smooth muscle serve the same role as the Z discs in

skeletal muscle.5. Most myosin filaments have “sidepolar” cross-bridges; the bridges on one

side hinge in one direction and the other side hinge in the opposite direction. 80% length contracted.

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1. Slow cycling of the myosin cross-bridges- Frequency is 1/10 to 1/300th of that in skeletal muscle. Yet the fraction of time is greatly increased.2. Energy required to sustain smooth muscle contraction-Same as frequency required to maintain muscle tone.3. Slowness of onset of contraction and relaxation of the total smooth muscle tissue-A. BEGINS TO CONTRACT 50 TO 100 MILLISECONDS AFTER IT IS EXCITEDB. REACHES FULL CONTRACTION ABOUT 0.5 SECOND LATER,C. THEN DECLINES IN CONTRACTILE FORCE IN ANOTHER 1 TO 2 SECONDSD. GIVING A TOTAL CONTRACTION TIME OF 1 TO 3 SE. DUE TO VARIETY OF TYPES OF SMOOTH MUSCLE AVG TIME 0.2-30 S. THIS IS ABOUT 30 TIMES AS LONG AS SKELETAL MUSCLES4. Force of muscle contraction-MAX FORCE OF CONTRACTION OF SMOOTH MUSCLE IS 4 TO 6 KG/CM2 VS 3 TO 4 KG/CM2 FOR SKELETAL MUSCLE.

COMPARISON OF SMOOTH MUSCLE CONTRACTIONAND SKELETAL MUSCLE CONTRACTION

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5. “Latch” mechanism for prolonged holding of contractions of smooth muscle-

Even after deactivation of enzymes ,Myosin kinase and Myosin phosphatase, myosin heads remain attached for prolonged period (few hrs) creating “static” force of contraction utilizing minimal energy ATP

6. The phenomena of stress relaxation and reverse stress-relaxation Ex.urinary bladder

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REGULATION OF CONTRACTION BY CALCIUM IONS Increase in Ca2+ due to:- Nervous or Hormonal Stimulation,

stretch of the fiber or change in chemical environment. Combination of Calcium Ions with Calmodulin—causes

activation of Myosin Kinase , Phosphorylation of 1 light chain of each Myosin head, cause intermittent pulls.

Cessation of Contraction—Role of Myosin Phosphatase, splits phosphate group from head, level of Ca2+ very low.

Neuromuscular Junctions of Smooth Muscle- different from skeletal, autonomic nerve fibers innervate-secrete Ach and NA, penetration in diffuse junctions.

Ach and NA secrete from different fibers always and can be excitatory/inhibitory as per receptor protein.

Fine terminal axons have multiple varicosities ,secretion of transmitter through walls.

Contact junctions 20-30 nm wide act as the NM junction space between muscle membrane and varicosities.

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NERVOUS AND HORMONAL CONTROL OF SMOOTH MUSCLE CONTRACTION

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MECHANISM OF ACTION

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MEMBRANE POTENTIALS AND ACTIONPOTENTIALS IN SMOOTH MUSCLE

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In unitary smooth muscle-A. Spike Potential 10-50 msecB. Slow wave rhythm of smooth muscle

fibers, not self regenerating, but localized, pacemaker waves in gut.

C. It has plateau phase which can account for prolonged contraction, large no. voltage gated Ca2+ ch. more 1000 msec.(cardiac,uterus,vascular muscles)

D. Muscle stretch causes automatic generation of muscle A.P

In multi unit smooth muscle-A. The local depolarization (the junctional potential) by nerve fibers on small muscle fibers

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EFFECT OF LOCAL TISSUE FACTORS AND

HORMONES TO CAUSE SMOOTH MUSCLE

CONTRACTION WITHOUT ACTION POTENTIALS Hormones norepinephrine, epinephrine,

acetylcholine,angiotensin, endothelin, vasopressin, oxytocin, serotonin, and histamine.

Lack of O2, Excess CO2, Increased H+, Adenosine, lactic acid, increased K+, diminished Ca2+, and increased body temperature can all cause local vasodilatation.

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REFERENCES

GUYTON TEXTBOOK OF MEDICAL PHYSIOLOGY,11TH ED.

MARIEB HUMAN ANATOMY AND PHYSIOLOGY,10TH ED.

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“THE ONLY PERSON YOU SHOULD TRY TO BE BETTER THAN, IS THE PERSON YOU WERE

YESTERDAY.”

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