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MUSCLE II Skeletal muscle Muscle force, muscle work, muscle fatigue Smooth muscle Heart muscle

MUSCLE II Skeletal muscle Muscle force, muscle work, muscle fatigue Smooth muscle Heart muscle

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MUSCLE II

Skeletal muscleMuscle force, muscle work, muscle fatigue

Smooth muscle

Heart muscle

Skeletal muscle – types of contraction

Isometric contraction• Muscle contracts against a force transducer• Muscle does not shorten during contraction

Isotonic contraction• Muscle shortens against a fixed load• Muscle shortens, tension remains constant

– Myographic curve on kymograph

Types of myographic curves - tetanus

Muscle contraction – twitch•Summation•SuperpositionTetanus•Smooth - multiple summation•Undulating – multiple superposition

Relation of muscle length to force of contraction

Resulting force of contraction is a sum of:• Active tension during contraction• Passive tension (resting tension) (sarcolemma, vessels, nerves)

Relation of velocity of contraction to load

maximal – when muscle contracts against no load

zero - when muscle contracts against maximal load

Muscle force

• Muscle force depends on the number of motor unit• Motor unit – all muscle fibers innervated by a

single motoneuron (from 2-3 up to several hundred muscle fibers)

• The total strength of the muscles in the human body is 250,000 N (Newtons).– 1 Newton (N) = 1 kg.m/s2

• Muscle force is influenced– genetically– hormonally – testosterone, anabolics

Muscle hypertrophy

Enlargement of a total mass of muscle• Increased number of myofibrils • Increased amount of mitochondrial enzymes - up

to 120% • Increased amount of ATP and kreatinphosphate -

up to 160 - 180% • Increased amount of glycogen - up to 150%• Increased amount fat deposits - up to 175-200%

Types of skeletal muscle fibers

• Fast fibers – whiteLonger fibers for great strength of contractionExtensive sarcoplasmic reticulum for rapid release of Ca2+

Large amounts of glycolytic enzymes for rapid release of energy

Less excessive blood supply (oxidative metabolism is of secondary importance)

Fewer mitochondriaAdapted for rapid, powerful contraction (jumping, sprints)

Every muscle is composed of a mixture of fast and slow muscle fibers

Types of skeletal muscle fibers

• Slow fibers - redSmaller fibersInnervated by smaller nerve fibersMore capillaries – to supply extra amount of oxygenuIncreased numbers of mitochondrias (high level of oxidative

metabolism)Large amount of myoglobin (Fe containing protein, similar to

hemoglobin - red color)Adapted for prolonged, continuous muscle activity (antigravity

muscle, long-distance races)

Every muscle is composed of a mixture of fast and slow muscle fibers

Muscle work

• Muscle work = the effect of the muscle force on a certain path (A = F . s) (J)

• Dynamic (during isotonic contraction, movement)More extensive blood supplyIntensive circulation during relaxation• Static (changes of muscle tension without the

shortening of the muscle)Lesser blood supply Limited circulation - accumulation of metabolites –

development of muscle fatigue

Muscle fatigue• Acute (recovery - within 24 hours) and chronic (may be

followed by a complete exhaustion)• Decrease force of muscle contraction• Fatigue is localized in the neuromuscular junction

– Accumulation of extracellular K+ may lead to a disturbance in depolarization, reduction of the amplitude of the action potential and conduction velocity

• Muscle fatigue is increasing parallel to decreasing amounts of muscle glycogen

• Accumulation of lactate – lower pH, increase of K+, stimulation of the free nervous endings – pain, edemas

• Contracture – long-lasting muscle contraction – without action potentials, exhaustion of ATP

Orbeli phenomenon

• Effect of epinephrine – a transitory increase of muscle force contraction

• appearance of the fatigue is postponed by the sympathetic stimulation

• Fatigue appears before the energetic reserves of the organism are exhausted

• Fatigue is a physiological protective phenomenon preventing the damage to the organism

Sources of energy for muscle contraction• ATP – maintains contraction for 1 to 2 seconds• phosphocreatine – 5 times as great as ATP, sufficient for 7-8

s contraction• Glycogen

– Enzymatic breakdown of the glycogen to pyruvate and lactate liberates energy that is used to convert ADP to ATP, glycolysis can sustain contraction for about 1 min

• Twofold importance of glycolysis– Reactions occurs in the absence of oxygen (muscle

contraction can be sustained for a short time when oxygen is not available)

– The rate of formation of ATP is 2.5 times as rapid as ATP formation with oxygene

– Accumulation of many end-products• Oxidative metabolism – the final source of energy

– 95% of all energy used by the muscle

Function of ATP

ATP is necessary for• Muscle contraction – detachment of the head of

myosin from the actin• Function of Na+/K+ pump• Function of Ca++ pumpPhysiological depletion of sources of ATP (reversible)

– contractureIrreversible loss of all ATP – rigor mortis

– Lack of energy for the separation of cross-bridges– Rigor is faster after muscle fatigue and exhaustion– Muscles remain in rigor until muscle proteins are

destroyed by autolysis (15-25 hours)

Smooth muscle - structure

Contains actin and myosin, it does not contain troponin

Dense bodies – analog of Z-lines – attachment of actin filaments

Actin – long filaments

15 times as many actin as myosin

•Contraction of smooth muscle is about 30 times slower than contraction of skeletal muscle•Great ability to shorten with full force of contraction•Some contractile units of smooth muscle have optimal overlapping of their actin and myosin filaments at one length of the muscle and others at other length

Types of smooth muscles

• Single-unit (visceral)– Hundreds to millions of muscle fibers contract together as a

single unit – syncithial smooth muscle– The cell membranes are joined by gap junction – through

which ions can flow freely from one cell to the next cell– Visceral organs of GIT, gut, bile ducts, ureters, uterus,

vessels• Multiunite

– Is composed of discrete smooth muscle fibers– Each fiber operates independently of others– Is innervated by a single nerve ending– The ciliary muscle of the eye (parasympathetic control)– The piloerector muscles (sympathetic control)

Contraction of smooth muscle

• Initiating event in smooth muscle contraction is an increase in intracelullar Ca2+ ions cause by:– Nerve stimulation– Stretch of the fiber– Hormonal stimulation– Changes in the chemical environment of the fiber

• Strength of contraction depends on extracellular Ca2+ • Removal of Ca2+ ions is achieved by calcium pump,

calcium pump is much slower in comparison with a pump of skeletal muscle – longer contraction

Mechanism of contraction

• Beginning of contraction4 Ca2+ bind with regulatory protein calmodulin

Complex Ca-calmodulin activates enzyme miosin kinase (a phosphorylating enzyme)

Light chain of of each myosin head (regulatory chain) become phosphorylated, the head has the capability of binding with the actin filaments

• Cessation of contraction:When the concentration of Ca2+ falls bellow a critical level,

all processes automatically reverse except for the fphosphorilation of myosin head

Enzyme myosin phosphatase splits the phosphate from the regulatory light chain

Smooth muscle - contraction

Smooth muscle – membrane potential

Smooth muscle has more voltage-gated calcium channels and very few voltage-gated sodium channels than skeletal m.

Importance of Ca2+ ions in generating smooth muscle action potential – phase plateau of AP, contraction

Slow wave•Resting potential –50 to –60 mV•Spontaneous slow wave (some smooth muscle is self-excitatory)•Slow wave can initiate action potentials (-35 mV)•The more AP, the stronger contraction

Contraction without action potentials

• In multiunite smooth muscle, Ca2+ ions can flow into the cell through the ligand-gated Ca2+ channel – ligand – acetylcholine, norepinephrine

• Action potentials most often do not develop• Membrane potential do not reach a critical

level for generating action potential because the Na+ pump pumps sodium ions out of the cell

Regulation of smooth muscle

Smooth muscle are regulated by autonomic nervesNerve fibers do not make direct contact with smooth muscle fibers – they formed so-called diffuse junctionTerminal axons have multiple varicosities, containing vesiculesIn the multiunite type of smooth cells, the contact junctions are similar to the end plate of skeletal muscle

Heart muscle - structure

•The cardiac muscle is striated

as skeletal muscle•Has typical myofibrils that

contain actin and myosin

filaments•Heart muscle is a syncitium --

gap junctions connection•Intercalated discs –

membrane with low resistance •AP travel from one cardiac

muscle cell to to another

Heart muscle - action potential

•Resting potential –80 to –95 mV

•Amplitude 105 mV

•Phase of plateau

•Duration 200 – 300 ms

Regulation of cardiac muscle

Sympathetic nervous system

• Norepinephrine binds to adrenergic receptors and activates Ca2+ channel (Ca2+ flows into the cell – increases positivity inside)

• Increases frequency • Increases strength of

contraction• Increases conductance• Increases excitability

Parasympathetic nervous system

• Acetylcholine binds to muscarine cholinergic receptors and activates K+ channel (K+ flow from the cell – increases negativity inside)

• Hyperpolarization makes excitable tissue much less excitable

• Slowing transmission• Decrease of frequency

Skeletal m. Smooth muscle

Sarcomere yes no

Nucleus a lot of one

Sarkopl. reticulum well developed moderately

T-tubules yes no (caveoli)Content of actomyosin higher lower

Ratio A:M 2:1 15:1

Actin filaments short long

Plasticity low high (10x)

Conductance faster slowerVelocity of contraction rapid slower

Resting potential -80 to –90 mV

-50 to –60 mV unstable

Skeletal m. Smooth muscle

Excitation-contraction coupling

Ca- TN-C

TN-I

Ca-calmodulin

myozin kinase

Cessation of contraction

Ca decrease

spontaneously

Ca decrease

myozin phosphatase

Energy from ATP high lowAutonom. contraction no yes pacemaker

Control motoneurons autonomic NS

humoral

mechanical

Fatigue yes practically not