20457035 muscle physiology 2

8/8/2019 20457035 Muscle Physiology 2

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MUSCLE PHYSIOLOGY

Characteristics of Muscle

Excitability - responds to Stimuli (eg. nervous)

Contractibility - able to shorten in length

Extensibility - stretches when pulled.

Elasticity - able to return to original shape andlength after contraction or extension


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F UNCTION O F MUSCLE

Motion

Maintenance of posture

Heat Production


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TYPES O F MUSCLES

SkeletalStriated muscleVoluntary muscle

Attached to bones and moves skeleton

SmoothNon striated muscleInvoluntary muscleMuscle of viscera (blood vessel wall, other hollow

structures/organs).CardiacStriatedInvoluntary


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MUSCLE CELLS

Consist of myofibrils

Myofibrils - made up of myofilaments- 2 types of myofilamentsThick myofilamentsThin myofilaments

Cell Membrane - called sarcolemma


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SARCOLEMMA

Has holes that lead into transverse

tubulesT - tubules - conduct impulses from

cell surface (sarcolemma) down into

the cell to the sarcoplasmic reticulum


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SARCOMERE

RELAXED

A bandCONTRACTED

H band I band

Actin M Line Myosin

Z-line


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SARCOPLASMIC RETICULUM

Primary function = Store Ca++ ions

Closely associated with myofilaments

Active pumps that transport Ca++ fromsarcoplasm into sarcoplasmic reticulum.

Relaxed muscle - very high Ca++ inSR low in sarcoplasm.


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THICK MYO F ILAMENT

Thick myosin filament (myosin molecules )

Tail

Head - 2 binding sitesa) ATP binding site - energyb) Actin binding site

Hinge - Junction between head and tail- Allows head to swivel back tocause muscle contraction


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THICK MYO F ILAMENT

Tail

ATP bindingsite

Actin binding site

Light Chain

(have regulatory function)Hinge


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SARCOTUBULAR HEAD

Ryanodine receptor

Ca++

Terminal Cisterna

Sarcoplasmicreticulum

Dihydropyridine receptor

T- Tubule


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THIN MYO F ILAMENT

G -actin molecules

Tropomycin

Troponin


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ACTIN INTERACTION

Actin combines with Myosin HeadATP in myosin head breaks down to ADP(ATP - ADP + P)

Energy released - Myosin head swivel.


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TROPOMYOSIN

Relaxed muscle : Myosin heads in contactwith tropomyosin.

As long as Myosin heads remain in contactwith tropomyosin, muscle remains relaxed.


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TROPONIN

Troponin I - tightly bound to myosin and tropomyosin inrelaxed muscle.

- Inhibits interaction between actin and Myosin- Covers sites where myosin heads bind to actin.

Troponin C - Binding site for Ca++- Ca++ weakens binding of troponin I to actinand moves tropomyosin laterally, exposingbinding sites for myosin heads


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MUSCLE CONTRACTION I

1 . Nerve impulse transferred from neuron tosarcolemma.

2. Acetyl Choline released at motor end-plate

3. ACh bind to nicotinic acetyl choline receptor

4. Increased Na+ and K+ conductance atmotor end plate - End plate potential.

5. Action Potential in muscle fibres


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MUSCLE CONTRACTION 2

6 . Inward spread of depolarisation down T-tubules

- activates sarcoplasmic reticulum via dihydro-

pyridine receptor.

7. Release of Ca++ from terminal cisternae into

sarcoplasm via Ryanodine receptor.

8. Binding of Ca++ to Troponin C, exposing myosin

binding sites on action.9. Formation of cross-linkages between actin and

myosin.

10. Sliding of thin and thick filaments p shortening


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MUSCLE RELAXATION

1 . Sarcoplasmic Ca ++ pumped back intosarcoplasmic reticulum (active Ca Pump)

2. Release of Ca ++ from Troponin

3. Interaction between actin and myosin ceases


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ACTIN MYOSIN CYCLEActin Myosin

Myosin ADP Pi+ potential energy

Myosin - ATP ACTIN MYOSIN ADP-Pi+ Potential energy

ACTIN - MYOSIN

Energisedcrossbridge

Contraction

ADP + Pi+ heat

Ca++

ACTINCa++

Actin


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MYOSIN ACTIN CONTRACTION

Stage 1 : Myosin still attached to actin ATP binds

to Myosin, Myosin released from Actin -

Ca++ released.

Stage 2 : ATP hydrolysed

Myosin energised but ADP & Pi still

bound to myosin.

Stage 3 : Myosin head reattached in presence of Ca++ and Actin.

Stage 4 : Contraction or Power stroke.

ADP + Pi released.


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EXCITATION - COUPLING

Sarcoplasmic Ca++ concentration

resting state = 10 -7 10 -8 mol/Lcontraction = 10 -5

During relaxation reuptake bysarcoplasm (energy required)troponin C release Ca++Myosin detaches from Actin


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SINGLE MUSCLE CONTRACTION

Muscle contraction = Combined response of

many individual muscle fibres.

= Temporal summation of

response fibres to multiple action potential.

= Tension developed varies with time.= Length may change depending on load


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TYPES O F MUSCLE CONTRACTION

Isometric - Contraction with no change in length

Isotonic - Contraction with constant load but

shortening length.

Lengthening contraction - Contraction in whichthe external load is greater than tension

developed by muscle, causing muscleto increase in length in spite of contraction


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ISOMETRIC TWITCH /CONTRACTION

tl

0 50 100 150

tl = Content periodtc = Contraction time

Tension

msecs

tc


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ISOTONIC TWITCH/CONTRACTION

0 50 100 150

Light load

Heavy load

Distancecontracted


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SE

CE

DIFF ERENCE BETWEEN ISOMETRIC

AND ISOTONIC CONTRACTION

ISOMETRIC ISOTONIC

1 . Increase tension

No change in length

Constant Tension

Shortening of muscle

2.

3. Latent periodshort.

Latent periodIncreased with increasedload

PERestingtension

Shorten

lengthen

CE

SE

Shortens

Lengthens

Load lift

PEshorten


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DIFF ERENCE BETWEEN ISOMETRIC

AND ISOTONIC CONTRACTION (CONTD)

ISOMETRIC ISOTONIC

4. Contraction time varies Latent period increased withIncreased load

5. Long twitch duration Short twitch duration

6 . No external work External work done (35%)Heat (75%)


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MUSCLE SPINDLE

EFF ERENT

K Dynamic

K Static

AFFERENT

Ia

II

Nuclear Chain

Nuclear bag


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MUSCLE SPINDLES (INTRA F USAL)

Proprioceptive sensory organs parallelto contractile muscle fibres (extrafusal)

Attached to connective tissue surroundingextrafusal fibre

Less striations


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MUSCLE SPINDLES (STRUCTURE)

Spindle shaped

2 types :

- Nuclear Bag (dilated Central area)

- Nuclear Chain (thin, Ends not striated)


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MOTOR SUPPLY TO MUSCLE SPINDLE

1 . G amma efferent fibres innervate striated poles.

2. Dynamic K - efferents to Nuclear bag fibres.

3. Static K - efferent to Nuclear chain fibres

4. K - Efferent stimulation : stretch the nuclear bagand regulates sensitivity of muscle spindle .


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SENSORY (A FF ERENT) SUPPLY TOMUSCLE SPINDLE

1 . Ia Primary Afferent- Innervate centre of nuclear bag of chain

(Annulospiral or Primary ending )- Large fibre with rapid conduction- Function : Dynamic response, responds to

velocity of muscle stretch

2. Secondary or Flower-spray (II) nerve ending~ Smaller fibres- More abundant on nuclear chain fibres- Function - Static response respond to change

in muscle length.


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F UNCTIONS O F MUSCLE SPINDLE

1. Primary Annulo-spiral endings

Stretch of annulospiral endings

Potentials in Ia fibres

Reflex to E motor neurons

Antagonist muscles contract

Overall Role :Serves to maintain muscle length


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F UCTIONS O F MUSCLE SPINDLE

2. Primary Endings

~ Dynamic & Static Response- Respond to tonic and phasic responses~ Phasic change responses dampen oscillations

or jerkiness of movement mediated by delay infeedback loop

Overall Role

Smoother muscle movement / contraction


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F UNCTION O F MUSCLE SPINDLE

- K - Efferent discharge from cerebellar activity

- Increased K efferent activity enhances musclespindle sensitivity.

- Important for maintenance of tone and posture.

Overall function of muscle spindle :

Sensory or proprioceptive organ that modifiesreflexes via servomechanisms.

Enable fine control of body movements


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GOLGI TENDON ORGAN

Sensory fibres in tendon

Act as tension receptor within tendon

located near or at its junction with themuscle.

Increase muscle tension - straightencollagen fibres - distort sensory

fibres within tendon


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GOLGI TENDION ORGAN - F UNCTION

1 . Informs CNS of muscle tension

2. Inhibits motor neurons of contractingmuscle (via interneurons).

3. Activates antagonist muscle.

4. Main role: protect muscle damageduring strong contraction


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MUSCLE METABOLISM

ATP - Immediate energy source for musclecontraction

3 Sources :

a) Phosphorylation of ADP by creatininephosphate.

b) Mitochondrial oxidative phosphorylation

c) G lycolytic phosphorylation in cytoplasm


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MUSCLE CREATINE PHOSPHATE

Very rapid

Occurs at onset of contraction.

Lasts only few seconds

Limited content of creatinine phosphate


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MUSCLE OXIDATIVEPHOSPHORYLATION

Occurs in mitochondria

Requires Oxygen

Supplies ATP during moderate exercise


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GLYCOLYTIC PHOSPHORYLATION

IN MUSCLE

Occurs in cytoplasm

Does not require oxygen

Produces small number of ATPper mole of glucose

Produce lactic acid incursoxygen debt


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METABOLIC SUBSTRATESDURING CONTRACTION

G lycogen mobilisation via Ca++ andadrenaline

Muscle glycogen last 10 mins. Duringmoderate exercise.

Blood glucose and fatty acids - next30 min.

After that fatty acid - predominantfuel.


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MUSCLE F IBRE TYPES

Based on mechanical performance andmetabolic performance.

Mechanical performance depending onmyosin ATPase activity Maximalshortening velocity.

Metabolic performance - oxidative or glycolytic source of ATP .


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MUSCLE F IBRE TYPES

Type I - Slow oxidative fibres- Red in colour - Associated with continuous slow sustained

contractions.eg. para-spinal muscles for posture control.

Type II - Fast glycolytic fibres- white in colour

- Fast metabolically and mechanically- Short rapid movements extraocular muscle.

Type III - Fast oxidative fibres- Red in colour.


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PROPERTIES O F MUSCLE F IBRES

Property Type I Type II Type IIIColour Red White Red

Diameter Small Large Intermediate

ATP Oxidative G lycolysis Oxidative

formation phosphorylation phosphorylation

Mitochondria Abundant Few Abundant

G lycogen Low High Intermediate

G lycolysis Low High Intermediate

Myosin-ATPase Low High High

Myoglobin High Low High

Contraction Slow Fast Fast

Fatigue rate Slow High Intermediate


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SMOOTH MUSCLE

Involuntary muscle

Autonomic nervous system innervation

Spindle shaped cell

Single nucleus

Vital role in hollow organ function


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SMOOTH MUSCLE

Contain actin, myosin and tropomysin but noregular arrangement.

No troponin

Poorly developed sarcoplasmic reticulum

Do not contain sarcomeres

Calcium bind to calmodulin ( NOT troponin )


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TYPES O F SMOOTH MUSCLES1 . Single Unit eg. G IT, ureter

- Spontaneous action potential- May contain pacemaker - Undergo electrical and mechanical activity in

synchronous manner.- low resistance gap junctions

2. Multi-Unit Smooth Muscles eg. ciliary muscle airway.- Muscle fibre respond independently- Densely innervated by autonomic nervous system- Do not have spontaneous activity- Contractility depends on frequency of nerve

stimulation and number of fibres activated.


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SMOOTH MUSCLE CONTRACTION

Release of Ca++ from sarcoplasmic reticulum or Ca++entry via voltage gated Ca++ channels in cell membrane.

Ca++ binds to calmodulin

Activates Myosin light chain kinase .

ATP phosphorylates myosin cross bridges which bind to Actin filaments to produce contraction .


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SINGLE UNIT CONTRACTION

Unstable membrane potential

Recurrent depolarisation

Voltage-gated Ca++ channels open

Ca++ action potentials generated atdepolarisation.


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SMOOTH MUSCLE EXTRINSICCONTROL

1 . Autonomic Control

Ach - contraction Adrenaline - relaxation

2. Hormones - via intracellular calcium

3. Local factors influencing intracellular Ca, pH, O 2 ionic composition,


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SINGLE UNIT vs MULTI-UNIT SMOOTHMUSCLE

PROPERTY SIN G LE UNIT SM MULTI-UNIT SM

G ap Junction Yes Few

Pacemaker pot Yes No

Tone Yes No

Neuro Control Yes Yes

Hormone Control Yes Yes

Stretch induced contraction Yes No


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SkeletalMuscle

CardiacMuscle

SmoothMuscle

Structure

Motor and Plate Yes None None

Mitochondria Few Many Few

Sarcomere Yes Yes None

Sarcoplasmic development Well Developed Nonedeveloped

Syncytium none Yes Yes

F unction

Pacemaker No Yes (fast) Yes (slow)

Response All or none All or none G raded

Tetanic Contraction Yes No Yes

20457035 muscle physiology 2

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