2nd Lecture on Skeletal Muscle Physiology by Dr. Mudassar Ali Roomi

7/29/2019 2nd Lecture on Skeletal Muscle Physiology by Dr. Mudassar Ali Roomi

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Skeletal muscle Physiology

By

Dr. Mudassar Ali Roomi (MBBS, M. Phil)


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2 Types of muscle contraction:

ISOTONIC CONTRACTION

ISOMETRIC CONTRACTION


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TYPES OF CONTRACTION

ISOTONIC (same tone)

Muscle length

decreases but muscletension constant.

Work is done in thistype of contraction.

Example: lifting of bookfrom a table.

ISOMETRIC

(same length)

No appreciable change

in length of muscle butmuscle tensionincreases.

Work is not done.

Example: heavy weightlifting by body builders.


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Isotonic contraction
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http://www.google.com.pk/url?sa=i&rct=j&q=isometric+exercises+vs+isotonic+exercises&source=images&cd=&cad=rja&docid=Iw_FSYX1Z3kFyM&tbnid=GaHzJs8dFJowAM:&ved=0CAUQjRw&url=http%3A%2F%2Fwww.chla.congenital.org%2F%3Fid%3Daorticstenosis7&ei=q5QwUY_vOYXHswa3gIG4Dg&bvm=bv.43148975,d.Yms&psig=AFQjCNGe8dP9VQaMxPF0fES-0Ew9Kq1bJg&ust=1362224643784366


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Isometric contraction


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Body movements are a mixture of isotonic and isometricmovements.

In muscle 2 types of elements:

1) Contractile elements:

(thin/actin &thick/myosin filaments)

2) Elastic elements:

(tendons & sarcolemmal ends of muscle fibers

attached to tendons)

Elastic component is in series with contractile component.

Contractile component undergoes shortening &

elastic component undergoes stretching.


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Isometric contraction (cont)

Isometric exercise orisometrics are a type ofstrength training in which thejoint angle and muscle lengthdo not change during

contraction (compared toconcentric or eccentriccontractions, calleddynamic/isotonicmovements).

Isometrics are done in staticpositions, rather than beingdynamic through a range ofmotion.

Examples:

e.g. holding a weight in afixed position


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Isometric contraction (cont)

Muscle must shorten 3-5% extra to neutralize

the stretching of elastic component.

In isometric exercise, only 3-5% muscle

shortening, tendons are stretching & this

shortening neutralizes the stretching no

change in length.


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FENN EFFECT

Greater the work done

by muscle, greater will

be the amount of ATP

hydrolyzed to ADP withemission of energy.

(the more you work, the

more you are paid)!
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Motor unitMotor unit:Single motor neuron along with muscle fiber it innervates.

There is overlapping of adjacent motor unit.

Number of muscle fibers in a motor unit vary.

In muscles concerned with fine skilled movements3-6muscle fibers in a motor unit.Example: ocular muscles, laryngeal muscles, small muscles ofhand.

In muscles concerned with prolong posture maintenance100-150 muscle fibers in a motor unit.Example: muscles of back & gastrocnemius.


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Motor unit
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Motor Unit Ratios

Back muscles

1:100

Finger muscles

1:10

Eye muscles

1:1


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Macro-motor unit

Increased number of muscle fibers in a motor

unit (seen in regeneration of poliomyelitis).

There is paralysis recovery / regeneration

terminal nerve fibers give more branches

to supply muscle fibersmacro-motor unit.


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TETANIZATION:

Summation of contraction/twitches Sustainedcontraction without relaxation.

Complete tetanus is produced when a muscle isstimulated at a very rapid rate. Example: 60-70

stimuli/sec. Muscle tension produced in complete tetanization is

greater than that in single muscle twitch.

Frequency of stimulation at which complete tetanus

is produced is called tetanizing frequency. Tetanus bacillus alpha motor neuron repeated

discharge.


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INCOMPLETE & COMPLTE TETANUS

INCOMPLETE

Repeated stimuli at a

fast raterelaxation of

each twitch remainsincomplete

incomplete tetanus.

COMPLETE

Repeated stimuli at still

higher rate

relaxation phasedisappears altogether

sustained

contraction phase is

obtained completetetanus.


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TETANY (increased excitability of motor

nerves)

CAUSES:

Parathyroidectomy (during thyroid surgery) lack of PTHplasma Ca+ level falls signs of neuromuscularhyperexcitability appear.

Alkalosis plasma proteins behave as anions bind cationsincluding Ca++decreased ionized calcium hypocalcemia less calcium available in ECF for membrane stabilizationno blocking of sodium channels by calcium cations (negativelycharged on inside) increased excitability of motor nerves tetany.

Hyperventilationhypocapnia (decreased CO2concentration) respiratory alkalosisplasma ionizedcalcium falls carpopedal spasm, a positive Chvostek sign &other signs of tetany).


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SIGNS OF TETANY:

CHVOSTEKS SIGN: A quick contraction of ipsilateralfacial muscles elicited by tapping over the facialnerve at the angle of the jaw.

TROUSSEAUS SIGN: A spasm of muscles of the upperextremity that cause flexion of the wrist & thumbwith extension of fingers.

In individuals with mild tetany with no obviousspasm, trousseaus sign may be produced byoccluding circulation for few minutes with a B.P cuff.


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TROUSSEAUS SIGN


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TREPPE / STAIRCASE PHENOMENON

Definition: When a muscle is stimulated bymaximum stimuli at a frequency less than tetanizingfrequency progressive increase in muscle tensionwith repeated stimuli, till it becomes constant.

If threshold stimuli are applied so that each stimulusreaches the muscle when the muscle twitch due toprevious stimulus has completed each successivetwitch shows an increased amplitude till a maximum

height is reached this is called as staircasephenomenon.


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Mechanism of Treppe

Greater availability of

calcium in sarcoplasm.

Accumulation of

metabolites. Rise in local

temperature.

All these exert

beneficial effect on

contraction.
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Muscle Atrophy

Definition of atrophy:

decrease in the size of a

tissue due to decrease in

size of its cells.

Weakening and shrinking of

a muscle

May be caused by:

Immobilization of muscles

e.g. in cases of bed riddenpatient

Loss of neural stimulation e.g.

in cases of nerve injury
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Muscle Hypertrophy

Definition of hypertrophy:

increase in the size of a

tissue due to increase in

size of its cells.

Enlargement of a muscle

More capillaries

More mitochondria

Caused by:

Strenuous exercise

Anabolic Steroid hormones


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RIGOR MORTIS:

after deathmuscles of dead body become

rigid rigor mortis.

Its onset depends on:

Temperature: increased temperature rapid

onset.

Activity: Vigorous activity of muscle before

death rapid onset.


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Mechanism of Rigor Mortis

After death ATP is notavailable nodetachment ofcrossbridges of myosinfrom active site of actinfilaments contracture/rigidity.

After 16-24 hrs rigor mortisdisappears due to autolysis

of muscle proteins(resulting from hydrolyticenzymes released fromlysosomes).


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SIGNIFICANCE of Rigor Mortis

Forensic significance: Cause of death. If

suicide, the gun or dagger is locked in the

hand.

Duration of death: it gives us some idea about

time since death. rigor mortis remains for

about 16 hours after death. It disappears after

16-24 hrs of death.


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What was the cause of death?

2nd Lecture on Skeletal Muscle Physiology by Dr. Mudassar Ali Roomi

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