Skeletal Muscle cnMechanics

8/11/2019 Skeletal Muscle cnMechanics

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MECHANICS OF SKELETAL

MUSCLE

Dr. Ayisha Qureshi

Assistant ProfessorMBBS, MPhil


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The Muscle Twitch

A single action potential causes a brief contractionfollowed by relaxation in the muscle. This is called a

single Muscle twitch.

Electrical and mechanical events in a musclealways occur in relation to one another: Theelectrical event (Action potential) is followed bythe mechanical events (contraction). The wholeprocess is called Excitation-contraction coupling.

Twitch starts 2 ms after depolarization of themembrane, before repolarization is complete-----Why the delay?


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Contractile activity and

electrical activity in

skeletal muscle:

A single action potential in a

skeletal muscle fiber lasts only 1 to2 msec, while a skeletal musclecontraction and relaxation lasts forabout 100 msec.

The onset of the resultingcontractile response lags behindthe action potential because the

entire excitationcontractioncoupling must occur before cross-bridge activity begins. In fact, theaction potential is completed beforethe contraction even begins.

Time is take for the followingprocesses:

AP to spread down the t-tubule.

Release of Ca2+

Ca2+to attach to Troponin C

Power stroke

Ca2+uptake by the ATPasepump in the SR.


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LENGTH & TENSION RELATIONSHIP:


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Length & Tension Relationship

A relationship exists between the length of the musclebefore the onset of contraction and the tension (forcedeveloped in the muscle) that each contracting fiber candevelop at that length.

For every muscle there is an optimal length (lo) at whichmaximal force can be achieved on a subsequentcontraction.

More tension can be achieved when beginning at theoptimal muscle length than when the contraction begins

with the muscle less than or greater than its optimal length.This lengthtension relationship can be explained by thesliding filament mechanism of muscle contraction.


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Length & Tension relationship

Length (L) and Force (F) or tension of a muscle are closely related:1. Optimal length (lo): (In the previous slide seen as point A) This is the point where thin

filaments optimally overlap the thick filaments. This is also the normal length of the

sarcomere. At this point, maximal no. of cross-bridges & actin filaments are accessible

to each other for binding & bending.

2. At lengths greater than Optimal length (lo): (in the previous slide seen as point C) This

is when the muscle is passively stretched. The thin filaments are pulled out from

between the thick filaments, decreasing the number of actin sites available for cross-

bridge binding. So some of the cross-bridge and actin sites do not match up and go

unused. So, NO actin myosin overlap, tension developed by the muscle is zero.

3. At lengths less than Optimal length (lo): (in the previous slide seen as point D) If a

muscle is shorter less tension is developed for the following reasons:

- The thin filaments from the opposite sides become overlapped.

- The ends of the filament become forced against the z-discs so no further

shortening can take place.


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Length-Tension Relationship

Points to Remember:

1. When the muscle is at its Optimal length, it

contracts with the maximum tension.

2. Force of contraction (tension generated) is

maximal at the resting (Optimal) length &

decreases if the muscle is longer or shorter.


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ENERGETICS OF MUSCLECONTRACTION:


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Energy sources

The main source of energy for muscle contraction is ATP. ATP is usedin 3 different steps in contraction-relaxation process. These stepsare:

1. Splitting of ATPby myosin ATPase provides the energy for the

power stroke of the cross bridge.

2. Binding (but not splitting) of a fresh molecule of ATPto myosin lets

the bridge detach from the actin filament at the end of a power stroke

so that the cycle can be repeated. This ATP is later split to provide

energy for the next stroke of the cross bridge.

3. Active transport of Ca2+back into the sarcoplasmic reticulumduring relaxation depends on energy derived from the breakdown

of ATPand is used by the ATP- dependant Calcium Pump.

The concentration of ATP in a Muscle fiber= 4mmole. It is sufficient to

maintain full contraction for only 1 to 2 seconds at most.


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SOURCES OF ATP

There are 3 main sources of ATP:1. Creatine Phosphate/ Phosphagen Energy system:

- takes place within the muscle

-uses the Phosphate bond from Creatine phosphate

- First source of ATP when exercise begins; instantaneous energy available.

- short bursts of high-intensity exercise. E.g. high jump, sprints

2. Oxidative phosphorylation: aerobic or endurance type exercise.

- takes place in the mitochondria

- requires oxygen & uses fatty acids, glucose in blood and glycogenstores

- to sustain long duration mild to moderate aerobic exercise. E.g. walks,

jogging, swimming, marathon runners.3. Glycolysis: anaerobic or high-intensity exercise

- when oxygen demands are not met & oxygen NOT available.

- uses glycogen stores of the muscle

- proceeds very rapidly and leads to formation of lactic acid.

- moderate to severe exercise. E.g. 800 meter run. Cannot be sustained forlong time.


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CHARACTERISTICS/ PROPERTIES OF WHOLE

MUSCLE CONTRACTION :

We have been talking about muscle fibers as asingle muscle cell..

Now we will consider Muscle as a wholeconsisting of several to several hundred musclefibers.


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1. MUSCLE FATIGUE

Definition:Fatigue occurs when prolonged & strong stimulation of anexercising muscle reaches a stage when the muscle is nolonger able to respond to the stimulation with the same

degree of contractile activity. Is of 2 main types:1. Muscle fatigue: occurs in the muscle & is a defense

mechanism that protects the muscle by preventing itfrom reaching a point where no ATP will be available.

2. Central fatigue: more psychological. Occurs when CNSno longer activates the motor neurons supplying themuscles. Person stops exercising even though themuscles can still perform.


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1. MUSCLE FATIGUE

CAUSES:

1. Depletion of Glycogen energy stores.

2. Accumulation of Hydrogen ions from lactic acid-

interfere with cross- bridge functions.3. Intracellular acidosis from lactic acid inhibits

glycolysis enzymes & slows ATP production.

4. NT depletion at the NMJ.

5. Central fatigue- lack of will & sleep.

6. Accumulation of extracellular K+


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2. OXYGEN DEBT

The body normally

contains about 2 liters

of oxygen:

0.5 litersAir in lungs

0.25 litersBody Fluids

1 literHb of Blood

0.3 litersMuscle withMyoglobin


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2. OXYGEN DEBT

During muscular exercise, a lot more Oxygen is suppliedto the muscle than is present.

O2consumption = energy expended

All stored O2 is used within a minute or so

After exercise is over: 2 liters of normally present blood must be replenished

9 liters extra must be provided for:

1) Resynthesis of the Creatine Phosphate.

2) Conversion of lactate into pyruvate.3) Form fresh supplies of ATP through oxidative

phosphorylation.


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2. OXYGEN DEBT

All this extra Oxygen that must be repaid(11.5liters) to the body is called the OxygenDebt.

SO,A person must breathe rapidly even after the

exercise is over!


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3. MUSCLE TONE

Even when muscles are at rest, a certain amountof tautness usually remainsThis is calledMuscle Tone.

Cause:Low rate of nerve impulses coming from thespinal cord which are controlled by the:

1. Signals from the brain to the spinal cord-

anterior motor neurons2. Signals that originate in the muscle spindles

located in the muscle itself-Intrafusal fibers


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4. MOTOR UNIT

Definition:All the muscle fibers innervated by a single nerve fiber are called a MOTOR

UNIT.OR

Each single motor neuron plus all the muscle fibers it innervates is called aMOTOR UNIT.

One motor neuron innervates a number of muscle fibers, but each musclefiber is supplied by only one motor neuron. When this neuron isstimulated, all the muscle fibers supplied by it contract together.

Each muscle consists of a number of mixed motor units.

For a weak contraction of the whole muscle, only one or a few of

its motor units are activated.

The number of muscle fibers per motor unit and the number of motorunits per muscle vary widely, depending on the specific function of the

muscle. E.g. the kind of work that the muscle performs..


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4. MOTOR UNIT

Number of muscle fibers in a motor unit vary indifferent muscles from 2 or 3 to more than 1000.

Average: 80-100 muscle fibers to a motor unit.

Muscles which have to perform fine grade, intricatemovements have motor units with as few as 3-5muscle fibers to a unit .e.g. hand, eye

Muscles with relatively crude movements, number of

muscle fibers is quite large. E.g. muscles of lowerlimbs

In one whole muscle, different motor units overlap


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5. ALL OR NONE LAW

In a single muscle fiber exactly the same as in the singlenerve fiber.

A sub-threshold stimulus does not produce a responsewhile a threshold or supra-threshold stimulus produces amaximal response.

In whole muscle the response is different. A gradual in stimulus strength causes a gradual in

muscle contraction till a maximum is obtained. This isbecause with each in stimulus strength more & moremotor units are stimulated.

When all motor units are activated---all muscle fibers arecontracted , then a further in the strength of thestimulus is without any additional contractile effect.


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6. Force of Contraction Summation:

Summation: is the process of adding together of

individual twitch contractions to increase the

intensity of whole muscle contraction.

There are 2 types of summation:

1. Multiple Fiber Summation (No. of motor

units stimulated)

2. Frequency Summation


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6. a: Multiple Fiber Summation

Definition:

It is the summation of individual muscle fiber contractions byincreasing the numberof motor units contracting simultaneously.

Initially, with a weak signal from the CNS-only smaller units are

stimulated. Later, when signal from CNS becomes stronger, larger motor units

are excited----This is called SIZE PRINCIPLE.

Importance:

It allows gradation of force to occur for weak & strong contractions.

Cause:Smaller motor units are driven by smaller motor nerves & are moreexcitable than large ones---so are excited first! Then, if greater strengthis required, then larger motor units are recruited.


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6. b: FREQUENCY SUMMATION

Definitions:

Force of contraction increases by increasing the frequencyof contractions.

Two twitches from 2 action potentials add together to produce greater

tension in the fiber than produced by a single action potential. This is called

twitch summation or frequency summation.

Force generated by the contraction of a single muscle fiber can be by

increasing the rate at which the action potentials stimulate the muscle

fiber.

If repeated APs are separated by long intervals of time, muscle fibers have

time to relax completely between stimuli.

If interval of time between AP shortened, the Muscle fiber will not have

relaxed completely at time of 2ndstimulus, resulting in a more forceful

contraction.


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A single action potential in a muscle fiber producesonly a twitch. Let us see what happens when a secondaction potential occurs in a muscle fiber. If the musclefiber has completely relaxed before the next action

potential takes place, a second twitch of the samemagnitude as the first occurs. The same excitation-contraction events take place each time, resulting inidentical twitch responses. If, however, the muscle fiberis stimulated a second time before it has completely

relaxed from the first twitch, a second action potentialcauses a second contractile response, which is addedpiggyback on top of the first twitch.


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FREQUENCY SUMMATION

When APs come one after theother after the relaxation of the

muscle is complete.

When APs come one after theother before relaxation of the

muscle is complete


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6. b: FREQUENCY SUMMATION

If APs continue to stimulate the muscle repeatedly at shortintervals, there is no time for complete relaxation between

contractions

Individual twitches fuse into one continuous contraction

Whole muscle contraction appears to be smooth, sustained & ofmaximal strength

This is called TETANIZATION or TETANUS(A tetanic contraction is usually three to four times stronger than

a single twitch.)


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Physiologic basis of twitch summation & Tetanus:

The main reason is the sustained elevation in cytosolicCa2+ permitting greater cross-bridge cycling. As thefrequency of action potentials increases, the duration ofelevated cytosolic Ca2+ concentration increases, andcontractile activity likewise increases until a maximumtetanic contraction is reached. With tetanus, the

maximum number of cross-bridge binding sites remainuncovered so that cross-bridge cycling, and consequentlytension development, is at its peak.


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7. THE STAIRCASE/ TREPPE EFFECT

DEFINITION:When a series of maximal stimuli are delivered to the muscle at

a frequency just below tetanizing frequency(when muscle twitch due to previous stimulus has just

completed), the tension/amplitude developed during each

twitch increases till a max. height is reached & a plateau isformed. This is called the Treppe/ staircase effect.

Because the tension rises in stages, like the steps in a staircase,this phenomenon is called treppe, a German word meaning"stairs."

CAUSE:The rise is thought to result from a gradual increasein the concentration of calcium ions in the sarcoplasm, inpart because the ion pumps in the sarcoplasmic reticulumare unable to recapture them in the time betweenstimulations.


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Treppe Effect


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8. ISOTONIC VS. ISOMETRICCONTRACTION


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ISOTONIC CONTRACTION

There are two primary types of contraction, depending onwhether the muscle changes length during contraction.They are: Isotonic contraction: occurs when muscle contracts with

shortening of length but against a constant load, thus,the tension on the muscle remains constant (iso= same,tonic= tension)

ORA contraction that creates force & moves a load.

Isotonic contractions are used for body movements and formoving external objects. E.g. picking up a book, a box.


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ISOMETRIC CONTRACTION

Isometric contraction: occurs when muscle contractswithout shortening in length.

(iso= same, metric= measure or length)

OR

A contraction that creates force without movement.Isometric contractions can be seen in 2 cases:

1. If the object you are trying to lift is too heavy.

2. If the tension developed in the muscle is deliberately

less than needed to move the load. E.g. standing forlong time or holding up a glass of water while takingsips.


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Physiologic basis of Isometric & Isotonic contractions:

The same internal events occur in both isotonic and isometriccontractions:

Muscle excitation starts the sliding filament cycling; the cross bridgesstart cycling; and filament sliding shortens the sarcomeres, which exert

force on the bone at the site of the muscles insertion.

During a given time, a muscle may shift between isotonic & isometriccontractions. E.g. when you lift a book up it is isotonic contraction andwhen you keep holding the book up while reading it is isometriccontraction.

NOTE:

Since Work=Distance X Load,

Isotonic contractions do work where as Isometric do not.


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9. ELECTROMYOGRAPHY

Activity of motor units can be studied byelectromyography, the process of recording theelectrical activities of the muscle on a cathode rayoscilloscope.

No anesthesia is required. Small metal discs areplaced on the skin overlying the muscle as pick-upelectrodes or hypodermic needle electrodes areused.

The record obtained with such electrodes is theElectromyogram (EMG).


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10. RECRUITMENT

If each motor unit contracts in an all-or-none manner,how then can muscle create graded contractions ofvarying force & duration?

The answer lies in the fact that muscles are composed ofmultiple motor units of different types. This allows the

muscle to vary contraction by:1. Changing the types of motor units that are active OR2. Changing the number of motor units that are respondingat any one time.

For a weak contraction of the whole muscle, only one or afew of its motor units are activated. For stronger & strongercontraction, more & more motor units are recruited. This iscalled Motor Unit Recruitment.


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10. RECRUITMENT

At rest EMG shows little or no activity

With minimum voluntary activity a few motor units discharge, & with

increasing voluntary effort more & more are brought into play-----

Recruitment of motor units

Asynchronous Recruitment:One way that CNS avoids fatigue in a

sustained contraction

The CNS alternates between the different motor units supplying the same

muscle so that some of the motor units rest between contractions,

preventing fatigue. e.g. during a sustained contraction, only a portion of

the muscles motor units is involved as is necessary in muscles supporting

the weight of the body against the force of gravity. The body alternates

the motor units as shifts at a factory, to give the motor units that have

been active an opportunity to rest while others take over. Changing of the

shifts is carefully co-ordinated so that the sustained contraction is smooth

rather than jerky.


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11. FAST vs SLOW FIBERS

The skeletal muscle fibers are mainly of 2 types:

1. SLOWor REDor TYPE I MUSCLE FIBERS

2. FASTor WHITE or TYPE II MUSCLE FIBERS

Every muscle of the bodyis composedof a mixtureof both fast & slow fibers.

Simply: Fibers that react rapidly are Fastfibers &muscles that react slowly with long contractions are

Slowfibers Color is determined by the protein myoglobin


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11. FAST vs SLOW FIBERS

SLOW-TWTCH/ RED/ Type I

Small diameter

More myoglobin

Fatigue resistant

Mostly Oxidative

Slow rate of contraction

Myosin ATPase activity LOW

no. of myofilaments Red

Posture maintenance

FAST-TWITCH/ WHTE/Type II

Large diameter

Less myoglobin

Easily fatigue

Mostly glycolytic & oxidative

Fast rate of contraction

Myosin ATPase activity HIGH

no. of myofilaments

White

Forceful & rapid movements


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12. MUSCLE HYPERTROPHY

Definition:When the total mass of a muscle increases, this is called Muscle

Hypertrophy. The resulting muscle enlargement comes froman increase in diameter of the muscle fibers. It is in response

to a regular & intensive use of that particular muscle. e.g.

body building.Physiologic Basis:

in the number of actin & myosin filaments causing increasein thickness of individual muscle fibers---called fiberhypertrophy

Rate of synthesis of actin & myosin far greater Signaling proteins triggered that turn on genes that direct the

synthesis of more of these contractile proteins.


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13. MUSCLE ATROPHY

Definition:When the total mass of a muscle decreases, it is called Muscle

Atrophy. If a muscle is not used, its actin and myosin contentdecreases, its filaments become smaller and the muscle decreases

in mass and becomes weaker.

Physiologic Basis:1. When the muscle is prevented from doing work even though the

nerve supply is intact. e.g. in bed-ridden patients, in a limb in aplaster of Paris cast. This type is thus called Disuse Atrophy.

2. Atrophy also seen nerve supply to the muscle is lost. This can bedue to an accident or when motor neurons supplying a muscle are

destroyed .e.g. Poliomyelitis. Muscle fiber becomes thin & low in proteins, glycogen and ATP.

When muscle continuously shortened then sarcomeresat the endof the muscle fiber actually disappear


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14. MUSCLE HYPERPLASIA

Under rare conditions of extreme muscle force

generation, the actual number of muscle

fibers increase, in addition to the fiber

hypertrophy ----This increase in fiber numberis called Muscle Hyperplasia.

Mechanism: Linear splitting of previouslyenlarged fibers


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MUSCLE DISEASES


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MUSCLE CRAMPSDefinition:

Painful, sustained & involuntarycontractions of the muscle with

motor units contractingrepeatedly.

CAUSE: There can be manycauses the most common ofwhich are:

Due to increased excitabilityof the peripheral parts of thenerves

Electrolyte disturbance

Nocturnal cramps (nightcramps)

Cramps due to strenousexercise

Dehydration.


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DUCHENNE MUSCULAR DYSTROPHY


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Duchenne Muscular

Dystrophy

Definition:It is a fatal muscle-wasting disease thatprimarily strikes boysand leads to their death

before the age of 20.There is progressivedegeneration ofcontractile proteins ofthe muscle and theirreplacement withfibrous tissue.

It is a genetic X-linkeddisease.


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DUCHENNE MUSCULAR DYSTROPHY

Mutation in the Dystrophin gene located on X-chromosome

Skeletal muscle lacks protein dystrophin (a large protein that provides

structural stability to the muscle cells plasma membrane)

Its absence leads to constant leakage of Ca into the muscle cell

Ca activates proteases that start damaging the muscle

Leads to increasing muscle weakness & fibrosis

Symptoms start at 2-3 years, patient wheel-bound at 10-12 years

Usually die at about 25-30 years of age (usually Males)

Death is usually due to respiratory failure or heart failure as therespiratory or heart muscles become too weak.

Milder disease is Beckers muscular dystrophy


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Skeletal Muscle cnMechanics