Skeletal Muscle Mechanics

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7. Isotonic vs. isometric contraction

ISOTONIC CONTRACTIONThere are two primary types of contraction, depending on whether 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)

OR A contraction that creates force & moves a load.

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

ISOMETRIC CONTRACTION Isometric contraction: occurs when muscle

contracts without 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:i. If the object you are trying to lift is too heavy.ii. If the tension developed in the muscle is

deliberately less than needed to move the load. E.g. standing for long time or holding up a glass of water while taking sips.

Physiologic basis of Isometric & Isotonic contractions:The same internal events occur in both isotonic and isometric contractions: Muscle excitation starts the sliding filament cycling; the cross

bridges start cycling; and sliding filament shortens the sarcomeres, which exert force on the bone at the site of the

muscle’s insertion.During a given time, a muscle may shift between isotonic & isometric contractions. E.g. when you lift a book up it is isotonic contraction and when you keep holding the book up while reading it is isometric contraction.

NOTE: Isotonic contractions do work where as Isometric do not.

8. ELECTROMYOGRAPHY Activity of motor units can be studied by

electromyography, the process of recording the electrical activities of the muscle.

No anesthesia is required. Small metal discs are placed on the skin overlying the muscle as pick-up electrodes or hypodermic needle electrodes are used.

The record obtained with such electrodes is the Electromyogram (EMG).

9. RECRUITMENTIf each motor unit contracts in an all-or-none manner, how then can muscle create graded contractions of varying force & duration?The answer lies in the fact that muscles are composed of

multiple motor units of different types. This allows the muscle to vary contraction by:

i. Changing the types of motor units that are active ORii. Changing the number of motor units that are

responding at any one time.

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

9. 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 muscle’s 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.

10. FAST vs SLOW FIBERS The skeletal muscle fibers are mainly of 2

types:1. SLOW or RED or TYPE I MUSCLE FIBERS2. FAST or WHITE or TYPE II MUSCLE

FIBERS Every muscle of the body is composed of a

mixture of both fast & slow fibers. Simply: Fibers that react rapidly are Fast

fibers & muscles that react slowly with long contractions are Slow fibers

Color is determined by the protein myoglobin

10. FAST vs SLOW FIBERSSLOW-TWTCH/ RED/

Type ISmall diameterMore myoglobinFatigue resistantMostly Oxidative Slow rate of contractionMyosin ATPase activity

LOW↓ no. of myofilamentsRed Posture maintenance

FAST-TWITCH/ WHTE/Type II

Large diameterLess myoglobinEasily fatigueMostly glycolytic &

oxidativeFast rate of contractionMyosin ATPase activity

HIGH↑ no. of myofilamentsWhite Forceful & rapid

movements

11. MUSCLE HYPERTROPHYDefinition: When the total mass of a muscle increases, this is

called Muscle Hypertrophy. The resulting muscle enlargement comes from an 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 increase in thickness of individual muscle fibers---called fiber hypertrophy

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.

12. MUSCLE ATROPHYDefinition:

When the total mass of a muscle decreases, it is called Muscle Atrophy. If a muscle is not used, its actin and myosin content

decreases, 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 a plaster of Paris cast. This type is thus called Disuse Atrophy.

2. Atrophy also seen nerve supply to the muscle is lost. This can be due 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 sarcomeres at the

end of the muscle fiber actually disappear

13. 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 number is called Muscle Hyperplasia.

Mechanism: Linear splitting of previously enlarged fibers

MUSCLE DISEASES

MUSCLE CRAMPS

Definition:Painful, sustained &

involuntary contractions of the muscle with motor units

contracting repeatedly.CAUSE: There can be many causes the most common of which are: Due to increased

excitability of the peripheral parts of the nerves

Electrolyte disturbance Nocturnal cramps (night

cramps) Cramps due to strenuous

exercise Dehydration.

DUCHENNE MUSCULAR DYSTROPHY

Duchenne Muscular Dystrophy

Definition: It is a fatal muscle-wasting disease that primarily strikes boys and leads to their death before the age of 20. There is progressive degeneration of contractile proteins of the muscle and their replacement with fibrous tissue. It is a genetic X-linked disease.

DUCHENNE MUSCULAR DYSTROPHYMutation in the Dystrophin gene located on X-chromosome

↓Skeletal muscle lacks protein dystrophin (a large protein that

provides structural stability to the muscle cell’s 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

↓Death is usually due to respiratory failure or heart failure as

the respiratory or heart muscles become too weak.↓

Milder disease is Becker’s muscular dystrophy

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