Skeletal Muscle Mechanics About Disease.co

7. Isotonic vs. isometric contraction

A contraction that creates force & moves a load. ISOTONIC CONTRACTION There 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. Not all muscle contractions shorten muscles. For a muscle to shorten during contraction, the tension developed in the muscle must exceed the forces that oppose movement of the bone to which the muscle’s insertion is attached. What is important to remember is that the muscle does not always shorten when it generates force or tension. The muscle can be generating force, creating tension and at the same time stay the same length, shorten or even lengthen. Considering your biceps as an example, assume you are going to lift an object. When the tension developing in your biceps becomes great enough to overcome the weight of the object in your hand, you can lift the object, with the whole muscle shortening in the process. Because the weight of the object does not change as it is lifted, the muscle tension remains constant throughout the period of shortening. This is isotonic contraction….

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: If the object you are trying to lift is too heavy. 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. What happens if you try to lift an object too heavy for you (that is, if the tension you can develop in your arm muscles is less than required to lift the load)? In this case, the muscle cannot shorten and lift the object but remains at constant length despite the development of tension, so an isometric (“constant length”) contraction occurs.

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. RECRUITMENT If 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: Changing the types of motor units that are active OR 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. A good example would be of holding a mike while talking. Although u can go on holding it for a long time there is little fatigue as the CNS keeps on alternating b/w different motor units so that the same muscles do not fatigue themselves.....

10. FAST vs SLOW FIBERS The skeletal muscle fibers are mainly of 2 types: SLOW or RED or TYPE I MUSCLE FIBERS 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

FAST-TWITCH/ WHTE/Type II 10. 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

11. MUSCLE HYPERTROPHY Definition: 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 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 content decreases, its filaments become smaller and the muscle decreases in mass and becomes weaker. Physiologic Basis: 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. 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 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 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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