Muscle

introduction, morphology, structure, epimysium , perimysium, endomysium, organization of contractile unit, cross- bridge interaction, types of muscle contraction, eccentric, concentric, isometric, isokinetic, isointertial, isotonic, passive tension, active tension, length- tension relationship, load- velocity relationship, force- time relationship, effect of skeletal muscle architecture, effect of pre- stretching, effect of fatigue, effect of temperature, muscle fibre differentiation

1. MUSCLE Vibhuti Nautiyal MPT (Musculoskeletal)

2. INTRODUCTION Three different types of muscles: Skeletal muscle: attached to the bones Cardiac muscle: found in the heart Smooth muscle: found in the internal organs and the walls of the blood vessels Skeletal muscle are attached to the bones on the either side of the joint by the tendons Agonists or prime movers: acts as the primary activators of motion Antagonists: counteracts the agonists and oppose the motion Skeletal muscle can also be classified as spurt or shunt muscles

3. Spurt muscles: originate far from the joint of rotation but are inserted close to the joint Short moment of arm Move the limb very rapidly Relatively limited in the magnitude of effective strength (e.g., biceps) Shunt muscles: Origin close to the joint and insertion far from the joint Large moment arm Relative stabilizing effect on the joint (e.g., brachioradialis )

4. MORPHOLOGY Two common shapes of muscle: fusiform and pennate muscle Fusiform: fibres running parallel to each other and to the central tendon; pull longitudinally; maximum velocity and ROM; e.g., biceps brachii Pennate muscle: fibres approach the central tendon obliquely; further classified into unipennate , bipennate and multipennate (depending on the number of similarly angled sets of fibres that attach to the central tendon)

5. STRUCTURE Muscle fibre is the structural unit of muscle Thickness ranges from 10- 100 mm Length from about 1- 50 cm Each muscle fibre is an individual cell (long cylindrical) with multiple nuclei Connective tissue within muscle consists of fibres, most fibres are collagen and the remaining fibres are elastin The combination of these two proteins provides strength, structural support, and elasticity to muscle Muscle consists of epimysium , perimysium and endomysium

6. Epimysium : Tough structure Surrounds the entire surface of the muscle belly Separates it from the other muscles Gives form to the muscle belly Contains tightly woven bundles of collagen Highly resistive to stretch Perimysium: Lies beneath the epimysium Divides muscles into fascicles Proved a conduit for blood vessels and nerves Tough and thick Resistive to stretch

7. Endomysium : Surrounds individual muscle fibre Composed of relatively dens meshwork of collagen fibrils Partly connected to the perimysium Through lateral connections to the muscle fibre, the endomysium conveys part of the contractile force to the tendon A muscle fibre consists of many myofibrils, which are invested by a delicate plasma membrane called the sarcolemma Sarcolemma is connected via vinculin - and dystrophin rich costameres with the sarcometric Z line, which represents a part of the extramyofibrillar cystoskeleton The myofibril is made up of several sarcomeres that contain thin (actin), thick (myosin), elastic ( titin ) and inelastic ( nebulin ) filaments Actin and myosin are the contractile part of the myofibrils Titin and nebulin are part of the intramyofibrillar cystoskeleton Myofibrils are the basic unit of contraction

8. ORGANIZATION OF CONTRACTILE UNIT The portion of the myofibril that is located between two Z disk is called the sarcomere Z disks serves as the boundaries for the sarcomere and also link the thin filaments together Areas of the sarcomere called bands or zones help to identify the arrangement of the thick and thin filaments The portion of the sarcomere that extends over both the length of the thick filament and a small portion of the thin filaments is called A or anisotropic band Areas that include only actin filaments are called isotropic or I bands The central portion of the thick filament (A band areas) in which there is no overlap with the thin filaments is called the H zone The central portion of the H zone, which consists of the wide middle portion of the thick filament, is called the M band

9. CROSS- BRIDGE INTERACTION: Arrival of a nerve impulse at the motor end plate→ evokes an electric impulse or action potential→ travels along the muscle fibre→ interaction between the thick and thin filament of the sarcomere→ muscle contraction Action potential→ release of calcium ions→ cause troponin to reposition the tropomyosin molecules→ receptor site on the actin are free→ head groups of the myosin can bind with actin→ this bonding is called cross- bridge Tension is generated with the inclusion of the hydrolysis of adenosine triphosphate (ATP) and the release of adenosine diphosphate (ADP) from the myosin head.

10. TYPES OF MUSCLE CONTRACTION: Concentric contraction: The sliding of thin filaments toward and past the thick filaments, accompanied by the formation and reformation of cross- bridges in each sarcomere, will result in shortening of the muscle fibre and the generation of tension The muscle fibre will shorten if a sufficient number of sarcomeres actively shorten and if either one or both ends of the muscle fibres are free to move Eccentric contraction: The thin filaments are being pulled toward the thick filament Cross- bridges are broken and reformed as the muscle lengthens Tension is generated by the muscle as cross- bridges are reformed Isometric contraction: Muscle fibre will not change length If the force created by the cross- bridge cycling is matched by the external force

11. Isokinetic contraction : Dynamic muscle work Movement of the joint is kept at a constant velocity, hence the velocity of shortening or lengthening of muscle is constant Muscle energy can not be dissipated through acceleration of the body part and is entirely converted to a resisting moment The muscle force varies with changes in its lever arm throughout the ROM Isoinertial contraction: The resistance against which the muscle must contract remains constant If the moment (torque) produced by the muscle is equal to or less than the resistance to be overcome, the muscle length remains unchanged and the muscle contracts isometrically If the moment is greater than the resistance, the muscle shortens and causes acceleration of the body part

12. Isotonic contraction: The tension is constant throughout a ROM Because the muscle force moment arm changes throughout the ROM, the muscle tension must do change In the truest sense this contraction does not exist in the production of joint motion

13. FORCE PRODUCTION IN MUSCLE Passive tension: Refers to tension developed in the parallel elastic component of the muscle Created by lengthening the muscle beyond the slack length of the tissues The total tension develops during an active contraction of a muscle is a combination of the passive tension added to the active tension Active tension: Refers to the tension developed by the contractile components of the muscle Initiated by cross- bridge formation and movement of the thick and thin filaments Amount of active tension depends on neural factors and mechanical properties of the muscle fibres Neural factors include: frequency, number and size of motor units that are firing Mechanical properties: length- tension relationship and force- velocity relationship

14. Length- tension relationship: The force or tension that a muscle exerts varies with the length at which it is held and stimulated Maximal tension is produced when the muscle fibre is approximately at its “slack” or resting length If the fibre is held at shorter lengths, the tension falls off slowly at first and then rapidly If the fibre is lengthened beyond the resting length, tension progressively decreases The changes in tension when the fibre is stretched or shortened primarily are caused by structural alterations in the sarcomere Maximal isometric tension can be exerted when the sarcomeres are at their resting length (2.0- 2.25 µm), because the actin and myosin filaments overlap during their entire length and the number of cross- bridges is maximal If the sarcomeres are lengthened, there are fewer junctions between the filaments and the active tension decreases At sarcomere length 3.6 µm, there is no overlap and hence no active tension

15. Load- velocity relationship: The velocity of shortening of a muscle contracting concentrically is inversely related to the external load applied The velocity of shortening is greatest when the external load is zero, but as the load increases the muscle shortens more and more slowly When the external load equals the maximal force that the muscle can exert, the velocity of shortening becomes zero and the muscle contracts isometrically When the load is increased still further, the muscle contracts eccentrically: it elongates during contraction The muscle eccentrically lengthens more quickly with increasing load

16. FORCE- TIME RELATIONSHIP The force or tension generated by a muscle is proportional to the contraction time: the longer the contraction time, the greater is the force developed, up to the point of maximum tension Slower contraction leads to greater force production because time is allowed for the tension produced by the contractile elements to be transmitted through the parallel elastic components to the tendon The tension in the tendon will reach the maximum tension developed by the contractile element only if the active contraction process is of sufficient duration

17. EFFECT OF SKELETAL MUSCLE ARCHITECTURE The arrangement of the contractile components affects the contractile properties of the muscle The more sarcomere lies in series, the longer the myofibril will be; the more sarcomere lie parallel; the larger the cross- sectional area of the myofibril will be The force the muscle can produce is proportional to the cross- section of the myofibril The velocity and excursion that the muscle can produce are proportional to the length of the myofibril Muscles with short fibres and a large cross- section area: force production (quadriceps); muscles with long fibres: excursion and velocity (Sartorius)

18. EFFECT OF PRE STRETCHING A muscle performs more work when it shortens immediately after being stretched in the concentrically contracted state than when it shortens from a state of isometric contraction Changes in the intrinsic mechanical properties of myofibrils are important in the stretch- induced enhancement of work production EFFECT OF TEMPERATURE A rise in muscle temperature causes an increase in conduction velocity across the sarcolemma, increasing the frequency of stimulation and hence the production of muscle force A rise in temperature also causes greater enzymatic activity of muscle metabolism, thus increasing the efficiency of muscle contraction

19. Rise in temperature increases elasticity of the collagen in the series and parallel elastic components, which enhances the extensibility of muscle- tendon unit EFFECT OF FATIGUE The ability of muscle to contract and relax is dependent on the availability of ATP If a muscle has an adequate supply of oxygen and nutrients that can be broken down to provide ATP, it can sustain a series of low- frequency twitch responses for a long time The frequency must be low enough to allow the muscle to synthesize ATP at a rate sufficient to keep up with the rate of ATP breakdown during contraction The drop in tension following prolonged stimulation is muscle fatigue If the frequency is high enough to produce tetanic contractions, fatigue occurs even sooner

20. MUSCLE FIBRE DIFFERENTIATION Type I slow twitch Oxidative Type II A fast twitch Oxidative glycolytic Type II B fast twitch glycolytic Speed of contraction Slow Fast Fast Primary source of ATP Oxidative phosphorylation Oxidative phosphorylation Anaerobic glycolysis Glycolytic enzyme activity Low Intermediate High Capillaries Many Many Few Myoglobin content High High Low Glycogen content Low Intermediate High Fibre diameter Small Intermediate Large Rate of fatigue Slow Intermediate fast

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