Muscle Contraction Objectives Describe the process of excitation

Muscle Contraction

Objectives • Describe the process of excitation and contraction coupling and muscle relaxation • Practical – In a muscle tracing, identify the following phenomena, muscle twitch, summation, tetanus, staircase phenomenon, muscle fatigue, effect of temperature on muscle contraction

Skeletal Muscle Fiber Structure Pg. 401 Figure 12 -3 b: ANATOMY SUMMARY: Skeletal Muscle

Muscle Fiber Structure Figure 12 -4: T-tubules and the sarcoplasmic reticulum Pg. 402

Muscle contraction • Depolarisation of the muscle membrane spreads through the muscle • Causes muscle contraction (mechanical event)

Muscle contraction • Excitation - contraction coupling – Excitation : electrical event – Contraction : mechanical event

Important structural details • sarcolemma – conduct AP over the surface of the muscle fibre • t tubules – Invagination of sarcolemmal membrane – conduct AP deep into the muscle fibre

Important structural details • sarcoplasmic reticulum (SR) – ends dilated as terminal cisternae – contains abundance of Ca++ ions bound to calsequestrin – Release Ca++ in response to AP in t tubules – Remove Ca++ back in to SR (resequester)

• AP spreads through t tubule into the muscle tissue • Close to the sarcoplasmic reticulum DHP receptor (dihydropyridine receptor) senses the membrane depolarization • alters its conformation • activates the ryanodine receptor (Ry. R) • that releases Ca 2+ from the SR • • Ca flows to the myoplasm in the vicinity of actin & myosin

THIN FILAMENT (Actin) THICK FILAMENT (Myosin)

Actin THIN FILAMENT Tropomyosin THICK FILAMENT Myosin Troponin

Actin Troponin Tropomyosin ATP • Actin Myosin – Composed of 3 different proteins: actin, tropomyosin & troponin – Actin has myosin binding sites – They are normally covered by tropomyosin – Troponin contains Ca++ binding sites • Myosin – Myosin contains protein chains with bent heads which forms the cross bridges – Myosin head contain ATPase. An ATP molecule is attached to it. ATP is broken down during sliding

• Ca++ binds to troponin • Troponin shifts tropomyosin • Myosin binding sites in actin filament uncovered • Myosin head binds with actin • Cross bridges form • Filaments slide with ATP being broken down • Muscle shortens • New ATP occupies myosin head • Myosin head detaches • Filaments slide back • Cycling continues as long as Ca is available Actin Troponin Tropomyosin Myosin ATP

Actin Troponin Ca 2+ Tropomyosin Myosin Binding Myosin binding sites Detachment Ca 2+ Sliding Ca 2+

Ca++ binds to troponin Tropomyosin exposes actin Ca++ Myosin ATP Actin Troponin myosin head binds to actin & cross bridge forms Filaments slide ATP is broken down New ATP comes, Ca is removed, ready to detach

ATP as the source of energy for contraction �the heads of the cross-bridges bind with ATP �ATPase activity of the myosin head immediately cleaves the ATP �Ca++binds with troponin-tropomyosincomplex, active sites on the actin filament are uncovered �bond between head of the cross-bridge & the active site of the actin filament

• once the head of the cross-bridge tilts, this allows release of the ADP & phosphate ion • a new molecule of ATP binds • binding of new ATP causes detachment of the head from the actin • the new molecule of ATP is cleaved to begin the next cycle

The process by which depolarization of the muscle fiber initiates contraction is called excitationcontraction coupling.

Walk-along theory of contraction • Heads of myosin filament is known to forms cross bridges by attachment • Then head bends causing actin filament to slide • Then head detaches from the actin filament and walked to a new site in actin filament and attaches again • This process continue to happen • As if myosin head walk-along actin filament

• Relaxation – This occurs when Ca++ is removed from myoplasm by Ca++ pump located in the sarcoplasmic reticulum – When Ca++ conc is decreased – Troponin returns to original state – Trpomyosin covers myosin binding sites – Cross-bridge cycling stops

Timing of Electrical & Mechanical Events Myogram of Single Muscle Twitch

Dystrophin • Dystrophin is a rod-shaped cytoplasmic protein, and a vital part of a protein complex that connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane • It provides an anchoring function to the muscle proteins

• Dystrophin deficiency causes rare muscle diseases known as muscular dystrophies

Muscle twitch

Muscle twitch • A single action potential causes a brief contraction followed by relaxation • This response is called a muscle twitch. • The twitch starts about 2 ms after the start of depolarization of the membrane, before repolarization is complete

Muscle twitch • Duration of the twitch varies with the type of muscle being tested • Fast muscle fibers, primarily those concerned with fine, rapid, precise movement, have twitch durations as short as 7. 5 ms • Slow muscle fibers, principally those involved in strong, gross, sustained movements have twitch durations up to 100 ms • The strength of twitch depends on the number of motor units activated

Summation and tetanus

Summation • If 2 stimuli are delivered in rapid succession the second twitch will be greater than the first • This only occurs if repolarization is not complete • The tension developed during summation is considerably greater than that during the single muscle twitch

Tetanus • With rapidly repeated stimulation, activation of the contractile mechanism occurs repeatedly before any relaxation has occurred • Individual responses fuse into one continuous contraction • Such a response is called a tetanus or tetanic contraction

Tetanus • It is a complete tetanus when there is no relaxation between stimuli and an incomplete tetanus when there are periods of incomplete relaxation between the summated stimuli • During a complete tetanus, the tension developed is about four times that developed by the individual twitch contractions

Staircase phenomenon (treppe) • When a series of stimuli is delivered to skeletal muscle, there is an increase in the tension developed during each twitch until, after several contractions, a uniform tension per contraction is reached • This phenomenon is known as treppe, or the "staircase" phenomenon • This is the basis of “warm up” • Treppe is believed to be due to increased availability of Ca 2+ for binding to troponin C, accumulation of heat or effect of p. H • It also occurs in cardiac muscle although cardiac muscles cannot be tetanised

Staircase phenomenon (treppe)

Staircase effect, summation and tetanus

Tetanus

Isotonic contraction • Produces movement • Most of the time movement is of this type • Used in – Walking – Running – Movement of a part of the body (eg. Hand movement)

Isotonic contraction

Isometric contraction • Muscular contraction involves shortening of the contractile elements, but because muscles have elastic and viscous elements in series with the contractile mechanism, it is possible for contraction to occur without an appreciable decrease in the length of the whole muscle • Such a contraction is called isometric ("same measure" or length)

Isometric contraction

Isometric contraction • Produces no movement • Used in – Standing – Sitting – Postural control