Phyg 16693 Module 1 Skeletal Muscle PDF

Module 1 Structure and Function of Exercising Muscle 1 Learning Objectives w Learn the structure of skeletal muscle, the muscle fiber, and the myofibril. w Learn the cellular events leading to a basic muscle contraction. w Disco...

Module 1 Structure and Function of Exercising Muscle 1 Learning Objectives w Learn the structure of skeletal muscle, the muscle fiber, and the myofibril. w Learn the cellular events leading to a basic muscle contraction. w Discover how muscle functions during exercise. w Consider the differences in fiber types and their impact on physical performance. w Learn how muscles generate force. 2 Types of Muscles Skeletal w Voluntary muscle; controlled consciously  Over 600 throughout the body Cardiac w Controls itself with assistance from the autonomic nervous and endocrine systems w Only in the heart Smooth w Involuntary muscle; controlled unconsciously by the autonomic nervous system  In the walls of blood vessels and internal 3  SKELETAL MUSCLE STRUCTURE Epimysium – CT surrounding the entire muscle Endomysium – CT surrounding each muscle fibre Perimysium – CT surrounding each fasciculus Fasciculus – Bundle of muscle fibres Muscle Fibre – single muscle Sarcomere cell Myofibril – contractile element of muscle fibre Sarcomere – smaller branch of myofibril, the smallest functional 4 unit of a muscle.  STRUCTURE OF A SINGLE MUSCLE FIBER or Sarcolemma 5  Key Points: Skeletal Muscle Fiber Structure An individual muscle cell is called a _____________ A muscle fiber is enclosed by a plasma membrane called the ___________________________________ The cytoplasm (fluid portion) of a muscle fiber is called the _____________________________. Within the__________________________, the extensive T-tubules allow transport of substances throughout the muscle fiber The _______________________________________ stores calcium 6 ELECTRON MICROGRAPH OF MYOFIBRILS Myofibrils - Each muscle fibre contains several hundred to several thousand myofibrils. - Under a microscope, muscle fibres have a striped appearance and are called “striated” muscle (actually Each myofibril is composed of numerous sarcomeres looking joinedat the end to ©end Custom Medical Stock Photo sarcomeres) between the 2 Z lines (or Z disk) (see next slide) 7 ARRANGEMENT OF FILAMENTS Sarcomere found between each Z disk and consists of (in this sequence): - I band (light) -A band (dark) - H zone (middle of A band) - rest of the A band - second I band 2 types of protein filaments are found in the sarcomere: - Actin (thin) 8 - Myosin (thick) ARRANGEMENT OF FILAMENTS: notes page Light I band: indicates the region of the sarcomere where there are only thin actin filaments Dark A band: represents the regions that contain both thick myosin and thin actin filaments. 9 ARRANGEMENT OF FILAMENTS: notes page H Zone: – central portion of the A band - only has thick myosin filaments - Absence of the actin filaments causes the H zone to appear lighter than the adjacent A band H zone Appears only when the sarcomere is in a resting state (relaxed), because the sarcomere shortens during contraction and the actin filaments are pulled into this zone, giving it the same 10 appearance as the rest of the A band ARRANGEMENT OF FILAMENTS IN A SARCOMERE Actin filament is composed of: Actin Troponin Tropomyosin Myosin Filament 2/3 of muscle protein Heads form cross- bridges that interact during muscle contraction with the active sites on actin filaments 11  ARRANGEMENT OF FILAMENTS IN A SARCOMERE Actin filament - one end of the actin filament is attached to the Z-disk - The other end extends towards the sarcomere, between the myosin filaments Actin Molecule: backbone of the actin filament -active binding sites are on the actin molecule -the binding sites serve as a point of contact with the myosin heads for muscle contraction 12 Troponin and Tropomyosin - Tropomyosin wraps around actin - Troponin is attached to tropomyosin - Work together with calcium to maintain relaxation or initiate contraction 13  ARRANGEMENT OF FILAMENTS IN A SARCOMERE Nebulin_– an anchoring protein for actin - plays a role as a mediator in the actin and myosin interactions Myosin – 2 protein strands twisted together - one end of each strand is folded into a globular head, called the myosin head Titin – stabilizes the myosin - The myosin head forms filaments in the longitudinal axis cross-bridges 14  The Myofibril: Key Points  ___________________ are made up of sarcomeres A sarcomere is composed of protein filaments of __________________ and ________________ Interactions between the filaments is responsible for _________________ ____________________ _____________, the thick filament, is composed of two protein strands, each folded into a globular head at one end The ______________ head forms ___________________________ The thin filament is composed of ______________________, ___________________, and _______________, with one end attached to a _________________. 15 Muscle Fibre Action An action potential (electrical signal from MOTOR NEURON and brain or spinal cord) is sent to the -motor MOTOR UNIT neuron. The dendrites sends the signals down the axon to the axon terminal When a motor neuron enters a muscle, it branches, with each axon terminal Axon hillcock- supplying a single muscle fibre. Site where action potential When a motor neuron is activated, all of the is initiated muscle fibres are stimulated to contract simultaneously. This is called the All or None Principle All-or-None Principle An excitable membrane either responds to a stimulus with a maximum action potential that spreads throughout the membrane, or it does not respond with an action potential at all. 16 Muscle Fibre Action Different # of -motor neurons for MOTOR NEURON and different types of muscles....examples MOTOR UNIT Muscles controlling fine movements, such as those controlling the eyes, have a small number of muscle fibers per motor neuron (about 1 neuron for every 15 muscle fibers). Muscles with more general function, such Axon hillcock- as those controlling the calf muscle, have Site where many fibers per motor neuron (about 1 action potential is initiated neuron for every 2,000 muscle fibers). One -motor neuron plus all of the muscle fibers it innervates – is called a motor unit 17 EVENTS LEADING TO MUSCLE CONTRACTION Neuromuscular junction (NMJ) Plasmalemma/ sarcolemma 18 EVENTS LEADING TO A MUSCLE CONTRACTION NMJ A-1. A motor neuron, with signals from the brain or spinal cord, releases the neurotransmitter acetylcholine (ACh) at the neuromuscular junction (NMJ) A-2. ACh crosses the junction and binds to receptors on the sarcolemma (plasmalemma) A-3.This initiates an action potential providing there is sufficient ACh. 19 EVENTS LEADING TO MUSCLE CONTRACTION B- Step -4. The action potential travels along the sarcolemma (plasmalemma) and through the T tubules to the SR releasing Ca2+. 20 EVENTS LEADING TO MUSCLE CONTRACTION C-5.The Ca2+ binds to troponin on the actin filament, and the troponin pulls tropomyosin off the active sites, allowing myosin heads to attach to the actin filament. C-6.Once a strong binding state is established with actin, the myosin head tilts, pulling the actin filament inward towards the middle of the sarcomere (this is called the power stroke). C-7.The myosin head binds to ATP, and ATPase found on the head splits ATP into ADP and Pi, releasing energy. C-8.Muscle action ends when calcium is actively pumped out of the sarcoplasm 21 back into the sarcoplasmic reticulum for storage.  Sequential Events Sliding of Muscle ADP Myosin head (high-energy Pi Filament Contraction configuration) Theory 1 Myosin head attaches to the actin myofilament, forming a cross bridge. Thin filament ATP ADP ADP Muscle Thick filament hydrolysis Pi Contraction 2 phosphate (Pi) generated in the 4 “Cocking” of the myosin head previous contraction cycle is released, initiating As ATP is split into ADP and P i, the myosin the power (working) stroke. The myosin head head is energized. pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released. ATP Myosin head ATP (low-energy configuration) 3 As new ATP attaches to the myosin head, the link between myosin and actin weakens, and the cross bridge detaches. 22  Sequential Events of Contraction - Sliding Filament Theory - Summary 1. Cross bridge formation 2. Power stroke 3. Cross bridge detachment 4. “cocking of the myosin head” 23 Sliding Filament Theory: Sarcomere Filaments During Contraction 24 CONTRACTING MUSCLE FIBER: Simplified 25 Put the pictures in the correct order: 1 = relaxed 2 = contracting 3 = fully contracted 26  Muscle Fibre Type Slow Twitch vs Fast Twitch Fibres ST/Type I – darkest (black) FTa /Type IIa - lightest (white) FTb/FTx/Type IIx – middle colour (gray) FTc/Type IIc - (can’t identify) 27  Type I and Type II differ in their speed of contraction, Why? This is due to different forms of myosin ATPase - Myosin ATPase is the enzyme that splits ATP to release energy. - Type I have slow forms of myosin ATPase, therefore ATP is split more slowly - Type II have fast forms of myosin ATPase, ATP is split more rapidly 28  Muscle Fibers (Type I – Slow Twitch) High aerobic (oxidative) capacity and fatigue resistance Low anaerobic (glycolytic) capacity and motor unit strength Slow contractile speed (110 ms to reach peak tension when stimulated) 10 –180 fibers per motor neuron Low sarcoplasmic reticulum development Most muscles are approximately 50% ST Human Body-------Soleus and muscles in the back are very high in Type I oxidative fibres, because they are needed to maintain posture continuously 29 Food ----- dark meat Muscle Fibers (Type IIa – Fast Twitch) w Moderate aerobic (oxidative) capacity and fatigue resistance w High anaerobic (glycolytic) capacity and motor unit strength w ATPase in Type IIa act faster than in ST fibres, therefore, there is a fast contractile speed (50 ms to reach peak tension) w 300–800 fibers per motor neuron  High sarcoplasmic reticulum development enhancing calcium delivery to muscle  Most muscles are composed of approximately 25% Type IIa NOTE:Type IIa can utilize BOTH anaerobic and aerobic metabolism - Therefore known as intermediate fast twitch fibres - Smaller muscles that fatigue easily contain Type IIa fast twitch 30 Muscle Fibers (Type IIx or IIb – Fast Twitch) w Low aerobic (oxidative) capacity and fatigue resistance w High anaerobic (glycolytic) capacity and motor unit strength  ATPase in Type IIx fibres act faster than in ST fibres, therefore, Type IIx fibres have a fast contractile speed (50 ms to reach peak tension) w 300–800 fibers per motor neuron  High sarcoplasmic reticulum development enhancing calcium delivery  Type IIx make up 25% of the muscle  Type IIc make up only 1% - 3% of the muscle (difficult to detect)  Note: the eye has mainly Type IIx but also Type IIa fibres31  Determining Muscle Fibre Type Muscle Biopsy Single Muscle Fibre Physiology Gel Electrophoresis 32 Muscle Biopsy to Determine Fibre Type w Hollow needle is inserted into muscle to take a wsample.Sample is mounted, frozen, thinly sliced, and examined under a microscope. w Allows study of muscle fibers and the effects of acute exercise and exercise training on fiber composition. Vastus lateralis is often used. Lidocaine is used to numb the area 33 SINGLE MUSCLE FIBER PHYSIOLOGY to determine fibre type A muscle biopsy is done and then a single muscle fibre is dissected. The muscle fibre is suspended between force transducers. A computer program analyzes the strength and velocity of the fibre to determine its fibre type. 34  Side Note Fact: SINGLE MUSCLE FIBER PHYSIOLOGY to determine fibre type The difference in force development between FT and ST motor units is due to: - the number of muscle fibers per motor unit and - the larger diameter of the FT fibers. **All fibre types tend to reach their peak power at approximately 20% of peak force 35  GEL ELECTROPHORESIS to determine fibre type Chemically separate different types of myosin molecules. Can detect fibres that have 2 or more forms of myosin. The fibres may be classified as 7 different types: I I/IIa IIa I/IIx IIx I/IIa/IIx IIa/IIx 36  Muscle Fibre Types: Summary Type IIx is also known as Type IIb…..Type IIc difficult to detect 37 Muscle Fibre Types: Summary 38  Review Questions: Fibre Type Answer the questions with one of the following responses: – Type I, Type II, both 1. Found in most skeletal muscles. _____________________________________ 2. Takes longer than other muscle fibres to reach peak tension. ______________ 3. Predominantly contains a slow form of myosin ATPase. __________________ 4. Has a more highly developed sarcoplasmic reticulum, which helps deliver calcium to the muscle cell when stimulated. ______________________________ 5. Has a motor neuron with a small cell body and can innervate a cluster of 10 to 180 muscle fibres. __________________________________________________ 6. Predominantly contains a fast form of myosin ATPase. ___________________ 7. Reaches peak tension faster and collectively generates more force than other fibre types. ___________________________________________________ 8. Has three subtypes. ___________________________________________ 39  Motor Unit Recruitment When an  motor neuron carries an action potential to the muscle fibres in the motor unit, ALL fibres in the unit develop force Activating MORE motor units produces more force Muscle contraction involves a progressive orderly recruitment of type I and then Type II motor units (depending on the exercise) 40 Orderly Recruitment of Muscle Fibers  Principle of orderly recruitment states that motor units are activated in a fixed order. - As intensity , the # of fibres recruited  (in the following order): Type I Type IIa Type IIx  Size principle states that the order of recruitment is directly related to their motor neuron size. - ST fibers, which have smaller motor neurons, are recruited before FT fibers. - Motor units supplying FT fibres are larger (eg – more fibres per motor neuron) than those supplying ST fibres, therefore FT motor units can recruit more fibres 41 Orderly Recruitment of Muscle Fibres ST fibres responsible for force (strength) at a lighter x load – therefore endurance type, can lift light loads for a long period of time FT fibres are recruited as the load gets heavier, - therefore are used for strength, explosive movements (lifting a heavy weight, sprinting, etc) 42 What Determines Fiber Type? w Genetics determine which type of motor neurons innervate our individual muscle fibers. w Muscle fibers become specialized according to the type of neuron that stimulates them.  Endurance training and muscular inactivity may result in small changes (less than 10%) in the percentage of FT () and ST () fibers.  Strength training may  Type IIa  Endurance training has been shown to  the percentage of Type IIx fibers, while  the fraction of Type IIa fibers.  Aging may result in changes in the percentage of FT () and ST fibers (). 43  Fibre Type in Athletes Fibre Type in Athletes varies: - males and females - Different muscle groups - Different sports World Champion Marathon Athletes 93% to 99% ST fibres (gastrocnemius) World Class Sprinters 25% ST fibres (gastrocnemius) 44 % Type I % Type II 45  The Effect of Endurance and Sprint Training on Muscle Fibre Type Black Bars – pre-training Open Bars – post-training Muscle fibre composition % Following a 6-week endurance training program: Type I (ST) fibres increased 18% Type IIa (FT)decreased 12% Type IIx (FT) decreased 6% Following a 6-week sprint training program: Type I fibres decreased 7% I IIa IIx I IIa IIx Type IIx fibres decreased 3% Type IIa fibres increased 10% Endurance Sprint 46 Athletes and Fibre Type: Key Points Muscle fiber composition differs in athletes by sport and event Speed and strength events are characterized by a higher percentage of type II fibers Endurance events are characterized by a higher percentage of type I fibers Fibre type alone is NOT a reliable predictor of the athlete’s success (what is??) 47 Functional Classification of Muscles Agonists—prime movers; responsible for the movement Antagonists—oppose the agonists to prevent overstretching of them Synergists—assist the agonists and sometimes fine-tune the direction of movement 48 Types of Muscle Contraction Concentric: muscle shortening Isometric: muscle is generating force but there is no change in muscle length Eccentric: muscle generates a force while lengthening 49 TYPES OF MUSCLE ACTION 50  Types of Muscle Contraction: Concentric actin and myosin filaments slide across each other because joint movement is produced, concentric actions are considered “dynamic actions” 51  Types of Muscle Contraction: Eccentric actin filaments are pulled farther away from the centre of the sarcomere, essentially stretching it 52  Types of Muscle Contraction: Isometric joint angle does not change, therefore consider “static movement” the myosin cross bridge (CB) form and are recycled, producing force, but the external force is too great for actin filaments to be moved – they remain in their normal position, so shortening can’t occur If enough motor units can be recruited to produce sufficient force to overcome the resistance, a static action (isometric) can become a dynamic one (concentric) 53 Types of Muscle Contraction: Isokinetic accomplished by the aid of a mechanical device (Biodex, Cybex) constant speed, variable resistance throughout the full ROM (semi-accommodating resistance) Activates the largest # of motor units and consistently overloads the muscle at their force output capacities during movement, even at the relatively “weaker” joint angle 54 Types of Muscle Contraction: Omnikinetic Accomplished by the aid of a mechanical machine (hydraulics, Hydra-Fitness/Hydra- Gym) Variable speed, variable resistance Works concentric movement in two muscle groups (ie – quadriceps, hamstrings) Activates the largest # of motor units and consistently overloads the muscle at their force output capacities during movement, even at the relatively “weaker” joint angle 55 Generation of Force The amount of force developed by a muscle (or muscle group) is dependant on: - joint angle -speed of contraction -muscle fibre and sarcomere length -number and type (Type I or Type II) of motor units activated -size of the muscle -frequency and stimulation of each motor unit 56  Generation of Force: Joint Angle All joints have an optimal angle at which the muscles crossing the joint produce maximal force. The angle of maximal force depends on the relative position of the muscle's insertion on the bone and the load placed on the muscle. Speed of action affects the amount of force produced. (faster, less force --- isokinetic machines (Biodex) can measure force with varying speeds) 57 Force- generating capacity of a muscle or muscle group varies in relation to joint angle throughout the ROM in both flexion and extension. 58 Speed of Contraction: Relationship between muscle lengthening and shortening velocity, and force production. Concentric: greater force at slower speeds Eccentric: greater force at faster speeds The capacity for muscle to generate force is greater during eccentric (lengthening) actions than during concentric (shortening) contractions. 59  Why Eccentric Contractions are the Strongest 1.) “Braking” action of stretched cross-bridges Stretched cross-bridges are opposed to an external force “braking” action increases force above isometric level 2.) Increased # of attached cross-bridges In eccentric contractions, approx 75% of CB’s are attached at any time (only 50% in isometric) Therefore, more attached CB’s, more force produced. 60 Muscle Fibre and Sarcomere Length The optimal sarcomere length is defined as that length where there is optimal overlap of actin and myosin, thus maximizing cross-bridge formation When a sarcomere is fully stretched (A) or fully shortened (E), little or no force can be developed because there is very little cross-bridge interactions C or B is best (remember eccentric contractions!!!) 61 Number and Type of motor units activated  motor units activated,  force Type II generate more force than Type I --- Why? – because Type II have more muscle fibres than Type I) Larger muscles produce more force—Why? (because they have more muscle fibres) 62 Frequency and stimulation of each motor unit  in stimulation,  force Twitch – response to a single electrical stimulus Summation – series of 3 stimuli in rapid sequence Tetanus – continued stimulations at higher frequencies Rate Coding – term used to refer to the process by which the tension of a motor unit can vary from a twitch to a tetanus by increasing the frequency of stimulation 63 Muscle Size Larger muscles (and muscle groups) produce more force Examples: Arm vs Leg Strength Quadriceps vs Hamstrings 64 Golgi Tendon Organ  Lies within a tendon near its attachment to the muscle  Sensory receptor  Detects tension applied to the tendon as a result of muscle contraction  If tension is extreme, it sends a signal to the muscle to relax (protective function by reducing potential for injury)  When stimulated, Golgi Tendon Organs inhibit the contracting (agonist) muscles and excite the antagonist muscles 65 Review: Generation of Force 1. ___________________________ is the term used to describe the process by which tension of a given motor unit can vary from a twitch to tetanus by increasing the frequency of stimulation of a motor unit. a. Frequency adjustment b. Stimulation synchronization c. Rate-coding d. Summation 2. Which statement is true? a. More force can be generated when more motor units are activated. b. Type I motor units contain more muscle fibers than type II motor units. c. Larger muscles tend to have fewer muscle fibers. d. Type II motor units generate less force than type I motor units. 3. A sarcomere generates the most force when ________________________________. a. it is fully stretched, producing more pulling power b. it has optimal overlap of the thick and thin filaments, producing good cross-bracing interaction c. it is in its shortest, most powerful position d. its thick and thin filaments are not touching 4. Maximal force development decreases progressively at higher speeds in _________________ contractions. a. eccentric b. isometric c. static d. concentric 66