Skeletal Muscle: Structure And Function PDF

SKELETAL MUSCLE: STRUCTURE AND FUNCTION Chapter 8 Lecture Outline Structure of Skeletal Muscle. Neuromuscular Junction. Muscular Contraction. Exercise and Muscle Fatigue. Exercise-Associated Muscle Cramps. Fiber Types. Muscle Actions. Speed of Muscle Action and Relax...

SKELETAL MUSCLE: STRUCTURE AND FUNCTION Chapter 8 Lecture Outline Structure of Skeletal Muscle. Neuromuscular Junction. Muscular Contraction. Exercise and Muscle Fatigue. Exercise-Associated Muscle Cramps. Fiber Types. Muscle Actions. Speed of Muscle Action and Relaxation. Force Regulation in Muscle. Force-Velocity/Force-Power Relationships. Muscles Functions of skeletal muscle Force generation for locomotion and breathing Force generation for postural support Heat production during cold stress Tissues in skeletal muscle Muscle cells (fibers) Nerve tissue Blood vessels Connective tissue  Connective tissue surrounding skeletal muscle 1. Fascia Separates individual muscles and holds them in position 2. Epimysium Surrounds entire muscle 3. Perimysium 1 Surrounds bundles of muscle fibers (fascicles) 4. Endomysium 2 Surrounds individual muscle fibers 3 4 3b 5. External lamina (basement membrane) Just below endomysium 6 6. Sarcolemma Muscle cell membrane Structural Organization of Skeletal Muscle  Muscle cells (fibers) Unique shape Contain same organelles with other cells mitochondria, etc But they are multinucleated (have many nuclei) Striated appearance Stripes produced by alternating dark and light bands Each muscle fiber extends the entire length of the muscle Cell membrane is called the sarcolemma Above the sarcolemma and below the external lamina are a group of muscle precursor cells called satellite cells  Satellite cells Undifferentiated cells involved in muscle growth and repair Satellite cells can be activated to increase number of nuclei Increasing the number of nuclei allows for greater protein synthesis can be important for adaptations to strength training Each nucleus can support the gene expression (protein synthesis) of a limited volume of sarcoplasm surrounding each nucleus called myonuclear domain  During growth, nuclei from satellite cells are incorporated to maintain a constant ratio of cell volume/nucleus (myonuclear domain) Muscle hypertrophy→ increased number of nuclei by activating satellite cells Muscle atrophy→ decreased number of nuclei Microstructure of muscle fibers Myofibrils Threadlike structures contain contractile proteins 2 major filaments: Sarcomeres -myofibrils subdivided into sarcomeres -divided by thin sheet of structural proteins called a Z-line or Z-disk Microstructure of skeletal muscle A-band: myosin filaments in the dark region of the sarcomere I-band: actin filaments in the light regions H-zone: center of sarcomere with no overlap of actin Sarcoplasmic reticulum & transverse tubules Structures that play an important role for muscular contractions: sarcoplasmic reticulum (SR): a network of channels that surrounds each myofibril and is the storage site for calcium transverse tubules: another set of channels extends from sarcolemma into the muscle fiber and passes completely through the fiber. transverse tubules pass between two enlarged SR portions called the terminal cisternae  Neuromuscular junction: Neuromuscular Junction where motor nerve and muscle fiber meet Motor end plate: pocket formed around motor neuron by sarcolemma Synaptic cleft: short gap b/n neuron & muscle fiber When an action potential reaches synaptic knob: 1) Acetylcholine (Ach) is released from synaptic vescicles 2) Ach diffuses across synaptic cleft & binds to receptors on motor end plate. 3) ↑ sarcolemma sodium permeability 4) depolarization of the muscle fiber (end- plate potential; EPP) EPP always exceeds threshold and is signal to begin contraction Check your understanding ▪ Skeletal muscle performs three major functions: (1) ______________________________ (2) ______________________________ (3) _______________________________ ▪ Individual muscle fibers are composed of hundreds of threadlike protein filaments called _______________. ▪ Myofibrils contain two major types of contractile protein: (1) ________ (part of the thin filaments) and (2) ______ (major component of the thick filaments). Check your understanding ▪ The region of cytoplasm surrounding an individual nucleus is termed the __________ domain. ▪ The importance of the myonuclear domain is that a single nucleus is responsible for the _________________ for its surrounding cytoplasm. ▪ Motor neurons extend outward from the spinal cord and innervate individual muscle fibers. The site where the motor neuron and muscle cell meet is called the ___________________. ▪ _______________ is the neurotransmitter that stimulates the muscle fiber to depolarize, which is the signal to start the contractile process. Muscle Contraction Best explained by the sliding filament/swinging lever arm model of contraction Muscle shortening occurs due to the movement of actin over myosin There is a reduction in the distance between z-lines Muscular contraction and cross-bridges Heads of myosin cross-bridges are oriented towards actin. During muscular contraction, actin and myosin slide across each other due to the action of numerous cross-bridges extending out as arms from myosin and attaching to actin. Binding of the myosin cross-bridge to actin results in an orientation of cross- bridges so that they can pull on actin from each side of the sarcomere towards the center. This “pulling” of actin over myosin results in muscle shortening and the generation of force. The sliding filament/swinging lever-arm model Myofilaments do not change length during muscle contraction Energy is required for muscle contraction Comes from breakdown of ATP by the enzyme myosin ATPase located at head of myosin cross-bridge When ATP → ADP + Pi , energy is released, the myosin cross-bridges become energized, which in turn pull actin over myosin and thus shorten the muscle A single contraction cycle or “power stroke” of all the cross-bridges in a muscle would shorten the muscle by only 1% of its resting length Because some muscles shorten up to 60% of their resting length, the contraction cycle must be repeated over and over again Sources of ATP for muscle contraction Relation between troponin, tropomyosin, myosin cross-bridges Regulation of contraction is controlled by 2 regulatory proteins: 1. Troponin 2. Tropomyosin The actin filament is formed by subunits arranged in a double row and twisted Tropomyosin is a thin molecule that lies in groove between double rows of actin Attached directly to tropomyosin is the protein troponin Troponin and tropomyosin work together to regulate the attachment of actin and myosin cross-bridges Relation between troponin, tropomyosin, myosin cross-bridges In a relaxed muscle, tropomyosin blocks the active sites on the actin molecule necessary for myosin cross-bridges to attach/form a strong binding state to produce a contraction. Contraction is triggered by release of calcium from terminal cisternae (region of sarcoplasmic reticulum). Calcium binds to troponin causing a position change in tropomyosin which uncovers the required active sites on actin. This allows the strong binding of an energized or cocked myosin cross-bridge on the actin molecule Regulation of excitation-contraction coupling Excitation 1. Nerve signal (action potential) arrives at the neuromuscular junction 2. Acetylcholine is released in the neuromuscular cleft and binds to receptors on the sarcolemma of the muscle fiber. This causes the movement of sodium into the fiber which depolarizes the fiber. 3. Wave of depolarization is conducted down the transverse tubules 4. When the action potential reaches the sarcoplasmic reticulum, the terminal cisternae (i.e., lateral sacs) release calcium which diffuses through the muscle fiber. Contraction 5. Ca++ binds to troponin and causes a position change in tropomyosin which exposes the myosin binding sites on actin. 6-8.An energized myosin cross bridge binds to the active site on actin and pulls on the actin molecule to produce movement (i.e., fiber shortening). This contraction cycle can be repeated as long as Ca++ can bind to troponin and ATP is available to energize the myosin cross-bridges. Failure of the muscle to maintain adequate Ca++ or to break ATP results in fatigue. Relaxation 9. The first step in muscle relaxation occurs when the motor neuron stops firing. Ach is no longer released and the muscle fiber is repolarized. 10. When this occurs, an energy-requiring Ca++ pump begins to move Ca++ back into the sarcoplasmic reticulum. Removal of Ca++ from troponin causes tropomyosin to move back to cover the binding sites on the actin molecule, and the muscle relaxes. Steps leading to muscle contraction via cross-bridge cycling 1. Resting muscle fiber; myosin head is not attached to actin (calcium remains in SR). 2. After calcium is released from SR, myosin head binds to actin and forms cross- bridge to actin. 3. Pi is released from myosin head causing a shift in position in myosin. 4. Power stroke causes myosin cross-bridge to pull actin inward and actin/myosin filaments slide across each other resulting in muscle shortening. ADP is released during this step. 5. Binding of a new ATP to myosin head allows it to release from actin. 6. Myosin ATPase breaks down the ATP causing the energized myosin head to return to its original position.  Check your understanding The process of muscular contraction can be best explained by the ___________________/_____________________ model, which proposes that muscle shortening occurs due to movement of actin filament over myosin filament. Excitation-contraction coupling refers to the sequence of events in which the nerve impulse (action potential) ______________ the muscle fiber, leading to muscle _______________ by cross-bridge cycling. The trigger to initiate muscle contraction is the depolarization-induced release of ________ from the sarcoplasmic reticulum. Muscular contraction occurs via the binding of the ___________ to __________ and the repeated cycling of myosin pulling on the actin molecule resulting in the shortening of the muscle fiber. Muscle relaxation occurs when the motor neuron _____________ the muscle fiber and ____________ is pumped backed into the sarcoplasmic reticulum. This removal of __________ from the cytosol causes a position change in __________, which blocks the ___________ binding site on the _______ molecule; this action results in muscle relaxation. Muscle fatigue Fatigue defined as a decline in muscle power output occurs due to: ↓ in muscle force production at cross-bridge level ↓ in muscle shortening velocity Short-term high intensity or prolonged submaximal exercise result in muscle fatigue which is reversible after rest. Cause of muscle fatigue dependent upon exercise intensity that produced fatigue.  Mechanisms of fatigue during heavy, very heavy, and severe exercise (1-10 min) ↑ key metabolites that contribute to fatigue: Pi , H+, & free radicals. H+ ions bind to Ca+2 binding sites on troponin → prevent Ca+2 binding and contraction. Both Pi and radicals modify cross-bridge head → reduce number of cross-bridges bound to actin. Mechanisms of fatigue during prolonged moderate intensity exercise (>60 min) Radical accumulation in muscle fibers modifies cross-bridge head → reduces number of cross-bridges bound to actin ↓ force production Depletion of muscle glycogen reduces Krebs cycle intermediates ↓ ATP production via oxidative phosphorylation  Central governor theory of exercise-induced fatigue Winning Edge 7.1 Research suggests that fatigue may be due to both peripheral (muscle- related) factors and central (neural-related) factors Central fatigue. Fatigue due to neuronal dysfunction within higher brain centers and/or motor neurons. Dysfunction may be due to depletion of excitatory neurotransmitters in the motor cortex resulting in reduced motor output to muscle. Central governor theory. Central control center regulates exercise performance. Reduces motor output to exercising muscle. Protects against catastrophic disruptions of homeostasis by promoting fatigue and cessation of exercise before damage occurs to the working muscle  Exercise-Associated Muscle Cramps Muscle cramps are spasmodic, involuntary muscle contractions Often associated with prolonged, high intensity exercise Most exercise-associated cramps do not appear to be caused by an electrolyte imbalance or dehydration Exercise-associated cramps are likely due to excessive firing of motor neurons in the spinal cord Rigorous exercise can alter muscle spindle and Golgi tendon organ function ↑ excitatory activity of muscle spindles ↓ inhibitory effect of the Golgi tendon organ Passive stretching often relieves this type of muscle cramp Muscle fiber types Muscle can be divided into several classes based on histochemical or biochemical characteristics of fibers. Muscle fibers are generally classified into 2 main categories: 1. slow-twitch fibers (type I) 2. fast-twitch fibers (type IIa and IIx) Most muscle groups contain an equal mixture of slow and fast fibers. The percentage of each fiber type in a muscle depends on genetics, hormones and exercise training. The fiber composition of skeletal muscles is important in both power and endurance events. Biochemical properties of muscle fibers Oxidative capacity More mitochondria provide greater capacity to produce ATP aerobically More capillaries ensure that enough O2 is available during muscular contraction More myoglobin improves delivery of O2 from capillaries to mitochondria A muscle fiber with high aerobic capacity will be fatigue resistant Type of myosin ATPase The speed of ATP degradation and in turn, the speed of muscular contraction depends on the isoform of ATPase Muscle fibers containing isoforms of high ATPase activity will degrade ATP rapidly and generate higher speed of muscle shortening compared to those fibers with isoforms of low ATPase activity. Abundance of contractile proteins (i.e., actin and myosin in muscle fiber). Contractile properties of muscle fibers Maximal force production Force produced per unit of fiber cross-sectional area (specific tension) Speed of contraction Maximal shortening velocity at which a fiber can shorten (called Vmax) Vmax is determined by the rate of cross-bridge cycling which is influenced by the type of myosin ATPase activity Maximal power output Determined by both force generation and shortening velocity Muscle fiber efficiency Energy used divided by force produced Efficient fibers require less energy to perform a certain amount of work Characteristics of individual fiber types Type IIx fibers Fast-twitch or fast-glycolytic fibers Type IIa fibers Intermediate or fast-oxidative glycolytic fibers Type I fibers Slow-twitch or slow-oxidative fibers The biochemical and contractile properties of type IIx, type IIa, and type I fibers represent a continuum instead of three neat packages. Characteristics of individual fiber types Type IIx Type IIa Type I Comparison of maximal shortening velocities between fiber types Comparison of Force Production and Power Output Between Fiber Types Characteristics of muscle fiber types Do fast fibers exert more force than slow fibers? Maximal force per cross-sectional area 10–20% higher in fast fibers (IIa and IIx) compared to slow (Type I) fibers Force production related to number of myosin cross-bridges bound to actin Fast fibers contain more cross-bridges per cross-sectional area compared to slow fibers Fiber types and performance Nonathletes Have approximately 50% slow and 50% fast fibers There are no differences related to sex Power athletes Sprinters- higher percentage of fast fibers Endurance athletes Distance runners - higher percentage of slow fibers Distribution of fiber type in athletes -Fiber type is not the only variable that determines success in an athletic event -Success due to complex interaction of psychological, biochemical, neurological, cardiopulmonary and biomechanical factors.  Check your understanding ▪ Human skeletal muscle fiber types can be divided into three general classes of fibers based on their biochemical and contractile properties. Two categories of fast fibers exist, _______and _________. One type of slow fiber exists, ______fibers. ▪ Remember that the biochemical and contractile properties of type IIx, type IIa, and type I fibers represent a _____________ instead of three neat packages. ▪ Successful power athletes (e.g., sprinters) generally possess a large percentage of _________ muscle fibers and, therefore, a low percentage of __________muscle fibers. ▪ In contrast to power athletes, endurance athletes (e.g., marathoners) typically possess a high percentage of ___________ muscle fibers and a low percentage of ____________ fibers. ▪ Although muscle fiber types are known to play a role in sport performance, considerable ____________ exists among successful athletes competing in the same sport.  Types of muscle action Isometric (static) No body part movement Muscle exerts force without large amount of muscle shortening Pulling against immovable object For example, postural muscles Isotonic (dynamic) Body part moves as force is produced Concentric Muscle shortens during force production Eccentric Muscle lengthens during force production Types of muscle actions Concentric Eccentric Isometric  Speed of muscle action and relaxation Muscle twitch Contraction resulting from single stimulus Latent period (~5 ms)-depolarization of muscle fiber Contraction-calcium released from SR Tension is developed due to crossbridge binding (40 ms) Relaxation (50 ms)-reuptake of calcium into SR Crossbridge detachment. Muscle fibers exhibit all-or-none responses to stimulation so to contract must receive appropriate amount of stimulation. Speed of shortening is greater in fast fibers because: Sarcoplasmic reticulum releases Ca++ at a faster rate Higher ATPase activity resulting in quicker release of energy required for contraction  Force generation in muscle depends on: Types and number of motor units recruited More motor units greater force Fast motor units greater force Force generation in muscle depends on: Initial muscle length at time of contraction; “optimal” length for force generation results in increased cross-bridge interaction Force generation in muscle depends on: Frequency of neural stimulation of the motor units As frequency of neural stimulation (firing rate of motor neurons) is increased, the muscle does not have time to relax between stimuli and the force produced is additive. This response is called summation. If frequency of neural stimulation is high enough then individual contractions are blended into a single sustained contraction called tetanus. Tetanus continues until stimuli are discontinued or the muscle fatigues. Tetanic contractions are the muscular contractions that occur during normal body movements and are the result of a series of rapidly repeated neural impulses that stimulate the motor units. Neural impulses do not arrive at same time. Some motor units are contracting while others are relaxing leading to coordinated smooth muscle contraction. Force generation in muscle depends on: Recent prior contractile activity of muscle Rested muscle versus muscle exposed to fatiguing exercise. Fatigued muscle will generate less force Warmup exercise consisting of non-fatiguing submaximal exercise results in “postactivation potentiation” and increased force generation Short period of low-intensity exercise increases force production due to phosphorylation of myosin light chains located at base of myosin crossbridge → ↑muscle sensitivity to Ca++ results in ↑ actin-myosin interaction and force generation  Muscle force-velocity relationship At any absolute force the speed of movement is greater in muscles with higher percentage of fast twitch fibers The greatest speed of movement (maximum force velocity) is generated at the lowest force (i.e., workloads) and this true for both fast and slow fibers.  Muscle power-velocity relationship At any given velocity of movement, the power generated is greater in a muscle with a higher percent of fast-twitch fibers The peak power increases with velocity up to 200–300 deg second–1 and then it declines with faster movement velocities  Check your understanding ▪ The amount of force generated during muscular contraction is dependent on the: (1) _________________________ (2) __________________________ (3) __________________________. (4)_____________________________ ▪ The addition of muscle twitches is termed ___________. When the frequency of neural stimulation to a motor unit is increased, individual contractions are fused together in a sustained contraction called _______________. ▪ The peak force generated by muscle ____________ as the speed of movement increases. However, in general, the amount of power generated by a muscle group _________ up to a movement velocity of 200 to 300 degrees/second.