Rehab521 Autumn 2024 Lecture 25: Muscle Disuse, Training, Sarcopenia, and Fatigue PDF

Muscle module: Responses to disuse, training, and ageing (sarcopenia); and Mechanisms of fatigue Mary Beth Brown, PT, PhD Rehab 521 Pathophysiology Recall MUSCLE ATROPHY VS. MUSCLE HYPERTROPHY this from last lecture degradation...

Muscle module: Responses to disuse, training, and ageing (sarcopenia); and Mechanisms of fatigue Mary Beth Brown, PT, PhD Rehab 521 Pathophysiology Recall MUSCLE ATROPHY VS. MUSCLE HYPERTROPHY this from last lecture degradation synthesis Powers SK et al., 2007 -Sarcopenia (Aging) -Disuse (bedrest, limb immobilization, denervation, unloading, spaceflight) -Burn Injury -Fasting -Sepsis -Neurodegenerative Diseases (e.g. ALS) -Diabetes -Renal Failure -Chronic Obstructive Pulmonary Disease (COPD) -AIDS -CANCER and ANTI-CANCER TREATMENTS (i.e. Cachexia) Recall this from last lecture Skeletal muscle illness- atrophy related, e.g. cancer  The mechanisms responsible for age-related muscle loss Recall this (Sarcopenia) and illness-related (Cachexia) are slightly different from last lecture Effects of Disuse (generalizations) Are generally the opposite of those induced by training (see next several slides) Are reversible More marked in animals than in humans Depend on the experimental model employed- absence of load bearing is most important Weightlessness: humans and animals Bed rest: humans Immobilization: humans and animals Hind-limb suspension: animals  Adaptations to training Talking about changes at the level of the muscle cell, things like: Muscle fiber size ie. hypertrophy/hyperplasia Enzyme activity ie. oxidative vs. glycolytic metabolism Isoform expression ie. myosin ATPase, Ca++ ATPase Organelles ie. Mitochondria Extracellular structures ie. scaffolding/tethering (matrix) proteins, capillarization  Adaptations to training Talking about changes at the level of the muscle cell, things like: Muscle fiber size ie. hypertrophy/hyperplasia Enzyme activity ie. oxidative vs. glycolytic metabolism Isoform expression ie. myosin ATPase, Ca++ ATPase Organelles ie. Mitochondria Extracellular structures ie. scaffolding/tethering (matrix) proteins, capillarization  Adaptations to training Kreb’s cycle (AKA TCA cycle or citric acid cycle) Adaptations to endurance training Are especially metabolic adaptations- oxidative vs. glycolytic enzymes AKA ‘aerobic’ c o lytic e (Gly mes ar y enz - non tive, a oxid AKA robic) e an a  Adaptations to training Adaptations to endurance training Are especially metabolic adaptations- Fuel and storage use Only accessible via oxidative metabolis  Adaptations to training Talking about changes at the level of the muscle cell, things like: Muscle fiber size ie. hypertrophy/hyperplasia Enzyme activity ie. oxidative vs. glycolytic metabolism Isoform expression ie. myosin ATPase, Ca++ ATPase Organelles ie. Mitochondria Extracellular structures ie. scaffolding/tethering (matrix) proteins, capillarization  Recall Removal of Calcium Ions to Stop Muscle this from Contraction muscle intro 1. Sodium-calcium lecture exchanger and calcium pump of sarcolemma rid cell of calcium How fast 2. Calcium pump this enzyme located in wall of is able to SR (Ca-ATPase) work also greatly 3. Calcium binding influences proteins the speed of Function to remove contraction free Ca++ in order to restore resting intracell. Ca++ levels Normal resting Ca++ levels are too low to initiate muscle  Recall Cross-Bridge Cycle this from muscle Step 2: ATP hydrolysis intro lecture Breakdown of ATP to ADP + Pi by myosin ATPase ADP + Pi stay on myosin head. How fast this enzyme Myosin head pivots to cocked is able to position, perpendicular (90 deg) work to filaments and lines up with a largely new actin molecule (but doesn’t determines the speed of attach yet!) contraction  I IIa IIb So what do we mean by fiber type? To what is ‘I, IIa, and IIb’ referring?? The next several slides will review this important Recall this from muscle intro lecture Motor Unit One muscle may have SPINAL CORD many motor units of different fiber types. Each motor unit is a group of muscle fibers Neuron 1 that will all Neuron 2 Neuron 3 contract together Motor nerve Each muscle KEY fiber is only Muscle fibers Motor unit 1 innervated by Motor unit 2 one somatic Motor unit 3 motor neuron Motor Unit One muscle may have SPINAL CORD many motor units of Motor unit- different fiber types. motor neuron + the fibers innervated by that motor neuron's Neuron I axonal terminals Neuron IIa Neuron IIb Motor pool- all Motor nerve the motor units within a muscle KEY Muscle fibers Groups of motor Motor unit I units work together to Motor unit IIa coordinate the Motor unit IIb contractions of a single muscle  Fiber type classification schemes Color Contraction speed Fatigability Metabolism Oxidative Non-oxidative machinery machinery Metabolism  Classification by contraction speed So which Rm is membrane input out of these resistance. If this is high, two excites then is easier to excite first? So what is happening here to cause force to exponentially increase? Classification by contraction speed Classification by Fatigability but Very fatigue- resistant! Ok now back to what we were talking about…. Adaptations to training Talking about changes at the level of the muscle cell, things like: Muscle fiber size ie. hypertrophy/hyperplasia Enzyme activity ie. oxidative vs. glycolytic metabolism Isoform expression ie. myosin ATPase, Ca++ ATPase Organelles ie. Mitochondria Extracellular structures ie. scaffolding/tethering (matrix) proteins, capillarization Better capillarization is a big reason why endurance training improves muscle’s max arterial-venous O2  Adaptations to training Adaptations to endurance training capillarization and skeletal muscle’s max arterial- venous O2 difference VO2max is measured by metabolic exercise testing, it is the maximal rate of oxygen uptake by the body Fick Equation: VO2max = HRmax x SVmax x a-vO2 diffmax  Adaptations to training And… the adaptations are reversible… sadly they go away when training stops Pictorial summary of some of the adaptations from endurance exercise, over time and then with stopping training Now back to the opposite of training- what happens with Effects of Disuse (adaptations at DISUSE… muscle level) Both type I and type II fibers are susceptible to disuse atrophy Disuse atrophy always greatest in antigravity muscles vs. their antagonists e.g. worse in quads than in hams In an immobilization model, the length at which muscle is ‘fixed’ affects amount of atrophy shorter length, more sarcomere resorption  A comparison of the effects of spaceflight in rhesus monkeys (14-day flight) and HUMANS (17-day flight) on the diameter and functional properties of single type I Sol fibers. Disuse atrophy (peak force) (peak (peak velocity) force/cross Fitts R H et al. Am J Physiolsectional area) Regul Integr Comp Physiol 2000;279:R1546-R1557 ©2000 by American Physiological Society  Disuse atrophy Experimental weightlessness- Why do induced you think gastroc didn’t atrophy much?  Longitudinal sections of soleus (Sol) muscle fibers from the 2 flight animals pre - and postflight. Disuse atrophy Monkey A Monkey B Pre- flight Post- flight Fitts R H et al. Am J Physiol Regul Integr Comp Physiol 2000;279:R1546-R1557 ©2000 by American Physiological Society  Disuse atrophy Thinner myofibrils in post- compared to pre- flight  A comparison of the effects of spaceflight in rhesus monkeys (14-day flight) and HUMANS (17-day flight) on the diameter and functional properties of single type I Sol fibers. What’s up with this?? (see next slides) (peak force) (peak (peak velocity) force/cross Fitts R H et al. Am J Physiolsectional area) Regul Integr Comp Physiol 2000;279:R1546-R1557 ©2000 by American Physiological Society Effects of Disuse (generalizations) Increased Vmax of muscle fibers Reduced force and power Change in the metabolic processes fueling exercise Greater dependence on non-oxidative (glycolytic) metabolism Greater fatigability The motor innervation AND the voluntary activation of the disused muscle may be affected  Increased Hind-limb suspension increases Vmax Vmax of of both slow and fast fibers muscle fibers These graphs are just typical force-velocity curves, showing that as velocity of contraction goes up (up on y axis), amount of force that can be produced goes down Note that(to left on disuse x axis) causes muscle to be able to produce a little more force (relative to the non-disuse control) as  Increased Hind-limb suspension increases Vmax Vmax of of both slow and fast fibers muscle WHY fibers increased??? Effects of Disuse (generalizations) Increased Vmax of muscle fibers Reduced force and power Change in the metabolic processes fueling exercise Greater dependence on non-oxidative (glycolytic) metabolism Greater fatigability The motor innervation AND the voluntary activation of the disused muscle may be affected  Reduced force and Weightlessness decreases power power in Type I and II fibers  Change in the metabolic processes fueling exercise *Because accumulates, not necessarily a good thing *rancid kitchen oils- see notes  Greater fatigability Weightlessness decreases fatigue resistance Effects of Disuse (generalizations) Increased Vmax of muscle fibers Reduced force and power Change in the metabolic processes fueling exercise Greater dependence on non-oxidative (glycolytic) metabolism Greater fatigability The motor innervation AND the voluntary activation of the disused muscle may be affected  The motor innervation AND the voluntary activation of the disused muscle may be affected Helps explain why force loss often exceeds that which is expected based on visible atrophy At the surface of disused fibers there is a spread of ACh receptors beyond the NMJ, indicating diminished resting membrane potential  ber , em Central m re ry d An unta on ! o l t i N S v iva c t res C a ui req Events that Take Place at the Neuromuscluar Peripheral Junction Somatic motor neuron Diminished muscle membrane Axon terminal potential Process of Excitation-Contraction Coupling ACh Ca2+ Ca2+ Action potential Acetyl + choline would Voltage-gated Ca2+ channel impact how Skeletal AChE muscle fiber Nicotinic Motor end 3 Action potential in t-tubule alters well we can plate conformation of DHP receptor. receptor proceed 4 DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters through this 3 4 cytoplasm. component 5 Ca2+ binds to troponin-C, allowing actin-myosin binding. in the steps 7 5 Ca2+ released 6 Myosin heads execute power of skeletal stroke. 6 muscle Actin filament slides toward center Myosin thick filament 7 of sarcomere. contraction Distance actin moves K EY (b) Exc itation-c ontrac tion c oupling DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel  The motor innervation AND the voluntary activation of the disused muscle may be affected Helps explain why force loss often exceeds that which is expected based on visible atrophy, as well as early gains in function with exercise! Day 1 of PT, post surgery/immobiliz. The next day Patient can Patient can do SLR barely contract quads, cannot do No visible change even 1 SLR in muscle size to explain ↑ strength Treated with NMES and quad So what explains sets, sent home it?? with quad sets for HEP  The motor innervation AND the voluntary activation of the disused muscle may be affected Helps explain why force loss often exceeds that which is expected based on visible atrophy, as well as early gains in function with exercise! Day 1 of new One week later you weight lifting feel like this at the routine at gym gym So what explains it?? Clinical consideration The therapist must always keep in mind that skeletal muscle immobility can contribute substantially to muscle wasting even in the absence of systemic inflammatory changes Muscle atrophy begins within hours of bed rest or deep sedation, and even healthy people can have large loss of muscle mass and strength within 10 days of bed rest, particularly from the lower limbs in antigravity muscles Clinical consideration Patients (of any illness) often c/o weakness and increased fatigability Recognize some may be due to disuse rather than to the primary illness muscle fiber atrophy and loss of myofibrillar protein less effective muscle metabolism inability of the motor centers in the brain to recruit motor neurons fully due to disuse of the descending motor pathways increased fatigability of motor units Muscle module: Responses to disuse, training, and ageing (sarcopenia); and Mechanisms of fatigue Mary Beth Brown, PT, PhD Rehab 521 Pathophysiology We slow down Sarcopenia Aging and Skeletal Muscle Denervation After the age of 60: Decreased number of motor neurons in the lumbosacral spinal cord. Decreased number of large and intermediate myelinated axons in the ventral roots. No change in the number of small fibers. Decreased axonal conduction velocity. As number of motor neurons decrease so do the number of motor units, in (Type IIa) (Type I) (Type IIb)  FOG (Type IIa) Especially lose the SO (Type larger units I) FG (Type IIb)  Can also occur in denervation atrophy, like t example below Patien t with Healt spinal hy muscu lar atroph y (SMA) Sarcopenia Sarcopenia  Sarcopenia So besides fiber atrophy and neuro/innervation changes, is there anything else that contributes to force loss as we age? Impairment at excitation-contraction coupling (AKA- the connection between the electrical signal and the calcium release to cause contraction…. Reminder next slide!) Recall Process of Excitation-Contraction this from Coupling muscle intro lecture Axon terminal of somatic motor neuron (terminal bouton) Muscle fiber 1 1 Somatic motor neuron releases ACh ACh at neuromuscular junction. 2 Na+ 2 Net entry of Na+ through ACh receptor-channel initiates a local muscle action potential 3 Motor end plate RyR 3 If sufficient Na+ entry, will trigger voltage T-tubule gated Na+ channels which will propagate Ca2+ Sarcoplasmic the action potential down into the t-tubule reticulum Z disk DHPR KEY Troponin Actin Tropomyosin M line DHPR = dihydropyridine receptor, an L-type Ca++ channel Myosin head Myosin thick filament RyR = ryanodine receptor, a Ca++ release channel Initiation of muscle action potential Muscle twitches become smaller and slower with aging Calcium release is impaired SR volume and Ca++ pumping capacity are reduced Decreased density of voltage sensitive channels (called DHPRs) needed to activate SR calcium release (via RyR) in response to AP Muscle twitches become smaller and slower with aging Red boxes indicate that machinery pairings needed for excitation- Calcium release is impaired contraction coupling is reduced SR volume and Ca++ pumping capacity are reduced Decreased density of voltage sensitive channels (called DHPRs) needed to activate SR calcium release (via RyR) in response to AP Muscle twitches become smaller and slower with aging Figure shows lower maximal calcium release And Shift to worsened sensitivity for calcium release (takes more voltage for a given amt of calcium release) Calcium release is impaired SR volume and Ca++ pumping capacity are reduced Decreased density of voltage sensitive channels (called DHPRs) needed to activate SR calcium release (via RyR) in response to AP  Sarcopenia So besides fiber atrophy and neuro/innervation changes, is there anything else that contributes to force loss as we age? Impairment at excitation-contraction coupling (AKA- the connection between the electrical signal and the calcium release to cause contraction) Impairment at the crossbridge cycle Less myosin heads in a force producing state (example figures next 3 slides) The transition speed of going from strong binding state to weak binding state (gapp) is sped up, so overall less are found in the strong Sarcopenia Aged muscles show other degenerative features Hyaline degeneration, vacuoles at the ends of fibers, loss of myofibrils Replacement of fibers by fat and connective tissue Necrosis, with infiltration of macrophages You don’t Groups of small fibers with dense nuclei need to Central nucleation of fibers memorize this list, is Splitting of fibers just FYI Disorganization of sarcomere spacing Reduced mitochondrial size Accumulation of reticular material from SR and t-tubules Sarcopenia Clinical implications Loss in muscle mass (i.e., atrophy) accounts for the age-associated decreases in basal metabolic rate contributing to metabolic disorders such as type 2 diabetes mellitus and osteoporosis and decreases in muscle strength and activity levels, which, in turn, are the cause of the decreased energy requirements of the aging adult Sarcopenia Clinical implications Loss of muscle mass (i.e., atrophy) and loss of muscle function resulting in subsequent muscle weakness are implicated in difficulty accomplishing activities of daily living (e.g., rising from a chair, climbing stairs, carrying groceries), slow gait speed, impaired balance reactions, and increased risk of vertebral compression (and other) fractures. Sarcopenia Clinical implications Aging workers notice increasing difficulty continuing a job they have previously performed without trouble. Slowing down of reflexes and coordination combined with loss of muscle mass and strength can make it difficult to remain in the same job or train for a new job. Sarcopenia Clinical implications By age 65, changes in the muscle mass, muscle weakness, and decreased levels of physical activity are evident in the increased numbers of falls and injuries. Injuries in an aging musculoskeletal system take longer to recover, contributing to further physical deconditioning, potentially creating additional comorbidities. Sarcopenia Clinical implications High-resistance training exercise has been of significant benefit to sarcopenia. Exercise training, resistance exercise has been shown to reverse mitochondrial dysfunction for genes that are affected by both age and exercise. Combinations of resistance exercise, aerobic exercise, and stretching have shown beneficial effects on sarcopenia, but the optimum regime for older adults remains unclear. Sarcopenia Clinical implications Many older adults would like to be more physically active but do not have the experience or knowledge to develop and build up an exercise regimen without appropriate supervision such as the physical therapist can offer. Others have participated in athletics throughout adulthood and continue to train and remain in good health. The therapist can help educate older adults about the importance of exercise to maintain strength and endurance, with the emphasis on strength training, since muscular strength decreases more rapidly than endurance as a consequence of aging. Muscle module: Responses to disuse, training, and ageing (sarcopenia); and Mechanisms of fatigue Mary Beth Brown, PT, PhD Rehab 521 Pathophysiology Mechanisms for skeletal muscle fatigue Muscle fatigue definition: The transient failure to maintain the required or expected force or power output.  Cav Ass eat: Mechanism: Stu ociat Probl Varies with type and intensity of exercise. d i e ed em s Wi s Subjects: th Fat Motivation, nutrition, environment, training status igu Protocols: e Each lab uses different protocols to induce fatigue Interpretation: Do results obtained in vitro apply in vivo, apply to humans, and under ‘real life’ conditions? How do you differentiate between muscle fatigue and exercise- induced muscle damage?  “The first blind man, grasping the ear of the elephant, remarked that an elephant was very much like the sail of a ship. The second blind man, holding the leg of the creature, compared the elephant with the trunk of a tree. The third blind man, clutching the trunk, argued that an elephant was similar to a snake, and the fourth blind man, grasping the tail, considered all of his fellows to be incorrect, because it was very clear that an elephant was like a thick piece of rope. Using a reductionist approach, all of these men were correct, in part. The elephant is like all, yet like none, of these individual observations. However, it is only by ‘‘seeing’’ the entire elephant that one can explain the individual contribution of each of these parts to the whole.” Caveat: Beware of the Reductionist ’s interpretati on of physiologica l phenomena Mechanisms for skeletal muscle fatigue 1. Maximal The 3 Calcium determinates of Activated contractile force Force Force 2. Calcium Sensitivity Calcium 3. How much Ca++ is delivered to the contractile proteins Mechanisms for skeletal muscle fatigue The 3 1. Decreased determinates of Maximal Ca2+ contractile force Activated Force Force Are all possible 2. Decreased Ca2+ Sensitivity factors of Activation contributing 3. Decreased SR Ca2+ Release to fatigue- induced [Calcium] force decline Mechanisms for skeletal muscle fatigue The 3 1. Decreased determinates of Maximal Ca2+ contractile force Activated Force Force Are all possible 2. Decreased Ca2+ Sensitivity factors of Activation contributing 3. Decreased SR Ca2+ Release to fatigue- induced How? [Calcium] force decline Mechanisms for skeletal muscle fatigue The 3 1. Decreased determinates of Maximal Ca2+ contractile force Activated Force Force Are all possible 2. Decreased Ca2+ Sensitivity factors of Activation contributing 3. Decreased SR Ca2+ Release to fatigue- induced How? Central or [Calcium] force decline Peripheral  Central Events that Take Place at the Neuromuscluar Peripheral Junction Somatic motor neuron Axon terminal Process of Excitation-Contraction Coupling ACh Ca2+ Ca2+ Action potential Acetyl + choline Voltage-gated Ca2+ channel Skeletal AChE muscle fiber Motor end 3 Action potential in t-tubule alters Nicotinic plate conformation of DHP receptor. receptor 4 DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters 4 cytoplasm. 3 5 Ca2+ binds to troponin-C, allowing actin-myosin binding. 7 Ca2+ released 5 6 Myosin heads execute power stroke. 6 Myosin thick filament 7 Actin filament slides toward center of sarcomere. Distance actin moves K EY (b) Exc itation-c ontrac tion c oupling DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel Mechanisms for skeletal muscle fatigue Central (CNS) Fatigue Decrease in the # of motor units recruited Includes psychological/motivational issues as well as central (brain and spinal cord) biochemical changes Examples suggesting Central fatigue: A sudden loud noise ’s tension during a fatiguing protocol Electrical stim. immediately after a maximum voluntary contraction (MVC) restores tension in a fatigued muscle Bigland-Ritchie et al. found that e-stim of fatigued muscles  tension in about half of NOTE: In the posted Mechanisms for skeletal muscle fatigue video that was filmed in classroo Central (CNS) Fatigue m, there was Decrease in the # of motor units recruited some audio trouble Includes psychological/motivational issues that happene as well as central (brain and spinal cord) d when I biochemical changes was at the tail end of Examples suggesting Central fatigue: explaini ng this slide, So, A sudden loud noise ’s tension during a I’m fatiguing protocol inserting what I Electrical stim. immediately after a said in maximum voluntary contraction (MVC) the notes restores tension in a fatigued muscle section Bigland-Ritchie et al. found that e-stim of below. fatigued muscles  tension in about half of Mechanisms for skeletal muscle fatigue NOTE: In the posted video that was filmed Central (CNS) Fatigue in the classroom, there is completely missing audio for this one slide. So, I Biochemical changes in CNS inserted what I said into the Slide’s notes Examples section below, please be sure to see it. Ammonia produced by working muscles circulates to the brain where it can alter neurotransmitter levels and contribute to central fatigue Tryptophan accumulates during prolonged exercise in direct proportion to blood free fatty acid levels Plasma Tryptophan Converted Decreases Decreases to Serotonin arousal statearousal state Branched Chain Amino Acids (BCAA) compete with Tryptophan for brain uptake & therefore may delay fatigue. Blomstrand E. Amino acids and central fatigue. Amino Acids. 2001;20(1):25-34.  Central Fatigue ↓ # Motor units activated Biochemical changes in CNS Events that Take Place at the Neuromuscluar Peripheral Fatigue Junction Somatic motor neuron Axon terminal Process of Excitation-Contraction Coupling ACh Ca2+ Ca2+ Action potential Acetyl + choline Voltage-gated Ca2+ channel Skeletal AChE muscle fiber Motor end 3 Action potential in t-tubule alters Nicotinic plate conformation of DHP receptor. receptor 4 DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters 4 cytoplasm. 3 5 Ca2+ binds to troponin-C, allowing actin-myosin binding. 7 Ca2+ released 5 6 Myosin heads execute power stroke. 6 Myosin thick filament 7 Actin filament slides toward center of sarcomere. Distance actin moves K EY (b) Exc itation-c ontrac tion c oupling DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel  Central Fatigue ↓ # Motor units activated Biochemical changes in CNS Peripheral Fatigue Events that Take Place at the Neuromuscluar Junction Somatic motor neuron Axon terminal Process of Excitation-Contraction Coupling ACh Ca2+ Ca2+ Action potential Acetyl + choline Voltage-gated Ca2+ channel Skeletal AChE muscle fiber Motor end 3 Action potential in t-tubule alters Nicotinic plate conformation of DHP receptor. receptor 4 DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters 4 cytoplasm. 3 5 Ca2+ binds to troponin-C, allowing actin-myosin binding. 7 Ca2+ released 5 6 Myosin heads execute power stroke. 6 Myosin thick filament 7 Actin filament slides toward center of sarcomere. Distance actin moves K EY (b) Exc itation-c ontrac tion c oupling DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel Mechanisms for skeletal muscle fatigue Peripheral Fatigue Ion shifts: Sodium and potassium shifts resulting from repeated action potentials alter the resting membrane potential in a manner that makes it more difficult to achieve further action potentials. Metabolic insufficiency: Energy demands can not be met by energy production. Metabolic by-products accumulate and impair muscle function. Reactive oxygen species( ROS): Increased energy production leads to the production of ROS, which can damage proteins and lipids. Ion Shifts and Metabolite Changes Associated with Muscle Fatigue Resting Fatigued ATP 6 mM 4.5 mM (lower in very extreme cases) ADP 0.05 mM 0.7 mM PC 15 mM 1 mM Creatine 11 mM 22 mM Pi 2 mM in fast 30 mM 6 mM in slow Lactate 2 mM 30 mM pH 7.0 6.3 [K+]i 145 mM 100 mM [K+]o 2.5 mM 8 mM [Na+]i 15 mM 50 mM [Na+]o 120 mM ??? [Ca2+]i 100 nM 160 nM [Ca2+]o 2 mM ??? Note - these numbers are dependent on the type of muscle studied and the protocol used to induce fatigue, they should be considered only very general values. ↓ skeletal muscle pH with fatigue What are the peripheral sites of action at which some of these metabolic and ionic changes could have effect? Potential Anatomical Sites of Impairment in Peripheral Skeletal Muscle Fatigue 1. Neurotransmitter release/ receptor activation and surface membrane action potential (AP) 2. T-tubule AP conduction Calcium signal (AKA ‘Ca++ handling’): 3. DHPR voltage sensor & link with RYR Ca2+ release channel 4. RYR Ca2+ release channel 5. SR Ca2+ pump 6. Ca2+ binding to troponin 7. Actin-myosin cross bridge  Central Fatigue Events that Take Place at the Neuromuscluar Peripheral Fatigue Junction Somatic motor neuron Axon terminal Ca2+ Ca2+ Action potential ACh Acetyl + choline Voltage-gated Ca2+ channel Process of Excitation-Contraction Coupling Skeletal AChE muscle fiber Motor end Nicotinic plate receptor 3 Action potential in t-tubule alters conformation of DHP receptor. 4 DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters 4 cytoplasm. 3 5 Ca2+ binds to troponin-C, allowing actin-myosin binding. 7 Ca2+ released 5 6 Myosin heads execute power stroke. 6 Myosin thick filament 7 Actin filament slides toward center of sarcomere. Distance actin moves K EY (b) Exc itation-c ontrac tion c oupling DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel Mechanisms for skeletal muscle fatigue Peripheral Fatigue Fatigue at NMJ  Events that Take Place at the Neuromuscluar Junction Somatic motor neuron Axon terminal Example of study showing NMJ Ca2+ Ca2+ Action potential transmission failure in Peripheral Fatigue ACh Voltage-gated Ca2+ channel Acetyl + choline Skeletal AChE muscle fiber Motor end Nicotinic plate receptor Nerve Muscle Stimulation Stimulation Kuei, et al. J. Appl. Physiol. 68: 174-180, 1990. In situ diaphragm prep. Notice that over repeated contractions, the decrement in force following nerve stim is worse than the decrement in force following direct muscle fiber stim. What does this mean?  Central Fatigue Events that Take Place at the Neuromuscluar Peripheral Fatigue Junction Somatic motor neuron Axon terminal Ca2+ Ca2+ Action potential ACh Acetyl + choline Voltage-gated Ca2+ channel Process of Excitation-Contraction Coupling Skeletal AChE muscle fiber Motor end Nicotinic plate receptor 3 Action potential in t-tubule alters conformation of DHP receptor. 4 DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters 4 cytoplasm. 3 5 Ca2+ binds to troponin-C, allowing actin-myosin binding. 7 Ca2+ released 5 6 Myosin heads execute power stroke. 6 Myosin thick filament 7 Actin filament slides toward center of sarcomere. Distance actin moves K EY (b) Exc itation-c ontrac tion c oupling DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel Reminder of Process of Excitation- Contraction Coupling 4 Action potential in t-tubule alters conformation of DHP Receptor. 5 Excitation-contraction coupling DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters 4 cytoplasm. 3 6 Ca2+ binds to troponin-C, allowing actin-myosin binding. 7 Ca2+ released 5 Myosin heads execute power 7 stroke. 6 Myosin thick filament 8 Actin filament slides toward center of sarcomere. Distance actin moves KEY DHP = dihydropyridine receptor, an L-type Ca++ channel RyR = ryanodine receptor, a Ca++ release channel  Reminder of Process of Excitation-Contraction Coupling 1 Cytosolic Ca2+ 1 Ca2+ levels increase in cytosol. 2 3 Tropomyosin shifts, exposing binding site on actin 2 Ca2+ binds to troponin C TN 3 Troponin-Ca2+ 5 complex (TN with green Actin dot) pulls moves tropomyosin ADP away from actin’s myosin-binding site. Power stroke 4 Pi 4 Myosin binds to actin and completes power stroke. 5 Actin filament moves. Mechanisms for skeletal muscle fatigue Twitch (single Peripheral Fatigue fiber) Changes in characteristics of twitch and of tetanus, including ↓force, ↓rate of contraction, ↓ rate of relaxation Mechanisms for skeletal muscle fatigue Tetany (whole Peripheral Fatigue muscle) Changes in characteristics of twitch and of tetanus, including ↓force, ↓rate of contraction, ↓ rate of relaxation Mechanisms for skeletal muscle fatigue Peripheral Fatigue Fatigue decreases membrane excitability Pre-Fatigue: muscle responds to each stimuli with an action potential Fatigued muscle does not respond to each stimuli with an action potential Mechanisms for skeletal muscle fatigue Peripheral Fatigue Fatigue decreases membrane excitability The extensive network formed by the t-tubules may limit diffusion of ions into and out of the t- tubules Ionic changes associated with repeated action potentials may be greater in the depths of the t- tubules than at the surface Mechanisms for skeletal muscle fatigue Peripheral Fatigue Fatigue alters the intracellular Ca++ transient In this experiment, repeated stimulation did not alter the t- tubule charge [Ca++] i movement, but deceased the intracellular Ca2+ transient … T-tubule indicates a Ca2+ charge channel (RYR) release problem movement (see next slide) Stim Gyorke, J. Physiol. 464: 699, 1993. Mechanisms for skeletal muscle fatigue pH Cyto- SR plasmic Lumen Peripheral Fatigue Fatigue alters ability for RyR (ryanodine receptor) to release Ca2+ from SR Why? Metabolic changes and ion shifts that occur during fatiguing exercise ie. Ca2+ release ↓ in low pH This figure is an example of how ion channels can be studied in the lab using measures of their open and closed behavior. The lines represent current flowing through the open channel. Compare open probability (PO) top and bottom panel. Mechanisms for skeletal muscle fatigue Peripheral Fatigue Fatigue alters ability for RyR (ryanodine receptor) to release Ca2+ from SR Why? Metabolic changes and ion shifts that occur during fatiguing exercise ie. Ca2+ release ↓ in low pH ie. Ca2+ release ↓ with the dropping levels of magnesium and ATP during fatiguing exercise Blazev and Lamb J Physiol 520:203, 1999. Mechanisms for skeletal muscle fatigue Low glycogen Peripheral Fatigue Fatigue alters ability for RyR (ryanodine receptor) to release Ca2+ from SR Why? Metabolic changes and ion shifts that occur during fatiguing 50% loss in exercise Higher glycogen force by rep ie. Ca2+ release ↓ in low pH 17 ie. Ca2+ release ↓ with the dropping levels of magnesium and ATP during fatiguing exercise ie. Ca2+ release ↓ with the dropping levels of fuel (glycogen) during fatiguing exercise 50% loss in force Caption is on next not until rep 25 slide if you’re Low glycogen impairs Ca2+ release, fatigue earlier Low glycogen 50% loss in Higher glycogen force by rep 17 50% loss in force not until rep 25  Central Fatigue Events that Take Place at the Neuromuscluar Peripheral Fatigue Junction Somatic motor neuron Axon terminal Ca2+ Ca2+ Action potential ACh Acetyl + choline Voltage-gated Ca2+ channel Process of Excitation-Contraction Coupling Skeletal AChE muscle fiber Motor end Nicotinic plate receptor 3 Action potential in t-tubule alters conformation of DHP receptor. 4 DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters 4 cytoplasm. 3 5 Ca2+ binds to troponin-C, allowing actin-myosin binding. 7 Ca2+ released 5 6 Myosin heads execute power stroke. 6 Myosin thick filament 7 Actin filament slides toward center of sarcomere. Distance actin moves K EY (b) Exc itation-c ontrac tion c oupling DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel Mechanisms for skeletal muscle fatigue Peripheral Fatigue Metabolic By-Products and ROS accumulation during fatiguing exercise impact Cross-Bridge detrimentally too! Why? Metabolic changes and ion shifts that occur during fatiguing exercise How do we know this? We measure maximal Ca++ activated force and Ca ++ sensitivity of contraction in a single muscle fiber in experimental preps that simulate fatiguing conditions. Mechanisms for skeletal muscle fatigue Peripheral Fatigue Metabolic By-Products and ROS accumulation during fatiguing exercise impact Cross-Bridge Single fiber detrimentally too! tension Why? Metabolic changes and ion shifts that occur during fatiguing exercise ie. Low pH decreases both maximal Ca++ force (peak of line) and Ca ++ sensitivity of contraction (curve is shifted to right) Mechanisms for skeletal muscle fatigue Peripheral Fatigue Metabolic By-Products and ROS accumulation during fatiguing Single exercise impact Cross-Bridge fiber 0.2 mM Pi detrimentally too! tension Why? Metabolic changes and ion 0.7 mM Pi shifts that occur during fatiguing exercise ie. ↑ing Pi decreases both 13.6 mM Pi maximal Ca++ force (peak of line) and Ca ++ sensitivity of contraction (curve is shifted to right) Mechanisms for skeletal muscle fatigue in vitro evidence for a Role of ROS Peripheral Fatigue in Fatigue Metabolic By-Products and ROS accumulation during fatiguing exercise impact Cross-Bridge detrimentally too! Why? Metabolic changes and ion shifts that occur during fatiguing exercise ie. ↑ing ROS decreases single fiber force response Application of the ROS scavenger to calcium ‘Tiron’ slows muscle fiber fatigue in vitro. Moopanar, TR and Allen, DG J Physiol 564: 189-199, 2005 Mechanisms for skeletal muscle fatigue Peripheral Fatigue Metabolic By-Products and ROS ROS are high chemically reactive accumulation during fatiguing forms of oxygen (because they exercise impact Cross-Bridge have an extra electron) detrimentally too! About 2% of the electrons going Why? Metabolic changes and ion through the electron transport chain escape, react with oxygen shifts that occur during fatiguing and form ROS. exercise In a test tube ROS damages lipid ie. ↑ing ROS decreases and protein via oxidation reactions. single fiber force response However, cells contain to calcium antioxidants and is debated how readily ROS can damage cellular components in vivo.  e m g So sin or clo od f ht fo o u g th So what causes fatigue??Is it…. Conscious Chemical? decision? Fatigue Mechanical? Reflexive? Energy Nerve deficiency? transmission trouble? Heat impairment? Etc etc….  “Fatigue is hypothesised as being the result of the complex interaction of multiple peripheral physiological systems and the brain. In this new model, all changes in peripheral physiological systems such as substrate depletion or metabolite accumulation act as afferent signallers which modulate control processes in the brain in a dynamic, nonlinear, integrative manner.” So what causes fatigue??