Functions of Skeletal Muscle Skeletal Muscle Tissue Keri Muma Bio 6 Movement muscles attach directly or indirectly to bone, pull on bone or tissue when they contract Maintain posture / body position muscles are continuously contracting to make adjustments to maintain posture Stabilize joints tendons crossing joints and muscle tone Thermogenesis - generate heat when contracting Functional Properties of Muscle Tissue Excitable respond to nerve stimulus Contractible shorten when stimulated Extensible can stretch beyond resting length when relaxed Elastic can recoil/rebound to original resting length after contraction or stretching Gross Anatomy of Skeletal Muscle Endomysium surrounds individual muscle fibers Perimysium surrounds bundles of muscle fibers called fascicles Epimysium dense irregular CT, surrounds the entire muscle Sarcolemma plasma membrane surrounding muscle fiber Sarcoplasm Contains a lot of mitochondria, glycogen, myoglobin, and contractile organelles called myofibrils Myofibril Long fiber-like organelle that fills the sarcoplasm of the cell Runs parallel to muscle fiber 1
Sarcomere Aligned end to end along the length of the myofibril Z-line boundary at the end of each sarcomere Myofilaments Contractile proteins in the sarcomere Arrangement gives muscle its striations Summary: Myosin (thick) filaments Tail with split head Arranged in bundles Heads contain: ATP binding site ATPase enzyme which splits ATP to provide energy during contraction Actin binding site Arrangement of Actin and Myosin Actin (thin) filament Attached to Z line and extends towards the center of the sarcomere Contains active binding site for myosin heads 2
Proteins that regulate the binding of myosin Tropomyosin spirals around actin, blocks active site when muscle is relaxed Troponin contains three binding sites Binds to actin Binds to tropomyosin and controls its position on active binding site Contains calcium binding site Titin (elastic) filaments Large protein attached to z-line and runs through the center of thick filaments Gives muscle elastic property; uncoils when muscle stretches yet stiffens to avoid over extension, and recoil when muscle relaxes Titin Microscopic Anatomy of Skeletal Muscle Sarcoplasmic reticulum (SR) specialized smooth endoplasmic reticulum Surrounds each myofibril Terminal cisternae expanded ends of SR Stores and releases calcium when muscle fiber is stimulated to contract Microscopic Anatomy of Skeletal Muscle Transverse Tubules (T-tubule) deep indentations of the sarcolemma into the muscle fiber Lies between two terminal cisternae Conducts electrical impulse from the sarcolemma into the muscle fiber Coordinates muscle contraction by triggering calcium release from the terminal cisternae Skeletal Muscle Contraction In order to contract, a skeletal muscle must: Be stimulated by a somatic motor neuron Propagate an electrical current, or action potential, along its sarcolemma Have a rise in intracellular Ca 2+ levels, the final trigger for contraction Linking the electrical signal to the contraction is excitation-contraction coupling Neuromuscular Junction The neuromuscular junction is formed from: Axon terminals, which have synaptic vesicles that contain acetylcholine (ACh) Synaptic cleft The motor end plate, which is a specific part of the sarcolemma that contains ACh receptors 3
Neuromuscular Junction When a nerve impulse reaches the end of an axon at the neuromuscular junction: Voltage-regulated calcium channels open and allow Ca 2+ to enter the axon Ca 2+ inside the axon terminal causes vesicles to fuse with the axon membrane and release ACh into the synaptic cleft by exocytosis Neuromuscular Junction ACh diffuses across the synaptic cleft to ACh receptors on the motor end plate Binding of ACh to its receptors initiates an action potential in the muscle ACh bound to ACh receptors is quickly destroyed by acetylcholinesterase Excitation-Contraction Coupling Once generated, the action potential: Is propagated along the sarcolemma Travels down the T tubules Triggers Ca 2+ release from terminal cisternae Excitation-Contraction Coupling Ca 2+ binds to troponin and causes: A conformational change in troponin Tropomyosin rolls off the actin active binding sites allowing them to be exposed for myosin to attach Sliding Filament Theory Sliding Filament Theory Cross bridge formation myosin cross bridge attaches to actin filament Working (power) stroke myosin head pivots and pulls actin filament toward M line 4
Sliding Filament Theory Sliding Filament Theory Cross bridge detachment ATP attaches to myosin head and the cross bridge detaches Cocking of the myosin head energy from hydrolysis of ATP cocks the myosin head into the high-energy state Muscle Relaxation Muscle Contraction When the nerve stimulation ceases: Ca 2+ is removed and actively transported back into the SR (requires ATP) Tropomyosin roles back over the binding sites, and the muscle fiber relaxes Tension force muscle exerts on an object when contracted Load or resistance is the opposing force exerted on the muscle Muscle tension must over come the force of the load in order to shorten Phases of Muscle Contraction Latent period first few milliseconds, excitationcontraction coupling Period of contraction cross bridge cycling, tension increases Period of relaxation calcium transported back into SR, cross bridge cycling ends, tension decreases Graded Muscle Response Individual muscle fiber contraction is an all or none response, but whole muscles can vary in tension produced and length of contraction Graded muscle responses are: Variations in the degree of muscle contraction Required for proper control of skeletal movement Responses are graded by: Changing the frequency of stimulation Changing the strength of the stimulus 5
Factors Effecting Tension Intensity of stimulus number of motor units Motor unit a single motor neuron and all the muscle fibers it supplies Recruitment calling additional motor units, therefore stimulating more fibers and increasing muscle tension Factors Effecting Tension Intensity of stimulus Threshold stimulus minimal stimulus needed to invoke visible muscle contraction Maximal stimulus all motor units are recruited, strongest contraction produced Asychronous recruitment of motor units alternates motor units Factors Effecting Tension Frequency of stimulation Twitch single impulse, contraction followed by relaxation Wave summation when impulses are delivered in succession the second twitch will be stronger then the first Complete tetanus rapid stimulation results in sustained smooth contraction without periods of relaxation Factors Effecting Tension Frequency of stimulation Refractory Period in Skeletal Muscle Contractile response lasts longer, far beyond the refractory period of the action potential This is important in skeletal muscle s ability to produce tetanus Treppe: The Staircase Effect Treppe increased contraction in response to multiple stimuli of the same strength Different than summation because relaxation occurs 6
Treppe: The Staircase Effect Factors Effecting Tension Contractions increase because: There is increasing availability of Ca 2+ in the sarcoplasm Reduced slack of the elastic series component Muscle enzyme systems become more efficient because heat is increased as muscle contracts Size of Muscle Number of muscle fibers per muscle Size of individual muscle fibers fibers produce more myofilaments in response to demands placed on them. Fibers hypertrophy. Factors Effecting Tension Optimal operating length the resting length in which maximum contraction can be generated (70-130%) Occurs when the muscle is slightly stretched and filaments barely overlap Types of Contractions Isotonic same tension Muscle tension remains constant during contraction Muscle length changes during contraction, shortens or lengthens Isotonic Contractions Concentric - muscle shortens and does work Examples: pick up pencil, kick soccer ball Isotonic Contractions Eccentric muscle contracts as it lengthens Helps counter act gravity or prevent joint injury muscle braking Example: squats quadriceps stretch but are contracted to counter act gravity and control movement 7
Types of Contractions Isometric same length Tension increases but muscle length remains the same Muscle is unable to produce enough force to overcome the load Example: pushing against a stationary wall Muscle Tone Constant low level of tension in relaxed muscles Maintained by spinal reflexes that activate alternating motor units Keeps muscles firm and ready to respond to stimuli Muscle Metabolism Role of ATP - muscles need a constant supply of ATP to carry out contractions For cross bridge formation and power stroke For disconnecting of cross bridges For active transport of calcium back into the terminal cisternae Sodium-potassium pumps Muscles only have enough ATP stored for 4-6 seconds worth of contraction Therefore ATP must be constantly regenerated Sources of ATP Three pathways that supply additional ATP: Creatine phosphate (direct phosphorylation) Oxidative phosphorylation Glycolysis Sources of ATP Direct phosphorylation Creatine phosphate, transfers energy and a phosphate to ADP forming ATP Creatine phosphate + ADP Creatine + ATP Creatine phosphate is stored in the muscle fibers Provides a rapid source of energy for 10 15 seconds of contraction Glycogen Stores 2 ATP O 2 Lactic Acid Glycolysis Glycolysis Pyruvic Acid Glucose from blood O 2 - stored by myoglobin or delivered by the blood Aerobic Respiration 8
Sources of ATP Oxidative Phosphorylation - Aerobic Respiration Occurs in the mitochondria Main source when O2 is present Fueled by glycogen stores and glucose and fatty acids delivered by the blood Can provide hours of muscle contraction for prolonged moderate activity Slower because it requires the delivery of oxygen and glucose Sources of ATP There are cardiovascular limits to the amount of nutrients that can be delivered to muscle CV system cannot keep up with O2 demands Blood vessels in the muscle are compressed during maximal contraction Oxidative phosporylation may not be able to produce enough ATP quick enough to keep up with the demands Sources of ATP Glycolysis - anaerobic Glucose is broken down to pyruvic acid and produces 2ATP In the absence of O 2 pyruvic acid is converted to lactic acid Produces minimal amounts of ATP but occurs quickly Provides 30-60 seconds of high level activity Muscle Fatigue Fatigue - decline in muscle tension as a result of previous activity Muscles are unable to contract despite being stimulated Results from a deficit of ATP (not a total absence) Anaerobic respiration becomes less efficient as lactic acid accumulates and ph drops in the muscle fiber Muscle fibers lose K+ as the Na-K pump is unable to restore ion balance since it requires ATP Neuromuscular fatigue caused by a shortage of neurotransmitters at the NMJ Muscle Fatigue Oxygen Debt Oxygen debt - the amount of extra oxygen the body must take in to restore muscle chemistry back to resting state The liver converts lactic acid in the blood to pyruvic acid which can be converted to glucose or enter aerobic respiration now that O 2 is available Glycogen stores are replenished in muscles and liver Creatine is re-phosphorylated into creatine phosphate and stored in muscles O 2 rebinds to myoglobin 9
Summary of Muscle Metabolism Muscle Fiber Types Muscle fibers differ in their methods of metabolism based on: Pathways they use to produce ATP Duration of muscle contraction How quickly their ATPases work Speed (velocity) of contraction Muscle Fiber Types Slow oxidative (red) Slow to contraction but most resistant to fatigue Good for endurance and continuous contraction Better equipped for oxidative phosphorylation Numerous mitochondria and rich supply of capillaries Small in diameter High myoglobin content Slow myosin ATPase activity Muscle Fiber Types Fast oxidative (pink) Fast to contraction but resistant to fatigue Also equipped for oxidative phosphorylation Fast myosin ATPase activity Muscle Fiber Types Fast glycolytic fibers (white) Fast to contract but fatigue quickly Good for power and speed for short durations High glycogen reserves and relies mainly on glycolysis Fatigue quickly due to lactic acid build up Large fibers generate more force but poor nutrient diffusion Light in color due to reduced myoglobin Fewer capillaries and mitochondria Effects of Exercise on Muscle Fibers Aerobic Exercise Results in more efficient muscle metabolism and resistance to fatigue Increases capillaries, mitochondria and myoglobin Also increases efficiency of the heart, lungs, body metabolism, and neuromuscular coordination 10
Effects of Exercise on Muscle Resistance weight lifting and isometric contractions Fibers produce more myofilaments and myofibrils causing muscle fibers to hypertrophy Increases glycogen stores Results in increased muscle size and strength Smooth Muscle Tissue Chapter 12 Smooth Muscle Composed of spindle-shaped fibers Organized into two layers (longitudinal and circular) Found in walls of hollow organs (except the heart) Innervation of Smooth Muscle Smooth muscle lacks neuromuscular junctions Innervating nerves have bulbous swellings called varicosities that release neurotransmitters Some neurotransmitters are excitatory and some are inhibitory, depending on the receptor Smooth Muscle Structural Characteristics SR is less developed than in skeletal muscle and lacks a specific pattern T tubules are absent Thin filaments only contain tropomyosin (NO troponin) Thick and thin filaments are arranged diagonally, causing smooth muscle to contract in a corkscrew manner Contraction Mechanism Actin and myosin interact according to the sliding filament mechanism Calcium influx from the extracellular space triggers Ca 2+ release from the SR The trigger for contractions is a rise in intracellular Ca 2+ 11
Role of Calcium Ion Ca 2+ binds to calmodulin Activated calmodulin activates the myosin light chain kinase enzyme which transfers phosphate from ATP to myosin cross bridges Phosphorylated cross bridges interact with actin to produce shortening Smooth muscle relaxes when intracellular Ca 2+ levels drop Smooth Muscle Contraction Calmodulin Ca 2+ Inactive myosin kinase Ca 2+ -calmodulin Inactive myosin Active myosin kinase P i Phosphorylated myosin (can bind with actin) Types of Smooth Muscle: Single Unit Visceral smooth muscle is autonomous Smooth muscle pacemaker cells display rhythmic, spontaneous variations in membrane potentials." Known as slow wave potentials Self-excitable (myogenic) can produce spontaneous action potentials without external stimulation Cells are electrically coupled to one another via gap junctions and contract rhythmically as a unit Smooth Muscle Activity Pacemaker smooth muscle cells (Interstitial cells of Cajal) membrane potential oscillates closer and further away from threshold If threshold is reached a burst of action potentials is triggered causing rhythmic smooth muscle contractions Drive several digestive processes (e.g., peristalsis and segmentation)" Gap Junctions Smooth Muscle Activity Pacemaker smooth muscle cell Spontaneous action potential induced by pacemaker potential Action potential spread to nonpacemaker cell Hormones, paracrines, mechanical stress, and nerve stimuli determines the starting point of the slow wave potentials Example: Food in the GI tract closer to threshold Empty GI tract further away from threshold Gap junction Nonpacemaker smooth muscle cell 12
Response to Stretch Smooth muscle exhibits a phenomenon called stress-relaxation response in which: Smooth muscle responds to stretch only briefly, and then adapts to its new length The new length, however, retains its ability to contract This enables organs such as the stomach and bladder to temporarily store contents Types of Smooth Muscle: Multiunit Multiunit smooth muscles are found: In large airways to the lungs, large arteries, arrector pili muscles attached to hair follicles, and in the internal eye muscles Their characteristics include: Rare gap junctions Structurally independent muscle fibers A rich nerve supply, forms motor units Graded contractions in response to neural stimuli 13