BIO305 Muscles Exam PDF

[Muscles Exam] Muscular System - Three basic muscle types in the body - Skeletal - Cardiac - Smooth Skeletal Muscle - Characteristics - Most are attached by tendons to bones - Cells are multinucleated - Striated -- have visible banding - Volun...

[Muscles Exam] Muscular System - Three basic muscle types in the body - Skeletal - Cardiac - Smooth Skeletal Muscle - Characteristics - Most are attached by tendons to bones - Cells are multinucleated - Striated -- have visible banding - Voluntary -- subject to conscious control - Functions - Support body to remain upright - Provides movement (bones) - Provides pressure (blood/lymph) - Protects internal organs - Stabilizes joints Connective Tissue of Skeletal Muscle - Cells are surrounded and bundled by connective tissue - Endomysium -- encloses a singular muscle fiber - Perimysium -- wraps around a fascicle (bundle) of muscle fibers - Epimysium -- covers the entire skeletal muscle - Fascia -- on the outside of the epimysium Anatomy of Muscle - Muscle - Composed of bundles of skeletal muscle fibers called fascicles - Fascicles - Surrounded by connective tissue as are the muscle fibers composing them - Muscle fiber - Each muscle fiber is a cell with a plasma membrane (sarcolemma), a cytoplasm (sarcoplasm), and an endoplasmic reticulum (sarcoplasmic reticulum) Microanatomy of Skeletal Muscles - Sarcolemma - Specialized plasma membrane - Myofibrils - Long organelles inside muscle cell - Aligned to give distinct bands - I band = light band - Contains only thin filaments - A band = dark band - Contains the entire length of the thick filaments - Sarcoplasmic reticulum - Specialized smooth endoplasmic reticulum Sarcomeres - Sarcomere -- contractile unit of a muscle fiber - Organization - Myofilaments - Thick filaments = myosin filaments - Thin filaments = actin filaments Myofilament Structure - At rest, there is a bare zone that lacks actin filaments called the H zone - Sarcoplasmic reticulum (SR) - Stores and releases calcium - Surrounds the myofibril Sliding Filament Model of Muscle Contraction - Muscles stimulated by CNS - Impulses travel down T-Tubules releasing Ca ions from the SR - Muscle contracts because the sarcomeres shorten - The sarcomere shortens because the actin filaments slide past the myosin filaments approaching each other from the ends of the sarcomere - The I-band shortens while the Z-bands move inward as the H-zone disappears - ATP supplies the energy for the Myosin filaments to do work for the muscle contraction Stimulation to Contraction - Excitability (also called responsiveness or irritability) -- ability to receive and respond to a stimulus - Contractility -- ability to shorten when an adequate stimulus is received - Extensibility -- ability of muscle cells to be stretched - Elasticity -- ability to recoil and resume resting length after stretching - Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract - Motor unit -- one motor neuron and all the skeletal muscle cells stimulated by that neuron Nerve Stimulus to Action Potential - Neuromuscular junction - Association site of axon terminal of the motor neuron and muscle - Synaptic cleft - Gap between nerve and muscle - Nerve and muscle do not make contact - Area between nerve and muscle is filled with interstitial fluid Transmission of Nerve Impulse to Muscle - Neurotransmitter -- chemical release by nerve upon arrival of nerve impulse - The neurotransmitter for skeletal muscles is acetylcholine (ACh) - Acetylcholine attaches to receptors on the sarcolemma - Sarcolemma becomes permeable to sodium (Na+) Motor Units - Many muscle fibers innervated by one nerve fiber axon - Nerve fiber + all fibers it innervates = motor unit - Motor unit obeys all-or-non law: All muscle fibers in a motor unit are stimulated at once so that they are all contract of don't contract - Smaller ratio of nerve fibers to muscle fibers for eyes, etc. because they require finer muscle movement - When motor unit is stimulated by infrequent electrical impulses a single contraction takes place lasting a fraction of a second = muscle twitch Tetanus - If maximal contraction occurs, a sustained contraction called a Tetanus is achieved - Tetanus continues until fatigue sets in and the muscle relaxes even when stimulated - Normally, tetanus doesn't happen under most circumstances because some motor units are contracting maximally while others are resting. This allows for sustained contractions - Muscle tone is when some motor units are always contracted but not enough to cause movement Energy Sources for Muscle Contraction - Sources of energy stored in muscle: Glycogen and fat (triglycerides) - Sources of energy derived from blood: Blood glucose and plasma fatty acids - Anaerobic pathways to produce ATP for energy for muscle contraction: Creatine PO~4~ Pathways (no need for O~2~); Fermentation (no need for O~2~) - Aerobic pathway to produce ATP for muscle contraction: Cellular respiration - Muscle cells have 3 ways to acquire more energy which they need because they have limited amounts of ATP stored Muscle Twitch - Three staged of muscle twitch: - Latent period -- period of time between CNS stimulation and the beginning of muscle contraction - Contraction period -- muscle is shortening - Relaxation period -- muscle is returning to its original length - Ca ions are released by the SR for contraction and returns to the SR for muscle relaxation - Nerve fibers can give rapid series of stimuli to the motor unit -- the summation of these stimuli can result in increased muscle contraction Aerobic vs Anaerobic - Creatine PO~4~ Pathway: Fastest and simplest way for muscles to produce ATP for energy to contraction of muscles -- CP formed during muscle cell resting and storage is limited - CP is use for high-intensity exercise before cellular respiration takes over - Fermentation: produces ATP by breaking down glucose producing Lactate -- begins by breaking down glycogen to glucose - Lactate formation also results in muscle aches and fatigue -- with rest and O~2~ intake lactate is converted back to glucose - O~2~ debt is what limits fermentation - Cellular respiration: more likely to supply energy when exercise is sub-maximal in intensity -- not an immediate source Fast Twitch vs Slow Twitch - Muscle cells contain myoglobin which combines with and stores O~2~ - Fast twitch muscle fibers differ from slow twitch in that they are light in color (fewer mitochondria) have little myoglobin (to store O~2~), and have fewer blood vessels - Fast twitch: Develops maximum tension more quickly and to a greater degree -- however, fatigue quickly and lactate builds up quickly because they depend on anaerobic energy -- sprinting and power sports - Slow twitch: Darker in color (more mitochondria) , lots of myoglobin, and are surrounded by dense capillary beds to take in lots of O~2~ -- more endurance (long distance running, swimming) because they produce most of their energy aerobically -- low maximum tension but resistant to fatigue -- substantial reserves of glycogen and fat Types of Contractions - Concentric contraction of muscles: muscles shorten during contraction - Eccentric: contraction of muscles: muscles lengthen during contraction (negative phase of the lift) - Benefits of eccentric contraction: - Improves strength - Good for reducing inflammation - Some studies show that it may help prevent Type 2 diabetes - Strengthens knee muscles - Treats tennis elbow - Lower strength losses during reduced training periods (in-season competition) - Improved performance - Prevents injuries - Lengthens careers Five Rules of Skeletal Muscles 1. With few exceptions, all skeletal muscles cross at least one joint 2. Typically, the bulk of a skeletal muscle lies proximal to the joint crossed 3. All skeletal muscles have at least two attachments: the origin and the insertion 4. Skeletal muscles can only pull; they never push 5. During contraction, a skeletal muscle insertion moves toward the origin Muscles and Body Movements - Movement is attained due to a muscle moving an attached bone - Muscles are attached to at least two points - Origin - Attachment to a moveable bone - Insertion - Attachment to an immovable bone Types of Body Movements - Flexion - Decreases the angle of the joint - Brings two bones closer together - Typical of hinge joints - Extension - Opposite of flexion - Increases angle between two bones - Rotation - Movement of a bone around it longitudinal axis - Common in ball-and-socket joints - Example is when you move the atlas around the dens of the axis (shake you head "no") - Abduction - Movement of a limb away from the midline - Adduction - Opposite of abduction - Movement of a limb towards the midline - Circumduction - Combination of flexion, extension, abduction, and adduction - Common in ball-and-socket joints Special Movements - Dorsiflexion - Lifting the foot so that the superior surface approaches the shin - Plantar flexion - Depressing the foot (pointing the toes) - Inversion - Turn sole of foot medially - Eversion - Turn sole of foot laterally - Supination - Forearm rotates laterally so palm faces anteriorly - Protonation - Forearm rotates medially so palm faces posteriorly - Opposition - Move thumb to touch the tips of other fingers on the same hand Types of Muscles - Prime mover -- muscle with the major responsibility for a certain movement - Antagonist -- muscle that opposes or reverses a prime mover - Synergist -- muscle that aids a prime mover in a movement a helps prevent rotation - Fixator -- stabilizes the origin of a prime mover Naming Skeletal Muscles - By direction of muscle fibers - *Rectus* -- straight - By relative size of the muscle - *Maximus* -- largest - By location of the muscle - *Temporalis* -- temporal bone - By number of origins - *Triceps* -- three heads - By location of the muscle's origin and insertion - *Sterno* -- on the sternum - By shape of the muscle - *Deltoid* -- triangular - By action of the muscle - *Flexor* -- flexes a bone - *Extensor* -- extends a bone Comparison of Skeletal, Cardiac, and Smooth Muscle - Skeletal - Connective tissue components - Epimysium - Perimysium - Endomysium - Regulation of contraction - Voluntary; via nervous system controls - Speed of contraction - Slow to fast - No rhythmic contraction - Cardiac - Connective tissue components - Endomysium attached to the fibrous skeleton of the heart - Regulation of contraction - Involuntary; the heart has a pacemaker; nervous system controls; hormones - Speed of contraction - Slow - Rhythmic contraction - Smooth - Connective tissue components - Endomysium - Regulation of contraction - Involuntary; nervous system controls; hormones, chemicals, stretch - Some rhythmic contraction Cardiac Muscle - Striations - Usually a single nucleus - Branching cells - Joined to another muscle cell at an intercalated disc - Involuntary - Found only on the heart Smooth Muscle - Lacks striations - Spindle-shaped cells - Single nucleus - Involuntary -- no conscious control - Found mainly in the walls of hallow organs Skeletal Muscle Types - Skeletal muscles fibers differ in the power, speed, and duration of the muscle contraction generated - Power is related to the diameter of a muscle fiber - Larger fibers have a larger number of myofibrils in a parallel resulting in a more powerful contraction - Speed depends on whether the muscle fiber expresses a slow or fast genetic variant of myosin ATPase (the enzyme that splits ATP) - Those with a fast variant are called fast-twitch fibers - Those with a slow variant are called slow-twitch fibers - Fast-twitch fibers also have: 1. A fast rate of action potential propagation along the sarcolemma 2. Are quick on their Ca^++^ release and re-uptake by the SR - Duration has to do with how long the contraction lasts - Fast-twitch fibers initiate a contraction more quickly (0.01 msec) compared to slow-twitch fibers (0.02 msec). - The duration of the contraction is 7.5 msec for fast-twitch and 100 msec for slow-twitch - Fast-twitch fibers exhibit both power and speed because they possess all three characteristics: 3. Stronger contraction 4. Initiate contraction quicker 5. Produce a contraction of shorter duration - Ways ATP can be supplied (another criterion differentiating muscle types) - Is the ATP supply more from aerobic cellular respiration or glycolysis? - Oxidative fibers specialize in providing ATP through aerobic cellular respiration - Other characteristics of oxidative fibers is that they are accompanied by: 6. An extensive capillary network 7. A large number of mitochondria 8. A large supply of red-pigment myoglobin -\> red fibers - The higher level of ATP generated provide energy for oxidative fibers to continue contracting for longer periods of time without fatigue -\> fatigue resistant - Glycolytic fibers specialize in providing ATP more rapidly through glycolysis - Glycolytic fibers have fewer structures (the ones needed for aerobic cellular respiration) 1. A less extensive capillary network 2. Fewer mitochondria 3. Smaller amounts of myoglobin -\> white fibers - Glycolytic fibers for have a large glycogen reserve fir supplying glucose for glycolysis, which is useful when O~2~ stores are low - Glycolytic fibers fatigue easily -\> fatigable Muscle Fiber Sub-Types - Three sub-types of muscle fibers - Slow-oxidative (SO) fibers also called Type I 1. Half the diameter 2. Slow myosin ATPase 3. Slower, less powerful contractions - Fast-oxidative (FO) fibers, also called Type II 4. Intermediate size 5. Fast myosin ATPase 6. Fast, powerful contractions primarily through aerobic cellular respiration - However, the vascular supply to (FO) fibers is less extensive than that of the (SO) fibers so the delivery rate of nutrients and O~2~ is lower - These fibers contain lots of myoglobin but less than (SO) fibers - Fast-glycolytic (FG) fibers, also called Type lib 7. Fast anaerobic fibers 8. Most prevalent muscle fiber type 9. Largest in diameter 10. Contains fast myosin ATPase 11. Provides both power and speed 12. Can only contract for short bursts (because ATP is provided primarily through glycolysis) 13. Fibers appear white because of the lack of myoglobin and mitochondria Skeletal Muscle Composition/Action - Tendon: at the end of a muscle, the three connective tissue layers (epimysium, perimysium, and endomysium) merge to form a fibrous tendon - Tendons are thick, cord-like structures that attach muscle to bone, muscle to skin, or muscle to muscle - In some cases, the tendon forms a thin, flattened sheet called an aponeurosis - The less movable attachment is called the origin and the more movable attachment is called the insertion - Sometimes the terms 'superior attachment' or 'inferior attachment' are used to describe axial muscles, and 'proximal attachment' and 'distal attachment' when discussing the appendicular muscles Skeletal Muscle Organizational Patterns - The organization fascicles in different muscles varies even though the fascicles, themselves, lie parallel to each other - There are 4 different patterns of fascicle arrangement: - Circular muscle (also called a sphincter): has concentrically arranged muscle fascicles around an opening or recess - Contraction decreases the diameter of the passageway - Parallel muscles: have a central body (belly) - When it contracts, it shortens and increases in diameter - High endurance but not strong - Convergent muscles: widespread muscle fascicles over a broad area that converge on a common attachment site - The attachment site could be 1. A single tendon 2. A tendinous sheet 3. A slender band of collagen fibers called 'raphe' - A convergent muscle is versatile in its direction of pull and the direction can be modified merely by activating a specific group of muscle fibers - They do not pull as hard on the tendon as 'parallel muscles' because they are pulling in different directions - Pennate muscles: the fascicles exhibit the same angle with respect to their tendon - Pennate muscles have one or more tendons extending through their body - The fascicles are arranged obliquely to the tendon (pulls at an angle to the tendon) - Because of this angle, pennate muscles do not move its tendon as far as parallel muscles - 3 types: 4. Unipennate muscle: all of the muscle fascicles are on the same side of the tendon 5. Bipennate muscle (most common): fascicles on both sides of the tendon 6. Multipennate muscles: have branches of tendon within the muscle and fascicles arranged on both sides of each tendon branch Skeletal Muscle Action - Agonist - AKA the prime mover - Contracts to produce a certain movement - Antagonist - Actions oppose those of the agonist - Contraction of the agonist lengthens the antagonist, and vice-versa - The lengthened muscle, usually does not relax because its tension is used to control the speed of the movement and make it smooth - Synergist - Assists the agonist - Most useful at the start of a movement (extended agonist can't produce power - May also assist an agonist by preventing movement at a joint (fixator) Muscles of Facial Expressions - Muscles facial expression attach from the superficial facia or skull bones to the superficial facia of the skin - When they contract, they pull the skin - Most are innervated on the 7^th^ cranial nerve (CN VII) which is called the facial nerve - Epicranius: composed of the occipitalis muscle and the epicranial aponeurosis - Raises the eyebrows and wrinkles the skin on the forehead - Corrugator supercilia - Draws the eyebrows together - Orbicular oculis - Circular muscle closing the eyes - Levator palpebrae superioris - Elevates the eyebrows when eyes open - Nasalis - Elevates the corners of the mouth - Procerus - Wrinkling of the nose - Orbicular oris - Circular muscle that closes the mouth - Depressor labii - Pulls the lower lip inferiorly - Depressor anguli oris - Frown muscles - Levator labii superiorly - Pulls the upper lip superiorly - Levator anguli oris - Pulls the corners of the mouth superiorly and laterally - Zygomaticus major/minor - Work with levator aguli oris to smile - Risorius - Pulls the corner of the lips laterally (closed mouth smile) - Mentalis - Protrudes the lower lip - Platysma - Tenses the skin of the neck and pulls the lower lip inferiorly - Buccinator - Compresses the cheek against the teeth when you chew Muscle Aches and Soreness - Muscle aches involve muscles, tendons, ligaments, and facia - Occurs from tension, overuse, muscle injury, etc. - Can be a sign of flu, lupus, fibromyalgia (tenderness, fatigue, soreness, etc.) - DOMS: Delayed Onset Muscle Soreness - Microtears in muscle fibers are often the cause - Causes inflammation 24-48 hours afterwards and thus the soreness pain - Some research says that eccentric contraction exercises cause more tears - Estrogen increases repair - Active recovery is best - Need blood moving through muscles - Foam rolling Muscular Paralysis - Tetanus - Clostridium tetani - Toxin blocks release of glycine, the inhibitory neurotransmitter in the spinal cord \> over stimulation of motor neurons of muscle cells \> excessive muscle contraction - Botulism - Clostridium botulinum (anaerobic) - Toxin prevents release of ACh at synaptic terminals \> muscular paralysis - Get it by ingesting toxin in canned foods where the food product was not heated enough to kill spores produced by bacteria - Botulism A: repeated application to reduce wrinkles - Reduce spasticity: the over contraction of muscles and used to treat cerebral palsy, multiple sclerosis, strokes, and spinal cord injuries Skeletal Muscle Relaxation - Termination of rapid nerve signals - Nerve signal stops \> ACh releases stops \> ACh receptors close on sarcolemma \> CA^++^ channels in the sarcoplasmic reticulum and T-tubules decrease Ca^++^ and return it to storage instead \> Troponin (actin) return to original shape because of no Ca^++^ - ATP is used to cause Ca^++^ pumps to be active in keeping Ca^++^ in the cytosol low \> prevents Ca^++^ from binding with PO~4~ ions that were released from ATP to form hydroxy apatite \> calcifies and hardens muscles - ATP is required for both contraction and relaxation of muscles - If not enough ATP available (as in death) \> muscles relaxation doesn't occur \> stay contracted (rigor mortis) Skeletal Muscle Metabolism - Most of the ATP produced for muscle cells is to re-set the myosin (thick) heads \> contraction - Requires lost of ATP - ATP is required for Ca^++^ pumps within the sarcoplasmic reticulum to retain Ca^++^ for storage - Limited ATP within the skeletal muscle fibers - An additional, small amount of ATP is available from transfer of P from ADP \> ATP - Enzyme involved is myokinase - This amounts of ATP is good for 5-6 sec of maximum exercise - Need ATP from other sources - Creatine PO~4~: high energy bonds with PO~4~ in tissue (muscle and brain cells) - The phosphorus in creatine PO~4~ is transferred to ADP \> ATP - This allows for an additional 20-25 seconds of maximum exertion Rest - During rest the limited stores of ATP and Creatine PO~4~ are replenished - ATP formed in cellular respiration helps to restore ATP - Some ATP is used to build more creatine PO~4~ - Creatine kinase is involved - Glycolysis - Glucose from glycogen stores within muscle fibers or delivered by blood - Advantage of ATP from glycolysis - No O~2~ needed - Rapid rate of production (twice as fast as aerobic cellular respiration Aerobic Cellular Respiration - Aerobic respiration - Within the mitochondria - Requires O~2~ from blood or myoglobin - Advantage - Variety of nutrients (glucose, fatty acids, amino acids, etc. can be oxidized) - Greater amounts ATP - Lactate formation (low O~2~ from increased intensity) - Enzyme involved: lactate dehydrogenized - What happens to the lactate? - It either enters the mitochondria within the muscle fiber and the blood \> taken up by the cardiac muscle \> pyruvate is oxidized - Taken up by the liver \> glucose (glucogenesis) Energy Supply and Varying Intensity of Exercise - At rest: skeletal muscle obtains the needed ATP almost exclusively from aerobic cellular respiration - Examples: - 50-yard sprint: takes 5-6 seconds \> ATP is supplied almost entirely from stored ATP and the P transfer between two ADP molecules and between creatine PO~4~ and ADP - 400-meter: ATP is initially supplied by the above, then by glycolysis, and then by lactic acid fermentation - 1500-meters: ATP is supplied by all three means, but primarily from aerobic cellular respiration Oxygen Debt - There are limitations to how much O~2~ can be supplied to skeletal muscles in a given time period - During exercise, when the demand for O~2~ exceeds the available O~2~, an O~2~ debt is incurred - By definition, O~2~ debt is the amount of additional O~2~ that is consumed following exercise to restore pre-exercise conditions - Additional O~2~ is required primarily by: - Skeletal muscle fibers to replace O~2~ on myoglobin molecules - Replenish ATO and Creating PO~4~ - Replace glycogen stores - By the liver to convert lactate back to glucose through glycogenesis - Also required by respiratory muscles engaging in forced breathing - The heart to pump blood - Overall higher metabolic rate: pay off the debt Nerve Stimulus and Neuromuscular Junction - Somatic motor neurons - Nerve cells that activate skeletal muscle fibers (cells) - Motor neurons reside in the brain and spinal cord - Branches leading to the end of the axon (axon termini) collectively are referred to as the neuromuscular junction or motor end plate for each individual fiber - Each muscle fiber has only one neuromuscular junction, located midway along itd length - The neurotransmitter is acetylcholine (ACh) which binds to ACh receptors is quickly terminated by the enzyme acetylcholinesterase located in the synaptic cleft - Homeostatic imbalance - Toxins, drugs, and diseases interfere with neuromuscular junction activities - Ex) Myasthenia gravis is characterized by drooping upper eyelids, swallowing and talking difficulties, and generalized muscle weakness caused by a shortage of ACh receptors - Because Abs to ACh were found in blood serum samples, it is believed to be an autoimmune disease in which ACh receptors are destroyed Generation of the Action Potential - Like all plasma membranes, a resting sarcolemma is polarized with the inside of the cell being negative relative to the outer membrane face - An action potential results from a sequence of electrical charges across the sarcolemma - Once the AP is initiated, it moves along the entire surface of the sarcolemma causing the AP - ACh molecules bond to ACh receptors \> opens the ligand (gated ion channels for Na^+^ and K^+^) \> depolarization (-90 mV to +30 mV) which is the end plate potential - End plate potential \> AP \> Na^+^ enters - Repolarization (K^+^ ion channels open) - During repolarization the muscle fiber is in a refractory period meaning the cell cannot be stimulated again until repolarization is complete - Na^-^K pump correct ionic imbalances - Thousands of APs can occur before imbalance can affect the contractile activity Excitation-Contraction Coupling - Action potentials are brief and end before contraction - The electrical signal (AP) causes a rise in intracellular Ca^++^ levels which in turn allows the myofilaments to slide (sliding filament theory) - 1^st^: Nerve impulse reaches the axon terminal opening voltage-gated Ca^++^ channels which triggers release of ACh into the synaptic cleft - 2^nd^: ACh binds with ACh receptors in the sarcolemma opening Na^+^ and K^+^ gates (end plate potential) - 3^rd^: Transmission of the AP along the T-tubules which causes SR to release Ca^++^ into the cytosol - Ca^++^ is required for the cross-bridge formation - When an intracellular Ca^++^ levels are low, the muscle cell is relaxed and tropomyosin molecules block the myosin-binding sites on actin - The tropomyosin "blockage" is removed when sufficient Ca^++^ is provided Rigor Mortis - Most muscles begin to stiffen 3-4 hours after death - Peak rigidity: 12 hours and then gradually dissipates over the next 48-60 hours - Dying cells are unable to get Ca^++^ intracellularly which is needed for formation of myosin cross bridges - ATP synthesis ceases at death but remaining ATP continues to be consumed and cross bridge detachment is impossible \> actin and myosin become irreversibly cross-linked \> stiffness \> muscle proteins break down and stiffness disappears (forensics) - Cross bridge cycle 1. Cross bridge formation: Energized myosin head attaches to an actin myofilament forming a cross bridge 2. The power stroke: ADP and P are released and the myosin head pivots and bends, changing to its bent low-energy state \> pulls the actin filament 3. Cross bridge detachment: After ATP attaches to myosin, the link between myosin and actin weakens and the myosin head detaches 4. Cocking of the myosin head: As ATP is hydrolyzed to ADP and P, the myosin head returns to its prestrike high-energy (clocked) position Wave Summation and Recruitment - The force exerted by a contracting muscle is called muscle tension - The opposite force on the muscle by the weight of the object is called the load - The contracting muscle does not always shorten and move the load - If muscle develops tension but the load does not move, the contraction is called isometric - If the muscle tension developed overcomes the load and muscle shortening does occur, the contraction is called isotonic - Graded muscle responses - Healthy muscle contractions are relatively smooth, varying in strength as different demands are placed on them - These variations controlling the movement are called graded muscle responses - Muscle contractions can be graded in two ways - Change in frequency of stimulation - Changing the strength of stimulation Muscle Contraction Changes/Stimulus Frequency - Greater muscular force is generated by the nervous system by increasing the firing rate of motor neurons - When two identical stimuli are delivered to a muscle in rapid succession, the 2^nd^ twitch will be stronger = wave or temporal summation - The 2^nd^ contraction occurs before the muscle has completely relaxed - Since the muscle is already partially contracted, and more Ca^++^ being reclaimed by the SR, the 2^nd^ contraction causes more shortening than the 1^st^ (contractions are added together) - The refractory period is always honored: If a 2^nd^ stimulus arrives before repolarization is complete, no wave summation occurs - How does this work? - The relaxation time between twitches becomes shorter and shorter - Ca^++^ concentration in the cytosol increases higher and higher - The degree of wave summation becomes greater and greater \> unfused, or incomplete contraction - If stimulation frequency continues to increase \> maximal tension \> sustained contraction plateau called fused or complete tetanus (infrequent and prolonged tetanus will eventually lead to muscle fatigue) Muscle Response/Stimulus Strength - Recruitment (multiple motor unit summation) - Controls the force of contraction more precisely than stimulus frequency - Calls for more and more muscle fibers into play - Recruitment of muscle fibers is not random but instead is dictated by the 'size principle' - The motor units with the smallest muscle fibers are activated first because they are controlled by the smallest,, most highly excitable motor neurons - As motor units with larger and larger muscle fibers begin to be excited, contractile strength increases - The largest motor units are activated only when the most powerful contraction is necessary - The size of the motor units is important because it allows the increases in force during weak contractions (posture and slow movements) to occur in small steps - More common for motor units to be recruited asynchronously instead of all at once Aerobic Metabolism - Is the primary energy source of resting muscles (to convert glucose into glycogen and to create energy storage compounds as CP) - During rest and light to moderate exercise, aerobic metabolism contributes 95% of the necessary ATP - It breaks down fatty acids, pyruvic acid (made via glycolysis), and amino acids - Produces 34 ATP molecules per glucose molecule Muscle Tone - Skeletal muscles are almost always slightly contracted = muscle tone - Muscle tone does not produce active movements but it does keep muscles firm and ready to respond to stimulation and help stabilize joints and maintain posture - Isometric contractions: occurs when a muscle attempts to move a load that is greater than the force (tension) the muscle is able to develop - Isotonic contractions are of two types: - Concentric contractions = muscle shortens during contraction to do work - Eccentric contractions = muscles generate force as it lengthens - Eccentric contractions are about 50% more forceful than concentric contractions at the same load Muscle Fatigue - Physiological inability to contract despite continued stimulation - Occurs when - Ionic imbalances (K^+^, Ca^2+^, P~i~) interfere with E-C coupling - Prolonged exercise damages SR and interferes with Ca^2+^ regulation and release - Total lack of ATP occurs rarely, during states of continuous contraction, and causes contractures (continuous contractions) Lactate - Lactate vs Lactic acid - The main difference is that lactic acid contains one more proton (H^+^) than lactate making lactate a base and lactic acid and acid - Lactate can act as a buffer for lactic acid in certain circumstances - Lactic acid donates the proton to lactate (buffer) - The body produces and uses lactate (not lactic acid) - Hard exercise \> not enough O~2~ (anaerobic mode) \> pyruvate to lactate (energy for a brief time) - Intense muscle work causes muscles to become acidic, interfering with contraction - Lactate helps to counter act depolarization (the burn feeling) = no more reps (protective device) when lactate can't continue (lactate threshold) - It is believed that microtears (and acid) are responsible for DOMS - Lactate helps bolster the mitochondria \> improve stamina and strength - High intensity interval training near the lactate threshold with a brief rest \> develops a higher lactate threshold - 75% of lactate is processed while 25% leaks into the blood and can therefore be tested - ATP hydrolysis is what puts H^+^ ions into the blood \> pH drops \> muscle acidosis \> lactate buffers by excepting a proton - Lactate clears within minutes via - Oxidative muscle fibers (H^+^ ions + oxygen \> H~2~O) - Cardiac muscle uses lactate for energy - Cori cycle in liver - Brain ATP for Muscle Contraction - What is ATP's role in muscle contraction? - Supplies energy to move and detach cross bridges - Energy to operate the Ca^2+^ pump in the SR - Energy to return Na^+^ and K^+^ to the cell exterior and interior, respectively after excitation-contraction coupling (Na^+^/K^+^ pump) - ATP storage (limited) - Muscles store very little ATP (4-6 sec worth at most) - Since ATP is the only energy source used directly for contractile activities, it has to be regenerated quickly, as fast as it is broken down, to continue contraction - How is ATP regenerated? - ATP is regenerated within a fraction of a second by one or more of three pathways - Direct phosphorylation of ADP by creatine phosphate - Anaerobic glycolysis converting glucose to lactic acid - Aerobic respiration Three Pathways to Regenerate ATP - Creatine PO~4~ pathway (direct phosphorylation of ADP) - Begin vigorous exercise \> stored ATP used up in a few twitches \> creatine PO~4~ (stored in muscles) is used to regenerate ATP while the metabolic pathways adjust to the new higher demand for ATP - CP is coupled with ADP, transferring energy and a PO~4~ to ADP \> ATP via the enzyme creatine kinase (creatine PO~4~ + ADP \> Creatine + ATP) - Muscle cells can store up to 3 times more CP than ATP (very efficient) - Together ATP and CP can provide maximum muscle power for 15 seconds - This is a reversible reaction and keeps CP reserves replenished during rest - Anaerobic pathway (glycolysis and lactic acid formation) - As ATP and CP are spent \> ATP generated by breaking down (catabolizing) glucose from the blood or glycogen stored in the muscle - First step is glycolysis which occurs with or without O~2~ - Since O~2~ is not used it is an anaerobic pathway - Each 6C glucose molecule \> 2 3C pyruvates = 2 ATP - Sub-maximal exercise the pyruvates enter the mitochondria (aerobic respiration) - When muscles contract vigorously (contractile activity reached 70% of max) the bulging muscles compress the blood vessels within them, impairing blood flow and O~2~ delivery \> most of the pyruvate is converted to lactic acid (O~2~ deficit \> lactic acid) - Lactic acid diffuses out of the muscles onto the bloodstream which carries the lactic acid to the liver, heart, or kidney cells which can use it as an energy source \> liver cells can reconvert it to pyruvate or glucose \> release it back to the bloodstream for muscles to use or convert it to glycogen for storage - Accounts for 5% as much ATP as aerobic pathway but 2½ times faster - Stored ATP + CP + anaerobic pathway = strenuous activity up to 1 minute - Negatives: large amounts of glucose used to produce small amount of ATP and muscle soreness from accumulated lactic acid - Aerobic respiration - Because ATP stores and CP is limited muscles must metabolize nutrients from food to ATP - Mitochondria can do this during rest and light to moderate exercise (95% of ATP generated) - Aerobic respiration: Glucose + O~2~ \> CO~2~ + H~2~O + ATP - Exercise begins \> muscle glycogen provides most of the fuel - Next: blood glucose, pyruvate from glycolysis, and free fatty acids are the major fuel sources - 30 minutes later: fatty acids are the major energy fuels - Aerobic respiration is slow because it depends on the delivery of O~2~ and the many chemical steps to break down glucose for fuel - Aerobic endurance: length of time a muscle can continue to contract using aerobic pathway - Anaerobic threshold: the point at which muscle metabolism converts to anaerobic glycolysis - Surge of power activities: rely entirely on ATP and CP stores (weight training, sprints) - Sligtly longer bursts of activity: fueled entirely by anaerobic glycolysis (soccer, tennis) - Prolonged activity: depend mainly on aerobic respiration using glucose and fatty acids Muscle Fatigue - Defined as a state of physiological inability to contract even though the muscle is receiving stimuli - Causes are not well understood but may be: - Ionic imbalances contribute to muscle fatigue - As action potentials are transmitted, K^+^ is lost from the muscle cells and accumulates in the fluids of the T-tubules halting Ca^2+^ release from the SR - Accumulation of inorganic phosphate from CP and ATP breakdown may interfere with calcium release from the SR - Lactic acid intracellular accumulation - Raises the H^+^ and alters contractile proteins - Still debatable because extracellular lactate actually counteracts the high K^+^ levels that lead to muscle fatigue - Summary of muscle fatigue - Intense exercise \> fatigue via ionic disturbances \> alter E-C coupling \> fast recovery - Slow developing fatigue from prolonged low-intensity exercise \> requires several hours for complete recovery \> damages SR interfering with Ca^2+^ regulation and release Forced Muscle Contraction - Force of muscle contraction depends on the number of myosin cross bridges that attach to the actin myofilaments, which is affected by four factors: 1. Number of muscle fibers recruited - The more motor units recruited, the greater the force of contraction 2. Size of the muscle fibers - The bulkier the muscle and the greater the cross-sectional area, the more tension it can develop - Increased resistance exercise \> muscle cell hypertrophy \> more power 3. Frequency of stimulation by the CNS - Contractions are summed as muscle is stimulated more frequently 4. Degree of muscle stretch - Tension that a muscle can generate varies with length - The ideal length-tension relationship occurs when the muscle is slightly stretched and the myosin and actin filaments overlap optimally - Too much stretch makes it difficult for the myofilaments to link - Too little stretch and the sarcomere is compressed (inefficient) Velocity and Duration of Contraction - Classification of muscle fiber type - Two functional characteristics of muscle fiber type - Speed of contraction - Slow fibers - Fast fibers - The difference are in how fast their myosin ATPases split ATP and the pattern of electrical activity of their motor neurons - Duration of the contraction depends on how quickly Ca^2+^ moves from the cytosol into the SR - Muscle cell's major pathways for forming ATP - Oxidative fibers: the muscle fiber cell that depends on the O~2~ using aerobic pathways to generate ATP - Glycolytic fibers: skeletal muscle cells relying more one anaerobic glycolysis and CP to generate ATP - From this criteria skeletal muscle cells can be classified into three groups: - Slow oxidative fibers - Fast oxidative fibers - Fast glycolytic fibers Muscle Fiber Type - Slow oxidative fiber (Type I) - Myosin ATPase are slow \> contracts slowly - Depends on O~2~ delivery - Resists fatigue and has high endurance - A thin cell with very little power with limited number of myofibrils - Many mitochondria, rich in capillary supply, red from abundant supply of myoglobin (muscle's O~2~-binding pigment that stores O~2~ reserves) - Fast oxidative fiber (Type IIa) - Intermediate between oxidative and glycolytic - Fast glycolytic fibers (Type IIb) - Fast myosin ATPases so contracts rapidly - Uses little O~2~ - Depends on plentiful glycogen reserves for fuel rather than on blood delivered nutrients - Fatigues quickly because glycogen reserves are short lived - Large diameter (large number of myofibrils) contracts powerfully - Few mitochondria, little myoglobin, and few capillaries (white fibers) Recruitment and Skeletal Muscle Response to Exercise - All muscle fibers in a particular motor unit are from the same type - Muscle fiber types differ genetically from person to person - Muscle fiber types can be modified by exercise to an extent - A greater load that the muscle has to deal with results in a longer latent period, slower shortening, and a briefer duration of shortening - Muscles work more quickly and can work longer with more motor units contracting - Forcing a muscle to work hard increases its strength and endurance - Muscles adapt to greater demands and must be overloaded to produce further gains - Muscle inactivity \> muscle weakness and atrophy Skeletal Muscle Response to Exercise - Aerobic (endurance) exercise - Number of capillaries surrounding the muscle fibers increases - Number of mitochondria within the muscle fibers increases - Muscles fibers synthesize more myoglobin - These changes occur in all fiber types but are most dramatic in the slow oxidative fibers - Resistance exercise - Muscles hypertrophy results from high-intensity resistance exercise under anaerobic conditions - Addition bulking of muscles is due mostly hypertrophy rather than recruitment - Some of the bulk may result from longitudinal splitting of fibers - Resistance activities can also convert fast oxidative fibers to fast glycolytic fibers - If the specific exercise routine is discontinued, the converted fibers revert back to their original metabolic properties Inactivity \> Unhealthy Muscles - Ex) enforced bed rest (immobilization) \> disuse atrophy (degeneration and loss of mass) \> occurs as soon as muscles are immobilized - Same for cases of loss of neural stimulation - Muscle strength can decline at a rate of 5% per day - Even at rest, muscles normally receive weak intermittent stimuli from the nervous system - A paralyzed muscle (total loss of stimulation) may atrophy to ¼ of its initial size - Fibrous connective tissue replaces the lost muscle tissue making muscle rehabilitation Muscular Dystrophy - Muscular dystrophy: group of inherited muscle-destroying diseases - Generally appear during childhood - Affected muscles first enlarge dur to fat and connective tissue deposits in place of muscle fibers that atrophy and degenerate - Ex) Duchenne muscular dystrophy (DMD) - Most common and most serious - Inherited as a sex-linked recessive disease - Expressed almost exclusively in males (1 of every 3600 male births) diagnosed between the ages of 2-7 - Affected children become clumsy and fall as their skeletal muscles weaken - Progresses relentlessly from the extremities upward until it reaches the head and chest muscles and the cardiac muscles - Rarely reach their 20s dying of respiratory failure - Caused by a defective gene for dystrophin (links the cytoskeleton to the ECM to stabilize the sarcolemma which: the weakening of the sarcolemma then tears during contraction allowing excess Ca^2+^ in to damage the contractile fibers) - No cure - Treatments are the steroid prednisone to reduce spine and joint deformities to remain mobile Aging - As we age \> number of muscle fibers decreases replaced by increased connective tissue \> muscles become stringy - By age 30 \> gradual loss of muscle mass (sarcopenia) begins - By age 80 \> muscle strength usually decreases by about 50% - Serious health implications for the elderly \> falling - Regular exercise helps reverse sarcopenia and rebuild muscle mass - Smooth muscle is not affected Developmental Aspects of Muscles - All three types of muscle tissue develop from embryonic mesoderm cells called myoblasts - Skeletal muscle tissue is formed when myoblasts fuse to form multinuclear myotubes - Integrins (cell adhesion proteins) guide this process and form functional sarcomeres - Skeletal muscle fibers are contracting by week 7 (embryo is 2.5 cm long) - Skeletal muscles stop dividing early, however satellite cells help repair injured fibers and allow limited regeneration of dead skeletal muscle (declines with age) - Myoblasts producing cardiac and smooth muscle cells do not fuse but develop gap junctions - Cardiac cells divide at a moderate rate, however injured heart muscle is repaired mostly by scar tissue - Smooth muscles have a good regenerative capacity, and smooth muscle cells of blood vessels divide regularly throughout life - At birth, a baby's movements are uncoordinated and largely reflexive - Both skeletal muscle and cardiac muscle retain the ability to lengthen and thicken in a growing child and to hypertrophy in response to increased load Anabolic Steroids - Anabolic steroids ('juice') are drugs that are variants of testosterone - Introduced in the 1950s - Athletes began using anabolic steroids heavily in the 1960s (mega doses) - Drug testing programs - Barry Bonds, Mark McGuire, Marion Jones, Lance Armstrong - Estimated that 1 in 10 young men have tried them and increasing among young women - Underground suppliers keep producing new versions of designer steroids that evade standard antidoping tests - 1980 Olympic Trials: 196 banned substances - Anabolic steroids enhance muscle mass and strength and raises O~2~- carrying capabilities because of greater volume of RBCs - Anabolic steroids cause: - Bloated faces, acne, hair loss, shriveled testes, infertility, liver damage (cancer), and changes in blood cholesterol levels (predisposed to heart disease) - Psychiatric hazards include: one-third of users suffer serious mental problems like depression, delusions, and manic behavior (roid-rage) Androstenedione - Nutritional performance-enhancer - Newest on the market and sold over-the-counter and not regulated by the FDA - Androstenedione is converted to testosterone in the body - The liver destroys most of it as soon as it is ingested but a few mg can temporarily boost testosterone levels - Reports of 5^th^ graders are using it - Studies have shown that users have developed higher levels of estrogen as well as testosterone - Risk of feminizing effects such as enlarged breasts - Youths may enter puberty early, stunts bone growth (shorter height)