I. Overview of Muscle Tissues

Transcription

1 I. Overview of Muscle Tissues A. Types of Muscle Tissue 1. Terminology 1. Muscle fibers = muscle cells are greatly elongated therefore known as fibers; true for skeletal and smooth muscles only 2. Myo or mys = word roots meaning muscle 3. Sarco = word root meaning flesh or muscle 2. Skeletal Muscle 1. Skeletal = attach to and cover the bony skeleton 2. Striated = obvious stripes due to cellular anatomy 3. Voluntary = under conscious control 4. Contracts rapidly 5. Produces tremendous force and power 3. Cardiac Muscle 1. Cardiac = only found in the heart walls 2. Striated 3. Involuntary = no conscious control 4. Smooth Muscle 1. Visceral = found in the walls of the visceral organs (stomach, urinary bladder, etc ) 2. Non striated 3. Involuntary B. Functional Characteristics of Muscle Tissue 1. Excitability (responsive or irritable) 1. Receives and responds to internal/external stimuli 2. Stimuli is a chemical response to a change 3. Response is an electrical impulse that causes a contraction 2. Contractibility 1. Shortens forcibly when stimulated 3. Extensibility 1. Ability to be stretched or extended while at rest 4. Elasticity 1. Muscle fiber recoil to resume resting length after being stretched C. Muscle Function 1. Producing Movement 1. Contraction of muscles to produce skeletal movement for locomotion and manipulation 2. Movement of fluids and foodstuffs through organs and body tracts

2 II. 2. Maintaining Posture 1. Continual muscular contractions that enable us to maintain erect posture 3. Stabilize Joints 4. Generate Heat 1. Heat is a by product of muscle contraction and release of energy Skeletal Muscle A. Gross anatomy of Skeletal Muscle (a discrete organ made up of several kinds of tissues; muscle fibers, blood vessels, nerve fibers, and connective tissues) 1. Nerve and Blood Supply 1. One muscle = one nerve = one artery = one or more veins 2. All enter and exit in central part of muscle 3. Skeletal muscles must be stimulated 4. Muscle contraction require large amounts of energy 1. Need continuous delivery of oxygen and nutrients 5. Produce a lot of metabolic waste that must be removed 2. Connective Tissue Sheaths 1. Support each muscle fiber and reinforce the whole muscle organ 2. Connective Sheaths 1. Endomysium sheath that surrounds each muscle fiber 2. Perimysium sheath that surrounds fascicle of muscle fibers 1. Fascicle = a group of muscle fibers 3. Epimysium sheath that surrounds the whole muscle 3. All connective sheaths are connective with one another and with the tendons that attach muscle to bone 1. Muscle fiber contraction pulls on the sheaths which transmit the pulling force to bone 3. Attachments 1. Attached to two places across a joint 1. Insertion = the point of attachment that moves during a muscle s contraction

3 2. Origin = the point of attachment that is less movable 1. origin is usually proximal to insertion in the limbs B. Microscopic Anatomy of Skeletal Muscle Fibers 1. General Organelles 1. Sarcolemma = the muscle fiber s plasma membrane 2. Multinucleated 3. Sarcoplasm = muscle fiber cytoplasm 4. Myoglobin = oxygen storing red pigment in muscle; similar to hemoglobin 5. Other characteristic organelles as well (especially mitochondria) 2. Myofibrils = rod-like, contractile elements of the muscle fibers 1. Striations = what causes the bands evident along the length of myofibrils? 1. Myofilaments = cellular structures made up of proteins 1. Thick filaments made up primarily of the protein myosin 1. H-zone (myosin filaments only) 2. M-line (proteins that hold filaments in place) 2. Thin filaments made up primarily of the protein actin 1. I-bands (light bands) 2. Attached to Z lines 3. A-zone (dark band where actin and myosin filaments overlap) 2. Myofibrils are divided into regions called sarcomeres (region from one z-line to another) 1. smallest contractile unit of muscle 2. Myofilaments arranged the same in each successive sarcomere 3. Arrangement of sarcomeres and myofilaments creates striations 2. Molecular composition of myofilaments 1. Thick filaments 1. Each thick filament contains 300 myosin molecules bundled together

4 2. Each myosin molecule has a rod-like tail terminating in two globular heads 1. forms cross bridges between actin and myosin molecules 2. Thin filaments 1. The thin filaments are composed of two interwoven actin filaments (helix) 1. the actin filaments are composed of polypeptides with kidney shaped subunits 2. The subunits are binding sites for the myosin heads 2. Regulatory proteins 1. Tropomyosin = spirals through actin molecule 1. block myosin binding sites in a relaxed muscle 2. Troponin another myosin interaction complex 3. Sarcoplasmic Reticulum and Transverse Tubules 1. Sarcoplasmic Reticulum 1. Smooth endoplasmic reticulum that surrounds each myofibril 2. Regulates intracellular levels of Ca ions 1. stores and releases for contraction 2. Transverse Tubules 1. Extension of sarcolemma that penetrates into the cell interior 2. Conduct nerve impulses deep into muscle cell C. Sliding Filament Model of Contractions 1. Contraction Defined = activation of myosin heads to form cross-bridges with actin 2. Sliding Filament Theory 1. States that during contraction, the thin filaments slide past thick ones 2. Myosin heads latch on to myosin binding sites on the actin molecule 1. Sliding begins when force of myosin head moves actin filament

5 2. Cross-bridges are formed and terminated several times during a contraction; ratchet like motion 3. Thin filaments move toward center of sacromere 1. Z-lines move toward thick filaments 2. I-bands shorten and H-zone disappears

6 D. Physiology of Skeletal Muscle Fibers 1. Nerve Stimulus and Events at the Neuromuscular Junction 1. Skeletal Muscles are stimulated by Motor Neurons 1. Motor Neurons reside in the brain or spinal cord 2. Axons of neurons extend to the muscle cells they serve 1. Axons divide profusely inside each muscle 2. Neuromuscular Junction = point where axon ending joins with a muscle fiber 1. Each muscle fiber has only one n.m. junction 3. Synaptic cleft = small space that separates axon terminal and muscle fiber 4. Synaptic vesicles = located at distal end of axon = contain neurotransmitter = acetylcholine (ACh) 5. Motor End Plate = Location of neuromuscular junction on muscle fiber sarcolemma 1. Highly folded to provide large surface area for ACh receptors 2. Contraction of muscle fiber by stimulation of motor neuron 1. Nerve impulse travels from brain to motor neuron axon 2. Vesicles release neurotransmitter into synaptic cleft 3. ACh receptors trigger contraction of muscle fiber 2. Excitation-Contraction Coupling (E-C) 1. E-C coupling = sequence of events by which transmission of an action potential along the sarcolemma which leads to a muscle contraction 1. action potential = electrical event that spreads in all direction from the neuromuscular junction across the sarcolemma 2. Caused by the me binding of ACh at the motor end plate 2. Steps in the E-C coupling

7 1. Action potential propagates along the sarcolemma and down T-tubules (once started it is unstoppable) 2. Transmission of signal into T-tubules causes Sarcoplasmic Reticulum to release Ca into the sarcoplasm available to myofilaments 3. Calcium binds to troponin 1. changes shape and removes blocking action of tropomyosin 4. Myosin heads attach and pull actin filaments toward center of sarcomere 1. Muscle is active 5. Ca signal ends = an active pump (ATP dependent) moves Ca back into sarcoplasmic reticulum 6. Tropomyosin blockade is re-established = muscle relaxation 7. Relaxation occurs when: 1. Action potential ceases 2. Acetylcholine is decomposed by the enzyme = Acetylcholinesterase 3. Calcium Ions return to SR 4. Actin binding sites are blocked 3. Muscle Fiber Contraction (once binding sites of actin are exposed due to a presence of Ca ions) 1. Cross bridge formation = myosin heads are attracted to actin binding sites 2. Power (working stroke) = myosin head pivots which pulls on the actin filaments 1. With the use of energy being released from the breakdown of ATP 3. Cross bridge detachment = myosin heads detach from actin 1. Another ATP attaches to myosin head 4. Cocking of myosin head = myosin head returns to original position 5. Single power stroke by cross bridges results in a muscle shortening by only 1% of original length 1. Contracting muscles usually shorten by 30-35% of original length

8 2. Cross bridges must attach and detach many times during a contraction 3. Half of myosin heads forming cross bridges at given time = no slippage 4. All requires energy provided by ATP E. Contraction of a Skeletal Muscle 1. The Motor Unit = a motor neuron and all of the muscle fibers it supplies 1. Motor neuron transmits impulse = all muscle fibers contract 2. Muscle with Fine motor control = smaller motor units 1. movements more precise 3. Muscle with less control = larger motor units 1. large, weight bearing muscles 2. Muscle Twitch = the response of a motor unit to a single action potential of its motor neuron (recorded in a myogram) 1. Laboratory use only 2. Distinct phases of the muscle twitch 1. Latent period = milliseconds required for E-C coupling to occur 2. Period of contraction = cross bridge formation is active; if muscle tension overcomes load resistance, muscle shortens 3. Period of relaxation = time required for movement of Ca into SR; muscle tension decreases; muscle lengthens 3. Muscle twitch varies from muscle to muscle depending on metabolic properties of the myofibrils (Shier pg 180) 3. Graded muscle response = healthy muscle response graded by demands placed on muscle 1. Muscle Response to Changes in Stimulation Frequency 1. Rapid successions of stimuli to a muscle produces twitch of different magnitude 1. Successive twitches will be stronger than predecessors 2. Wave Summation = the second contraction occurs before muscle had completely relaxed from the first

9 1. Second contraction will cause more shortening than the first = contractions are summed 2. Stimuli at increasingly faster rate = less relaxation = more Ca in sarcoplasm = degree of summation increases 3. Tetanus = sustained contraction 1. maximal tension reached = relaxation disappears = smooth, sustained contraction reached 2. leads to muscle fatigue 2. Muscle response to stronger stimuli 1. Recruitment = the summation of motor neurons in response to stronger stimulus of contraction (more motor units = greater force of contraction) 2. Threshold stimulus = the minimal strength of a stimulus required to cause a contraction 3. Beyond the threshold = muscle contractions are more and more vigorous as stimulus is increased 4. Maximal stimulus = the point at which all motor units are recruited 5. The recruitment process (dictated by size) 1. Motor units with small smallest muscle fibers recruited first (highly excitable motor neurons) 2. Contractile strength is increased when the larger muscle fibers are recruited 6. Treppe (staircase effect) 1. Muscle contractions after long periods of rest may not be as strong as those that occur later in response to stimuli of same strength 2. Probably reflects the amount Ca in sarcoplasm 3. The basis for warm-ups in athletics 4. Muscle Tone 1. Relaxed muscles being slightly contracted 2. Muscle tone is the result of spinal reflexes activating groups of motor units

10 3. Helps keep muscles firm, healthy, and ready to respond 4. Helps stabilize joints and maintain posture 1. Loss of muscle tone (unconsciousness) = body collapse 5. Isotonic vs. Isometric contractions 1. Isotonic contraction (same tension) = muscle length changes and moves load 1. Types of isotonic contractions 1. Concentric contraction = muscle shortens and does work 1. Kicking ball, picking up a book 2. Eccentric contraction = muscle generates force as it lengthens 1. Hold a load while the muscle stretches 3. Eccentric and Concentric working together (Squats) 1. Flexion of knee to begin squat = eccentric contraction of quads as they lengthen but control the movement 2. Extension of knee to squat up = quads contract to force body into an upright position 3. Eccentric contractions put body in position to contract concentrically 2. Isometric contraction (same measure) = muscle does not lengthen or shorten as tension builds up 1. Muscle attempting to move a load that it cannot overcome 2. Maintaining stationary positions (balance and posture, core strength etc ) F. Muscle Metabolism the mechanism by which the body provides energy needed for contraction 1. Providing Energy for Contraction 1. Stored ATP 1. Uses of ATP 1. Energy for cross bridge movement and detachment 2. Operation of calcium pumps

11 2. Muscles only store enough ATP for 4-6 seconds of activity 3. Regeneration of the ATP must occur quickly 1. ADP/creatine phosphate interaction 2. anaerobic glycolysis 3. aerobic respiration 2. Direct Phosphorylation of ADP by Creatine Phosphate 1. Creatine Phosphate = high energy molecule stored in muscle cells 1. regenerated during periods of inactivity 2. ADP can be phosphoralized by a transfer of a phosphate group from the CP to form ATP 1. CP + ADP creatine + ATP 3. Muscles store CP to energize the muscle while the metabolic pathways are prepared 4. Stored ATP and CP provide muscle power for seconds 3. Anaerobic Mechanism = glycolysis 1. ATP production continues when glucose from blood stream or glycogen in muscle is catabolized 2. Glycolysis is the initial step in this process 1. Does not require oxygen 2. Muscles that are under vigorous activity tend to bulge (swell) constricting blood vessels, thus restricting oxygen delivery 3. Pyruvic acid is the by-product of glycolysis 1. In the absence of oxygen, pyruvic acid is converted to lactic acid 1. Lactic acid diffuses out of muscle 1. Picked up by liver, heart, kidneys (for energy supply) 2. Liver converts back to pyruvic acid or glucose 2. Accumulating lactic acid contributes to muscle fatigue and muscle soreness during intense exercise 4. Produces small amounts of ATP fast

12 1. Good source when large amounts of ATP are required for moderate periods (30-40 sec) 2. High amount of glucose used for small amounts of ATP 4. Aerobic Mechanism = respiration 1. Occurs in mitochondria 1. Requires oxygen 2. Utilizes a series of chemical reactions in which bond of fuel molecules are broken and the energy is used to make ATP 2. Fuel sources 1. Muscle glycogen 2. Glucose in blood 3. Pyruvic acid from glycolysis 4. Free fatty acids 3. Produces high amounts of ATP, but is slow because of the many chemical reactions that take place and the high amount of oxygen and nutrient fuels 1. Good for resting and light exercise 5. Pathways used during Activities 1. With oxygen, aerobic reactions produce ATP 1. Body is capable of using aerobic pathways while delivering adequate amounts of oxygen 1. Conditioning and aerobic endurance 2. When exercise demands exceed the ability to use oxygen pathways, anaerobic systems prevail 1. Anaerobic threshold = the point at which the muscle metabolism converts to anaerobic glycolysis 3. Examples 1. ATP and CP stores used for energy 1. Activities that require short bursts of energy 2. Weight lifting, diving, sprinting 2. Anaerobic glycolysis 1. Tennis, soccer, longer sprints

13 2. On and off burst-like activities 3. Aerobic respiration 1. Endurance rather than power 2. Jogging 2. Muscle Fatigue = the state of physiological inability to contract even though the muscle is receiving stimuli 3. Oxygen Debt = the extra amount of oxygen that the body must take in for a muscle to return to its resting state 1. Resting state of muscle 1. Oxygen must be replenished 2. Accumulated lactic acid must be pushed out for reconversion to pyruvic acid 3. Glycogen stores must be replaced 4. ATP and CP reserves must be re-synthesized 2. Oxygen debt represents the difference between the amount of oxygen needed for totally aerobic muscle activity and the amount actually used 4. Heat Production During Muscle Activity 1. Only 40% of the energy released during muscle contraction is converted to useful work 2. 60% given off as heat G. Force of Muscle Contraction 1. Number of Muscle Fibers Stimulated 2. Size of the Muscle Fibers Stimulated 3. Frequency of Stimulation 4. Degree of Muscle Stretch H. Influences on Velocity and Duration of Contraction 1. Muscle Fiber Type 1. Classifications 1. Speed of contraction (slow and fast fiber types) 1. Speed of contraction reflects how fast the myosin use ATP to pull on actin and the patterns of electrical activity in the motor neuron 2. Major pathways use for forming ATP 1. Muscle fibers use different pathways for ATP formation 2. Muscles contain a mixture of fiber types which gives them a range of contractile speeds and fatigue resistance

14 III. 3. Genetic differences can cause people to have relatively more of one type than another 1. Determine athletic capabilities I. Effects of Exercise on Muscles *** when used actively or strenuously, muscle may increase in size or strength, become more efficient and fatigue resistant 1. Adaptations to Exercise 1. Endurance exercise = aerobic exercises 2. Increase in the number of capillaries surrounding muscle fiber 3. Number of mitochondria increase 4. Fibers synthesize more myoglobin 5. Results in more efficient muscle metabolism 1. Greater endurance, strength,a nd resistance to fatigue 2. Does not promote skeletal muscle hypertrophy 2. Resistance exercise = weight lifting or isometric exercises (anaerobic) 1. Strength, not stamina is important 2. Muscle bulk increase 1. Size of muscle fibers increase 2. Splitting or tearing of muscle fiber which increases number of muscle fibers 3. Vigorously stressed muscle fibers contain more mitochondria, form more myofilaments and myofibrils, and store more glycogen Developmental Aspects of Muscles A. Muscular Development 1. Reflects the level of neuromuscular coordination 1. Develops head-to-toe 1. Lift head before it can walk 2. Proximal-to-distal 1. Gross movements precede fine ones 3. Reach peak of natural neural control of muscles in mid-adolescence 1. Improve level of development by training B. Differences between males and females 1. Muscle Mass 1. Women = 36% of body mass is skeletal muscle

15 2. Men = 42% 2. Hormone differences 1. Due to testosterone, men s skeletal muscles enlarge to a greater degree