Chapter 10: Muscles and Muscle Tissue Chapter Objectives OVERVIEW OF MUSCLE TISSUE 1. Describe the three key functions of muscle. 2. Describe the four special properties of muscle tissue. SKELETAL MUSCLE TISSUE 3. Describe the difference between a fascicle, a myofiber, a myofibril and a myofilament. 4. Explain what is meant by muscle atrophy and hypertrophy. 5. Describe and give the functions of the four kinds of proteins found in myofibrils. 6. Describe the different bands found in a sarcomere. 7. Explain how the arrangement of the thick and thin myofilaments forms the observed sarcomere structural patterns of the myofibril. 8. Discuss the connection between the sarcolemma and the Transverse tubules. 9. Describe and give the function and location of the sacroplasmic reticulum and the terminal cisternae. 10. List the three layers of connective tissue that surrounds skeletal muscle and give a function for the tissue. THE NEUROMUSCULAR JUNCTION 11. Show the general features of the neuromuscular junction that allows signals coming from the brain to be conveyed across the gap between the neuron motor cell and the sarcolemma of the muscle cell. 12. Describe the effects of certain drugs, bacterial toxins and disorders on the function of the neuromuscular junction (NMJ). CONTRACTION AND RELAXATION OF SKELETAL MUSCLE FIBERS 13. Describe the conditions inside a skeletal muscle cell at rest in regards to ATP, Calcium, and the state of the myofilaments. 14. Explain why the process of muscle contraction is called Excitation-Contraction Coupling. 15. Describe what is happening in a skeletal muscle cell during the Sliding Filament Mechanism in regards to ATP, Calcium and myofilament function. 16. Illustrate the progressive overlap of the thick and thin filaments as they pull the Z disc toward the center of the sarcomere, and the result on the length of the fibril, fiber, and muscle. 17. Define a power stroke cycle and explain the steps of the cycle. Include the factors that maintain continuous cycling of muscle contraction.
18. Describe the condition called rigor mortis and its cause. MUSCLE METABOLISM 19. Describe energy use in muscle cells and list the three sources for ATP regeneration in muscle cells. 20. Discuss the relative durations of time that each form of ATP production provides for sustained activity. 21. Describe the use of creatine phosphate in muscles to produce ATP. 22. Define oxygen debt and discuss the purpose of the elevated use of oxygen after exercise. CONTROL OF MUSCLE TENSION 23. Define the All or None Principal 24. Define a motor unit and indicate the relationship between a motor neuron, the number of muscle fibers it innervates, and the strength of contraction by the motor unit. 25. Discuss the effects of increasing the number of active motor units on contraction force. 26. Define muscle tone and note how it normally works in body posture maintenance. 27. Discuss the activities that occur in each period of a twitch contraction as shown in a myogram. 28. Define Refactory period. 29. Define twitch, treppe and tetanus and explain the conditions that cause them. 30. Compare and contrast isotonic and isometric contractions. TYPES OF SKELETAL MUSCLE FIBERS 31. Discuss the slow oxidative fibers based on their cellular components, fatigue resistance, duration of contraction and ATPase reaction rate. 32. Discuss the fast glycolytic based on their cellular components, fatigue resistance, duration of contraction and ATPase reaction rate. 33. Discuss the intermediate fatigue resistant fibers based on their cellular components, fatigue resistance, duration of contraction and ATPase reaction rate. 34. Examine how various types of exercise can induce changes in the fibers in skeletal muscle. SMOOTH MUSCLE TISSUE 35. Compare skeletal and smooth muscle cells with respect to organelles, cytoskeleton, and contractile filaments. 36. Contrast the source and nature of differences in contraction between smooth and skeletal muscles. CARDIAC MUSCLE TISSUE
37. Compare and contrast differences in cardiac and skeletal muscle cell structure and physiology. Chapter Lecture Notes Functions of Skeletal Muscle Movement of body Posture maintenance Heat production - 85% of body heat is generated by skeletal muscle 25-40 % of energy from nutrients is converted to ATP by cellular respiration 60-75 % of energy from nutrients is converted to heat Properties of Skeletal Muscle Excitability - ability to receive and respond to a stimulus stimulus = change in environment strong enough to initiate an electrical signal, action potential In skeletal muscle, stimulus is usually acetylcholine (ACh, neurotransmitter) Contractility - ability to shorten (can shorten up to 50% of resting length) Extensibility - ability to stretch (can stretch up to 20% of resting length) Elasticity - ability to return to original length after contraction Histology Common root words for muscle sarco = fleshy myo = muscle Large Small: fascicle myofiber myofibril myofilament (Table 10.3) Muscle consists of elongated cells called muscle fibers or myofibers (Fig 10.1 & 10.2) A bundle of these fibers is called a fascicle Inside of a muscle cell there are small cylinders called myofibrils which may number several 100 to several 1000/cell (Fig 10.3)
Exercise increases myofibril production (hypertrophy); lack of exercise decreases myofibrils (atrophy) Each myofibril consists of myofilaments (proteins) (Fig 10.3, 10.4 and Table 10.2 & 10.3) thick myofilaments = myosin thin myofilaments = actin, troponin, tropomyosin Sarcomere - myofilaments don't extend entire length of muscle fiber; they are stacked into compartments called sarcomeres (Fig 10.3 & Table 10.1) Sarcomeres are the functional unit of a skeletal muscle (contractile unit) Parts of a sarcomere A band - myosin + overlapping actin I band - only actin, troponin, tropomyosin Z disc through center of I band H zone - only myosin M line middle of sacromere Sarcomere extends from Z disc to Z disc - 2 I bands / sarcomere Sarcoplasm cytoplasm Sarcolemma - cell membrane (Fig 10.2) T tubule - tubular invagination of sarcolemma that surrounds each myofibril at the A band - I band junction Sarcoplasmic reticulum (SR) - smooth endoplasmic reticulum that stores Ca 2+ Each myofibril is surrounded by SR Terminal cisternae dilated end sacs of the SR Triad - 2 terminal cisternae + 1 T tubule located at the A band - I band junction Entire muscle is wrapped in fibrous connective tissue which is continuous with tendons that insert skeletal muscle into periosteum of bone
Connective tissue holds muscle together as well as serving to transmit blood vessels and nerves to inner muscle cells Connective tissue layers of skeletal muscle (Fig 10.1) Epimysium - outside of entire muscle Can be called deep fascia (sheet of fibrous connective tissue) as compared to superficial fascia also known as subcutaneous layer Perimysium - divides muscle cells into fascicles (bundles) Endomysium - covers individual muscle cells Neuromuscular Junction A synapse or site of communication between a neuron and muscle is usually named a neuromuscular junction (NMJ) (Fig 10.9) Structure Neuromuscular junction = axon terminal (synaptic end bulb) of a motor neuron + synaptic cleft + motor end plate (sarcolemma under motor neuron) One per muscle fiber and usually in middle Function Acetylcholine (Ach) is released from the axon terminal (synaptic end bulb) of the motor neuron by exocytosis Ach diffuses across the synaptic cleft to the motor end plate Ach binds to receptors on the motor end plate Ach binding initiates a change in the electrical state of the muscle cell called an action potential Problems at the NMJ Curare - binds to ACh receptors in skeletal muscle membrane Competes with ACh but does not stimulate the ACh receptor Muscle paralysis
Myasthenia gravis - antibodies destroy ACh receptors Muscle paralysis Botulism (from the bacteria Clostridium botulinum) - toxin inhibits ACh release Muscle paralysis A dilute solution of botulinum toxin (Botox) can be injected into a muscle that is in spasm to help it relax Tetanus - (from the bacteria Clostridium tetani) - this anaerobic bacteria produces a toxin that blocks an inhibitory neurotransmitter in the central nervous system Causes muscle spasms and painful convulsions Tetanus shots immunize against the toxin Organophosphates (in some pesticides) - inhibits acetylcholinesterase Acteylcholinesterase enzyme that inactivates acetylcholine Muscle spasms Sliding Filament Mechanism Sliding Filament Mechanism means: myosin (thick myofilaments) cross bridges pull actin (thin myofilaments) toward H zone during contraction (Fig 10.5) I band and H zone may disappear in a contracting muscle A band stays same length during contraction At rest Calcium in SR (terminal cisternae) Troponin-tropomyosin prevents myosin from binding to sites on actin ATP bonded to myosin cross bridges (concentration of ATP is high in relaxed muscle) Excitation-Contraction Coupling (Fig 10.10) Muscle action potential will be generated at the NMJ due to neural stimulus - Ach binds to its receptor on the motor end plate
Action potential - rapid change in electrical polarity at the plasma membrane of a muscle or neuron Will travel along the sarcolemma down into the T tubules Production of AP in muscle leads to contraction; Excitation-Contraction Coupling The action potential running through the T tubules causes the sarcoplasmic reticulum s terminal cisternae to release calcium into the sacroplasm The calcium will then bind to troponin Tropomyosin changes shape so myosin binding sites on actin are uncovered (Fig 10.7) Myosin then can bind to actin The calcium will also activate ATPase ability of myosin The ATPase activity of myosin activates its power stroke movement (Fig 10.6) Pulls actin inward (H zones and I bands narrow and may disappear; A band does not change its length) At end of power stroke, a new ATP binds to ATP binding site on the myosin cross bridge, resulting in detachment of myosin from actin Attach, pull, detach (release) = steps in contraction If lack ATP - rigor mortis To relax following contraction ACh is inactivated by acetycholinesterase (from sarcolemma surface) Calcium is actively transported back into SR ATP is required to pump calcium back into the SR ATP attaches to myosin cross bridge and it releases from actin Troponin-tropomyosin covers myosin binding sites on actin preventing binding of myosin cross bridges ATP and Muscle Function ATP required for
Power stroke - movement of myosin cross bridges pulls actin inward in a sarcomere At end of power stroke, ATP binds to ATP binding site, resulting in detachment of myosin from actin (ATP needed to release cross bridges) Pumping Ca 2+ back into terminal cisternae of the SR Sources of ATP (Fig 10.11) Stored ATP - lasts only 6 seconds during bursts of muscle contraction ATP generated from creatine phosphate (CP) (CP + ADP creatine + ATP) Together ATP that is stored and CP provide muscle power for 10-15 sec CP replenished during resting periods Even as ATP and CP are being used, ATP is generated by aerobic respiration and anaerobic respiration Resting and slowly contracting muscles obtain bulk of ATP via aerobic respiration of fatty acids (enters as acetyl-coa) Aerobic pathway: glucose + O 2 CO 2 + H 2 O + 36ATP In actively contracting muscles, glucose (from blood and breakdown of glycogen) is primary fuel supply Anaerobic pathway: glucose lactic acid + 2 ATP Aerobic pathway produces 20X more ATP than anaerobic respiration but takes 2 1/2 times longer Anaerobic respiration causes oxygen debt to occur Oxygen Debt Oxygen Debt: Amount of oxygen needed to metabolize the accumulated lactic acid and to restore ATP levels Muscle fatigue is result of ATP depletion and accumulation of lactic acid 80% of lactic acid diffuses from skeletal muscle to blood and is transported to liver where it is converted back to glucose
20% remains in muscle - processed to CO 2 and H 2 O + ATP Oxygen debt results in labored breathing in order to pay back the O 2 debt Lactic acid decreases ph of blood which stimulates increased respiration All or None Principal All or None Principle - individual muscle fibers of a motor unit will contract to its fullest extent of its immediate ability when stimulated by a nerve impulse (action potential) of threshold level The principle does not apply to the entire muscle but only to motor units Motor unit - motor neuron + all the skeletal muscle fibers it services (5 fibers to 2000 muscle fibers) (Fig 10.12) One entire muscle has many motor units Not all are stimulated at same time The smaller the number of muscle fibers/motor unit, the more precise the control of the muscle fibers So the strength of a contraction of an entire muscle depends on Size of load - heavier the load, stronger the contraction (pencil vs book) Number of motor units activated Initial length of fibers - more a muscle is stretched, to a point, the greater the contraction (Fig 10.8) Muscle tone - partial sustained contraction important in maintenance of posture Only a fraction of motor units are activated at any one time Produces tautness instead of recognized contraction Maintain tone without fatigue because there is a system of rotation; at any one moment some fibers are contracted and others are relaxed & resting, ready to take up the work next (motor units innervate scattered muscle cells and not clustered)
If muscle tone is not maintained, possibly due to nerve damage, the muscle has less than normal tone, or is flaccid If flaccidity is maintained for an extended period of time the muscle will atrophy, a decrease in muscle mass by a decrease in myofibrils Leg in cast Bedridden The opposite of atrophy is hypertrophy, an increase in diameter of muscle from forceful muscle activity or repetitive muscle activity; there is an increase in number of myofibrils, mitochondria, SR, blood vessels, connective tissue Kinds of Contractions Muscle twitch - rapid jerky response to single stimulus Can be traced on a myogram (Fig 10.13) Three phases: latent, contraction, relaxation Length of twitch varies for different muscles Short in eye muscle Long in leg muscle Refractory period - period of lost irritability (excitability) - short period of time during which there is no response to additional stimuli Short in skeletal muscle (.005 sec) Long in cardiac (.3 sec) Wave Summation (Treppe) - staircase phenomenon - reoccurring stimuli results in increasingly stronger twitch contractions (Fig 10.14) Increased heat in warmed up muscle increases speed of all reactions Increased calcium in sarcoplasm activates sliding filament mechanism faster Practical application: warming up of athletes
Tetanus - (not to be confused with bacterial disease of same name) - fusion of twitches (Fig 10.14) Relaxation is not allowed to occur Body normally delivers volleys of impulses in rapid succession, giving us smooth sustained contractions Voluntary contractions (normal movements) are tetanic contractions Isotonic contraction - muscle shortens and can move a load (Fig 10.15) Tension, force exerted by a contracting muscle on some object, has to exceed load to move it Tension remains constant as the muscle shortens Isometric contractions - develops increased tension in muscle but muscle does not shorten or there is minimal shortening (Fig 10.15) Cross bridges formed but unsuccessful in moving thin myofilaments (cross bridges spinning wheels on same cross bridge binding site) Length remains about the same Types of Skeletal Muscle Fibers Slow twitch - fatigue resistant red (SO) (Table 10.4) Contract more slowly Slower myosin ATPase Fuel source: fatty acids Aerobic respiration More mitochondria Low glycogen High myoglobin Myoglobin - similar to hemoglobin, hence red in color; binds O 2 and acts as a reservoir for O 2 until needed by mitochondria
More capillaries Small diameter ex. - large postural muscles, long distance runners Fast twitch fatigable white (FG) Contract more rapidly Faster myosin ATPase Fuel source: glucose Anaerobic respiration Fewer mitochondria High glycogen Low myoglobin Fewer capillaries Large diameter ex. muscles of arm and legs Intermediate intermediate fatigue resistant pink (FOG) Contract more rapidly Faster myosin ATPase Fuel source: glucose Anaerobic and aerobic respiration More mitochondria High glycogen High myoglobin More capillaries Intermediate diameter Most muscles have a combination of both types - all fibers of a motor unit are of same type
Endurance type exercises (running, swimming) can transform fast twitch - fatigable into intermediate by producing more mitochondria and increased myoglobin Cannot transform slow twitch to fast twitch fibers Smooth Muscle Smooth Involuntary, nonstriated, visceral (Fig 10.16) Located in walls of organs and blood vessels Has actin and myosin but not arranged orderly into sarcomeres Contract and relax more slowly than skeletal muscle due to arrangement of actin and myosin and a different calcium regulation molecule (calmodulin instead of troponintropomyosin) Contracts in waves as impulse spreads from one cell to another ex. - propulsion of food through digestive system Cardiac Muscle Cardiac Involuntary, striated (Table 10.5) Same arrangement of actin and myosin as skeletal muscle but not organized into discrete myofibrils Has own intrinsic rhythm Own electrical system Cardiac cells can contract without nerve stimulation Source of stimulation is specialized cells that initiate contraction, pacemaker Nerve stimulation merely speeds up or slows down rate of contraction