Muscle Overview

1. Muscle Overview • 3 different types of muscle tissue provide movement: • Skeletal • attached to the bones of the skeleton • controlled consciously (voluntary) • Cardiac • heart • controlled unconsciously (involuntary) • Smooth • airways of the lungs, blood vessels, the digestive, urinary, and reproductive tracts • controlled unconsciously (involuntary)

2. Characteristics of Muscle Tissue • Excitability, or irritability • the ability to receive and respond to stimuli • Conductivity • the ability to create and conduct an action potential along the cell membrane • Contractility • the ability to shorten forcibly through the hydrolysis of ATP by contractile proteins • Extensibility • the ability to be stretched or extended • Elasticity • the ability to recoil after being stretched

3. Muscle Terminology • Prefixes • sarco-“flesh” • sarcolemma = muscle plasma membrane • sarcoplasm = cytoplasm of a muscle fiber (cell) • my- “muscle” • myocyte = muscle fiber • epimysium = the sheath of connective tissue that surrounds a skeletal muscle

4. Motor Unit: The Nerve-Muscle Functional Unit • A skeletal fiber will contract only after it is excited • A skeletal fiber is excited by the exocytosis of the neurotransmitter acetylcholine from a motor neuron at a synapse called the neuromuscular junction (NMJ) • generates a graded potential which can lead to an action potential in the fiber to trigger contraction • A single motor neuron is capable of stimulating multiple skeletal muscle fibers to contract simultaneously • one axon branches creating multiple axon termini • the anatomical relationship between a motor neuron and all skeletal fibers that it causes to contract is called a motor unit

5. Motor Unit: The Nerve-Muscle Functional Unit

6. Motor Unit: The Nerve-Muscle Functional Unit • The number of muscle fibers per motor unit can range: • few (small motor unit) • control fine movements (fingers, eyes) • several hundred (large motor unit) • control gross movements (arms, legs) • large weight-bearing muscles (back)

7. Muscle Twitch • The contraction followed by the relaxation of a muscle fiber to a single, brief threshold stimulus by a motor neuron is called a twitch • There are three phases of a muscle twitch • Latent (lag) period • time between the stimulation by a motor neuron and the beginning of contraction (few milliseconds) • Contractile period • contractile proteins within the fiber hydrolyze ATP causing the fiber to shorten resulting in an increase in tension (force) • Relaxation period • fiber lengthens causing tension to decrease

8. Muscle Twitch

9. Contraction of Skeletal Muscle • The two types of muscle contractions are: • Isometric contraction = “samelength” • muscle contracts and produces tension, but does not shorten • trying to lift a car • Isotonic contraction = “sametension” • muscle contracts and produces tension • shortens as it contracts • lifting a pencil

10. Isometric Contractions • Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens • Occurs if the load is greater than the tension the muscle is able to develop

11. Isotonic Contractions • In isotonic contractions, the muscle changes in length and moves the load

12. Types of Skeletal Muscle Fibers • There are 3 different types skeletal muscle fibers based on the duration of a twitch and the method of ATP production • slowoxidativefibers • fast oxidativefibers • fast glycolyticfibers • Skeletal muscles of your body contain a combination of all three fiber types, but their ratio determines the overall function of that muscle

13. Oxidative vs. Glycolytic fibers • Oxidative fibers contain greater amounts of mitochondria compared to glycolytic fibers • Oxidative fibers contain an oxygen-binding protein called myoglobin to maintain a high concentration of oxygen within the fiber for aerobic respiration • similar in structure to the blood protein hemoglobin • provides a red color to oxidative fibers • a lack of myoglobin in glycolytic fibers results in a white color

14. Characteristics of Skeletal Muscle Fiber Types • Slow oxidative fibers: • have a slow twitch (use ATP slowly) • fatigue resistant • muscle fibers used to maintain posture • Fast oxidative fibers: • have a fast twitch (use ATP quickly) • moderate resistance to fatigue • muscle fibers used for non-exertive movement (walking) • Fast glycolytic fibers: • have a fast twitch (use ATP quickly) • easily fatigued • muscle fibers used for powerful movements (jumping and sprinting)

15. Fatigue • Weakening of contracting muscle caused by: • the rate of ATP hydrolysis exceeds the rate of synthesis • lactic acid accumulation (↓ pH) inhibits muscle protein function • motor neurons run out of acetylcholine

16. Resistance to Fatigue • Fibers that use ATP slowly and have a high capacity to synthesize ATP are resistant to fatigue • Fibers that use ATP quickly and have a high capacity to synthesize ATP have moderate resistance to fatigue • Fibers that use ATP quickly and have a low capacity to synthesize ATP have no resistance to fatigue

17. Variety of Muscle Responses • Variations in the force of muscle contraction is required for proper control of skeletal movement • moving a pencil vs. a textbook with your hand uses the same muscles, but requires a different amount of force • Skeletal muscle contractions are varied by: • altering the number of muscle fibers that contract • determined by thenumber of motor units that are propagating action potentials to a muscle and which muscle fiber types are contracting to perform a particular task • altering the frequency of muscle stimulation • determined by the frequency of action potentials traveling down a motor neuron arriving at a fiber

18. Muscle Response: Motor Unit Recruitment • The first observable muscle contraction occurs following a threshold stimulus • activates one motor unit • As stimulus strength is increased more motor units are activated • recruitment • The maximum force that a muscle is capable of generating is reached when all motor units are activated • an increase in stimulus intensity results in no further increase in force generated

19. Stimulus Intensity and Muscle Tension

20. Motor Unit Recruitment • Slow oxidative fibers are first stimulated to contract • provide basal muscle tension (tone) • If additional muscle tension is required, fast oxidative fibers are stimulated to contract • Finally, the fast glycolytic fibers are stimulated to bring muscle tension to maximum

21. Muscle Response: Stimulation Frequency • Rapidly delivered stimuli result in the summation of muscle twitches creating an incomplete (unfused) tetanus (constant submaximal contractile force where each twitch is visibly distinct) • muscle tension does not return to baseline • If stimuli are given quickly enough, complete (fused) tetanus is observed where the contractile force reaches a maximum, but individual twitches blended together

22. ATP Sources During Muscle Contraction • Resting muscle fibers synthesizes and stores enough ATP (by cellular respiration) for 5 seconds of maximal sustained contraction. After that the muscle must make ATP in order to continue contraction • During resting periods, skeletal muscle uses ATP that it synthesizes to energize the amino acid derivative creatine into creatinephosphate which can be stored • during contraction creatine phosphate is converted back into creatine as ADP is converted to ATP • Glucose delivered to the muscle as well as stored glycogen (once hydrolyzed) is used by the muscle for additional ATP synthesis via glycolysis and oxidative phosphorylation

23. 3 Sources of ATP Formation in Skeletal Muscle

25. Monitoring of Muscle Length and Tension • Within skeletal muscle are 2 sensory receptors that monitor muscle length and tension • Muscle spindles are modified muscle fibers called intrafusal muscle fibers that are wrapped around by a neuron which sends information to the brain/spinal cord about the length of a muscle and the speed at which the length changes during contraction or stretching • extrafusal fibers are those that contract to produce tension and movement • Golgi tendon organs are neurons that are wrapped around the collagen fibers of a tendon near the attachment to muscle which sends information about the tension that a muscle produces during contraction

26. Muscle Spindles and Golgi Tendon Organs • Neurons associated with the spindle will generate additional or fewer APs which propagate to the brain/spinal cord when the length of the muscle (spindle) increases or decreases, respectively • Tension within a tendon (by either contraction or passive stretching) generates APs in the neuron which propagate to the brain/spinal cord

27. Muscle Spindles • A lengthened spindle generates more APs, a shortened spindle generates fewer APs • The brain/spinal cord interprets the change in the AP frequency from the spindle as a change in length

28. Myotatic Reflex • Reflex that causes the contraction of a muscle following an increase in that muscles length • APs from the lengthened spindle synapse with neurons in the spinal cord causing: • contraction of the extensors (pathway A and C) • relaxation of the opposing flexors (pathway B • sensory (pathway D) for perception by the brain

29. Golgi Tendon Organ • Tension within a tendon generates APs in the neuron which propagate to the brain/spinal • The greater the tension the higher the frequency of APs are generated so the brain/spinal cord can monitor the amount of stress in the tendon

31. Golgi Tendon Reflex • Protective reflex that prevents over contraction of a muscle resulting in damage to the muscle, tendon or bone • Contraction of the extensor muscle on the thigh stretches the Golgi tendon organ and generates APs causing: • inhibition of the motor neurons that innervate the extensor (A) • excitation in the opposing flexor’s motor neurons (B)

32. Microscopic Anatomy of a Skeletal Muscle Fiber • Each fiber is long (up to 30 cm) and cylindrical with multiple nuclei just beneath the sarcolemma • the sarcolemma contains both voltage-gated Na+ and K+ capable of generating an action potential • portions of the sarcolemma called transverse (t) -tubules fold inward toward the center of the fiber • propagate APs to the center of the muscle cell • Muscle fibers contain an elaborate, smooth sarcoplasmic (endoplasmic) reticulum(SR) • physically associated with the t-tubules • storage site of intracellular calcium (Ca+2) • An action potential in the t-tubules causes the release of from the SR into the sarcoplasm which increases the cytoplasmic level of Ca+2 • triggers the contraction of a muscle fiber

33. Microscopic Anatomy of a Skeletal Muscle Fiber

35. Contractile Proteins • Occupying most of the space within the cell, long filamentous contractile proteins are arranged in long bundles called myofibrils • composed of 2 types of contractile proteins (myofilaments) that overlap and slide past one another during contraction and relaxation • “thin” • “thick”

36. Structure of Thin Filaments • Thin filaments are composed of 3 proteins • F (fibrous) Actin is a helicalpolymer of G (globular) actin protein subunits • each subunit contains a binding site for the protein myosin of the thick filaments • Tropomyosin blocks the interaction between actin and myosin • prevents an unstimulated muscle from contracting • Troponin C is attached to tropomyosin • binds to Ca2+ in the sarcoplasm during contraction

37. Structure of Thin Filaments

38. Structure of Thick Filaments • Thick filaments are composed of many molecules of the protein myosin • Each myosin protein has a rodlike tail and two heads • Myosin heads: • hydrolyze a molecule of ATP • uses the chemical energy to contract • Temporarily bind to actin • pull on actin causing the shortening sarcomere

39. Structure of Thick Filaments

40. Arrangement of the Filaments in a Sarcomere

41. Striations of Skeletal Muscle • The overlapping arrangement of myofilaments creates a repeating pattern of striations (stripes) called sarcomeres when viewed longitudinally

42. Segments of a Sarcomere • Z disc • constitutes the end of a sarcomere • anchors the thin filaments • A band • the length of the thick filaments • I band • the length of thin filaments within a sarcomere that is not overlapping with the thick filaments • H (bare) zone • the length of thick filaments within in a sarcomere that is not overlapping with the thin filaments • During contraction, the thin and thick filaments slide past one another as the sarcomere shortens

43. Sarcomeres

44. Sliding Filament Model of Contraction • In the relaxed state, thin and thick filaments overlap only slightly • Upon stimulation, the thick filaments pull the thin filaments toward the center of the sarcomere • filaments overlap to a greater degree • shortening the sarcomere • As all of the sarcomeres in a muscle shortens, the entire muscle shortens

45. Skeletal Muscle Contraction • In order to contract, a skeletal muscle must be stimulated by a motor neuron • generates an action potential in the muscle fiber • causes an increase in the amount of cytoplasmic Ca2+ • causes the muscle fiber to contract • Linking the action potential to the contraction of a muscle fiber is called excitation-contraction coupling

46. Neuromuscular Junction • The axon termini have synaptic vesicles that contain the neurotransmitter acetylcholine(ACh) • ACh receptors (ligand-gated Na+ channels) are localized to a portion of the sarcolemma called the motor end plate

47. Neuromuscular Junction

48. Neuromuscular Junction

49. NMJ Function

50. Excitation-Contraction Coupling • Binding of ACh to its receptors opens the channel and allows both Na+ and K+ to diffuse • diffusion of more Na+ than K+ causes the membrane potential to depolarize (endplate potential)