Skeletal Muscles

1. Skeletal Muscles • Attach to bones • Produce skeletal movement (voluntary) • Maintain posture • Support soft tissues • Regulate entrances to the body • Maintain body temperature

2. Properties of Skeletal Muscles Electrical excitability -ability to respond to stimuli by producing action potentials -two types of stimuli: 1. autorhythmic electrical signals 2. chemical stimuli – e.g. neurotransmitters Contractility -ability to contract when stimulated by an action potential -isotonic contraction: tension develops, muscle shortens -isometric contraction: tension develops, length doesn’t change Extensibility -ability to stretch without being damaged -allows contraction even when stretched Elasticity -ability to return to its original length and shape

3. Muscles: Gross Anatomy • muscles with similar functions are grouped and held together by layers of deep fascia e.g. biceps femoris and brachialis – forearm flexion • three layers of connective tissue surround a muscle • Epimysium – divides a muscle into fasicles • Perimysium – divides a fasicle into muscle fibers • Endomysium – divides a muscle fiber into myofibrils

4. Gross Anatomy • muscles are surrounded by a fascia (areolar tissue) • remove the fascia – epimysium that surrounds the muscle and divides it into groups called fascicles • each individual fascicle is surrounded by a perimysium • perimysium divides the fascicle into muscle fibers • muscle fiber = individual muscle cell • each muscle fiber/muscle cell is surrounded by an endomysium • endomysium divides the muscle fiber into protein filaments = myofibrils • myofibrils contain a “skeleton” of protein filaments (myofilaments) organized as sarcomeres

5. Muscles: Gross Anatomy • epimysium and perimysium extend off the muscle to become organized as a tendon – attaches to the periosteum of the bone

7. Microanatomy of Skeletal Muscle Fibers • large, multinucleated cells called fibers • mature muscle fibers range from 10 to 100 microns in diameter • typical length is 4 inches - some are 12 inches long • muscle fibers increase in size during childhood = human GH & testosterone • testosterone also increases muscle size • embryonic development – stem cells (satellite cells) differentiate into immature myoblasts which begin to make the proteins of the myofibrils (i.e. the myofilaments) • myoblasts mature into myocytes • multiple myocytes fuse to form the muscle cell (muscle fiber) • muscle fibers cannot divide through mitosis • number of muscle cells predetermined before birth – they get larger as we grow • satellite cells can repair damaged/dying skeletal muscle cells throughout adulthood

8. Muscle Cell Anatomy • new terminology • cell membrane = sarcolemma • cytoplasm = sarcoplasm • large amounts of glycogen and myoglobin • surrounds the myofibrils • myofibrilsmade up of myofilaments • actin & myosin • transverse tubules • ingrowths of the sarcolemma • carry the action potential deep into the fiber • flanks the sarcoplasmic reticulum • novel internal membrane system = sarcoplasmic reticulum • for calcium storage

9. The Proteins of Muscle • contractile elements of the myofibrils = myofilaments • -2 microns in diameter • -give the muscle its striated appearance • myofilaments are built of 3 kinds of protein • contractile proteins • myosin and actin • regulatory proteins which turn contraction on & off • troponin and tropomyosin • structural proteins which provide proper alignment, elasticity and extensibility • titin, myomesin, nebulin, actinin and dystrophin • two kinds of myofilaments • Thin - actin, troponin and tropomyosin • Thick – myosin only

10. M line Sarcomere Structure • myosin/thick filament only region = H zone • thin filament only region = I band • length of myosin/thick filaments = A band • contraction = “sliding filament theory” • thick and thin myofilaments slide over each other and sarcomere shortens

11. Sarcomere Structure • sarcomere = regions of myosin (thick myofilament) and actin (part of thin myofilament) • bounded by two Z lines • thin filaments project out from Z line – actin attached via actinin (structural protein) • myosin/thick filaments lie in center of sarcomere - overlap with thin filaments and connect to them via cross-bridges • myosin/thick filaments are held in place at the M line at the center and by titin at the Z-line

12. Contraction: The Sliding Filament Theory • Actin proteins in the thin filament have myosin binding sites • these sites are “covered up” by troponin and tropomyosin in relaxed muscle • removal of troponin/tropomyosin from these sites is required for contraction • removal of troponin/tropomyosin is done through binding of calcium to troponin • calcium is released from the sarcoplasmic reticulum upon the action potential

13. Contraction: The Sliding Filament Theory • myosin/thick myofilament is a bundle of myosin molecules • myosin looks like a “golf club” with a head, a hinge region and a shaft • each myosin protein has a globular “head” with a site to bind and breakdown ATP (ATPase site) and to bind actin (actin binding site) • binding of actin and myosin binding sites = cross-bridging

14. Increase in Cai Removal of troponin-tropomyosin RESETTING of system -for cross bridging- you will need two things: 1. calcium – uncovers the myosin binding sites on actin – “pushes aside” the troponin- tropomyosin complex 2. myosin head bound to ADP -for contraction – i.e. pivoting of the myosin head into the M line – the myosin head must be empty -to “reset” for a new cycle of cross-bridging – the myosin head must detach and pivot back -the myosin head must bind ATP -once the myosin head pivots back – the ATP is broken down to ADP – head is ready to cross-bridge again – if actin is “ready” CONTRACTION Sliding of actin along myosin

15. Contracted Sarcomere • http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__myofilament_contraction.html • http://www.youtube.com/watch?v=EdHzKYDxrKc • https://www.youtube.com/watch?v=4201SrN0WlY • https://www.youtube.com/watch?v=Ct8AbZn_A8A

16. The Neuromuscular Junction • the motor neuron’s synaptic terminal is in very close association with the muscle fiber • distance between the bulb and the folded sarcolemma = neuromuscular junction • neurotransmitter released = acetylcholine https://www.youtube.com/watch?v=7wM5_aUn2qs

17. The “Big Picture” • AP generated at trigger zone in pre-synaptic motor neuron 2. AP arrives in synaptic end bulb – causes entry of calcium into end-bulb release of of acetylcholine/Ach • Binding of Ach to ligand-gated Na channels on muscle sarcolemma (i.e. Ach receptors) • Na+ enters muscle cell = depolarization • Muscle membrane potential reaches threshold Action Potential 6. AP travels along sarcolemma into T-tubules 7. AP “passes by” sarcoplasmic reticulum  release of calcium into sarcoplasm 8. Ca binds troponin-tropomyosin complex & “shifts” it off myosin binding site 9. Cross-bridging between actin and myosin, pivoting of myosin head = Contraction (ATP dependent)

18. Contraction vs. Relaxation • Acetylcholinesterase (AchE) breaks down ACh • calcium pumped back into sarcoplasmic reticulum • Limits duration of contraction

19. Action Potentials in Nerve and Muscle • Entire muscle cell membrane versus only the axon of the neuron is involved • Resting membrane potential • nerve is -70mV • skeletal & cardiac muscle is closer to -90mV • Duration • nerve impulse is 1/2 to 2 msec • muscle action potential lasts 1-5 msec for skeletal & 10-300msec for cardiac & smooth • Fastest nerve conduction velocity is 18 times faster than velocity over skeletal muscle fiber

20. Motor Units • Each skeletal fiber has only ONE NMJ • motor unit = somatic motor neuron + all the skeletal muscle fibers it innervates • number and size of motor units indicate precision of muscle control • Motor units fire asynchronously • some fibers are active others are relaxed • delays muscle fatigue so contraction can be sustained • motor units are also intermingled • the net distribution of force applied to the tendon remains constant even when a fraction of muscle fibers are relaxed Muscle twitch Single momentary contraction of a motor unit Response to a single stimulus

21. Muscle Metabolism • Production of ATP: -contraction requires huge amounts of ATP -muscle fibers produce ATP three ways: 1. Creatine phosphate 2. Anaerobic metabolism 3. Aerobic metabolism

22. Creatine Phosphate • Muscle fibers at rest produce more ATP then they need for resting metabolism • Excess ATP within resting muscle used to form creatine phosphate • creatine – arginine, glycine and methionine • made by kidneys and liver • phosphorylated by the enzymecreatine kinase • takes a phosphate off of ATP and transfers it creatine • takes the phosphate off of creatine phosphate and transfers it back to ADP – to make ATP • Creatine phosphate: 3-6 times more plentiful than ATP within muscle • Sustains maximal contraction for 15 sec (used for 100 meter dash). • Athletes often use creatine supplementation • gain muscle mass but shut down bodies own synthesis (safety?)

23. Anaerobic Cellular Respiration • Muscles deplete creatine – can make ATP in anaerobically or aerobically • Glycogen converted into glucose first • ATP produced from the breakdown of glucose into pyruvic acid (sugar) during glycolysis • if no O2 present (anaerobic) - pyruvic converted to lactic acid which diffuses into the blood • Glycolysis can continue anaerobically to provide ATP for 30 to 40 seconds of maximal activity (200 meter race) • If O2 is present the pyruvic acid is converted into acetyl coA (aerobic)

24. Aerobic Cellular Respiration • ATP for any activity lasting over 30 seconds • if sufficient oxygen is available, pyruvic acid is converted into acteyl coA • Acetyl coA enters the mitochondria to enter the Kreb’s cycle • Kreb’s cycle generates NADH and FADH2 (electron carriers) • The electrons are carried to an enzymes located on the inner mitochondrial membrane • The electrons are then transported along three complexes of enzymes – ultimately transported to oxygen • This results in the synthesis of ATP, water and heat • Provides 90% of ATP energy if activity lasts more than 10 minutes • Each glucose = 36 ATP • fatty acids and amino acids can also be used by the mitochondria • Fatty acid = ~100 ATP • Sources of oxygen – diffusion from blood, released by myoglobin

25. Types of Muscle Fibers • classified on how they make their ATP • Fast fibers = fast glycolytic • Slow fibers = slow oxidative • Intermediate fibers = fast glycolytic-oxidative • Percentage of fast versus slow fibers is genetically determined • Proportions vary with the muscle • neck, back and leg muscles have a higher proportion of postural, slow oxidative fibers • shoulder and arm muscles have a higher proportion of fast glycolytic fibers

26. Fast Fibers: Large in diameter • Contain densely packed myofibrils • Large glycogen reserves • powerful, explosive contractions • fatigue quickly • Fast oxidative-glycolytic (fast-twitch A) • red in color (lots of mitochondria, myoglobin & blood vessels) • split ATP at very fast rate; used for walking and sprinting • Fast glycolytic (fast-twitch B) • white in color (few mitochondria & BV, low myoglobin) • anaerobic movements for short duration; used for weight-lifting • Slow fibers: Half the diameter of fast fibers • Three times longer to contract • Continue to contract for long periods of time • longest to fatigue • e.g. marathon runners

27. Classification • According to arrangement of fibers and fascicles • Parallel muscles • Parallel to long axis of muscle • Convergent muscles • Fibers converge on common attachment site • Pennate muscles • One or more tendons run through body of muscle • Unipennate, bipennate, multipennate • Circular muscles • Fibers concentrically arranged

28. Bones & Muscles: Origins and Insertions • Origin – portion of the skeletal muscle that attaches to the more stationary structure • Insertion - portion of the skeletal muscle that attaches to the more moveable structure • majority of muscles originate or insert from bony markings on the skeleton • markings: specific elevations, depressions, and openings of bones • bony markings provide distinct and characteristic landmarks for orientation and identification of bones and associated structures. • many markings come together to form a joint • many markings are often the sites for the origins or insertions of muscles, or a channel for the passage of nerves and vessels • 3 kinds of bony markings: • depressions – fossa, fissure • openings – foramen, canal, meatus • processes – condyles, spines, crests, heads, lines, ridges, trochanters, tubercles

29. Muscle Names • Yield clues to muscle orientation, location or function • Biceps brachii (two heads, arm) • Vastus femoris (large, femur) • Orbicularis oculi (circular, eye) • Rectus abdominus (erect, abdomen)

30. Musculoskeleton: Axial Division • Axial skeleton: 80 bones • main axis of the body • forms a framework for the protection of delicate organs • including the sense organs • Axial musculature • axial musculature arises from and inserts on the axial skeleton • responsible for positioning the head and spinal column – postural muscles • also moves the rib cage, assisting in breathing • grouped into 4 groups: • 1. muscles of the head and neck • 2. muscles of the vertebral column • 3. the abdominals - oblique and rectus muscles • 4. muscles of the pelvic floor

33. Muscles of the Head and Neck • Muscles of facial expression – know for practical • Extrinsic eye muscles – know for practical • Muscles of mastication – check your list for practical • Muscles of the tongue – check your list for practical • Muscles of the pharynx – don’t worry about these • Muscles of the anterior neck – know for practical

34. Muscles of Facial Expression • Originate on surface of skull • Largest group associated with mouth • Orbicularis oris • Buccinator • Occipitofrontalis muscle • Movement of eyebrows, forehead, scalp • Platysma • Skin of neck, depresses mandible

35. Muscles of Facial Expression • Orbicularis oris - puckering • Buccinator - whistling • Occipitofrontalis muscle (Epicranius) • movement of eyebrows, forehead, scalp • Front belly = Frontalis (moves eyebrows and forehead) • Back belly = Occipitalis (moves back of scalp) • Zygomaticus Major and Minor - smiling • Risorius - grinning • Orbicularis oculi – closes eye • Depressor anguli oris - – lowers angle of mouth • Depressor labii inferioris –depresses lower lip • Levator labii superioris – raises upper lip • Mentalis - pouting • Platysma – pouting & depresses mandible originate on surface of skull – insert on the skin of the face

36. zygomaticus minor zygomaticus major

38. Muscles of Mastication • Act on the mandible • Temporalis – elevates mandible • Masseter – elevates mandible • Medial pterygoid & Lateral pterygoid • protract the mandible and move it side to side • Don’t need to know for practical

39. Six Extra-Ocular (Oculomotor) Muscles • Inferior rectus • Superior rectus • Medial rectus • Lateral rectus • Superior oblique • Inferior oblique

40. Muscles of the Tongue • Necessary for speech and swallowing • Genioglossus • Hyoglossus • Palatoglossus • Styloglossus palatoglossus temporalis styloglossus hyoglossus mylohyoid

41. Anterior Muscles of the Neck • foundation for the muscles of the tongue and pharynx, move the hyoid up or down for swallowing & breathing, depress the mandible • Digastric – two bellies • Mylohyoid • Stylohyoid • Sternohyoid • Sternothyroid • Thyrohyoid • Omohyoid

42. Anterior Muscles of the Neck • Foundation for the muscles of the tongue and pharynx • Digastric • Mylohyoid • Stylohyoid • Sternocleidomastoid omohyoid thyrohyoid sternohyoid thyroid gland

43. Anterior Muscles of the Neck • Sternocleidomastoid • two sites of origin – clavicle and sternum • inserts onto mastoid process • both together – flexes head forward • individually – laterally flexes & rotates head to the opposite side

44. Anterior Muscles of the Neck

45. Muscles of the Vertebral Column • covered by a superficial layer of back muscles • Trapezius • Latissimus dorsi • superficial and deep layers • quite complex – multiple origins and insertions + overlapping • 5 Groups: • Splenius group • Erector spinae group • Scalenes • Transversospinalis group • Segmental group

46. Muscles of the Vertebral Column • Superficial muscles: • Splenius group – together they extend the head and neck, individually they laterally flex and rotate to the same side • Splenius capitis • Splenius cervicis

47. Muscles of the Vertebral Column • Superficial muscles: • Erector Spinae - spinal extensors • comprised of capitis, cervicis, thoracic and lumborum portions • Spinalis – medial group of muscles • capitis, cervicis and thoracis groups • Iliocostalis – lateral group of muscles • cervicis, thoracis and lumborum • Longissimus – in between these two (intermediate) • capitis, cervicis and thoracis • lumborum portion fuses with iliocostalis

48. Deep muscles: • Interconnect and stabilize the vertebrae • Scalenes – flexes and rotates neck, used in deep inspiration • Anterior • Medial • Posterior • Transversospinal group – extends the vertebral column and rotates it to the opposite side • Semispinalis – capitis, cervicis and thoracis portions • runs from transverse to spinous processes • Multifidus – below the ribcage semispinalis is called mutifidis • Rotatores Muscles of the Vertebral Column

49. Deep muscles: • Quadratus lumborum - flexes the lumbar spine • Segmental group – extends and laterally flexes vertebral column • Interspinales – run between spinous processes • Intertransversarii – run between transverse processes Muscles of the Vertebral Column

50. The Oblique and Rectus Muscles • Abdominal Oblique and Rectus Muscles • Rectus abdominus – partitioned into 4 sections by fibrous “tendinous intersections” • 3 Abdominal oblique muscles • External oblique (down and in) • Internal oblique (up and in) • Transversus abdominis (side to side) • Compress underlying organs • together – flex the vertebral column • singly - rotate the vertebral column to the opposite side -the oblique muscles are connected to their opposite partners via an aponeurosis (broad, flat tendon) -the 3 aponeuroses form an “envelope” that encloses the rectus abdominus = rectus sheath