W14 - Skeletal Muscle Anatomy And Physiology

Question 1

Differentiate between axial and appendicular skeleton

Answer 1

Axial: axis of skeleton (main body)
- skull, vertebrae, ribs and sternum

Appendicular: appendages (limbs)
- bones of upper limb (including clavicle and scapula)
- bones of lower limbs (including pelvic bones)

Question 2

Describe the main features of a long bone

Answer 2

• Diaphysis (shaft)
  
  • Medullary (marrow) cavity: empty space in shaft
  • Filled with bone marrow (red: blood; yellow: fat)
• Metaphysis (area between epiphysis and diaphysis)
• Epiphysis (wider upper and lower region)
  
  • Proximal: Upper region (closer to axial region)
  • Distal: Lower region (further from axial region)
    (Check Notion image)

Question 3

Explain how compact bone is arranged

Answer 3

Compact bone: hard, outer layer of bone, surrounds marrow cavity
- Cancellous bone: loose, internal spongy (mesh work of thin, bony struts)

• Osteons & Haversian system
  
  • 1 Osteon = concentric layers of bone (lamellae)
    
    • lacunae = tiny spaces between lamellae
      
      • each lacuna contains osteocyte (living bone cells)
  • Has central Haversian canal
    
    • contains blood vessels and nerves
  • External environment: bone lined by periosteum
  • Internal environment: marrow cavity
    (Check Notion image)

Question 4

Explain the role of osteocytes in bone

Answer 4

• surrounded by bone matrix
• have cytoplasmic processes
  
  • extend from their cell bodies & are contained in tiny canals (canaliculi)
  • the processes of multiple osteocytes are connected
    
    • enables nutrients and oxygen to pass from central vessels in Haversian canal -> osteocytes in the outermost concentric lamella
      (Check Notion image)

Question 5

State the stages of fracture repair

Answer 5

Fracture -> haematoma (most pool of blood clot around fracture) -> cartilage (fibrovascular tissue) -> callus (matrix, thick and surrounds bone) -> bone remodelling

Question 6

Explain the structure of the upper limb

Answer 6

Top to bottom (check Notion image)
- Pectoral Girdle (shoulder region): scapula (shoulder blade), clavicle (collar bone)
- Humerus (upper arm)
- Radius (outer) and Ulna (inner)
- Bones of hand

Question 7

Explain the structure of the lower limb

Answer 7

Top to bottom (check Notion image)
- Pelvic girdle (hips region): pelvic bone
- Femur (thigh)
- Tibia (inner) and Fibula (outer)
- Bones of the foot

Question 8

Classify the different types of muscles

Answer 8

• Skeletal muscle - “voluntary” muscle (has the ability to move)
• Cardiac muscle - “involuntary” muscle (can not move)
  
  • makes up the muscular walls of the heart (myocardium)
• Smooth muscle - “involuntary muscle”

Question 9

Explain the structure of muscles

Answer 9

(Check Notion image)
- muscles are aligned either parallel or oblique (slanting, not parallel nor fully 90°) in line of action

• Fusiform or Flat sheet (parallel)
  
  • eg. Biceps brachii (tendon, belly, tendon)
  • contraction = greater distance being moved (shortens) -> increased speed (broad sheet
• Multipennate (oblique)
  
  • eg. Deltoid muscle (shoulder)
  • has more muscle fibres/myofibrils/sarcomeres than parallel muscles, despite being the same size
    
    • i.e. don’t shorten, produces greater force

Question 10

Compare and contrast tendon and ligament

Answer 10

• Both are composed of dense connective tissue w/ high collagen content
• Very similar structural composition, w/ minor diff.
  
  • tendons have more type 1 collagen, but lesser type 3 collagen and ground substance than ligaments
• Tendons
  
  • Found b/w bone and muscle
  • Help generate movement by transferring force between the 2 structures
• Ligaments
  
  • Found b/w bone and bone
  • Generally stiff, inelastic = stabilises joints by limiting movement in specific directions

Question 11

Explain the structure of a tendon

Answer 11

Parallel collagen fibres, arranged in a wave-like pattern, and fibroblast nuclei between the collagen fibres
(Check Notion image)

Question 12

Explain how tendons are inserted into bone (?)

Answer 12

• Enthesis
• parallel collagen fibres of the tendon
• transition from unmineralised to mineralised fibrocartilage
• cortical bone

Question 13

Explain ligaments in the elbow joint

Answer 13

• Synovial hinge joint
• Ligaments on the medial (inner elbow) and lateral side (outer elbow)
  
  • Lateral collateral ligament (connect more to radius)
  • Medial collateral ligament

Question 14

Explain ligaments in the knee joint

Answer 14

• inferior end of of femur doesn’t fit with superior end of tibia (low congruency) = not much stability
• Lateral Collateral Ligament (thin, outer of the leg)
• Medial Collateral Ligament (inner leg)
• Anterior and Posterior Cruciate ligament (from a cross between the bones, anterior goes from back to front, posterior goes from front to back)

Question 15

Explain ligaments in the intervertebral joints

Answer 15

• Ligamentum Flavum has a higher content of elastin fibres (less stiff)
  
  • connects the anterior parts of the laminae to next vertebrae
• Anterior longitudinal ligament (lines the front of vertebral bodies/column)
• Posterior longitudinal ligament (lines back of vertebral body/column, in between the gap of the laminae)

Question 16

How is muscle contraction triggered?

Answer 16

Contraction for all three types (skeletal, cardiac, smooth) is triggered by an increase in Ca2+

Question 17

How does skeletal muscle contract?

Answer 17

• in response to neuromuscular synaptic transmission
• 1 skeletal muscle cell (fibre) -> 1 NMJ w/ concentrated ACh receptors
  
  • receptors: non-selective cation channels that open in response to ACh binding -> depolarisation -> epp -> if epp > threshold -> V-gated Na+ -> action potential generated -> ACh is inactivated by ACh-esterase (quickl) -> cycle begins again

Question 18

Describe the structures of myofibrils in relation to the whole muscle

Answer 18

Muscle -> Multiple muscle fibres layered w/ connective tissue -> muscle fibre -> made of multiple myofibrils

• Myofibrils have dark A bands followed by light I bands
  
  • Sarcomere: layers of thin (actin) and thick (myosin) filament (myosin and actin = protein molecules) (filaments = cytoskeletal elements)
    
    • Cross bridges: arm like attachments that extend from the thick to thin filaments
    • Z-line: separates sarcomeres
    • M-line: mid-line of sarcomeres
    • A band: main expanse of sarcomere body
    • I band: ends of sarcomere (includes Z-line)

Question 19

Explain how skeletal muscle fibres are striated by a highly organised internal arrangement

Answer 19

• Single skeletal muscle cell (muscle fibres are striated by highly organised internal arrangement)
  
  • 10-100 μm in diameter and up to ~75 cm in length
  • multiple nuclei and lots of mitochondria
  • have 100s-1000s of myofibrils
    
    • myofibrils: specialised contractile elements that extend throughout muscle fibre

Question 20

Explain the arrangement of cytoskeletal element actin in thin filaments

Answer 20

• Actin molecules come together to form a helix
  
  • they contain binding sites for attached with myosin cross bridge
  • (tropo)myosin with the help of binding molecules troponin in wraps around the helical structure
• Tropomyosin blocks binding site on actin
• This is the structure of thin filaments

Question 21

Explain the function of troponin

Answer 21

• Heterotrimer
  
  • 1x troponin interactions with 1x tropomyosin -> interacts directly with 7x actin monomers
  • Three types:
    
    • Troponin T: binds to 1x tropomyosin
    • Troponin C: binds Ca2+
    • Troponin I: binds to actin and inhibits contraction

Question 22

Explain the structure of myosin in thick filaments

Answer 22

-100 nm helical tail made of titin, with heads
- head contain Actin binding sites (binds to actin molecules) and myosin ATPase sites (hydrolyses ATP)

• made of multiple myosin-II molecules
  
  • double trimmer, containing 2x of each of the following:
    
    • intertwined heavy chains, intertwined light chains, alkali/essential light chains
      
      • the two heavy chains are made up of a tail, hinge and head region
        
        • tails: α-helices that intertwine
        • hinge joint opens up -> 2x globular heads
        • head regions: S1 fragments (cross bridges) b/w thick and thin filaments
          
          • made of 2 light chains, 1 alkali/essential and 1 regulatory (attached on hinge region)
            
            • alkali light chain = stabilises myosin head
            • reg. light chain = regulates ATPase activity
              
              • via phosphorylation by kinases

Question 23

Explain the process of muscle contraction

Answer 23

• myosin-II heads bind to action -> cross-bridges become distorted -> detach
  
  • energy for this comes from hydrolysis of ATP
  • increase in [Ca2+]i triggers contraction by removing the inhibition of cross-bridge cycling
    
    1. Upon stimulation, [Ca2+]i may rise from resting level of < 10^-7 to > 10^-5 M
    2. Subsequent decrease in [Ca2+] -> relaxation, cross-bridge cycling relaxes

Question 24

Explain the role of Ca2+ in cross-bridge cycling

Answer 24

• modulates contraction via regulatory proteins rather than interacting directly with the contractile proteins
• In absence of Ca2+: regulatory proteins inhibit actin-myosin interactions -> contractile process inhibited
• Ca2+ binding to one of more proteins: conformation change in regulatory complex -> inhibition stopped
  (Check Notion image)

Question 25

Explain what each Troponin C molecule is composed of + its role

Answer 25

• 2x high affinity Ca2+ binding sites
  
  • help bind Troponin C to thin filaments
  • Ca2+ doesn’t affect muscle activation
• 2x low affinity Ca2+ binding sites
  
  • binding of Ca2+ -> conformational change
    
    • Troponin I shifts -> tropomyosin molecule can now move, and with the help of Troponin T, tropomyosin is moved away from the myosin-binding site on the actin and into the actin groove
• myosin is able to interact with actin and engage in cross bridge cycling

Question 26

Explain the cross-bridge cycle ???? (SLIDE 23)

Answer 26

Contractile proteins convert the ATP hydrolysis energy -> mechanical energy
1. ATP binding
2. ATP hydrolysis
3. Cross-bridge formation
4. Release of Pi from myosin
5. ADP release

Question 27

Explain what is meant by the sliding filament hypothesis of muscle contraction

Answer 27

• cross-bridge interaction b/w in actin and myosin -> muscle contraction with the sliding filament mechanism
  
  • thin filaments on each side of a sarcomere slide inwards over stationary thick filaments
    
    • as they slide inwards, the thin filaments, which are attached to Z lines, pull them closer together (power stroke) -> sarcomere shorten -> detaches -> cycle repeats
  • all sarcomeres shorten simultaneously -> entire fibre shortens (not the actual filaments)
    
    • A band: determine by thick filaments: stays same
    • I band: thin filaments doesn’t overlap thick: width decreases
  • H zone : within A band, thick filaments don’t overlap thin, width decreases
  • Distance between Z lines: decreases

Question 28

Understand how and when rigor mortis occurs

Answer 28

• begins 3-4 hours after death and finished in 12 hours