L31, 36: Movement Generating Tissue / Excitation & Contraction of muscles

Question 1

Components of muscle cell

Answer 1

• Sarcoplasm (cytoplasm)
• Sarcolemma (cell membrane)
• Myofibrils: Thin filament (G-actin subunits —> F-actin strand + troponin / tropomyosin: backbone) + Thick filament (Myosin)

Question 2

Types of muscle

Answer 2

1. Striated (cross striation at light microscopic level)

2. Smooth

Question 3

Organisation of muscle cells / myocyte / muscle fibre

Answer 3

Myofilament (Actin + Myosin)
—> Myofibril
—> Myocyte / muscle fibre (separated by Endomycium: CT)
—> Muscle fascicle (functional unit, fusion of myoblast, separated by Perimycium)
—> surrounded by Epimysium

Muscle satellite cell interposed between muscle fibre and external lamina

• multipotent
• myogenic precursor

Mitochondria + glycogen, Sarcoplasmic reticulum surrounding myofilament

Question 4

Classification of skeletal muscle cells

Answer 4

1. Type I (slow oxidative, small)
   - many mitochondria, oxidative enzyme (TCA cycle + oxidative phosphorylation)
   - many cytochrome complex
   - many myoglobin
   - fatigue resistant
   - less tension
   - slow, long contraction
2. Type IIa (fast oxidative, intermediate)
   - many mitochondria
   - many myoglobin
   - fatigue resistant
   - high peak tension
3. Type IIb (fast glycolytic, large)
   - less mitochondria, oxidative enzyme
   - less myoglobin
   - many glycogen
   - fatigue prone
   - rapid contraction
   - high peak tension
   - fine movement
   - great number of neuromuscular junctions

Question 5

What is sacromere

Answer 5

Functional unit of myofibril

Between adjacent Z-lines

Question 6

Cross-striation in myofibril

Answer 6

A band: myosin length
H band: only myosin
I band: only actin

A + I band = whole length

M line: middle of myosin
Z line: middle of actin

Question 7

Describe ultrastructure of skeletal muscle fibre

Answer 7

Sarcoplasmic reticulum:

• One network surrounding A band
• One network surrounding I band
• Terminal cisterna (SR: storage of Ca): at AI junction

Mitochondria + glycogen:
- Surrounding myofibril

T tubule:

• Invagination of sarcolemma in AI junction
• 2 tubules per sarcomere
• Transmission of action potentials

2 terminal cisterna + T tubule:
- Triad transverse tubular system: contain voltage sensor protein
—> T tubule transmit depolarisation to terminal cisterna to release Ca
—> muscle contraction

Question 8

Sarcomere at different functional stages

Answer 8

Resting: H-band, I-band wide
Contraction: Z line move closer, H-band, I-band decrease in width
Stretched: Thin and thick filament do not interact

A-band always remain same length (because its the length of myosin only)

Question 9

Release, synthesis and degradation of ACh

Answer 9

1. Action potential arrives —> Depolarisation
2. Ca enter pre-synaptic terminal
3. ACh released into synaptic cleft (Ca removal —> ACh release stop)
4. ACh diffuse across synaptic cleft and bind to ACh receptors on post-synaptic membrane
5. Post-synaptic ligand-gated sodium channel open —> graded depolarisation
6. ACh broken down into Acetate + Choline by AChE
7. Pre-synaptic terminal reabsorb choline to make ACh

Question 10

Contractile cycle

Answer 10

*Excitation-contraction coupling:
—> Action potential arrival
—> Na influx, depolarisation
—> voltage-sensitive protein at T-tubule changes shape
—> trigger Ca release from SR
(Cardiac muscle: —> Ca-induced Ca release: Depolarisation by Na causes Ca go into sarcoplasm (Dihydropyridine receptor) and trigger Ca release from SR (via Ryanodine receptor) —> Contraction of cardiac muscle)
—> Actin-myosin interaction
—> Muscle fibre contraction
—> Ca reabsorption into SR via Calsequestrin / pumped to extracellular space

1. Binding of Ca to Troponin C
2. Rotate and swings Tropomyosin away
3. Expose myosin binding site to actin —> begin contractile cycle
4. Myosin head uncoupled from actin after binding to ATP
5. Hydrolysis of ATP advances myosin head by short distances (recock head)
6. Cross-bridge formation: myosin head tightly bound to actin by releasing Pi
7. Force generation: power stroke after releasing ADP, Myosin head move towards M-line
8. Reattachment: Myosin bind tightly to new actin

Question 11

Nerve innervation of muscle fibre

Answer 11

1. Motor: innervate muscle cell, motor neurones via motor unit, neuromuscular junction
2. Sensory: innervate sensory muscle spindles, Dorsal root ganglion

Question 12

Neuromuscular junction

Answer 12

• At motor end plate
• End of myelin sheath of axon
• Axon branches in to small branches
• innervate many muscle fibres: more delicate —> fewer muscle fibre per neurone
• Pre-synaptic terminals release synaptic vesicles containing ACh into synaptic cleft
• ACh receptors at motor end plate / post-synaptic terminal

Question 13

Regeneration and development of skeletal muscle, cardiac muscle, smooth muscle

Answer 13

Skeletal muscle

• Stop proliferating after 24th embryonic week
• Response to hypertrophy rather than hyperplasia
• Satellite cells have limited regenerative capacity

Cardiac muscle

• do not proliferate nor regenerate
• replaced by fibrous tissue

Smooth muscle

• Capable of dividing, mitosis
• Regularly replicating
• from undifferentiated mesenchymal stem cells

Question 14

Muscular hypertrophy, atrophy, dystrophy

Answer 14

Hypertrophy: small tear in muscle fibre
Atrophy: decrease in mass of muscle
Dystrophy: exhausted satellite pool, myogenic cells recruited from bone marrow

Question 15

Describe Cardiac muscle

Answer 15

• Striated muscle
• Nucleus located at the centre
• Cell-cell attachment via intercalated disc
• Branched fibre
• Involuntary spontaneous contraction

Question 16

Describe ultrastructure of cardiac muscle

Answer 16

1. Intercalated disc (transverse + lateral components)
   —> Fascia adherens (transverse)
   —> Macula adherens (transverse + lateral)
   —> Gap junctions (lateral)
2. T-tubule:
   - at Z line (instead of AI junction)
   - only 1 tubule per sarcomere
   - Diad instead of triad
   - Sarcoplasmic reticulum network extend from Z-line to Z-line

Question 17

Innervation of cardiac muscle

Answer 17

Sympathetic + Parasympathetic
Innervate Cardiac conducting cells
- Nodal cells
- Conducting fibres (Purkinje fibre)

Question 18

Describe smooth muscle

Answer 18

• No striation
• No myofibrils
• Bundles of elongated tapering cells
• Nucleus located at the centre
• Interconnected by gap junction: also regulate contraction
• Slow long contraction
• Innervated by sympathetic + parasympathetic nervous system
• Stimulated by mechanical, chemical and electrical stimulus

Question 19

Describe ultrastructure of smooth muscle

Answer 19

• No T tubule system —> replaced by Invaginations resembling caveolae
• Gap junction: regulate contraction and connect cells
• Actin attached dense bodies (Desmin + Vimentin)
• Myosin
• Sparse sarcoplasmic reticulum

Question 20

Comparison between skeletal, cardiac, smooth muscle

Answer 20

Fibre:
Single skeletal muscle cell
Linear branched arrangement
Single smooth muscle cell

Striation:
Present
Present
Absent

Nucleus:
Peripheral
Centre, surrounded by juxtanuclear region
Centre

T tubule:
AI junction, 2 tubule per sarcomere, triad
Z line, 1 tubule per sarcomere, diad
No T system, only SR

Cell-cell junction:
No
Intercalated disc (transverse + lateral)
Gap junction

Innervation:
Motor plate, Motor unit, dorsal root ganglion, NMJ
ANS
ANS

Response to demand:
Hypertrophy
Hypertrophy
Hypertrophy + Hyperplasia

Regeneration:
Satellite cell
No, fibrous tissue replacement
Undifferentiated mesenchymal stem cells

Question 21

ATP generation in muscle

Answer 21

1. Aerobic: within mitochondria
   - produce large amount of ATP
   - break down fatty acid
   - main energy source of resting muscle
   - ATP is used to produce creatine phosphate and glycogen
2. Anaerobic: within cytoplasm
   - glycolysis
   - break down glucose from glycogen
   - break down creatine phosphate
   - 2 net ATP
   - lactate as by-product —> acidic
   - inefficient energy production
   - main energy source of peak muscular activity