Skeletal, smooth, (cardiac) muscle

Question 1

What is the ultrastructure of skeletal muscle

Answer 1

Multi nucleated
10-100mm diameter, up to 20cm long
Bundles of fibres encased in connective tissue sheaths
Attached to bones by tendons
Striated

Question 2

Describe the growth/repair of muscles

Answer 2

Fibre size increase during growth
Myoblasts not replaced if damaged
Satellite Cells replaced cells after injury
Satellite cells differentiate to form new muscle fibres
Other fibres have hypertrophy to compensate
Muscle will never completely recover

Question 3

Describe striations

Answer 3

Question 4

What is the structure of sarcomeres

Answer 4

Question 5

Describe sliding filaments

Answer 5

Question 6

Describe the cross bridge cycle

Answer 6

1. As Ca+ rises energized cross bridges bind to actin
2. ADP + Pi are released and cross bridge moves
3. ATP binds to myosin, causing cross bridge to detach
4. ATP hydrolysis to ADP + Pi energizeses cross bridge

Question 7

What is the purpose of Troponin, tropomyosin, Ca2+

Answer 7

Tropomyosin partially covers myosin binding site
Held in blocking position by troponin
Co-operative block
Calcium binds to troponin
Troponin alters shape – pulls tropomyosin away
Remove calcium – blocks sites again

Question 8

What is Excitation-contractile coupling

Answer 8

Question 9

What are the different muscle mechanics

Answer 9

Force exerted by muscle = TENSION
Force exerted on muscle = LOAD
Contraction with constant length = ISOMETRIC
Contraction with shortening length = ISOTONIC
Contraction with increasing length = LENGTHENING

Question 10

What are twitch contractions

Answer 10

Single AP -> Muscle fibre -> TWITCH
Latent period - time before excitation contraction starts
Contraction time - occurs between start of tension and peak tension
Muscle fibres have different contraction times (10-100ms)
Contraction time depends on [Ca2+]
Isometric has shorter latent period, longer contraction event
As load increases, contraction velocity and distance shortened decreases

Question 11

Describe Tetanic muscle contraction

Answer 11

Summation of contractile events by increase in frequency of APs
Greater than twitch tension since [Ca2+] never gets low enough to allow troponin/tropomyosin to re-block myosin binding sites

Question 12

Describe the length-tension relationship

Answer 12

Less overlap of filaments = less tension
Too much overlap = filaments interfere with each other
Optimal length = muscle length for greatest isometric tension

Question 13

What are motor units

Answer 13

Motor neurons + muscle fibres = MOTOR UNIT
Muscle fibres within a unit may be scattered throughout muscle

Question 14

Explain fatigue

Answer 14

Repeated muscle stim. = muscle fatigue
Depends on fibre type, length of contraction and fitness of individual
Muscle rest, can contract again

Fatigue prevents muscles using too much ATP, would cause rigor (muscles would not be able to activate new X-bridge cycles and become stiff)

Question 15

What are the different types of skeletal muscle fibres

Answer 15

Slow oxidative (I) - resist fatigue

Fast oxidative (IIa) - intermediate resistance to fatigue

Fast glycolytic (IIb) - fatigue quickly

Question 16

What are the aerobic and anaerobic properties of muscle

Answer 16

Aerobic exercise increase mitochondria, vascularisation, fibre diameter
Anaerobic (strength) exercise increases diameter, glycolysis

Question 17

Describe oxidative muscle fibres

Answer 17

Increased mitochondria, oxid. phosphorylation, vascularisation to deliver more O2 and nutrients
Contain myoglobin increases O2 delivery
Fibres are red and have low diameters

Question 18

Describe glycolytic fibres

Answer 18

Few mitochondria
Increase glycolytic enzymes and glycogen
Lower blood supply
White fibres with larger diameters

Question 19

Describe the structure of smooth muscle

Answer 19

No striations
Innervated by ANS
X-bridge cycle and uses Ca2+
Exists in hollow organs (e.g. GI tract, uterus, airways, ducts)
Spindle-shaped (2-10mm diameter)
Mononucleate
Thick myosin and thin actin filaments

Question 20

Describe smooth muscle filaments

Answer 20

Arranged diagonally across cells and anchored to membranes and cell structures by dense bodies (like Z-lines)
Filaments slide together to contract cell

Question 21

Describe Smooth Muscle X-bridge cycle activation

Answer 21

1. Increase Ca2+ binds to calmodulin
2. Ca2+ - Calmodulin binds to mysoin light chain kinases
3. Kinase phosphorylates myosin cross bridges with ATP
4. Binds to actin and causes contraction
5. Dephosphorylation of cross bridges relaxes smooth muscle

Question 22

What are factors affecting contractile activity

Answer 22

Spontaneous electrical activity in muscle membranes
Pacemaker activity
Autonomic neurotransmitters from varicosities
Hormones
Local factors (pH, O2, osmolarity, ions, NO)
Stretch

Question 23

Descirbe Ca+ stimulation in smooth muscle

Answer 23

Persistent stimulation and increase in [Ca2+] can occur
Phosphorylated X-bridges may be dephosphorylated when still bound to actin
Decrease rate of ATP splitting
Slows X-bridge cycle
Can maintain tension for long time with low ATP
Useful in blood vessel walls that have to stay open

Question 24

What are the different types of smooth muscle fibres

Answer 24

Single or multiunit smooth muscle
Most smooth muscles in organ have both
Organ can have mixture of properties in different areas