Skeletal, smooth, (cardiac) muscle Flashcards
What is the ultrastructure of skeletal muscle
Multi nucleated
10-100mm diameter, up to 20cm long
Bundles of fibres encased in connective tissue sheaths
Attached to bones by tendons
Striated
Describe the growth/repair of muscles
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
Describe striations
What is the structure of sarcomeres
Describe sliding filaments
Describe the cross bridge cycle
- As Ca+ rises energized cross bridges bind to actin
- ADP + Pi are released and cross bridge moves
- ATP binds to myosin, causing cross bridge to detach
- ATP hydrolysis to ADP + Pi energizeses cross bridge
What is the purpose of Troponin, tropomyosin, Ca2+
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
What is Excitation-contractile coupling
What are the different muscle mechanics
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
What are twitch contractions
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
Describe Tetanic muscle contraction
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
Describe the length-tension relationship
Less overlap of filaments = less tension
Too much overlap = filaments interfere with each other
Optimal length = muscle length for greatest isometric tension
What are motor units
Motor neurons + muscle fibres = MOTOR UNIT
Muscle fibres within a unit may be scattered throughout muscle
Explain fatigue
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)
What are the different types of skeletal muscle fibres
Slow oxidative (I) - resist fatigue
Fast oxidative (IIa) - intermediate resistance to fatigue
Fast glycolytic (IIb) - fatigue quickly
What are the aerobic and anaerobic properties of muscle
Aerobic exercise increase mitochondria, vascularisation, fibre diameter
Anaerobic (strength) exercise increases diameter, glycolysis
Describe oxidative muscle fibres
Increased mitochondria, oxid. phosphorylation, vascularisation to deliver more O2 and nutrients
Contain myoglobin increases O2 delivery
Fibres are red and have low diameters
Describe glycolytic fibres
Few mitochondria
Increase glycolytic enzymes and glycogen
Lower blood supply
White fibres with larger diameters
Describe the structure of smooth muscle
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
Describe smooth muscle filaments
Arranged diagonally across cells and anchored to membranes and cell structures by dense bodies (like Z-lines)
Filaments slide together to contract cell
Describe Smooth Muscle X-bridge cycle activation
- Increase Ca2+ binds to calmodulin
- Ca2+ - Calmodulin binds to mysoin light chain kinases
- Kinase phosphorylates myosin cross bridges with ATP
- Binds to actin and causes contraction
- Dephosphorylation of cross bridges relaxes smooth muscle
What are factors affecting contractile activity
Spontaneous electrical activity in muscle membranes
Pacemaker activity
Autonomic neurotransmitters from varicosities
Hormones
Local factors (pH, O2, osmolarity, ions, NO)
Stretch
Descirbe Ca+ stimulation in smooth muscle
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
What are the different types of smooth muscle fibres
Single or multiunit smooth muscle
Most smooth muscles in organ have both
Organ can have mixture of properties in different areas
