Muscles Flashcards
Describe the structure of skeletal muscles
Striated - voluntary muscles, diaphram
muscles fibres multinucleate
for in utero from mononucleate myoblasts
increase fibre size during growth
bundles of fibres encased in connective tissue sheaths
cells replaced after injury - satellite cells differentiate to form new muscle fibres
other fibres undergo hypertrophy to compensate
sarcomere
what is a sarcomere
repeating unit within muscle that allows it to complete its function (contraction)
combination of thick myosin filament (cropss bridge) and think actin filament(troponin & tropomyosin)
repeating arrangement - Z lines (thick deposit of protein holding together muscle unit)
Process of excitation-contraction coupling
- muscle action potentially propagated (travels deep within muscle core - transverse tubules)
- Ca ion released from lateral sac (Ca from ryanodine receptors)
- Ca ion binding to troponin removes blocking action of tropomyosin
- Cross bridge moves (contraction)
- Ca ion pumped out of cellular environment by ATP through Ca ATPases
- Ca ion removal from troponin restores tropomyosin blocking action (relaxation)
Explain the mechanics of skeletal muscle contraction
cross bridge move by grabbing onto actin filament
actin has binding sites attach to cross-bridge head on myosin
cross bridge cycle:
1. cross bridge binds to actin - Ca rises
2. cross bridge moves + ADP + Pi
3. ATP binds to myosin causing cross bridge to detach
4. Hydrolysis of ATP energizes cross bridge
back to 1.
Define “isometric” and “isotonic” twitches
Isometric - the muscle tension changes but the muscle length remains the same
isotonic - muscular contraction against resistance in which the length of the muscle changes
Explain the sliding-filament theory of muscle contraction and its relationship to the length-tension properties of muscle
less overlap of filaments - less tension (over-stretching)
too much overlap = filaments interfere with each other (over-contraction)
muscle length for greatest isometric tension = optimal length
Describe the generation of controlled force by recruitment of motor units
motor unit - motor neurons + muscle fibre
increase load = increased need to activate more motor units
increased number of active motor units = recruitment
slow oxidative fibres activated first, then fast oxidative, with fast glycolytic last
neural control of muscle tension depends on - frequency of AP’s to motor units, recruitment of motor units
Explain tetanus
AP is 1-2ms long but twitch may last up to 100ms
may get more AP’s during contraction, these add up = summation
longer sustained contractions
tetanic tension greater than twitch tension since Ca never gets low enough to allow troponin/trypomyosin to re-block myosin binding sites (proper-relaxation)
Explain fatigue
repeated muscle stimulation = muscle fatigue
depends on fibre type, length of contraction and fitness of individual
rests - able to contract again
fatigue prevents muscles using up vast amount of ATP, which would cause rigor (ie. muscle would not be able to activate new cross bridge cycle)
Describe the factors that cause fatigue
during high intensity short duration exercise - conduction failure due to K - depolarisation, increased lactic acid - acidifies proteins, increased ADP + Pi inhibits cross bridge cycle, dleaying myosin detachment from actin filament
during long term, low intensity exercise - decreases muscle glycogen, decreased blood glucose, dehydration
central command fatigue - cerebral cortex cannot excite motor neurons, unable to use CNS to excite neurons
Describe the aerobic and anaerobic properties ofd mucles
aerobic - increased mitochondria, increased vascularisation, increased fibre diameter (not massive due to O2 diffusion)
anaerobic - increased diameter, increased glycolysis
Describe the structure and function of smooth muscles
no striations
innervated by ANS, not somatic NS
has cross bridge cycle and uses Ca
filaments and excitation - contraction coupling are different
exists in hollow organs (eg. GI tract, uterus, airways, ducts), lines hollow organs and tubes
single celled and mononucleated
spindle-shaped
thick myosin and thin actin filaments, however filaments arranged diagonally across cells and are anchored to membranes and cell structures by dense bodies
Explain contraction in smooth muscle
increased Ca
Ca binds calmodulin
Ca calmodulin binds to myosin light chain kinase
kinase phosphorylates myosin cross bridge with ATP
phosphorylated cross bridge bind to actin filaments
contraction and tension
Explain relaxation in smooth muscle
smooth muscle relaxes via action of myosin light chain phosphatase - dephosphorylates cross bridges
persistent stimulation increases Ca in some smooth muscles
Factors affecting contractile activity
spontaneous electrical activity in muscle membranes = pacemaker activity
autonomic neurotransmitters from varicosities
hormones eg. oxytocin (ie breast milk lactation)
local factors (paracrine agents, pH, O2, osmolarity, ions, NO- nitric oxide)
stretch
