Skeletal Muscle Pt2 Flashcards
Contraction of motor unit initiated from
CNS, occurs as all or noting twitch response
Contraction of motor unit steps
- Alpha-motor neuron propagates actio potential to muscle
- Acetylcholine released, binds at muscle endplate
- Channels in nicotinic receptors open -> Na+ diffusion into cell
- depolarization at motor endplate
- If enough miniature endplate potentials -> full fledged end plate potential
- Voltage gated Na+ channels open -> self propagating action potential spreads involving entire plasma membrane including T-tubule system membrane invaginations
- Voltage gated Ca2+ channels in T-tubules open
- Voltage-induced confrontational change t-tubule channel -> Ca2+ release channels opening
- Ca2+ flows out SR through release channels
- Large rapid increase in cytoplasmic calcium concentration
- Ca2+ binds troponin C
- Conformational change in troponin complex -> change troponin complex
- Conformatinal change troponin complex pulls tropomyosin out of normal position on actin
- myosin heads bind acting forming cross-bridges
- myosin head hydrolyzes ADP uses energy ATP hydrolysis to walk along actin filament
- Ca2+ removed from cytoplasm by Ca2+-ATPase of SR membrane
- Cytosolic Ca2+ concentration decreases
- Ca2+ released from troponin C
- Tropomyosin returns to resting portion blocking myosin binding sites on actin filaments
acetyl choline released from____ at ____
-released from presynatptic alpha-motor neuron at neuromuscular junction
acetylcholine diffuses across____ to bind to ____ in ___
acetylcholine diffuses across synaptic cleft to bind at nicotinic acetylcholine receptors in the muscle endlplate
nicotinic Ach receptors are
ligand-gated ion channels
Depolarization event at motor endplate is NOT
an action potential
action potenital occurs only if
enough miniature end plate potentials to summate to full-fledged end plate potential
Voltage gated Ca2+ channels in T-tubules closely associated with
calcium release channels in SR membrane which is why voltage-induced conformational change in T-tubule channel causes Ca2+ release channel to open
the linkage between voltage gated Ca2+ channels and Ca2+ release channels in SR is
- mediated direct mechanical interactions between channels on T-tubule and SR
- specifically by large cytoplasmic extension (foot process) of Ca2+ release channel
conformational change in troponin fx
- pulls tropomyosin out of normal position on actin allowing for interactions between actin and myosin
myosin heads biding actin form
cross bridges
single step of movement down actin filament produced by
each cycle of ATP binding, hydrolysis, and phosphate release
cross-bridge cycle continues until
Ca2+ removed from cytoplasm by Ca2+-ATPase of SR membrane (SERCA) ultimately leading to Ca2+ release from troponin C -> tropomyosin returning to resting position -> blocking myosin bing sites on actin filaments
sequence of events in excitation-contraction coupling
- action potential
- increase cytosolic Ca2+ concentration
- Muscle contraction
summation
- duration of contraction (twitch) long releative to duration exciting action potential so can initiate second action potential before first fully subsides causing second twitch to superimpose on residual tension first twitch this = summation
- if frequency stimulation great enough individual twitches occur v close together in time and then are no longer distinguishable from each other
ATP use muscle contraction
- Cross-bridge cycling
- moving Ca2+ from cytoplasm back into SR
- myocytes use ATP maintain and replenish membrane potentials
- in exercise muscle use ATP can exceed 100x basal rate
ATP needed for contraction and relaxation
small ATP stores so have to regenerate ATP needed in contraction and relazation
energy pathways in muscle fiber
- stores ATP and phosphocreatinin
- Anerobic glycolysis
- Oxidative phosphorylation
Phosphocreatinin
- most available pool energy= high energy bond phosphocratinine
phosphocreatinin produced by/ steps use
- transfer high energy phosphate from ATP to creatinine by creation kinase in mitochondria
- Phosphocreatinin diffuses from mitochondria -> major sites ATP utilization
- Phosphate transferred from phosphocreatinin to ADP
- dephosphorylated creatinine diffuses back to mitochondria where can be rephosphorylated
major sites ATP utilization
- myofibrils
- SR
- Sarcolemma
what allows for rapid regeneration ATP phosphocreatinin steps
phosphate transferee phosphocraatinin to ADP
anaerobic glycoysis
produces ATP quickly to support muscle activity for few minutes but end products impair contractile fx
end products anaerobic glycolysis
- H+
- lactate
- both impair contractile fx
oxidative phospohrylation
provides energy for muscle usage at intensities that can be sustained for longer than a few minutes
fuels used by muscle fibers to produce ATP
- main: carbohydrates and fats
- small contributor to total energy production: metabolism of amino acid
- selection of fuel depends on intensity and duration of exercise
carbohydrates for fuel muscle fibers for ATP production in what forms
- plasma glucose
- muscle glycogen
fats for fuel muscle fibers for ATP production in what forms
- free fatty acids
- muscle triglycerides
fuel for high intensity exercise
- glycogen stores= primary fuel
fuel for low intensity exercise or long duration exercise
- free fatty acid metabolism= major source
affect of myosin and actin isoforms on cross-bridge cycling
- isoforms affect speed of cross bridge cycling
- some muscle fibers contract more quickly than others
- speed contraction influences what metablic pathways used by cell
2 main types muscle fiber
- type 1
2. type 2
type 1 muscle fiber
- slow
- oxidative
- fatiuge- resistant
- postural muscles contain more type 1
- shorten less rapidly than type 2?
type 2 muscle fiber
- fast
- glycolytic
- easily fatigued
- fast-twitch shorten more rapidly than slow twitch (type 1?)
virtually all muscles contain what muscle fibers
mix type 1 and type 2 allowing any muscle to be used in different activities with different mechanical and metabolic demands
muscle fibers vary in what phenotypes
biochemical mechanical and metabolic phenotypes
sarcomere
basic contractile unit skeletal muscle
motor unit
- basic functional unit of skeletal muscle
- defined as single motor neuron and all the fibers it innervates
- cells with in motor unit contract synchronously when motor neuron fires
- vary in size
small motor units
necessary for fine, precise movements
muscles with large motor units
control course mvoements
all fibers within motor unit=
same fiber type
motor neuron pool
group of all motor neurons that innervate single muscle
motor units and fasicles
- motor units do not coincide with fascicular organization of muscle
- each fasicle contains fibers belonging to many different motor units
force generated by muscle depends on
- degree of activation by nervous system
- size of muscle
- architecture of muscle (especially pinnation angle)
- number actin-myosin cross-bridges formed
- force generated by each cross-bridge
muscles can generate increasing force how
frequency summation
multiple-fiber summation aka spatial summation
frequency summation
- at level of single fiber
- force increased by summing multiple twitches over time
multiple-fiber summation or spatial summation
- at level of whole muscle
- force can be increased by recruiting more motor units and summing contractions of multiple fibers
muscle fiber activation during muscle activation
- brain recruited small motor units with slow oxidative fibers 1st
- excitatory input increases
- larger motor units containing fast-twitch fiber recruited -> increased force generation
why can’t you sustain maximal tension for more than brief period of time
b/c fast twitch fibers fatigue more rapidly due to glycogen depeltion
How are gradations in force accomplished at levels of force production lower than upper recruitment
- changing number active motor units
- changing firing rate in those that have been recruited
- once all motor units have been recruited force can be increased only by increasing firing rate
as more motor units recruited force
increases
fluid and energetically efficient movements require
learning in order to achieve optimal recruitment patterns of motor units
